Mechanism of the a-silylcarbinol to silyl ether rearrangement1

A. G. BROOK, G. E. LEG ROW,^ AND D. M. MACRAE Department of Chemistry, University of Toronto, Toronto, Ontario

Received August 5, 1966

The rates of rearrangement of a wide variety of a-silylcarbinols have been measured. Hammett plots of log k vs. u have been made for several systems, and values for p, for the energy of activation, and for the entropy of activation have been obtained. A mechanism which is consistent with the above data, and which involves a cyclic three-membered transition state, has been proposed for the a-silylcarbinol to silyl ether rearrangement.

Canadian Journal of Chemistry. Vo lume 45, 239 (1967)

In recent years considerable study has been given to the base-catalyzed rearrange- ments of a-silylcarbinols, which usually are rapidll- and quantitatively converted into the isomeric silyl ethers. In addition to

product analysis studies (1-3), the stereo- chemistry of the rearrangement both a t an asymmetric silicon center (4) and a t an asymmetric carbinol-carbon center (5) has been investigated and shown to involve retention of configuration in both cases. The present paper reports a kinetic study of this reaction.

Kinetic Results I t \\;as found experimentally that the

rates of rearrangement of a wide variety of a-silylcarbinols were first - order in carhinol and pseudo first order overall, since the basic isomerizing agent is not consumed during the reaction. The kinetic data for most of these rearrangements mere obtained by nuclear magnetic resonance (n.m.r:) techniques (described more fully in the Experimental), wherein the peak height of an appropriate signal (frequently methyl attached to silicon or carbon) was followed with time for the silylcarbinol reagent, the silyl ether product, or both. A typical result is shown in Fig. 1, which clearly shorn-s, on a figure resulting from horizontal displacement of the n.m.r. chart

'From the Ph.D. thesis of G. E. LeGrow, Univer- sity of Toronto, Toronto, Ontario, 1964.

2Dow-Corning Silicones of Canada Fellow, 1962-1964.

pen after each recording of the CHrSi signals in the 6 0.6 to 0.7 p.p.m. region of the spectrum, the exponential decrease in the silylcarbinol concentration, and con- currently, the exponential increase in the silyl ether concentration. Plotting the logarithm of the peak height against time gave pseudo first order plots, of which Fig. 2 is typical. The rearrangements are also first order in base, since division of the pseudo first order rate constant kl' by the catalyst concentration yields a second- order rate constant kz which is independent of the initial concentrations of both the a-silylcarbinol and base. Typical data are given in Table I. -

Wide variations in the rates of rearrange- ment were observed depending on the structure of the a-silylcarbinol. In a series of diphenylcarbinols, in which only the nature of the groups attached to silicon were changed (Table 11), the rates of rearrangement decreased linearly by about a factor of 6 as the three phenyl groups of triphenylsilyldiphenylcarbinol were succes- sively replaced by methyl groups, an overall decrease in the rate by a factor of about 240 being observed in going from triphenyl- silyl- to trimethylsilyldiphenyl-carbinol.

A very limited investigation mas made of the effect on the rate of rearrangement of 9-substituents on the phenyl groups attached to silicon. Under identical con- ditions, with diethvlamine as catalvst and dimethyl sulfoxide as solvent, tris(p:chloro- phenyl)silylmethylphenylcarbinol re- arranged about 13 times faster than triphen~~lsilylmethylphenylcarbinol a t 36.5 O C , the second-order rate constants being

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240 CANADIAN JOURNAL OF CHEMISTRY. VOL. 45. 1967

TlME (rnin) 0 5 10 15

TlME (min)

FIG. 1. The decrease in the a-silylcarbinol concentration and the increase in the silyl ether concentration with time for the reaction Ph2MeSiCOHPhz + PhzMeSiOCHPh2 with diethylamine in CHCl, a t 34.5 OC. FIG. 2. Plot of log concentration of a-silylcarbinol vs. time for the reaction PhsSiCMePhOH +

PhaSiOCHMePh with diethylamine in dimethyl sulfoxide a t 26.5 "C.

TABLE I

Rates of rearrangement of triphenylsilyldiphenylcarbinol by diethylamine in chloroform a t 27.5 OC

Initial carbinol Initial base concentration ( M ) concentration (M) k ~ ' (s-l) k2 (1 mole-' 6-1)

TABLE I1 Rates of rearrangement of or-silyldiphenyl- carbinols by diethylamine in chloroform

a t 27.5 "C

Carbinol k2 (1 rn~le-~s-l)

9.53 X and 7.41 X 1 mole-I s-l, respectively. Attempts to measure the rate of rearrangement of tris(p-t-butyl- phenyl)silylmethylphenylcarbinol were un- successful, since the n.m.r. signals for the t-butyl group in the carbinol and silyl ether overlapped seriously.

Much greater changes in the rate were observed when the nature of the groups attached to carbinol-carbon were altered. Although direct measurement of the series

~triphenylsilyldiphenylcarbinol, triphenyl- silylmethylphenylcarbinol, and triphenyl- silyldimethylcarbinol could not be made under the same experimental conditions, replacement of phenyl by methyl produced a decrease in the rate by a factor of about 1000. In addition, replacement of a di- phenylmethyl group by a 9-fluorenyl group resulted in a significant rate increase, as shown in Table 111.

TABLE I11 Rates of rearrangement of or-triphenylsilylc~binols

by diethylamine in chloroform a t 27.5 OC

Carbinol k2 (1 mole-' s-1)

9-Triphenylsilyl-9- hydroxyfluorene 1.18

PhaSiCPhzOH 6.10 X PhaSiCPhMeOH 4.3X10-6* PhaSiCMezOH Too slow to measure

*This rate constant was not measured directly, but has been calculated from the rate constant of this compound in dimethyl sulfoxide, using the data from Table IV.

