Silylenes multiple bond insertion reactions

Not all cycloadducts with Si—Si bonds are formed by addition of a disilene to multiple bonds. Thus, reactions of phosphaalkynes with the trisilane 115, usually used for the generation of the disilene 9, are assumed to proceed via addition and insertion of two silylene molecules, Mes2Si , to yield the formal [2 + 2] adducts 116 (equation 25)118. 

Many of the addition reactions of the silylenes 83-85 with molecules having multiple bonds and also many of their insertion reactions into activated (T-bonds are thought to proceed via cyclic transition states. For that reason these reactions are summarized in this section. It should be emphasized, however, that in only a few cases mechanistic investigations either by theoretical means or by experimental methods have been undertaken to substantiate the claim for a one-step cycloaddition. 

Several of the reactions described in Section 6.16.2.4.6.1 are two-step reactions. After the initial [2+1] addition of the silylene to the multiple bond, a second insertion reaction of a silylene into a reactive Si-X bond of the cyclopropane derivative takes place yielding four-membered ring compounds. Examples are the reactions with alkynes, nitriles, imines, and ketones. 

Major reaction patterns of silylenes are insertion into the O-H (equation 82), Si-H (equation 59) and Si Si bonds (equation 60), addition to multiple bonds (equation 83), rearrangement (equation 84), and dimerization. Dimerization of the bis(mesityl)silylene (equation 85) opened a pathway to the first isolable disilene. ... 

During the past three years, we have studied the reactions of thermally generated silicon atoms with low molecular weight reaetants. Reaction products were isolated in argon matrices and identified by means of IR spectroscopy, aided by calculated vibrational spectra. The method turned out to be very versatile and successful [1-4]. When the reactions of silicon atoms with unsaturated substrates are compared, a general sequence of products can be outlined as follows if a heteroatom is participating in the multiple bond, the silicon atom is bound to a lone pair in the primary product. This primary product can be isomerized photochemically to a cyclic silylene, the formal addition product of the Si atom to the Jt bond, which is the first product in case of substrates with C-C multiple bonds. Consecutive photoisomerizations lead to the formal insertion product of the Si atom into a C-H bond and, finally, to the product(s) of (successive) migration of H atoms to the Si atom. 

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