2- Bromo-l-alkenes

There are numerous examples of the addition of HBr to alkynes.10 Addition to acetylene is difficult, tends to produce mixtures of bromide products, and requires a catalyst.94,10 1 22 Simple terminal alkynes react with HBr in the absence of peroxides to produce mixtures of 2-bromo-l -alkenes and geminal dibromides (equation 110).85,159 The latter can be produced in high yield when excess HBr is used. 60... 

Recently, two very useful procedures for the conversion of terminal alkynes to 2-bromo-l-alkenes have been reported (equations 123 and 124).173,174 Both procedures accommodate alcohol groups as well. 

The addition of HBr to alkynes, as to alkenes, can be so directed that it leads (a) to 2-bromo-l-alkene or 2,2-dibromoalkane or (b) to 1-bromo-l-alkene or 1,2-dibromoalkane ... 

Bromo-l-alkenes. Hydrogen bromide adds to terminal acetylenes to form 1-bromo-l-alkenes. However, it adds to 1-trimethylsilyl-l-alkynes to form 2-bromo-l-alkenes. Although the reaction is a free radical reaction, a peroxide initiator is not required and may be deleterious. Trimethylsilyl bromide is the... 

Bromo-l-alkenes can be prepared regioselectively and in high yield by direct hydrobromination of terminal acetylenes with tetraethylammonium hydrogen dibromide in dichloromethane. The reagent, which is prepared in situ from tetraethylammonium bromide and hydrogen bromide, also hydro-brominates acetylenic alcohols and ethers. 

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

IKROMOHYDRINS FROM ALKENES AND N-BROMO-SUCCINIMIDE IN DIMETHYL SULFOXIDE erythro-2-BROMO-l,2-DIPHENYLETHANOL... 

Trisubstituted alkenes. The (Z)-2-bromo-l-alkenylboranes (1), obtained by bromoboration of 1-alkynes with BBr, (13, 43), undergo coupling with organozinc chlorides in the presence of Cl2Pd[P(C6H5)3]2 to provide, after protonolysis, disub-stituted alkenes (3). However, the intermediate alkenylborane (2) can undergo a... 

The addition of bromine monofluoride to alkenes using equimolar quantities of /V-bro-moamides in excess anhydrous hydrogen fluoride gives the expected l-bromo-2-fluoroalkanes and small amounts (3-8 %) of isomeric 2-bromo-l-fluoroalkanes.29 1-Bromo-2-fluoroheptane can be prepared in 60-77% yield from hept-l-cnc, /V-bromoacetamide and hydrogen fluoride in diethyl ether at — 78"C.30 The bromofluorination of methyl methacrylate (1) with 1,3-di-bromo-5.5-dimethylhydantoin (DBH) and liquid hydrogen fluoride exemplifies this procedure.31... 

In accordance with the Markovnikov rule, terminal alkynes are converted to 2-halo-l-alkenes. The reagents prepared in the reaction of anhydrous HF with organic bases and used in hydrofluorination of alkenes can add to the carbon-carbon triple bond producing Markovnikov products.94-96,98 99 However, 1-bromo-l-alkenes and... 

In the presence of peroxides, 1-bromo-l-alkenes or 1,2-dibromoaIkanes are produced depending on the amount of HBr employed (equation 111).83-139 60... 

Before discussing examples of the reactions, it should be pointed out that most types of vinylic halides used in this reaction are easily available. The UV-catalyzed addition of HBr to terminal acetylenes forms the 1-bromo-l-alkenes. The cis isomer is formed almost exclusively if the addition is carried out at dry ice temperature. The 2-bromo or 2-iodo-l-alkenes are obtained from reacting aqueous hydrogen halides with alkynes. The 2-substituted-1-bromo-l-alkenes are available by the bromination-base dehydrobromination reactions. 

Z)-2-bromo-l-phenylthio-l-alkenes to afford (E)- and (Z)-vinyl sulfides, respectively. 

The (E)-bromides in 1,1-dibromo-l-alkenes can be stereoselectively coupled with aryl- or al-kenylboronic acids to give the corresponding (Z)-l-aryl(or alkenyl)-l-bromo-l-alkenes. Tris(2-... 

