Silatropic shift

EXSY NMR spectroscopy to undergo 1,5-shifts. Silatropic rearrangements of tetrahydro-4,4,8,8-Me4-4,8-disila-5ym-indacene have been established using... 

The preparation of cyclopentadienes with up to four trimethylsilyl groups can be performed easily on a large scale starting with monomeric cyclopentadiene by repeated metalation with n-butyllithium and treating the resulting anion with chlorotrimethylsilane [84], Any complication caused by formation of regioisomers does not occur, since all trimethylsilyl-substituted cyclopentadienes are fluxional by virtue of proto- and silatropic shifts [85], Upon deprotonation with n-butyllithium the thermodynamically most favorable anion is formed selectively (Eqs. 20, 21). Thus, metalation of bis(trimethylsilyl)cyclopentadiene 74, which exists preferentially as the 5,5-isomer, selectively affords the 1,3-substituted anion 75. Similarly, tris(trimethylsilyl)cyclopentadiene 76, which is found to be mainly as the 2,5,5-isomer, affords the 1,2,4-substituted anion 77. 

SCHEME 12. Silatropic shifts and deprotonation/silylation reactions in silylated cyclopentadienes... 

Polymer-supported azomethine ylids generated from a-silylimines through a 1,2-silatropic shift, are shown to be versatile reagents suitable for the synthesis of libraries of pyrrolidine derivatives after 1,3-dipolar cycloaddition with a series of dipolarophiles. Effectively substituents R1, R2 and dipolarophiles A=B and A=B can be chosen to get the desired adduct.146... 

The oxidation of furans has often been used to express the latent functionality present within this heterocyclic framework, and it results particularly attractive for the synthesis of the widespread butenolides [8c,56]. Dye-sensitized photo-oxygenation of furfural, furoic acid and a- or a,a -unsubstituted furans leads directly to butenolides [8c,56,60d,63]. The presence of a silyl group in a-position enhances the rate of endoperoxide formation and controls the regiochemical outcome of the second-step leading to the related butenolide via a rapid intramolecular silatropic shift [64]. An application of this reaction is shown in Sch. 39 as a step in the synthesis of spongianolide A, an antitumoral natural sestertepenoid [65],... 

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. 

CMe-SiFe(CO)induces a 1,3-silatropic shift to the acyl oxygen 229 ... 

A 1,2-silatropic shift was a key step when generating resin-bound azomethine ylides from a-silylimines (Scheme 11.8). A 1,3-dipole was captured with maleimide, and a polymer-bound pyrrolidine was obtained. Acidic or alkylative cleavage of the N-Si bond released A-unsubstituted or A -substituted pyrrolidines, respectively, from the resin in a traceless manner. 

Scheme 11.8. Generation of an azomethine ylide via a 1,2-silatropic shift.

In addition, pyrazoles have been synthesized in a traceless manner from resin-bound azomethine imines that were generated via a 1,4-silatropic shift (Scheme 11.18). 

Scheme 11.18. Traceless synthesis of pyrazoles via a 1,4-silatropic shift.

M. Komatsu, H. Okada, T. AkaM, Y. Oderaotoshi, S. Minakata, Generation and cycloaddition of polymer-supported azomethine yhde via a 1,2-silatropic shift of a-silylimines traceless synthesis of pyrrolidine derivatives. Org. Lett. 2002, 4, 3505-3508. 

The thermal [l,4]-silatropic shift of the silyl group on to the oxygen atom of the nitroso group of Q -silylnitrosamines (171) to give an azomethine imine intermediate (172) has been reported. These intermediates have been shown to undergo 1,3-dipolar cycloadditions with dimethyl acetylenedicarboxylate to afford pyrazoles (173) see Scheme 42. 

Cobalt - Variable temperature NMR spectroscopy has shown restricted phenyl rotation in [(tetraarylporphyrin)CoCH=CH(tetraarylporphyrinH2)]. [(riCindenyl)SiMe2(C = CSiMe3)] and its Co2(CO)6 complex have been shown by EXSY to undergo 1,5-silatropic shifts. ... 

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