Insertion of alkynes into the heterocycle of (η5-pentaalkyl-2,3-dihydro-1,3-diborolyl)(η5-pentamethylcyclopentadienyl)ruthenium: Formation and characterization of 4-borataborepine ruthenium complexes

The violet ruthenium complex [(η5-C5Me5)Ru(η5-C3B2Me4R1)] (2a, R1 = Me) reacts with terminal alkynes R2Ctriple bond; length of mdashCH to give yellow 4-borataborepine compounds [(η5-C5Me5)Ru{η7-(MeC)3(R1B)2(R2C2H)}] (4c, R1 = Me, R2 = Ph; 4d, R1 = Me, R2 = SiMe3; 4e, R1 = Me, R2 = H). The insertion of alkynes into the folded C3B2 heterocycle of 2a causes some steric hindrance, which yields with elimination of the distant boranediyl group the corresponding boratabenzene complexes 5 as byproducts. The analogous reactions with internal alkynes R2Ctriple bond; length of mdashCR2 proceed slowly and afford predominantly the boratabenzene complexes [(η5-C5Me5)Ru{η6-(MeC)3(MeB)(R2C)2}] (5f, R2 = Et, 5g, R2 = p-tolyl), respectively. In the latter case, three byproducts are formed: methylboronic acid and 1,2,3,4-tetra-p-tolyl-1,3-butadiene (9) due to hydrolysis of the postulated 2,3,4,5-tetra-p-tolyl-1-methylborole (10) and unexpectedly, the cationic triple-decker complex [{(η5-C5Me5)Ru}2{μ,η7-(MeC)3(MeB)2(CH)2}]Cl (11) having two separated CH groups. The new compounds were characterized by NMR, MS, and single-crystal X-ray studies of 4c, 5f, 9 and 11.