Conformer conjugated

CONFORMATIONS Conjugated double bonds, DELOCALIZATION Conjugate force,... 

Cyclohexenones are even flatter than cydohexenes, but it is convenient to draw them in a similar conformation. Conjugate addition to this substituted cyclohexenone gives the trans product. 

The addition of large enolate synthons to cyclohexenone derivatives via Michael addition leads to equatorial substitution. If the cyclohexenone conformation is fixed, e.g. as in decalones or steroids, the addition is highly stereoselective. This is also the case with the S-addition to conjugated dienones (Y. Abe, 1956). Large substituents at C-4 of cyclic a -synthons direct incoming carbanions to the /rans-position at C-3 (A.R. Battersby, 1960). The thermodynamically most stable products are formed in these cases, because the addition of 1,3-dioxo compounds to activated double bonds is essentially reversible. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

The two most stable conformations of conjugated dienes are the s cis and s trans The s trans conformation is normally more stable than the s cis Both conformations are planar which allows the p orbitals to overlap to give an extended tt system... 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Conformational effects appear to be important in determining which tertiary a-hydrogen is removed. For example, 4-phenyl-, 4-p-nitrophenyl-, and 4-/)-methoxyphenylquinolizidine (83) all are oxidized to the corresponding /j < -iminium salts (84) and not to the conjugated zJ -iminium salts (85) (86). The authors Judged that steric hindrance was responsible or that the conformation of the 4-sabstituted quinolizidines did not contain ideal... 

For acyclic allylic substrates die situation is mote complex, since a larger number of reactive conformations, and betice corcesponding transition states, compete. Hius, mediyl ciimamyl derivatives 163 tX= O.Acj, upon treatment witli litliiiim dimetliylcuprate, mainly gave tlie S 2 substitution product 166 fentry 1, Tab. 6.6 and Sdieme 6.34) [80]. Hie preference for die S 2 product is expected, since de-conjugation of die alkene system is electronically imfavorable. 

Syntlietic cliemists can now work witli tlitee-dlmensional pictures of tlie conjugate addition available on a website [80]. In tlie absence of steric hindrance i5-nietliylcydobexenone, for example), an "axial attack" tlitougb a balf-diait conformation is favored, while in tlie corLisone syntliesis an "equatorial attack" tliroLigli a half-boat conformation is favored because of tlie constraint imposed by tlie bicydic tings [83]. 

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

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