Understanding Initiation with Triethylboron and Oxygen: The Differences between Low-Oxygen and High-Oxygen Regimes

A unified kinetic theory for both initiation and autoxidation reactions of Et3B and O-2 is put forth, and then divided into low-oxygen and high-oxygen experimental regimes for application of Et3B/O-2 as an initiating system. In the low-oxygen regime, only long, efficient chains can be initiated. In the high-oxygen regime, less efficient chains can be initiated but they must compete with autoxidation. We apply the analysis along with new experimental results to show why AIBN and Et3B/O-2 give different stereochemical results in hydrostannation reactions of propargyl silyl ethers. Counterintuitively, AIBN is the better initiator, initiating both the rapid chain hydrostannation and the subsequent slow E/Z isomerization. AIBN gives the thermodynamic results. Et3B/O-2 is the poorer initiator, initiating only the hydrostannation and not the isomerization. Et3B gives the kinetic result. We further apply the analysis to understand recent results in Et3B/water reductions.