Asymmetric Catalysis Stereocontrolled by Entropy : A New Aspect for Chiral Tethered Reactions

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The studies to develop asymmetric catalytic system basing on the chiral tethered reaction brought us the following results.1)The chiral tethered reactions with 2,4-pentanediol (PD) and its analogues were studied for meta-arene-alkene photocycloaddition, rhodium carbenoid addition to olefin or arene, and ketene-olefin cycloaddition. By the analysis of their stereoselectivities, either of the two methyl groups in PD tether was found to be indispensable factor for the strict stereocontrol, and the methyl group of the chiral source can be displaced to the other smaller than isopropyl group without loosing the efficiency of the intramolecular reaction.2)A new kinetic parameter, chiral perturbation factor, was introduced to clarify the relation between the substituents on the tether and differential activation entropy caused by them. By the analysis of the rhodium carbenoid addition to arene with chiral perturbation factor, one of the methyl groups on the PD-tether was found to accelerate the major process in entropy term, while decelerate the minor one to result in 30 cal/mol K, which is assumed to be a major reason of the strict stereocontrol of the chiral tethered reaction.3)Dimerization reaction of phenoxyl radical was studied in the presence of cyclodextrins (CDs). The stereoselectivity of 1 to 1 in the absence of CD in 42% yield becomes 47% diastereomeric excess when the tether has a benzoyloxy substituent at the 2 position.4)The PD tethered reaction was extended to a vapor phase pyrolysis, where ketene generated adds to olefin under strict stereocontrol even at 400 The reactions with PD-tethered diazo esters resulted in many optically active products, which were converted to varied compounds such as an insect pheromone, sperapyrone, and several polyketides.