Syn coplanar transition state

Three-center SN2 displacement and anti-eliminations from unsaturates are obvious examples of the coplanarity principle. DePuy et al. (1965) noted that when anti eliminations cannot have coplanar reacting centers the syn coplanar-transition state may become more favorable. Syn bimolecular eliminations had been noted in various systems previously, e.g. haloethenes (Miller, 1961), but these were generally slower than anti eliminations. There were, however, syn bimolecular eliminations whose rates approached that of the anti form or exceeded it. The relative rates of elimination of 2-phenylcyclopentyl and cyclohexyl tosylates with t-butoxide in t-butyl alcohol at 50° are as follows syn-cyclopentyl, 3 awii-cyclopentyl, 26 syw-cyclohexyl, 0 and anii -cyclohexyl, 2. In the cyclopentyl system in which the torsional angle (r) between the leaving groups approaches zero the syn rate is close to the anti rate. In the cyclohexyl system in which t of the stable form is ca. 60° the syn rate is 0. LeBel et al. (1964), report that in the reactions of t-butoxide with the 2,3-dihalobornanes, 92-95,... 

Syn coplanar transition state (only with certain rigid molecules)... 

The syn coplanar transition state occurs only with rig)d molecules that are unable to assume the anti arrangement. The reason the anti coplanar transition state is staggered (and therefore of lower energy), while the syn coplanar transition state is eclipsed. Practice Problem 7.9 will help to illustrate this difference. 

An alternative elimination in solution, i.e., nitrile formation from the 2,4-dimethoxy-syn-benzaldoxime pseudosaccharyl ether was found to be first order.43 The modest influence of the solvent on the rate of reaction was interpreted254 in terms of its stabilizing effect on an all planar conformation.14 If the mechanism is similar to type (ii) a coplanar transition state would be suited for an intramolecular elimination reaction. 

Consider a simple molecule such as ethyl bromide and show with Newman projection formulas how the anti coplanar transition state would be favored over the syn coplanar one. 

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... 

Because adjacent bonds are eclipsed when the H—C—C—X unit is syn coplanar a transition state with this geometry is less stable than one that has an anti coplanar rela tionship between the proton and the leaving group... 

DeDnA in the lUPAC system. The elimination must be syn and, for the four- and five-membered transition states, the four or five atoms making up the ring must be coplanar. Coplanarity is not required for the six-membered transition state, since there is room for the outside atoms when the leaving atoms are staggered. 

Despite mechanistic complications, however, it appears very likely that most, if not all, of the facile and synthetically attractive carbometallation reactions involve, at a critical moment, concerted addition of carbon-metal bonds where the synergistic HOMO-LUMO interactions shown in Scheme 4.3, akin to those for the concerted hydrometallation reactions, provide a plausible common mechanism. This mechanism requires the ready availability of a metal empty orbital. It also requires that addition of carbon-metal bonds be strictly syn, as has generally been observed. Perhaps more important in the present discussion is that concerted syn carbometallation must proceed via a transition state in which a carbon-metal bond and a carbon-carbon bond become coplanar. Under such constraints, one can readily see how chirally discriminated carbon-metal bonds can select either re or si face of alkenes. In principle, the mechanistic and stereochemical considerations presented above are essentially the same as for related concerted syn hydrometalla-tion. In reality, however, carbometallation is generally less facile than the corresponding hydrometallation, which may be largely attributable to more demanding steric and... 

E2 eliminations as in Figure 4.26 usually proceed with anti- instead of. syn-selectivity. This is true although both eliminations benefit from transition states with 7t-like bonding interactions between the MOs of coplanar, breaking C-H- and C-X bonds (also cf. the discussion of Figure 4.20). The crucial factor is a steric effect that favors the anti- over the yyn-transition state, since the spectator substituents at C and adopt a nearly staggered structure in the anti-tran-... 

Of these possible conformations, the anti-coplanar arrangement is most commonly seen in E2 reactions. The transition state for the anti-coplanar arrangement is a staggered conformation, with the base far away from the leaving group. In most cases, this transition state is lower in energy than that for the syn-coplanar elimination. 

The E2 reaction takes place through a concerted mechanism that requires a coplanar arrangement of the bonds to the atoms being eliminated. The transition state is usually anti-coplanar, although it may be syn-coplanar in rigid systems. 

The syn and the anti conformations leading to (R,R)-31, illustrated in Table 3, are calculated (mechanics) to be the two lowest-energy transition states for the cyclization of 29. Of the two, the anti conformation (Rh carbene and carbonyl coplanar but pointing in opposite directions) is the more stable, by 3.37 kcal/ mol. If steric factors alone governed the outcome of these cyclizations, we would expect that the anti transition state leading to (R,R)-31 would be competing with the syn transition state leading to (S,S)-31. The former would be favored by... 

Such a mechanism requires that the major product, 1,2,4-trichlorobenzene should contain deuterium when the reaction is performed in a deuterated medium , but the amount of label found was no greater than predicted from the rate of exchange of the product under the reaction conditions . Presumably fairly extensive double-bond character is exhibited in the transition state for bimolecular elimination from the benzene hexachlorides and the abnormally high activation energy for the beta isomer is most plausibly attributed to the lack of coplanarity in the iy -c//nfl/-elimination or the high energy and low population of the boat conformer required for jy/i-elimination. Alternatively, a sparsely populated conformation for a concerted syn-clinal-... 

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