Bond reductively induced

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. 

Similarly, C—Se bonds are formed by an internal homolytic substitution of aryl radicals at selenium, with the preparation of selenophenes and benze-neselenophenes [75]. Scheme 7.7 illustrates the reaction of aryl iodides 61 with (TMS)3SiH, which afforded benzeneselenophenes in good yields. The presence of (TMS)3SiI after the reduction induced the final dehydration of the intermediate 3-hydroxyselenophenes, presumably through an intermediate silyl ether. 

With an ot, y-ketodiol, cyclization to produce a 3-furanone derivative is feasible, as is shown for the synthesis of ascofuranone (71) and geiparvarin (72) (Scheme 6.57) (286). The precursor for 71 was prepared by the cycloaddition of diene 66 to nitroalcohol 67. In this case, regioselective attack occurred only on the terminal double bond. Reductive cleavage-hydrolysis of the isoxazoline adduct 68 with Mo(CO)6 followed by acid-induced cyclization led to the furanone intermediate (286). A similar strategy was used for the synthesis of geiparvarin (72) (Scheme 6.58) (286). 

Scheme 10. Thermochemical cycle for determination of reductively induced C-H bond activation in dinuclear ethylidyne complexes.

The ability of the calixarene complex (12) with its Nbln=Nbln bond to induce four-electron reduction of N2, aldehydes, and ketones has been discussed in Sections 4.5.2.1.2, 4.5.2.2.4, and 4.5.2.4.5.(ii) (Scheme 32). 

The presence of metallic Mo and MoOt on Si0 was found to be a prerequisite to the development of activity and selectivity for alcohol synthesis from C0-Hj. The gradual increase in alcohol production during the reaction is ascribed to the formation of CO-reduction induced defects on MoC. Experiments performed by adding olefins.to CO-H2 revealed that CO insertion into a metal-alkyl like bond constitutes the reaction pathway to alcohols. The role of K and Cl in the promotion of alcohol production is discussed. 

Incoming nucleophiles may also cause cleavage of the M-Si bond by inducing a reductive elimination reaction. Thus, MeLi and phosphines displace silanes HSiR3 from Cp(CO)2Mn(H)(SiR3) compounds11 195. 

Cleavage of p-nitrobenzyl group. Protection of carboxylic acids and amines as the esters and carbamates can take advantage of the selective reduction-induced fragmentation by zinc dust, as C=C bonds, S-N bonds, benzyloxycarbonyl, and diphenylmethyl groups are not affected during the operation. 

Studies of the reactivity of Cp2Sm(THF)2 compounds with CO, RC=CR, RN=NR and R2CN2R2 (Evans and Drummond 1989) have shown the variety of useful transformations induced by Sm on these unsaturated substrates. In its reaction with azine (which contains a C=N double bond), in contrast to the two electron multiple bond reduction observed in the previously quoted substrates, the azines are reduced by one electron... 

C-C bonds can be formed by reaction with alkyl iodides or more usefully by reaction with metal carbonyls to give aldehydes and ketones e.g. Ni(CO)4 reacts with LiR to form an unstable acyl nickel carbonyl complex which can be attacked by electrophiles such as H+ or R Br to give aldehydes or ketones by solvent-induced reductive elimination ... 

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