Siletane

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

This work has been compared with analogous cyclobutane thermolytic decompositions. The siletanes were found to fragment more readily than the cyclobutanes. Although fragmentation via propene formation (most substituted C—C bond) was favored in both classes of compounds, it was more dominant with the siletanes. These effects are apparent from the kinetic data in Table IV.144-147... 

TABLE 8. Standard enthalpies of formation (A//f°/kJ mol 1) of some siletanes and their strain enthalpies... 

Strained siletanes may also be used in the Tamao Oxidation instead of halosilanes these intermediates offer a comparable Lewis acidity because coordination of the fluoride ion releases angle strain. 

The fact that oxidation of carbon-silicon bond is possible in the presence of a silyl ether increases the attractiveness of siletanes as masked hydroxyl groups. 

Siletane oxidation can be used as a trigger to promote cleavage of/>-siletanylbenzyl (PSB) ethers <2005TL3283>. Mild oxidation of the arylsiletane yields the />-hydroxybenzyl ether, which can be easily hydrolyzed to release the alcohol. This methodology appears to be most efficient for the protection and deprotection of phenols and primary alcohols (Scheme 28). 

It was mentioned in Section 2.11.6.3.1 that the reaction of siletanes with thermolytically generated carbenes produces mixtures of products arising from competing Si-C and C-H bond insertions (Scheme 56) <1967JA1538,1971JA3709>. 

For siletanes with tertiary carbons, insertion into a C-H is favored over a Si-C insertion <1967JA1538>. 

Phosphoran [l-(l-Methyl-l-sileta-nyl)-ethyliden]-triethyl- El, 648 (R3P = CH-R + 1-C1-1-CH3-siletan)... 

From methylenephosphoranes (R = H) both mono- or bis-silylated products may result. Schmid-baur et al. have used a variety of compounds for the silylation of ylides. a-Silylated alkylidenephos-phoranes are also formed in the reaction of simple ylides widi 2-chloroethylsilane (equation 38), chloromethylsilane (equation 39), siletanes, 1,3-disiletanes (equation 40) and the highly strained hexa-methylsilirane (equation 41). ... 

Syntheses of complex ylides by alkylation of simple ylides have been achieved in great var-iety,i 10.111.132 using not only alkyl halides (usually containing another functional group) but also other alkylating reagents (Mannich bases,strained cyclic compounds like epoxides," aziridines ° and siletanes, pyrylium salts and others). Equations (54)-(56), (57), (58), (59), (60) and (61) give some selected examples. 

With Pd insertion into a C—Si bond of a siletane elements of the small ring add to conjugated carbonyl compounds, eight-membered unsaturated O,Si-heterocycles are produced. ... 

A limited amount of data exists for silicon-containing ring compounds. This is shown in Table 5. All the studies involved used combustion with an auxiliary fluorine compound, and so it is disappointing that for the four-membered ring compounds (siletane, disiletane) the agreement is so poor. Steele s values are taken to be the most reliable, both... 

A direct approach to silacyclobutane (siletane) derivatives is the reaction of meth-ylsilenes with other unsaturated compounds. Thus, the silaethylene (silene) (326), formed from pivaloyl-tris(TMS)-silane (325) via thermolysis (10h/140°C), reacts with 1-phenylprop-l-yne to yield the silacyclobutene (silete) derivative 327 (equation 148)172. 

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