7244 Chem. Commun., 2010, 46, 7244–7246 This journal is c The Royal Society of Chemistry 2010

Competing ring cleavage of transient O-protonated oxaphosphirane

complexes: 1,3-oxaphospholane and g2-Wittig ylide complex formationwzJanaina Marinas Perez, Carolin Albrecht, Holger Helten, Gregor Schnakenburg and

Rainer Streubel*

Received 8th July 2010, Accepted 6th August 2010

DOI: 10.1039/c0cc02436h

O-Protonation and ring cleavage of oxaphosphirane complexes

1a,b enabled the synthesis of novel compounds such as the

bicyclic 1,3-oxaphospholane complex 5 and the Z2-Wittig ylide

complex 7, which demonstrate the emerging chemistry of this

new reactive intermediate. Whereas P–O bond cleavage occurred,

in the first case, thus revealing the superior ability of the

P-bonded Cp* group to stabilize cationic charge, in the second

case competing C–O and P–O bond cleavages occurred, thus

leading to a mixture of complexes 3, 4 and 7.

Phosphorus ylides I were introduced by Wittig et al.1 into

olefination chemistry and have become an indispensable tool

in academic and industrial synthesis.2,3 The bonding can be

described by the canonical ylide I and ylen I0 structures

and substituent effects have been widely studied including

structures II and III with C-4–6 and P-metallo7–9 substituents

(Scheme 1). Early on, Schmidbaur realized the potential of I as

ligands10 to create complexes IV11–13 thus opening up a rich

new field of coordination chemistry. Surprisingly and to the

best of our knowledge, no Z2-Wittig ylide complexes of type V

are known, so far.

In light of the studies on side-on methylene phosphonium

complexes, recently obtained via C–O bond cleavage of oxa-

phosphirane complexes using very strong acids with weakly

coordinating anions,14 we became interested in studying acids

that might lead to bond formation of the anion to the carbon

or the phosphorus center. If phosphorus would be preferred

for the newly formed bond one might obtain the first example

of a Wittig ylide complex of type V.

We report studies on the reactivity of transient O-protonated

oxaphosphirane complexes, either leading to the mixture of a

Z2-Wittig ylide and P–F phosphane complexes or a bicyclic

1,3-oxaphospholane complex. The product formation takes

advantage of the high reactivity of two transient species, one

being the O-protonated oxaphosphirane complex and the

other its acyclic isomer, the methylene phosphonium complex.

Reaction of the oxaphosphirane complexes 1a,15 b16 with

HBF4�Et2O in dichloromethane yielded complexes 3, 4, 5 and

7 (Scheme 2). Presumably, O-protonation of 1a,b occurred in

the first step to yield transiently 2a,b, which can undergo either

P–O bond cleavage to give a diastereomeric mixture of fluoro-

phosphane complexes 3 and 4 in the case of complex 2a (i) or

furnish selectively the novel bicyclic 1,3-oxaphospholane

complex 5 (ii) in a multi-step process (from 2b). In contrast,

the formation of the Z2-Wittig ylide complex 7, obtained as a

mixture with 3 and 4 (ratio 3 : 4 : 7 10 : 65 : 25), requires a

preceding C–O bond cleavage step and a haptotropic shift of

the W(CO)5 moiety to give presumably the transient complex

6a (cf. ref. 14), which then adds fluoride (iii) to give complex 7;

complex 6a could not be detected.

The complex 3 showed a 31P NMR doublet at 198.1 ppm

with a 1JW,P value of 296 Hz and a 1JP,F value of 826.0 Hz. A

very broad signal (Wh1/2 = 420 Hz) at 201.0 ppm with

approximately the same 1JP,F coupling value was tentatively

assigned to complex 4, though its weak intensity and the decompo-

sition of the compound during the column chromatography did

Scheme 1 Wittig ylides I, I0, metallo derivatives II, III and Z1 and Z2

complexes IV and V.

