Metal Carbene Chemistry

I’m beginning to wonder if I chose the wrong school…Northwestern straight-up dominated Purdue this week to move to 6-1. Who would’ve thought? This has been a rough couple of weeks for sports fans in the C-U area, begun by the Cubs’ abysmal playoff performance and capped by an embarrassing UI homecoming loss to Minnesota. At least Ron Zook seems to be taking it well!

Every organic chemist is at least mildly familiar with the chemistry of metal carbenes, complexes containing a metal-carbon double bond. Their propensity to do [2+2] cycloadditions and retro-[2+2] processes renders them ideal for olefin metathesis, a process that was deemed Nobel-worthy in 2005. Although a metal-carbon double bond may look somewhat mystical at first, demystifying metal carbenes is certainly possible.

Complexes with an electron-withdrawing heteroatom attached to the double bond are known as “Fischer carbenes.” They behave, to a first approximation, like carbonyl compounds…the double-bound carbon is electrophilic, and the whole metal fragment just acts like a carbonyl oxygen! The substitution of Fischer carbenes is easily changed by adding a nucleophile through a transesterification-esque process. Deprotonation generates an enolate-type species that can perform aldol reactions, alkylation, Michael reactions, etc. Fischer carbenes are unique, however, for their ability to perform cyclopropanation of double bonds, a reaction that bare organic carbenes are famous for. One of the generally accepted mechanisms is somewhat odd; nucleophilic attack by the olefin on the carbene generates a zwitterionic intermediate reminiscent of the famous “tetrahedral intermediate” of carbonyl chemistry, then the cyclopropane is liberated by attack of the remaining carbon-metal bond on the positive olefin carbon. [2+2] reactions under similar conditions tend to give olefin metathesis products, not cyclopropanes, so choosing an adequate olefin partner is important.One of the most intriguing reactions of chromium carbenes is the Dotz reaction, the regioselective conversion of a chromium carbonyl vinyl carbene and an alkyne into a substituted phenol. Extensions of this reaction can generate highly substituted aromatic systems. There you go; the lesser known side of a giant of organic chemistry, the metal carbene!

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One Comment

  1. Fischer carbenes are useful, but I think they’re air sensitive. Arduengo-type carbenes are air-stable and have been known to catalyze Lewis-basic chemistry.

    In fact, if you take Grubbs-II, remove the imidazole ligand and replace it with an Arduengo-type carbene, the yields can jump ~10-20% in a cross metathesis between Type I and Type II (at least that’s the rumor in our department).

    There’s a really interesting intellectual property backstory to this chemistry.. we should meet up for drinks.. at some point.

    Reply

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