1- bromo-1-alkenes

E-1,4-bis(trimethylstannyl)-1,3-butadiene 174 bis(triphenylphosphane) palladium(O) 100 bombykol 64, 65 a-bond metathesis 37 9-borabicyclo[3.3.1]nonane 49 boron-zinc exchange 389, 396 (5)-(/ )-BPPFA 29 Bredt s rule 243 bromide-zinc exchange 404 1,4-bromoacetoxylation 366 1 -bromo-1 -alkenes 63 bromoboration of acetylene 51... 

In the chromium-mediated reaction of 1-fluoro-l-bromo-l-alkene with an aldehyde, the ( )-isomer reacts more quickly than the (Z)-isomer. Therefore, when a mixture of stereoisomer 42a was subjected to the reaction with benzaldehyde in the presence of CrCl2 and Ni catalyst, a (Z)-isomer of allylic alcohol 51 was selectively formed [77, 78] (Scheme 21). The reaction proceeds through an alkenylchromium species 52, and the formation of the ( )-alkenylchromium species is much faster than that of the (Z)-isomer. Actually, the reactimi of a pure (Z)-isomer of 1 -fluoro-1 -bromo-1-alkene with aldehyde is sluggish, and the corresponding ( )-isomer of the product was obtained in low yield or not obtained at all. Therefore, a good method for the synthesis of ( )-51 has not yet been reported. 

Sonogashira, K., Tohda, Y., Hagihara, N. Convenient synthesis of acetylenes. Catalytic substitutions of acetylenic hydrogen with bromo alkenes, iodo arenes, and bromopyridines. Tetrahedron Lett. 1975, 4467 470. 

Damborsky J, Berglund A, Kuty M, Ansorgova A, Nagata Y, et al. Mechanism-based quantitative structure-biodegradability relationships for hydrolytic deha-logenation of chloro- and bromo- alkenes. Quant Struct-Act Rel 1998 17 450-8. 

Homolytic cycUzation of a bromo-alkene yields dthydropyrroles but bromo-alkynes by the same mechanism form a mixture of pyrrole and 3-methyIene-pyrrole in high yield (the ratio is not stated) [3434]. Aryl radicals produced by the action of samarium di-iodide on an aryl Iwomide react with an o-propynyl-amino (or propynyloxy—see Section 1.3) side-chain to form a new fus pyrrole (or furan) under mild conditions. The corresponding allylamino bromides give... 

LUMO mixing. The photochemical C-Br fission in the bromo-alkenes (217) continues to be of synthetic value. Thus Irradiation of (217) in a nixed solvent, water / methylene dichloride, with a phase transfer agent and potassium isoeyanata affords the isoquinollnones (218). The reactions are quite... 

Bromodesilylation. Alkenylsilanes are converted into the corresponding bromo-alkenes by NBS/... 

Introduction of a triple bond into a pre-existing carbon skeleton is almost always effected by a method that starts from compounds containing an ethylenic bond or proceeds through such compounds as intermediates.177 The reaction most commonly used is dehydrohalogenation of appropriate halogen compounds, namely, either of chloro- or bromo-alkenes (1 or 2) or of dichloro-or dibromo-alkenes (3, 4, or 5) carrying the two halogen atoms on vicinal or the-same carbon atom. 

Lithium n butyl (phenylthio) cuprate has been used in nucleophilic substitution reactions of arenesulfonyl fluorides, al-lylic acetates, 9 BBN, propargyllc carbamates, and bromo-alkenes, as well as in nucleophilic additions to acetoxy-epoxides. It Is a good choice for 1,4-addItIon of an n-Bu group, having been used in 1,4 addition-elimination reactions of a-oxoketene dithloacetals and 3-halo-2-cycloalkenones, and in tandem vicinal dialkylation reactions of 5-methyleneoxa-zolones and alkynes. A typical example Is the use of the reagentin the stereospecific synthesis of (, -2-heptenoicacidfrom acetylene (eq 1). ... 

Sonogashira, K. Tohda, Y. Hagihara, N. Convenient Synthesis of Acetylenes. Catalytic Substitutions of Acetylenic Hydrogen with Bromo Alkenes, lodo Arenes and Bromopyridines Tetrahedron Lett. 1975,15, 4467—4470. 