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BROOK ET AL.: REARRANGEMENT MECHANISM 241

T o learn something about the transition state and the effect of substituents on the reaction, several series of triphenylsilyl-p- substitutedphenylcarbinols were synthe- sized, and their rates of rearrangement, catalyzed by diethylamine, were measured in suitable solvents by the n.m.r. method. The data for three of these series (the tri- phenylsilylmethylphenylcarbinols, the tri- phenylsilyldiphenylcarbinols, and the tri- phenylsilylphenylcarbinols) are given in Table IV.

For the methylphenylcarbinols, i t is apparent that the rate of the rearrangement is increased by electron-withdrawing p-sub- stituents, and that, as shown in Fig. 3, a

linear relationship between log kz and the Hammett p-substituent constant (a,) exists for the Br, C1, H, and t-butyl compounds. The slope of this line (p, the reaction constant) is +4.63 (+4.60 a t 34.5') with a correlation coefficient of 0.999. This very large and positive value for p indicates that the transition state for the reaction is highly sensitive to the electron-withdrawing properties of the substituents, a large negative charge being developed either on the aromatic ring bearing the substituent or on the adjacent carbon atom (6), and the transition state being stabilized when the charge can be delocalized by electron- withdrawing substituents.

TABLE IV Rates of rearrangement of triphenylsilyl-p-X-phenylcarbinols by diethylamine

kz (1 mole-' s-1)

Temperature (OC) 27.5 34.5 34.5 36.0 Solvent* DMSO DMSO CHCla DMSO P 4.63 4.60 1.54 3.66 Correlation

coefficient 0.999 0.999 0.999 0.99 *DMSO, dimethyl sulfoxide.

-1.2 - -1.2 - B r

- -

-2.0 -

- - 4 -

I I I I I I >

-3.6

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

OP OP

FIG. 3. Plot of log k 2 vs. Hammett up constants for rearrangements of PhaSiCOHMeC8H4-p-X. FIG. 4. Plot of log k2 vs. Hammett up constants for rearrangements of 0, PhaSiCOHPhCeH4-fi-X; and

0, PhaSiCHOHCsHrp-X.

-4.4

- 3 - .OMe

I I I I I I

-4.4

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242 CANADIAN JOURNAL OF CHEMISTRY. VOL. 45. 1967

The points for p-methoxy and p-fluoro deviate noticeably from the above corre- lation, and, in fact, the p-fluoro compound rearranges more slowly than the parent hydrogen compound despite the fact that the Hammett substituent constant for p-F is positive. These deviations mill be dis- cussed in further detail later.

Table IV and Fig. 4 display the rates of rearrangement for two other series of p-substituted silylcarbinols (the triphenyl- silyldiphenylcarbinols in, chloroform, and the triphenylsilylphenylcarbinols in di- methyl sulfoxide). Again, linear relation- ships of log rate constant with up were found for the p-t-butyl, hydrogen, chloro, and bromo compounds, but again the points for both methoxy and fluorine fall off the line, the fluoro compound again rearranging more slo11~1y than the parent compound. For the diphenylcarbinol series, the slope of the line is +1.54 with a correlation coefficient of 0.999. The smaller magnitude of p ~mplies that, in this series of compounds, the transition state is less sensitive to sub- stituent effects and that there is less localization of the negative charge on the carbon-atom adjacent to the ring bearing the substituent. For the simple phenyl- carbinol series, o is t 3 . 6 6 , a value inter- mediate between those observed for the diphenylcarbinol and phenylmethylcarbinol series.

In Table V are listed rate data for three series of triphenylsilylbenzylcarbinols whose rates of rearrangement were also measured in dimethyl sulfoxide. Hammett

substituent plots for the benzyl-p-sub- stituted phenj~lcarbinols and for the p-sub- s t i t u t ed phenyl-p-subst i tuted benzyl- carbinols showed a minimum of scatter, and in both series large positive reaction constants of 3.40 and 4.14, respectively, were found. In the series of p-substituted phenylbenzylcarbinols, considerably more scatter in the Hammett substituent plot was observed. In fact, the rates appeared to vary according to the size of the p-sub- stituent on the benzyl group, rather than according to the u constant of the sub- stituent. This would suggest that the methylene in the substituted benzyl group is acting as an effective insulator against inductive or mesomeric effects on the reaction center. On the other hand, i t is not clear why the addition of substituents on the benzyl group in the p-substituted benzylphenylcarbinols should raise the magnitude of p in this new series by the significant amount observed. This implies that an electronic effect acting through the methylene of the benzyl group is felt by the reaction center.

Table VI reports the rate data and the related activation parameters obtained from a study of the effect of temperature on the rearrangements of two isomeric silylcarbinols (triphenylsilylmethylphenyl- carbinol and diphenylmethylsilyldiphenyl- carbinol), using an n.m.r. instrument equipped with a variable- temperature probe. Linear plots of log ks vs. l / T were obtained over the 30 degree temperature range studied.

TABLE V Kates of rearrangement of triphenylsilylbenzylphenylcarbil~ols containing p-substituents, by diethylamine in

dimethyl sulfoxide at 36.0 "C

kz (I mole-' s-l)

P Correlation

coefficient

3.40 Uncertain correlation 4.14

0.999 0.999

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BROOK ET AL.: REARRANGEMENT MECHANISM

TABLE VI Rates and activation parameters for isomeric silylcarbinols

Ph3SiCMePhOH PhzMeSiCPhzOH

kz (1 mole-' s-'1 T ("C) kz (1 mole-' s-I) 7- (OC)

Catalyst Diethy lamine Solvent Dimethgl sulfoxide Arrhenius energy of

activation (E,) 11.16 kcal/mole Correlation coefficieilt 0.999

Diethylamine Chloroform

Entropy of activation (AS*) - 3 4 . 3 f 0.1e.u. -45.6~kO.le .u .

The two isonxeric silvlcarbinols were chosen for this study so that there ~vould be no effect of substrate mass on the activated complexes of the two rearrange- ments, except for solvent coordination differences. The relatively low energies of activation of about 11 and 8.5 kcal/mole are coilsistent with a transition state in xvhich considerable progress toward new- bond formation has occurred before old- bond distortion,- and the large negative entropies of activation of - 34 and - 45 e.u. are consistent with a highly oriented, stericallq- crowded, activated complex in which there may be considerable restriction to free rotation about single bonds, as in a cyclic transition state (7).

