C-Br bond formation

As C-Br bond formation occurs by back-side attack, inversion of the configuration at carbon is anticipated. However, both racemization and rearrangement are observed as competing processes.10 For example, conversion of 2-butanol to 2-butyl bromide with PBr3 is accompanied by 10-13% racemization and a small... 

One example of C-Br bond formation by cleavage of a C-Hg bond with bromine has been reported in moderate yield. ... 

Scheme 28 Chelate ring size-controlled oxidative addition - C-Br bond formation in bromination of Pt(II) diaryl complexes with Bt2...

Since C—Br bond formation occurs by back-side attack, inversion of configuration at carbon is anticipated. However, both racemization and rearrangement can be... 

First, look at the reaction and identify the bonding changes that have occurred. In this case, a C—Br bond has broken and a C-C bond has formed. The formation of the C-C bond involves donation of an electron pair from the nucleophilic carbon atom of the reactant on the left to the electrophilic carbon atom ol CH Br, so we draw a curved arrow originating from the lone pair on the negatively charged C atom and pointing to the C atom of CH3Br. At the same time the C—C bond forms, the C-Br bond must break so that the octet rule is not violated. We therefore draw a second curved arrow from the C-Br bond to Br. The bromine is now a stable Br- ion. 

X-ray diffraction analysis of crystalline poly(schiff base)s and their low molecular models shows that the formation of molecular complexes is accompanied by an increase in interplanar distances and, in a number of cases, by complete amor-phization. Molecular complexes of poly(schiff base)s with Br2 decompose with time, because of the bromination of the donor components, forming C—Br bonds. Substitution of hydrogen by bromine in phenyl groups occurs only in cases in which these groups are not included into the main polymeric chain. 

Electrochemical methods allowed to shed light on the different reaction mechanisms, both in homogeneous and heterogeneous (Ag20 promoted) systems. Furthermore, electroreduction reverses the C-Br bond polarity, allowing the formation of a C-C bond with an electrophile (f.ex. CO2). 

The linear appearance of the plot shows that this reaction obeys a first-order rate law. Additional mechanistic studies suggest that alkene formation proceeds in a two-step sequence. In the first step, which is rate-determining, the C — Br bond breaks to generate a bromide anion and an unstable cationic intermediate, hi the second step, the intermediate transfers a proton to a water molecule, forming the alkene and H3 ... 

At first sight the formation of 147 from 145 might be explained by a mechanism in which stretching of the (C—Br) bond is coupled with a symmetry allowed... 

Figure 5. Attack of bromide ion on the 1-methylpropyl carbocation Attack from the top leading to S products is the mirror image of attack from the bottom leading to R product. Since both are equally likely, racemic product is formed. The dotted C-Br bond in the transition state indicates partial bond formation. 

Bromocyclopentitols and amino(or amido)bromocyclopentitols having a C—Br bond trans to two vicinal hydroxy groups show selectivity in base-promoted epoxide formation, e.g. (52) gives (53) by path a, rather than (54) by path... 

The nucleophilic hydroxide ion attacks the C atom in the bromoethane from the side opposite to the C-Br bond and begins to form a covalent bond with it. At the same time, the C-Br bond begins to break. A transition state is then reached in which the new 0-C bond is partially formed and the C-Br bond is partially broken. The reaction is completed by the formation of the full 0-C bond and the complete break-up of the C-Br bond. 

Thus, if the monochromator is set to a wavenumber of 650cm", the formation of the C—Br bond can be recorded as a function of time during a chemical reaction between bromine and an olefin. Buehler used this method to follow the bromine/octadecene reaction. It should be pointed out that the Raman scattering is a function of droplet size as well as composition, and unlike bulk Raman it is complicated by the morphological resonances... 

Mason 61) has suggested that this may take place in a single-step multicentre reaction. The positive end of the polarised Br2 molecule attacks the terminal carbon of the olefinic group (in both vinylphenyl and aUyl compounds) while the negative end adds to the gold. In both cases, in terms of shifts of electron density, this localises deviations from neutrality on the non-terminal carbon atom of the double bond. The formation of the Au—Br, Au—C and C—Br bonds and the breaking of the Br—Br bond occur simultaneously. 

In order for C-alkylation to occur, the p orbital at the a carbon must be aligned with the C—Br bond in the linear geometry associated with the SN2 transition state. When the ring to be closed is six-membered, this geometry is accessible, and cyclization to the cyclohexanone occurs. With five-membered rings, colinearity cannot be achieved easily. Cyclization at oxygen then occurs faster than does cyclopentanone formation. The transition state for O-alkylation involves an oxygen lone-pair orbital and is less strained than the transition state for C-alkylation. 

A more detailed picture could be obtained from the results of stereochemical studies.3111 Since 1,2-dimethylcyclohexene, 1,6-dimethylcyclohexene, and 2-methylmethylenecyclohexane give different proportions of cis- and tram-1,2-dimethylbromocyclohexanes, simultaneous formation of the C-H and the C-Br bonds was suggested.112 A similar conclusion was arrived at on the basis of the stereoselective, predominantly anti addition of HC1 and HBr to 1,2-dimethylcyclo-pentene in pentane.113... 

The cis isomer is not formed at all. To give the trans isomer, the two new C-Br bonds have to be formed on opposite sides of the double bond by antarafacial addition. But this is impossible by a one-step mechanism because the Br-Br bond would have to stretch too far to permit the formation of both C-Br bonds at the same time. 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

Tri-0-acetyl-2-amino-2-deoxy-a-D-glucosyl bromide hydrobromide (LXVa)169 170 is more stable than the V-acyl analogs, possibly because heterolysis of the C—Br bond with the generation of a carbonium cation proceeds less readily as a result of the presence of the neighboring—NH3 center. /3-u-Glycoside formation proceeds normally with alcohols,169 and treatment with an equivalent of sodium ethoxide under anhydrous conditions liberates170 the free base (LXVb). 

Due to the same compulsory anti-selectivity the potassium alkoxide-mediated HBr eliminations from trans-1,2-dibromocy clohexane ultimately results in the formation of 1,3-cyclohexa-diene rather than 1-bromocyclohexene (Figure 4.25). The reason is that an anh -selective elimination occurs initially, trans-1,2-Dibromocyclohexane—except in very polar solvents— prefers a chair conformation with axial C-Br bonds (as it is only in this conformation that the... 

A photochemical cyclization involving a phosphorus atom occurs in the reaction of diethyl 8-bromo-2, 3, 0-isopropylideneadenosine 5 -phosphite (212) upon irradiation in acetonitrile610. The primary step is homolysis of the C—Br bond and this is followed by intramolecular attack of the adeninyl 8-radical on the phosphite group with the formation... 

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