1.3- Silyl migrations thermal

Although silyl migrations are usually acid- or base-catalyzed, they have been observed to occur thermally. 

Silyl migration in chloromethylpentamethyldisilane 33 X=Y=Si has been known to occur thermally or more easily in the presence of aluminum chloride70. Similar rearrangements promoted by a Lewis acid were observed in (chloromethyl)pentamethylgermasilanes71. Presumably, the reaction does not involve a carbenium or a silicenium cation as a discrete reactive intermediate but proceeds via the cyclic transition state 34 involving aluminum chloride participation (equation 22). 

Photochemical 1,2-silyl migration from l-tris(trialkylsilyl)cyclotrisilene 72 occurred to give the corresponding bicyclo[1.1.0]tetrasilane 70 (equation 54)132. During the photochemical and thermal interconversion among the SLiRg isomers 69, 70 and 72, the corresponding 1,3-tetrasiladiene derivative was never detected. 

Sekiguchi and coworkers observed the thermal and photochemical 1,2-silyl migrations in persilyl-l-disilagermirene 73 (equation 55)133. 

This type of thermal 1,3-silyl migration is mechanistically very interesting, because two typical intramolecular 1,3-silyl migrations in allylsilanes 190 (equation 118) and /3-ketosilanes 191 (equation 119) were shown to occur concertedly but with different stereochemistry at the silicon center1. 

A number of anionic 1,4-silyl migrations between N and N were documented in the review of BB1. Mack and coworkers found a similar N to N migration during the thermal decomposition of an /V.iV-dilithium salt of /V,/V-bis(tri methyl si ly I )ethylenedi amine 297 (equation 184)445. 

Thermal or photochemical activation of the complexes results in the formation of siloxycarbenes 13 via diradicals 12. Carbenes 13 lie in a very shallow minimum and are stabilized by a [1.2]H shift, if an a-hydrogen atom is available, or by [l,2]silyl migration Vinyloxysilanol 8 or formylsilanol 11 are the final oxydation products in argon matrices. 

Komatsu et al. have developed unique methods for the generation of 1,3-dipoles from organosilanes (Scheme 10.222). Linder thermal conditions, N-(a-silylbenzyl) imines and -amides are converted, via 1,2- or 1,4-silatropic shift of the silyl group, into azomefhine ylides (153 from the amide) which react with dipolarophiles [578]. Similar thermal 1,4-silyl migrations of a-silylnitrosamines and S-a-silylben-zyl thioesters provide convenient routes to azomethine imines 154 [579] and fhio-carbonyl ylides 155 [580], respectively. 

Reactions Involving A-Bis(trimethyl ilyl)methyl Imine Derivatives. Bis(trimethylsilyl)methylamine condenses with aldehydes and ketones to form stable imines, which are useful intermediates in a variety of synthetic transformations. Thus, various A-bis(trimethylsilyl)methyl-l-aza-l,3-dienes underwent thermally mediated, frans-stereoselective C->N silyl migration to furnish l,2-bis(trimethylsilyl)-2,3-dihydropyrroles in good yields (eq 3). These products can be A-desilylated to afford 2-trimethyl-silyl-3,4-dihydropyrroles (68-93%), which can be reduced with lithium aluminum hydride to afford pyrrolidine derivatives. 

Under thermal conditions, the silyl group of conjugated SMA imines undergoes 1,2-migration from C to N with concomitant cyclization when the structure is favorable. 

The silyl group in an SMA derivative is able to migrate from C to N under thermal conditions (see Section VI.B. 4). This is the case with SMA imines, and such rearrangement leads directly to the corresponding azomethine ylid which may be intercepted by a dipolarophile.402... 

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