One of the misconceptions in the academic literature is that the well-defined (Schrock and Grubbs) olefin metathesis catalysts need to be used in dilute solutions. This isn’t surprising given the way in which these catalysts were developed, but it’s a misconception that has taken a while to debunk.
There have been examples in the literature of the use of Schrock and Grubbs catalysts in solvent-free conditions,1 but they are application-focused, and a systematic examination of metathesis with and without solvent has yet to be published. That is, until the recent report by the Meier group.2
In this paper, the Meier group studied the “standard” ring closing metathesis (RCM of diethyldiallyl malonate) and cross metathesis (CM of allyl benzene and cis-1,4-diacetoxy-2-butene) reactions under both solvent and solvent-free conditions using the three ruthenium catalysts shown below.
In all six substrate/catalyst permutations studied, the metathesis event proceeded well without solvent. The biggest benefit was observed at low catalyst loadings (<0.5 mol%), where the solvent-free reactions performed better across the board! I’ll let you all speculate as to why that is, but the bottom line is that while trying to be green* and avoid using solvent, Meier discovered that he could be even greener* and use less catalyst.
* This is one of the rare cases where the words “cheap” and “cheaper” could be substituted for “green” and “greener” – it’s not often that both objectives align!
1 Pederson, R. L.; Fellows, I. M.; Ung, T. A.; Ishihara, H.; Hajela, S. P. Adv. Synth. Catal. 2002, 344, 728-735.
2 Kniese, M.; Meier, M. A. R. Green Chem. 2010, DOI: 10.1039/b921126h.