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In a 2013 study, an iridium catalyst and a chiral amine worked together to control the two neighboring stereocenters formed when branched aldehydes underwent α-allylation. By changing the catalysts’ configurations, the researchers could direct formation of each of the four stereoisomers described in the report.
What the catalyst pair does
The method joins a secondary allylic alcohol to an α-disubstituted, or branched, aldehyde. The two chiral catalysts activate different partners: an iridium phosphoramidate catalyst activates the allylic alcohol, while a chiral amine activates the aldehyde by forming an enamine. Their coordinated influence guides the stereochemistry at the two adjacent centers created during bond formation.
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This division of roles is the key idea: rather than asking one catalyst to control both new stereocenters in the same way, the reaction uses a catalyst associated with each reacting partner. The 2013 paper describes the approach as enantio- and diastereodivergent dual catalysis.
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A molecule with two stereocenters can have four possible stereoisomers. The report describes access to all four by selecting the configurations of the chiral catalysts. In practical terms, the catalyst choice can steer the reaction toward different combinations of configurations at the two centers, rather than simply favoring one stereochemical outcome over all others.
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This is the meaning of stereodivergence here: the same general bond-forming transformation can be directed toward distinct stereoisomeric products by changing the catalyst pair. The result depends on the specific combination of the chiral iridium and amine catalysts; it is not a claim that any two chiral catalysts will provide independent control.
What selectivity the study reported
Chemistry World’s 2013 account reports that many of the examined examples gave yields above 70%, more than 20-fold excess of the desired diastereomer, and enantiomeric excess above 99%. These are reported outcomes for many studied substrates, not guarantees for every substrate or for α-allylation in general. Substrate-specific figures should be checked against the paper’s experimental data.
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The primary report is Simon Krautwald, David Sarlah, Michael A. Schafroth, and Erick M. Carreira, “Enantio- and diastereodivergent dual catalysis: α-allylation of branched aldehydes,” Science 340(6136), 1065–1068 (May 31, 2013), doi:10.1126/science.1237068. Chemistry World’s account provides the reported scope and selectivity figures, along with comments from the researchers and an outside chemist: Chemistry World’s 2013 report.
What the result does—and does not—establish
The demonstrated reaction is specifically the α-allylation of branched aldehydes with secondary allylic alcohols using paired chiral iridium and amine catalysis. The result illustrates a strategy for controlling two newly formed stereocenters in that transformation; it should not be generalized to all aldehydes, allylations, or dual-catalytic reactions.
At the time, the work was presented as progress toward fully stereodivergent reactions. The cited accounts do not establish how widely the strategy was adopted afterward or whether it applies broadly to other organic reactions. As Eric Jacobsen observed in the 2013 coverage, whether the strategy would transfer to other reactions remained an open question.
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