Complex radical processes

Irradiation of diethyl oxalate in cyclohexane produces a variety of products which must be formed by complex radical processes.100-101 Cleavage of the ester to form radicals is a reasonable first step since hydrogen abstraction by... 

Hirooka (29) has proposed that copolymerizations of this type be named "complex copolymerization. Russian workers (55) have suggested that polymerizations initiated by radical catalysts in the presence of a complexing agent be called "complex-radical processes. Both polymerizations are more appropriately considered as polymerizations through activated charge transfer complexes, in which the Lewis acid or metal halide catalyzes the formation of the complex, and free radicals may or may not be necessary to initiate the homopolymerization of the complex. 

Dimethyl-I,l -biphenyl has been prepared by a wide variety of procedures, but few of these are of any practical synthetic utility Classical radical biarjl syntheses such as the Gomberg reaction or the thermal decomposition of diaroyl peroxides give complex mixtures of products m which 4,4 dimethyl-l.l -biphenyl is a minor constituent A radical process maj also be involved in the formation of 4,4 dimethyl-1, l -biphenyl (13%) by treatment of 4-bromotoluene with hydrazine hydrate 5 4,4 -Dimethyl-l,l -biphenyl has been obtained in moderate to good yield (68-89%) by treatment of either dichlorobis(4-methyl phenyl)tellurium or l,l -tellurobis(4-methylbenzene) with degassed Raney nickel in 2 methoxyethyl ether 6... 

CHARGE TRANSFER COMPLEXES AS INTERMEDIATE IN RADICAL PROCESSES... 

Other metals can also be used as a catalytic species. For example, Feringa and coworkers <96TET3521> have reported on the epoxidation of unfunctionalized alkenes using dinuclear nickel(II) catalysts (i.e., 16). These slightly distorted square planar complexes show activity in biphasic systems with either sodium hypochlorite or t-butyl hydroperoxide as a terminal oxidant. No enantioselectivity is observed under these conditions, supporting the idea that radical processes are operative. In the case of hypochlorite, Feringa proposed the intermediacy of hypochlorite radical as the active species, which is generated in a catalytic cycle (Scheme 1). 

In another example of a radical process at the pyrrole C-2 position, it has been reported that reductive radical cycloaddition of l-(2-iodoethyl)pyrrole and activated olefins, or l-(oj-iodo-alkyl)pyrroles 34 lead to cycloalkano[a]pyrroles 35 via electroreduction of the iodides using a nickel(II) complex as an electron transfer catalyst <96CPB2020>. Thus, it appears the radical chemistry of pyrroles portends to be a fertile area of research in the immediate or near future. 

Atom transfer radical polymerization, ATRP, is a controlled radical process which affords polymers of narrow molecular weight distributions. Strictly this is not a coordinative polymerization, but its dependency upon suitable coordination complexes warrants a brief discussion here. 

The mechanism of the reaction of secondary alkyl halides with low-valent transition metal complexes is considerably more complex, and radical processes have been clearly identified in some cases (13, 14). 

In general, Ti appears to display the widest range of reactivity among the three members of the Ti triad. The most common oxidation number for all three members is +4. Their complexes in which they exist in the +2 oxidation state have also been implicated in many cases. Furthermore, organotitanium complexes of the +3 oxidation states have been much more widely observed than the corresponding complexes of Zr and Hf.14 The relatively ready accessibility of the +3 oxidative state along with the +4 and +2 oxidation states implies that Ti is more prone to one-electron transfer or radical processes than Zr or Hf, and this indeed has been the case. Undoubtedly, this is one of the main reasons for the versatile reactivity of Ti, which has led to a number of both favorable and unfavorable consequences relative to Zr or Hf. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Recently, intermolecular hydrophosphination of alkynes was also reported with ytterbium-imine complex catalyst precursors [20]. Aromatic alkynes react at room temperature to afford ( )-isomers, while aliphatic ones require heating at 80 °C and, quite surprisingly, (Z)-isomers (trans-addition products) are formed preferentially (Table 4). In this respect the ytterbium-catalyzed reactions are different from the radical process, in which the ( )-isomer formed initially isomerizes to the (Z)-isomer. 

When (40) is irradiated by Hg lamp at room temperature in the presence of pentamethyl-cyclopentadienyl dicarbonyl cobalt(III), Co(III) dithiolato complexes (45) and (46) are formed implying involvement of benzonitrile sulfide as an intermediate <92CL243). Thermolysis of (40) (and its phenyl ring substituted derivatives (40a)) at 110-140 °C in aromatic solvents results in formation of another heterocyclic mesoionic structure (47) and appears to proceed as a radical process (Scheme 2) (91TL4023). The reaction is inhibited by radical scavengers. 

On the basis of kinetic studies, a mechanism for the radical oxidation of thioether with 36 has been proposed and is indicated in Scheme D ". The key step involves the formation of a radical cation-anion pair within the solvent cage. The presence of the pic ligand in the coordination sphere of the metal reduces the electrophilicity of the peroxo complex, thus allowing the competitive radical process to take place. 

Astruc, D. Electron Transfer and Radical Processes in Transition-Metal Complexes VCH New York, 1995. 

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