Chemists have conducted much research on the synthesis of molecules that can trigger unprecedented molecular transformations. Moreover, well-designed molecules can provide insight into reaction mechanisms that have so far remained unclear. Indeed, a variety of phosphine imides (R′-NPR3), also known as iminophosphoranes, have been prepared and used in multiple ways, for example, as nitrogen-based Brønsted bases, donor ligands for main-group and transition-metal complexes, polyphosphazenes, and organocatalysts. The multipurpose utility of phosphine imides comes from the characteristic electronic properties of their P-N bonds which have resonance structures including a multiple bond P=N form and a zwitterionic P + -N -form (Figure 1A left). Theoretical calculations suggest the predominant contribution from the zwitterionic form that includes a negative hyperconjugation from a lone pair on the N atom to an empty orbital on the P atom. The reactivity of phosphine imides should be thus designed and modulated based on the manipulation of the aforementioned donor-acceptor interactions via the introduction of appropriate functional groups on the N and P atoms, and/or by the structural constraint (i.e. an acyclic or a cyclic structure) of the molecule. In this context, N-boronsubstituted phosphine imides, such as B(NPPh3), [B(NP t Bu3)2][B(C6F5)4], and [B2(NPEt3)4]Br2, have been investigated because the empty p orbitals on the B atoms can regulate the reactivity of the phosphine imides via delocalization. Stephan et al. reported the preparation of acyclic Nboryl phosphine imides (R″2B-NPR3), and demonstrated the contribution of a multiple-bond character between the B and N atoms and a zwitterionic P + -N -character to the bonding of these molecules (Figure ). The corresponding resonance structures were also proposed for the cyclic N-boryl phosphine imides reported by Nöth et al. The synthesis of cyclic/acyclic adducts of the type R3P(R′)N-BR″3 comprised of phosphine imides and boranes (BR″3) was also studied by Erker et al., Bourissou et al., and Slootweg et al; however, studies on their reactivity have been limited (Figure ). Given this background, we envisioned the synthesis of N-borane-substituted cyclic phosphine imides (BCPIs, Figure ) for the exploration of unprecedented molecular transformations utilizing phosphine imides. BCPIs should have sufficient basicity and/or nucleophilicity at their phosphinimidoyl nitrogen atoms due to the absence of the obvious  interaction between the adjacent B atoms, which would clearly distinguish their reactivity, and thus utility from that of N-boryl phosphine imides. The reactivity of BCPIs can be finely tuned via the introduction of appropriate substituents on not only the phosphinimidoyl N/P atoms but also the B atoms. Moreover, the combination of these substituents is a crucial factor in modulating the spatial environment around the phosphinimidoyl moiety.