Olefin Metathesis, p‐Cresol, and the Second Generation Grubbs Catalyst: Fitting the Pieces

p‐Cresol as additive to the Grubbs second generation catalyst (GII) allows the cross‐metathesis of acrylates with prop‐1‐en‐1‐ylbenzenes under conditions that only give the prop‐1‐en‐1‐ylbenzene self‐metathesis product in the absence of cresol. NMR and IR spectroscopy, MALDI‐TOF MS and XPS supported the formation of a ruthenium benzylidene with hydrogen bonds between p‐cresol and the chloride ligands of GII. XPS furthermore confirmed p‐cresol to increase the binding energies of the GII Ru 3d5/2, 3d3/2, 3p3/2 and 3p1/2 photoelectron lines, whereas 1H NMR spectroscopy indicated the carbene carbon and hydrogen to be shielded. It is thus postulated that p‐cresol allows for more facile interaction between electron‐deficient compounds and the ruthenium benzylidene by decreasing the electron density on the metal center and increasing the electron density on the carbene. Additional roles for p‐cresol in the cross‐metathesis of prop‐1‐en‐1‐ylbenzenes and α,β‐unsaturated carbonyl compounds catalyzed by the second generation Grubbs catalyst (GII) have been determined. According to NMR, UV‐Vis and XPS spectroscopy, p‐cresol decreases the electron density on the metal center of GII and increases the electron density on the carbene, thus allowing for more facile interaction between acrylates and the ruthenium benzylidene.