Silver-catalysed [3 + 2] annulation reaction of aryldiazonium salts with allenes enabled by boronate direction†

Allenes are a unique type of powerful synthon used for the construction of various carbocycles and heterocycles, but their annulation reactions with N–N triple bond electrophiles have not been disclosed. Here we report an efficient silver-catalysed boronate-directed [3 + 2] annulation reaction of aryl diazonium salts with allenylboronates. This transformation offers unprecedented access to a wide scope of N¹-aryl-1H-pyrazoles with high regioselectivities under mild conditions. Preliminary experimental and computational studies support a transmetalation/stepwise cycloaddition/pyrazolyl silver hydrolysis pathway involving propargyl silver species as the key intermediate in enabling reactivity and controlling regioselectivity.