Scheme 2 Reaction of oxaphosphirane complexes 1a,b with

HBF4�Et2O.

Institut fur Anorganische Chemie der RheinischenFriedrich-Wilhelms-Universitat Bonn, Gerhard-Domagk-Str. 1,53121 Bonn, Germany. E-mail: r.streubel@uni-bonn.de;Fax: +49 228 739616; Tel: +49 228 735345w The work is dedicated to Prof. H. Karsch on the occasion of his 65thbirthday.z Electronic supplementary information (ESI) available: Detailedanalytical data of complexes 1, 3, 4, 5 and 7, further explanations onthe computational results. CCDC 776722 (5). For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/c0cc02436h

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 7244–7246 7245

not allow further data to be obtained. The Z2-Wittig ylide

complex 7 displayed a 1JW,P value of 144.0 Hz, which is quite

close to the values of recently reported P–OH substituted

methylene phosphonium side-on complexes that displayed

values between 98–118 Hz (cf. ref. 14). Further strong evidence

for the constitution of 7 came from the large 1JP,F value of

1043.0 Hz, indicating the presence of another electronegative

group being the P–OH group, the presence of which

was independently deduced from the IR spectra through its

absorption at 3373 (br) cm�1.17 The P–F functionality would

also help to understand the relative increase in the 1JW,P value.

A further strong argument for the side-on coordination of the

W(CO)5 moiety in complex 7, besides the observed phosphorus

tungsten coupling, came from the 13C{1H} NMR spectra,

which revealed the presence of only one resonance for the

pentacarbonyltungsten group, typical for a side-on bonded

W(CO)5 group (cf. ref. 14), at 197.5 ppm having a 2JP,C value

of 10.9 Hz. The only example of a (thermally labile) ylide

tungsten complex of type IV18 did not show any phosphorus–

carbon couplings for the carbonyl groups and, instead,

exhibited the cis-CO and trans-CO resonances in the 13C NMR

spectra as expected. The 31P{1H} NMR spectra of complex 5

showed a doublet at 215.6 ppm (1JW,P = 288.4 Hz) due to

the 1JP,F coupling (892.6 Hz). The structure of 5 was also

unambiguously confirmed for the solid state by single-crystal

X-ray crystallography (Fig. 1).

The bicyclic structure in complex 5 shows the transforma-

tion of the (former) Cp* group into a partially unsaturated

five-membered ring having a C–C double bond (C(10)–C(11)

1.331(4) A) and a (newly) established C–O bond (O(1)–C(9)

1.461(3) A), which is slightly longer than the O(1)–C(1) bond

with 1.419(3) A; the reason for the latter was not apparent.

In order to find an explanation for the divergent behavior of

the two oxaphosphirane complexes 1a,b with respect to P–O

vs. C–O bond cleavage, both reactions were computed by

means of DFT calculations starting from 1a,b–HOEt2+,

where the acid proton, mediated through diethyl ether,19 is

bound to the oxaphosphirane oxygen center via O–H–O

hydrogen bonding (Scheme 3); relative free energies are given

in Table 1. For the C–O bond cleavage (reaction II) both the

free energy of activation and the free energy of reaction are

almost equal for the two differently substituted oxaphosphirane

complexes. However, a remarkable difference is found for the

P–O bond breaking mode (i). The barrier is significantly lower

in the case of R= C5Me5 and the reaction is exergonic for this

derivative. The reason derives from the different extent of

positive charge stabilization in the reactive intermediates 8a

and 8b, respectively. Complex 8a constitutes a phosphenium

complex (see ESIz, Fig. S2), wherein the phosphorus–metal

bonding situation is characterized by cooperative s-donor andp-acceptor bonds, thus giving rise to a rather short P–W

distance (2.438 A).20 In complex 8b the P–W bond is con-

siderably longer (2.592 A), thus being in the range of W(CO)5phosphirane complexes. Here, the Cp* ring is Z2-bound to

phosphorus, as it was found also for the non-coordinated

phosphenium ion [P(NHtBu)(C5Me5)]+ by Niecke et al.21 The

positive charge of 8b is transferred to the former Cp* ring,

where it is stabilized through delocalization. Our calculations

further revealed that 8b should have a fluxional structure with

a sequence of degenerate [1.5]-sigmatropic rearrangements

(circumambulatory rearrangement),22 for which the barrier

was predicted to be only DGa298 = 31.3 kJ mol�1.

