Synthesis and Structure of N-Heterocyclic Carbene Complexes with Tethered Olefinic Groups: Application of the Ruthenium Catalyst in Olefin Metathesis

Silver(I) and rhodium(I) complexes bearing the bisallyl-substituted N-heterocyclic carbene ligand (4 R,5 S )-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (allyl 2 SIMes) have been prepared in a straightforward synthesis. The reaction of (4 R,5 S )-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3 H -imidazol-1-ium tetrafluoroborate ( 1a ) with Ag 2 O affords the ionic biscarbene complex [(allyl 2 SIMes) 2 Ag] + BF 4 - ( 2 ), while the reaction of (4 R,5 S )-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3 H -imidazol-1-ium chloride ( 1b ) with Ag 2 O leads to the monocarbene complex (allyl 2 SIMes)AgCl ( 3 ). Sequential treatment of 1a with KO t Bu and dimeric [RhCl(cod)] 2 (cod = cyclooctadiene) yields the rhodium carbene complex (allyl 2 SIMes)RhCl(cod) ( 4 ). However, the reaction of 1a with the first-generation Grubbs catalyst (PCy 3 ) 2 Cl 2 Ru C(H)Ph (Cy = cyclohexyl) leads to ring-closing metathesis of the two allylic groups, yielding 1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3 H -benzimidazol-1-ium tetrafluoroborate ( 5 ). Subsequent reaction of this new imidazolium salt with KO t Bu and 1 equiv of (PCy 3 )Cl 2 Ru C(H)(C 6 H 4 O i Pr-2) forms [1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3 H -benzimidazolin-2-ylidene]dichloro(2-isopropanolatobenzylidene)ruthenium(II) ( 8 ). All new complexes have been thoroughly characterized, including X-ray crystallographic analyses of 2, 3, and 8 . The most intriguing feature of 8 is the presence of an innocent C C bond that is part of a highly active olefin metathesis catalyst, which offers many options for further functionalization of the ligand backbone. The catalytic activity of complex 8 has been evaluated for the ring-closing metathesis of N, N -diallyl-4-toluenesulfonamide.