Friday, November 6, 2009

All-Up Reactive Conformation of Chiral Rhodium(II) Carboxylate Catalysts

Vincent N. G. Lindsay, Wei Lin, and Andre´ B. Charette*
doi: 10.1021/ja9044955

Chiral Rh(II)-carboxylate catalysts have found widespread use in the field of metal carbene transformations, including asymmetric cyclopropanation reactions.1 Though several enantioselective transformations have been developed to date, little evidence is known
on how the chirality is projected near the reaction center by the chiral carboxylates. Davies et al. have suggested that four main possible symmetries have to be considered, from which only two
possess equivalent catalyst faces (Figure 1, C2 and D2).2 It has been postulated that catalysts having two different carbene-formation sites should not be effective in inducing enantioselectivity, since the more kinetically active and accessible face is apparently achiral (C1 andC4).

However, Fox et al. recently contradicted this concept by reporting the highly efficient asymmetric cyclopropanation of alkenes with R-alkyl-R-diazoesters using such a catalyst, where DFT calculations demonstrated that the all-up conformation of the catalyst plays a prominent role in these reactions.

All our experiments suggest that the halogenated rhodium carboxylate catalysts used in this process react through an all-up conformation, which is consistent with Fox’s DFT calculations made on non-halogenated analogues.

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