Aldehydes, freezing point depression

The 3,3 -bis-triarylsilyl BINOL ligands 98 were first introduced by Maruoka, Itoh, Shirasaka, and Yamamoto for the heteroatom Diels-Alder reaction of electron-rich dienes with unactivated aldehydes [76,77], The catalyst was prepared by treating the BINOL 97 with trimethylaluminum in dichloromethane at room temperature for 1.5 h. The catalyst is pink to wine red and was found, by freezing point depression, to be a monomer. A typical procedure for the Diels-Alder reaction is illustrated for the reaction of the trisubstituted diene 371 and cyclohexylcarboxaldehyde. The reaction is complete with 10 mol % catalyst at -20 °C in 2 h in toluene and, after treatment with TFA to effect elimination of the methoxyl group, the product of the reaction is predominately the cii-pyrone 373. The triphenylsilyl substituted catalyst gives the product in 93 % ee (Sch. 47). 

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