The flash vacuum pyrolysis (FVP) of ortho-substituted trimethylsilyl(phenyl)acetylenes resulted in the formation of indene and naphthalene derivatives not possessing silicon. The loss of silicon was proposed to occur after cyclization of the starting material;Flash vacuum pyrolysis of acetylenes with a dimethylsilyl substituent beta to the acetylene group provided an unprecedented and convenient method of silacycle formation. Utilizing this new method of silacycle formation, 1,1-dimethyl-1-silaindene and 3,3-dimethyl-4-oxa-3-silacyclopentene were formed in 84% and 32% yield, respectively. Deuterium labeling studies revealed pathways competitive with alkylidenecarbene formation and trapping which led to the products;Alkylidenecarbene cyclization was also applied to the synthesis of furan and pyran derivatives. The FVP of o-(trimethylsilylethynyl)-phenol afforded the anticipated 3-trimethylsilylbenzofuran, but competitive and secondary processes led to the formation of two additional isomers. FVP of 2-trimethylsilyl-4,5-dihydrofuran resulted in an unprecedented gas-phase reductive elimination to form an alkylidenecarbene. The attempted synthesis of benzopyran derivatives by alkylidenecarbene cyclization failed to yield the desired pyrans. The products obtained were the result of the extrusion of carbon monoxide or dimethylsilylene from the starting material;The FVP dimethyl(propargyl)silane led to the formation of 1,1-dimethyl-1-silacyclobutene during the interconversion of dimethyl-(propargyl)silane to allenyltrimethylsilane. Mechanisms involving 1,2-shifts were proposed which account for the products;The FVP of 2-dimethylsilyl-4,5-dihydrofuran led, in part, to the formation of dimethylsilylketene which apparently decomposed to acetytlene and the elements of dimethylsilanone. This unprecedented decomposition was subsequently verified in the pyrolysis of dimethylsilyl(trimethylsilyl)ketene which produced trimethylsilylacetylene in 76% yield.