Allylsilanes cationic cyclization

Similarly, at a carbon anode in 1 1 MeOH-THF, anodic cyclization of allylsilane enol ether (XCIX) proceeded stereoselectively to give (C) [Eq. (63)]. The use of allyl silanes as the unsaturated nucleophilic component in such radical-cation cyclizations proved to be beneficial, though the exact mechanistic details remain somewhat speculative [147]. The method represents an improvement over earlier methods involving anodic cyclization of alkenyl-substituted enol acetates [148]. 

Murai et al. showed that the cycloisomerization of enynes catalyzed by PtCl2 has several feasible pathways (1) to 1,3-dienes via a formal metathesis, (2) to a 1,4-diene if the enyne substrates contains an allylsilane or stannane, (3) to a homo-allylic ether if it the reaction is performed in an alcoholic medium, or (4) to bicycle[4.1.0]heptene derivatives (Scheme 4) [26]. Further studies conducted by other groups have indicated the cyclization might proceed via a cationic mechanism triggered by coordination of Pt(II) with the alkyne moiety [27, 28]. Very recently, Oi and coworkers also observed a formal metathesis reaction mediated by a cationic Pt complex [29]. 

The electron-transfer initiated photoaddition of allylsilanes to iminium salts has been examined in detail. Irradiation of l-methyl-2-phenylpyrrolinium perchlorate (177) and the allylsilanes (178) in methanol, for example, gave the 2-allylpyrrolidines (179) " the proposed pathway is outlined in Scheme 11. Photoaddition is initiated by electron transfer and driven to completion by desilylation of the allylsilane-derived cation radical. Analogous conversions have been reported in the allyl iminium perchlorates (180), " and cyclization to the spiro-compounds (181) occurs on irradiation of the /3-enaminone-derived allyliminium salts (182). " A photocyclization of this type has been... 

A related version of this cyclization is the reaction illustrated in Eqs (170) and (171) [428], in which methylenecyclopropane was cleaved with TiCU to give the similar allyl cationic intermediate which adds to the double bond of allylsilane to give the cyclopentane framework. 

Scheme 59. Cation radical cyclizations using allylsilanes as neutral components.

Allylsilanes are good acceptors of 1,3-dipolar compounds such as nitrones [503] and oxyallyl cations [504]. West et al. have used allylsilanes to trap oxyallyl cations generated during the Nazarov cyclization of l,4-dien-3-ones (Scheme 10.188). The tandem bicyclization provides bicyclo[2.2.1]heptanes with high diastereoselectivity. 

In addition to their use in Mannich (and variant) reactions, iminium ions are useful for other cationic type cyclizations. Corey employed a novel tandem iminium ion cyclization as part of an elegant cascade used for the synthesis of aspidophytine. The reaction of tryptamine 292 and dialdehyde 293 in CH3CN at ambient temperature afforded the pentacyclic skeleton of the alkaloid (296 Scheme 54) (99JA6771). Condensation of the free amino functionality of 292 with the dialdehyde produced a dihydropyridinium intermediate 294 that then cyclized onto the indole n-bond to give 295. The iminium ion so produced underwent a second cyclization with the tethered allylsilane moiety to give 296. Protonation of the enamine in 296 provided still another iminium ion (297) that was then reduced with NaCNBH3 to furnish 298 in 66% yield. All of the above reactions could be made to occur in a single pot. 

The occurrence of a fast cationic aza-Cope reairangement has been proven in the case of vinylsilane cyclizations (equation 109). Enantiomerically pure ethoxycarbamate (148) cyclizes under the influence of BF3-Et20 to a I I mixture of racemic tetrahydropyridines (149) and (150). Thus, the equilibration of iminium ions (151) and (152), attended with racemization (equation 110), occurs much faster than ir-cy-clization. Because allylsilanes are more nucleophilic than vinylsilanes, ir-cyclization probably takes place from (152). The fast rate of cyclization of allylsilanes is apparent from the cyclization via (140), where cationic aza-Cope rearrangement cannot compete (equation 105). 

In situ-generated iminium cations undergo rapid cyclization with on-ring allylsilanes, as shown in Scheme 3.39 (Mannich cyclization reaction). The substrates are prepared from the allyl vinyl dihromide 189 via substitution of sihcon and subsequent introduction of the amino appendage (190 — 192) [62]. 

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