DNMR, DFT and preparative study on the conformation of (Z)-4,5,6,7-tetrahydropyrazolo[1,5-e]benzo[g][1,5]diazonin-8-ones and (Z)-4,5-dihydropyrazolo[1,5-d]benzo[f][1,4]diazocin-7(6H)-ones

By means of base-catalyzed ring enlargement of triazaindenoindenes and pentalenoindenes obtained from anhydride-induced ring transformation of 3,4-dihydro-2H-pyrimido- and 2,3-dihydroimidazo[2,1-a]phthalazinium-olates, respectively, a series of pyrazolo[1,5-e]benzo[g][1,5]diazonin-8-ones and pyrazolo[1,5-d]benzo[f][1,4]diazocin-7(6H)-ones were obtained. Alternative pathways and energetics for the ring inversion of symmetrically substituted medium-size ring systems were determined by combined use of DNMR measurements and B3LYP/6-31G(d,p) calculations using the IEFPCM solvent model. One pyrazolobenzodiazonine carrying hydrogen at the C1 position was found to undergo facile ring inversion by a two-step mechanism, while 1-Me and 1-Ph substituents rendered complete rigidity to this ring system. A three-step mechanism was revealed for the ring inversion of the two studied pyrazolobenzodiazocines with energetics practically invariant to the investigated C1-substituents (H and Me). The attempted RCM of the N,O-diallyl derivative of a selected rigid model effected by Grubbs II catalyst led to deallylation and olefin isomerization avoiding the formation of bridged products with enhanced skeletal strain. A tolerable degree of ring strain associated with negligible skeletal distortion could be introduced into the same benzodiazonine by N,O-dialkylation with 1,3-bis(bromomethyl)benzene.