The hydrolysis of ( ~ F - C ~ H ~ ) C ~ ( ~ - C ~ F ~ C ~ ~ C ~ H ~ ) : an unexpected decarboxylation

MICHAEL J. MCGLINCHEY AND HAO NGUYEN Department of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4MI

~ece'ived November 5, 1985

This paper is dedicated to Professor Arthur N . Bourns

MICHAEL J. MCGLINCHEY and HAO NGUYEN. Can. J. Chem. 64, 1170 (1986). The attempted basic hydrolysis of the ester sandwich compound (C6H6)Cr(C6F5CO2Et) did not yield the expected carboxylic

acid but instead produced (C6H6)Cr(C6F5H) in good yield together with traces of (C6H6)Cr(C6HF40Me). Attempts to trap a benzyne intermediate were unsuccessful and the mechanism of decarboxylation is discussed in terms of internal chelation at the chromium centre.

MICHAEL J. MCGLINCHEY et HAO NGUYEN. Can. J. Chem. 64, 1170 (1986). On a tent6 une hydrolysk en milieu basique de compos6 ester sandwinch (C6H6)Cr(C#5C02Et); toutefois, au lieu d'obtenir

I'acide carboxylique attendu, il y a eu formation du (C6H6)Cr(C#5H), avec un bon rendement, ainsi que des traces du (C6H6)Cr(C6HF40Me). On a tent6 sans succks de pieger I'intermediaire benzynique et on discute du mecanisme de d6carboxylation en fonction d'une chklation interne au niveau du chrome central.

[Traduit par la revue]

Introduction While the ferrocenyl moiety has now become a standard

organic functional group (1) and the (CSHS)Fe fragment has been exploited for synthetic purposes (2,3), the chemistry of the analogous bis(arene)chromium complexes (chromarenes) (4) is much less understood. Initially, the problems were synthetic in that the Friedel-Crafts route (5) to (C6H6)2Cr - viz., C I - C ~ ~ / C ~ H ~ / A ~ C ~ ~ / A ~ - would not tolerate functional groups on the arene. Thus, arenes such as chlorobenzene or anisole, which possess non-bonding electron pairs, react with the Lewis acid catalyst (AlCl,) and undergo side reactions which do not yield the desired sandwich compounds (6). This problem has been largely circumvented by the metal atom vapour technique (7-10) whereby the gaseous metal and the ligand vapour are cocondensed onto a cold (77 K) surface and direct combination occurs to give the required products. By this means, a wide variety of organic functional groups can be incorporated into the chromarene system (1 1).

A further development was the discovery that in the mixed sandwich compound 1,2,3,4,5-pentafluorochromarene, 1, the unique hydrogen was readily removable using an alkyl lithium (12) and the corresponding anion 2 reacted with a variety of electrophiles (4). In particular, one can produce the ester 3 in high yields (13). It therefore seemed not unreasonable that one could hydrolyze the ester to the corresponding carboxylic acid 4; subsequent measurement of the pKa of 4 and also of C6F5C02H should allow a direct evaluation of the electronic effect of the (C6H6)Cr moiety on another arene ring.

1: X = H 2: X = Li 3: X COOC2H5 4: X = COOH

Results and discussion The electronic effect of an organometallic fragment on an

organic molecule was probably first studied by Nicholls and Whiting (14) who noted that the pKa of (C6H5C02H)Cr(CO)3

was similar to that of para-nitrobenzoic acid. He proposed that the tricarbonylchromium moiety was strongly electron- withdrawing and should hence facilitate nucleophilic attack on aromatic rings. This perceptive prediction has since been amply verified and much elegant chemistry has resulted (15). Owing to the non-availability of chromarenyl carboxylic acids, such an approach has not been possible and so more indirect probes have been used. Typically, a comparison of the vco values for C6FsM(CO), and the corresponding chromarenes (C~H~)C~(C~FSM(CO),) indicated that the (C6H6)Cr moiety was a net donor of electron density to the fluorinated ring (4).

For these reasons, we attempted a standard basic hydrolysis of 3 using methanolic KOH. To our surprise, the product after neutralization and extraction was not the anticipated acid but rather pentafluorochromarene, 1, with traces of a methoxy- tetrafluorochromarene, 5, which was identified mass spectro- scopically.

