Acylsilanes structure

Heat treatment of PVTMSK films resulted in spectral changes similar to those after mid- or deep-UV exposure. Thus, the intensity of the IR band at 1636 cm" (vc=o) nd of the UV band at 365 nm (n tt ) decreased after a PVTMSK film was kept for several hours at >80 C. Both exposure to UV radiation and increased temperature gave rise to a new three-component band at 1718 cm in the IR spectrum and caused an increased absorption at X < 325 nm. Thus, these treatments lead to similar transformations of the acylsilane structure. 

The unusual l-sila-l,2-diol (CH2=CH)CH(OH)SiPh2OH, prepared by hydrolysis of the acylsilane (CH2=CH)C(0)SiHPh2, has a structure comprising hydrogen-bonded double chains as shown in Fig. 9. Only COH - HOSi hydrogen bonds are present, with the bonds between the chains being shorter than those that build up the chain, 2.666 and 2.739 A, respectively (131, 210). 

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

The structural chemistry of silicon bonded to special functional groups such as acetyl group, cyanide and isocyanide deserves special attention. Acylsilanes having the general formula l SiC.OR constitute an interesting class of chemical compounds. They are sensitive to light and rather unstable, particularly in a basic environment, where they react... 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

The very long endocyclic C—C bond in 362 (1.66 A) indicates already relatively facile homolytic C—C bond cleavage38. The solution of the dimer and the irradiated pivaloyl-acylsilane 147 gave rise to a strong broad ESR signal without a fine structure. Attempts to intercept a putative biradical like 363 failed, but they do not rule out the possible existence as an intermediate in low concentrations. A biradical species like 363 would also account for dimerization products isolated from other silenes (see below)119. 

Photolysis of a,/i-unsaturated acylpolysilanes of the general structure (Me3Si)3-SiCOCR=CR2 failed to yield significant amounts of the isomeric siladienes, and gave instead products derived from the Norrish type 1 cleavage of the acylsilane into the radicals (Me3Si)3Si and COCR=CR, 124. 

Phenyllithium (and lithiophosphites) also bring with them sufficient stabilisation of the organolithium for Brook rearrangement to occur readily.40 With acylsilane 49, intramolecular Michael addition leads to cyclic structures 50. 

Two simple a, P-unsaturated acylsilanes, l-trimethylsilyl-2-propen-l-one (III) and l-trimethylsilyl-2-methyl-2-propen-l-one (IV) were chosen for polymerization studies. The polymerization of the carbon analogues of these a,p-unsaturated acylsilanes, that is, 4,4-dimethyl-2-propen-3-one (vinyl tert-butyl ketone, V) and 2,4,4-trimethyl-2-propen-3-one (isopropenyl tert-hutyl ketone, VI) has been studied by Willson et al. 16, IT), These authors reported that whereas V readily polymerizes under free-radical-polymerization conditions, VI undergoes polymerization only under anionic-initiation conditions in the presence of a crown ether as a complexing reagent. On the basis of UV and NMR spectroscopic data, Willson et al. (i6, 17) ascribed the difference in polymerization behavior to the nonplanar, unconjugated structure of ketone VI brought about by steric hindrance caused by the methyl group at C-2. 

In a few cases acylsilanes have shown dual behavior. Thus some alkylacylsilanes when photolyzed- in alcohols give products derived from both siloxycarbenes and radicals101,103. The cyclic acylsilane, l,l-diphenylsila-2-cyclohexanone, when photol-yzed in cyclohexane gave some diphenylsilacyclopentane, obviously derived from loss of carbon monoxide from an acyl radical, and two dimers whose structures showed clearly that they arose from a cyclic siloxycarbene (which had also been trapped by alcohol or oxygen under other conditions)104 (equation 68). 

The nucleophilic and electron-accepting properties of heterocyclic nucleophilic carbenes 36 were also used in combination with the electrophilic/nucleophilic character of acylsilanes via Brook rearrangement, leading to the invention of a sila-Stetter reaction by Scheidt and coworkers fScheme 6.24). The iminium structure in 37, generated by addition of the carbene catalyst 36 to the acylsilanes, promotes a Brook rearrangement to afford enol silyl ether 38. The alcohol additive present in the reaction causes desilylation to produce nucleophilic enaminol 39, which adds to a,p-unsaturated ketones to give 40. The formation of aryl ketone expels the carbene catalyst and produces 1,4-diketone 41. 

An intermediate siladiene 153 is formed during the thermolysis of an o , -unsaturated acylsilane 154. Addition of (Me3Si)3Si and subsequent hydrogen abstraction led to the isolation of the enol ether 155 formed, presumably, via radical 156 (equation 38). The identity of 155 was confirmed by an X-ray crystal structure . 

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