Synthetic and Mechanistic Investigation of an Oxime Ether Electrocyclization Approach to Heteroaromatic Boronic Acid Derivatives

A range of functionalized heteroaromatic boronic acid derivatives are readily accessed by a diboration/6π‐electrocyclization sequence. This study revealed the surprising observation that there is a direct relationship between oxime ether stereochemistry and reactivity towards electrocyclization. Specifically, E‐oxime ethers are found to be significantly more reactive than their Z‐counterparts (stereochemistry relative to azatriene scaffold). In contrast, the configuration at the azatriene alkene terminus has little impact on reaction rates. Computational analysis offers a rationale for this observation; a Nlone pair→C=C π* orbital interaction lowers the energy of the transition state in the electrocyclization of E‐oxime ethers. Finally, unreactive Z‐oxime ethers can be converted to the corresponding heterocyclic products by a photolytically promoted E→Z isomerization and electrocyclization sequence. It's not all E/Z! E‐oxime ethers are found to be significantly more reactive in electrocyclizations than their Z‐counterparts because of an available transition state stabilizing orbital interaction in the former case. The scope of this chemistry to deliver useful heterocyclic products is also described.