FURTHER REACTIONS OF IMINOBORANES RBNR' WITH NEOPENTYLIDENETANTALUM COMPLEXES

The azaboratantalacyclobutanes [-NR'-BR-CH(tBu)-Ta-CpCl-] (3 b, d, f, h-1; R/R' = Pr/ tBu, iPr/ tBu, iPr/iPr, tBuCH2/tBuCH2, Me3SiCH2/Me3SiCH2, tBu/Me, tBu/iPr, tBu/SiMe3) are formed from the corresponding iminoboranes RB=NR' and the neopentylidenetantalum complex [tBuHC=TaCpCl2] (2a) by [2 + 2] cycloaddition. The elimination of HCl from 3b, d, f, h and from the four known azdboratantalacyclobutanes 3a, c, e, g [R/R'= Et/tBu, Bu/tBu, Ph/tBu, iBu/iBu] gives the azaboratantalacyclobutenes [-NR'-BR-C(tBu)=TaCpCl-] (4a-h). X-Ray structural analyses of 4e, f show these cyclic carbenetantalum complexes to contain short Ta-C and Ta-N bonds and perhaps some transannular Ta-B interaction. The complex [tBuHC=TaCl3(THF)2] (2b) adds iminoboranes RB=NtBu with unbranched alkyl groups (R = Et, Pr, Bu) in a 1:2 molar ratio to yield the diazoniadiboratatantalacyclohexadienes 5a-c; the product 5a adopts a non-planar ring structure in the crystal. The 1:2 reaction of 2b with tBuB=NMes, instead, results in the cyclobutane [-N(Mes)-B(tBu)-N(Mes)-TaCl3(THF)-] (7m).