Subject metal halide elimination

A 2-haloarylmetal may either be subjected to electrophilic substitution and addition, without loss of the halogen, or serve as the precursor to an aryne that is set free by elimination of metal halide. In either case, it is crucial to know the threshold temperature below which the organometallic intermediate is safe and above which the elimination of the metal halide is unleashed. 

The presence of group 4 metals P to a carbenium ion has a dramatic effect on the stability of the carbenium ion, a remarkable phenomenon known as the P-effect.12 29, 30 The silicon P-effect was discovered in 1937 by Ushakov and Itenberg,31 who noted the exceptional reactivity of P-silyl-substituted halides R3SiCH2CH2X towards elimination (Scheme 1). The effect has since been the subject of many mechanistic32-36 and theoretical studies37-39 and several reviews.10-12,29... 

The catalytic activity of Hg in elimination and substitution reactions of transition-metal complexes continues to be the subject of study. Kinetic studies on the Hg" catalysed aquation of both cis-[Co(en)2(CN)Cl], cis-[(chloro)(aniline)CO(en)2], and [PdLX] (L = tetramethyldiethylenetriamine X = Cl, Br, or I) are consistent with intermediate adduct formation at the halide substituent. Mercury(ii) reacts reversibly with the [Co(NH3)5SCN] ion to yield [Co(NH3)5(SCN)Hg] which irreversibly undergoes either aquation to yield [Co(NH3)sH20] or isomerization to yield [Co(NH3)5(NCS)Hg] at approximately equal rates. The kinetics of the reaction... 

Olefins can be prepared by the dehydrogenation of paraffins, dehydration of alcohols, or decomposition of ethers and halides, if vapours of these substrates are passed over metals or metal oxides at elevated temperatures (300-600°C). Dehydration reactions have been most widely studied and by careful selection of the catalyst and the reaction conditions the direction and stereochemistry of elimination can be controlled. However, dehydration often has to compete with dehydrogenation, and isomerisation of olefinic products by the acidic sites on the catalyst can reduce the synthetic utility of these reactions. Most frequently alumina has been used as the catalyst and the advantages and complexities of the method are amply illustrated by the dehydration of alcohols. Surface-catalysed eliminations have been the subject of several reviews "". ... 

When the substrate is a triflate, or when a halide is subjected to appropriate reaction conditions in the presence of a halide scavenger (such as silver or thallium salts), the reaction proceeds via the cationic manifold. After oxidative addition, dissociation of X yields cationic intermediate 67. Alkene coordination provides 68 and migratory insertion delivers 69. /S-Hydride elimination then yields the desired product 66. Of importance in the pathway 62- 7— 68— 69— 66 is that both phosphines maintain contact with the metal throughout the process. This is ostensibly the factor responsible for the high enantioselectivities observed for reactions that are thought to proceed along this pathway. This contrasts with the course of events in the neutral pathway, where phosphine dissociation is thought to be responsible for low enantioselectivities. 

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