1-Hexene, vinylic substitution

Table III. Vinylic Substitution Productions from 1-Hexene and Various Vinylic Bromides and Morpholine ...

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

Substitution reactions with (E)- or (Z)-vinylic halides usually show predominant retention of structure in the olefin substitution as they do in the carboalkoxylation, but the specificity is quite dependent on reaction conditions. Low reaction temperature, excess organophosphine, and dilution with excess trialkylamine and/or olefin all appear to improve the specificity. Under favorable conditions, for example, (Z)-l-bromo-l-hexene and methyl acrylate give an 82% yield of the (E, Z) and only 10% of the (E, E) isomer of methyl 2,4-nonadienoate 29). 

The non-polar olefins or only mildly polar substituted olefins cited include ethylene, propylene, isobutylene, 1-hexene, 2-methyl-1-pentene, a-methylstyrene, isopropenyltoluene, and vinyl ethers. 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

More recently, Hodgson et al. have found that aziridinyl anions can also undergo a diastereoselective intramolecular cyclopropanation reaction to give 2-aminohicyclo[3.1.0]hexenes in good yield <20060L995>. Reversing the addition order so that the aziridine was added dropwise to the hase led to increased yields of the hicyclic amine. When the dienyl-substituted aziridine 386 was used, an 85% yield of the 2-amino hicyclo[3.1.0]hexane 387 was obtained, which contains the potentially useful vinyl cyclopropane moiety (Scheme 98). 

Tolman has shown that the equilibrium constants for the reactions of 38 substituted alkenes with Ni[P(0-o-tolyl)3]3 (13) in benzene, to form (ENE)bis(tri-o-tolylphosphite)nickel complexes (14), are sensitive to the structure of the alkene (equation 13). Values of K[ at 25 °C vary from 10 to 4 x 10. The stability of the complex is enhanced by electron-withdrawing substituents such as cyano and car-boxy and lowered by alkyl groups. That resonance involving unshared electrons on the oxygen of an al-koxy group overpowers the inductive effect is indicated by the relative values of Ki for allyl methyl ether, 1-hexene and vinyl butyl ether which diminish in that order by factors of 3 1 0.006. 

In addition, the vinyl radical can rearrange, before accepting an electron and a proton, to afford either a cis or trcms alkene as a final product. Among compounds that have been investigated are aryl-substituted vinyl bromides [96], cis- and tra w-3-iodo-3-hexene [97], bromomaleate and bromofumarate [98], and aryl-substituted 3-chloroacrylonitriles [99]. Yoshida and coworkers [100] electrolyzed vinyl halides at platinum in DMF in the presence of trimethylchlorosilane to obtain silylation products. 

For cases in which vinylation with higher olefins has been studied, conflicting results have been reported. In one case, it has been reported (29) that olefins give predominantly 2-substitution—e,g.y l-penten-2-yl acetate (I) from pentene when a palladium acetate-acetic acid system is used. On the other hand, a buffered (sodium acetate) acetic acid solution of palladium chloride has been reported (58) to give 1-substitu-tion—e.g., 2-hexen-l-yl acetate (II) from hexene. Propylene has been... 

Several approaches were taken. Capillary-column gas chromatography on a vinylation reaction product showed 24 separate peaks present in the hexenyl acetate fraction however, six of the peaks accounted for about 90% of the total sample. Hydrogenation of the reaction mixture (hydrogen over platinum on carbon) reduced the hexenyl acetates to a mixture of three hexyl acetates and thereby greatly simplified determining the position of oxygen substitution with, however, loss of information on olefin position. We tried to synthesize the specific hexenyl acetate isomers by the ester interchange reactions (Reactions 7a, 7b, and 7c). Mixtures of isomers were obtained, but they corresponded to the main components of the vinylation reaction mixture. For example, the main products isolated from the vinylation of hexene-1 corresponded to the products from Reactions 7a and 7b—i.c., vinyl rather than allyl esters. 

The reactions and product distributions thus far reported have been exclusively concerned with hexene. It was of interest to see whether the high specificity of positional substitution could be maintained with the other hexene isomers. By positional substitution specificity is meant ester attachment on ether of the carbons involved in the original carbon-carbon double bond. Table VII shows the results of these studies. The internal olefins reacted more slowly than the a-olefin, and with both palladium chloride-cupric chloride and 7r-hexenylpalladium chloride-cupric chloride systems high substitutional specificity (> 95% ) was also maintained with 2-hexene (Table VII). However, with 3-hexene the specificity is considerably lower (80%). Whether this is caused by 3-hexene isomerization prior to vinylation or by allylic ester isomerization is not known. A surprisingly high ratio of 2-substitution to 3-substitution is found ( 7 1) in the products from 2-hexene. An effect this large... 

In principle nitroalkenes should be available from direct nitration of vinyl carbanions. However, this reaction is not practical due to the potential anionic polymerization. But vinylstannanes obtained from ketones can be used instead of vinyl carbanions. Tetranitromethane is effective in replacing tin by nitro at unsaturated carbon. This method has been used to prepare several alkyl-substituted 1-nitrocyclo-hexenes and 1-nitrocycloheptenes (Scheme 2). ... 

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