Acidities of hydrocarbons and sulfur-containing hydrocarbons in dimethyl sulfoxide solutions

Abstract

The relative p£a values in Me2SO solution of 13 hydrocarbons that form carbanions with highly dispersed negative charges on deprotonation were found to differ from literature relative ion-pair acidities in cyclohexylamine (CHA) by only ±0.7 unit, or less, when the ion-pair pA"a's were anchored to the pATa of 17.9 for 9-phenylfluorene in Me2SO. The relative p£a's for phenylacetylene, cyclopentadiene, and indene were found to be 2.4, 2.7, and 0.9 units higher, respectively, in Me2SO than in CHA. Higher relative p£a's were also observed for PhCH2S02Ph, PhS02CH3, and PhS02CH2Me, by 1.9, 2.7, and 2.8 units, respectively, and for 2and 1,3-dithiane, by 0.4, 0.65, 0.8, and ~1.9 units, respectively. The apparent higher relative acidities for these compounds in CHA is probably caused by stronger ion pairing or aggregation of the ions derived therefrom than for the hydrocarbon indicator ions with which they are being compared. These perturbations of the equilibria probably arise because of differences in the extent of charge dispersion in the acid and indicator ions. The p£a of 1,3-dithiane itself is estimated from an average of three extrapolations to be 39 ± 2 in Me2SO. Acidities in Me2SO for 1,3-dithianes with electron-withdrawing groups in the 2-position were observed to increase in the following order: 2-C6H5 < 2-p-C6H5phenyl < 2-CONMez < 2-£-PhCH=CH < 2-CH=C-S(-CH2)3-S < 2-COzMe < 2-CN. The order of acidities of 3 alkylthio alkyl sulfoxides was found to be MeSCH2S(0)Me < EtSCH2S(G)Et < Z-BuSCH2S(0)z-Bu.In previous papers we have reported equilibrium acidities in MezSO solution for a large number of hydrocarbons and sulfur-containing hydrocarbons.1 In this paper we turn our attention to a number of types of sulfur compounds that are of particular interest to synthetic organic chemists. The carbanion derived from 1.3-dithiane was the first of several strongly basic ions that were shown to be useful synthetically as acyl anion equivalents since the two sulfur atoms can serve as a masked carbonyl group.2 Reactions of anions derived from 2-substituted 1,3-dithianes with alkylating agents or Michael acceptors have been studied extensively.3 The stereochemistry of these reactions has also received considerable attention following the discovery that 2-lithio-tis-

Related Organocatalysts (2): Urea Derivatives

“…Thiophenol has greater acidity than phenol by 10.7 kcal/mol in dimethyl sulfoxide (DMSO); the difference is explained by unfavourable lone pair-lone pair interactions in the larger thiolate ion (PhS À ) ion [14]. Thus, thioureas are, as expected, more acidic than the corresponding urea derivatives (Figure 9.4) [15].…”

2-aryl-1,3-dithianes and -dithiolanes: A nearly ideal series for relating the energies for bond breaking to electron transfer

“…The thermodynamic stabilities of dithianyl R-are in relatively good shape. Bordwell's group determined pK,,s for a number of sulfur-containing hydrocarbons, including the parent 2-phenyldithiane, in DMSOK'DMSYL- [9,36]. This compound has a pKHA = 30.65, within experimental error of that (30.62) of triphenylmethane.…”

“…The pK, for 2-@-fluoropheny1)-1,3-dithiane was determined using 2-phenyl-l,3-dithiane (pK, = 30.7 [36]) as the indicator acid. Integration of the appropriate proton resonances gave a pK, for 2-(p-nitrophenyl)-l,3-dithiane of 3 1.1.…”

Bis(phenylthio)methane

“…Application of (1) to the synthesis of aldehydes involves a sequence of steps (eqs 1-3) analogous to those of 1,3-dithiane. With a pK a of 30.8 (in DMSO), 7 (1) is a somewhat stronger acid than 1,3-dithiane (pK a = ca. Nevertheless, there are a number of novel transformations involving derivatives of (1) and it is the applications of these reactions that comprise the major portion of the present account.…”