Couplings radical-like

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Sn—Sn bond formation can be achieved by indirect electrolysis considering the relative ease of SnH-bond activation. Tributylin hydride is a known H atom donor. It is attacked by radicals like Mn(CO)3P(OPh)3]2, electrogenerated from the anion Mn(CO)3P(OPh)3]2p. The kinetics of hydrogen transfer and coupling of Ph3Sn and Mn(CO)3P(OPh)3]2 was studied188. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serving as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. 

Numerous examples exist of combining CRP methods with other polymerization techniques for preparation of block copolymers. Non-living polymerization methods like condensation, free-radical, and redox processes can easily be combined with CRP to produce novel materials. Transformation chemistry may be the only route to incorporate polymers like polysulfones (as described above), polyesters, or polyamides that are prepared solely through condensation processes into subsequent CRP to form block copolymers with vinyl monomers. The same can be said of polymers prepared through coupling techniques, like po-ly(phenylenevinylene) and poly(methylphenylsilylene), which can maintain their conductive or photoluminescence properties, but become easier to process... 

Tertiary alkyl halides usually give only a few percent of the tert-butyl derivative upon interaction with an alkali-organic compound. The incidental couplings with satisfactory yields are likely to be the result of a radical-like mechanism. 

CO insertion prior to the transmetallation step. The mechanism of nickel-catalyzed coupling reactions is less established. Early studies indicated that homocoupling processes occur by oxidative addition through radical intermediates and possible intermediacy of Ni(I) and Ni(III) complexes. The copper-catalyzed cross-coupling reactions likely occur by transmetallation prior to oxidiative addition of the aryl halide. Iron-catalyzed reactions likely occur by low-valent, even sub-valent, species. 

It is now known that the photoreduction of benzophenone is a reaction of the n-7T triplet state (T ) of benzophenone. The h-tt excited states have radical character at the carbonyl oxygen atom because of the unpaired electron in the nonbonding orbital. Thus, the radical-like and energetic Tj excited-state species can abstract a hydrogen atom from a suitable donor molecule to form the diphenylhydroxym-ethyl radical. Two of these radicals, once formed, may couple to form benzpinacol. The complete mechanism for photoreduction is outlined in the steps that follow. 

The mechanism is believed to begin by SET from the base to the aryl iodide to form a radical anion. Inter-molecular reaction can occnr through a HAS, the radical variant of the more commonly known EAS reaction. For intermolecular reaction, addition of the radical to the coupling partner resnlts in a resonance-stabilized radical. Deprotonation with base forms a biaryl radical anion which propagates the mechanism by donating an electron to another aryl haUde. For intra-molecular reaction, the aryl radical likely adds to the ipso position of the tether. Subsequent ring expansion and re-aromatization yields the prodnct. ... 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

Each radical undergoes reaction with another dimerization, disproportionation, or the like. The cross-coupling process between R1 and R2 also occurs. With these, the reaction scheme takes the following form ... 

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