Palladium-Catalyzed Regioselective B(9)-Amination of o-Carboranes and m-Carboranes in HFIP with Broad Nitrogen Sources

Abstract

Amination of carboranes has a good application prospect in organic and pharmaceutical synthesis. However, the current methods used for this transformation suffer from limitations. Herein, we report a practical method for a highly regioselective formation of a B−N bond by Pd(II)-catalyzed B( 9)-H amination of oand m-carboranes in hexafluoroisopropanol (HFIP) with different nitrogen sources under air atmosphere. The silver salt and HFIP solvent play critical roles in the present protocol. The mechanistic study reveals that the silver salt acts as a Lewis acid to promote the electrophilic palladation step by forming a heterobimetallic active catalyst PdAg(OAc) 3 ; the strong hydrogen-bond-donating ability and low nucleophilicity of HFIP enhance the electrophilic ability of Pd(II). It is believed that these N-containing carboranes are potentially of great importance in the synthesis of new pharmaceuticals.

Visible-light-induced 1,2-diphenyldisulfane-catalyzed regioselective hydroboration of electron-deficient alkenes

“…These methods using a radical initiator usually require elevated temperature, which may limit the functionality and applicability, and the use of a metal photocatalyst under visible-light irradiation will lead to residual metal in the reaction system. We have long been working in the area of boron chemistry [33][34][35][36][37][38][39] and recently have had a special interest in photocatalysis, 40 so we wondered whether the method of formation of NHC-boryl radicals could be developed in milder and greener conditions. Disulfide has an important role in the photocatalysis reaction, 41,42 and the disulfide could generate electrophilic thiyl radicals under UV light irradiation.…”

Highly selective electrophilic B(9)-amination of o-carborane driven by HOTf and HFIP

“…Based on our previous work, 18,19 we started our investigation with 1,2-Me 2 - o -carborane 1a as the standard substrate, electron-deficient diethyl azodicarboxylate (DEAD) as the electrophilic reagent, 20 and HFIP as the solvent. However, no product was obtained under these conditions (Table 1, entry 1).…”

“…17 Very recently, we achieved the regioselective B(9)-amination of o -carboranes and m -carboranes in HFIP with a wide range of nitrogen sources through Pd( ii )-catalyzed B–H activation (Scheme 1a). 18 HFIP was found to be a critical solvent in this transformation due to its strong hydrogen bond donating ability and low nucleophilicity, which greatly increased the electrophilicity of Pd( ii ). Additionally, HFIP was also used in the electrophilic halogenation of o -carboranes, which shows its irreplaceable role.…”

Pd-Catalyzed C–O Bond Formation Enabling the Synthesis of Congested N,N,O-Trisubstituted Hydroxylamines

“…The efficiency of the reaction slightly improved when NaO t Bu was used as the base, but replacing the tert -butoxide salt with a non-nucleophilic weak base, such as Li 2 CO 3 , did not give any target product (entries 3 and 4, respectively). These results show that O -benzoyl hydroxylamine, which is commonly used as an electrophilic amination reagent via the cleavage of N–O bond, instead served as a precursor to the O -nucleophile in our reaction. The putative nucleophile should be the oxyanion of hydroxylamine generated in situ by the nucleophilic attack of O -benzoylhydroxylamine by tert -butoxide during the reaction.…”