Meghan A. Dureen

Terminal Alkyne Activation by Frustrated and Classical Lewis Acid/Phosphine Pairs

Frustrated and classical Lewis pairs arising from combinations of Lewis acids and phosphines react with terminal alkynes either via C-H activation forming an alkynylborate salt or by addition to alkyne giving a zwitterionic phosphonium borate.

B-H activation by frustrated Lewis pairs: borenium or boryl phosphonium cation?

Catechol borane reacts with the frustrated Lewis pairs tBu2RP (R = tBu, 2-C6H4(C6H5)) and B(C6F5)3 to give the species [(C6H4O2)BPtBu2R][HB(C6F5)3] that can formally be described as either borenium cation or boryl-phosphonium salts; the nature of these species was probed with DFT calculations.

Heterolytic cleavage of disulfides by frustrated Lewis pairs.

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C(6)F(4))B(C(6)F(5))(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(SPh)(C(6)F(4))B(SPh)(C(6)F(5))(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(C(6)F(4))B(donor)(C(6)F(5))(2)]. The reaction of 1 with S(8) gave tBu(2)P(S)(C(6)F(4))B(C(6)F(5))(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C(6)F(5))(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C(6)F(5))(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu(3)P horizontal lineS, Bn(2)S, and B(C(6)F(5))(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(C(6)F(5))(4)] (7) and [NBu(4)][(p-tolylS)B(C(6)F(5))(3)] (9) was performed. The former species was prepared by a reaction of 4 with [Ph(3)C][B(C(6)F(5)) (4)], while cation exchange of [(Et(2)O)(2)Li( p-tolylS)B(C(6)F(5))(3)] (8) with [NBu(4)]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(C(6)F(5))(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.

New Strategies to Phosphino–Phosphonium Cations and Zwitterions

By employing strategies based on frustrated Lewis pair chemistry, new routes to phosphino-phosphonium cations and zwitterions have been developed. B(C(6)F(5))(3) is shown to react with H(2) and P(2)tBu(4) to effect heterolytic hydrogen activation yielding the phosphino-phosphonium borate salt [(tBu(2)P)PHtBu(2)] [HB(C(6)F(5))(3)] (1). Alternatively, alkenylphosphino-phosphonium borate zwitterions are accessible by reaction of B(C(6)F(5))(3) and PhC[triple chemical bond]CH with P(2)Ph(4), P(4)Cy(4), or P(5)Ph(5) affording the species [(Ph(2)P)P(Ph)(2)C(Ph)=C(H)B(C(6)F(5))(3)] (2), [(P(3)Cy(3))P(Cy)C(Ph)=C(H)B(C(6)F(5))(3)] (3), and [(P(4)Ph(4))P(Ph)C(Ph)=C(H)B(C(6)F(5))(3)] (4). A related phosphino-phosphonium borate species-[(Ph(4)P(4))P(Ph)C(6)F(4)B(F)(C(6)F(5))(2)] (5) is also isolated from the thermolysis of B(C(6)F(5))(3) and P(5)Ph(5).