A CHEMICAL AND THERMOCHEMICAL STUDY OF NON-OBSERVED SYMMETRY ALLOWED REACTIONS

Article abstract of DOI:10.1016/0040-4020(81)85008-9

A variety of reactions, whilst predicted to be symmetry allowed, are not observed experimentally.Several such non-observed yet allowed reactions have been studied in order to understand them and to possibly bring about, with suitably modified substrates, their observation.For example, although ozone adds to ?-bonds, the isoelectronic nitro group does not, due to a high activation energy barrier.This reaction is of the type A<*>B<*>C.Molecular orbital calculations of a series of 1,3-dipolar species reveal a correlation of the ? bond order with their effectiveness in <4+2>-additions.Based on this, the nitro group as well as ozone should be considered as poor dipolar agents.The exceptionally high ΔH^o_f of ozone (+34.00 kcal mol^-1) as compared with that of nitrobenzene (+3.8 kcal mol^-1) could possibly explain the reactivity of the former.This is reflected in an estimated ΔH^o value of -45.5 kcal mol^-1 for the ethylene-ozone addition as compared with +7.5 kcal mol^-1 for the non-occuring ethylene-nitrobenzene addition.Thermochemical calculations demonstrate that the nitrogroup is more prone to undergo addion as a 2? partner than as a 1,3-dipolar reagent.The thermal, nitro group-olefin addition has been examined using a variety of electrophilic nitrobenzenes with tetracyclone as the common substrate.The extent of cyclo-addition was assessed on the basis of the isolation of 2-benzoyl-3,4,5-triphenylfuran. 4-Nitro-4'-methyl diphenyl sulfone and 4,4'-dinitro diphenyl sulfone have been identified as efficient reagents for these reactions.The former compound has been found to add to even non-conjugated olefins.The clean thermal transformation of crystalline 7,7-dimethyl-bicyclo<2.2.1>heptane-1-nitrile oxide to the corresponding isocyanate is an example of the observable A<*>B<*>C change.In this case, the oxazirine corresponding to B, arising from conrotatory cyclisation, undergoes irreversible rupture (B<*>C).Such transformations are not observed with the stereochemically more favoured nitrones since these belong to the type A<*>B.The ?⁴s+?²s addition of carboxylates to olefins would lead to a carbon base at the expense of an oxygen one.This is clearly unfavourable and is not observed.On the other hand, the reverse of this reaction, namely , the fragmentation of ketal conjugate bases to carboxylates would be expected to be facile.This is indeed so and the process has been shown to be concerted.Ketals from p-nitrobenzaldehyde are shown to undergo fragmentation with 1,5-diazabicyclo<3.4.0>nonene-5 (DBN) to olefins.The carboxylate-olefin addition belongs to the general category ?+A^-<*>B^-, that encompasses cycloadditions, electrocyclic reactions and sigmatropic shifts.An equation ΔG deg=<1.4(pK^BH_a-pK^AH_a)+X>kcal mol^-1, wherein X is dependent on the reaction type has been derived for predicting the ΔG^o for these ?+A<*>B processes.Of practical interest is the development of perturbed ?+A₁<*>B₁ systems that are predicted to occur in the forward direction, where the parent type ...