The formation of complexes 3, 4, 5 and 7 can be explained

by assuming O-protonation of 1a and 1b to be the first step,

which induces a kinetically preferred P–O bond cleavage.

However, in the case of 1a the reaction is endergonic and,

hence, complex 8amay partly react back to 1a and via reaction

II to give complex 6a. A transfer of fluoride from tetrafluoro-

borate onto either 7a to give 3 or onto 6a to afford complex 7

occurs. In the case of 1b, the reaction I is irreversible and rapid

fluoride transfer onto complex 8b seems likely to occur, which

Fig. 1 Structure of complex 5 (50% probability level; hydrogen

atoms except H1 and H12 are omitted for clarity). Selected bond

lengths [A] and angles [1]: W–P 2.4431(6), P–C(1) 1.843(3), P–C(8)

1.843(3), P–F 1.5975(15), C(1)–O(1) 1.419(3), O(1)–C(9) 1.461(3),

C(8)–C(9) 1.573(4), C(9)–C(10) 1.512(4), C(10)–C(11) 1.331(4),

C(11)–C(12) 1.507(4), C(8)–C12 1.558(4); F–P–W 107.86(6),

C(1)–P–F 97.95(10), C(1)–P–C(8) 91.14(12), P–C(1)–O(1) 114.13(17),

C(1)–O(1)–C(9) 114.12(19), O(1)–C(9)–C(8) 111.2(2), C(9)–C(8)–P

102–92(17), C(12)–C(8)–P 111.72(16).

Scheme 3 Computed acid-induced ring opening of complexes 1a,b.

Table 1 Calculated thermochemical data for reactions shown inScheme 3 (all values in kJ mol�; B3LYP/aug-TZVP/ECP-60-MWB(W),COSMO (CH2Cl2)//RI-BLYP/aug-SV(P)/ECP-60-MWB(W), COSMO(CH2Cl2))

Reaction a: R = CH(SiMe3)2 b: R = C5Me5

I DGa298 +19.3 +7.6

DRG298 +2.5 �15.2II DGa

298 +51.3 +54.9

DRG298 �40.2 �46.5

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7246 Chem. Commun., 2010, 46, 7244–7246 This journal is c The Royal Society of Chemistry 2010

then is followed by 1,4-addition of the pending OH group

to the butadiene system of the Cp* substituent to yield

complex 5.

Complexes 3, 4, 5 and 7: complex 1a or 1b (150 mg,

0.240 mmol) was dissolved in CH2Cl2 (1.5 mL) and HBF4�Et2O(33 mL, 0.240 mmol) was added at ambient temperature. The

mixture was stirred for 10 min and the solvent was then

removed in vacuo (4 � 10�2 mbar). In the case of complex 3,

the residue was subjected to low-temperature chromato-

graphy, whereby the last fraction yielded complex 3 as a white

oil. As complex 7 decomposed during the chromatography it

was characterized from a mixture with 3 and 4. In the case of

complex 5, washing of the residue with n-pentane yielded the

product as a white powder.y3 (91.4 mg, 59%) (found: C 35.82; H 4.20%. C19H26FO6PSi2W

requires C 35.65, H 4.09%); dC (75.5 MHz, CDCl3; 30 1C) 1.6

(dd, 3JP,C 1.9, 4JP,F 1.2, Si(CH3)3), 2.0 (dd, 3JP,C 1.9, 4JP,F1.2, Si(CH3)3), 30.0 (mc br, CH((SiCH3)3)2), 78.1 (quasiq,1JP,C E 2JF,C 17, PCOH), 126.6 (dd, JP,C 3.8, JF,C = 2.3,

Ph), 127.8 (d, JP,C 1.9, Ph), 128.0 (d, JP,C 2.3 Hz, Ph), 136.7

(s; i-Ph), 195.0 (dd, 2JP,C 7.7, 3JF,C 3.5, cis-CO), 197.5

(dd, 2JP,C 30.1, 3JF,C 0.9, trans-CO).

