THE OUT,OUT TO OUT,IN TRANSITION FOR 1,(N+2)-DIAZABICYCLO[N.3.1]ALKANES

Hexahydropyrimidines N,N-bridged by a chain of n methylene groups (1,(n + 2)-diazabicyclo[n.3.1]alkanes) adopt out,out (axial,axial) structures for n = 2, 3, and 4. When n = 5, the photoelectron spectrum shows evidence of the presence of some of the out,in (axial,equatorial) isomer in the gas phase, although none can be found in solution. When n = 6, the compound is apparently entirely out,in in the gas phase but exists as a mixture of out,out and out,in conformers in solution. For n = 7, only the diamond lattice out,in isomer can be detected in solution. These experimental data are correlated with,force field (MM2) calculations; multiple minimum search methods have been used to locate all low-energy conformations. Semiempirical calculations (MNDO, AMI, and PM3) have been carried out on model systems. Related tricyclic bis-aminals having 10- and 12-membered rings have also been studied. They adopt [2323] and [3333] conformations, respectively, each having out,in (equatorial,axial) bridged hexahydropyrimidine rings. For several of the compounds, dynamic NMR processes are observed, and possible mechanisms for these are discussed.