SHAPE-SELECTIVE AND ASYMMETRIC CYCLOPROPANATION OF ALKENES CATALYZED BY RHODIUM PORPHYRINS

Rhodium porphyrins catalyze the cyclopropanation of simple alkenes by diazo esters. Functionalized catalysts that mediate selective cyclopropanation reactions might be of use in organic synthesis, particularly since the porphyrin catalysts generally provide syn cyclopropyl esters as the major product. In this report we examine the possibility of engineering shape selectivity and asymmetric induction into this system. We show that the iodorhodium derivative of an optically active macrocycle, the ''chiral wall'' porphyrin, does mediate the enantioselective cyclopropanation of prochiral olefins. Enantiomeric excesses of 10-60% are observed. In addition, the substrate selectivities of hindered (RhTMPI) and unhindered (RhTTPI) simple porphyrins were assessed. In both cases, mono-, di-, and trisubstituted aliphatic olefins undergo reaction at nearly equivalent rates but tetrasubstituted alkenes are cyclopropanated poorly, especially when the crowded catalyst is employed. In the case of bulkier aromatic alkenes, extremely high cis/trans substrate selectivity is observed. These observations have led to a model for the geometry of interaction between the putative metallocarbene and the alkene that rationalizes the relative reactivity of the many substrates investigated. The relevance of these results to the rational design of more selective asymmetric catalysts is also discussed.