Silaallenes reactivity

Silaallenes 613 have been proposed by Ishikawa, Kumada and coworkers as transient reactive intermediates in the photolytic or thermolytic degradation of alkynyldi-silanes 614 as minor byproducts, the main product being silacyclopropenes 615184,192,287 (equation 203). 

Of the 2-silaallenes no sufficiently stable species have yet been isolated, so that no spectroscopic verification is available up to date. The synthesis of possible 2-silaallenes is therefore treated along with the description of their reactivity in Section n.C below. 

The polarity of the Si=C bond in 1-silaallenes is reduced compared with that in silenes. Thus, for the parent 1-silaallene H2Si=C=CH2 604 the calculated Mullikan atomic charges on Si and on the central C are +0.17 and —0.10293, respectively, compared to 0.46 and —0.65 in H2Si=CH2 25. This is expected to reduce the reactivity of 1-silaallenes... 

The two silaallenes 26a and 26b exhibit lifetimes in air-saturated hexane solution of ca 2.5 gs and ca 10 gs, respectively. Saturation of the solutions with oxygen leads to shortening of the lifetime in both cases, and allows estimates of ko2 1.1 x 108 and 1.5 x 107 M 1 s 1 for the rate constants for oxygen quenching of 26a and 26b, respectively. Interestingly, silaallene 26a appears to be somewhat more reactive than 26b toward 02. 

A mixture of reactive intermediates, including l,l-dimethyl-3,3-bis(trimethylsilyl)-Tsilaallene and dimethylsilylene, along with l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silacyclopropene 86 were formed and detected from the direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane in hydrocarbon solution (Equation 21). These species were detected and identified using laser flash photolysis. They were trapped as their methanol adducts in steady-state irradiation experiments. Steady-state irradiation in the presence of methanol affords MeOH-addition products which are consistent with the formation of the silaallene, silacyclopropene, and silylene along with bis(trimethylsilyl) acetylene as the major product <1997JA466>. 

Summary Reactions of dichlorodivinylsilane and LitBu in a molar ratio 1 1 and 1 2 lead to highly reactive intermediates, which can be trapped by suitable trapping agents. From that mono and double addition products are formed, which provide experimental hints for the intermediate formation of the neopentylsilene H2C=CH(Cl)Si=CHCH2/Bu 3 and the 2-silaallene tBuCH2CH=Si=CHCH2tBu 4. In particular, the formation of double cycloadducts from 2 with Lifflu is a preparatively facile route for the synthesis of silaspirocycles such as 12, 14, and 15, which could be characterized by single crystal X-ray structure analysis. 

Direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane (94) afforded a mixture of reactive intermediates, which were detected and identified by flash-photolysis techniques and trapped as methanol adducts in continuous irradiation experiments. Amongst the intermediates identified was the 1-silaallene (95), which, although a minor photoproduct, was readily observable owing to its relatively long lifetime. The stereochemistry of the addition of carbonyl compounds to silenes generated by photolysis of meso- and rac-1,2-diethyl-1,2-dimethyldiphenyldisilane has been investigated. ... 

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