1-Siloxy-l-silanes

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

To disfavor unwanted cyclization entropically, this general reaction scheme was extended to include monomers 6-hex-1-enyltris(dimethyl-siloxy)silane and 8-oct-l-enyltris(dimethylsiloxy)silane. The resulting polymers gave no indications of cyclization and resulted in molecular weights of about 12,000. However, SEC traces were multimodal in nature caused by monomer contamination with internal olefins (notoriously sluggish or completely unreactive towards hydrosilation reac-... 

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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