We recently disclosed anew ruthenium-catalyzeddehydrogenative cyclization process (CDC) of diamine–monoboranesleadingtocyclic diaminoboranes. In the pres-ent study,the CDC reactionhas been successfully extendedto alarger number of diamine–monoborane s(4–7)and toone amine–borane alcohol precursor (8). The correspondingNB(H)N-and NB(H)O-containingcyclic diaminoboranes (12–15)and oxazaborolidine (16)were obtained in good to highyields. Multiple substitution patterns on the startingamine–borane substrates were evaluated andthe reactionwas alsoperformed with chiral substrates. Efforts have been spent tounderstand the mechanism of the rutheniumCDC process.In addition to acomputational approach, astrategy enablingthe kinetic discrimination on successive eventsofthe cata-lytic process leading to the formationofthe NB(H)N linkagewas performed on the six-carbon chain diamine–monobor-ane 21 and completed with a15NNMR study.The long-lifebis-s-borane ruthenium intermediate 23 possessing areac-tive NHMe ending was characterizedinsitu and proved tocatalyze thedehydrogenativecyclization of 1,ascertainingthat bis s-borane ruthenium complexes are key intermedi-ates in the CDC process.