JACS: preparation of aromatic amines by decarboxylation of carbamates catalyzed by nickel

Aromatic amine is an important chemical raw material, which is widely used in medicine, agricultural chemicals and organic materials The catalytic cross coupling (Buchwald Hartwig amination) of aryl halides and amines is a common method for the synthesis of aromatic amines In addition, phenol can be aminated by transforming into corresponding trifluoromethane sulfonate In this way, oxidation addition occurs between aryl C-O bond and catalyst, which will generate stoichiometric fluorinated waste (Figure 1a) Although this problem has been partially solved by using non fluorinated active groups, such as p-toluenesulfonate, methanesulfonate, carboxylate, carbamate, etc., the use of active groups cannot be avoided Recently, researchers from Osaka University and Hokkaido University in Japan reported the method of preparing aromatic amines by decarboxylation of arylcarbamates catalyzed by nickel The only by-product of the reaction was carbon dioxide In addition, amination is carried out under neutral conditions without the use of free amines, which increases the compatibility of functional groups The biphosphonic ligands immobilized on polystyrene (ps-dppbz) are the key to the success of the reaction, and the catalytic substances produced are more active than the simple non carrier variants Relevant research results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b02751) (source: j.am Chem SOC.) it has been reported that Ni (0) complexes with strong σ - donor ligands can catalyze various cross coupling reactions of phenol derivatives which can not be catalyzed by common palladium catalysts Therefore, the author speculates that the C (aryl) - O bond of arylcarbamate may decarboxylate after oxidation addition, and form C-N bond (Figure 1b) through reduction elimination In order to test the feasibility of this hypothesis, the author investigated the decarboxylation amination of carbamate 1a in the presence of nickel catalyst and a series of σ - donor ligands (Figure 2) The results of ligand screening showed that ps-dppbz (3G), a polymer supported phosphine ligand, had excellent catalytic activity with a yield of 94% Ligands 3G can be easily synthesized from commercial reagents in two steps The catalyst can be recycled twice without significantly reducing the yield of 2A (second operation: 96%; third operation: 89%) (source: J am Chem SOC.) next, the author used Ni (0) / 3G catalyst system to study the decarboxylation range of arylcarbamate (Figure 3) Aryl carbamates containing various functional groups, including ether (1c), fluorinated substituents (1D and 1E), carbonyl (1F - 1I, 1p - 1z), borate (1K) and heteroaromatics (1l-1n), were successfully tolerated in the reaction It is worth noting that the formyl group (1g, 1H, 1p - 1z) which can react with free amine is also compatible These results highlight the advantages of decarboxylation in the synthesis of formyl substituted aniline derivatives This decarboxylation also allows the introduction of a series of amino groups, including pyrrolidine (2S), piperidine (2q, 2R and 2U), piperazine (2t) and aromatic amines of 1,4-azacycloheptane (2V) ring system (source: J am Chem SOC.) the author studied the reaction mechanism through a series of control experiments (Figure 4) The substituent effect was examined by comparing the reaction rate constant (k OBS) The results show that the oxidation addition of C (aryl) - O bond is the rate limiting step of the reaction, rather than the elimination of CO 2 extrusion or reduction (source: J am Chem SOC.) all kinds of n-arylpiperidine (Figure 5) can be obtained by assembling phenol, pyridine and organometallic nucleophiles In this synthesis scheme, the phenol part can be used not only as the activation group of pyridine ring, but also as the arylation reagent (source: J am Chem SOC.) conclusion: researchers from Osaka University and Hokkaido University in Japan reported the decarboxylation of arylcarbamates catalyzed by nickel, which provides a new method for the synthesis of aromatic amines, and the reaction has a wide range of functional compatibility.