1,2-Silyl migrations radical

The rearrangement 12 13 is not straightforward and two alternatives routes have been suggested. One of them involves a 1,3 transfer from silicon to oxygen to give the silyl radical 15 followed by Sui reaction on the peroxide moiety [15]. The other indicated by PM3 calculations is a dioxirane-like three-membered intermediate 16 having a pentacoordinated central silicon followed by a 1,2 silyl migration to afford radical 13 [19]. 

A number of disilanes have been investigated for their suitability as potential silene precursors. The formation of silenes by the 1,3-silyl migration pathway competes with dehydrosilylation, which also gives silenes, and with homolytic Si—Si bond cleavage to give silyl radicals. 

In addition to the radical Brook rearrangement from a-silylalkoxy radical 45 to the siloxyalkyl radical 46 shown in equation 37, several 1,2-silyl migrations to a radical... 

The authors used (5)-carvotanacetone (dihydrocarvone) as starting material (Scheme 34). To prepare the linearly conjugated sUylenol ether, they used the Kharash protocol and attained y-alkylation by Mukaiyama aldol reaction with trimethylorthoformate (195). The ketoacetal 295 was a-hydroxylated according to Rubottom by silylenol ether formation followed by epoxidation and silyl migration. Acid treatment transformed 296 to the epimeric cyclic acetals 297 and 298. endo-Aceta 297 was equilibrated thereby increasing the amount of exo-acetal 298. The necessary unsaturated side chain for the prospected radical cyclization was introduced by 1,4-addition of a (trimethylsilyl)butynylcopper compound. 

Under slightly different conditions both disilene extrusion and rearrangement were observed. Evidence was presented that the rearrangement occurred as the result of radical cleavage of a carbon-silicon bond, followed by a 1,2-silyl migration in the resulting allylsilane system87 (equation 54). 

Intramolecular radical cyclizations of acylsilanes containing the methyldiphenylsilyl moiety were investigated by the Tsai group (eq 29). As indicated in eq 29 products of radical addition to the carbonyl group involve silyl migration, whereas attack at the alkene center gives simple adducts. 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

We suggest that the ejected thiyl radical undergoes a fast 1,2-migration of silyl group from silicon to sulfur (Reaction 85), affording a new silyl radical that either reacts with (TMSlsSiH (Reaction 86) which completes the reaction cycle, or replaces the (TMSlsSi radical in the above described reaction sequence. 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

With the functional groups at different silicon atoms as in 247 only the migration to the vinyl group occurs. A competitive reaction is the cleavage to yield the silyl radicals 248 and 249. 

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