Conformational ring-flip mechanisms

The isotropic ESR spectra of 101 at temperatures accessible to the radical show signs neither of the existence of separate rotamers nor of line-broadening effects suggestive of their rapid interconversion. An INDO MO study has shown that the radical probably exists in two preferred propeller-like conformations (that of C3 symmetry is shown in 101 that of Cj symmetry is obtained by a rotation of any single thiophene ring in 101 through 180°). The calculations indicate the C3 conformer to be more stable by 0.75 kcal mol" and the barrier to interconversion of the two forms, by a preferred two-ring-flip mechanism, to be about 6 kcal mol . This low barrier to interconversion is consistent with the failure to observe effects of the isomerism in the ESR spectra at temperatures above those at which the radical dimerizes. 

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). 

Quite an important feature of the phenyl ring 7r-flip mechanism of the BPA-PC p transition is the intermolecular cooperativity that can exist between the flip of a phenyl ring and the motion of another phenyl ring as far away as 7 A (centre-to-centre distance), or that of a carbonate group in an out-of-equilibrium conformation. 

Prototropic tautomerism obstructs rationalization of the ee sensing mechanism in OxP [26]. A further complication exists in the macrocyclic inversion of OxP caused by a ring-flipping process between different saddle-like conformations... 

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. 

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