BIS(DI-T-BUTYLPHOSPHINO)METHANE COMPLEXES OF RHODIUM - HOMOGENEOUS ALKYNE HYDROSILYLATION BY CATALYST-DEPENDENT ALKYNE INSERTION INTO RH-SI OR RH-H BONDS - MOLECULAR-STRUCTURES OF THE DIMER [(DTBPM)RHCL](2) AND OF THE SILYL COMPLEX (DTBPM) RH[SI(OET)(3)](PME(3))

The homogeneous, Rh-catalysed hydrosilylation of but-2-yne with triethoxysilane has been studied. All rhodium complexes employed as catalyst precursors contain (t)Bu(2)PCH(2)P(t)Bu(2) (''dtbpm'') as a chelating ligand. The crystal and molecular structure of the dimer [(dtbpm)RhCl](2) (10) has been determined by X-ray diffraction. Complex 10 is shown to be a sluggish catalyst in hydrosilylation reactions of hex-1-ene, whereas but-2-yne is hydrosilylated more rapidly. A much more efficient and highly selective catalyst is 10 with added PPh(3), equivalent to the use of monomeric (dtbpm)RhCl(PPh(3)). (E)-2-Triethoxysilylbut-2-ene is formed exclusively and with high turnover numbers in this case. For both 10 and its PPh(3) derivative, the 14-electron fragment [(dtbpm)RhCl], formed by dissociation processes, is the most likely active intermediate in a Harrod-Chalk-type catalytic cycle. The PPh(3) dissociation equilibrium has been studied in detail for (dtbpm)RhCl(PPh(3)) and its thermodynamic parameters have been determined. With rhodium alkyl complexes as catalyst precursors, a different type of alkyne hydrosilylation catalysis, involving direct alkyne insertion into the Rh-Si bond of an intermediate rhodium silyl complex, (dtbpm)Rh[Si(OEt)(3)](PMe(3)) (14), has been found. Complex 14 was synthesized independently from (dtbpm)RhMe(PMe(3)) and characterized by X-ray diffraction. It is an equally active catalyst itself, yielding (E)-2-triethoxysilylbut-2-ene as the major product (90%) from but-2-yne and HSi(OEt)(3) (turnover number 1000 per 30 min). The insertion step of the alkyne into the Rh-Si bond of 14 and the formation of two stereoisomeric rhodium vinyl complexes were established independently for MeO(2)CC=CCO(2)Me as a more reactive alkyne substrate. A catalytic cycle is proposed for this unprecedented hydrosilylation reaction. The synthesis of the eta(3)-benzyl complex (dtbpm)Rh(eta(3)-CH2C6H5) (23) is described. This compound allows an alternative, more efficient access to the new silyl complex (dtbpm)Rh[Si(OEt)(3)](PMe(3)).