Our initial thoughts focussed on the possibility of a benzyne intermediate, 6, produced via decarboxylation and elimination of fluoride ion. Subsequent attack by methanol should then yield the ortho isomer 5a, as in Scheme 1. Indeed, it is well established that pentafluorophenyl lithium is an excellent precursor for tetrafluorobenzyne (16) which undergoes Diels- Alder reactions with thiophene and benzene (17,18). However, in our hands, repeated attempts to trap 6 as its furan adduct 7 were unsuccessful and we are not of the opinion that a benzyne intermediate is formed. We note that other attempts to detect n-complexed arynes have not been successful (19-21) except where the aryne binds to a metal triangle as in 8 (22). In these latter molecules the ligand functions as a cycloalkyne rather than as an aryne. Recently, however, it has been shown that a benzyne-zirconocene adduct has a transient existence (23) and can be used in synthesis (24).

Eventually, after numerous reactions, the available quantities of 5 were such as to allow the measurement of its 1 9 ~ nmr spectrum which showed only two fluorine environments. We thus assign 5 as the para isomer since the other possibilities would exhibit four fluorine resonances. As we have noted pre- viously (25-27), the fluorine chemical shifts of n-complexed fluoroarenes are very markedly shielded relative to those of the free arenes themselves. It seems most likely that 5 arises via a nucleophilic attack on the complexed fluoroarene and is probably unrelated to the mechanism of decarboxylation. As

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McGLINCHEY AND NGUYEN

CH30H F a OCH,

F T F 90 -C02 F ' F F ' F C; ' -FO ' 5' 5'

with their non-complexed analogues (28), nucleophilic attack to displace fluoride from pentafluorochromarenes occurs predomi- nantly at the para position (29).

Finally, one might speculate as to the reason'for the unexpec- tedly facile decarboxylation during ester hydrolysis. Clearly, since this process is not observed with the free arene, the metal must be implicated either directly or indirectly in the elimination of carbon dioxide. We note that decarboxylation processes are sometimes catalyzed by the presence of transition metal ions, such as Cu+ or ~ n ' + , and chelate formation is almost invariably invoked (30). In fluorochromarene sandwiches, such as those discussed here, the chromium is generally considered to be rather more positive than its formal zero oxidation state would indicate. Thus, the relative difficulty of oxidizing the chromium (31), the decreased infrared frequency of the ring-breathing

mode of the fluorinated ring (32), as well as ESCA data (unpublished data from this laboratory), all indicate that there is a net transfer of electron density into the fluorinated ring and that the chromium atom bears a partial positive charge. It is then reasonable to visualize chelation by the carboxylate oxygen, thus placing a formal negative charge on the fluorinated ring as in 9. Subsequent protonation and elimination of C02 from the cyclohexadienyl complex 10 would lead directly to the observed product. A tentative mechanism is presented in Scheme 2.

To conclude, complexation of Ca5C02Et to a T - ( C ~ H ~ ) C ~ moiety reduces its susceptibility towards nucleophilic displace- ment of fluoride. However, upon hydrolysis to the correspond- ing carboxylate, loss of C02 is greatly facilitated. There is no evidence in favour of a T-complexed benzyne intermediate.

Experimental section All reactions were carried out under an atomosphere of nitrogen.

Mass spectra were obtained using a VG-7070F mass spectrometer fitted with a VG 2035 data system. "F nmr spectra were obtained on a Bruker WH90 spectrometer operating at 84.66 MHz using CFC13 as an external reference.

Using the metal atom vapour apparatus previously described (25), pentafluorobenzene, benzene and chromium vapour were cocondensed at -196°C to yield 1 which was purified by repeated sublimations.

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1172 CAN. 1. CHEM. VOL. 64, 1986

As described elsewhere (4), treatment of 1 with tert-butyl lithium at -78°C and subsequently with an excess of ethyl chlorofomate yielded ethyl pentafluorochromarenoate, 3. Attempted hydrolysis of 3 (500 mg, 1.35 rnmol) with a six-fold excess of KOH in methanol gave a slightly greenish coloured solution. After neutralisation with HC1, filtration and ether extraction, the products were chromatographed on alumina and eluted with benzene to give 1 (221 mg, 0.76 mmol; 55%), identical in all respects with an authentic sample, and traces of a yellow compound 5 showing mass spectral peaks at m / z 310, C13HloF40CrC; 279, C ~ ~ H ~ F ~ C I +; 261, CI2HsF3CrC ; 242, C12H8F2Cr+; 180, C7H4F40+; 149, C6HF4+; 131, C 6 ~ 2 F 3 + ; 130, C&~CI+; 78, C a 6 + ; 52, Cr+. The accumulated samples of 5 from six experiments gave multiplets in the 19F nmr spectrum at - 173 and - 178 ppm (i.e., to high field of CFCI3).

When the hydrolysis was repeated in a ten-fold excess of furan, the same products were obtained and no evidence for the production of 7 could be obtained.

Acknowledgements We thank the Natural Sciences and Engineering Research

Council of Canada for financial support and Professor G. Jaouen (Univ. d e Paris) for helpful discussions.

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