Finallv, Table VII summarizes the rate A ,

data obtained with two carbinols where the solvent and the catalyst were varied. As might have been anticipated for a polar reaction, the change from a relatively non- polar solvent such as chloroform to the

polar solvent dimethyl sulfoxide resulted in rate enhancements by factors of 200 to several thousand. The rate of reaction also shows some dependence on the basicity of the isomerizing agent, the stronger base effecting more rapid rearrangement, par- ticularly in the less-polar solvent, but the effects observed do not vary linearly with the pKb of the base.

The Mechanism of the Rearrangement A general mechanism for the silylcarbinol

to silyl ether rearrangement must be con- sistent with the foregoing data, and with the known stereochemistry of the reaction (4, 5). In particular, the stereochemistry of the reaction involves retention of con- figuration a t both asymmetric silicon and carbinol-carbon atoms, and the kinetics, \vlxich are first order in carbinol, involve processes having low energies of activation but very high entropies of activation, where the transition state appears to

TABLE VII Rates of rearrangement for silylcarbinols in dimethyl sulfoxide (DMSO) or CHCll with different

amines as catalyst

kz (1 mole-Is-')

Carbinol Solvent (CH2)sSH (CzH6)zNH (CzHd3N

PhzMeSiCPhZOH DIMSO 2.41 1.31 1.33XlO-I CHC13 9.27X10-3 1 . 3 1 ~ 1 0 - ~ 3.4X10"*

PhaSiCOHMeCsHa-b-Br DMSO 1.59XlO-1 7.79X10-2 6.51X10-3

*The rates of these reactions were too slow to give reproducible results; the values shown are estimates based on trends shown in the rest of the table.

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244 CANADIAN JOURNAL O F CHEMISTRY. VOL. 45, 1967

involve considerable development of the negative charge on the carbon atom ad- jacent to the substituted aromatic ring.

A mechanism consistent with these data is shown in eqs. [I]-[3]. The first step, which involves removal of the carbinol

.. BH+ [3 bl RQSi C-R - RaSiOCHRR1 \o/k/ Fast

proton, is known to be rapid and reversible (from n.m.r. data). The hydroxyl hydrogen of or-silylcarbinols appears as a sharp peak in the region 250-360 c.p.s. below tetra- methylsilane, when dimethyl sulfoxide is used as solvent (8). When as little as equivalent of base is added to the silyl- carbinol solution, the hydroxyl resonance signal completely disappears, although the n.m.r. spectrum shows that no rearrange- ment to ether has occurred. Thus the hydroxyl proton must be undergoing a rapid base-catalyzed exchange, and hence both k1 and k-1 are large.

The second, rate-controlling step involves intramolecular attack by oxygen on silicon, probably facilitated by the energetically available d orbitals on silicon, and accom- panied by weakening of the carbon-silicon bond. The species formed in step [2] may be a pentacovalent silicon intermediate, or the species written may better be regarded as the transition state. The cyclic, highly oriented structure is consistent with the high entropies of activation observed, particularly if, as seems possible from the high entropy of activation, and the high

stereospecificity of the rearrangement, the protonated base is an intimate part of the activated com~lex. The existence of Denta- covalent silicon species seems clearly estab- lished (9), and they have frequently been proposed (10) as intermediates in displace- ment reactions, but no data are available in the present study which require the forma- tion of an actual sp3d intermediate.

The low energies of activation observed for the reaction are consistent with con- siderable progress toward new-bond forma- tion between silicon and ox\-aen in the - transition state. Such bond formation, in- volving sp3d hybridization of silicon, would involve the development of a negative charge on silicon, and to the extent that this could be delocalized, the reaction should be facilitated. I t is, in fact, observed that electron-withdrawing substituents. such as chlorine, on phen$ groups attached to silicon lead to rate enhancement by a factor of about 13. The large positive values of p found for the methylphenyl- and phenyl- hydrogen-carbinols imply a considerable development of negative charge on the carbon atom adjacent to the aromatic ring, which would result from substantial break- ing of the silicon-carbon bond. Although this charge would appear to be highly de-

. veloped in the methylphenylcarbinols (p = 4.63), in the diphenylcarbinols (p = 1.54) the smaller value of the reaction constant is consistent either with a smaller degree of bond rupture or, more probably, with significant delocalization of the charge into the second unsubstituted phenyl ring. In all cases, electron-withdrawing groups, which would reduce the charge on the carbon atom, facilitate the rearrangement. I t is evident that the transition state for the rearrangement has considerable carb- anion character, and that the more stabi- lized the species is the more readily the rearrangement occurs.

The final s t e ~ in the reaction involves collapse of the activated complex or penta- covalent intermediate to silyl ether. This may occur directly (step [3a]) or could proceed via a carbanion-like species, which must subsequently be rapidly and irrevers- ibly protonated (step [3b]). The optically

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BROOK ET AL.: REARR AITGEMENT MECHANISM 245

active silyl ether produced stereospecifically from optically active silylcarbinol is not racemized by prolonged treatment with diethylamine or other isomerizing agents such as sodium-potassium alloy (11), as would be expected if step [3b] were revers- ible. Further, since the reaction yields silyl ether with complete retention of configura- tion regardless of whether the rearrange- ment is carried out in diethyl ether, di- methyl sulfoxide, or methyl cellosolve (solvents of considerably different polarity and protonating ability), any carbanion- like species involved in step [3b] would be unusual because of its lack of racemization or inversion in the more polar solvents (12). I t is therefore impossible to decide with the available data whether the transition state collapses directly by stereospecific protona- tion to product, or whether the transition state collapses to an intimate ion pair involving protonated base and carbanion which rapidly undergoes stereospecific pro- tonation, regardless of the nature of the solvent-system.

