Metathesis reactions for making compounds that contain carbon-carbon multiple bonds are one of the most important sets of processes in the chemical industry. The alkene or alkyne products are widely used, for example, in polymerizations to make plastics.
Chemists have been interested in extending metathesis methods to carbon’s heavier analogs, such as silicon, which one day could lead to new types of polymeric materials. The holdup has been the lack of suitable silicon precursors. A research team led by Vladimir Ya. Lee and Akira Sekiguchi of the University of Tsukuba, in Japan, has now cleared that hurdle (J. Am. Chem. Soc., DOI: 10.1021/ja401072j).
To make their precursors, the researchers coupled a bicyclic tetrasilicon molecule to titanocene dichloride to form a Ti=Si complex called a silylene. This species is a heavy analog of the alkylidene metal complexes that are catalysts for alkene and alkyne metathesis reactions.
When they tried mimicking alkyne metathesis by adding an alkyne to their silylene, the researchers generated a silicon analog of the key cyclobutane metathesis intermediate for the first time.
For now, this cyclic silicon compound is a “frozen” intermediate of the metathesis process, Sekiguchi explains. That’s because the team hasn’t worked out exactly how to prompt the intermediate to complete the catalytic cycle and shed the final product, he says. This molecule would be a Ti=C–C=Si species, which may possibly react further with an alkene to form a new type of 1,3-diene with a C=C–C=Si core.
“This is a significant report, in that it provides direct observation of a [2+2] cycloaddition involving a silylene complex and an alkyne,” says T. Don Tilley of the University of California, Berkeley. Tilley’s group has reported the only other example of a silylene cycloaddition, which involved a ruthenium silylene and a nonmetathesis mechanism.
“This type of reaction has long been postulated for metal-mediated silylene-transfer reactions,” Tilley adds. “It’s nice to see a well-characterized example.”
Sekiguchi is optimistic about the prospects of using the approach to make heavy alkenes and dienes containing silicon, or even extending the method one row lower on the periodic table to germanium.
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