Bromide possible mechanisms

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

This type of coupling reaction is extended a further step to a doublecoupling reaction if 4 moles of vinylmagnesium bromide and 1 mole of titanium tetrachloride are combined (34). A possible mechanism for this reaction is summarized in Fig. 5. 

Scheme 5. Possible mechanism for the synthesis of carba-zoles from 2-chloroanilines and aryl bromides (ring substituents omitted for clarity).

The possible mechanism for the synthesis of alkenyl bromides involves the usual trans-addition followed by case induced nms-elimination 1741 as illustrated in Eq. 110. 

Lemieux has given a detailed account of a possible mechanism for this reaction. It is suggested that absorption of a 1,2-cis-poly-O-acetylglycosyl bromide on the silver chloride facilitates the formation of carbonium ions which are stabilized (immediately they are formed) by assuming a 1,2-cyclic structure. Reaction of this cyclic intermediate with chloride ion then yields the 1,2-[Pg.220]

Another possible mechanism starts with nucleophilic reaction by 3-63 at the epoxide. The alkoxide ion then displaces bromide to produce a new epoxide. 

The paper discussed possible mechanisms for nucleophilic substitution at a cyclopropane carbon with particular attention to retentive replacements. From the NMR spectra of the syn- and anti-bromides and acetates shown above, it was concluded that smooth stereospecific substitution of bromide by acetate had taken place under typical Sis 2-type conditions. For the anti-pair, comprising two solids (one member of the syn pair is an oil), this conclusion was confirmed by X-ray structure determination of both educt and product. 

Allyl chloride and j3-phenylallyl bromide have been converted into ailyl azide and )3-phenyIallyl azide, but to differentiate between possible mechanisms involved in these reactions labelling studies would be necessary. The primary and secondary allylic chlorides, geranyl chloride and a-butylallyl chloride appear to undergo normal... 

It has been suggested that the oxidation of alcohols is catalyzed by a small amount of bromide ion remaining in [emim][BF ]. A possible mechanism is described that... 

Reaction of ethyl diazoacetate with sr-allylnickel bromide (2, 291). Ethyl diazoacetate reacts with 7r-allylnickel bromide (0°, ether) with evolution of nitrogen to give ethyl tranr-/3-vinylacrylatc (69% yield), ethyl cw-/3-vinylacrylate (19% yield), and ethyl A4-pentenoate (8% yield).1 A possible mechanism is suggested based on... 

Another possible mechanism involves halide-assisted protonation. The electrostatic effect of a halide anion can facilitate proton transfer. The key intermediate in this mechanism is an ion sandwich involving the acid anion and a halide ion. Bromide... 

Similarly, the oxidation of benzoylthiourea by bromine yields, in the first place, dibenzoyldithioformamidine dihydrobromide (50). On attempted recrystallization, this is converted, with loss of the elements of ammonium bromide to 5-benzoylamino-3-benzoylimino-3//-1,2,4-dithiazole (51) together with small quantities of 3,5-bis(benzamido)-l,2,4-thiadiazole. The preferential loss, from 50, of ammonium bromide rather than sulfur was ascribed to the influence of the benzoyl groups, and possible mechanisms were discussed.66... 

Based on solvent isotope effects and on the sensitivity of reaction rates to solvent, it has also been proposed that solvent molecules can provide nucleophilic assistance to the conversion of the CT complex to thebromonium ion, as shown in Figure 9.8. This process is analogous to that proposed for one possible mechanism (Figure 9.9) for the bromide-catalyzed addition to highly deactivated alkenes such as cinnamic acid. ... 

The possible mechanism for present investigation involves the nucleophilic attack of a bromide ion at the less substituted C-3 position of the spiro epoxide leading to the formation of an intermediate (II and 12). Further, as sulfur is abetter nucleophile than nitrogen, in the next step cyclization leads to formation of an oxathiolane ring (Scheme 8.57). 

The following is a possible mechanism for a reaction involving hydrogen peroxide in aqueous solution only a small amount of sodium bromide was added to the reaction mixture ... 

The reaction of azadiene 382 with difluorocarbene gives fluoropyrrole 385 as a major product. Difluorocarbene was generated by redaction of dibromodifluoro-methane with active lead in dichloromelhane in the presence of tetrabntylanunonium bromide under ultrasound irradiation. A possible mechanism includes the formation of difluoroazomethine yhde 383, 1,5-cyclization of the latter into diflnoropyrroline 384, and subsequent HF elimination [125]. 

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