Synthesis of monomeric and dendritic ruthenium benzylidene cis-bis-tertiobutyl phosphine complexes that catalyze the ROMP of norbornene under ambient conditions

The synthesis of monomeric and dendritic ruthenium benzylidene cis-bis-phosphine complexes that catalyze norbornene ROMP previously reported with dicyclohexyl bis-phosphines has now been extended to monomeric and dendritic bis-tertiobutyl phosphines. The reaction of Hoveyda's catalyst [RuCl2(=CH-o-O-iPrC6H4)PPh3] (1) with the diphosphine PhCH2N(CH2PtBu2)2 (2) gives the new air-stable green ruthenium carbene complex (3) in which (2) models a dendritic branch of poly(diphosphine) dendrimers DAB-dendr-[N(CH2PtBu2)2]n (G1, n = 4; G2, n = 8; G3, n = 16). Metallodendrimers DAB-dendr-[N(CH2PtBu2)2Ru(=CHAr)(Cl)2]n (4)-(6) derived from the three first generations of DAB polyamines containing, respectively, 4, 8 and 16 ruthenium branches have been synthesized. These dendritic ruthenium-benzylidene complexes initiate the ROMP of norbornene at room temperature to form star-shaped metallodendritic polymers slightly more rapidly than the analogues with bis-cyclohexyl phosphines. Interestingly, the metallodendrimers G1 (4) initiates the ROMP of norbornene much faster than the model ruthenium complex (3) the overall rate order being G1 > G2 > G3 > model, these positive and negative dendritic effects being comparable with those found for the dicyclohexyl bis-phosphine complexes.