Silacyclopropene, ring opening

At the time, these suggestions could not be tested as silacyclopropenes were unknown, but an alternative mechanism was proposed involving the initial formation of the 1,2-disilacyclobutene, which ring opens to the 1,4-disilabuta-l,3-diene reversibly to add a second molecule of alkyne. The disilacyclobutene (27), formed from 2-butyne, adds 3-hexyne to give the disilin (28 Scheme 37) (76JA7746). 

Silacyclopropenes are commonly formed from the addition of a silylene to an alkyne, or in some cases as the result of photolysis of an alkynyldisilane (see Section III.C). Substituted silacyclopropenes have been shown to undergo both 1,2- or 1,3-shifts when photolyzed, yielding silyl-substituted allenes or alkynes, respectively2. More complex behavior was observed with methylenesilacyclopropenes such as 2323 which ring-opened to a diene, as shown in Scheme 4. 

Recently, detailed mechanistic studies have been made of the photolysis of l-aryl-4-(pentamethyldisilanyl)-l,3-butadiynes 96 in various solvents. Photolyses in the presence of methanol led to the isomeric products 97 and 98, derived from solvolytic ring opening of the initially formed silacyclopropene, 99, resulting in turn from 1,2-silyl migration48, while photolyses in acetone led to the products 100 and 101 arising from two-atom insertion into the three-membered ring49 (Scheme 14). 

The first synthesis of unsubstituted silacyclopropene 8 was achieved recently, along with a novel route to obtain this class of substances. High vacuum pyrolysis of l,l,l-trimethyl-2-vinyldisilane 80 produces the corresponding silacyclopropene by elimination of trimethylsilane (Scheme 23). A ring-opening reaction of the silacyclopropene produces the other product <1995AGE1439>. 

A computational study has shown that the barrier to the concerted ring opening of silacyclopropene to vinylsilylene is lower than the barrier to forming silylacetylene via silylvinylidene. ... 

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