Dienes cisoid conformation

As illustrated in Scheme 8.1, both 2-vinylpyrroles and 3-vinylpyiroles are potential precursors of 4,5,6,7-tetrahydroindolcs via Diels-Alder cyclizations. Vinylpyrroles are relatively reactive dienes. However, they are also rather sensitive compounds and this has tended to restrict their synthetic application. While l-methyl-2-vinylpyrrole gives a good yield of an indole with dimethyl acetylenedicarboxylate, ot-substitiients on the vinyl group result in direct electrophilic attack at C5 of the pyrrole ring. This has been attributed to the stenc restriction on access to the necessary cisoid conformation of the 2-vinyl substituent[l]. 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

The diene must be in the cisoid conformation. If it is frozen into the transoid conformation, as in 84, the reaction does not take place. The diene either must be frozen into the cisoid conformation or must be able to achieve it during the reaction. 

The explanations for the relative rates of reaction have been based on three factors (1) The rate of reaction increases as the electron density in the diene system increases thus isoprene reacts faster than butadiene and a complex electron-rich 2-silylmethylbutadiene reacts even faster. (2) The rate of reaction increases as the steric hindrance due to the diene substituents decreases thus frans-piperylene reacts more slowly than dimethylbu-tadiene or isoprene. (3) A decrease in the equilibrium concentration of the cisoid conformer results in a slower reaction rate thus cw-piperylene or cis/trans-2,4-hexadiene react more slowly than /rans-piperylene or transltrans-2,4-hexadiene, respectively.175177... 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

Substituents in the diene may also affect the cycloaddition sterically, through influencing the equilibrium proportion of the diene that is in the required cisoid conformation. Thus bulky 1-ci s substituents (28) slow the reaction down, whereas bulky 2-substituents (29) speed it up, through this agency ... 

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

The Diels-Alder product 324 resulting from the diene 322 and the allenyl sulfone 323 can be used together with potassium tert-butoxidc directly in a Ramberg-Back-lund reaction [333]. In this case, a diene permanently in the cisoid conformation is regenerated to make the polycyclic compounds 326 and 327 available by the same Diels-Alder/Ramberg-Backlund sequence. 

The reactivity of dienes in Diels-Alder reactions is also controlled by the diene conformation. The two planar conformations of 1,3-butadiene are referred to as s-trans and s-cis (equation 16). Calculations have shown the s-trans conformation to be 2-5 kcalmol-1 more stable than the s-cis conformation. Open-chain dienes can only react in their cisoid conformation. Thus, 2-substituted dienes are generally more reactive than 1,3-butadiene due to their stronger preference for the s-cis conformation. 1 -Cis substituted 1,3-butadienes are almost exclusively in the s-trans conformation and are not reactive in Diels-Alder reactions. Highly substituted dienes may, however, be present in the s-cis conformation during a sufficient amount of time to participate in Diels-Alder reactions, even if a 1 -cis substituent is present62. 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

In [4 + 2]-cycloadditions the diene reacts in the cisoid conformation,530 which is essential for p-orbital overlapping in the diene and the dienophile and to form the new double bond in the six-membered ring. Diels-Alder reactions are regioselec-tive. Of the two possible isomers formed in the cycloaddition of nonsymmmetri-cally substituted reactants [Eq. (6.87)], usually one predominates depending on the structure of the reacting molecules and reaction conditions ... 

Certain chromium complexes also exhibit similar selectivities in the hydrogenation of conjugated dienes.139 140 Coordination of the diene in the cisoid conformation and 1,4 addition can account for the selectivity. 

The Diels-Alder reaction is highly stereospecific. The diene reacts in an unfavorable conformation in which its double bonds lie in a plane on the same side (cis) of the single bond connecting them. This s-cis (or cisoid) conformation is required to give a stable product with a cis double bond. Addition of ethene to the alternate and more stable (transoid) conformation would give an impossibly strained Pimv-cyclohexene ring. Possible transition states for reaction in each conformation follow, and it will be seen that enormous mo-... 

Selective 1,4-hydrogenation of 1,3-dienes to (Z)-alkenes is possible with benzenechromium tricarbonyl as a catalyst precursor. Only those conjugated dienes that can adopt a cisoid conformation, and hence chelate to the metal, are hydrogenated. Methyl sorbate (68) is reduced to methyl (Z)-3-hexenoate (70) [46,47]. The reaction... 

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

The [4+2] cycloaddition of the standard silaethene with trans.trans- and cw,/ra 5-hexadiene occurs exclusively with stereospecifity, whereby the latter diene reacts 10 times more slowly than the former on grounds of unfavorable cisoid conformation [28]. 

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