New N-Heterocyclic Carbene Ligands in Grubbs and Hoveyda-Grubbs Catalysts

A series of N-heterocyclic carbene (NHC) ligands bearing aliphatic amino side groups were synthesized and reacted with the Grubbs first generation catalyst. Reactions involving symmetrical, aliphatic NHCs did not allow the isolation of any pure NHC substituted complexes due to their instability. Unsymmetrical NHCs having a planar mesityl group on one amino side reacted with Grubbs catalyst in a favorable manner, and the resulting complexes were stable enough to be isolated. X-ray crystallographic analysis demonstrated that the mesityl group is co-planar with the phenyl ring of the benzylidene, which indicates that a pi-pi interaction between the mesityl arm and the benzylidene moiety might constitute an important structural element. Catalysts substituted with an NHC derived from a primary or secondary amino-group were found to surpass the parent-complex for the ROMP of cycloocta-1,5-diene. The catalyst substituted with an NHC derived from tBu-NH2 was considerably less metathesis active. Also new N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbenes were synthesized. These NHC ligands revealed a different reactivity towards Grubbs complexes than the hitherto reported imidazolinylidenes: (i) facile bis(NHC) coordination was found, and (ii) both NHCs on the bis(NHC) complexes can be exchanged with a phosphine, thereupon regenerating the Grubbs first generation complex. Furthermore, a comparison between the classical Hoveyda-Grubbs complexes and complexes substituted with N-alkyl-N'-(aryl) heterocyclic carbenes demonstrates that the introduction of one aliphatic group into the NHC framework does not improve the catalytic activity in any of the tested metathesis reactions. The introduction of two aliphatic amino side groups enhances the reactivity in the ROMP reaction while the increase of steric interactions lowers the CM activity. The lower activity of the N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbene complexes compared with the N-alkyl-N'-mesityl heterocyclic carbene complexes, may analogously be attributed to a more demanding steric environment. While small differences in donor capacities might cause a significantly different catalytic behavior, it is thus plausible that subtle steric differences exert a more determining influence on the activity of the catalysts. In addition, the obtained results confirm that the NHC's amino side groups play a pivotal role in determining the reactivity, selectivity as well as the stability of the corresponding catalysts.