Assessment of Stereoelectronic Effects in Grubbs First-Generation Olefin Metathesis Catalysis Using Molecular Electrostatic Potential

Quantum mechanically derived molecular electrostatic potential (MESP) based descriptors have been proposed for the assessment of both the steric and electronic effects of phosphines in several first-generation Grubbs olefin metathesis catalysts. The MESP at the P nucleus of the active form of the catalyst Cl-2(PR3)Ru=CH2 (1) and its ethylene-coordinated complex (2) are determined. Further, frozen structures corresponding to 1 and 2 are located by replacing the P-R bonds with P H bonds. The MESP at the P of a frozen geometry is free from the electronic effect of R but is influenced by steric effects due to the structural restrictions imposed in the geometry. The difference between the MESP values at the P nucleus of Cl-2(PH3)Ru=CH2 and Cl-2(PR3)Ru=CH2 is taken as a measure of the combined steric and electronic effects of PR3 (V-SE1) in 1. Similarly, the combined steric and electronic effect of PR3 in 2 (V-SE2) is also calculated. The frozen structures allowed calculation of the steric-only effects for 1 (V-S1) as well as 2 (V-S2). Thus, the electronic effect of PR3, V-E1, in 1 is V-SE1 - V-S1 and that of PR3, V-E2,in 2 is V-SE2 - V-S2. Both V-S1 and V-S2 showed impressive linear correlations with popular geometric steric parameters: viz. the Tolman cone angle (theta) and the symmetric deformation coordinate (S4'). Moreover, V-E1 and V-S1 showed linear relationships to the binding energy of ethylene (E-1), suggesting that the steric effect is 1.88 times more dominant than the electronic effect in the olefin binding process. Similarly, both V-E2 and V-S2 showed linear correlation with the activation energy (E-2) for the formation of metallacyclobutane. In both the olefin binding process and the transition state formation leading to C-C bond coupling, a drastic reduction in E-1 as well as in E-2 is observed with an increase in the steric bulkiness of,PR3, while only a moderate decrease in energy parameters is observed with an increase in the electron-rich character of PR3. The stereoelectronic correlation studies presented herein demonstrate that the success of the first-generation Grubbs catalysts is Primarily due to the choice of the right mix of steric (bulky R substituents on P) and electronic (electron-donating R substituents on P) effects of the PR3 ligand.