What Are Climate Models Missing?

1053 PERSPECTIVES ner. In one example, two chiral catalysts were used to induce conjugate addition reactions of cyanide (7). Only one stereocenter is created in that process, but substantial enhancement in enantioselectivity was observed through the " matching " of the two chiral catalysts. In another example, researchers used two different chiral secondary amine catalysts in the same reaction to effect a sequential enantioselective conjugate addition and then trapping of the intermediate enolate (8). Through ingenious reaction design, the fi rst catalyst was induced to fall off after the initial step, and the second catalyst entered and controlled the diastereoselectivity of the second stereocenter formation. As such, both stereo-centers of the product can be controlled, and thus all four of the possible stereoisomers could be accessed through proper choice of the two chiral catalysts. That work represented an extraordinary illustration of a well-established strategy in asymmetric synthesis: chiral catalyst-controlled diastereoselectivity in a reaction of an enantioenriched substrate. Krautwald et al. also use two different chi-ral catalysts to control the formation of two stereocenters. However, their approach is fundamentally different from previous efforts in that it relies on the independent activation of two distinct reacting partners in a single reaction (see the fi gure, panel B). The goal is not for one catalyst to overcome or complement the effect of the other, but rather for the two catalysts to induce stereoselectivity independently yet simultaneously. The concept is illustrated in the α-allylation of branched aldehydes, a carbon-carbon bond-forming reaction that generates two contiguous stereocenters (see the fi gure, panel C). Relying on previously established catalytic reactivity principles, the aldehyde is activated by an amine catalyst, and the allylic electrophile is activated by an iridium catalyst. The authors show in an unambiguous set of experiments that each catalyst controls the stereoselectivity of one of the substrates but has essentially no effect on the stereocenter derived from the other reacting partner. Then, in a stunning set of results, they show that two chiral catalysts working together achieve the desired effect of nearly perfect and independent control of each center. Selective access to every possible stereoisomer can thus be achieved in a single transformation from the same set of substrates simply by choice of a distinct catalyst combination. In principle, this has the potential to emerge as a powerful new strategy for reaction design, applicable to a wide range of important reactions. Many of the …