Two-ring flip mechanism

Quantitatively, line-shape analysis was used to determine rate data for these stereoisomerizations in terms of the two-ring flip mechanism. 32> The associated free energies of activation for the various exchange processes at 20 °C are shown schematically in Fig. 10. For the equilibrium BB AA, AG°2o is 0.3 kcal/mol. For the conversion of BB to AA, the calculation yielded AG 0 16.2 kcal/mol. and for the reverse reaction (AA- -BB), AGt0 15.9 kcal/mol. The barrier to enantiomerization of B and B is AG%B 14.6 kcal/mol. Thus, at 20 °C the enantiomerization of B and B is energetically more favorable by 1.6 kcal/mol than that of A and A. 

Variable temperature xH-nmr studies of other hindered (in both ortho positions) triarylboranes have revealed 32> that these compounds are also mixtures of propeller-like stereoisomers on the nmr time scale at low temperatures, and that interconversion of these stereoisomers occurs with activation energies of ca. 14—16 kcal/mol (Table 3). These isomerizations are also interpretable in terms of two-ring flip mechanisms. 

We turn to empirical force field calculations in order to choose between these two mechanisms. 44 Such calculations indicate that the two-ring flip mechanism is the lowest-energy pathway, and yield a barrier of 20 kcal/mol for the two-ring flip of 7. 44> The experimental free energy of activation for stereoisomerization derived from the temperature-dependent 1H-nmr spectrum, is AG 67 21.9 kcal/mol 43, in excellent agreement with the calculated value. This high barrier admits of the possibility that 7 is separable into its optical antipodes at moderately low temperatures. 

The stereochemical correspondence between trimesitylmethane and trimesitylsilane extends to other Group IVA elements. Thus the barrier to enantiomerization by a two-ring flip mechanism for trimesitylgermane is AG 80 9.2 kcal/mol. 47>... 

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). 

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