Although i t is not clear whether a "carbanion" is involved in the reaction, i t appears quite clear that the transition state possesses considerable carbanion character, and that the stability of this species strongly influences the rates of the reactions. Thus, the rates of rearrangement shown in Table I11 exactly parallel the stabilities expected for the related carbanions, charge delocali- zation being most effective in a fluorenyl system, and least effective in an isopropyl carbanion. Indeed, i t is significant that an approximately linear cor;elation exists between the p values determined for the phenyl, benzyl, hydrogen, and methyl p-substituted phenylcarbinol series and the Taft polar substituent constants (13) for these groups.

The data in Table I1 indicate that, for a series of related diphenylcarbinols, the rates of rearrangement are somewhat increased as the methyl groups on silicon are replaced by phenyl groups, but this effect is much smaller than similar replacements on the a-carbon atom. This rate enhancement probably arises in part from stabilization of the cyclic sp3d hybridized transition state

by inductive electron withdrawal by the phenyl groups (p-chlorophenyl groups are even more effective), but i t is conceivable that the rate enhancement may also have its origin in part from steric effects, a bulkier triphenylsilyl group resulting in more carbon-silicon bond stretching in relief of steric strain in the activated complex than occurs with the smaller trimethylsilyl group.

I t was noted earlier that, in each of the silylcarbinol systems investigated, the points for the 9-fluoro and p-methoxy comp&nds consistently fell off the line, in a direction in- dicating that these compounds reacted more slowly than would have been expected on the basis of the published u9 values.

The possibility that these deviations were caused by hydrogen bonding between the catalvst diethvlamine and the unshared elections of the fluorine or methoxyl groups was eliminated by showing that the same relative rates of rearrangement were ob- - tained for triphenylsilylmethylphenylcar- bin01 and its p-fluoro derivative, with either diethvlamine or triethvlamine. The latter catalyst, lacking hydrogen atoms attached to nitrogen, could not hydrogen bond to fluorine or methoxyl groups. The rates for the hydrogen compound and its p-fluoro derivative with diethylamine in dimethyl sulfoxide a t 34.5' were 6.53 X and 2.47 X (ratio 2.64), whereas with tri- ethylamine the rates were 5.37 X and 2.17 X 1 mole-' s-I (ratio 2.48), res~ectivelv.

Similar deviations of fluoro and methoxy compounds from Hammett plots have previously been reported by Eaborn and his co-workers in the base-catalvzed cleav- ages of benzyltrimethylstanna&s (14) and benzyltrimethylsilanes (15), by Berliner (16) in the reaction of piperidine with 2-nitro-4-substituted bromobenzenes, and by Streitwieser (20) on the rate of reaction of 9-fluorotoluene with lithium cyclohexyl- amide. Larne deviations of this sort seem - to arise only when p for the reaction is large and where the transition state bears the charge either directly on the aromatic ring or on the adjacent carbon atom. In these systems, which are considerably different

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246 CASADIAN JOURNAL O F CHEMISTRY. VOL. 45. 1967

from the benzoic acid systems with lo117 p

values, from which the normal values of u were derived, the resonance interaction of p-methoxyl or p-fluorine with the ring must affect the delocalization of the developing electron density on the adjacent carbon atom to a greater extent than is accounted for in the normal u values. This should result in less stabilization of the carbanion, and a slower rate of rearrangement. Com- menting on the deviation in his system, Streit~vieser (20) has suggested that adding a U- component to the normal u value, in a Yukawa-Tsuno type of correction, may be helpful. In this connection AlcKeever and Taft (21) have recently proposed u- values of -0.03 and -0.47 for 9-F and p-3Ie0, respectively, values much more negative than those usually ascribed to these groups. I t is significant that, if the log rate values for p-F and p-llle0 in the present work are forced onto the observed linear correlations of lon k with u bv allowing u to varv, the - - range of values found for u, 9-F =

+0.005 to -0.13 and p-Me0 = -0.30 to -0.45, are in reasonable agreement with Taft's U- values. Thus concerted inductive- mesomeric interaction (22) may account for the observed effects3

The preceding data indicate the remark- able facility with which a-silylcarbinols rearrange to their isomeric silyl ethers, and strongly suggest the involvement of a cyclic transition state in the rearrangement. Sub- sequent papers in this series will indicate that the silylcarbinol to silyl ether re- arrangement is but one of a family of related reactions in which stereospecific silicon- oxygen bond formation occurs with accom- panying silicon-carbon bond cleavage.

EXPERIMENTAL

All reactions involving organometallic compounds were carried out under a nitrogen atmosphere.

The carbinols utilized in this study were syn- thesized by one of several standard methods: (a) the reaction of 3 Grignard reagent with a be~~zoy l - silane, ( b ) the reductiou of a benzoylsilane with lithium aluminium hydride, or (c) the hydrolysis of a benzylic bromide (made by the reaction of a hydrocarbon with N-bromosuccinimide) with silver acetate. Typical examples of each of these methods

3Last three sentences added in proof

are given below, and the various new compoullds prepared are tabulated, together with their proper- ties, in Tables 1'111 and X . The sily1 ethers, which were the products of the rearrangements, were normally isolated from the reaction, purified, and characterized, and are given in Table IX. In a few cases they were also syuthesized directly.

The following compounds have been described previously: triphenylsilyl- and diphenylmethylsilyl- diphenylcarbinols and their ethers (I) , trimethyl- silyldiphenylcarbinol and its ether (2) , triphenyl- silylmethylphenylcarbinol and its ether (2), p-sub- stituted benzoyltriphenylsilanes (X = F, CI, Br, t-butyl, or OMe) (17), and @-substituted benzhydryl- triphenylsilanes (X = F, Cl, t-butyl, or OR'Ie) (18).

Triphenylsilyl-p-bromophenylnzethy2carbzol T o a solution of 1.60 g (0 0036 mole) of p-bromo-

benzoyltriphenylsilane (17) in 25 ml of dry ether, 5.00 ml (0.0072 mole) of a 1.45 Msolution of methyl- magnesium bromide was added dropwise, with vigorous stirring. On complete addition the yellow color of the ketone had disappeared. The reaction mixture was then added to dilute hydrochloric acid, and the ether layer was washed with water and dried with magnesium sulfate. After the ether l a ~ e r was filtered, the solvent was removed from the filtrate and the residue was crystallized from hot 1:1 ethanol-water to give 1.32 g (80yo) of tripheng Isilyl- p-bromophenyln~ethylcarbinol, m.p. 130-130.5 O C .

