Gammill, Gold, Mizsak / a-Ketoketene Dithioacetals: LiAIH4 Reduction Modes 5,5'dithiobis(2-nitrobenzoic acid), Whitesides et al. (ref 24)found a single line with slope pnW= 0.36,whereas Wilson et ai. (ref 35)found two parallel lines of slope pnuc = 0.48 with the rates for aromatic thiols being about fivefold higher than those for aliphatic thiols of the same pK,. (42)For purposes of reference, some values of for reactions of thiol anions follow: with acetaldehyde (ref 43),0.1;with benzene oxide (ref 44) or ethylene oxide (ref 38),0.2 and 0.3;with pnitrothiophenyl acetate (ref 45), 0.27;with pnitrophenyl acetate (ref 39),0.38;with 5,5'-dithiobis(2-nitrobenzoic acid) (note 41),0.36or 0.48:with N-p-2-benzimidazolylphenylmaleimide (ref 46).0.42:with acrvlonitrile (ref 47).0.45:with chloroacetamide (ref'37), -1; with 1,2dithiblane (ref 34),-1. (a) G. E. Lienhard and W. P. Jencks, J. Am. Chem. SOC.,88,3982-3995 (1966);(b) R. E. Barnett and W. P. Jencks. ibid., 91,6758-6765 (1969). D. M. Reuben and T. C. Bruice, J. Am. Chem. SOC., 98, 114-121
Pnuc
(1976). D. J. Hupe and W. P. Jencks. J. Am. Chem. SOC., 99,451-464 (1977). T. Sekine, K. A. Kato, K. Takamori, M. Machida, and Y. Kanaoka, Biochim. Biophys. Acta, 354,139-147 (1974). M. Friedman, J. F. Cavins, and J. S.Wall, J. Am. Chem. SOC.,87,3672-
3681 (1965). J. 0. Edwards, J. Am. Chem. Soc.,76, 1540-1547 (1954). R. E. Davies, Surv. Prog. Chem., 2, 189-238 (1964),especially (a) pp 199-212,(b) p 197. (a) J. F. Harris, Jr., J. Am..Chem. SOC.,82, 155-158 (1960).(b) Review: G. Zumach and E. Kuhle, Angew. Chem., lnt. Ed. Engl., 9, 54-63
(1970).
-
+
+
+
Le., R'SH R2SSCONHR R'SSR2 COS H2NR, with no RISSCONHR formed. Note the close analogy of this result to the reported thiol-induced fragmentation of sulfenylthiocarbonatesto give unsymmetrical R2SSCOOCH3 R1SSR2 COS CH30H. See S. disulfides: R'SH J. Brois. J. F. Pilot, and H. W. Barnum, J. Am. Chem. Soc., 92,7629-7631
+
-
+
+
(1970). The trend of increased k2 with increased mole fraction of water in mixed aqueous media (Table I, lines 31-35) supports the suggestion that the driving force of the second step has an electronic origin, namely, the acidity of the thiocarbamate leaving group, since ionization constants are lower under entirely aqueous conditions. By the same token, the relative insensitivity of kl to the mole fraction of water as demonstrated in the same experiments indicates the unimportance of electronic factors in governing reactivities of the first step of the reaction mechanism. J. E. Leffler, Science, 117,340-341 (1953). L. Eldjarn and A. Pihl , J. Biol. Chem., 225, 449-510 (1957). P. C. Jocelyn, Eur. J. Biochem., 2, 327-331 (1967). W. L. Zahler and W. W. Cleland, J. Biol. Chem., 243,716-719 (1968). In water, K 375 is unusually high, meaning that the carbamoyl disulfide intermediate [Carb] reacts much more readily with thiol anions than does
-
3095
the starting Dts amine. As is described in the text, the factors governing the rates k l and k 2 are quite distinct, so it should not be surprising that under other conditions, specifically nonaqueous ones, K is often
