Olefins methoxy-bromination

A number of methods of bromine production in situ have been reported. A regioselective and stereoselective methoxy-bromination of olefins has been accomplished using LiBr and (diacetoxyiodo)benzene as oxidants [3]. Zinc bromide... 

The bromination products of a-methylstilbenes, XC6H4C Me= C HC6H4Y, have been extensively studied in methanol as a function of X and Y (Ruasse and Argile, 1983). In agreement with kinetic data, which establish that the intermediates are exclusively tertiary and/or secondary bromocarbocations, the bromination of these olefins is completely regio-selective but hardly stereoselective at all. The two diastereoisomeric di-bromides and methoxy-... 

Allylic and vinylic ethers via methoxytelluration of olefins (general procedure) To a solntion of phenyltellnrinm tribromide (2.22 g, 5 mmol), prepared from diphenyl diteUnride and bromine in methanol (5 mL), is added the olefin (10 mmol) and the mixture refluxed for 1 h. On cooling, the ()3-methoxy)alkyl or cycloalkylphenyltellnrinm dibromide precipitates and is separated by filtration. 

Preussomerin I 697 and ( )-preussomerin G 698 were obtained from 620 with a five- and six-steps sequence in 15% and 12% overall yield, respectively, through modifications of substituents of the dioxocin ring. Thus, attack of lithium methoxide from the less hindered face of the enone 620, followed by protection of the phenolic oxygen as its methyl ether provided the methoxy adduct 692. The ketone 693 was obtained through a benzylic bromination-solvolysis-oxidation protocol, which required only a single purification. The C(2)-C(3) olefin was introduced by selective silylation of the C-l carbonyl of diketone 693 and oxidation of the silyl enol ether with Pd(OAc)2. Enone... 

Electrophilic additions also occur to cumulated dienes (allenes). Caserio and co-workers established that the mechanisms of both the oxy-mercuration reaction and the electrophilic addition of bromine to allenes are similar to the corresponding additions to olefins. The oxymercuration of ( )-(—)-2,3-pentadiene (82) in methanol produced 83% of (S)-traMS-3-acetoxy-mercuri-4-methoxy-2-pentene (83), confirming the anti pathway for the addition. Also formed was 17% of cis-3-acetoxymercuri-4-methoxy-2-pentene (84), which was presumed to have the R configuration. As shown in Figure 9.48, the product ratios can be explained on the basis of preequilibrium formation of mercurinium ions resulting from attachment of mercury to either the top or the bottom of one of the double bonds. Subsequent rate-limiting attack of methanol on the mercurinium ions is easier for the pathway... 

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