A Simple Test for Diffusion-Limited Reactions. Near-Diffusion-Controlled Nitrene Insertion into C-H Bonds

Article abstract of DOI:10.1021/ja00203a021

A test for diffusion-controlled reactions is described.It is based on a simple experiment, wherein a reactive intermediate competes for two reagents containing a different number of reactive sites.This approach allows one to estimate rate constants of rapid reactions (i.e., k_r ca. k_d without absolute rate measurements.The test is applied to reaction of (diethoxyphosphoryl)nitrene with hydrocarbons in perfluorinated solvents.The photogenerated singlet nitrene undergoes efficient insertion into C-H bonds, and its reactivity is unaffected by the presence of oxygen.The insertion of the nitrene into a tertiary C-H bond is found to be near-diffusion-controlled with k_obs ca. 0.3k_dif = 3 * 1E9 M^-1 s^-1.Increased selectivity for insertion into tertiary versus primary C-H bonds is observed in perfluorohexane (9.7) as compared to a hydrocarbon solvent (5.7).This diminished reactivity of the nitrene is considered as evidence for complex formation between the singlet nitrene and a solvent molecule.The rate of the complexed-singlet to triplet intersystem crossing is estimated to be 8 * 1E8 s^-1.The triplet nitrene reacts by hydrogen abstraction and is scavenged by oxygen.