Don P. Duncan

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.

Novel two atom insertions into the silacyclopropane and silacyclopropene rings

Abstract The reactions of hexamethylsilirane and 1,1-dimethyl-2,3-bis(trimethylsilyl)-1-silirene with aromatic and α,β-unsaturated aldehydes and ketones gave insertion products having the 1-sila-2-oxacyclopentane and 1-sila-2-oxacyclopent-4-ene structures, respectively. Benzaldehyde methylimine reacted with hexamethylsilirane in similar fashion, giving 1,1,2,4,4,5,5-heptamethyl-3-phenyl-1-sila-2-azacyclopentane. Reactions of the silirane and silirene with phenylacetylene gave both ring insertion and ring opening products. 1,1-Dimethyl-3,4-diphenyl-1-silacyclopentadiene was formed in the reaction of the silirene with an excess of phenylacetylene in the presence of a catalytic quantity of bis(triphenylphosphine)palladium dichloride.

Reactions of siliranes with elemental sulfur and with t-butyl mercaptan. Preparation of the 2,3-dithia-1-silacyclopentane ring system

Abstract The reaction of 1,1-dimethyl- trans -2,3,-bis(2′,2′-dimethylcyclopropylidene)-1-silirane with S 8 gives a mixture of four isomeric products derived from incorporation of one sulfur atom and a cyclopropylcarbinyl-to-butenyl type rearrangement. This silirane reacts with t-butyl mercaptan to give a product of mercaptan addition in which a cyclopropylcarbinyl-to-butenyl rearrangement also has occurred. Hexamethylsilirane reacts with S 8 to give 1,1,4,4,5,5-hexamethyl-2,3-dithia-1-silacyclopentane in high yield. These reactions are discussed in terms of free radical mechanisms.

Reactions of silacyclopropanes with 1,2-dipolar reagents: diazomethane and methylenetrimethylphosphorane

Abstract The reaction of diazomethane with hexamethyl-, 1,1,2,2,3-pentamethyl-2-ethyl- and 1,1,2,2,3-pentamethylsilirane in diethyl ether solution resulted in (formal) CH2 insertion into the SiC2 ring to give silacyclobutanes as products. The reaction of methylenetrimethylphosphorane with hexamethylsilirane produced a new phosphorus ylide, Me2CHCMe2SiMe2 C H P + Me3.

Two atom insertions into the silacyclopropane and silacyclopropene rings: mechanistic considerations

Abstract Novel two atom insertion reactions into the SiC2 rings of hexamethylsilirane and 1,1-dimethyl-2,3-bis(trimethylsilyl)-1-silirene involving the C bonds of styrene and α-methylstyrene, 1,3-butadiene and its 2-methyl and 2,3-dimethyl derivatives, and, in the presence of ultraviolet radiation, of aliphatic aldehydes and ketones, are reported. A radical mechanism is suggested tobe operative in these reactions. Also described are novel palladium-catalyzed reactions of the silirene with terminal and internal acetylenes.

A gas kinetic study of the pyrolysis of octamethyl-1,2-disilacyclobutane

Abstract Pyrolysis of octamethyl-1,2-disilacyclobutane(I) has been shown to proceed by a unimolecular isomerisation involving a biradical which undergoes internal hydrogen-abstraction. Kinetic data were obtained for this pyrolysis and for that of the oxidation product of I, octamethyl-1,3-disila-2-oxacyclopentane(II). A few additional experiments indicated that intermediates with π-bonds to silicon may be generated by photolysis of I. The energetics of these reactions were discussed.

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.

Evidence for the generation of a disilacyclopropane in solution

Abstract 1,1,2,2-Tetramethyl-3-norcaranylidene-1,2-disilacyclopropane has been generated by the reaction of n-butyllithium with 1-chloro-2-(bromonorcaranyl)tetramethyldisilane. Its reaction with methanol results in cleavage of the Si—Si bond.

The reaction of the trimethylchlorosilane/magnesium reagent with gem- dibromocyclopropanes

Abstract The reaction of the trimethylchlorosilane/magnesium reagent with gem- dibromocyclopropanes in tetrahydrofuran medium results in the formation of silytated cyclopropanes and/or silylated ring-opened products, depending on the substituents on the cyclopropane ring.