Synthesis and Applications of RuCl₂(CHR‘)(PR₃)₂:  The Influence of the Alkylidene Moiety on Metathesis Activity

The reactions of RuCl₂(PPh₃)₃ with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl₂(CHR)(PPh₃)₂ (R = Me (1), Et (2)) and RuCl₂(CH-p-C₆H₄X)(PPh₃)₂ (X = H (3), NMe₂ (4), OMe (5), Me (6), F (7), Cl (8), NO₂ (9)) was observed for alkyl diazoalkanes RCHN₂ and various para-substituted aryl diazoalkanes p-C₆H₄XCHN₂. Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3−9 as catalysts have shown that initiation is in all cases faster than propagation (k_i/k_p = 9 for 3) and that the electronic effect of X on the metathesis activity of 3−9 is relatively small. Phosphine exchange in 3−9 with tricyclohexylphosphine leads to RuCl₂(CH-p-C₆H₄X)(PCy₃)₂ 10−16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51−1.63) and 1,5-cyclooctadiene (PDI = 1.56−1.67). The crystal structure of RuCl₂(CH-p-C₆H₄Cl)(PCy₃)₂ (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkylidene unit lies in the Cl−Ru−Cl plane. The benzylidenes RuCl₂(CHPh)(PR₃)₂ (R = Cy (cyclohexyl) (10), Cp (cyclopentyl) (17), i-Pr (18)) are quantitatively available via one-pot synthesis with RuCl₂(PPh₃)₃, PhCHN₂, and PR₃ as reaction components. 10 is an efficient catalyst for metathesis of acyclic olefins:  On reaction with excess ethylene, the methylidene complex RuCl₂(CH₂)(PCy₃)₂ (19) is formed quantitatively, and various alkylidene compounds RuCl₂(CHR)(PCy₃)₃ (R = Me (20), Et (21), n-Bu (22)) are isolated as the kinetic products from the reaction of 10 with an excess of the corresponding terminal or disubstituted olefins. Metathesis of conjugated and cumulated olefins with 10 results in the formation of vinylalkylidene and vinylidene complexes, as shown by the synthesis of RuCl₂(CHCHCH₂)(PCy₃)₂ (23) and RuCl₂(CCH₂)(PCy₃)₂ (24) from 1,3-butadiene or 1,2-propadiene, respectively. Also, functional groups such as −OAc, −Cl, and −OH can be introduced into the alkylidene moiety via cross metathesis with the appropriate alkene.