Triphenylsilyl-p-chlorophenylcarbinol T o a solution of 1.50 g (0.038 mole) of lithiutll

aluminium hydride in 150 ml of dry ether (prepared by refluxing lithium aluminium hydride ill ether for about 3 h) a t 0 'C was added rapidly, with stirring, in 10 min a yellow solution of 20.0 g (0.05 mole) of p-chlorobenzoyltriphenylsilane (17) in 200 ml of dry ether. The yellow color immediately disap- peared. T o this solution, 50 ml of ethyl acetate mas added rapidly, with stirring. The reaction mixture was added to dilute hydrochloric acid and the product was worked up in the same manner as described above. The residue was recrystallized from ~e t ro l eum ether i b . ~ . 90-100 "C) to give 17.9 P

Preparation of p-Chlorobenzhydryltriplzenylsilane Anhydrous boron trifluoride gas was bubbled into

a solution of 4.01 g (0.01 mole) of triphenylsilyl-p- chlorophenylcarbinol in 250 ml of dry benzene a t 5' for 10 min (18). The colorless reaction mixture was drowned it1 ice water and extracted with ether. After the ether layer was washed and dried, removal of the solvent left a crystalline residue, whose n.m.r. spectrum showed a singlet a t 6 4.25 p.p.111. attrib- utable to p-chlorobenzhydryltriphenylsilane and a doublet 6 3.9 p.p.m., Ja-s 7.5 c.P.s.,~ attributable to p-chlorobenzhydryldiphenylfluorosilane, the inten- sities of the bands being in the ratio of 64:36,

4We are indebted to Professor R. J. Gillespie of RiIcMaster University, Hamilton, Ontario, for the fluorine n.m.r. spectra.

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TABLE VI l I

a-Silylcarbinols

Carbon (O/ , )

Method of Yield synthesis (Oj,)

Melting poi11 t ("C) Formula

Calcu- lated Found

Calcu- lated Found

, "

( ~ - c I - c ~ H , ~ , s ~ c o H ( c H ~ ) P ~ A (p-t-B11-C~! Ia)3SiCOEI(CI13)P1~ A

68, 95

~ - - - --

*Crystallizrd from 1 :1 ethanol-watrr. +Crystallized from petroleum ether (h.p. 90-100 'C). $Crystallized from ethanol. $Crystallized from 1 :I ethanol - petroleum r t t ~ r r (b.p. 90-lCO OC). IlBoiling point 140 O C a t 0.20 mm, n ~ ~ ~ . V ] . 6 0 1 . TCrystal!ized from 3 :1 ethanol-benzene. **9-Hydroxy-9-fluoret1yl. NOTE: A, ketone + RMgX; B, ketone + 1,iAlHt; C. bromide $ AgOAc and water; D, hydrocarbon + N-brotnosuccinitnide.

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pUn0.g pale1 pun03 paw1 e l n m ~ o + ~ (3,) ~u!od (%) srsaq~uds punodwo3 rr: -nqe3 -n31~3 SU!TI~~ZI Pla!A 30 PoVJaM 3

(%) tIa80lpd~ z (%I UOCIJE3

s n s~ayza -4

5 XI 818V.L 0

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TABLE X Miscellaneous compounds

Carbon (%) Hydrogen (%I

Method of Yield Melting Calcu- Calcu- E 0

Compound synthesis (%) point ("C) Formula lated Found lated Found

Gt 59 M

Ph3SiCHPhCsH4-p-Cl 170-17211 C31H25SiCl 80.75 81.23 5.47 5.44 4 PhaSiCHPhCsH4-p-F 2 58 165-16711 C31H25SiF 83.37 83.72 5.64 5.67 $ Ph3SiCHPhCsH4-p-OMe 76 1651677 C32H2sSiO 84.14 84.47 6.17 6.30 :. Ph3SiCHPhCsH4-p-t-Bu H 54 165-166** C35H34Si 87.08 85.98 7.10 7.11 p PhaSiCBrPhCaHn-p-C1 H 85 200-202tt C31I-Iz,SiBrCI 68.95 68.82 4.48 4.53 g Ph3SiCBrPhCeH4-p-F H 82 198-200tt C31H24SiBrF 71.12 70.66 4.62 4.95 p PhMezSiCHPht I 69 - $1 C21H2~Si 83.38 83.41 7.33 7.17 PhaSiC13Hg* I 49 182-1837 C31H24Si 87.69 86.68 5.70 5.66 Ph3SiCBrClzHs* D 93 171-17211 C31H23SiBr 73.95 74.05 4.60 4.75 (p-t-BuC6H4)3SiCH2Ph J D 66 170-17258 C37H46Si 85.63 85.98 8.95 8.95 f (p-t-BuCsH4)3SiCBrzPh 90 258-26Ol111 C37H4,SiBi-2 65.67 64.73 6.57 6.46 M (p-t-BuC6H4)3S1COPht C 70 140-14211 C37H44Si0 83.39 82.56 8.34 8.29 ",

*CHCnHs = 9-fluorenyl. ?Infrared: C 4 stretching a t 6.18 p (1 618 cm-1) (CC14). Ultraviolet: X,,, (C6H12) 227.5 (e 38 000). 256 ( 6 17 700). 407 (r 286). 427 (s 400), and 448 mp ( e 250); A,,, (&OH)

(a 39 700). 258 (a 15 650). and 420 mp (e 346). D h e reaction gave a 64:36 mixture (as shown by n.m.r.) of P-chlorobenzhydryltriphenylsilane and P-chlorobenzhydryldiphenylfluorosilane. &The reaction gave a 60:40 mixture (as shown by n.m.r.) of fi-fluorobenzhydryltriphenylsilane (6 4.35 p.p.m. (1H)) and fi-fluorobenzhydryldipi~e~~ylfluorosilane (6 4.05 p.p.m

JH-F = 8 c.p.s.)).

