Structure-reactivity relationship in polyarylcarbocation systems

As described above, although the pi R+ values for symmetrically trisubstituted triarylmethanols [3(X = Y = Z)] give a linear Y-T correlation against a a scale with an r value of 0.76 (Yukawa et al., 1966), clearly non-linear Y-T correlations are obtained for the pK + values for monosubstituted triphenylmethanols [3(X,H,H)] or any non-equivalently substituted series [3C (X Y A Z)j. In any polyarylmethyl carbocation the molecule adopts a 

This kind of conformational dependence has been observed for many polyaryl systems. However, no explicit consideration has been given to its implications for the interpretation of correlations even in a qualitative manner. For this type of analysis, the trityl ion series [3C ] involves less than a sufficient number of substituents. Although the benzhydryl solvolysis series has sufficient substituent sets, the change in rotation appears too small to estimate the effect quantitatively. In practice, the solvolyses of a-trifluoromethyl-diarylmethyl tosylates [29C (X,Y)] best illustrate this analysis. 

The optimized structures at the 6-31G level, for typical carbocations [29C (X,Y)] are shown in Fig. 21 (Fujio et al, 1999). In the parent carbocation [29C (X = Y = H)], the two phenyls are rotated significantly from coplanarity with the sp carbocation centre rotation angles are 23° and 38° (MP2/6-31G ). 

Essentially the same structure was obtained for the symmetrically di-p-methoxy-substituted ion [29C (X = Y = MeO)]. On the other hand, the optimized structure of the mono-p-methoxy [29C (X,p-MeO)] has the phenyl ring twisted by 53° while the anisyl is coplanar with 6 60 = 0°. This conformation is 3.3 kcal moE more stable than the doubly twisted conformation the opposite conformation with the phenyl coplanar and the anisyl twisted (6h = 0° and 0Meo 50°) is a further 9 kcal mol less stable. 

The symmetrically disubstituted precursors [29(X = Y)] react through a transition state structurally close to the diaryl-carbocation [29C (X = Y)] of a preferred propeller shape denoted here as the E-conformer where both aryls are equivalently twisted from coplanarity with the reaction centre (0x = 0y = 30°). 

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Structure-reactivity relationship in polyarylcarbocation systems 334 Conformations of carbocations 334 Reactivity-conformation relationship 337 Stabilities of carbocations in the gas phase 343 Structural effects 343 Tlie resonance demand parameter 355 Theoretically optimized structures of carbocations 362 Reaction mechanisms and transition-state shifts 365 Extended selectivity-stability relationships 365 Ground-state electrophilic reactivity of carbocations 366 Sn2 reactions of 1-arylethyl and benzyl precursors 372 Concluding remarks 378 Acknowledgements 379 References 379... 

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