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

Abstract

Abstract The violet ruthenium complex [(η 5 -C 5 Me 5 )Ru(η 5 -C 3 B 2 Me 4 R 1 )] ( 2a , R 1 = Me) reacts with terminal alkynes R 2 C CH to give yellow 4-borataborepine compounds [(η 5 -C 5 Me 5 )Ru{η 7 -(MeC) 3 (R 1 B) 2 (R 2 C 2 H)}] ( 4c , R 1 = Me, R 2 = Ph; 4d , R 1 = Me, R 2 = SiMe 3 ; 4e , R 1 = Me, R 2 = H). The insertion of alkynes into the folded C 3 B 2 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 R 2 C CR 2 proceed slowly and afford predominantly the boratabenzene complexes [(η 5 -C 5 Me 5 )Ru{η 6 -(MeC) 3 (MeB)(R 2 C) 2 }] ( 5f , R 2 = Et, 5g , R 2 = 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 [{(η 5 -C 5 Me 5 )Ru} 2 {μ,η 7 -(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 .