4 Crystallized from ethanol. Crystallized from petroleum ether (b.p. 90-100 "C) .

**Crystallized from 1 :1 ethanol - petroleum ether (b.p. 90-100 'C). y y s t a l l i z e d fromacetone. t Bolllng polnt 128 130" a t 0.25 mm. n~2s.s 1.596. $$Crystallized from 1 :1 ethanol-benzene. (IIICrystallized from ethyl acetate. NOTE: C. bromide +AgOAc and water; D. hydrocarbon +N-bromosuccinimide; G. PhaSiCHOHCsHrX + CsHs + BF3; A. PhsSiCHOHPh f C6HsX + BF3; I. &Sic1 +

J. RSiC13 + 3R'Li.

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250 CANADIAN JOURNAL O F CHEMISTRY. VOL. 45, 1967

respectively. The mixture was dissolved in 50 ml of acetone and 5 ml of water, and 0.89 g (0.005 mole) of silver acetate was added. The resultant suspension was stirred in the dark for 6 h and then was filtered to remove silver fluoride and excess silver acetate. Removal of the solvent from the filtrate left a color- less oil, which was crystallized from hot ethanol to give 2.70 g (5970) of p-chlorobenzhydryltriphenyl- silane, m.p. 170-1729

p- Chloro-a-bro~zobenzhydryltriphenylsilane To a solution of 2.30 g (0.005 mole) of p-chloro-

benzhydryltriphenylsilaile in 50 ml of carbon tetra- chloride was added 1.00 g (0.0055 mole) of N- bromosuccinimide and a trace of benzoyl peroxide. The mixture was refluxed for 2 h and cooled; 0.50 g (10070) of succinimide was filtered off. The filtrate was evaporated under reduced pressure and the residue was crystallized from acetone to give 2.30 g (85%) of p-chloro-or-bromobel~zh~~dryltriphenyl- silane, m.p. 200-202 O C .

Triphenylsilyl-p-chlorodiphenylcarbinol A mixture of 2.30 g (0.0043 mole) of 9-chloro-a-

bromobenzhydryltriphenylsilane and 1.00 g (0.0055 mole) of silver acetate in 50 ml of acetone, 50 ml of benzene, and 2.0 ml of water was stirred in the dark a t room temperature for 12 h. The mixture was then filtered t o remove silver bromide and excess silver acetate. The filtrate was evaporated to near dryness and the residue was crystallized from hot petroleum ether (b.p. 90-100 OC) to give 1.40 g (70v0) of triphenylsilyl-p-chlorodipheny lcarbinol, m.p. 159- 161 "C.

Preparation of Benzhydryldimethylphenylsilane To a solution of 490 ml (0.375 mole) of benzhy-

dryllithium in ether (prepared from 126 g (0.75 mole) of diphenylmethane in 450 ml of dry ether and 515 ml (0.75 mole) of 1.45 M n-butyllithium in ether), 64.0 g (0.375 mole) of dimethylphenyl- chlorosilane was added dropwise, with stirring. The red color of the organornetall-ic compound faded to a pale orange and a precipitate formed. On complete addition of the chlorosilane, the reaction mixture was stirred for an additional 24 h, by which time a negative Gilman color test I was obtained. The reaction mixture was poured into dilute hydrochloric acid and extracted with ether. The ether layer was dried with sodium sulfate, and the solvent was removed under reduced pressure. The remaining liquid was distilled a t 0.25 mm to give 78.3 g (69.3y0) of benzhydryldimethylphenylsilane, b.p. 128-130 "C, n ~ ~ ~ . ~ 1.596.

9- Fluorenyltriphenylsilane To a solution of 145 ml (0.10 mole) of 9-fluorenyl-

lithium in ether (prepared by refluxing 16.6 g (0.10 mole) of fluorene in 160 ml of dry ether with 70 ml (0.10 mole) of 1.45 M n-butyllithium in ether for 24 h), a solution of 28.0 g (0.10 mole) of triphenyl- chlorosilane in 100 ml of dry ether was added drop- wise, with stirring. The red color of the organometal- lic compound faded to a bright orange and a precipitate formed. On complete addition of the chlorosilane, the reaction mixture was stirred for an additional 24 h, by which time a negative Gilman

color test I was obtained. The reaction mixture was poured into dilute hydrochloric acid and extracted with ether. The ether layer was dried with sodium sulfate, and the solvent was removed under reduced pressure. The resultant orange solid was recrystal- lized three times from hot petroleum ether (b.p. 90-100 "C), once from 1:1 ethanol-benzene, and finally once again from petroleum ether (b.p. 90- 100 "C) to give 20.7 g (49%) of colorless 9-fluorenyl- triphenylsilane, m.p. 182-183 "C.

9-Bromo-9-tripheny1silylJluorene To a solution of 10.6 g (0.025 mole) of 9-triphenyl-

silylfluorene in 100 ml of carbon tetrachloride was added 4.45 g (0.025 mole) of N-bromosuccinimide and a trace of benzoyl peroxide. The mixture was refluxed for 5 h and cooled; 2.20 g (98%) of suc- cinimide was filtered off. The filtrate was evaporated under reduced pressure to give a solid residue, which was recrystallized from hot ethanol to give 11.72 g (93v0) of 9-bromo-9-triphenylsilylfluorene, m.p. 171-172°C.

9- Triphenylsilyl-9-hydroxyJEuorene To a solution of 1.60 g (0.0032 mole) of 9-bromo-

9-triphenylsilylfluorene in 50 ml of acetone, 25 ml of benzene, and 1.0 ml of water was added 0.70 g (0.0032 mole) of silver trifluoroacetate. The mixture was stirred in the dark a t room temperature for 6 h. The mixture was then filtered to remove 0.60 g (10073 of silver bromide, and the solvent was removed from the filtrate under reduced pressure, leaving a pale-yellow oil, which was then dissolved in ether. The ethereal solution was extracted with water and dried with sodium sulfate, and the solvent was removed a t a reduced pressure. The product was recrystallized four times from n-heptane to give 1.25 g (78v0) of colorless 9-triphenylsilyl-9-hydroxy- fluorene, m.p. 165-167 "C (decomp.).

