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June 5, 2013
Quaternary Chiral Centers Made to Order
Chiral catalyst reactions seem to show up on both lists when you talk about new reactions: the list of "Man, we sure do need more of those" and the "If I see one more paper on that I'm going to do something reckless" list.
I sympathize with the latter viewpoint, but the former is closer to reality. What we don't need are more lousy chiral catalyst papers, though, on that I think we can all agree. So I wanted to mention a good one, from Erick Carreira's group at the ETH. They're trying something that we're probably going to be seeing more of in the future: a "dual-catalyst" approach:
In a conceptually different construct aimed at the synthesis of compounds with a pair of stereogenic centers, two chiral catalysts employed concurrently could dictate the configuration of the stereocenters in the product. Ideally, these would operate independently and set both configurations in a single transition state with minimal matched/mismatched interactions. Herein, we report the realization of this concept in the development of a method for the stereodivergent dual-catalytic α-allylation of aldehydes.
Shown is a typical reaction scheme. They're doing iridium-catalysed allylation reactions, which are already known via the work of Hartwig and others, but with a chiral catalyst to activate the nucleophilic end of the reaction and a separate one for the electrophilic end. That lets you more or less dial in the stereocenters you want in the product. It looks like the allyl alcohol need some sort of aryl group, although they can get it to work with a variety of those. The aldehyde component can vary more widely.
You'd expect a scheme like this to have some combinations that work great, but other mismatched ones that struggle a bit. But in this case the yields stay at 60 to 80%, and the ee values are >99% across the board as they switch things around, which is why we're reading this in Science rather than in, well, you can fill in the names of some other journals as well as I can. Making a quaternary chiral center next to a tertiary one in whatever configuration you want is not something you see every day.
I think that chiral multi-catalytic systems will be taking up even more journal pages than ever in the future. It really seems like a way to get things to perform, and there's certainly enough in the idea to keep a lot of people occupied for a long time. Those of us doing drug discovery should resist the urge to flip the pages too quickly, too, because if we really mean all that stuff about making more three-dimensional molecules, we're going to have to do better with chirality than "Run it down an SFC and throw half of it away".
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