One-ring flip mechanism

The structure correlation method has been applied to derive information about a variety of other reaction paths, for example, for weakening and ultimate fission of one bond of a tetrahedral MX4 molecule to give a planar MX3 species (5n1-reaction type) [48] and nucleophilic addition at carbonyl C-atom [49]. From analysis of the conformations of Ph3P=0 fragments [50], the stereoisomerization path could be mapped and identified as corresponding to a two-ring flip mechanism [51]. 

Lindner (171) developed his own tt-SCF MO force field that is similar to MMPI in construction. This program was applied to simulate racemization of metacyclophane (48) and hexahelicene (50). In metacyclophane the m-phenylene ring flips readily at room temperature. Two mechanisms can be conceived one operates by way of a high steric energy conformation (48b) the other involves a biradical intermediate (49). The calculated activation energies are 17 and 32 kcal/mol, respectively. The experimental value is 17.7 kcal/mol, in accord with the first mechanism (172). The structures and energies of seven types of cyclophane have been calculated (172). 

There are numerous other examples of stereochemical correspondence in propeller molecules. The propellane 19> 1e has only one energetically reasonable isomerization mechanism available a twisting motion about the C3 axis which corresponds to the three-ring flip as well as to the trigonal twist rearrangement (Fig. 5). Similarly, only one isomerization mechanism (stereochemically correspondent in the permutational sense to the zero-ring flip) is energetically reasonable for tri-o-thymotide (Id). 8,20)... 

This is not a problem in this example, because flipping of the ring and inversion of the nitrogen are fast, and enough of the starting material is in this conformation at any one time for the reaction to take place. But compare this bicyclic acetal whose fragmentation (actually just an acetal hydrolysis) looks possible by this mechanism. 

The fact that only one spectral set is observed for both of the chelate rings in 4 - 5 in the and C NMR spectra is evidence that (a) the equilibrium between 4 and 5 is rapid relative to the NMR time scale and, (b), that the open and closed chelate moieties in 5 are equivalent, i.e. they also undergo simultaneous rapid equilibration. The equilibration of the chelate rings must follow a rapid flip-flop type mechanism, by which one NMe2 group displaces the other, as described previously for other systems [8]. 

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