1-Azabicyclo[1.1.0]butanes (ABBs) on the Map: Mechanistic Pathways for Catalytic Ring Opening and Functionalizations

Given the prominence of azetidine derivatives in pharmaceutical chemistry, developing their catalytic synthesis continues to attract strong attention from synthetic chemists. While the strain-release ring opening and functionalizations of 1-azabicyclo[1.1.0]butanes (ABBs) has become a promising route to access diverse azetidines and 1-azabicyclic molecules since their discovery in the late 1960s, such transformations were mostly operated under stoichiometric conditions, and reactions facilitated by catalysis have only emerged recently. The integration of catalysis in ABB chemistry has enabled innovative reaction development, particularly through the interception of transient reaction intermediates-for instance, C3-radical and N1-anion-to allow divergent transformations, thereby achieving unprecedented synthetic versatility. This mini-review summarizes recent advances in catalytic strain-release ring opening and functionalizations of ABBs from a mechanistic-centered perspective. The reaction pathways are categorized into polar ring opening, radical ring opening, and polar ring opening with radical crossover. The features of each class of reaction are presented for the identification of mechanistic commonalities and differences across diverse transformations.