Metallacyclobutanes

Metallacyclobutanes are four-membered rings that include one metal atom and three carbon atoms. They are key intermediates in several organometallic reactions, most prominently in olefin metathesis, where they link the catalytic cycle to the exchange of alkene partners.

Structure and formation. The metallacyclobutane forms by a [2+2] cycloaddition between a metal-carbene (alkylidene) complex and an alkene. The resulting cyclic intermediate contains a metal center fused to a three-carbon framework. The exact bonding within the ring is subject to fluxionality and depends on the metal and ligands, but the essential feature is a four-membered M–C–C–C cycle. After formation, the metallacyclobutane can undergo cycloreversion, breaking the ring to yield two new olefins, thereby achieving olefin metathesis.

Mechanistic role and scope. In the Chauvin mechanism, the formation and breakdown of metallacyclobutanes govern the distribution and rate of metathesis products. The process is reversible and sensitive to catalyst structure, substrate substitution, and reaction conditions, which in turn influences cis/trans geometry and selectivity. Metallacyclobutanes are not limited to classic ruthenium catalysts; analogous intermediates are invoked for molybdenum, tungsten, and other metal systems, and related metathesis processes such as alkyne metathesis may involve similar cyclic intermediates.

Study and significance. Metallacyclobutanes are often short-lived but have been detected or trapped under certain conditions and studied by spectroscopy and computational methods. Their concept provides a unifying framework for understanding catalyst design, reactivity, and selectivity in metathesis chemistry, and remains central to explaining how redistributions of carbon–carbon double bonds occur in these reactions.