Bromination neighboring groups

The bromine atom is much more selective than the chlorine atom. As indicated on page 906, it is often possible to brominate tertiary and benzylic positions selectively. High regioselectivity can also be obtained where the neighboring-group mechanism (p. 899) can operate. 

A cyclic complex derivable by addition of positive bromine to a double bond has already been discussed as an example of the neighboring group effect. The same intermediate, except perhaps for the nature of the bonds to bromine, is formed in the addition of bromine to olefins and is responsible for the stereochemistry of the addition reaction and the nature of the by-products.232... 

Examples of neighboring group participation in isoxazole bromination have been reported. When the isoxazolylpropanol (80) was treated with bromine in pyridine, it gave the spiro (furan-2,5-isoxazole) (81) (74BAU2566). The corresponding propenoic acid gave 82 [76JCS(P1)1694], whereas a similar process accounts for the reaction of... 

The formation of the dimer 48 from 47 a differs from the formation of the dimer 43 of 41 in that 48 is produced even in the early stage of the oligomerization, whereas 43 is formed after the middle stage of the oligomerization. This difference suggests different mechanisms operative in the cyclodimerizations of 47a and 41. As illustrated in Scheme 10 [50], neighboring group participation of the axially oriented bromine atom of 47 a facilitates the formation of the cyclic dimer 48. 

In the oligomerization of 47 e, the neighboring group participation of the equatorially located bromine atom is impossible, unless the oxonium ion is unimolecularly ring-opened to form an oxacarbenium ion. The electron withdrawing bromine atom disfavors such an unimoleeular ring-opening reaction. Therefore, the cyclic dimer cannot be formed by a mechanism similar to that described for the cyclodimerization of 47 a. 

If a neighboring group is to form a bridged cation, it must have electrons to form the extra bond. These may be unshared pairs on atoms like sulfur, nitrogen, oxygen, or bromine electrons of a double bond or aromatic ring or even, in some cases, a electrons. 

A cyclopropylfluorosilane was formed in excellent yield on reaction of a l-(allyldimethyl-silyl)bicyclo[4.1. Ojheptane derivative 1 with a slight excess of bromine and then with pyridinium poly(hydrogen fluoride). The reaction appears to be sensitive to the nature of the neighboring groups and the quality of the fluorinating agent (see Section 5.2.5.1.9.2.). ... 

Another aspect of the mechanism is the reversibility of formation of the bromonium ion. Reversibility has been demonstrated for highly hindered alkenes, and attributed to a relatively slow rate of nucleophilic capture. However, even the bromonium ion from cyclohexene appears to be able to release Br2 on reaction with Br. The bromonium ion can be generated by neighboring-group participation by solvolysis of frfln -2-bromocyclohexyl triflate. If cyclopentene, which is more reactive than cyclohexene, is included in the reaction mixture, bromination products from cyclopentene are formed. This indicates that free Br2 is generated by reversal of bromonium ion formation. Other examples of reversible bromonium ion formation have been found. " ... 

The ethyl 2,3,4,()-tetra-0-acetyl-l-thio-a- and d-D-glucopyranosides react with bromine with net inversion, and the product from the a-D anomer is the unstable tetra-0-acetyl- 8-D-glucopyranosyl bromide, A detailed mechanistic study of the reaction indicated that the net steric course is, in other instances, dependent on conformational factors and neighboring-group effects, and steric hindrance when bulky groups are... 

Bromination of Olefins. In the Corey synthesis of epibati-dine the cyclohexene (19) reacts with NBS to give bromination with neighboring group participation, producing 20 (eq 47). This reaction has been studied in detail by Vasella. ... 

There is direct evidence for the existence of halonium ions such as those that are invoked in bromination and chlorination reactions. They can be formed from dihalides by ionization with neighboring-group participation and can be observed by NMR measurements in nonnucleophilic solvents ... 

Scheme 7.21. A representation of the addition of radiolabeled bromine (indicated as Br2> to 3-bromo-l-propene (allyl bromide, H2C=CHCH2Br). It is important to note that the initially formed bromonium ion is capable of being opened by internal (backside) attack from the bromine atom on the neighboring carbon. This is an example of the process called neighboring group participation (this chapter, Rearrangement). In this particular case, the newly formed bromonium ion is, except for the position of the label ( Br), the mirror image of the bromonium ion from which it was generated. Each of these bromonium ions is capable of being attacked by exogenous labeled bromide anion. Thus, depending on the specific bromonium ion, the final relationship of the two isotopically unique bromine atoms in the product (1,2,3-tribromopropane) will be either 1,2 or 1,3.

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