Shape and stereoselective cyclopropanation of alkenes catalyzed by iron porphyrins.

Abstract

Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate. Fe(TIP) (TIP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products. The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1, 1-disubstituted alkenes. Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes. A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein. Disciplines Chemistry CommentsReprinted (adapted) Am. Chern. Soc. 1995, 117, 9194-9199 Shape Abstract: Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate. Fe(TIP) (TIP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products. The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1, 1-disubstituted alkenes. Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes. A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein.The metal-catalyzed cyclopropanation of substituted olefins by diazo esters is a reaction commonly employed in organic synthesis. 1 Chiral copper 2 -4 and rhodium 5 • 6 catalysts have recently been reported to effect cyclopropanation with high enantioselectivity. However, the diastereoselectivities obtained in these reactions are generally poor. Mixtures of the trans and cis cyclopropyl esters are produced, with the trans isomer obtained in only a slight excess. A notable exception is a recently reported ruthenium catalyst/ which provided high enantioselectivities and trans/cis ratios of 10:1 using ethyl diazoacetate (EDA) as the carbene source.We are interested in the development of metalloporphyrin cyclopropanation catalysts, since such reactions often exhibit unique stereoselectivities and exceedingly high catalyst turnover numbers. For example, rhodium(III) porphyrin-catalyzed cyclopropanation reactions using EDA as the carbene source produced the cis cyclopropyl ester as the major product when bulky ligands such as the tetramesitylporphyrin (TMP) (see Figure 1) were employed. 8 -11 To the best of our knowledge, this is the only exampl...

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Cyclopropanation Reactions Mediated by Group 9 Metal Porphyrin Complexes

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