Conformations anti-coplanar

Nearly all cyclohexanes are most stable in chair conformations. In the chair, all the carbon-carbon bonds are staggered, and any two adjacent carbon atoms have axial bonds in an anti-coplanar conformation, ideally oriented for the E2 reaction. (As drawn in the following figure, the axial bonds are vertical.) On any two adjacent carbon atoms, one has its axial bond pointing up and the other has its axial bond pointing down. These two bonds are trans to each other, and we refer to their geometry as trans-diaxial. 

The anti coplanar conformation is the preferred transition state geometry. 

E2 reactions are stereospecific, because they generally occur via the anti-coplanar conformation, rather than the syn-coplanar conformation. Small deviations from coplanarity can be tolerated, and it is sufficient for the proton and the leaving group to be anti-periplanar. 

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... 

However, Aae can still be used in suitable cases as an indication of conformation, particularly for polycyclic compounds. Thus Aae measurements on TV-CH2X protons may be used to establish the cis or trans nature of the ring fusion in 1,3-heterocyclic systems. For example, the trans-fused perhydro-pyrido[3,2,l-iJ][3,l]benzoxazine (21) shows Aae 0.84 ppm, indicative of the anti-coplanar CH-nitrogen lone-pair geometry, whereas the cis-fused isomer (22), in which the nitrogen lone pair bisects the CH2 group, shows Aae 0.12 ppm.45... 

The chemical shift of the angular proton in benzo[c]quinolizidines will differ from that in quinolizidine itself as a result of delocalization of the lone pair electrons over the aromatic ring in certain conformations. Examples are provided by 29 and 30. In trans-fused 29, the nitrogen lone pair is delocalized over the aromatic ring and thus the anti-coplanar shielding mechanism is lost and the angular proton absorbs to low field of that in the cis conformation 30.51... 

When each of the rotors has a unique substituent (as in —CX2Y)— which are not necessarily the same—Vx and V2 terms are required. The Vx term reflects the interactions of the two substituents when they are coplanar and, thus, differences between syn (3) and anti (4) conformations. A positive Vx corresponds to an unfavorable, or respulsive, interaction... 

In the anti-periplanar conformation, the C—H and C—L sigma bonds are coplanar but on opposite sides of the C—C bond. Again, as the bonds break, the carbons change hybridization, and the sp3 orbitals change to the parallel p orbitals of the pi bond. 

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. 

It is apparent that the ( )- and (Z)-vinyl sulfoxides adopt different reactive conformations in their reactions with amines. Studies on the conformational preference of vinyl sulfoxides bearing dipolar substituents suggest that the (/ . )- and (Z)-vinyl sulfoxides favor the s-cis-25 (S = 0 and C = Ci>v -coplanar) and the s-trans-25 (S = 0 and C = C ann -coplanar) conformations, respectively, in solution. The stereochemical outcome of the reactions in ethanol can be readily accounted for by inferring nucleophilic attack by amine on the s-cis and s-trans conformations of the chiral ( )- and (Z)-vinyl sulfoxides, respectively, from the least hindered rc-face (i.e., anti to the 4-methylphenyl group)96. 

Over the past few years, the debate over the origin of the p-silicon effect on carbocations has narrowed to one of the relative magnitudes of inductive and hyperconjugative factors. Theory and experiment are finally in agreement that hyperconjugation is by far the dominant factor—29 kcal/mol calculated to be from P-stabilization ( ) versus 9 kcal/mol from induction and polarization. The realization of these effects is dramatically revealed in the SnI solvolyses of the conformationally locked cyclohexyl trifluoroacetates (OTFA) (3-5), The relative solvolysis rates at 25 °C for compounds 3-5 are 1, 4 X 10, and 2.4 X 10, respectively. Compound 4 cannot attain the necessary anti-coplanar relationship of the Si-C and C-O bonds, which is present in 5 and required for full hyperconjugative interaction with the cation formed as the C-O bond suffers heterolysis. 

In E2, the two groups to be eliminated must be coplanar. In conformationally mobile systems like acyclic molecules, or in cyclohexanes, anti-coplanar is the preferred orientation where the H and leaving group are 180° apart. In rigid systems like norbomanes, however, SYN-coplanar (angle 0°) is the only possible orientation and E2 will occur, although at a slower rate than anti-coplanar. 

Therefore, only molecules having accessible H-X anti- conformations can react via this route. The anti-coplanarity restriction can dictate the non-respect of Zaitsev s rule. 

Bromine is axial and anti coplanar to two axial hydrogens in the most stable conformation of c/ -4- r butylcyclohexyl bromide and has the proper geometry for ready E2 elimination. The transition state is reached with little increase in strain, and elimination occurs readily. 

Stereoelectronic effects are also important in the dehydrohalogenation of acyclic alkyl halides by an E2 pathway. Again, the most favorable arrangement for the hydrogen and the halide being lost is anti coplanar. In the formation of 2-methyl-2-butene from 2-bromo-2-methylbutane shown on page 208, the elimination of HBr occurs readily from the conformation on the left but not from the one on the right. 

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