Halides benzylic, reduction

In related Pd(Xantphos)-catalyzed cross-couplings of benzyl halides, reductive elimination was proposed as the slow step, since the overall rate increased with large bite angle ligands [98], P-C bond formation with isolated diastereomers of dinucleoside H-phosphonates was stereospecific, likely with retention of configuration at phosphorus, as above (Scheme 58). 

However in aprotic media the reactions of R are more varied. Thus in the presence of carbon dioxide, benzyl halide reduction in aprotic solvents produces some phenyl acetate. In aqueous solutions only toluene is formed at negative potentials. Rifi has reported the synthesis of bicyclobutane derivatives from the corresponding 1,3-dihalo-cyclo butanes at a mercury pool cathode in DMF containing LiBr e.g. 

The reductions in aqueous methanol of a number of alkyl halides by Co(CN)5 have simple second-order kinetics. For methyl and benzyl halides (RX) the stoichiometries are " ... 

The reduction of transition metal halides with Li has been recently extended by Boudjouk and coworkers for Ullman coupling (benzyl halide to bibenzyl) by Cu or Ni, using a low intensity cleaning bath (5J.). Ultrasound dramatically decreased the time required for complete reduction of the metal halides ( 12 h without, <40 minutes with ultrasound). The subsequent reactivity of the Cu or Ni powders was also substantially enhanced by ultrasonic irradiation. This allowed significant increases in the yield of bibenzyl (especially for Ni) at lower temperatures, compared to simple stirring. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Experimental tests of the theoretical predictions have involved the electrochemical reduction of alkyl and benzyl halides as well as their reduction by homogeneous electron donors.22,29-31 In the first case, AG° = E - rx r.+x=f where E is the electrode potential and rx r.+x=f is the standard potential of the RX/R + XT couple. In the homogeneous case, AG° = E q — rx r-+xt> where E Q is the standard potential of the outer-sphere electron donor or acceptor couple P/Q, and + stands for a reduction and — for an oxidation. 

In the electrochemical case, the predicted and experimental values of the intrinsic barrier for the reduction of butyl and benzyl halides on a glassy carbon electrode were found to agree satisfactorily.22,31... 

Water-soluble transition-metal complexes have been used recently for transfer hydrogenolysis of halocarbons. Paetzold and Oehme [110] have realized the reductive dehaiogenation of allyl or benzyl halides in the presence of [(phosphine) 2PdCl2] complexes with sulfonated phosphines as ligands (e.g., Ph2P(CH2)3S03K) by... 

In a similar type of reaction, polymer-supported hydridoiron tetracarbonyl anion reacts with simple non-benzylic aliphatic bromides and iodides to produce aldehydes (Table 8.15), presumably through the intermediate formation of RCOFeH(CO)3, which undergoes reductive extrusion of the aldehydes [3], In contrast, benzylic halides and a-halocarbonyl compounds are reductively dehalogenated by the HFe(CO)4 anion (see Chapter 11). 

The yields of ketones, isolated from the reductive debromination of a-bromo-ketones by dicobalt octacarbonyl under basic phase-transfer conditions are good (Table 11.13), but are improved (>95%) by the use of stoichiometric amounts of the quaternary ammonium catalyst. Somewhat unexpectedly, in the case of the reductive dehalogenation of secondary benzylic halides, the yields of the coupled alkanes are... 

Mechanistic aspects of the reduction of benzyl halides at mercury have been extensively investigated [35, 38]. From the reduction of benzyl iodide at platinum, Koch and coworkers [39] obtained toluene, bibenzyl, and hydrocinnamonitrile. Electrolysis of benzyl chloride in the presence of acyl chlorides can be used to synthesize alkyl benzyl ketones [40], whereas alcohols are formed by electrolysis of... 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

A variety of alkyl halides have been reduced at room temperature, including benzyl halides, primary, secondary and tertiary alkyl halides. The reaction times depend on the halide, and vary between 20min (benzyl bromide, 0.5 mol% 28) up to several days (iodopentane, fluoropentane). The reactivity of alkyl halides decreases in the order R—Br > R—Cl > R—1 when reductions are performed in separate flasks. Several mechanistic details of the reaction have been uncovered by in situ monitoring of the reaction by NMR spectroscopy. The precatalyst 28 appeared to be activated by a rapid reduction of the coordinated acetone to PrO—SiEt3 and concomitant coordination of an alkyl halide (H Scheme 12.11). This complex represents a resting state that is in equilibrium with a o-silane... 

Reduction of alkyl and benzyl halides proceeds in two one-electron addition steps. The first detectable product is the alkyl or benzyl radical and this is reduced further to the carbanion. Some alkyl iodides show two polarographic waves corresponding to the two steps. Alkyl bromides show only one two-electron wave and alkyl chlorides are not reducible in the available potential window. Benzyl halides also show only one wave and benzyl chlorides are reducible in the available potential range. Reduction potentials measured in dimethylformamide are collected in... 

Reduction potentials for alkyl and benzyl halides measured by cyclic voltammetry at 0.1 or 0.2 V s" with a glassy carbon electrode in dimethylformamide. 

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