Allylsilanes radical

In 1982, Ohga and Mariano first reported the photoallylation of iminium salts. The photoreaction of iminium salts with allyl-trimethylsilane in acetonitrile affords allylated compounds via the addition of an allyl radical to an a-aminomethyl radical. Allyl radicals are produced via the cleavage of C-Si bonds of allylsilane radical cations and the a-aminomethyl radicals are generated by one-electron reduction of iminium salts as shown in Scheme 4. 

Bock et al. studied the cation radicals of various organosilanes having a -system in the / position extensively and found that the charge is delocalized considerably into the silyl groups [9,10]. Kira and Sakurai et al. also carried out an ESR study of cation radicals of allylsilanes and revealed the large polarization of the SOMO (singly occupied molecular orbital) as a consequence of the a-% interaction [11],... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Closely related reactions have been accomplished by photoelectron-transfer reactions of allylsilanes and benzylsilanes, and a similar mechanism involving the cation radical intermediate is suggested [29]. Chemical oxidation of allylsilanes [27] and ferrocenylsilanes [30] also cleaves the C-Si bond and mechanism of these reactions seem to closely relate to that of the electrochemical process. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Radical allylations with allylsilanes 71 occur under mild conditions in good to excellent yields, provided that the radical precursor and the silane have the appropriate electronic pairing [85]. The two examples in Reactions (7.75) and (7.76) show the reactivity matching of the allylating agent with the radical. These reactions offer tin-free alternatives for the transformations that are currently carried out by allyl stannanes. 

In addition to allylsilanes, CM can also be applied to allylstannanes, which serve as valuable reagents for nucleophilic additions and radical reactions.To date, only eatalyst 1 has been shown to demonstrate CM reactivity in the preparation of 1,2-disubstituted allylstannanes, as ruthenium catalysts were found to be inactive in the presence of this substrate class.Poor stereoselectivities were generally observed, with the exeeption of one instance of >20 1 Z-selectivity in the reaction of allyltributylstannane with an acetyl-protected allyl gluco-side. 

A three-component radical coupling reaction involving allylsilanes has been employed leading to 145 (equation 121)213. It is noteworthy that the corresponding carbon-silicon bond remains intact in this reaction. 

Annelation.1 This reagent is used for conversion of 3-substituted aldehydes (or acetals) into methylenecyclohexanes. The conversion involves addition of the allylsilane group to the aldehyde or acetal to provide an adduct that undergoes radical cyclization via the allylic sulfide group. The phenylthio group is used to enhance 6-endo cyclization over the usual 5-exo cyclization. In addition it allows use of bis(tributyltin) as the initiator (14,173-174). Unfortunately the radical cyclization shows only slight stereoselectivity. 

In photochemical reactions, some examples have been known of fragmentation of cation radicals of silyl compounds into carbon radicals. Mizuno and colleagues reported a photochemical addition reaction of allylsilane to dicyanophenylethene. Photochemical electron transfer between excited phenanthrene E50 and dicyano-... 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

Radical allylation of bromides derived from p-substituted, a,p-unsaturated /V-oxazohdinones with several allylsilanes have been carried out with 1 equiv of Lewis acid and ligand in dichlorometh-ane initiated by Et3B at —78 °C. The use of the (5,5)-/-Bu-box ligand in combination with Mgl2 proceeds with good selectivity and yield (eq 21). 

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... 

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

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]. 

The preparative electrochemical oxidation of allylsilanes proceeds smoothly and the C-Si bond is cleaved selectively without affecting other allylic C-H bonds [110-113]. This selectivity is ascribed to the selective cleavage of the C-Si bond in the cation radical intermediate. The resulting allyl radical intermediate is further oxidized to give the allyl cation intermediate, which is trapped by nucleophiles such as alcohols, water, carbamates, and tosylamides to give the corresponding allylic substitution products as shown in Eq. (25). Usually, the nucleophiles are introduced to both ends of the allyl cation, and therefore a mixture of two regioisomeric products is formed. 

Diastereoselective radical allylations have been studied in many different contexts, and a plethora of information exists regarding stereocontrol in these reactions. Allylations have been performed using the traditional trapping and )9-elimination sequence occurring typically with allylstannanes as well as a stepwise atom transfer/ elimination sequence found to occur with allylsilanes. Stereochemistry is commonly controlled through the use of chiral auxiliaries or by 1,2-induction, and functionalized anh -aldol and amino acid products are available using this established methodology. 

In the presence of Mn(OAc)3- 2ll,() in AcOH, allyltrimethylsilane reacts with 1,3-diketones and /I-ketoesters to give trisubstituted dihydrofurans in good to high yield (Scheme 10.187) [502], Mn(OAc) j serves as an oxidizing agent to generate an electrophilic carbon radical intermediate from the substrate. It is probable that annulation of the carbon radical with the allylsilane followed by oxidation with another equivalent of Mn(III) forms the silicon-containing cyclic products. 

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