Pyridines bromination, mechanism

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

General Method of Preparation of Tin Triphenyl Halides — Tin tetraphenyl (85-4 grams) is dissolved in 700 grams of hot pyridine with mechanical stirring and the solution rapidly cooled in ice, so that a very fine suspension of the compound is produced. At a temperature of —48 C.j 32 grams of bromine in 150 grams of pyridine are added in 6... 

Two surprising observations were made in the course of this work first that the enol acetate (5) is stable under the conditions for formation of (6) from (4) second, that the course of the buffered bromination of (5) depends on the conditions used. Thus, in the presence of epichlorohydrin, (7) is the sole isomer produced, whereas in pyridine-acetic acid approximately equal amounts of (7) and (8) are formed. It was suggested that this difference is inherent in the mechanism and not a result of isomerization of (7) to (8) during the course of the reaction. 

Current views (100) on the mechanism of bromination by NBS invoke the formation of molecular bromine and bromine atoms in low concentration, which subsequently act as the brominating agent. The bromination reaction was studied in detail in this laboratory under a variety of conditions using 93 (R = Ms) as a model. The product 94 (R = Ms) was indeed formed (42%) when NBS was substituted by 1.1 equivalents of bromine which was added at a slow rate to the reaction mixture. The yield was 68% when benzoyl peroxide was used as a catalyst. Using NBS alone or in the presence of reagents such as barium carbonate, pyridine, or s-trinitrobenzene, the yield was 60-70%. 

These results have been rationalized74 by Heasley and coworkers by assuming that the primary function of the complexes is to limit the concentration of free halogen. In the reaction of free bromine where the reaction is second order in bromine, two or more molecules of halogen participate in the transition state while the halogen complexes with pyridine or amines impose a first-order mechanism by limiting the availability of free halogen (equation 40). 

The rate law for the oxidation of [Ru(NH3)5(FlL)] + (HE = isonicotinamide) by I2 in acidic solution contains two terms, one depending on P2] and one depending on [I3 ] and [Ru complex]. An outer-sphere electron-transfer mechanism is proposed for each term. Reduction of [Ru (NFl3)5L] + (TIL = nicotinamide or isonicotinamide) to [Ru (NH3)5L]+ is accompanied by an isomerization from the amide-bonded L to pyridine-bonded FIL. Bromine oxidation of... 

Q Suggest a possible mechanism for the conversion of coumarin into 3-bromocoumarin by the action of bromine and then pyridine. 

Certain convenient brominating agents, such as pyridine hydrobromide perbro-mide (PyHBr3) and tetramethylammonium tribromide, may be used to transform alkenes to vicinal dibromides. They often give better yields than does liquid bromine, but may react by different mechanisms. Isomeric 1-phenylpropenes, for instance, react nonstereoselectively with bromine, but exhibit near-exclusive anti addition with the other reagents 244... 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

Figure 4-5. Reaction mechanism for the derivatization followed by reductive cleavage based on Lu and Ralph (1997). Reaction of the lignin with acetylbromide (AcBr) results in the acetylation of the y-carbon, while the a-carbon is brominated. Zinc (Zn) catalyzes the cleavage of the ether bond between the P-carbon of one residue and the 0-4 position of the adjacent residue. The resulting monomer is acetylated with acetic anhydride (Ac20) and pyridine (Py). R can be a proton or an aryl group. In H-residues R3 and R5 are protons, in G-residues R3 is a methoxyl group and R5 is a proton, whereas in S-residues both R3 and R5 are methoxyl groups. The wavy bonds indicate that both the S- and R- (4.26, 4.27) or E- and Z-stereo-isomers (4.28, 4.29) are present.

---