Silylcarbenes intramolecular

When two carbene functions are separated by one or more silicon atoms, one can expect them to enter independently the usual inter- or intramolecular reactions. Among the intramolecular reactions, extensions of those which have been discussed in Sections III.B and IILC are particularly appealing, namely silylcarbene-to-silene rearrangement at one or both carbene centers and intramolecular carbene dimerization to form a C,C double bond and thus an unsaturated silaheterocycle. 

By far the best source for 3a is (trimethylsilyl)diazomethane (19). It has already been mentioned that gas-phase pyrolysis of 19 alone yields products which are derived from intramolecular carbene reactions such as 1,3-C,H insertion and silylcarbene-to-silene rearrangement (see equation 20). Also, copyrolysis of 19 with alcohols or benzaldehyde allowed one to trap the silene but not the carbene 33 (see equation 5). Furthermore, solution photolysis of 19 in the presence of alcohols or amines did not give the X,H insertion products of the carbene but rather trapping products of the silene . On the other hand, photochemically generated carbene 3a did undergo some typical intermolec-ular carbene reactions such as cyclopropanation of alkenes (ethylene, frani-but-2-ene, but not 2,3-dimethylbut-2-ene, tetrafluoroethene and hexafluoropropene), and insertion into Si—H and methyl-C—H bonds (equation 39). The formal carbene dimer, trans-1,2-bis(trimethylsilyl)ethene, was a by-product in all photolyses in the presence of alkenes it is generally assumed that such carbene dimers result from reaction of the carbene with excess diazo compound. 

Remarkably, the thermal decomposition of (l-diazo-2-oxoalkyl)silanes 186 does not generate silylcarbenes. Instead, a 1,3(C—>-0) silyl shift takes place under rather mild conditions (20-80 °C, depending on the substituents) from which ( -siloxy)diazoalkenes 192 result (equation 59). These short-lived cumulenic diazo compounds rapidly split off molecular nitrogen, and the resulting -siloxyalkylidene carbenes undrago typical intramolecular reactions, from which siloxyalkynes, l-oxa-2-sila-4-cyclopentenes or heptaful-vene derivatives result. The presumed diazoalkenes 192 can be trapped by [3 + 2]... 

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