Kinetic Studies on the Thermal Decomposition oftrans-2,3-Dimethyl-1-tri-tert-butylsilyl-1-triisopropylsilylsilirane and the Mechanism of Silylene Generation

Abstract

trans-2,3-Dimethyl-1-tri-tert-butylsilyl-1-triisopropylsilylsilirane (1) has been prepared in 98% yield by the treatment of ( t Bu 3 Si)( i Pr 3 Si)SiBr 2 with activated magnesium in the presence of trans-2-butene. Trapping studies suggested that silylene ( t Bu 3 Si)( i Pr 3 Si)Si: (2) was extruded by thermolysis of 1. Without added trapping agents self-trapping of silylene 2 formed a stable product, 1,1-di-tert-butyl-4,4-dimethyl-2-triisopropylsilyl-1,2-disiletane (5), by intramolecular C-H insertion. Thermolysis of 1 presented an ideal reaction for a kinetic study of the thermal decomposition of a silirane. It was found that thermal decomposition of 1 is stereospecific and first-order in benzene solution. From the rates of reaction at various temperatures activation parameters were obtained. The results were consistent with a nonleast motion pathway for silylene retroaddition and hence addition.The thermally induced extrusion of silylenes from siliranes was discovered by Seyferth in 1975 1 and has often been used for thermal generation of silylenes ever since. 2 Both a concerted mechanism for silylene extrusion, the reverse of single-step singlet state silylene addition to a CdC bond, and a stepwise process proceeding via a diradical formed by Si-C homolysis were initially considered for the pyrolysis of siliranes. 1,3 Seyferth et al. found, in 1982, that transfer of Me 2 Si from hexamethylsilirane to olefins generating new siliranes was stereospecific. 4 Boudjouk et al. found stereospecific extrusion of di-tert-butylsilylene from diastereomeric siliranes. 5 In 1991, experimental results from the Gaspar group showed that the decomposition of cis-and trans-2,3-dimethyl-1,1-diadamantylsilirane to the corresponding 2-butene was stereospecific, as was the transfer to diastereomeric olefins of the diadamantylsilylene unit Ad 2 Si. 6 Stereospecific transfer of silylene units upon photolysis of their non-silirane precursors had been established by Jones et al. 7,8 and by Kumada, Ishikawa et al. 9 for Me 2 Si, 7,8 Ph 2 Si, 8 and PhMeSi. 9 The stereospecificity of silylene transfer from siliranes and to olefins was interpreted in terms of a concerted mechanism: single-step thermal extrusion of free silylenes as the inverse of single-step addition of free silylenes to olefins.Despite initial doubts regarding the formation of free silylenes upon the pyrolysis of siliranes and photolysis of oligosilanes, 10 clear evidence for the intermediacy of free silylenes has accumulated over the years. 2 This can be summarized as being comprised of the clear indication of a common reactive intermediate from quite different plausible precursors of the same silylene and the direct detection of several silylenes by electronic spectroscopy upon the photolysis of a variety of oligosilanes. 2,8 The case for concerted addition and retroaddition reactions of silylenes is more tenuous. Scheme 1 compares concerted and stepwise silylene addition and retroaddition mechanisms.The general acceptance of the concerted addition (k add )-retro...

Substituent Effects on the Reactions of Diarylgermylenes and Tetraaryldigermenes with Acetic Acid and Other Lewis Bases in Hydrocarbon Solution

“…For example, they all undergo facile insertion into the O−H bonds of alcohols and carboxylic acids, form Lewis acid−base complexes (“ylides”) with ethers, amines, and other Lewis bases, and undergo (2+1)-cycloaddition reactions with alkynes, alkenes, and dienes. ,, It is generally recognized that one of the main variables throughout the series, all else being equal, is a systematic variation in overall reaction thermodynamics, with reaction exothermicities decreasing down the group. − One particularly good example of this is the (2+1)-cycloaddition with alkenes. While cyclopropanes are a common structural motif in organic chemistry, most of the known silacyclopropane derivatives enjoy only marginal stability, and only two germacyclopropane derivatives have ever been reported, to our knowledge; (2+1)-cycloreversion to generate an alkene and the corresponding metallylene is the prevalent mode of decomposition of sila- and germacyclopropane derivatives in the absence of secondary trapping reactions. ,− …”

iPr2N–P═Fe(CO)4 in Olefinic Solvents: A Reservoir of a Transient Phosphinidene Complex Capable of Substrate Hopping

“…Exemplary are the established low-valent carbenes (NHCs, CAACs), their heavier homologues (tetrylenes: Si, Ge, Sn, Pb), and the fast growing class of FLPs that activate dihydrogen and small unsaturated molecules . Reversible cycloadditions are known for silylenes and alkenes and even ethylene when using A or B , not yet for germylenes, but also stannylene C is in equilibrium with a stannacyclopropene. , Recently, even the stable carbene D was reported to give reversible cycloadditions with retroaddition requiring elevated temperatures . Such dynamic covalent chemistry under mild conditions has not yet been reported for the diagonally related phosphinidenes. , …”

Synthesis of vinyl germylenes

“…As the reaction of free silylenes and germylenes with alkynes was found to proceed to silirenes [21][22][23] and germirenes [24][25][26] we were interested to check for similar reactivity of base adducts 1a and 1b.…”