Radical chain reactions dehalogenation

Silylated 1,4-cyclohexadienes were introduced as reducing agents in radical chain reactions such as dehalogenation, deoxygenation via thionocarbonate ester and deselenization [120]. Two examples are given in Reactions (4.75)... 

Reagent for Polarity Reversal Catalysis in Radical Reactions. Polarity reversal catalysis (PRC) has been established by Roberts in free-radical chemistry as an efficient alternative to the use of stannanes (e.g., tri-/>butylstannane) and their associated toxicity and purification problems. Silyl radicals can be a valid alternative to tin radicals for one of the most common radical reactions, that is, radical dehalogenation, but silanes, contrary to stannanes, cannot sustain an effective radical chain reaction, due to the stronger Si-H bond. 

D. H. R. Barton, D. O. Jang, and J. Cs. Jaszberenyi, The invention of radical reactions. Part XXXI. Diphenylsilane A reagent for deoxygenation of alcohols via their thiocarbonyl derivatives, deamination via isonitriles, and dehalogenation of bromo- and iodo-compounds by radical chain chemistry, Tetrahedron, 49 (1993) 7193-7214. 

The radical chain dehalogenation with Bu3Sn-H is a very useful reaction. The ease of dehalogenation follows the C-X bond strength I > Br > Cl > F the weakest C-X bond is preferred. The initiator for this reaction is commonly AIBN. An example mechanism involving this reaction is shown in Section 11.7, Approaches to Radical Mechanisms. 

These reactions are relevant to this review in that they may be photoinduced, but as radical chain processes many of them can be initiated in other ways. Radical chain dehalogenation of aryl halides has been reviewed previously.The chain propagation sequence for the deiodination of aryl iodides with CH5O-/CH3OH is shown in Scheme 1. The species (Arl) is very short lived and deiodinates efficiently. Reactivity diminishes in the order Arl > ArBr > ArCl and is enhanced in cases where there is relief of steric strain. Photochemical dehalogenations of aryl halides with AIH4 and BH4 have been proposed to follow similar radical chain mechanisms. Evidence for electron transfer from BH4 to ArCl has been present by Freeman and Ramnath. ... 

Bunnett, J. E, Radical chain, electron-transfer dehalogenation reactions, Ace. Chem. Res., 25, 2, 1992. 

Although UGTs catalyze only glucuronic acid conjugation, CYPs catalyze a variety of oxidative reactions. Oxidative biotransformations include aromatic and side chain hydroxylation, N-, O-, S-dealkylation, N-oxidation, sulfoxidation, N-hydroxylation, deamination, dehalogenation and desulfation. The majority of these reactions require the formation of radical species this is usually the rate-determining step for the reactivity process [28]. Hence, reactivity contributions are computed for CYPs, but a different computation is performed with the UGT enzyme (as described in Section 12.4.2). 

The tin hydride reaction is a standard dehalogenation using the radical to sustain a chain reac -je (pp. 1040-1). The stereochemistry is that of the most stable product with the C02Me group on outside of the folded molecule. In fact, this stereochemistry is also unimportant as it disappear. s. the next reaction. 

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