Carbanion Stabilization by Adjacent Sulfur: Polarizability, Resonance, or Negative Hyperconjugation? Experimental Distinction Based on Intrinsic Rate Constants of Proton Transfer from (Phenylthio)nitromethane and 1-Nitro-2-phenylethane Claude F. Bernasconi* and Kevin W. Kittredge Department of Chemistry and Biochemistry of the University of California, Santa Cruz, California 95064 Received October 22, 1997 (Phenylthio)nitromethane, PhSCH 2 NO 2 , is about as acidic as PhCH 2 NO 2 and about 4 pK a -units more acidic than CH 3 NO 2 in water or aqueous DMSO, showing the well-known acidifying effect of thio substituents in the R-position of carbon acids. Over the years various interpretations have been offered for the acidifying effect of sulfur groups: d-p π-resonance, polarizability, and negative hyperconjugation. Assuming that the nature of the factors that potentially stabilize the transition state of the proton transfer from the carbon acid are the same as those that potentially stabilize the carbanion, we show that a distinction between these interpretations can be based on the effect of the phenylthio group on the intrinsic rate constants (k o ) of proton transfer. Such intrinsic rate constants were determined for the deprotonation of PhSCH 2 NO 2 and PhCH 2 CH 2 NO 2 by amines in water and 90% DMSO-10% water; in both solvents k o for PhSCH 2 NO 2 was found to be substantially higher than for PhCH 2 CH 2 NO 2 as well as for other nitroalkanes reported previously. Based on a detailed analysis of how various factors such as resonance, inductive effects, polarizability, and positive and negative hyperconjugation affect the intrinsic rate constants for proton transfer, it is concluded that the high k o values for PhSCH 2 NO 2 result from a combination of the inductive and polarizability effect of the PhS group and that d-p π-resonance and negative hyperconjugation play a minor role if any. Introduction It is well-known that R-alkylthio or R-arylthio groups increase the acidity of adjacent CH bonds. This increased acidity has led to numerous synthetic applications. 1 The acidifying effect has been attributed to the stabilization of the carbanion by sulfur. 2 The mechanism by which this stabilization occurs has generated much interest. Originally this stabilization was explained in terms of a resonance effect involving the d orbitals of sulfur, i.e., d-p π-bonding between the carbanion lone pair and sulfur 3d orbitals, 2-7 e.g., eq 1. However, theoretical work has challenged this notion, suggesting that the stabilization is mainly due to the polarizability of sulfur. 8-11 A third interaction mechanism, negative hyperconjugation, has also been invoked by several authors. 9,11-13 This mechanism involves double bond- no bond resonance structures (e.g., eq 2). 14 The d-p π-bonding mechanism is no longer strongly advocated; regarding polarizability and negative hyper- conjugation, they may well both be important. 9,11-13 However, based on a recent high level ab initio study, Wiberg et al. 15 have concluded that negative hypercon- jugation is the main factor in the stabilization of the dimethyl sulfide anion by sulfur. Similar conclusions were reached by Cuevas and Juaristi. 16 In the present study we attempt to evaluate the relative influence of the various interaction mechanisms by using an approach that combines kinetic and ther- modynamic data. It is based on the assumption that the kinds of the factors that potentially stabilize the transi- tion state of the deprotonation of a carbon acid are the same as of those that potentially stabilize the carbanion, i.e., polarizability, d-p π-bonding, negative hyperconju- gation, and possibly others (see below). It exploits the (1) (a) Corey, E. J.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1965, 4, 1075. (b) Seebach, D. Synthesis 1969, 1, 19. (c) Corey, E. J.; Erickson, B. W. J. Org. Chem. 1971, 36, 3553. (2) For reviews, see (a) Price, C. C.; Oae, S. Sulfur Bonding; Ronald Press: New York, 1962. (b) Cram, D. J. Fundamentals of Carbanion Chemistry; Academic Press: New York, 165; pp 71-84. (3) Oae, S.; Tagaki, W.; Ohno, A. Tetrahedron 1964, 20, 417. (4) Eliel, E. L.; Hartmann, A. A.; Abatjoglou, A. G. J. Am. Chem. Soc. 1974, 96, 1807. (5) (a) Wolfe, S.; LaJohn, L. A.; Bernardi, F.; Mangini, A.; Tonachini, G. Tetrahedron Lett. 1983, 24, 3789. (b) Wolfe, S.; Stolow, A.; LaJohn, L. A. Tetrahedron Lett. 1983, 24, 4071. (6) Bernardi, F.; Mangini, A.; Tonachini, G.; Vivarelli, P. J. Chem. Soc., Perkin Trans. 2 1985, 111. (7) Bordwell, F. G.; Bares, J. E.; Bartmess, J. E.; Drucker, G. F.; Gerhold, J.; McCollum, G. J.; Van der Puy, M.; Vanier, N. R.; Matthews, W. S. J. Org. Chem. 1977, 42, 326. (8) Streitwieser, A., Jr.; Williams, J. E. J. Am. Chem. Soc. 1975, 97, 191. (9) Lehn, J.-M.; Wipff, G. J. Am. Chem. Soc. 1976, 98, 7498. (10) Bernardi, F.; Csizmadia, I. G.; Mangini, A.; Schlegel, H. B.; Wangbo, M.-H.; Wolfe, S. J. Am. Chem. Soc. 1975, 97, 2209. (11) Schleyer, P. v. R.; Clark, T.; Kos, A. J.; Spitznagel, G. W.; Rohde, C.; Arad, D.; Houk, K. N.; Rondan, N. G. J. Am. Chem. Soc. 1984, 106, 6467. (12) Hopkinson, A. C.; Lien, M. H. J. Org. Chem. 1981, 46, 998. (13) Schleyer, P. v. R.; Kos, A. J. Tetrahedron 1983, 39, 1141. (14) Equation 2 can be visualized as donation of the carbanion lone pair to the C-S σ* antibonding orbital. (15) Wiberg, K. B.; Castejon, H. J. Am. Chem. Soc. 1994, 116, 10489. (16) Cuevas, G.; Juaristi, E. J. Am. Chem. Soc. 1997, 119, 7545. 1944 J. Org. Chem. 1998, 63, 1944-1953 S0022-3263(97)01946-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/26/1998