5 (99 mg, 67 %) (found: C 42.55, H 3.51%. C22H22FO6PW

requires C 42.88, H 3.60%); dC (75.5 MHz, CDCl3; 25 1C) 9.5

(s; C-CH3), 11.7 (d, 2JP,C 7.7, PC-CH3), 12.5 (s, C-CH3), 15.3

(d, 3JP,C 8.3, CH-CH3), 21.0 (d, JP,C 2.4, OC-CH3), 48.5

(d, JP,C 20.3, C-CH3), 57.5 (dd, JF,C 16.7, JP,C 8.9, PC-CH3),

85.7 (dd, JF,C 14.9, JP,C 11.9, PC-Ph), 98.6 (d, JP,C 3.6,

PC(H)OC), 128.5 (d, JP,C 1.8, Ph), 128.8–128.9 (m, Ph),

132.1 (d, JP,C 1.2, i-Ph), 132.3 (s, CQC), 138.6 (d, JP,C 4.2;

CQC), 194.4 (dd, 2JP,C 7.7, 3JF,C 3.0, JW,C 125.2, cis-CO)

197.2 (dd, 2JP,C 31.0, 3JF,C 1.8, trans-CO).

7 dC (75.5 MHz; CDCl3, 30 1C)�0.2 (d, 3JP,C 4.3, Si(CH3)3),

0.0 (d, 3JP,C 3.0, Si(CH3)3), 13.0 (d, 1JP,C 17, CH(SiMe3)2), 66

(broad; CHPh), 126.8 (s, Ph), 127.8 (d, 2JP,C 1.9, Ph), 127.3

(d, JP,C 2.2, Ph), 136.0 (s; i-Ph), 197.5 (d, 2JP,C 10.9, CO).

Financial support by Thermphos AG Intl. and the COST

action CM0802 ‘‘PhoSciNet’’ is gratefully acknowledged.

Notes and references

y X-Ray crystallographic analysis: colorless single crystals wereobtained from concentrated n-pentane/CH2Cl2 solutions upon slowcooling to 4 1C. C22H22FO6PW; crystal size 0.60 � 0.31 � 0.27 mm,monoclinic, P21/c, a = 10.4284(2) A, b = 6.56720(10) A,c = 33.0807(7) A, b = 96.3597(9)1, V = 2251.60(7) A3, Z = 4, 2ymax

561, collected (independent) reflections 33 860 (5407), Rint = 0.0727,m = 5.245 mm�1, 286 refined parameters, 0 restraints, R1

(for I > 2s(I)) = 0.0223, wR2 (for all data = 0.0589, max./min.residual electron density) 1.964/�1.590 e A�3.23

1 G. Wittig and U. Schollkopf, Chem. Ber., 1954, 87, 1318–1330;G. Wittig and W. Haag, Chem. Ber., 1955, 88, 1654–1666.

2 H. Pommer and P. C. Thieme, Topics in Current Chemistry:Industrial Applications of the Wittig Reaction, Springer,Berlin/Heidelberg, 1983, vol. 109, pp.165–188.

3 For a review see: B. E. Maryanoff and A. B. Reitz, Chem. Rev.,1989, 89, 863–927.

4 G. L. Crocco, K. E. Lee and J. A. Gladysz, Organometallics, 1990,9, 2819–2831.

5 X. Li, A. Wang, L. Wang, H. Sun, K. Harms and J. Sundermeyer,Organometallics, 2007, 26, 1411–1413; X. Li, A. Wang,L. Wang, H. Sun, S. Schimdt, K. Harms and J. Sundermeyer,Organometallics, 2007, 26, 3456–3460.

