Hydroboration of Alkynes: η4-Alkene–Borane versus η4-E-Boratabutadiene

Abstract

A series of hydroborated η4-σ,π-alkene–borane complexes have been synthesized from the reaction of ruthenium–bis(σ)borate complex [Cp*Ru(μ-H)2BH(S-CH═S)] (1) and terminal as well as internal alkynes. Likewise, the reactions of manganese–bis(σ)borate complexes [Mn(CO)3(μ-H)2BHNCSC6H4(NL)] (L = NCSC6H4 or H) were explored with terminal alkynes that yielded boratabutadiene complexes [Mn(CO)3{(NCSC6H4)2N}{(R1MeC)═B(HC═CHR1)}] [R1 = phenyl (4a) or p-tolyl (4b)] via triple hydroboration of alkynes. These complexes feature a boratabutadiene ligand that is coordinated to a metal through the η4-CBCC mode. To the best of our knowledge, these are the first examples of η4-E-boratabutadiene-coordinated manganese complexes generated by the trans-hydroboration of alkynes. The steric and electronic effects of the borate ligands have been demonstrated using a less sterically hindered manganese–bis(σ)borate complex, [Mn(CO)3(μ-H)2BH(HN2CSC6H4)], that generated monohydroborated complexes [(CO)3Mn(μ-H)2(HN2CSC6H4)B(R1C═CHR2)] (for 6, R1 = Ph and R2 = H; for 7, R1 = p-Tol and R2 = H; for 8, R1 = R2 = Ph). Theoretical studies using density functional theory methods and chemical bonding analyses established the bonding and stability of these species.