Dennis C. Annarelli

Hexamethylsilirane : I. Preparation, characterization and thermal decomposition

Abstract Hexamethylsilirane has been prepared by the action of magnesium on dimethyl-bis(α-bromoisopropyl)silane in tetrahydrofuran (THF) solution. It was found to be highly reactive toward atmospheric oxygen and moisture and to decompose when heated in solution at 60–75°C. Its decomposition results in the extrusion of dimethylsilylene which may add to the tetramethylethylene produced in the decomposition to regenerate the silirane, insert into the reactive SiC2 ring of the silirane to give octamethyl-1,2-disilacyclobutane or oligomerize to give (Me2Si)n oils. Dimethyldiisopropyl-, tetraisopropyl- and tert-butyltriisopropylsilane were prepared by catalytic hydrogenation of the corresponding isopropenylsilanes. Bromination of dimethyldiisopropylsilane at 65°C resulted in exclusive formation of dimethyl-bis(α-bromoisopropyl)silane.

Silacyclopropenes: I. Synthesis and properties of some silacyclopropenes

Abstract The thermal decomposition of hexamethylsilirane in the presence of selected disubstituted acetylenes (Me3SiCCSiMe3), Me2HSiCCSiMe2H, Me3SiCCCH3, Me3SiCCCMe3, Me3CCCCH3) resulted in Me2Si addition to the CC bonds to give the respective silacyclopropenes. These are thermally stable but are extremely reactive toward atmospheric oxygen and moisture and react readily with methanol and ethanol at room temperature with SiC (ring) bond cleavage. The very high field 29Si NMR resonance (−87 to −106 ppm) is a characteristic feature of the silacyclopropene ring.

7,7-Dimethyl-7-siladispiro[2.0.2.1]heptane derivatives. The first silacyclopropanes☆

Abstract The preparation of the first silacyclopropanes, all derivatives of 7,7-dimethyl-7-siladispiro[2.0.2.1]heptane, by magnesium-induced ring closure reactions of 1,3-dihalides, is described. Mass spectral and 1 H, 13 C and 29 Si NMR spectral data, as well as the unusually high chemical reactivity of these compounds support the assigned silacyclopropane structures.

Dimethylsilylene transfer from hexamethylsilirane to olefins

Abstract The thermolysis of hexamethylsilirane in the presence of cis - and trans -4-octene, cyclooctene, propenyltrimethylsilane and trimethylethylethylene resulted in dimethylsilylene transfer and formation of the respective silacyclopropanes. In contrast, silacyclopentane derivatives were formed when such thermolysis was carried out in the presence of styrene and α-methylstyrene. This is believed to be a result of the interception of the intermediate diradical from hexamethylsilirane ring opening by the styrene.

The extraordinary reactivity of the ring SiC bonds in 1,1-dimethyl-trans-2,3-bis(2′,2′-dimethylcyclopropylidene)-1-silacyclopropane

The reactions of the title silirane with oxygen, water, alcohols, primary and some secondary amines, hydrogen sulfide, carboxylic acids and hydrazoic acid proceeded exothermally at room temperature with opening of the SiC bond of the SiC2 ring. Hydrogen chloride reacted rapidly at −78°C, as did methyl- and phenyl-lithium and lithium dialkylamides. Ring cleavage of the silirane also could be effected with acetyl chloride, boron trichloride and lithium aluminum hydride. A comparison of the reactivity of the title silirane with that of 1,1-dimethyl-1-silacyclobutane shows the dramatic effect of the high ring strain in the former, which results in hyper-reactive ring SiC bonds.

Hexamethylsilirane: II. Ring-opening reactions with some simple reagents

Abstract Ring-opening reactions formally of the type Me 2 SiCMe 2 CMe 2 ⋆ X-H→ Me 2 XSiCMe 2 CMe 2 H occur on reaction of hexamethylsilirane with water, alcohols, ammonia, amines, HCl and car☐ylic acids and germanium and tin hydrides. More complicated ring opening reactions are observed with halogens, tetrahalomethanes and molecular oxygen. The oxidation and protolysis (H 2 O, ROH, HCl, RCO 2 H, NH 3 , amines) reactions of hexamethylsilirane are exothermic at room temperature.

Hexamethylsilirane: IV. Nucleophillic ring opening by alkyllithium reagents

Abstract Alkyllithium reagents (methyl-, n-butyl- and t-butyl-) react at 0°C with hexamethylsilirane to give RSiMe 2 CMe 2 CMe 2 Li as the initial product. In a second step this reagent metalates methyl substituents on silicon to give RSi(CH 3 )(CH 2 Li)CMe 2 CMe 2 H as the final product.