Unprecedented selectivity of ruthenium iodide benzylidenes towards unhindered olefins and its unpredictable consequences

Sulfur-chelated Ru benzylidenes facilitate anionic ligand exchange due to the enhanced trans influence of the strongly sigma donating NHC ligand; by this fashion Ru-S-I could be readily obtained (Figure 1). While Ru-S-I was extremely efficient in ring closing metathesis (RCM) reactions, it was completely inert towards ring opening metathesis polymerization (ROMP).¹ This never before seen behavior led us to investigate the reactivity of Ru-S-I towards several other reactive substrates. Indeed, all ROMP monomers were completely inert. Moreover, when DEDAM was combined with norbornene derivatives, only RCM occurred. However, a most intriguing outcome was obtained when cyclooctenes were mixed with DEDAM: the complete consumption of all metathesis substrates. When the DEDAM/COE ratio is large, 1,9-decadiene was the main product; while the opposite ratio lead to oligomers reminiscent of an ADMET mechanism. We propose that while the alkylidene complex is too bulky to coordinate a non-terminal olefin, the methylidene intermediate formed by the RCM reaction is just the right size to react with cyclooctene (but not the norbornenes). This unexpected behavior allows the ring-closing metathesis of 1,9-undecadiene to produce strained cis-cyclooctene with good selectivity. Moreover, a mixture of the diiodo complex with polybutadiene and a small amount of a terminal olefin initiator catalyzes a depolymerization process yielding small cyclic molecules, including the highly strain 1,5-cyclooctadiene. Perhaps the most fascinating result is that when mixing neat DCPD (the most reactive and useful ROMP monomer) with Ru-S-I, the liquid mixture remains unaffected, highlighting the complexes’ impressive latency for this important industrial monomer. To our great satisfaction, addition of (Bu)₄NCl even after one week standing, immediately activated the complex by a facile exchange of the anionic ligands forming Ru-S-Cl in situ and polymerizing the mixture!

Figure 1. anionic ligand exchange

[1] Ivry, E., Nechmad, N.B., Baranov, M., Goldberg, I. and Lemcoff, N.G., Inorg. Chem., 2018, 57, 15592-15599.

[2] Noy B. Nechmad, Ravindra Phatake, Elisa Ivry, Albert Poater, and N. Gabriel Lemcoff, Submitted.