Non-aromatic Systems

Structure of Five-membered Rings with Two or More Heteroatoms 

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Once again, we may identify the cis-1,2-difluoroethylene pi dication as a 4N pi electron Hiickel antiaromatic system and the trans isomer as a 4N pi electron non-aromatic system. 

While the greatest percentage of PTC-aided anionic substitutions involve non-aromatic systems (7-10), a number of liquid-liquid and solid-liquid, PTC-aided SnAr reactions have been reported (32-38). These reports involve a variety of substrates [unactivated (32,33), slightly activated (M), activated (35-37), and transition metal complexed 32,38)1, nucleophiles OMe (32,38), CN ( ), SR (34) SCN (36), SO (36), OR (37)] and PTCs... 

As has been pointed out in the past (e.g. concerning the linear-cyclic equilibrium in Ceand Cio carbon clusters (40)), Hartree-Fock underestimates the resonance stabilization of aromatic relative to non-aromatic systems (in the case at hand, between the N- and / -protonated isomers) and MP2 overcorrects. The structures are found to be nearly isoenergetic at the CCSD level inclusion of connected triple excitations favors the N-protonated ion. The direction of the effect of connected quadruples is somewhat unclear, and a CCSD(TQ) or CCSDT(Q) calculation impossible on systems this size, but the contribution will anyhow be much smaller in absolute magnitude than that of connected triple excitations, particularly for systems like these which are dominated by a single reference determinant. We may therefore infer that at the full Cl limit, the N-protonated species will be slightly more stable than its / -protonated counterpart. 

An alternative approach to the experimental estimation of REs utilizes equilibrium (protonation) data rather than thermochemical data, the idea being that comparisons of the basicities of pyrrole and its benzo fused analogues with those of non-aromatic systems which form cations of 7r-electron structure similar to the aromatic compounds should furnish a measure of the loss of RE accompanying protonation of the aromatic system (76T1767, 72CI(L)335). Thus, for the a-protonation of N-methylpyrrole, the model non-aromatic system was chosen as (20). Combination of pKa values for the protonation of the aromatic and non-aromatic molecules, taking into account the intrinsic resonance stabilization of the... 

In this section the reactivity of aromatic and non-aromatic systems are treated separately as in Chapter 4.02. For convenience, the former group is taken to comprise all compounds of structural types (5)-(ll), including benzo fused derivatives, even when there is little evidence of aromaticity as is the case with dioxazoles. Likewise, the nonaromatic systems will include dihydro and tetrahydro derivatives of structural types (16)-(24). 

Aromatic systems Structure type R1 Yield (%) Method Ref. X Y Z Non-aromatic systems (25) R1 Yield (%) Method Ref. 

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