J . Am. Chem. SOC.1985, 107, 303-306
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NMR Studies of Phenylsilyl Anions. Evaluation of Ion Pairing and Charge Distribution’ Erwin Buncel,*3aT. K. Venkatachalam,2aBertil Eliasson,zband Ulf Edlund*2b Contribution from the Departments of Chemistry, Queen’s University, Kingston, Canada K7L 3N6, and UmeA University, S-90187 UmeA, Sweden. Received May 14, 1984
Abstract: The results of an N M R study (IH, I3C, ’Li) of phenyl-substituted silyl anions are reported as a function of solvent, temperature, and the presence of crown ether complexing agents. Comparison of I3C chemical shifts for Ph,SiLi, Ph2MeSiLi, and PhMezSiLi, and for the corresponding silyl potassium compounds, with carbanion analogues is especially informative concerning the transmission of electronic effects in these systems. It is concluded that r-polarization of the phenyl rings (F,) is the major cause of the observed phenyl carbon shifts in the silyl anions relative to the neutral chlorides, in contrast to the arylmethyl carbanions. The minor I3C chemical shift changes observed for the phenyl carbons by addition of crown ethers or by varying cation or solvent can be interpreted as a cation-induced modulation of this effect. The narrow range for the ’Li chemical shifts observed downfield of a LiCl reference supports a significant silicon-lithium interaction in these anion species in the ethereal solvents under investigation (EtzO, MTHF, THF, DME).
There is considerable interest in ion-pair phenomena in carbanion systems, in part because of the consequences that these phenomena impart to reactivities, and due to their characteristic spectroscopic properties., The experimental findings gathered so far point toward equilibria of solvent-separated (SSIP) and contact ion pairs (CIP), where the relative proportions are determined by the choice of solvent, temperature, and counterion. UV,4 IR5,and more recently NMR6 have been the major spectroscopic techniques used to characterize these systems, which with a few exceptions have all been monocharged all-carbon systems, e.g., fluoreny14, and arylmethy14b.c carbanions. Recently, however, several investigations have been directed toward defining the role of px-pa or dx-pr conjugation in anions containing group 4 elements.’-I0 The first report on the electronic spectrum of silyl anions showed that (triphenylsily1)lithium in tetrahydrofuran ( T H F ) has a low-wavelength absorption of 335 nm.’ By comparing this result with the UV absorption for (triphenylmethy1)lithium (A, 500 nm) the authors concluded that the much lower A,, of the silyl compound is indicative of negligible delocalization of negative charge to the phenyl rings. Experiments were also conducted in different solvents, and the main conclusion was that only one type of ion pair exists in the (triphenylsily1)lithium system under the prevailing solvent con( 1 ) Carbanion Mechanisms. Part 14. For Part 13, see: Buncel, E.; Menon, B. C.; Venkatachalam, T. K. J . Org. Chem. 1984, 49, 413. (2) (a) Department of Chemistry, Queen’s University. (b) Department of Organic Chemistry, UmeA University. (3) (a) Szwarc. M., Ed. ‘Ions and Ion Pairs in Organic Reactions”; Wiley-Interscience: New York, 1972, 1974; Vol. 1, 2. (b) Buncel. E., Durst, T., Eds. “Comprehensive Carbanion Chemistry”;Elsevier: Amsterdam, 1980, 1984; Vol. 1, 2. (4) (a) Hogen-Esch, T. E.; Smid, J. J . Am. Chem. SOC.1966,88, 307. (b) Velthorst, N. H.; Hoijtink, G. J. J . Am. Chem. SOC.1965, 87, 4529. (c) Buncel, E.; Menon, B. C. J . Org. Chem. 1979.44, 317. (d) Buncel, E.; Menon, B. C.; Colpa, J . Can. J . Chem. 1979, 57, 999. (5) Menon, B. C; Shurvell, H. F.; Colpa, J. P.; Buncel, E. J . Mol. Srruct. 1982, 78, 29. (6) (a) Sandel, V. R.; Freedman, H. H. J . Am. Chem. SOC.1963,85,2328. (b) Cox, R. H. J . Phys. Chem. 1969, 73, 2649. (c) Grutzner, J. B.; Lawlor, J. M.; Jackman, L. M. J . Am. Chem. SOC.1972.94, 2306. (d) Burley, J. W.; Young, R. N. J . Chem. SOC.,Perkin Trans. 2 1972, 835. (e) Waack, R.; Doran, M. A.; Baker, E. B.; Olah, G. A. J . Am. Chem. SOC.1966, 88, 1272. (0 Takahashi, K.; Kondo, Y . ;Asami, R.; Inoue, Y . Org. Magn. Reson. 1974, 6, 580. (g) House, H. 0.;Prabhu, A. V.; Phillips, W. N. J . Org. Chem. 1976, 41, 1209. (h) OBrien, D. H.; Russell, C. R.; Hart, A. J. J . Am. Chem. SOC. 1979, 101, 633; 1976, 98, 7427; 1975, 97, 4410. (i) Fraenkel, G.; HalldenAbberton, M. P. J . Am. Chem. SOC.1981, 103, 5657. Bates, R. B., ref 3b, 1980, Chapter I . (k) Lambert, J. B.; Wharry, S . M. J . Am. Chem. SOC. 1982, 104, 5859. (7) Evans, A. G.; Hamid, M. A,; Rees, N. H. J. Chem. SOC.B 1971, 1110. (8) Cox, R. H.; Janzen, E. G.; Harrison, W. B. J . Magn. Reson. 1971.4,
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274. (9) Olah, G. A.; Hunadi, R. J. J . Am. Chem. SOC.1980, 102, 6989. (10) Batchelor, R. J.; Birchall, T. J . Am. Chem. Sor. 1983, 105, 3848.
0002-7863/85/1507-0303$01.50/0
Table I. I3C Chemical Shifts of Silyl Anions and Related Species in THF SolventQsb ipso ortho meta para Me C-a Ph3SiH 134.2 136.5 128.8 130.6 Ph3SiC1 133.8 135.9 128.9 131.5 Ph2MeSiClb 134.4 134.0 128.1 130.5 0.9 PhMe2SiC1 137.0 133.8 128.8 131.0 2.3
Ph3SiLi Ph2MeSiLi PhMe2SiLi
155.9 160.1 166.0
137.0 135.4 133.8
126.9 126.7 126.5
124.6 123.9 122.7
4.8 7.5
Ph3SiK Ph2MeSiK PhMe2SiK
158.6 163.2 170.1
136.9 135.1 133.6
126.7 126.5 126.4
123.8 123.0 121.6
5.8 9.1
Ph3CH 145.0 128.9 130.2 126.9 57.7 Ph3CC1‘ 145.3 129.7 127.7 127.7 81.3 Ph3CLi 150.0 124.2 128.1 113.2 90.3 Ph3CK 149.1 124.0 129.0 114.3 88.5 “Cyclohexane used as reference (6 27.7). Anion samples run in an unlocked mode. Reference 20. Reference 21. ditions. It was also found that temperature change has no significant effect on the UV absorption of the silyllithium compound. Using ‘H NMR, Cox et al. studied the lithium salts of the triphenyl group 4 species in THF.* A variation in the shielding of the para hydrogens was observed, but definitive interpretation of the results was uncertain since the range for the para proton shifts was found to be quite small (
