Mechanistic studies of the synthesis and decomposition of some amine-carbonyl condensation products
Condensation of benzylamine and glyoxal in a 2:1 molar ratio in acetonitrile containing a catalytic amount of acid is found to give the cage compound 2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazatetracyclo [18.104.22.168(^5,9).0(^3,11)] dodecane (hexabenzylhexaazaisowurtzitane). The yield of the isowurtzitane cage compound was optimised by varying the temperature and the acid catalyst. Eight phenyl ring substituted benzylamines were also found to give the isowurtzitane cage structure. Yields were found to be lower for benzylamines carrying electron-withdrawing ring substituents. A tetra-acetylated isowurtzitane derivative was also prepared and the NMR spectra were examined showing it to consist of a mixture of isomers due to restricted rotation about the N-acetyl bonds. (^1)H and (^13)C spectral assignments were aided by the use of deuteriated isowurtzitane cage compounds in which all the benzyl methylene hydrogens had been replaced by deuterium. N,N'-dibenzyl-l,2-ethane diimine and l,2-bis(benzylamino)- 1,2-ethanediol which are thought to be intermediates in the formation of the cage compounds were prepared and the mechanism of formation of the hexabenzylhexa- azaisowurtzitanes is discussed. The reaction mixture left after the precipitation of the cage compounds was examined by HPLC and NMR to ascertain if any other products could be isolated. Results indicated a complex mixture from which the diimine, protonated benzylamine and the diol could be identified. In acidic solution the cages are found to exist in either the monoprotonated or diprotonated forms. The monoprotonated species is found to be fairly stable whereas the diprotonated species is found to decompose fairly rapidly. (^1)H and (^13)C NMR show that the proton is held strongly in a position between the two nitrogen atoms in the two five-membered rings on one side of the molecule. The diprotonated species has two protons, one incorporated on each side of the molecule. Kinetic results show that decomposition of the isowurtzitane cages in acidic media occurs via an acid or water catalysed pathway. An approximate value of the K(_a) for hexabenzylhexaazaisowurtzitane of ≥4 has been calculated. The decomposition of some 3,7 di-substituted bicyclononanes in acidic media has been investigated. Kinetic results indicated two decomposition pathways involving reaction of the monoprotonated forms of DPTDNPT and DAPT with either water or acid. In the case of DNPT there was also evidence for reaction via a diprotonated species. The reaction of 2-aminobenzylamine with glyoxal is found not to give the expected cage structure but a novel cyclisation product 2,2' -bis( 1,2,3,4-tetrahydro- quinoxaline). Formation is thought to involve intramolecular cyclisation of the diimine intermediate. Reaction of 4-aminobenzylamine with glyoxal gave an unidentified solid which does not possess an isowurtzitane cage structure.