Benzhydvyloxydimethylphenylsilane Asolution of 8.52 g (0.05 mole) of dimethylphenyl-

chlorosilane, 9.20 g (0.05 mole) of recrystallized benzhydrol, and 5.0 g (0.06 mole) of pyridine in 150 ml of dry toluene was refluxed for 6 h and then cooled. The pyridine hydrochloride was filtered off, and the resultant filtrate was washed with dilute hydrochloric acid and water and then dried with sodium sulfate. The solvent was removed from the filtrate a t a reduced pressure, leaving an oil, which was distilled a t 0.25 mm to give 1.80 g (20%) of benzhydrol, m.p. 66-68 OC, and 11.60 g (73%) of benzhydryloxydimethylphenylsilane, b.p. 144 OC, m.p. 40-41 "C.

Nuclear Magnetic Resonance Spectra of Reagents and Products

From this study of approximately 25 new a-silyl- carbinols and their ethers, a large quantity of n.m.r. and infrared spectral data was obtained. Much of this is not highly significant and is briefly sum- marized in Table XI. The position of the hydroxyl proton resonance in dimethyl sulfoxide is a matter of some interest, and is the subject of a separate note (8). The n.m.r. data were referred to the position of the solvent peak, either chloroform (taken as 435 c.p.s. below tetramethylsilane) or

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BROOK ET AL.: REARRANGEMENT MECHANISM 251

TABLE XI

Nuclear magnetic resonance data for a-silylcarbinols and silyl ethers

Series Solvent Group Multiplicity* Chemical shiftt

P ~ ~ S ~ C L I ~ O H C G H ~ X S DMSO$ OH CHa

DWISO ' 3 3 . 3 d , J = 6 . 5 c . p . s . 1.37-1.45 CH q , J=6 .5c .p . s . 5.14-5.18

X = t-Bu s 1.37 X = OMe s 3.75

DMSO OH XiIefSi)

DMSO CH CHdSi)

CHC13 OH X = t-Bu X = OMe

Ph3SiOCHPhCeH4X CHC13 CH s 5.93-6.0 X = t-Bu s 1.35 X = OMe s 3.77

Ph3SiCHOHCcH4X DMSO CH d, J = 4.5c.p.s. 5.4-5.5 OH d, J = 4.5c.p.s. 5.5-5.8

X = t-Bu s 1.30 X = OMe s 3.40

P ~ ~ S ~ C O H ( C H Z P ~ ) ( C G H ~ X ) DMSO OH E

CH2 s X = t-Bu s X = OWIe s

Ph3SiCOH(CHzCgHqX)(CgHqX) DMSO OH CH2

4 . 1 6 4 . 8 5 3.52-3.70 (broad)

Ph3SiOCH(CH2C6H4S)(C~H4X) DMSO CH t , J = 6 . 5 c . p . s . 4.95-5.10 CHz d , J = 6 . 5 c . p . s . 3.00-3.08

X = t-BU X = OMe

*s = singlet. d = doublet, t = triplet, and q = quartet. ?Chemical shift (6) in p.p.m. below tetramethylsilane. $All substituents X are in the para position. BDMSO, dimethyl sulfoxide.

dimethyl sulfoxide (taken as 157 c.p.s. below and piperidine) were purified by refluxing them over tetra~nethvlsilane. sodium hydroxide pellets for 24 h. Constant-boiling

fractions,. collected from a 12 in. Vigreux colum< Kinetic iiIeasurements were then stored in a dry nitrogen atmosphere. The rates of rearrangement of a-silylcarbinols reagent anhydrous chloroform and with added basic catalyst were measured in a few ,Baker reagent dimethyl sulfoxide were experiments by infrared techniques, but chiefly by used directly in the kinetic work. n.m.r. methods.

Each a-silylcarbinol was rigorously purified by Kinetic Measurements by Infrared Spectroscopy recrystallization and thoroughly dried. The n.m.r. The measurements of the rates of rearrangement spectra of the carbinols showed that in every case of the four a-silylcarbinols listed in Table I1 and the carbinols were free of contaminants. the first in Table I11 were carried out with two

The three catalysts (diethylamine, triethylamine, matched sodium chloride cells and a Beckman IR4

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252 CANADIAK JOURNAL OF CHEMISTRY. VOL. 45, 1967

spectrophotometer. Individual solutions of the a-silylcarbinol, its isomeric ether, and the catalyst (diethylamine) were prepared in chloroform.

Solutions of the isomeric ether and the catalyst, such as would be obtained froni a rearrangement, were prepared by addition of a 0.05 ml aliquot of the catalyst solution to a 1.00 ml aliquot of the ether solution. The instrument was then calibrated with these solutions a t the wavelength of maximum absorption of the Si-0-C vibration (ca. 9.40 p). T o a 1.00 ml aliquot of the a-silylcarbinol solution was added a 0.05 ml aliquot of the catalyst solution. The resultant solution was thoroughly mixed and rapidly injected into the sample cell in the machine. The percentage transmission of the Si-0-C peak was then followed with respect to time. The times required for formation of known concentrations of the isomeric ether, a t points A and B of each run, were measured from the graph. The concentrations of isomeric ether a t A and B were then related to the concentrations of unreacted a-silylcarbinol still present. A plot of In [Ao/A] - [Bo] versus time for the data obtained from a number of runs at various a-silylcarbinol and catalyst concentrations was a straight line. The slope of this line is the average rate constant for the rearrangement.

