Rhodium (II) carboxylates have been widely used as transition metal catalysts in organic synthesis to catalyze reactions of diazo compounds in reactions like olefin cyclopropanation, aromatic cycloaddition, O-H, and N-H insertion reactions. Prior work including some from our group has been focused on using polymers as supports to facilitate the recycling of the metal catalyst. For example, our group has developed polyethylene (PE)-bound as well as polyisobutylene (PIB)-bound Rh(II) complexes for such reactions. Such polymer bound complexes can be used as homogeneous catalysts in a single phase with the substrates and then isolated by either thermomorphic or latent liquid-liquid biphasic strategies. However, these catalysts like low molecular weight Rh (II) carboxylate catalysts require the use of a syringe pump to add the diazo compound in order to minimize dimer formation due to rapid Rh(II)-catalyzed dimerization of the diazo compound. This exothermic side reaction limits the applicability of this chemistry. Here we describe a method that uses a heptane/acetonitrile biphasic system that suppresses this side reaction without syringe pump addition of the diazo compound. This reaction scheme uses a PIB oligomer-bound Rh (II) catalyst that is soluble in heptane but insoluble in a CH3CN phase that contains the diazo compound, ethyl diazoacetate  since this diazo reagent is not soluble in the heptane phase where the Rh(II) catalyst is phase-localized, the velocity of the bimolecular decomposition of the diazo compound is suppressed in heptane only encounters a low concentration of ethyl diazoacetate. However, while the cyclopropanation of styrene is slower too, that reaction is not suppressed to the same extent. Under these conditions, the Rh(II) catalyst is still recyclable. Examples applying this strategy to O-H insertions will also be discussed.

Pseudo biphasic Rh-catalyzed cyclopropanation using recyclable Rh(II) catalysts

247th National Meeting (2014)

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