Zur Kenntnis des Kohlenstoffringes VI. Über die relative Bildungsleichtigkeit, die relative Beständigkeit und den räumlichen Bau der gesättigten Kohlenstoffringe

Additions and corrections: Transverse compression and the secondary H/D isotope effects in intramolecular SN2 methyl-transfer reactions

Using ab initio molecular orbital theory mainly at the 3-21+G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3 kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3 -O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3 kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.

Transverse compression and the secondary H/D isotope effects in intramolecular SN2 methyl-transfer reactions

Using ab initio molecular orbital theory mainly at the 3-21 + G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3 kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3-O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3 kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.

Studies in polymerization. X. 'The chain-effect’

The polymerization of the N -carboxy anhydride of DL-phenylalanine initiated by polysarcosine dimethylamide has been studied. The reaction shows a number of remarkable features; for example, it is very much (e. g. 100 times) faster than expected, the rate depends on the degree of polymerization of the initiator, and the reaction is inhibited by the products. There is clearly a strong catalysis by the polysarcosine chain of the normal reaction between base and N -carboxy anhydride, and we have called this the ‘chain-effect’. The results are explained in terms of adsorption of the monomer by the polysarcosine chain; this leads to a very high collision frequency between the terminal base group of the polymer and the adsorbed monomer molecules, and consequently a very high rate of reaction. A simple kinetic scheme is developed on the basis of this mechanism which gives reasonable agreement with the experimental results. It is pointed out that the chain-effect may occur in many different systems where reactions of polymers are involved. Of particular interest are the analogies between the reactions described and those catalyzed by enzymes. The general reaction pattern and the kinetic behaviour are similar in the two cases, and it is likely that studies of the chain-effect will be of value in elucidating the problems of enzymic reactions.