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

Abstract

Silver(I) and rhodium(I) complexes bearing the bisallyl-substituted N-heterocyclic carbene ligand (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (allyl2SIMes) have been prepared in a straightforward synthesis. The reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborate (1a) with Ag2O affords the ionic biscarbene complex [(allyl2SIMes)2Ag]+BF4- (2), while the reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium chloride (1b) with Ag2O leads to the monocarbene complex (allyl2SIMes)AgCl (3). Sequential treatment of 1a with KOtBu and dimeric [RhCl(cod)]2 (cod = cyclooctadiene) yields the rhodium carbene complex (allyl2SIMes)RhCl(cod) (4). However, the reaction of 1a with the first-generation Grubbs catalyst (PCy3)2Cl2RuC(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-3H-benzimidazol-1-ium tetrafluoroborate (5). Subsequent reaction of this new imidazolium salt with KOtBu and 1 equiv of (PCy3)Cl2RuC(H)(C6H4OiPr-2) forms [1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-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 CC 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.