Alkenes conformations

An equally intriguing report concerned the rhodium(I)-catalyzed hydroformylation of C-glycoside alkenes 12 (Scheme 5.5) [5]. As depicted, aldehydes 13 were formed with excellent diastereoselectivity. It was proposed that the diastereoselectivity is due to a strong conformational bias favoring the depicted alkene conformation. 

In order to explain reactions of chiral alkenes like this, we need to assess which conformations are important, and consider how they will react, just as we have done for chiral carbonyl compounds. Much of the work on alkene conformations was done by K.N. Houk using theoretical computer models, and we will summarize the most important conclusions of these studies. The theoretical studies looked at two model alkenes, shown in the margin. 

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 regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

What alkene gives a racemic mixture of (2R 3S) and (2S 3R) 3 bromo 2 butanol on treat ment with Br2 in aqueous solution" Hint Make a molecular model of one of the enantiomeric 3 bromo 2 butanols arrange it in a conformation in which the Br and OH groups are anti to one another then disconnect them )... 

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. ... 

With more substituted terminal alkenes, additional conformations are available as indicated below for 1-butene. 

Allyl strain influences the conformation of Z-alkenes. A 4-substituted 2-alkene will prefer conformation C over D or E to minimize the steric interaction with the C—1 methyl group. ... 

Step through the sequence of structures depicting rotation about the carbon-carbon bond in the two dibromoethane isomers l,2-dibromo-l,2-diphenylethane A andfi). For each, plot energy (vertical axis) vs. BrCCBr torsion angle (horizontal axis), and identify all minimum-energy structures. Which of these are reactive conformers , that is, conformers which are set up for either syn or anti elimination of HBr Which are non-reactive conformers , that is, which do not meet the requirements for elimination Do the reactive conformers correspond only to syn elimination, only to anti elimination, or are both pathways represented Which alkene would these reactive conformers lead to Are your results consistent with the observation that each isomer of the starting material gives only one alkene Explain. 

Examine all of the low-energy (within. 004 au or 3 kcal/ mol of the lowest-energy conformer) conformers of cis-2-methylcyclohexyl tosylate. Identify every conformer that can undergo anti elimination of OTs and H+, and predict the alkene that will be produced. What alkenes will be obtained from the cis tosylate ... 

Analyze the low-energy conformers of trans-2-methylcyclohexyl tosylate in the same way. What alkenes will be obtained from the trans tosylate ... 

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. 

For historical reasons, there are a few alkenes whose names are firmly entrenched in common usage blit don t conform to the rules. For example, the alkene derived from ethane should be called etheite, but the name ethylene has... 

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). 

What stereochemistry do you expect for the alkene obtained by E2 elimination of, (lR /O-LZ-dibromo-l -diphenylethane Draw a Newman projection of the reacting conformation. 

The difference in reactivity between the isomeric menthyl chlorides is due to the difference in their conformations. Neomenthyl chloride has the conformation shown in Figure 11.20a, with the methyl ancl isopropyl groups equatorial and the chlorine axial—a perfect geometry for L2 elimination. Loss of the hydrogen atom at C4 occurs easily to yield the more substituted alkene product, 3-menthene, as predicted by Zaitsev s rule. 

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