Renaissance of Sandmeyer-Type Reactions: Conversion of Aromatic C-N Bonds into C-X Bonds (X = B, Sn, P, or CF3).

The Sandmeyer reaction represents an important organic transformation that converts an arylamine to an aryl halide using Cu(I) halide via a diazonium salt intermediate. The reaction was first reported by Sandmeyer in 1884, and a number of named reactions closely related to it have been developed and widely applied in organic synthesis throughout the 20th century. These include the Pschorr reaction for the synthesis of biaryl tricycles, the Gomberg-Bachmann reaction for biaryl formations, the Balz-Schiemann reaction for C-F bond formations, and the Meerwein reaction for arylation of α,β-unsaturated carbonyl compounds. However, all these reactions were discovered before 1940. In 1977, Doyle and co-workers reported an organic phase diazotization process, and Kikukawa and Matsuda used aryldiazonium salts in transition metal-catalyzed cross-coupling reactions. However, completely new processes involving diazonium salts have been seldom reported since then, although aryldiazonium salts are widely utilized in modern organic synthesis. In the past few years, diazonium salt chemistry has been revisited and become a fast-growing research topic. Several novel transformations based on diazonium salts have been developed and have been practiced in organic synthesis. In 2010, we reported a direct conversion of arylamines to pinacol boronates through the reaction of in situ generated aryl diazonium salts with B2pin2. This new strategy is under metal-free conditions and thus completely avoids contamination by transition metals in the boron products. From readily available arylamines various functionalized arylboronates, some of which are difficult to access by other methods, can be easily obtained with this reaction. Mechanistic investigations indicate the reaction likely follows a radical mechanism, which is similar to traditional Sandmeyer-type reactions. Subsequently, modified reaction conditions for this transformation appeared in the literature, which include light-induced reactions, aqueous-phase diazotization methods, and reactions with aryltriazenes as the arene diazonium salt surrogates. In addition to the borylation, we have also demonstrated the corresponding stannylation and phosphorylation of arylamines with similar Sandmeyer-type approaches. The stannylation of arylamines was achieved by the reaction of in situ generated diazonium salts with a distannane reagent, while phosphorylation is the reaction of arylamines with trimethyl phosphite in the presence of tert-butyl nitrite (t-BuONO). With the sequential borylation and stannylation approaches, the aromatic compounds bearing two amino groups are easily converted into trimethylstannyl arylboronates, which can be further used in consecutive Stille and Suzuki-Miyaura cross-couplings. Finally, direct conversion of the amino group of arylamines to the trifluoromethyl group has been developed through aryl diazonium salts almost simultaneously by several groups. These reactions represent a novel strategy to achieve trifluoromethylation of aromatic compounds. These developments show the revivification of this age-old chemistry, and this Account will summarize the Sandmeyer reaction-related transformations that have been developed since 2010.