Keywords: Cyclopropenes / Reactive intermediates / Carbenes / Unimolecular reactions / Silyl group effects / Quantum chemical calculationsSeveral tetrasubstituted cyclopropenes have been prepared and their pyrolyses and photolyses have been investigated. Tetrakis(trimethylsilyl)cyclopropene (10), which was obtained in 25% yield from tris(trimethylsilyl)cyclopropenylium hexachloroantimonate (9), gave tetrakis(trimethylsilyl)allene (12) as the sole product both thermally and photochemically. Kinetic studies in [D 8 ]toluene indicated first-order behavior with Arrhenius parameters log(A/s −1 ) = 11.75 ± 1.20 and E a = (37.5 ± 2.5) kcal mol −1 . All three new 3-alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes (17a−c, with C 1 -, C 2 -, and C 3 -alkenyl groups as tethers, respectively) gave allenes upon irradiation, but thermally only two (17a, 17c) gave allenes, whilst 17b yielded a bicyclo[4.1.0]hept-3-ene derivative 22 as a result of an intramolecular ene reaction. Photolyses of two further cyclopropenes (33a,b) bearing 1,2-bis(alkenyldimethylsilyl) substituents also gave the corresponding allenes as the sole products. For none of these tethered cyclopropenes was a product found that could have originated from intramolecular trapping of a cyclopropylidene intermediate. Quantum mechanical (ab initio) calculations have been carried out on the silyl-substituted cyclopropene model compounds 3,3-dimethyl-1-silyl-(36a), 3,3-dimethyl-1,2-disilyl-(37), and tetra-

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