Nanoreactor microenvironments rewire reaction-network kinetics by a mechanistic closure of barrier, dynamics, and encounter.

Nanoconfinement in porous hosts and nanoreactor microenvironments is routinely invoked to tune catalytic performance and selectivity, yet predictive control of microenvironment-driven kinetic selectivity in reaction networks remains limited. A major reason is that confined kinetics is still commonly rationalized as a "barrier-shift" effect, even though confinement simultaneously modifies (i) dynamical recrossing/friction and (ii) the probability that reactants access and remain within reactive regions near active zones. Here, we provide a quantitatively testable framework that elevates this intuition into a closure-validated kinetic theory and shows how confinement can reprogram network-level rate and selectivity outcomes. We introduce a mechanistic closure for confined observed rates, kobs ≈ kTSTκPenc, which factorizes each elementary-step rate into independently measurable contributions from barrier control, dynamical recrossing (κ), and encounter/residence gating (Penc). Using an encounter-gated slit-confined model designed to make each factor operational and measurable from the same dynamics, we validate this closure across broad confinement conditions (pore width, wall affinity, friction, and temperature). Building on the validated closure, we show that confinement can rewire reaction-network kinetics by reweighting step-specific effective edge rates, thereby inducing pathway switching, intermediate trapping, and sharp regime boundaries not predictable from barrier shifts alone. Finally, we derive low-dimensional control variables that collapse multi-parameter confinement dependencies into compact design rules for programming kinetic selectivity and intermediate trapping via pore size, wall affinity, and friction.