Reduction, propagation step

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

Hydrogen-Atom Transfer. Many oxidation and reduction reactions are free-radical substitutions and involve the transfer of a hydrogen atom. For example, one of the two main propagation steps of 14-1 involves abstraction of... 

As an example, the propagation steps for the reductive alkylation of alkenes are shown in Scheme 7.1. For an efficient chain process, it is important (i) that the RjSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene, and (ii) that the alkyl radical reacts faster with the alkene (to form the adduct radical) than with the silicon hydride. In other words, the intermediates must be disciplined, a term introduced by D. H. R. Barton to indicate the control of radical reactivity [5]. Therefore, a synthetic plan must include the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and that the concentration of silicon hydride often serves as the variable by which the product distribution can be influenced. 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Termination steps in SrnI reactions compete with the propagation steps and, although these processes have aroused considerable mechanistic and theoretical speculation (see Chapter 9 in ref. 18), their effects, with several important exceptions, are not significant. For example the self-coupling of aryl radicals (equation 11) does not appear to occur under the conditions used for the SrnI reaction. One potentially disruptive termination step is reduction of the intermediate aryl radical (equation 6). The source of the reducing electron can be a dissolving metal (or solvated electron), one of the radical anion intermediates in the reaction (ArNu- or ArX- ), an electrode, or the nucleophile itself. These termination... 

An intermediate reduction of aryldiazonium salts Ar-N =N to the diazo radicals Ar-N=N also occurs when aryldiazonium salts react with KI to yield aryl iodides (Figure 5.55). Therefore, aryl radicals Ar are obtained under these conditions, too. Their fate, however, differs from that of the aryl radicals, which are faced with nucleophiles in the presence of Cu(II) (cf. Figure 5.53) or H3P02 (cf. Figure 5.54) the iodination mechanism of Figure 5.55 is a radical chain reaction consisting of four propagation steps. 

Electron-deficient aryl diazonium salts such as the pentafluoro derivative can offer the attractive option to conduct radical arylations as chain reactions with an SrnI mechanism (Scheme 35) [151]. In these special cases, only catalytic amounts of an initiating reductant, such as sodium iodide, are required. In the propagation step, the diazonium salt 92 acts as oxidant for the cyclohexadienyl intermediate 93. Rearomatization of 93 to 94 as well as the generation of a new pentafluorophenyl radical are achieved through this step. 

The distinction between retarders and inhibitors is difficult to make with precision. Retarders give no induction period, their presence merely causing a reduction in rate. Inhibitors give rise to an induction period, although the initial period of no detectable reaction is subjective, the limit of detectability depending on the sensitivity of the measuring equipment. The mechanistic distinction usually assumed is that inhibitors interfere with the normal process of initiation retarders interfere only with the propagation steps. 

Many phosphane-substituted transition-metal clusters have been synthesized from late transition-metal carbonyl clusters and the appropriate phosphane using reductive ETC catalysis with reductive initiation [318-333]. Indeed such an initiation provides an exergonic cross electron-transfer propagation step. Most syntheses were carried out using a cathodic initiation or sodium benzophenone radical anion. The method was successful because it turned out that the first substitution of a carbonyl by a phosphane proceeds with high yield and coulombic efficiency in homoleptic metal carbonyl clusters and some others. 

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