Silane-initiated cascade cyclization

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

Rhodium-catalyzed, silane-initiated cascade cyclization of 1,6,11-triynes 83 was proposed to occur via a silane-initiated cascade carbocyclization to form the silylated bicyclic triene (Z,Z)-In. / -Migratory insertion of the silylated G=G bond into the Rh-G bond of (Z,Z)-In followed by / -hydride elimination from frans-lln could then form 84a. Alternatively, cisitrans isomerization of (Z,Z)-In followed by / -migratory insertion of the silylated G=G bond into the Rh-G bond of resulting isomer ( ,Z)-In could form cis-Wn. Subsequent / -silyl elimination from m-IIn would form unsilylated tricycle 84b (Scheme 21). 

Dodecacarbonyltriiron, characteristics, 6, 260 l-Dodecene-1,11-diynes, silane-initiated cascade cyclization, 11, 400-401... 

Generation of 3-indolylacyl radicals from the selenoesters 149, using either /j-Bu3SnH or tris(trimethylsilyl)silane (TTMSS) followed by reaction with various alkenes, offers a route to 3-acylindoles 150. On the other hand, the use of n-Bu Sn2 under irradiation gave cyclopent[6]indole derivatives such as 151 via a cascade involving initial addition of the acyl radical to the alkene, and a subsequent oxidative cyclization at the indole C-2 <02JOC6268>. 

Alkenyl epoxides have been analogously used in the synthesis of bicyclic compounds [69]. Addition of a thiyl radical to a suitably protected epoxyketone enolate generated the oxiranyl carbinyl radical which initiated a cascade of uni-molecular steps (i.e. ring-opening, 1,5-radical translocation, 5-exo cyclization, and ejection of the PhS radical) to produce the final enol product in good yield and high diastereoselectivity (equation (31)). The PhS radical-induced fragmentation of alkenyl epoxy silanes has been found to afford a-trimethylsilyl aldehydes [70]. 

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