Grubbs Ruthenium Catalysts Explained

In a bid to solve a long-standing chemical mystery and to help improve catalyst design, researchers led by Donald G. Truhlar of the University of Minnesota, Minneapolis, have used a new computational method to sort out the reactivity differences between first-generation and second-generation Grubbs catalysts (Organometallics, DOI: 10.1021/om200529m). Chemists have known that the first step in olefin metathesis using Grubbs catalysts (shown) is dissociation of a phosphine ligand. This dissociation is faster in Grubbs I than in Grubbs II, but Grubbs II has higher overall metathesis activity. Researchers have been unable to nail down the cause of this discrepancy, in part because computational methods were inadequate for studying actual catalysts and relied on simplified models. Truhlar and coworkers used a new density functional method named M06-L that can crunch numbers representing real catalysts. They found that the benzylidene ligand in Grubbs I and II rotates and serves as a toggle switch to trigger metathesis. The rotation precedes dissociation for the more readily formed Grubbs I, but occurs in synch with dissociation in Grubbs II. When an olefin substrate coordinates to ruthenium in Grubbs I, the catalyst must overcome electronic effects stemming from the rotation—a barrier that is lower in Grubbs II.