In the previous section we have discussed that 1-halo-1-alkenes can be conveniently synthesized. However, with these methods it is not possible to synthesize 2-halo-l-alkenes. Although the halometallation reaction would be a powerful tool for the preparation of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose 183). Recently, it has been reported that B-bromo-9-borabicyclo[3.3.1]-nonane (B-Br-9-BBN) and B-iodo-9-borabicvclo[3.3.1] nonane (B-I-9-BBN)1841 react with 1-alkynes, stereo-, regio- and chemoselectively, and after protonolysis, 2-halo-1-alkenes are obtained in excellent yields (Eq. 118)1851. 

In an approach similar to the Naso alkene synthesis described earlier, Suzuki has reported the stereospecific synthesis of alkenes from ( )-(2-bromo-l-alkenyl)dibromoborane by sequential cross-coupling reactions (equation 21). The reaction sequence can be carried out in one pot, without isolation of the intermediate. Transmetalation from boron to palladium occurs only after base has been added. 

Chloroperoxidase (CPO) has been used to enantioselectively epoxidize (o-bromo-2-methyl-l-alkenes (i.e., 40), the course of which is highiy dependent upon the chain length of the substrate <97JA443>. 

Most studies of bromine addition to alkenes have presumed that the intermediate proceeds to product and does not revert to alkene and bromine. Brown and co-workers determined that bromonium ions generated from the solvolysis of the fra s-2-bromo-l-brosylates of wclohexene or cyclopentene could react with added Br to produce Br2- Furthermore, erythro-2-hTomo-l, 2-diphenylethanol was foimd to react with anhydrous HBr (in 1,2-dichloroethane or chloroform) to produce both frans-stilbene and meso-l,2-dibromo-l,2-diphenylethane. The reaction of the erythro diastereomer can be explained by a mechanism involving anchimeric assistance in departure of water, which leads to a bromonium ion that reverts to the stilbene, as shown in Figure 9.10. ... 

One way to test for the intermediacy of carbocations in reaction mechanisms is to look for rearrangements, for example, from a 2° carbocation to a 3° carbocation. In the addition of bromine to 3,3-dimethyl-l-butene (7) in methanol, however, the only products observed were l,2-dibromo-3,3-di-methylbutane (8), 45%, and 2-bromo-l-methoxy-3,3-dimethylbutane (9), 44%. There was no evidence for products such as 10, which might have been expected if a free 2° carbocation were formed and then rmderwent a methyl shift to yield a 3° carbocation. Therefore, the intermediate in the addition of bromine to alkyl-substituted alkenes appears not to behave like a carbocation. 

While inter- and intramolecular Diels-Alder reactions normally require electron-deficient dienophiles, the 6it-electrocyclization proceeds with a large variety of substituents on a hexatriene. In one such approach, the intramolecular Heck-type reaction of a 2-bromo-l-en-(intermolecular Heck coupling with an alkene to form the conjugated 1,3,5-hexatriene 81, which eventually cyclizes in a 6it-electrocycHc process (Scheme 8.21) [319], In many cases, aromatization of the primarily formed cyclohexadiene 82 occurs to yield carbo- and heterobicyclic compounds of type 85 [319a). But with alkyl ethenyl ethers, the cyclohexadienes 83 can be obtained in moderate yields [319b]. 

Retrosynthetic analysis of nitrile 164 disconnects the C-CN bond because it is clear that the six carbons of the methylcyclopentene starting material are more or less intact in the remainder of the molecule. This disconnection requires a C-C bond-forming reaction involving cyanide. Because cyanide is associated with a carbon nucleophile, assign Cj to the cyanide and to the cyclopentene carbon. The synthetic equivalent for Cg is an alkyl halide, and 2-bromo-l-methylcyclopentane (168) is the disconnect product. Bromide 168 is obtained directly from the alkene starting material, but it requires the use of a radical process to generate the anti-Markovnikov product (see Chapter 10, Section 10.8.2). 

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