A wide variety of natural and unnatural compounds contain the cis cyclopropane group. Most importantly, cyclopropanes undergo ring-opening reactions which have significant synthetic importance. Among the methods available, the most attractive method is the transition-metal-catalyzed decomposition of diazo compounds in the presence of alkenes to yield cyclopropanes. Most of the catalytic systems used to date preferentially provide more stable trans cyclopropanes. Recently, we developed the iron Lewis acid, (CpFe(CO)2(THF)]+ (1), catalyzed cyclopropanation reactions with cis selectivity. Apparently, the reaction is the first successful catalytic cyclopropanation reaction involving an iron carbene complex. The reaction of phenyldiazomethane with alkenes in the presence of 1 gave almost all cis/syn cyclopropanes, providing the first example of an overwhelming cis preference in the catalytic cyclopropanation reactions. We plan to explore this unique catalytic stereoselective reaction, its potential generality and its application in organic synthesis. The three main goals of this proposal involve: (1) extending our study to a variety of diazo compounds and alkenes, (2) understanding the mechanism of this reaction, in particular, the stereoselectivity of cyclopropanation and (3) synthesizing cis cyclopropanes of synthetic interest. More active iron Lewis acids 36 and 37 are proposed to be synthesized by introducing an electron-withdrawing group at the Cp and/or at the iron. An analogous ruthenium Lewis acid will be synthesized to understand the role of the metal on reactivity and selectivity of catalytic cyclopropanation reaction. Synthetic objectives include preparation of 1,1-disubstituted cyclopropanes and cis vinyl cyclopropanes. Synthetic objectives also include the development of (a) first catalytic asymmetric cyclopropanation reactions with chiral iron Levis acids, and (b) the first catalytic intramolecular cyclopropanation reaction involving iron carbenes. Synthesis of the iron Lewis acid 1 is an efficient and short and also very economical procedure that makes any organic transformation mediated by iron reasonable and cost effective.
NIH ExPorter
NIH ExPorter