Slow inversion at nitrogen and its consequences for some azabicyclic systems.

The chemistry of a range of N-methyl-1,4-dihydro- and 1,2,3,4-tetrahydronaphthalen-1,4-imines has been investigated. Inversion at nitrogen is slow in these amines and can be observed by 1H nmr spectroscopy. The invertomer ratios of both series have been measured and, in each amine, the major invertomer has been assigned the syn-configuration (that with the N-methyl group syn- to the aromatic ring). On the basis of a comparative study, it was concluded that homoconjugative interaction of the lone pair and adjacent?-electrons is only a minor factor affecting the preferred configuration at nitrogen. The inversion barriers of the dihydronaphthalen-1,4-imines have been measured and the values obtained gave further proof of the barrier-raising ability of the 7-azabicyclo(2.2.1) heptadienyl group. The inversion barriers were found to vary slightly with the substituents on the aromatic rings but no clear reasons for this variation emerged. The members of both series of amines were quaternised with various alkylating agents. The ratios of the diastereoisomeric quaternary salts formed varied with the substituents on the aromatic rings. This variation was thought to be due to the prevailing electronic forces in view of the minimal steric differences between the amines in each series. The high inversion barriers (>95kJmol-1) of N-chloro-1,4-dihydro-naphthalen-1,4-imines allowed the study of the separate chemistries of the two invertomers. At 0°C with silver salts in methanol (conditions of negligible inversion), each invertomer solvolysed with assistance from adjacent ?-electrons. The structures and stereochemistries of the products reflected this assistance and further, the ability of the aryl group to participate in the heterolysis of the anti-N-C1 bond varied predictably as a result of substitution with electron-withdrawing or releasing groups. The anti-invertomer rearranged to the novel 6,7-benzo-1-azabicyclo(3.2.0) hept-3-ene system. Under conditions of free inversion (ambient temperature, no silver salt), the products from the methanolyses reflected the relative reactivities of syn- and anti-invertomers. The less reactive invertomer (anti-) "reacted" only via prior inversion to the more reactive (syn-).