Reduction potentials alkyl halides, 268, Table

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

Tlte reduction potential for an alkyl or benzyl radical, relative to that of the carbon-halogen bond from which it is derived, is important in determining the isolated products. Products are derived either by radical or by carbanion chemistry. The half-wave potential for the second polarographic wave of alkyl halides is connected with reduction of the radical. Sophisticated methods have been devised for deducing radical reduction potentials in cases where (his second wave is not seen. Values are collected in Table 4.4. 

Alkyl- and Aryl-Halides. Because the halo-groups of organic molecules have large electronegativities and electron affinities, all halo-carbon molecules are electrophilic. Their electrochemical reduction potential is a measure of their electrophilicity (and electron affinity), which is illustrated in Figure 12.1 for hexacWorobenzene (C6C16), 1,2,3,4-tetrachlorobenzene (1,2,3,4-C6H2C14), and n-butyl iodide (n-BuI).8,9 Table 12.1 summarizes the reduction potentials for several alkyl-halides and ary 1-chlorides.810... 

TABLE 13.7 Reduction Potentials for Alkyl Halides (RX) in Dimethylformamide"... 

Standard potentials ( ) have been estimated from thermodynamic properties 134,40.47,481, and also from analysis of the rtites of homogeneous electron transfer reductions of alkyl halides on the basis of Marcus theory [49]. In Table 2.5, some experimental reduction potentials and estimates of are summarized. Despite some puzzling inconsistencies, the experimental and theoretical potentials show general trends toward increasing ease of reduction for Cl < Br < I, and 1° < 2 < 3°. Allyl and benzyl htilides arc especially easily reduced. 

Additional support for an electron transfer-like mechanism comes from the observation that reactions with n-alkyl halides are incomplete, with ca- 40% of substrate unreacted, whereas reactions with alkyl iodides are nearly complete. This is consistent with the fact that the reducing power of n-type polyacetylene decreases as the carbanion concentration decreases, and that normal alkyl chlorides are the most difficult to reduce in the series employed in our experiments (see Table 1). (Note that the reduction potentials of polyacetylene and the alkyl halides cannot be compared directly as they were measured in different solvents.) Alkyl fluorides, as expected, do not react to a measurable extent. 

The catalytic cycles for reduction of alkyl and atyl halides using Ni(o), Co(i) or Pd(o) species are interrupted by added carbon dioxide and reaction between the first formed carbon-metal bond and carbon dioxide yields an alkyl or aryl car-boxylate. These catalyses reactions have the advantage of occuriiig at lower cathode potentials than the direct processes summarised in Table 4.14. Mechanisms for the Ni(o) [240] and Pd(o) [241] catalysed processes have been established. Carbon dioxide inserts into the carbon-metal bond in an intermediate. Once the carboxy-late-metal species is formed, a further electron transfer step liberates the carboxy-late ion reforming the metallic complex catalyst. 

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