The Course of (R_2R'SiO)_3TaCI_2 (R = ~tBu,R' = H,Me,Ph,~tBu (silox); R = ~iPr,R' = ~tBu,~iPr) Reduction Is Dependent on Siloxide Size

摘要

Various sized siloxides (Cy_3SiO > ~tBu_3SiO >~tBu_2PhSiO > ~tBu_2MeSiO - ~'Pr_2BuSiO > jPr_3SiO > ~tBu_2HSiO) were used to make (R_2R'SiO)_3TaCI_2 (R = ~tBu,R' = H (1-H),Me (1-Me),Ph (1-Ph),~tBu (1); R = ~iPr,R' = ~tBu (1-~jPr_2); R = R' - ip_r (1-~ipr_3); R = R' = ~cHex (Cy)).Product analyses of sodium amalgam reductions of several dichlorides suggest that [(R_2R'SiO)_3Ta]_2(mu-CI)_2 may be a common intermediate.When the siloxide is large (1-~tBu),formation of the Ta(lll) species (~tBu_3SiO)_3Ta (6) occurs via disproportionation.When the siloxide is small,the Ta(IV) intermediate is stable (e.g.,[('Pr3SiO)3Ta]_2(mu-CI)_2 (2)),and when intermediate sized siloxides are used,solvent bond activation via unstable Ta(lll) tris-siloxides is proposed to occur.Under hydrogen,reductions of 1-Me and 1-Ph provide Ta(IV) and Ta(V) hydrides [(~tBu_2MeSiO)_3Ta]_2(mu-H)_2 (4-Me) and (~tBu_2PhSiO)_3TaH_2 (7-Ph),respectively.