Alkenes stable conformations

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

The catalyst + aliphatic n-alkene system, for all of the olefins from propene to 1-decene, give almost 40,000 possible conformations. The most stable conformations for each of the olefins were selected (around 1,700) and their transition states were optimized at IMOMM level. The calculated enantiomeric excesses are shown in Fig. 13. Calculations are able to reproduce the observed increase in ee for short chains, and the presence of a ceiling value after which the increase in enantioselectivity is much smaller, in excellent agreement with experiment. 

Fig. 6.19. Isotactic polymerization of propene with rac-l,2-ethanediyl-bis(indenyl)zirconium complexes. The plane of the drawing coincides with the plane bisecting the two planes of the indenyl ligands. Propene coordination takes place with the alkene Tt-orbitals in the plane of the drawing. The carbon atoms of the growing chain are depicted in the plane of the figure in between insertions no rearrangements to more stable conformations have been drawn. Indenyl ligand above the plane drawn in full.

In the epoxidation of /ra/w- -caryophyllcnc (12)158,16fi, conformations 12 A, B for both modes of attack are readily accessible. Interestingly, the less stable conformer of the alkene 12 A is preferentially attacked by the peracid. The reason for this is that torsional interactions of the partially formed C —O bonds differ considerably in the two diastereomeric transition states. This should be taken as a warning when prognosticating diastereomeric ratios of epoxidation of medium and large ring (i )-alkcnes from the relative population of alkene conformers. In this case, the correlation with relative stabilities of the diastereomeric epoxides seems to be generally better. The transition state force field reproduces the reported diastereoselectivities of epoxidation (80 20) of the endocyclic double bond perfectly. 

The target alkene should contain one or more pronounced stereogenic centers that do not permit facile (total) ring inversion of the most stable conformer. 

Important information about the stereochemistry of dehydration has been obtained by studying the transformation of cyclic alcohols many of these reactions have proved suitable for the selective synthesis of alkenes [2]. The dehydration of menthol (5) and neomenthol (6) illustrates the usefulness of such processes (Scheme 3 axial hydrogens participating in water loss to form the major menthene isomers are shown) [55]. The regioselectivity observed points to an anti elimination mechanism. Isomer 6, with a trans OH/H configuration in the most stable conformation, reacts faster than compound 5. 

There are two families of conformations available to terminal alkenes. These are the eclipsed and bisected conformations shown below for propene. The eclipsed conformation is more stable by about 2kcal/mol. ... 

Cis alkenes are less stable than their trans isomers because of steric strain between the two larger substituents on the same side of the double bond. This is the same kind of steric interference that we saw previously in the axial conformation of methylcyclohexane (Section 4.7). 

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. 

The Z-alkene isostere mimics the cis-amide bond conformation. Its molecular volume and log p values are almost identical to those of the E-isostere (Table 1). Its use in bioactive peptides is very limited, probably because of the synthetic challenge involved. One study reports the migration of the double bond to the a,(3-position in an enkephalin analogue. 4 The Z-alkene in the Alat t[Z, CH=CH]Pro dipeptide isostere, however, was reported to be stable towards isomerization.1 1X1 orf/to-Substituted aromatic or tetrazole rings have been used more frequently as ds-amide bond mimics. 

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