Silyl groups 1,2-migration

In the well-known Brook rearrangment, silyl groups migrate from oxygen to carbon, but the following example is less obvious and not necessarily predictable ... 

The use of boron trifluoride-diethyl ether complex as the Lewis acid in these reactions promotes silyl group migration and gives rise to the formation of tetrahydrofurans with excellent stereoselectivity82. 

Similar results were obtained by Ogino and co-workers. After the initial cleavage of the CO from the metal, a silyl group migration occurs. It was shown by cross experiments that in this case the silyl substituent migrates and not the silylene. In a subsequent step, the silylene is then displaced by CO. Unfortunately, no trapping experiments of the silylenes in solution have been reported. 

The formation of the (SiN)2 ring can be explained by a nucleophilic 1,3-rearrangement of a methanide ion at silicon. The cyclic ylide is formed because the two nitrogen atoms are identically substituted and a silyl group migration would be without energy profit in such a molecule. 

Figure 2 Plausible mechanism of the silyl group migration in the reaction of silylborane with carbenoid.

Treatment of chiral (E)-crotylsilanes 47 with 43a in the presence of BF3-OEt2 gives tetrahydrofuran derivatives in good yield with 96% de. Interestingly, 1,2-silyl group migration competes favorably with elimination of the silyl group after condensation with 43a (equation 30)81. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

No examples were found in the literature of the reaction of the parent vinylfuran with azo compounds. Only the 2-(l-trimethylsilyloxy) derivative 112 is reported to react with diethyl azodicarboxylate (DEAZD) to give the open-chain adduct 238, presumably via a zwitterionic intermediate 239, followed by a silyl-group migration to the anionic center [83H1933 84JCR(S)218]. 

On prolonged treatment with rerr-butylchlorodimethylsilane and imidazole in DMF, silyl group migrations between vicinal cis [424, 428, 429] and even trans [430]... 

Silyl groups migrate to more negatively charged atoms or anions. The isomerization is electrophilic and follows an Sat2 mechanism. 

Acrylates polymerize two orders of magnitude faster than methacrylates by anion catalyzed GTP however, the polymerization dies at about 10,000 MW. During the anion catalyzed polymerization of acrylates the silyl ketene acetal end groups migrate to internal positions. These ketene acetals are too hindered to act as initiators for branch formation [9]. 

Reaction 5 is accompanied by intermolecular (6) and intramolecular (7) silyl group migration. 

When silylcarbinols are treated with catalytic amounts of base or active metal, the silyl group migrates from carbon to oxygen (Equations Si 1.2 and Si 1.3). Detailed studies of this reaction have been carried out by A.G. Brook whose name is now associated with the reaction. 

The stereochemical course of solvolysis of a diastereomeric /3-bromo-silane, which was found to be trans elimination, as well as the observation of 1,2-silyl group migration in a process of substitution at the 0 carbon 143) are evidence of anchimeric assistance of the silyl group in ionization of the bond to the /3 carbon. It was further noticed 144) that after quenching the solvolysis reaction of /3-bromo-/3-dideuterioethylsilane the substrate recovered is a mixture of a and /3 deuterated isomers [Eq. (33)]. These... 

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