Kinetic Measurements by n.m.7. Spectrometry Most of the rate constants that were determined

in this research were obtained from rate measure- ments made by this technique with a Varian 4-60 spectrometer. Sample solutions of the a-silylcarbinol t o be studied were prepared in the same manner as above, and either 0.50 or 1.00 ml aliquots of the solutions were put into a series of n.m.r. tubes. The n.m.r. spectrum of the a-silylcarbinol was then recorded, the reference peak being that of the so1v"et. T o one sample of the a-silylcarbinol was added a 0.05 ml aliquot of the catalyst solution. After 20 half-lives, the n.m.r. spectrum of the solu- tion of the isomeric ether and the catalyst was recorded. By comparison of the n.m.r. spectra, a choice of a peak or peaks whose intensity was to be followed could be made.

A peak to be used was chosen on the basis that i t was sharp and that i t was isolated from all other peaks in the spectra of both the a-silylcarbinol and its isomeric ether as well a s in those of the solvent and the catalyst.

Kinetic measurements were usually made by one of three techniques: following the disappearance of a peak characteristic of the a-silylcarbinol, following the disappearance and appearance of a pair of peaks characteristic of the a-silylcarbinol and the isomeric ether, respectively, or following the appearance of a peak characteristic of the isomeric ether. The second of these three methods gave the best results, since the rate constant could be determined by two methods simultaneously. However, not all a-silyl- carbinols allowed this type of kinetic study, and thus the first or third method had to be used.

After a peak or pair of peaks was chosen for use in the kinetic runs, the machine was optimized by the normal procedure and then set up for a kinetic run as follows.

A sample of the solution of the carbinol was inserted in the probe and allowed to come to the temperature of the probe. The sweep width and sweep offset controls were adjusted to position the peak or peaks to be used a t the left side of the chart. The spectrum amplitude control was adjusted to obtain approximately 75% of full-scale deflection for the peaks to be used. (If a peak characteristic of the isomeric ether was used, the last two operations above were carried out with a fully isomerized sample.) The peak to be used was then scanned several times. This provided a correlation between the peak height and the concentration of the a-silyl- carbinol. (If a peak characteristic of the ether was followed, then this operation was carried out after ca. 20 half-lives.)

The sample was withdrawn, a 0.05 ml aliquot of the catalyst solution was added, and the resultant solution was thoroughly mixed and then reinserted in the probe. The peak or peaks were then scanned a t regular intervals of time. The sweep offset control was adjusted between each scan so as to position each successive peak or peaks 5 or 10 c.p.s. to the right of the previous peak or peaks. The reaction was usually followed for approximately three half- lives unless a final reading was necessary. In this case, the final reading was made after 20 half-lives. Rate constants were obtained by plotting the logarithm of the peak height, which is directly proportional to the logarithm of the concentration (19), versus time, as in Fig. 2. Dividing the slope of the line obtained from the graph by the concentra- tion of the catalyst present gave the second-order rate constant. The temperatures a t which rearrange- ments were followed were determined by a methanol temperature calibration chart.6

ACKNOWLEDGMENTS This research was supported by the

National Research Council of Canada. The authors are also grateful to the Dew- Corning Silicones of Canada Ltd. for a fellowship to G. E. L., and to Professor Harry Walborsky for helpful discussions.

REFERENCES I . A. G. BROOK. J. Am. Chem. Soc. 80, 1886

(1958). 2. A. G. BROOK, C. M. WARNER, and M. E.

MCGRISKIN. J. Am. Chem. Soc. 81,981 (1959). 3. A. G. BROOK and B. IACHIA. J. Am. Chem. Soc.

83,827 (1961). 4. A. G. BROOK and C. M. WARNER. Tetra-

hedron Letters. 18.815 (19621. 5. A. G. ~ R 0 o ~ ' a n d w.' W. LIMBURG. J. Am.

Chem. Soc. 85,832 (1963). 6. C. EABORN and S. H. PARKER. J. Chem. Soc.

126 (1955). 7. L. STEIN and G. W. MURPHY. J. Am. Chem.

Soc. 74,1041 (1952). I. DOSTROVSKY and E. D. HUGHES. J. Chem. Soc. 157 (1946).

8. A. G. BROOK and K. H. PANNELL. J. Organo- metal. Chem. Amsterdam. In press.

%upplied by Varian Associates.

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BROOK ET AL.: REARRANGEMEKT MECHANISM 253

9. C. L. FRYE, G. E. VOGEL, and J. A. HALL. 15. H. R. ALLCOCK and E. EABORN. Unpublished J . Am. Chem. Soc. 83, 996 (1961); Chem. Eng. results. (See ref. 14.) Kews, 43,31 (1965). 16. E. BERLINER and L. C. MONACK. J. Am. Chem.

10. L. H. SOMMER, W. P. BARIE, and D. R. WEYEN- SOC. 74,1574 (1952). BERG. J. Am. Chem. Soc. 81, 251 (1959). 17. A. G. BROOK, R. KIVISIKK, and G. E. LEGROW. L. H. SOMMER. Stereochemistry, mechanism Can. J . Chem. 43,1125 (1965). and silicon. McGraw-Hill Book Co., Inc., New 18. A. G. BROOK, K. H. PANNELL, G. E. LEGROW, York. 1965. and J. J. SHEETO. J. Organometal. Chem.

11. W. W. LIYBURG. Ph.D. Thesis, University of Amsterdam, 2,491 (1964). Toronto, Toronto, Ontario. 1965. 19. J. A. POPLE, W. G. SCHNEIDER, and H. J.

12. D. J. CRAM. Fundamentals of carbanion BERNSTEIN. High resolution nuclear magnetic chemistry. Academic Press, Inc., New York. resonance. McGraw-Hill Book Co., Inc., New 1965. Chap. IV. York. 1959. p. 77.

13. R. W. TAFT, JR. I n Steric effects in organic 20. A. STREITWIESER, JR. and H. F. KOCH. J . Am. chemistry. M. Newman (Editor). John Wiley & Chem. Soc. 86,404 (1964). Sons, Inc., New York. 1956. Chap. 13. 21. L. D. MCKEEVER and R. W. TAFT. J. Am.

14. R. tlr. BOTT, C. EABORN, and T. W. SWADDLE. Chem. Soc. 88, 4544 (1966). J . Chem. Soc. 2342 (1963). 22. R. S. MULLIKEK. Tetrahedron, 5, 253 (1959).

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