Halides, alkyl, reaction with Bu3SnH

Radical reductions of alkyl halides in water.1 Radical reactions with Bu3SnH are limited to organic solvents, but this tin hydride (1) is sufficiently soluble in water to reduce alkyl bromides or iodides under free-radical conditions in water or in organic solvents. 

Radicals for addition reactions can be generated by halogen-atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism, shown in Fig. 10,3. The rates of each of these steps must exceed those of competing chain termination reactions in order for good yields to be obtained. The most important competitions are between the addition step and reaction of the intermediate R- with Bu3SnH and between the H-abstraction step ki and addition to another molecule of the alkene. If the addition step is not fast enough, the radical R- will... 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

The conversion of halides to alcohols is a typical SN1 or SN2 reaction in the polar reaction method, and generally the reactions require basic conditions. However, the conversion of halides to alcohols by the radical reaction method can be carried out under neutral conditions. The treatment of alkyl halides with Bu3SnH /AIBN in toluene under aerobic conditions (atmosphere) gives the corresponding alcohols, by means of the reaction of the alkyl radical with molecular oxygen, and the subsequent reduction of alkyl hydrogen peroxide (ROOH) with Bu3SnH (eq. 2.22) [52-57]. When 1802 is used instead of 1602 in... 

Addition reactions with Mode A are popular, and most addition reactions are classified into this group. The most typical experimental procedure is the treatment of alkyl halides (RBr, RI) and electron-deficient olefins with Bu3SnH in the presence of AIBN. Bu3GeH or Ph3GeH can be also used instead of Bu3SnH however, there are rather expensive and less reactive. Typical addition reactions are shown below, in... 

Ce4+ can be also used for the same type of reaction, since it is a strong one-electron oxidant. Generation of sp2 carbon-centered radicals such as aryl radicals, is not so easy, except for the reactions of aryl halides with Bu3SnH or Ph4Si2H2. However, treatment of arylhydrazines with Cu2+ generates aryl radicals through the initial oxidation to the arenediazonium ion with Cu2+, and subsequent SET from Cu+. Aryl radicals are much more reactive than alkyl radicals, and rapidly react with alkenes or imines as shown below (eq. 4.22) [60-63]. 

Tributyltin hydride (Bu3SnH) is used synthetically to reduce alkyl halides, replacing a halogen atom with hydrogen. Free-radical initiators promote this reaction, and free-radical inhibitors are known to slow it or stop it. Your job is to develop a mechanism, using the following reaction as the example. 

Radical substitution reactions can also be used to remove functional groups from molecules. A useful reagent for this (and, as you will see, for other radical reactions too) is tributyltin hydride, Bu3SnH. The Sn-H bond is weak and B SnH will react with alkyl halides to replace the halogen atom with H, producing BL SnHal as a by-product. 

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