Halogenation trans products

The addition of halogens and halogen acids to alkenes has been shown to be predominantly trans and where the results do not agree, explanations have been given in terms of steric factors. Dewar has proposed that in all electrophilic addition reactions where a classical carbocation is formed, cis addition is the rule and where there is the preponderance of the trans product, the effect is due to steric factors. 

How does the proposed mechanism invoking a bridged halonium ion intermediate explain the observed trans products of halogenation For example, chlorination of cyclopentene affords both enantiomers of rram-l,2-dichlorocyclopentane, with no cis products. 

Blake and Kubota have convincingly shown that in anhydrous benzene, chloroform and ether, as in the gaseous reaction on the solid complex, hydrogen halides (HX) and /m -[Iry(CO)(PPh3)2] (T = halogen) react to give octahedral cw-addition products (i.e. H and X cis). In contrast in wet solvents, and in such solvents as dimethylformamide, acetonitrile and ethanol, mixtures of cis and trans products are formed. Whether these solvents cause rapid halide-ion exchange before the addition reaction or in the products is yet to be demonstrated. ... 

Haloketenes, generated in situ, react with imines to afford halogenated /8-lactams. For example, reaction of chloroacetyl chloride or dichloroacetyl chloride with benzophenone anil in the presence of triethylamine at room temperature affords high yields of the cycloadducts The stereochemistry of the [2-1-2] cycloaddition of chloroketene to substituted benzalaniline was investigated by Nelson , who found that substituents in the o-position of the phenytring on carbon enhanced the formation of the cis isomers, while para substituted derivatives afforded exclusively the trans products. A two step cycloaddition process was also postulated for this reaction. Fluoroketenes, PhC(F)=C=CO and CHF=C=0, react with imines R CH=NR (R = Ar, Et, Me) to give [2-1-2] cycloadducts having cis stereochemistry . ... 

The possible presence in the 4-chloro-4-hexenyl trifluoroacetate of small amounts of two cis-trans pairs of products of addition of trifluoroacetic to the triple bond without concomitant halogen shift remains speculative. In any event these compounds would be removed as ketones upon hydrolysis of the trifluoroacetate. Both the 4-chloro-4-hexenyl trifluoroacetate and the alcohol resulting from its hydrolysis have been shown to contain 9% of the (E) isomer. In the present study the hydrogen decoupled magnetic resonance spectra of the ester and alcohol were shown to contain peaks attributable to approximately 9% of E) isomer. 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

Many compounds of rhodium (III) may readily be obtained by this general catalytic procedure. Rhodium(III) bromide may be substituted for the rhodium trichloride, and weakly basic nitrogenous ligands (2,2 -bipyridine, o- (or l,10-)phen-anthroline, or dioximes) for the pyridine. The products all have the halogen atom in trans positions. 

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