Benzyl halides, bond

Section 1115 The simplest alkenylbenzene is styrene (C6H5CH=CH2) An aryl group stabilizes a double bond to which it is attached Alkenylbenzenes are usu ally prepared by dehydration of benzylic alcohols or dehydrohalogena tion of benzylic halides... 

Various alkylating agents are used for the preparation of pyridazinyl alkyl sulfides. Methyl and ethyl iodides, dimethyl and diethyl sulfate, a-halo acids and esters, /3-halo acids and their derivatives, a-halo ketones, benzyl halides and substituted benzyl halides and other alkyl and heteroarylmethyl halides are most commonly used for this purpose. Another method is the addition of pyridazinethiones and pyridazinethiols to unsaturated compounds, such as 2,3(4//)-dihydropyran or 2,3(4//)-dihydrothiopyran, and to compounds with activated double bonds, such as acrylonitrile, acrylates and quinones. 

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... 

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. 

The high diastereoselectivity observed with benzylic halides has been explained by postulating an interaction between the aromatic ring of the halide and the conjugated double bonds of the enolate intermediate. One face of the enolate is shielded by the benzene ring of the phenyl-ethanamine moiety, thus hindering electrophilic attack. 

In the case of halocarbon matrices, the SOMO is of the a type and the cleavage is facilitated by the antibonding nature of the SOMO. In other ions, a or a orbitals of scissile bonds interact with a ti or 7i orbital, causing them to be weakened. Accordingly, radical anions of benzyl halides may generate benzylic radicals with loss of a halide ion. Conversely, benzylsilane radical cations may form benzylic radicals with loss of a silyl cation (Fig. 6.15). 

Although there are numerous investigations of concerted dissociative ET reported, particularly those involving alkyl and benzyl halides, examples of other molecular systems that undergo putative concerted dissociative ET are not as well documented. Peroxides and endoperoxides are another class of compounds where ET to the oxygen—oxygen bond has recently been shown conclusively to follow a concerted dissociative mechanism. The concerted nature of the dissociative ET is, in part, a result of the very weak 0—0 bond, which is a necessary condition, as illustrated by equation (41). 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

Organonickel(II) species are believed to be formed during the reaction between [Ni(TMC)] and primary alkyl halides, and subsequently undergo hydrolysis with cleavage of the Ni—C bond. Kinetic data measured in the presence of excess alkyl halide indicate a rate law -dlNi1 (TMC)+]/cft = MNi (TMCr][RX]. The rate constants increase for R and X in the order methyl < primary < secondary < allyl < benzyl halides and Cl < Br < I (133, 140). This suggests that the rate-determining step is electron transfer from the Ni(I) complex to R—X via an inner-sphere atom-transfer mechanism (143). 

Benzyl Halides The molecular ion peak of benzyl halides is usually detectable. The benzyl (or tro-pylium) ion from loss of the halide (rule 8, Section 2.7) is favored even over /3-bond cleavage of an alkyl substituent. A substituted phenyl ion (a-bond cleavage) is prominent when the ring is polysubstituted. 

The cationic polymerization of isobutylene (12) and styrene (13) is initiated readily by Et2AlCl in the presence of an alkyl halide, RC1. The interaction of the catalyst and cocatalyst is presumed to produce the carbonium ion R+, which initiates polymerization, and the corresponding gegenion Et2AlCl2". Alkyl halides with low R-Cl bond dissociation energies—e.g. tertiary, substituted allylic, and benzylic halides—are among the most effective cocatalysts. 

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