Heterolytic Cleavage of Disulfides by Frustrated Lewis Pairs

The addition of diphenyl disulfide (PhSSPh) to tBu2P(C6F4)B(C6F5)2 (1) affords the zwitterionic phosphonium borate [tBu2P(SPh)(C6F4)B(SPh)(C6F5)2] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu2P(C6F4)B(donor)(C6F5)2]. The reaction of 1 with S8 gave tBu2P(S)(C6F4)B(C6F5)2 (3). In a similar fashion, the frustrated Lewis pair of tBu3P/B(C6F5)3 reacts with RSSR to give [tBu3P(SR)][(RS)B(C6F5)3] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu3PS, Bn2S, and B(C6F5)3. Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu3P(SPh)][B(C6F5)4] (7) and [NBu4][(p-tolylS)B(C6F5)3] (9) was performed. The former species was prepared by a reaction of 4 with [Ph3C][B(C6F5) 4], while cation exchange of [(Et2O)2Li( p-tolylS)B(C6F5)3] (8) with [NBu4]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu3P(SR)]+ and anions [(RS)B(C6F5)3]−, R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4−8, and the natures of the P−S cations are examined via DFT calculations.