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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...

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The role of macrophytes in the biogeochemical cycle of mercury (Hg) in the Florida Everglades is poorly understood. Stable isotope tracer techniques were employed to investigate Hg uptake by sawgrass ( Cladium jamaicense) from soil and atmospheric pathways and the fate of Hg after absorption. Our results suggest that soil spiked Hg was rapidly taken up by roots and transported to aboveground parts. The spiked Hg that was transported to the aboveground parts was trapped; no release of the spikedHg from the leaf to the air was detected. Atmospheric Hg exposure experiments revealed that the majority of the previously deposited Hg taken into the leaf was fixed, with a very limited proportion (1.6%) available for re-emission to the atmosphere. The percentage of Hg fixed in the leaf will help reduce the model uncertainty in estimating the Hg exchange over the air-vegetation surface. We propose that sawgrass needs to be viewed as an important sink for atmospheric Hg in the regional Hg mass balance; this would have important implications for the critical loads of Hg to the Everglades. The multi-isotope tracer technique could be an effective tool to identify the role of plants in biogeochemical cycling of Hg in other ecosystems.