Silyl radical with halides

Table 4.5 Rate constants (M s ) at 27 °C for the reaction of phenyl substituted silyl radicals with a variety of halides [98,99]...

Scheme 4.8 shows that the CH2 of cyclohexadiene moiety acts as the H donor with formation of cyclohexadienyl radical as the intermediate, which rapidly ejects the silyl radical upon re-aromatization. The silyl radical is able to propagate the chain by reaction with a starting halide. The hydrogen donation of silylated cyclohexadienes toward primary alkyl radicals is reported to be 1 X 10 M s at 70 °C [120], which is in accord with the reported range of 10 -10" s at room temperature for the reaction of primary and second-... 

In the case of halocarbon matrices, the SOMO is of the a type and the cleavage is facilitated by the antibonding nature of the SOMO. In other ions, a or a orbitals of scissile bonds interact with a ti or 7i orbital, causing them to be weakened. Accordingly, radical anions of benzyl halides may generate benzylic radicals with loss of a halide ion. Conversely, benzylsilane radical cations may form benzylic radicals with loss of a silyl cation (Fig. 6.15). 

The radical-initiated allylation of alkyl halides with allyltris(trimethylsilyl)silanes proceeds via an SH2 process mediated by a tris(trimethylsilyl)silyl radical.225 The radical-allylating agents react with alkenes, alkynes, and aldehydes via a radical chain process to give the corresponding allylsilylation products.226... 

As stannyl or silyl radicals have high rates of halogen abstraction, alkyl and aryl halides are the most common sources of radicals [39]. The reactivity order is R-I>R-Br>R-Cl, with R being either alkyl or aryl. Radical generation by chlorine abstraction occurs only in activated alkyl chlorides, such as a-chloroketones (or esters), and polyhalogenated carbons, whereas vinyl and aryl chlorides tend to be nonreactive to organostannyl radicals. 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore sa-ve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides . Thus, thomolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

Our success in synthesizing silyl ketals containing an aryl halide with (+)-ethyl lactate led us to explore the intramolecular radical translocation reaction (Scheme 29). The term radical translocation is described by Robertson et al. as the intramolecular abstraction of an atom (usually hydrogen) or group by a radical center this results in a repositioning of the site of the unpaired electron which can lead to functionalization at positions normally unreactive towards external reagents or whose selective modification is difficult In the most common cases the abstraction occurs at a site that is five atoms away from the radical 1,6 atom abstraction are less common, and l,n-abstractions where n > 6 are rare. This is because the shortest chain length that can accommodate the trajectory for atom abstraction contains six atoms, as in the case of the 1,5 atom abstraction. Entropic factors usually result in the failure of the process in the cases where n > 6 atoms. 

Enol silyl ethers can lead to a-chloro ketones on treatment with anhydrous copper(II) chloride in DMF or iron(lll) chloride in acetonitrile (equation 13, Table 1). The chlorination of (36 equation 14) proceeds through a cation radical intermediate formed by an electron-transfer process with metal halides. 

According to Bard and Merz [108], in MeCN containing TBAP, allyl bromide and allyl iodide interact chemically with a mercury electrode to form allylmercury halides. These allylmercury halides undergo reduction to yield diallylmercury, which is itself electroactive. Allyl bromide and allyl iodide are reduced at platinum in MeCN in a two-electron process to give the allyl anion, and the allyl radical is not involved as an intermediate. Reduction of allyl halides at platinum in DMF containing TEAOTs and in the presence of trimethylchlorosilane results in silylated compounds [100]. 

Chatgilialoglu and Curran have found that allyltris(triniethylsilyl)silanes react with a variety of alkyl halides to provide allylation products via an Sh2 process mediated by the tris(trimethylsilyl)silyl (TTMSS) radical (Scheme 10.199) [523]. In this system the allylsilanes work as radical-allylating agents and TTMSS radical sources. We have used the reactivity of allyltris(trimefhylsilyl)silanes for allylsilyla-tion of alkenes and alkynes via a radical chain mechanism (Scheme 10.199) [524]. 

The electrophilic PhCOCH2 radical generated by photolysis of PhCOCH2HgCl in DMSO adds readily to enamine 10b to form the substituted enamine 18, which upon hydrolysis gives the 1,4-diketone 19 in 60% overall yield. In this free radical chain reaction (Scheme 2), the electron transfer from the easily oxidizable adduct radical 17 to PhCOCH2HgCl is facilitated by the fact that the irreversible half-wave reduction potentials of alkylmercury halides are typically more positive than — 0.6 Silyl derivative 20 of an reacts with morpholine enamine 2 in the presence of 2.5 equivalents of manganese(III) 2-pyridinecarboxylate [Mn(pic)3] to give... 

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