Reductive dimerization of aryl halides

Under the same conditions, the use of an iron anode is also necessary for the reductive dimerization of aryl halides (p-MeOPhBr, p-EtOCOPhBr, etc.). With a Mg or A1 sacrificial anode, or if electrolysis is conducted in a divided cell, aryl bromide (ArBr) is principally converted to the reduction product (ArH) [22]. 

This aspect is developed by addressing the following question Is there any chance that biaryls may be formed, via aryl radical dimerization, during the reductive electrolysis of aryl halides ... 

Electrochemical reduction of aryl halides in the presence of olefins (94), (equation 54) leads to the formation of arylated products (95). Electroreduction of several aralkyl halides at potentials ranging from -1.24 V to -1.54 V (see) gives products which involve dimerization, cyclization, and reduction to the arylalkanes. Carbanions and/or free radicals were again postulated as intermediates79. Aryl radicals generated from the electrochemical reduction of aryl halides have been added to carbon-carbon double bonds80,81. Electrochemical reduction of aryl halides in the presence of olefins leads to the formation of arylated products78. Preparative scale electrolyses were carried out in solvents such as acetonitrile, DMF and DMSO at constant potential or in liquid ammonia at constant current. The reaction is proposed to involve an S l mechanism. 

Although alkylmagnesium halides lead to reductive dimerization of the dienyl cations, synthetically useful alkylations and arylations have recently been achieved using zinc and cadmium reagents [Eqs. (41) and (42) (Birch and Pearson, 1975)]. 

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. 

The direct interaction of Al, H2, and olefin (hydroalumination) is used to give either the dialkyl hydrides or the trialkyls. These are also accessible by reduction of organoaluminum halides and exist as dimers or trimers, depending on the steric requirements of the alkyl or aryl group. Examples are [Bu2A1H]3, [mes AlH2]2, and mes GaH2.24... 

A radical species may also be generated by reduction of an electron-deficient compound and a classical entry to 4,4 -bipyridines is the reduction of a pyridine by sodium and subsequent rearomatization. Figure 8a illustrates the use of such a reduction in order to prepare the precursor of a sodium-ion molecular switch [30], A more general route, derived from the ancient copper-catalyzed Ullmann coupling, is the metal-induced dimerization of an aryl halide. The key step is a reductive elimination within the coordination sphere of the metal. A nickel(O) complex, in stoichiometric quantities, is usually selected for this purpose. Constable and Ward have used such a reaction to prepare a bis-terpyridine from an interesting synthon, which would have otherwise required a more specialized strategy with dedicated intermediates (Figure 8b) [31]. 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases." In the conventionally accepted reaction mechanism," a four-coordinate aryl-Pd(II) intermediate is formed by oxidative addition of the aryl halide to a Pd(0) complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd , and ligand dissociation combine to give a viable catalytic species." If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming, and an... 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

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