Expanding the Family of Hoveyda–Grubbs Catalysts Containing Unsymmetrical NHC Ligands

A series
of Hoveyda–Grubbs second-generation catalysts containing N-alkyl/N′-aryl N-heterocyclic carbene
(NHC) ligands were synthesized and investigated in representative
olefin metathesis reactions. Steric perturbations of unsymmetrical
NHCs were achieved through modulation of the hindrance of alkyl (neopentyl,
neophyl, cyclohexyl) and aryl (2-isopropylphenyl, mesityl) substituents
at the nitrogen atoms in combination with different backbone configurations
(syn and anti).
The NHC substitution patterns strongly influence the stability and
reactivity of the corresponding complexes. In general, complexes bearing
an anti NHC backbone are more stable and more active
than their corresponding syn isomers.
In both the series, the presence of bulky, highly branched N-alkyl groups tends to give reduced catalytic differences
between syn and anti isomers, whereas the nature of the N′-aryl
substituent (2-isopropylphenyl or mesityl) gives rise to different
activity and/or selectivity. Of note, an N′-mesityl
catalyst with anti backbone was found to be highly
competent in the ethenolysis of ethyl oleate, achieving up to 90%
selectivity for the formation of terminal olefins.