DIRECT-DETECTION AND CHARACTERIZATION OF A TRANSIENT 1-SILAALLENE DERIVATIVE IN SOLUTION

Direct irradiation of ((trimethylsilyl)ethynyl)pentamethyldisilane in hydrocarbon solution affords a mixture of reactive intermediates which have been detected and identified using laser flash photolysis techni ques and trapped as the methanol adducts in steady state irradiation e xperiments, Flash photolysis of air-saturated hexane solutions of the disilane allows detection of transient species assigned to 1,1-dimethy l-3,3-bis(trimethylsilyl)-1-silaallene and dimethylsilylene, along wit h a non-decaying species assigned to methyl-2,3-bis(trimethylsilyl)-1- silacyclopropene, These were identified on the basis of their UV absor ption spectra and reactivity toward various reagents, The 1-silaallene is a minor photoproduct, but it is readily observable in transient ab sorption experiments because it is relatively long-lived (tau 2.4 mu s in air-saturated solution) and absorbs strongly, with absorption maxi ma considerably to the red of those of the 1-silacyclopropene, The 1-s ilaallene exhibits characteristic silene reactivity; it reacts with me thanol, acetic acid, acetone (k = 10(6)-10(8) M(-1) s(-1)) and oxygen (k similar to 10(8) M(-1) s(-1)), exhibits low reactivity toward aliph atic dienes and tert-butyl alcohol, and decays with second-order kinet ics in the absence of quenchers, With methanol and acetic acid, it has been shown that reaction in hexane occurs with the monomeric ROH spec ies only. Steady state irradiations in the presence of methanol afford methanol-addition products consistent with the formation of the silaa llene, silacyclopropene, and silylene, along with bis(trimethylsilyl)a cetyiene as the major product.