Assessment of Steric and Electronic Effects of N-Heterocyclic Carbenes in Grubbs Olefin Metathesis Using Molecular Electrostatic Potential

Abstract

The steric and electronic effects of N-heterocyclic carbenes in the second-generation Grubbs olefin metathesis catalysts are quantified on the basis of molecular electrostatic potential at the carbene carbon of both the NHC ligand (VC1) and the alkylidene moiety (VC2). The quantity (VC1 + VC2), calculated separately for the active form of the catalyst, (sImR2NR2)Cl2Ru═CH2 (1), and the ethylene-bound complex, (sImR2NR2)Cl2Ru═CH2(C2H4) (2), reflected the combined steric and electronic effects of the NHC. For every 1 and 2 systems, R is replaced with H and all other nuclear coordinates are frozen to obtain respectively 1′ and 2′. The quantity (VC1′ + VC2′) of 1′ and 2′ served as a good measure of the steric effect of the NHC. For a normalization procedure, (VC1 + VC2) and (VC1′ + VC2′) for any R is considered relative to R = H to define the combined steric and electronic effect of NHCs in the complexes 1 (VSE1) and 2 (VSE2) as well as the steric effect in 1 (VS1) and 2 (VS2). Thus the electronic effect of the active form, VE1, is (VSE1 – VS1) and that of the olefin-bound complex, VE2, is (VSE2 – VS2). Both VS1 and VS2 showed good linear correlation with the volume-based steric parameter, the buried volume (%VBur) developed by Cavallo et al. In complexes with less bulky NHCs, ethylene is coordinated to the active form with an orientation parallel to the alkylidene moiety (mode A binding; binding energy, E1 is 5–9 kcal/mol), whereas in complexes with stericaly bulky NHCs, ethylene is coordinated to the active form only in a plane perpendicular to the alkylidine moiety (mode B binding; E1 is ∼10.0 kcal/mol). In mode A binding, E1 as well as E2 for the metallacyclobutane formation decreased with an increase in both the steric and electron-donating effects of the NHC ligand. The systems with −CF3 as N-substituent deviated significantly from the (E1, VE1) correlation line due to the presence of Ru···F interaction. In mode B binding, E1 and E2 increased with an increase in the steric effect, while the electronic effect showed no correlation with either E1 or E2. Thus, if the NHC is bulky, the steric effect will dominate over the electronic effect and will control the metathesis, whereas the electronic and steric effect will have almost equal importance in metathesis activity of the catalyst with less bulky NHCs. Lone-pair interactions of the type Ru···F have the ability to significantly alter the catalytic activity.