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

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_T ≈ k_d) without absolute rate measurements. The test is applied to reactions 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 ≈ 0.3k_dif = 3 × 10⁹M^-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 × 10⁸s^-1. The triplet nitrene reacts by hydrogen abstraction and is scavenged by oxygen.