Bromine alkenes

The first possibility envisages essentially the same mechanism as for the second-order process, but with Bt2 replacing solvent in the rate-determining conversion to an ion pair. The second mechanism pictures Bt2 attack on a reversibly formed ion-pair intermediate. The third mechanism postulates collide of a ternary complex tiiat is structurally similar to the initial charge-transfer complex but has 2 1 bromine alkene stoichiometry. There are very striking similarities between the second-order and third-order processes in terms of magnitude of p values and product distribution. In feet, there is a quantitative correlation between the rates of the two processes over a broad series of alkenes, which can be expressed as... 

Table 2 Formation constants of bromine-alkene charge-transfer complexes.

In accordance with an earlier suggestion by Olah and Hockswender,240 the initial 1 1 adducts have been shown to be n complexes572 and a second species, a 2 1 bromine-alkene complex was directly observed.573 In the course of the bromination of crowded alkenes, such as ( )-2,2,3,4,5,5-hexamethylhex-3-ene (46), the bro-monium ion intermediate was shown to be reversibly formed.574 However, steric hindrance prevents the formation of the usual dihalogenated product. Instead, it loses a P proton to give an allylic bromo derivative, which is then dehydrobromi-nated to diene 47 ... 

Bromination.2 This bromine-crown ether complex, like dioxane-bromine (5, 58), can brominate alkenes, but the stereoselectivity is greater than that with free bromine and is less sensitive to solvent effects. Thus, bromination of trans-ifi-methylstyrene with DBC Br2 occurs exclusively by anti-addition and bromination of dr-/J-methylstyrene occurs by anti-addition to the extent of 95-100%. The bromine complex of polydibenzo-18-crown-63 is a particularly useful reagent because it can be packed as a slurry in a chromatography column. The alkene is then placed on the column and eluted with CC14. 

Several chlorinated and brominated alkenes are present in emissions of Mt. Etna, Vulcano, Kuju, and Satsuma-Iwojima (217). In addition to vinyl chloride (10), TCE, PERC, and 1,1-dichloroethylene, compounds 11-57 were detected in these emissions. For many of these compounds, exact structures remain unknown. 

Allylic bromination alkene ----------- NBS + free-radical initiator... 

The electrogenerative mode has also proved successful in direct and indirect electrocatalytic brominations and fluorinations under mild conditions. Vaporized bromine in nitrogen was used at the cathode to brominate alkenes at the anode, in a process similar to the chlorination above (47). Since bromine can form polyhalogen ions in solution. Bra , Br, , which could react nonelectrochemically, the electrolyte was flowed over the catalyst to remove such ions. As with direct chlorination, bromohydrins and dibromoalkanes were formed at platinum anodes. 

A stirred suspension of benzoic acid and 6-nitro-l-(2-nitrophenylsulfonyloxy)benzo-triazole in dry dichloromethane treated slowly with triethylamine at 0°, and stirred at room temp, for 2 h - l-benzoyloxy-6-nitrobenzotriazole (Y 88%), in ethanol at 0° treated with NaBH4 over 15 min, and stirred for 2 h at room temp. - benzyl alcohol (Y 80%). The method is simple, mild, and leaves a number of functional groups (nitro, bromine, alkenes) unaffected. F.e. and via 3-acyl-6-nitrobenzotriazole 1-oxides s. T. Okawara et al., Chem. Pharm. Bull. 36, 3628-30 (1988). 

The iodination and bromination of vinylamides have been achieved using rhodium catalysts and NIS (Scheme 7.113) [178], The reaction occurred under mild conditions and generally afforded excellent yields of the iodinated and brominated alkenes. The process was highly regioselective for the formation of the p-substitution product. Control reactions revealed that the rhodium catalyst was critical to the success of the reaction as its removal leads to poor yields and mixtures of products. 

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