Alkenes from vinyl bromides

This reaction can be extended to di- and trisubstituted alkenes by use of substituted vinyllithium reagents. These can be prepared quantitatively from vinyl chlorides or bromides by reaction in ether with lithium powder (containing 2% sodium) or from vinyl bromides by halogen—metal exchange with an alkyl-lithium. 

Meerwein reactions can conveniently be used for syntheses of intermediates which can be cyclized to heterocyclic compounds, if an appropriate heteroatom substituent is present in the 2-position of the aniline derivative used for diazotization. For instance, Raucher and Koolpe (1983) described an elegant method for the synthesis of a variety of substituted indoles via the Meerwein arylation of vinyl acetate, vinyl bromide, or 2-acetoxy-l-alkenes with arenediazonium salts derived from 2-nitroani-line (Scheme 10-46). In the Meerwein reaction one obtains a mixture of the usual arylation/HCl-addition product (10.9) and the carbonyl compound 10.10, i. e., the product of hydrolysis of 10.9. For the subsequent reductive cyclization to the indole (10.11) the mixture of 10.9 and 10.10 can be treated with any of a variety of reducing agents, preferably Fe/HOAc. 

Being aware of the fact that a hetero-substituted carbon-carbon double bond is convertible into a carbonyl group, one can use a-hetero-substituted lithio-alkenes 2 as nucleophilic acylation reagents 142 and 143, which display the umpoled d reactivity, provided that the carbanionic character is effective. Depending on the hetero-snbstitnent X, the conversion of the vinyl moiety into a carbonyl gronp can be effected either by hydrolysis or by ozonolysis. The former procednre has been applied preferentially in the case of lithiated vinyl ethers, whereas the latter has been nsed in particnlar for cleavage of the double bond in such products that result from the reaction of hthiated vinyl bromides with electrophiles (Scheme 17). 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Additions to functionalized three-carbon olefins have been studied extensively. We have used methyl acrylate as a standard olefin since it always reacts only at the terminal carbon and the a,/3-double bond in the product is always trans. The stereospecificity of its reactions with vinylic halides varies with structure. The simple 1-halo-l-alkenes with methyl acrylate under normal conditions give mixtures of E,Z- and E,E-dienoates. The reaction is more selective with the bromides than with the iodides and the stereoselectivity increases with increasing triphenylphosphine concentration. This occurs because the excess phosphine displaces the hydridopalladium halide group from the diene 7r-complex before readdition to form the ir-allylic species occurs (see Equation 6). The disubstituted vinylic bromides react stereospecifically with methyl acrylate (4). 

Aryl and vinyl bromides and iodides have been employed most commonly, with 1-5 mol% Pd , and reaction temperatures ranging from ambient to 125 °C (equation 11). The Pd catalyst can be Pd(Pli3P)4 or the Pd species produced by in situ reduction of Pd. The alkene itself can serve as the reducing agent for Pd but the catalyst can also be produced by deliberate addition of a reducing agent, such as sodium borohydride, formic acid, or hydrazine. ... 

The reaction of a vicinal dibromide with triethylamine and DMF with micro-wave irradiation leads to vinyl bromide. Alkenes are formed from vicinal bromides by heating with iron in methanol " or samarium in the presence of TMSCl and a trace of water. a,(3-Dibromo amides are converted to conjugated amides upon photolysis in methanol. ... 

An early example having potential commercial importance comes from tlie Trost laboratory s synthesis of vitamin D analogs (Scheme 6-23) [51], Their combination of vinyl bromide 129 and alkyne 130 to form triene 131 led to a concise and efficient synthesis of (-i-)-alphacalcidiol (134). In this reaction, vinyl bromide 129 participates in a bimolecular Heck reaction with alkyne 130 and the resulting alkenylpalladium intermediate 133 undergoes subsequent intramolecular Heck reaction with the pendant terminal alkene to provide 131. Under the reaction conditions, some of the desired product undergoes a [1,7]-hydrogen shift to yield 132. After thermal recycling of the minor component, a remarkable 76% yield of 131 was obtained. 

What is fhe implication of our work wifh respect to the metal-catalyzed polymerization of polar vinyl monomers FirsL for fhe late metal compounds, fhe polar vinyl monomers can clearly outcompete efhene and simple 1-alkenes wifh respect to insertion. However, fhe ground-state destabilization of the alkene complex that favors the migratory insertion of fhe polar vinyl monomers is a two-edged sword because it biases the alkene coordination towards ethene and l-alkenes. Indeed, we have observed fhe near quantitative displacement of vinyl bromide by propene to form 7 from 3 (Scheme 9.1). Thus, the extent of incorporation of fhe polar vinyl monomer in fhe polymer will depend on the opposing trends in alkene coordination and migratory insertion. The above discussion does not take into account the problem of functional group coordination for acrylates or halide abstraction for vinyl hahdes. 

A bonanza of Heck reactions was used by Tietze25 in the synthesis of cephalostatin analogues. Corey-Fuchs reaction on the aldehyde 167 gives the alkene 168 from which the trans Br atom is stereoselectively removed by Pd-catalysed tin hydride reduction to give the Z-vinyl bromide 169. This reaction is discussed below under Stille coupling. 

Using a Suzuki disconnection, we intend to add an alkyl boronic acid 260 to an -vinyl bromide 259 (from but-l-yne) and the boronic acid will come from hydroboration of a simple alkene 261 rather than an alkyne. 

For the reaction of OH radicals with vinyl chloride and vinyl bromide the halogen elimination reactions are thermo-chemlcally favorable, the overall reactions being <11 kcal mole and <24 kcal mole exothermic for X = Cl and Br, respectively (203). The elimination of Br atoms from activated chloro-bromoalkyl radicals (206, 207), and of H, Cl, or Br atoms from activated fluoroalkyl radicals (208-213), have been studied using molecular beam techniques, these Intermediate radicals being produced by the reaction of Cl atoms with bromlnated alkenes (206, 207) or of F atoms with alkenes and halogen-substituted alkenes (208-213). For the elimination of Br atoms In the reactions... 

A tandem double Heck reaction was employed to build the steroid ring system from dibromide 5.196 and alkene 5.195 (Scheme 5.54). The first Heck reaction involves the vinyl bromide and results in alkene migration giving intermediate 5.197 in preparation for the second Heck reaction, involving the aryl bromide. 

The following reactions of olefins have also been studied formation of dialkyl adducts (517) from vinyl monomers and dienes with lithium metal and alkyl bromides in THF alkylsulphonium salts (518) from thioethers and protonated alkenes hydroformylation of styrene and a-methylstyrene in the presence of bis-(iV-a-methylbenzylsalicylaldiminato)cobalt(n) to give 2-phenylpropanal (optical purity 1.9%) and 3-phenylbutanal (optical purity 2.9%) various electron-rich olefins of the type (519) with primary amines... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

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