Isobutene, reactions with silene

For l,5-bis(pentamethyldisilanyl)naphthalene a 1,3-silyl migration does not occur. The products formed are analogous to 220 and 225. l-(Pentamethyldisilanyl)naphthalene undergoes silyl migration from position 1 to 8. 2-(Pentamethyldisilanyl)naphthalene 226 gives a silene 227 by a silyl shift from position 2 to 1. It can be trapped in an ene reaction with isobutene to yield 228 (equation 57). 

With dihydropyranyl-substituted phenyldisilane 240 Ishikawa and coworkers found the formation of both possible types of silenes 241 and 242134. Both were trapped by acetone, but 242, however, only through an ene reaction to give 549. With isobutene only the silene 242 gives an ene product 548 (equation 189). 

Ene reactions of monoolefins with silenes have been known for a long time308. The addition of isobutene to 54 (equation 136) has been observed to proceed at —10 °C243. A similar reaction occurs between 54 and other olefins (Table 4)246 see also Reference 266. The rates of these reactions appear to be somewhat sensitive to steric hindrance but even the stable hindered silene 41 still adds propene and isobutene115. They are very much slower than the analogous ene additions of carbonyl compounds containing a hydrogens, such as acetone. 

Silenes derived from aromatic di- or polysilanes have been characterized in particular by the ene-type reactions they undergo when treated with isobutene or acetone. Recently, Leigh80 observed the first reported case of one of these silenes undergoing [2+2] cycloaddition (21% yield) with acetone. The ene product, the only product previously detected from the reaction of such silenes, was formed in 41% yield, as shown in Eq. (33). 

In contrast, the study of isomeric bis(disilanyl)benzenes 208-210 showed that only products 211-213 from ene reaction of isobutene with rearranged silenes 214-216 were formed128. The silepines 217 and 218 result from a reaction of the initially formed silenes with isobutene (equation 52-54). 

Benzenoid disilanes are known to undergo photorearrangement via a silene intermediate. A conversion of this type (Scheme 4) has been proposed to account for the photoreaction of 1,2-bis(pentamethyldisilanyl)benzene (193) with isobutene ene-reaction of the photochemically generated silene (194) with isobutene affords 2-(isobutyldimethylsilyl)-1-(pentamethyldisilanyl) -3-(trimethyIsilyl)benzene (195).1,3-Bis(pentamethyldisilanyl) benzene undergoes an analogous photoreaction with isobutene to give 3-(isobutyldimethylsilyl)-1-(pentamethyldisilanyl) -4- (trimethylsilyl)benzene the silepin (196) is a minor product of this reaction and arises from the silene (197) as shown in Scheme 5. 

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