Substituted Acyclic Conjugated Dienes

The hydrogenation of substituted butadienes introduces another element into the problem of selectivity, that is the influence of the substituents on the degree of hydrogenation and the direction of hydrogen addition. The hydrogenation of 1,3-butadiene-2,3-dicarbonitrile (70) over Pd/C in THF at atmospheric pressure and room temperature gave almost exclusively the cis- and tra j-2-butene-2,3-dicarbonitriles (71 and 72), the products of 1,4-addition (Eqn. 15.43). °  

The hydrogenation of 1,3-pentadiene either neat or in methanol solution over noble metal catalysts gave results comparable to those observed in the 

The hydrogenation of isoprene, 75, however shows the reverse trend with the primary product from 1,2-addition being 2-methyl-l-butene (Eqn. 15.45), which is formed in about 75% yield over copper chromite catalysts. The hydrogenation of isoprene over a sulfided Raney nickel catalyst 3 took place primarily by way of a 1,4-addition process to give 2-methyl-2-butene (Eqn. 15.45). 

Apparently with isoprene, steric factors are more important than electronic considerations although the reason is not obvious because the steric differences between 1,3-pentadiene and isoprene are not great. It would seem from these results that some factors controlling the hydrogenation of these substituted conjugated dienes can be rather subtle. 

Further increases in the steric environment can give even more peculiar results. The hydrogenation of 2,5-dimethyl-2,4-hexadiene (76) proceeded by way of a 1,2-addition mode over palladium catalysts (Eqn. 15.46). In this case the 7t-allyl intermediate (77) required for 1,4-addition is evidently too sterically crowded because of the additional alkyl groups present, so this species is not a viable surface moiety and 1,2-addition is favored by default. 

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The position of the E.T. band depends in a predictable manner upon the extent of conjugation, the degree of substitution, etc., and may be calculated following rules which are analogous to those available for the prediction of absorption characteristics of conjugated dienes and which are set out in Table 3.9. The base values selected are 215 nm for an enone in an acyclic or six-membered ring system, or 202 nm for an enone system in a five-membered ring, or 207 nm for an a,/ -unsaturated aldehyde. 

Compelling evidence for stepwise C-O bond formation in [Mn(salen)] -catalyzed epoxidation is found in the formation of both cis- and trans-epoxides as primary products from acyclic ds-olefins [68]. The extent of frans-epoxide formation depends strongly on the nature of the substrate. Whereas simple alkyl-substituted ds-olefins are epoxidized stereospecifically, aryl-substituted cis-olefins afford mixtures of cis- and trans-epoxides with the ds-isomers being formed selectively. Epoxidations of conjugated dienes and enynes also afford cis/trans mixtures, with the frans-epoxide product predominating. These observations may be interpreted according to a stepwise mechanism in which a discrete radical intermediate undergoes competitive collapse to ds-epoxide and rotation/collapse to frans-epoxide (Scheme 4). 

Primary tosylates are more prone than bromides to form f-butyl ethers by Sn2 displacement by the f-butoxide anion in DMSO. Sulfonate esters of flexible cyclic and secondary acyclic alcohols give predominately alkenes in the presence of f-BuOK/DMSO. With sulfonate esters of 3-hydroxy steroids, there is competition between /S-elimination and attack of the f-butoxide ion on sulfur to form alcohols mesylates are more prone to this reaction than tosylates. Sulfonate esters of 3a-acetoxy-12a-hydroxycholanate undergo mainly /3-elimination with f-BuOK/DMSO (eq 4). In this case, substitution of various other aprotic solvents for DMSO and DMSO Na+ for f-BuOK was not as effective. Treatment of both the mesylate and the tosylate of cholesterol with f-BuOK/ DMSO gives the conjugated diene, 3,5-cholestadiene, in high yield. ... 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

The addition of O2 to acyclic dienes proved to be strongly dependent on terminal substitution and the substituents at other positions of the conjugated system, and, furthermore, it must be accompanied by photoisomerization of ( ,Z)-dienes to ( , )-dienes because singlet oxygen adds exclusively to ( , )-dienes to yield endoperoxides (cf Tables 6(a), 6(b), and 7, and references therein). 

Cobalt is another metal which has been successfully used in asymmetric cyclopropanation. A chirally modified catalytic system for selective cyclopropanation of phenyl-, vinyl- or alkoxy-carbonyl-conjugated terminal double bonds with diazoacetates is formed from cobalt(ll) chloride and (+)-a-camphorquinonc dioxime27,69 71 and similar systems 09. Best optical yields are achieved with styrene and the bulky 2,2-dimethylpropvl diazoacetate which gives 2,2-dimethylpropyl /ra .v-2-phenyl-l-cyclopropanecarboxylate in 88% ee and the as-isomer in 81%ee7n. No cyclopropanation occurs with alkyl-substituted or cyclic alkenes, cyclic or sterically hindered acyclic 1.3-dienes, vinyl ethers and phenylethyne. 

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