Alkenes conformational equilibria

The bonding in vinylcyclopropane (3) is such that an s-trans-gauche conformational equilibrium exists to allow for maximum orbital overlap of the asymmetric component of cyclopropane orbitals with the IT- or 7T -orbitals of the ethylene unit, as shown in (3a). From thermochemical stuches it appears that conjugation of an alkene with cyclopropane stabilizes the system by 1.2 kcal mol". The conformational equilibrium for vinylcyclopropane was shown to consist of an s-trans minimum (3b) and two gauche conformers that are equ in energy and destabilized by 1.43 kcal mol with respect to the s-trans conformation. The barrier to interconversion has been determined to be 3.92 kcal mol . ... 

The major product of olefination with phosphonate carbanions is usually the ( )-isomer.304 However, a (Z)-selective reaction has recently been developed that uses sodium iodide and DBU as the base.305 Speziale and Ratts suggested that increased amounts of (Z) alkene were obtained by increasing the steric bulk of the L and l1 groups in 557 and 560, formed by reaction of the phosphonate ester ylid with a carbonyl compound. This equilibrium favors the anti conformation(557 over 560 and elimination gives more of the (Z) product, 559.306 jjjjg model assumes that steric encumbrance is more important in 557 and 560 than in oxaphos-phetanes 558 and 561, respectively, that are required for syn elimination to the alkene. This equilibrium generally favors 559 over 562 due to the stereochemical preferences in the initially formed ylid products. 

When cyclopentene was replaced with 1,1-diethoxyethylene, only one regioisomer was detected with 168 and 175, and the highest diastereo-selectivity was 56% [151]. Results were rationalized on the basis of insufficient face differentiation owing to the conformational equilibrium. In the ground state, two main conformers are present in equal amounts. Attack of the alkene from the less-hindered side occurs from the opposite re and si diastereofaces of the two conformers respectively. 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

A number of cis/trans 4,6-dialkyl-2,2-dimethyl-l,3-dioxanes were studied by C NMR spectroscopy (93JOC5251). The C NMR shifts of C -Me groups (Scheme 8) were found to be very sensitive to the 1,3-dioxane conformation [chair form Me(ax) ca. 19 ppm and Me(eq) ca. 30 ppm— pure 30.89 ppm in the twist-boat form both methyl carbons resonate at ca. 25 ppm (pure 24.70 ppm)]. With these values, AG° of the chair to twist-boat equilibrium was calculated (Table IV). For 13a (nitrile), 13b (alkyne), and 13e (methyl ester) (Scheme 8) in CH2CI2, the temperature dependence of the AG° values was determined. Depending on the substituent, small negative or positive entropy terms were found generally the enthalpy term dominates the -AG° value. In the tram isomers 13, the cyano and alkyne substituents favor the chair conformation, but CHO, ester, alkene, and alkyl substituents, respectively, clearly favor the twist-boat conforma-... 

A pentopyranosyl radical is much more flexible than a hexopyranosyl radical. Because the alkyl-anchor at C-5 is absent, the radical is now so flexible that several species of similar energy can coexist. According to ESR spectroscopic data, the arabinopyranosyl radical 9 exists as an equilibrium between the 4C19a and the B03 9b conformation, which both realize a coplanar arrangement of the C-O bond and the SOMO [9] (Scheme 6). Reactions with alkenes are unselective. However, the arabinofuranosyl radical 10 reacts with high stereoselectivity [9]. This is due to its 2E conformation in which the si-side of the radical center is sterically hindered by the large benzoyl group. 

Ab initio calculations on the structures of ethylchlorocarbene and chloromethyl-chlorocarbene show that the equilibrium between cis and trans conformers is shifted moderately to the trans isomer for the ethyl derivative and strongly to the cis for the chloromethyl (due to a stabilizing carbene n — rearrangement transition states are product-like and the Z-isomers are stabilized by a through-space interaction of the pz orbitals localized on the terminal atoms. 

An interesting aspect of this reaction is the formation of substantial amounts of cw-2-butene, which would appear to require the intermediacy of the j-cis-1,3-butadiene anion radical, even though butadiene exists almost exclusively in the s-trans conformation (98 %). At —33°C, 13 % of the 2-butene mixture is the cis alkene, and at -78 °C 50 % of the mixture is cw-2-butene. In the case of 1,3-pentadiene, 68 % of the 2-pentene is the cis isomer. The most plausible explanation for these stereochemical results appears to be the reversible reduction of the diene to the diene anion radical at -78 °C by the pool of solvated electrons, which yields an equilibrium mixture of the s-cis and j-tran5-anion radicals (ca. 50 50), which are... 

Each alkene can be made stereospecifically (the synthesis of the Z-isomer 45 is given later in this chapter) and treatment of the cis alkene 45 under the same acidic conditions also gives the trans alkene 40. The alkenes are in equilibrium via the stable tertiary carbocation 43. The cation has two conformations 43a and 43b that can eliminate to give the -alkene 40 and free rotation about the central o-bond in this relatively long-lived intermediate 43 ensures that one of these conformations is accessible from all possible starting materials. 

Phenylmenthylalcohol has been widely employed as a chiral auxiliary in diastereoselective reactions [39]. Essential to achieving good diastereoselectivity in radical additions are proper rotamer distributions with respect to the 0=C-C=C bond and successful shielding of the alkene Ti-face. The conformation of the acrylate in this scenario exists as an equilibrium mixture of s-cis and s-trans isomers [40]. Reports indicate that the acrylate can be fixed in the s-trans conformation in the presence of Lewis acid [41]. 

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