The stereoselective synthesis of cyclopropanes is an important issue in synthetic chemistry due to the relevance of these three-member carbon rings which often represent the active part in biological and/or pharmaceutical compounds.^1,2 The one pot reaction of diazo compounds with alkenes represents a sustainable methodology to achieve cyclopropanes due to the formation of N₂ as the only stoichiometric by-product. The reaction is efficiently catalysed by metal porphyrins which promote the synthesis of cyclopropanes with excellent diastereo- and enantioselectivities. Considering the importance in developing eco-friendly catalytic procedures, we are currently studying the catalytic activity of iron porphyrins which display a cheap, efficient and low toxic metal centre.

The two iron complexes 1Fe and 2Fe are good catalysts for the alkene cyclopropanation and both have one C2 axis within the porphyrin plane and exhibit an open space on each side for the substrate access. Complex 1Fe displays a pre-organized structure which was responsible for excellent trans-diastereoselectivities (94-99%) in alkene cyclopropanations while the presence of chiral moieties in 2Fe allowed to perform the reactions with also a very good enantioselectivity (up to 87%).²

Experimental data disclosed that each ‘totem’ portion of 2Fe has a specific catalytic role: i) the porphyrin core containing the active iron centre is in charge to activate the diazo compound; ii) benzylic moieties are responsible for the reaction diasteroselectivity and iii) the two chiral binaphthyl groups allow the enantioselective synthesis of cyclopropanes.

The study of the reaction scope revealed that both 1Fe and 2Fe catalysts were active in promoting the cyclopropanation of several olefins by differently substituted diazo compounds.

References

1. a) C. A. Carson, M. A. Kerr, Chem. Soc. Rev. 2009, 38, 3051-3060. b) A. K. Kumar, Int. J. Pharm. Pharm. Sci. 2013, 5, 467-472.

2. D. Intrieri, S. Le Gac, A. Caselli, E. Rose, B. Boitrel, E. Gallo, Chem. Commun. 2014, 50, 1811-1813.