81981-54-6Relevant articles and documents
A CHEMICAL AND THERMOCHEMICAL STUDY OF NON-OBSERVED SYMMETRY ALLOWED REACTIONS
Ranganathan, S.,Ranganathan, D.,Ramachandran, P. V.,Mahanty, M. K.,Bamezai, S.
, p. 4171 - 4184 (2007/10/02)
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 ABC.Molecular orbital calculations of a series of 1,3-dipolar species reveal a correlation of the ? bond order with their effectiveness in -additions.Based on this, the nitro group as well as ozone should be considered as poor dipolar agents.The exceptionally high ΔHof 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 ΔHo 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-bicycloheptane-1-nitrile oxide to the corresponding isocyanate is an example of the observable ABC change.In this case, the oxazirine corresponding to B, arising from conrotatory cyclisation, undergoes irreversible rupture (BC).Such transformations are not observed with the stereochemically more favoured nitrones since these belong to the type AB.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-diazabicyclononene-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=BHa-pKAHa)+X>kcal mol-1, wherein X is dependent on the reaction type has been derived for predicting the ΔGo for these ?+AB processes.Of practical interest is the development of perturbed ?+A1B1 systems that are predicted to occur in the forward direction, where the parent type ...
