Filling gaps in asymmetric hydrogenation methods for acyclic stereocontrol: application to chirons for polyketide-derived natural products.
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abstract
The large volume of research studying hydrogenation catalysis might suggest that stereoselective hydrogenation of alkenes is a solved problem, but we believe the most important parts of asymmetric hydrogenation methodology remain unmastered. The most popular chiral catalysts, Rh- and Ir-diphosphine complexes, do not hydrogenate the largest categories of prochiral alkenes, hindered tri- and tetra-substituted ones, at useful rates unless the substrate has a "classical" coordinating functional group (CFG), for example, amides or homoallylic alcohols, to anchor the substrate to the metal. Therefore, while many methods are available for the asymmetric hydrogenation of alkenes with appropriate CFGs, synthetic chemistry would benefit from chiral hydrogenations of substrates with functional groups that typically do not coordinate in Rh- and Ir-diphosphine complexes. In this Account, we demonstrate the application of chiral analogues of Crabtree's catalyst to asymmetric hydrogenations of coordinating unfunctionalized, trisubstituted alkenes. Crabtree's catalyst, a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine, differs from Rh- and Ir-diphosphine complexes, which we broadly refer to as "chiral analogues of Wilkinson's catalyst." Crabtree's catalyst analogues hydrogenate substrates that do not contain functionalities generally recognized as CFGs, and we propose reasons for this chemistry based on the catalytic mechanisms. Thus, chiral analogues of Crabtree's catalyst facilitate many hydrogenations that would not be possible using Rh- or Ir-diphosphine complexes. Directed hydrogenations have been used in acyclic stereocontrol for decades, but the realization that these catalysts can be used for acyclic stereocontrol without the types of directing groups that are necessary for other hydrogenations significantly broadens the scope of hydrogenations for this purpose. Recently, we have prepared chirons for polyketide-derived natural products using an N,carbene-Ir complex (1). This approach has led to catalytic syntheses of several important chirons to facilitate preparations of these ubiquitous natural products.
