Figure 1. Introduction and background.

a. A = a potential double hydrogen atom (2H) acceptor, in which the accepting moiety could be either monoatomic (e.g., metal, metal-oxo, carbene, or nitrene) or polyatomic (e.g., a π-bonded species) in nature. b, The prototypical HDDA cascade. c, Intramolecular HDDA cycloisomerization followed by silyl ether trapping.⁶ d, Here we show that benzynes (like 3 or 6) will extract two hydrogen atoms from adjacent carbon atoms of suitable 2H-donor substrates.