Ketones silene cycloadducts

The Wiberg -type silenes like 92, available through salt elimination reactions from 93, react with nonenolisable aldehydes, ketones and the corresponding imino derivatives to give in a first step donor adducts 9459, which are then transformed to the [2 + 2] and [2 + 4] cycloadducts 95 and 96, respectively (equation 21)60-62. These cycloadducts may liberate the silene 92 upon heating and it can be trapped by suitable reagents. 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

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