Benzophenone-silene cycloadducts

The attempted synthesis of 104 from its LiF adduct by salt elimination leads exclusively to the silene 104a65,66. 104a can be reacted with benzophenone to give the [2 + 4] and [2 + 2] cycloadducts 105 and 10667. The [2 + 4] cycloadduct of silene 104 cannot be obtained directly. The adducts 105 and 106, however, rearrange to the thermodynamically more stable 107, probably via the donor adducts 108, 109 and the free silenes 104 and 104a (equation 24). 

The addition of N-(trimethylsilyl)benzophenone imine to 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54)243 proceeds in a fashion quite analogous to the reaction of benzophenone, but is much slower. Both the 2 + 2 (56) and the 4 + 2 (55) cycloadducts have been isolated (equation 74). The former is formed at only about half of the rate of the latter but dominates in the equilibrium. As described in Section III.A.2.C, the small amount of 54 present in the equilibrium can be trapped by other added reagents so that 56 represents a convenient means of storing the silene 54, which cannot be stored neat since it rapidly dimerizes244,246. Another such store is provided by the silenate adduct of 54 with NMe3, which reacts with N-(trimethylsilyl)benzophenone imine to yield 55 and 56266. 

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