Metal-catalyzed formation of 3- and 4-membered rings

Abstract

The catalytic activity of TBA[Fe] in the cyclopropanation of aryl olefins using diazo esters as carbene surrogates was investigated. By employing two modes of activation for the catalyst, i.e. thermal or photochemical, a variety of aryl olefins as well as 1,3-enynes were converted to the corresponding cyclopropanes in moderate trans-selectivity and good to excellent yields.The catalytic process exhibited good functional group tolerance and chemoselectivity, as triple bonds and non-conjugated olefins remained unaffected, and no competing X-H insertion reaction was observed. Mechanistic studies allowed to determine that the catalyst was activated by either photochemical or thermal decarbonylation. Moreover, the presence of 4-nitroanisole in sub-stoichiometric amounts was shown to accelerate decarbonylation of the catalyst under thermal conditions and enhance its activity. In addition, a novel silver-catalyzed oxidative ring expansion of propargyl cyclopropanes was developed to give a straightforward access to valuable synthetic intermediates. Enantiopure cyclobutenyl ketones were obtained from chiral cis-propargyl cyclopropanes under mild conditions with almost full retention of their enantiomeric excess and up to 92 % yield. The reaction exhibited so far good functional group tolerance and mechanistic investigations as well as expansion of the substrate scope are currently under way in our laboratories.