Bromine olefins with

The aHphatic iodine derivatives are usually prepared by reaction of an alcohol with hydroiodic acid or phosphoms trHodide by reaction of iodine, an alcohol, and red phosphoms addition of iodine monochloride, monobromide, or iodine to an olefin replacement reaction by heating the chlorine or bromine compound with an alkaH iodide ia a suitable solvent and the reaction of triphenyl phosphite with methyl iodide and an alcohol. The aromatic iodine derivatives are prepared by reacting iodine and the aromatic system with oxidising agents such as nitric acid, filming sulfuric acid, or mercuric oxide. 

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

Bromine-catalyzed Aziridination of Olefins with Chloramines... 

Oxyselenation is a powerful tool for functionalization in synthetic organic chemistry. Electrochemically generated [Br]+ or bromine reacts with (PhSe)2, producing PhSeBr. Attack of PhSeBr on the olefin (25) gives a selenated intermediate followed by solvolysis to provide (26) and a bromide ion (Scheme 11). A typical electrolysis is carried out in a MeOH-Et4NBr/H2S04 (trace)/(PhSe)2-(Pt) system in an undivided cell. Some results... 

Table 7.7 Relative Rates of Bromination of Alkyl-Substituted Olefins with Molecular Bromine in Methanolic Sodium Bromide at 25°C ...

Although the reaction in Scheme 10 is a highly efficient procedure, a two-step process was required to prepare aziridines from olefins. Two more convenient methods for the one-step aziridination using CT were discovered by the authors in 1998, one of which involves the iodine-catalyzed aziridination of unfunctionalyzed olefins with CT trihydrate [7b] (Scheme 11). The bromine-catalyzed aziridination of unfunctionalyzed olefins and allylic alcohols with anhydrous CT was reported at the same time [7c], though in this case phenyltrimethylammonium tribromide (PTAB), and not Br2, was used as a catalyst (Scheme 12). These two reactions are applicable to a wide range of olefins, and are considered to proceed by almost the same pathway. 

Isopropyl anisole (171) was converted to bromide (172) by metalation, formylation and bromination. Alkylation with cyclopropyl ketoester produced (173) whose transformation to alcohol (174) was achieved by saponification, decarboxylation and reduction.. Its conversion to homoallylic bromide (175) was accomplished by the method of Julia et al. [56]. Alkylation of ethyl acetoacetate with bromide (175) furnished p-ketoester (176). It was subjected to cyclization with stannic chloride in dichloromethane. The resulting tricyclic alcohol provided the olefinic ester (177) by treatment with mesylchloride and triethylamine. Epoxidation followed by elimination led to the previously reported intermediate (146) whose conversion to triptolide (149) has already been described. 

The phenyl tellurium tribromide can be prepared in situ from diphenyl ditellurium and bromine. Olefins thus far investigated include isobutene, (E)- and (Z) -butene, 1 -hexene, 2-methyl-l-pentene, 1-octene, (E)- and (Z)-4-octene, 1-decene, phenylethene, 1-phenyl-I-methylethene, cyclopentene, cyclohexene, cycloheptene, and cyclooctene. Most of the reactions were carried out with phenyl tellurium tribromide in methanol. 

Several conjugated diolefins have been made by heating bromo olefins with solid potassium hydroxide or excess quinoline. In the latter case, the bromo olefins were made available by allylic bromination of olefins with N-bromosuccinimide. /S-phenylbutadiene is obtained in 46% yield by the action of pyridine on the corresponding secondary chloride. Chlorination of n-butyl chloride gives an isomeric mixture of dichlorides from which low yields (18-30%) of butadiene are obtained by passing the vapors over soda lime at about 700°. ... 

It is known that pyridinium tribromide converts olefins with specific r/7 t/-dibromina-tion. Therefore, the v/c-dibromides thus obtained can be cathodically reduced to the original olefins without geometrical changes of molecular structure [218]. This stereospecific bromination-debromination may be useful as an olefin protection-deprotection method. 

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