Dimethylzinc can be used to methylate bromo alkenes (Scheme 5-74). In the case shows, a selective tram-coupling was followed by an alkylation. It should be noted at this point that depending on the zinc species, retention or inversion of the double bond occurs. ... 

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]. 

Heck type vinylation of 4-bromo-l-(4-methylphenylsulfonyl)-indole proceeds in good yield with such alkenes as methyl acrylate, styrene and N-vinylphthalimide using Pd(OAc)2 (5 mol%) and tri-o-tolylphosphine as the... 

What alkene gives a racemic mixture of (2R 3S) and (2S 3R) 3 bromo 2 butanol on treat ment with Br2 in aqueous solution" Hint Make a molecular model of one of the enantiomeric 3 bromo 2 butanols arrange it in a conformation in which the Br and OH groups are anti to one another then disconnect them )... 

Allylic brommations are normally carried out using one of a number of special ized reagents developed for that purpose N Bromosuccimmide (NBS) is the most fre quently used of these reagents An alkene is dissolved m carbon tetrachloride N bromo succimmide is added and the reaction mixture is heated illuminated with a sunlamp or both The products are an allylic halide and succimmide... 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

S-methyl dithiocarbonate (76S413). Stereoselective isomerization of 1,2-disubstituted alkenes may be achieved by a sequence such as the following fr<2n5-alkene bromo-hydrin -> /3-hydroxythiocyanate -> cw-thiirane -> cw-alkene (75TL2709). 

Irradiation of the A-bromo- or N- chloro-azetidin-2-ones (71) in the presence of alkenes, alkynes or radical donors induces rearrangement to the /3-haloalkyl isocyanates (72) via a... 

When 2-bromo-2-methylpentane is dissolved in DMF, the formation of 2-methyl-1-pentene (A) and 2-methyl-2-pentene (B) occurs. The ratio of alkenes formed is not... 

Predict the effect on the 1-butene Z-2-butene E-2-butene product ratio when the E2 elimination of erythro- i-deuteno-lAxomobutaae is compared with that of 2-bromo-butane. Which alkene(s) will increase in relative amount and which will decrease in relative amount Explain the basis of your prediction. 

The coupling of bromo- or iodobenzene to styrene yields regioselectively a mixture of E- and Z-stilbenes 12 and 13. An electron-withdrawing substituent at the olefinic double bond often improves the regioselectivity, while an electron-donor-substituted alkene gives rise to the formation of regioisomers. 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo-2-methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures ... 

Another method for preparing alkyl halides from alkenes is by reaction with jV-brotnosuccinimide (abbreviated NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the allylic position—the position next to the double bond. Cyclohexene, for example, gives 3-bromo-cyclohexene. 

In addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Because the unpaired electron is delocalized over both ends of the nr orbital system, reaction with Br2 can occur at either end. As a result, allylic bromination of an unsymmetrical alkene often leads to a mixture of products. For example, bromination of 1-octene gives a mixture of 3-bromo-l-octene and l-bromo-2-octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Although anti periplanar geometry is preferred for E2 reactions, it isn t absolutely necessary. The deuterated bromo compound shown here reacts with strong base to yield an undeuterated alkene. Clearly, a svn elimination has occurred. Make a molecular model of the reactant, and explain the result. 

Assume that you are carrying out the base-induced dehydrobromination of 3-bromo-3-methylpentane (Section 11.7) to yield an alkene. How could you use 1R spectroscopy to tell which of two possible elimination products is formed ... 

Bromohydrin (Section 7.3) A 1,2-disubstituted bromo-alcohol obtained by addition of I lOBr to an alkene. 

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

CHAIN ELONGATION OF ALKENES via gem-DIHALOCYCLOPROPANES l,l-DIPHENYL-2-BROMO-3-ACETOXY-l-PROPENE... 

The effect of the solvent on the product distribution is observed in the conjugate addition of amines to 1-bromo-l-(phenylsulfonyl)alkenes (54) (equation 54)46. When the reaction is conducted in benzene at room temperature for 4 days, the adduct 55 is formed in good yield. On the other hand, the reaction in DMSO at 80-90 °C for 2.5 h affords 2-(phenylsulfonyl)aziridines (56) and no adduct (55) is isolated. 

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