Halogenative cleavage

Acid, hydrogen haUde, or halogen cleavage of tetraorganotins is not used except on a laboratory scale because they are wasteful of tin—carbon bonds and uneconomical on a commercial scale. 

Electrolysis, Ni(II), Mg anode. DMF, it 77-99% yield. This method is not compatible with halogenated phenols, because of competing halogen cleavage. ... 

As the two diastereoisomers of each product have different lH-NMR spectra, the degree of stereoselectivity for all of the reaction steps in Scheme 20 can be determined by NMR integration. It turned out that, depending on the reaction type and the reaction conditions, different stereoselectivities were found. Hence, the bromination of the (-)365-iodo compound 30a has a stereoselectivity as high as 80%, whereas the meth-ylation of 30a and the halogen cleavage of the Fe—C bond in 31a occur with an optical yield of only —40% (58). 

Carbon-halogen bond cleavage in aryl halides is believed to involve initial excitation into an upper singlet state that is stable with respect to cleavage, followed by intersystem crossing to an upper triplet state that is either dissociative itself or can cross to an upper dissociative (a, a ) triplet [65-67]. The two-color approach has been used to demonstrate directly that excitation to an upper triplet state results in carbon-halogen cleavage. For example, in 2-bromonaphthalene (81) and 9-bromo-phenanthrene (82) [68], promotion of Tj to an upper triplet by dye laser excitation... 

Catalytic reactions of organohalides using palladium, nickel, or rhodium catalysts are well investigated. Interestingly, however, in the case of ruthenium-catalyzed reactions, only a few examples involving carbon-halogen cleavage have been reported. 

The part played by the more stable fragments resulting from carbon-halogen cleavage is again uncertain, but Heine (7) has indicated a reduced tendency for these radicals to add to unsaturated systems. Dimeric products have been isolated in the case of photolyses carried out in benzene, e.g. (7). 

Halogenative cleavage likewise appears completely general for both alkyl- and alkenyl-zirconium complexes (Scheme 10), and proceeds with complete retention of configuration at carbon. For the hydrozirconation products of 3-methyl-1,3-dienes, such as (40), cyclization competes (equation 43). Analogs without the 3-methyl subsitutent, or with more remote double bonds, give only the uncyclized products in good yields. " ... 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

Show that halogen cleavage of CpFe(CO)2CD2CH2Ph will give two Exercise 8-13... 

The dichotomy between retention and inversion during halogenolysis occurs with a number of a metal-alkyl and metal-alkenyl complexes. The factors mentioned earlier, such as solvent polarity, steric hindrance, and the nature of the HOMO, are influential in directing the stereochemical outcomes. It is only with the early transition metals that rather consistent stereochemical outcomes occur. As in protonolysis, it appears that halogen cleavage occurs by an SE2 mechanism with retention of configuration (path b, Scheme 8.9). Equations 8.61s7 and 8.6288 demonstrate this pathway for a-alkyl and cr-alkenyl Zr complexes, respectively. 

A complementary structural and kinetics characterisation study regarding the halogen cleavage reaction (by pyridines) in dimethylaminomethylphenyl-C N pallada-and platina-cycles has been reported (see Scheme 15).269 This work followed by 30 years a pioneering study of the kinetics of monomerisation of a dinuclear bromine-bridged platinum compound using pyridine.270... 

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