Hydrogen donors Bu3SnH radical

The results for the cyclization of the l,l,5-trimethyl-5-hexen-l-yl radical, formed from the reaction of l,l,5-trimethyl-5-hexenyl-l-bromide (3) with Bu3SnH/AIBN, are shown in eq. 3.2. These results indicate that the ratio of 5-exo-trig marmer/6-endo-trig manner depends on both the temperature and concentration of Bu3SnH. Thus, as the concentration of Bu3SnH at 40 °C is decreased (which means the decrease of the concentration of the hydrogen donor), both a decrease of the direct reduction product,... 

Refluxing or photolytic treatment of O-acyl esters (2) in the presence of a hydrogen donor such as Bu3SnH or ter -BuSH, provides the corresponding reduction products via alkyl radicals. This reaction can be applied to primary-, secondary-, and tertiary-alkyl chained carboxylic acids, and can also be used for steroids, sugars, and peptides as shown in eq. 8.4 [9-14]. Racemization does not occur at other chiral centers. 

The reactions shown in Scheme 2.140 involve the oxidation of radical species, for example 419a, as the termination step in a radical addition. An alternative and highly useful protocol was elaborated with a system composed of a radical initiator and a hydrogen donor capable of reducing a similar radical intermediate. Utilization of azobisisobutyronitrile (AIBN) plus Bu3SnH was shown to be especially efficient for this purpose. In this system, BusSnH fulfills a dual... 

In later chapters, we will see the evolution of free radical chemistry, starting from the typical Bu3SnH radical hydrogen donor in benzene to the use of... 

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... 

Vitamin E and vitamin C are also good hydrogen atom donors in living bodies. The rate constants for the reaction of an alkyl radical and an alkoxyl radical with vitamin E are 1.7 X 106 and 3.8 X 109 M-1 s-1, respectively [58, 59]. The rate constants of hydrogen atom abstraction from R-H such as cyclopentane, 1,4-cyclohexadiene, tetrahydrofuran, Bu3SnH by tert-BuO are shown in Table 1.12. 

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