6 G. Erker, P. Czisch, C. Kriiger and J. M. Wallis, Organometallics,1985, 4, 2059–2060.

7 W. V. Konze, V. G. Young and R. J. Angelici, Organometallics,1998, 17, 1569–1581.

8 I. P. Lorenz, W. Pohl, H. Niith and M. Schmidt, J. Organomet.Chem., 1994, 475, 211–221.

9 M. Yoshifuji, K. Shibayama, T. Hashida, K. Toyota, T. Niitsu,I. Matsuda, T. Sato and N. Inamoto, J. Organomet. Chem., 1986,311, C63–C67.

10 H. Schmidbaur, Angew. Chem., Int. Ed. Engl., 1983, 22, 907–927;H. Schmidbaur, Pure Appl. Chem., 1980, 52, 1057–1062;H. Schmidbaur, Pure Appl. Chem., 1978, 50, 19–25; H. Schmidbaur,Acc. Chem. Res., 1975, 8, 62–70.

11 H. El Amouri, M. Gruselle, Y. Besace, J. Vaissermann andG. Jaouent, Organometallics, 1994, 13, 2244–2251.

12 J. F. Hoover and J. M. Stryker, Organometallics, 1988, 7,2082–2084.

13 A. Wang, H. Sun and X. Li, Organometallics, 2009, 28, 5285–5288.14 J. M. Perez, H. Helten, B. Donnadieu, C. A. Reed and R. Streubel,

Angew. Chem., 2010, 122, 2670–2674 (Angew. Chem., Int. Ed.,2010, 49, 2615–2618).

15 R. Streubel, A. Kusenberg, J. Jeske and P. G. Jones, Angew.Chem., 1994, 106, 2564–2565 (Angew. Chem., Int. Ed. Engl.,1995, 33, 2427).

16 M. Bode, J. M. Perez, G. Schnakenburg, J. Daniels andR. Streubel, Z. Anorg. Allg. Chem., 2009, 635, 1163–1171.

17 Complexes of bifunctional trivalent phosphane complexeshaving P–F and P–OH functions were reported by:A. Marinnetti and F. Mathey, Phosphorus Sulfur Relat. Elem.,1984, 19, 311–317.

18 F. R. Kreissl, E. O. Fischer, C. G. Kreiter and H. Fischer, Chem.Ber., 1973, 106, 1262–1276.

19 Due to the acid strength of HBF4 it is conceivable to assume thatprotonated diethyl ether is the protonating agent; see e.g.,(a) D. Farcas-iu and D. Hancu, J. Chem. Soc., Faraday Trans.,1997, 93, 2161–2165; I. A. Koppel, P. Burk, I. Koppel, I. Leito,T. Sonoda and M. Mishima, J. Am. Chem. Soc., 2000, 122,5114–5124.

20 (a) A. H. Cowley and R. A. Kemp, Chem. Rev., 1985, 85,367–382; (b) M. Sanchez, M. R. Mazieres, L. Lamande andR. Wolf, in Multiple Bonds in Low Coordination PhosphorusChemistry, ed. M. Regitz, O. J. Scherer, Georg Thieme,Stuttgart, 1990, pp. 129–148; (c) D. Gudat, Coord. Chem. Rev.,1997, 163, 71–106; (d) W. W. Schoeller, Top. Curr. Chem., 2003,229, 75–94.

21 D. Gudat, M. Nieger and E. Niecke, J. Chem. Soc., Dalton Trans.,1989, 693–700.

22 R. E. Bulo, F. Allaart, A. W. Ehlers, F. J. J. de Kanter,M. Schakel, M. Lutz, A. L. Spek and K. Lammertsma, J. Am.Chem. Soc., 2006, 128, 12169–12173.

23 G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64,112–122.

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