Hydrogen bromide reaction with dienes

In previous experiments, Bald et al. [2] had tried to liberate 131 from 3-bromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene (134) by /3-elimination of hydrogen bromide with KOtBu. Both the trapping products 140 of the intermediate with DPIBF and the enol ether 138 appeared to provide evidence in favor of 131, but did not exclude the cycloalkyne 136 [94]. The generation of 136 in the presence of DPIBF by two routes that cannot lead to 131 gave rise to cycloadducts different from 140, which were converted into 140 on treatment with KOtBu, however [95, 96], On the basis of these results and further investigations [97, 98], the formation of 131 from 134 is considered unlikely. Instead of 131, 136 seems to arise and to be the source of the products observed. In contrast, the phenyl derivative 137 of 131 is the obvious intermediate in the reaction of 135 with KOtBu, which furnished the enol ether 139 [2]. 

Secondary and tertiary propargyl alcohols are directly converted to halo-allenes on reaction with concentrated aqueous hydrogen halides in the presence of the corresponding cuprous halide [60,72-73]. (See Table VII.) Better yields are obtained with hydrogen bromide. Hydrogen chloride yields chloroallene, propargyl chloride, and the chloro- 1,3-diene isomers (Eq. 60). 

This procedure illustrates a recently published, simple, general method for the synthesis of conjugated dienes from olefins. The scope of the reaction is shown in Table I.5 In most of these examples hydrogen bromide. .elimination can be effected by stirring a solution of the olefin-bromo-methanesulfonyl bromide adduct in methylene chloride with one equivalent of triethylamine at room temperature. Only two equivalents of the more costly potassium tert-butoxide are then needed in the second elimination step the yields using the two-base procedure are generally superior to that obtained using only potassium tert-butoxide. 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

Allyl radicals substituted at only one of the terminal carbon centers usually react predominantly at the unsubstituted terminus in reactions with nonradicals. This has been shown in reactions of simple dienes such as butadiene, which react with hydrogen bromide, tetrachloromethane or bromotrichloromethane to yield overall 1,4-addition products . The reaction of allyl radicals with hydrogen donors such as thiols or tin hydrides has been investigated and reviewed repeatedly. In most cases, the thermodynamically more favorable product is formed predominantly. This accords with formation of either the higher substituted alkene or the formation of conjugated tt-systems. Not in all cases, however, is the formation of the thermodynamically more favorable product identical to overall 1,4-addition to the diene. In those cases in which allyl radicals are formed through reaction of dienes with tin hydrides or thiols, the... 

Cyclopropyl thien-2-yl ketone (34), available commercially, was subjected to a Grignard reaction with 21 to yield a carbinol (31) in excellent yield. Bromotrimeth-ylsilane-mediated opening of the cyclopropyl ring led to the 4-bromo derivative 32. Condensation with ethyl i -(-)-pipeildme-3-carboxylate (16) was conducted in isopropyl acetate as a solvent with anhydrous lithium carbonate as the base. The transformation was cleaner and did not provide any of the diene (26) arising from elimination of hydrogen bromide. The alkylation of the nipecotate residue could also be directly effected with a mixture of the tartrate salt of 16, lithium carbonate, and isopropyl acetate. Compound 33 was readily isolated as the hydrochloride isopropyl acetate was superior to all other solvents used in this reaction. Since it is not itself prone to hydrolysis, it is preferable to ethyl acetate. Moreover, it appears to crystallize hydrochlorides much better. 

RhCCeFsXPPhaXcyclo-octa-l.S-diene)], respectively the former product is less reactive than its iridium analogue (see below), but its reaction with hydrogen bromide results in cleavage of the CeFs—Rh bond to give Rh(H)Br2(CO)(PPha)2. ... 

Neooctams, such as the unsubstituted parent compound (191), are easily obtained in cycloadditions. Isocyanates [125,126], sulphonyl isocyanates (43) [29,127] and especially chlorosulphonyl isocyanate (189) [128] react with alkenes to form variously-substituted j3-lactams [129—135]. Synthesis of compound (191) is an example of this reaction which has been discussed in detail [52]. The same compound (192) can be obtained in the photolytic rearrangement of the pyrazolone (194) [136]. Compound (196) is neooctam-5,7-diene, obtained from the azocine (195) and hydrogen bromide [137]. Lactam (196)is a partly saturated form of (199). The latter is an unstable neooctam derivative, formed in an intramolecular ketene-imine cycloaddition reaction (see (198))... 

Free-radical Reactions.—Publications have appeared dealing with the bidirectional addition of trifluoroiodomethane and hydrogen bromide across the C=C bond in the olefin CF3 CH CHMe, peroxide-initiated addition of 1,2-dibromotetrafluoroethane to ethylene, propene, and isobutene, the addition of pentafluoroiodoethane to 3,3,4,4-tetrafluorohexa-l,5-diene (see p. 29), peroxide-initiated cyclodimerization of 3,3,4,4-tetrafluoro-4-iodobut-l-ene (see p. 29), and telomers from tribromofluoromethane or tetrabromomethane and bromotrifluoroethylene as high-density fluids for gyroscope flotation. The telomerization of chloromethanes with tetra-fluoroethylene provides a measure of the relative reactivity for both chlorine and hydrogen abstraction by the CF2 CF2 radical. The chain-transfer... 

In the absence of a nucleophile, the reaction of allenes with aryl bromides provides 1,3-dienes in good yield (Scheme 16.27) [32], The reaction is very sensitive to the reaction temperature, solvent, base and amount of phosphine used. The formation of a 1,3-diene may be explained by either /3-hydrogen elimination or deprotonation at the a-carbon of the Jt-allylpalladium intermediate. 

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