Chiral C2-Symmetric Bis-Thioureas

The C2-symmetric chiral bis-thiourea catalyst 78 mediated the addition of cyclohexanone to a range of activated aldehydes in the presence of DMAP, or imida-... 

At almost the same time, Nagasawa s group developed the C2-symmetric chiral bis-thiourea-catalyzed asymmetric MBH reaction of cychc enones with aldehydes (Scheme 2.196). They found that the bis-thiourea catalyst 363 promoted the reaction of cyclohexenone with benzaldehyde in the presence of an additive (0.4 equiv) such as DMAP or imidazole to afford the allyhc alcohol (DMAP — 5 °C, 88%, 33% ee imidazole room temperature, 40%, 57% ee). Higher enantioselectivities were obtained in the reaction with aliphatic aldehydes (up to 90% ee). A transition state 364, in which both the aldehyde and the enone coordinate to the thiourea groups of 363 through hydrogen bonding interactions, was proposed to explain the stereochemistry of the product. 

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