Fragmentations are controlled by stereochemistry

The control of rearrangements can be stereoelectronic in origin—if a molecule is to rearrange, orbitals have to be able to overlap. This means that, for a Beckmann rearrangement, the migrating group has to be trans to the leaving group. Not surprisingly, the same is true for Beckmann fragmentations like the one at the end of the last section, where the green fragmenting bond is trans to the leaving group. Before we extend these ideas any further, consider these two quite different reactions of quite similar compounds.

Just as with the rearrangements, we need to draw these compounds in reasonable chair conformations in order to understand what is going on. In the cis isomer, both substituents can be equatorial; in the trans isomer one has to be axial, and this will be mainly the OTs group, since the two methyl groups of NMe2 suffer greater 1,3-diaxial interactions.

Now, the cis isomer has clearly undergone a fragmentation reaction and, as usual, number ing the atoms can help to identify the bond that breaks. The nitrogen lone pair pushes, the departing tosylate pulls, and the resulting iminium ion hydrolyses to the product aldehyde.

Yet the trans isomer does this only in very low yield. Mostly it eliminates TsOH to give a mixture of alkenes. Why? Well, notice that, in the cis isomer, the fragmenting bond is trans to the leaving group—indeed, it is both parallel and trans (in other words anti-periplanar) to the leaving group. Electrons can flow smoothly from the breaking σ bond into the σ* of the C OTs bond, forming as they do so a new π bond.

For the trans isomer, fragmentation of the most populated conformation is impossible because the leaving group is not anti-periplanar to any C–C bond. The only bonds anti-peri planar to OTs are C–H bonds, making this compound ideally set up for another reaction whose requirement for anti-periplanarity you have already met—E2 elimination.

The other conformation can fragment because now the OTs is anti-periplanar to the right C–C bond, and this is probably where the 11% fragmentation product comes from.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct