| Abstract
| - A general and efficient method for the rhodium-catalyzed enantioselective catalytic conjugate additionof organoboronic acids to α,β-unsaturated sulfones is described. The success of the process relies on theuse of α,β-unsaturated 2-pyridyl sulfones as key metal-coordinating substrates; typical sulfones such asvinyl phenyl sulfones are inert under the reaction conditions. Among a variety of chiral ligands, Chiraphosprovided the best asymmetric induction. This rhodium [Rh(acac)(C2H4)2]/Chiraphos catalyst system hasa broad scope, being applicable to the addition of both aryl and alkenyl boronic acids to cis and transα,β-unsaturated 2-pyridyl sulfones. In most cases, especially in the addition of aryl boronic acids, thereactions take place cleanly and with high enantioselectivity, affording chiral β-substituted 2-pyridylsulfones in good yields and enantioselectivities (70−92% ee). The sense and magnitude of thisenantioselectivity have been studied by DFT theoretical calculations of the aryl−rhodium insertion step.These calculations strongly support the formation of a five-membered pyridyl−rhodium chelated speciesas the most stable complex after the insertion into the CC bond. These highly enantioenriched chiralsulfones are very appealing building blocks in enantioselective synthesis. For instance, the straightforwardelimination of the 2-pyridylsulfonyl group by either Julia−Kociensky olefination or alkylation/desulfonylation sequences provides a variety of functionalized chiral compounds, such as allylic substitutedalkenes or β-substituted ketones and esters.
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