The model studied describes intramolecular stochastic dynamics in terms of diffusion in a parabolic potential along a coordinate of various possible physical interpretations. The chemical transformation is considered as a transition perpendicular to this coordinate. Equations of dynamics are formally equivalent to a Schrodinger equation with a spin that can take two values. In the slow diffusion limit the reaction can be either an ordinary activated process, studied already in the literature, or can be controlled by a nonchemical variable-the mean value of the diffusion coordinate. For the latter case an asymptotically exact nonlinear kinetic equation is derived and discussed. Importance of the initial stage of the reaction, when a local equilibrium is being reached between species for each value of the diffusion coordinate,is indicated. Results of the approximation proposed are compared with the actual course of reaction found by direct numerical solution of the starting equations. A distinct case of "gated" reversible reaction, when fast perpendicular transitions are sharply localized at a certain point on the diffusion coordinate, is considered separately.