We have noted that the substituents on the aromatic ring of arylamine substrates significantly affect the yields of borylation products. Therefore, we further devoted ourselves to developing more general conditions for the widespread aryl amine substrates. Gratifyingly, we found that simply carrying out the reaction in MeCN at 80 °C in the absence of BPO would expand the substrate scope.⁸ Importantly, the modified reaction conditions give slightly improved yields in most cases. The modified conditions turn out to be compatible with a broader substrate scope, including ortho-substituted aryl amines, amines with unprotected hydroxyl and amino groups, aldehydes, and more importantly, heterocyclic amines (Scheme 2). It has been observed that electron-deficient heterocyclic amines exhibit high reactivity in the formation of the borylation products, while electron-rich heterocyclic amines are prone to oxidation in the presence of t-BuONO, thus resulting in diminished yields. The reactions can be carried out on gram scale for selected substrates.

Following our initial report in 2010, many new Sandmeyer-type borylations under modified conditions have been developed. In 2012, Yamane group reported the BF₃-mediated synthetic method for deaminoborylation of aryltriazenes 4 with bis(pinacolato)diboron 2.⁹ Various arylboronic esters are prepared in moderate to good yields with this facile transition-metal-free procedure (Scheme 3). Mechanistic studies suggested that this borylation process might not involve an aryl free radical process.

In 2012, Yan and co-workers reported an organic dye (eosin Y)-catalyzed visible light-promoted reduction of stable aryldiazonium tetrafluoroborate salts 5 to generate aryl free radicals that are borylated with B₂pin₂ 2.¹⁰ Both electron poor and electron rich aromatics are tolerated and provide the corresponding borylated compounds in moderate to good yields (Scheme 4). Similarly, this photoredox protocol could be extended to in situ generated diazonium salt from aryl amine and t-BuONO.

In 2016, Ranu reported that using the same photocatalyst, eosin Y, a variety of anilines were borylated to give corresponding arylboronates in the presence of t-BuONO and B₂pin₂ 2 at room temperature with visible light irradiation (Scheme 5).¹¹ This procedure avoids the isolation and handling of hazardous diazonium salt and the use of any metal reagent.

In 2014, Pucheault and co-workers disclosed that a series of titanocene and zirconocene reagents were capable of promoting borylation of anilines to form boronates at room temperature (Scheme 6).¹² In contrast to previous works, this protocol uses diisopropylaminoborane 6 (H₂BNi-Pr₂) as the borylating agent, and the reaction is presumed to proceed through a radical chain mechanism.

In 2017, Wu and co-workers demonstrated a zinc-catalyzed borylation of aryldiazonium salts 5 and aryltriazenes 8 for arylboronate synthesis (Scheme 7).¹³ The reaction was carried out in methanol with 5% Zn(ClO₄)₂ catalyst at 40 °C, providing various arylboronates.

Diazotizations were typically conducted in aqueous solution in the early days. Although aqueous conditions have sometimes restricted the substrate scope due to functional group compatibility or solubility issues, there are also potential advantages of using water as solvent, such as low cost, safety, and environmental benefits. Thus, developing Sandmeyer-type borylation under aqueous conditions is desirable. In 2014, Xue and co-workers reported aqueous phase diazotization conditions for the conversion of aryl amines to arylboronates.¹⁴ In this procedure, the first diazotization step was conducted at 0−5°C in mixed solvent of MeOH/aq. HCl. For the second step, protic solvents such as alcohols and organic acids are superior to aprotic solvents. In addition, Xue and co-workers expanded the borylation to include B₂(OH)₄ 9 as the boron source, affording arylboronic acids 10 as the products (Scheme 8).

In 2014, Blanchet group disclosed the aqueous phase borylation of a wide range of diazonium salts with diboronic acid 9, leading to the facile synthesis of (hetero)arylboronic acids 10.^15a The most attractive advantage was the speed of the reactions, as all of the examples went to completion in approximately a half an hour (Scheme 9). Furthermore, the authors combined this fast one-pot borylation method with other reactions such as esterification, Suzuki−Miyaura coupling, hydrogenolysis, and Petasis borono-Mannich reaction for rapidly achieving molecular diversity.^15b

In 2017, Wu further demonstrated that the borylation of stable diazonium tetrafluoroborates 5 could proceed under catalyst-free conditions in acetone-water solution (Scheme 10).¹⁶ Control experiments indicate that water plays a crucial role in promoting the borylation of diazonium tetrafluoroborates. Mechanistically, the reaction proceeds through a radical pathway, as TEMPO or BHT completely inhibited the desired transformation.

In 2019, Miao and co-workers reported the metal-free transformation of arylamines to aryl naphthalene-1,8-diamino boronamides 12 with a nonsymmetrical diboron reagent B(pin)-B(dan) 11 (Scheme 11).¹⁷ This Sandmeyer-type borylation reaction represents an important supplement for the preparation of aryl-B(dan) compounds.

In summary, the Sandmeyer-type borylation of aryl amines and their derivatives for the synthesis of (hetero)arylboronic acids and boronates have been well studied in the past decade. A number of modified practical routes to arylboronic compounds have been established, and these methods are available for application in academic settings as well as industrial processes.