Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System.

The primary phosphines MesPH and tBuPH react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H C B H -9-PHR (R=2,4,6-Me C H (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H C B H -9-PHR(p-C F )BF(C F ) (2 a, b) through nucleophilic para substitution of a C F ring followed by fluoride transfer to boron. Further reaction with Me SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H C B H -9-PHR(p-C F )BH(C F ) (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R'=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H C B H -9-PR(p-C F )B(C F ) (4 a, b). Hydrolysis of the B-C F bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H C B H -9-PR(p-C F H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H C B H -9-PR(p-C F )BX(C F ) ] [DMSOH] (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R' at the boron atom yielding the anions [1,7-H C B H -9-PR(p-C F )BR'(C F ) ] (R=Mes, R'=Me (8 a), R=tBu, R'=Me (8 b); R=Mes, R'=Ph (9 a), R=tBu, R'=Ph (9 b)) with [MgBr(Et O) ] as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.