Cyclopropane ring switching

Another type of equilibrium which has been studied by NMR spectroscopy is valence tautomerism. This is illustrated by the example of 3,4-homo-tropilidene (Figure 3.44) in which bond migration leads to the cyclopropane ring switching between two alternative positions. Thus C-8 changes from a cyclopropyl to an allylic carbon. By lowering the temperature to below — 40°Q it is possible to freeze the equilibrium sufficiently to record the spectrum of a single structure rather than that of the equilibrium mixture. 

Enhancement of the acceptor quality can be gained by switching from the ester group to a keto function. This has been realized by treating 258 with sodium or lithium salts of sulfones. In the resulting p-ketosulfones 259 the cyclopropane ring is... 

Ab initio DFT calculations for all isomers of 21 (dimethylboryl derivatives) and the transition states for their interconversions have been used to clarify the reasons of the relative stabihty of 21b-exo and 2lb-endo. In the former compound the empty 2p-AO of the boron atom is ideally oriented for overlapping with the Walsh orbital of the cyclopropane ring. On the other hand, in 2lh-endo this interaction is elfectively switched off due to the perpendicular orientation of these orbitals emerging when the steric interactions of the alkyl groups and the cyclohexadiene ring are minimized. The thermodynamic parameters for the equilibrium between 21a and 21b were determined by NMR data from the linear dependence of In K versus /T AH =10.1 0.5 kJ mol L AS= -23 1 J mol K . These values are reasonably reproduced in the calculations (AH = 8.4 kJ mol AS = —15.5 J mol K ). 

---