Advanced 1,1-carboboration reactions with pentafluorophenylboranes
The 1,1 carboboration reaction of a variety of metal-substituted alkynes with simple trialkylboranes R3B yields the respective alkenylboranes (Wrackmeyer reaction). The use of the strongly electrophilic R-B(C6F5)2 reagents allows for much milder reaction conditions and gives good yields of the respective bulky alkenylboranes from conventional terminal alkynes by means of 1,2-hydride migration. Even internal alkynes undergo 1,1-carboboration with the R-B(C6F5)2 reagents, in this case yielding alkenylboranes by means of C–C bond cleavage. Phosphorus, sulfur or even boron containing substituents can serve as the migrating alkynyl substituents in the advanced 1,1-carboboration reactions using the R-B(C6F5)2 reagents. Sequential 1,1-carboboration of geminal bis(alkynyl) derivatives of these elements with the R-B(C6F5)2 boranes yields boryl substituted phospholes, thiophenes or even boroles in quite a variety. Vicinal bis(alkynyl)arenes or heteroarene substrates undergo benzannulation reactions in this way. Many of the -B(C6F5)2 substituted 1,1-carboboration products can be used as reagents in cross coupling reactions. A recently disclosed organometallic analogue, namely a 1,1-carbozirconation reaction is described.