Heterolytic Cleavage of Disulfides by Frustrated Lewis Pairs

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C6F4)B(C6F5)(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(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 [tBu(2)P(C6F4)B(donor)(C6F5)(2)]. The reaction of 1 with S-8 gave tBu(2)P(S)(C6F4)B(C6F5)(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C6F5)(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C6F5)(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:11 mixture of tBu(3)P=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 [tBu(3)P(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)(2)Li( 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 [tBu(3)P(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.