Periplanar conformation

Some examples of syn elimination have been found in molecules where H and X could not achieve an anti-periplanar conformation. 

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... 

The mechanism of these reactions is often El. However, in at least some cases, an E2 mechanism operates.It has been shown that stereoisomers of cyclic y-amino halides and tosylates in which the two leaving groups can assume an anti-periplanar conformation react by the E2 mechanism, while those isomers in which the groups cannot assume such a conformation either fragment by the El mechanism or do not undergo fragmentation at all, but in either case give rise to side products characteristic of carbocations. " ... 

It is noteworthy that the value of this substrate is smaller by one order compared to non-cyclic compounds. According to the discussions proposed above, this is considered to be due to its conformation already being fixed to the one that fits to the binding site of the enzyme. This estimation was demonstrated to be true by the examination of the effect of temperature on the kinetic parameters. Arrhenius plots of the rate constants of indane dicarboxylic acid and phenyl-malonic acid showed that the activation entropies of these substrates are —27.6 and —38.5 calmol K , respectively. The smaller activation entropy for the cyclic compound demonstrates that the 5yn-periplanar conformation of the substrate resembles the one of the transition state. 

ANTI elimination [(32) — (33)] was found to proceed only 14 times faster than SYN elimination [(31)— (33)] reflecting the fact that the energy needed to distort the ring, so that (32) can assume an approximately anti-periplanar conformation, almost outweighs the normal energetic advantage of the staggered conformation over the, syn-periplanar, eclipsed one, i.e. (31). 

High diastereofacial selectivities are observed in cycloadditions and Michael additions with ot,(3-unsaturated esters having chiral heterocyclic auxiliary at the p-position, as shown in Schemes 11.20, 11.21, and 11.25, and cannot be well-explained using Kozikowski s awfi-periplanar model (124,125) or Houk s inside alkoxy model (126,127). Both the anti-periplanar conformation and the syn-periplanar conformation of the acceptors participate in the transition structures, depending on nonbonding interactions in the dipole-chiral auxiliary pair (121). 

The experimental dependency of the /J-silyl effect on 0 in solvolysis reactions is sketched in Figure 764. Obviously, it differs from that anticipated for a k mechanism with rate-determining formation of siliconium ion or from the cosine-squared function expected for the pure hyperconjugative stabilization model. Apparently, the /J-silyl effect is operative in the solvolysis of both the syn- and rmt/ -periplanar conformations. The rate acceleration in the latter might be ascribed to a more favourable geometry for the (T-anchimeric assistance. 

The removal of a molecule of a hydrogen halide from an alkyl halide to yield an alkene is effected under strongly basic conditions, e.g. a concentrated alcoholic solution of sodium or potassium hydroxide or alkoxide. This overall reaction has been submitted to most rigorous mechanistic studies. Most of the factors (temperature, nature of base, structure of substrate, solvent, etc.) which control product composition have been evaluated. It thus appears that under the conditions noted above, an E2 process, in which the participating sites adopt an ann -periplanar conformation leading to an anti-elimination process, is generally favoured. 

The three-membered ring system, although the most highly strained of all the ring sizes, is readily formed in the cyclisation step. This arises from the fact that carbon-carbon bond formation in this case is an irreversible intramolecular S 2 displacement of halogen reaction, which is facilitated by the thermodynamically favourable anti-periplanar conformation of the anion. 

Vic -dihalides exhibit a single 2e polarographic wave, while nonvicinal ( >3) dibromides normally are reduced in two separate 2e transfers. The reduction potential of the dihalides is more positive (A E +0,4 to +0,7 V) than that of the monohalides 3,6>321 3221. A plot of the half-wave potentials of 21 vicinal dibromides versus the dihedral angle,, between the two C-Br bonds exhibits maxima (most positiveE t j2) for y = 180 0 (anti-periplanar conformation) and i/) = 0°... 

In the syn-periplanar conformation, the C—H and C—L sigma bonds are coplanar and on the same side of the C—C bond. As the bonds to the hydrogen and the leaving group start to break, the hybridization at each carbon begins to change to sp2, and the two sp3 orbitals, which have some pi-type overlap initially, change to the p orbitals of the pi bond. 

Mechanism of E2 elimination from syn-periplanar and anti-periplanar conformations. 

Closer examination reveals that the syn-periplanar conformation has all the bonds eclipsed, whereas the anti-periplanar conformation has these bonds staggered, as shown in the following Newman projections ... 

Upon E2 elimination. (I R.2R)-1 -bromo-1,2-diphenylpropane produces only (Z)-1,2-diphenylpropene. This result demonstrates that the reaction occurs entirely from the anti-periplanar conformation shown. The enantiomer of this compound, (IS,2S)-l-bromo-l,2-diphenylpropane, also produces only the (Z)-alkene when it undergoes an E2 elimination reaction. (To simplify viewing, the phenyl groups are shown as single blue atoms in the ball-and-stick models.)... 

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