Catalytic oxidative carbonylation of aliphatic secondary amines to tetrasubstituted ureas

doi:10.1016/S1381-1169(00)00164-3

Journal of Molecular Catalysis A: Chemical

Volume 159, Issue 1, 22 September 2000, Pages 11-17

https://doi.org/10.1016/S1381-1169(00)00164-3 Get rights and content

Abstract

Secondary amines can be catalytically carbonylated to symmetrical tetrasubstituted ureas using W(CO)₆ as the catalyst, I₂ as the oxidant, and CO as the carbonyl source. Preparation of the corresponding tetrasubstituted ureas from the aliphatic secondary amines HNR₂ (R=C₂H₅, n-Bu, i-Pr, PhCH₂) and HNRR′ (R,R′=(CH₂)₄; (CH₂)₅; PhCH₂, CH₃) was achieved in moderate yields. Aromatic secondary amines are unreactive.

Introduction

Substituted ureas have found widespread use as agricultural chemicals, pharmaceuticals, resin precursors, dyes, and additives to petroleum compounds and polymers [1]. Among the numerous methods for synthesis of N,N-disubstituted ureas are the reactions of primary amines with isocyanates, phosgene, or phosgene derivatives [2]. While reports describing the synthesis of disubstituted ureas are prevalent, methods for the synthesis of tetrasubstituted ureas are less common, due to the difficulty of converting secondary amines directly to tetrasubstituted ureas [3]. The best known method involves the reaction of a carbamoyl chloride with a secondary amine [4]. However, both experimental [3] and safety [2] problems with this method have been noted. Tetrasubstituted ureas can also be obtained in good yields from the reaction of lithium amides with carbon monoxide, followed by oxidation [5]. In addition, tetrasubstituted ureas have more recently been produced from reaction of phosgene derivatives, such as 1,1-carbonylbisbenzotriazole [3] and N,N′-carbonyldiimidazole, [6] with secondary amines.

Since phosgene is highly toxic and corrosive, and phosgene derivatives can be expensive to use on a large scale, there is continuing interest in the development of alternative systems for the synthesis of substituted ureas. This interest has led to exploration of the metal-catalyzed carbonylation of amines [7], [8], [9]. Transition metal complexes of Ni [10], Co [11], Mn [12], [13], Ru [14], and most commonly, Pd [15], [16], [17], have been demonstrated to catalyze oxidative carbonylation of primary amines to disubstituted ureas. However, these metal-catalyzed reactions generally require high temperatures and pressures. In addition, yields for aliphatic amines are usually lower than those for aromatic cases. Main group elements such as sulfur [18], [19] and selenium [20], [21], [22] can also serve as catalysts.

While transition metal-catalyzed carbonylation of aliphatic and aromatic primary amines to 1,3-disubstituted ureas is well known, the direct carbonylation of secondary amines to tetrasubstituted ureas is less well explored. More commonly, transition metal-catalyzed carbonylation of secondary amines selectively produces formamides [23], [24], [25], [26]. However, there is one example of direct conversion of secondary amines and CO to tetrasubstituted ureas, which involves Pd(OAc)₂ as the catalyst and I₂ as an oxidant [15]. Using this system, Alper converted several secondary amines to the corresponding tetrasubstituted ureas in yields that range from 67% for 1,3-dibenzyl-1,3-dimethylurea to 2% for 1,1,3,3-tetrabutylurea. Among the main group elements, selenium also serves as a catalyst [27] or stoichiometric promoter [28] for the conversion of secondary amines to tetrasubstituted ureas.

Although many transition metal carbonylation systems have been examined, carbonylation of amines involving Group 6 metals [29] has remained rare. We recently reported the catalytic oxidative carbonylation of primary amines to ureas using either [(CO)₂W(NPh)I₂]₂ or W(CO)₆ as the catalyst and I₂ as the oxidizing agent (Eq. 1) [30], [32]. In addition, [(CO)₂W(NPh)I₂]₂ was determined to be a stoichiometric reagent for the carbonylation of secondary amines to formamides (Eq. 2) [33].

Although [(CO)₂W(NPh)I₂]₂ and W(CO)₆/I₂ both exhibit similar behavior with primary amines, the W(CO)₆/I₂ carbonylation conditions do not convert secondary amines to the expected formamides as does [(CO)₂W(NPh)I₂]₂. We now report the catalytic oxidative carbonylation of cyclic and acyclic aliphatic secondary amines to N,N,N′,N′-tetrasubstituted ureas in moderate yields using W(CO)₆ as the catalyst, I₂ as the oxidant and CO as the carbonyl source (Eq. 3).

Section snippets

Materials and general methods

Tetrahydrofuran was distilled from sodium/benzophenone. Methylene chloride was distilled over calcium hydride. Acetonitrile was distilled from calcium hydride. Toluene was distilled over sodium. All other chemicals were purchased in reagent grade and used with no further purification unless stated otherwise. The tetrasubstituted urea products were identified by comparison to authentic samples purchased from Aldrich or by comparison of their spectral data to literature values.

¹H and ¹³C NMR

Results and discussion

Based on the similarity of the oxidation carbonylation chemistry of primary amines with [(CO)₂W(NPh)I₂]₂ and with W(CO)₆/I₂, reaction of secondary amines with CO in the presence of W(CO)₆/I₂ was expected to produce formamides. However, when W(CO)₆, 50 eq of piperidine, 25 eq of I₂, and 50 eq K₂CO₃ are placed in a 125-ml Parr high-pressure vessel and pressurized with 80 atm CO, dipiperidylurea is produced in 36% yield based on amine. The expected piperidine formamide was found in the reaction

Acknowledgements

Support of this work was provided by the Office of Naval Research.

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Catalytic oxidative carbonylation of aliphatic secondary amines to tetrasubstituted ureas

Abstract

Introduction

Section snippets

Materials and general methods

Results and discussion

Acknowledgements

Tetrahedron Lett.

J. Organomet. Chem.

J. Mol. Catal.

J. Organomet. Chem.

J. Catal.

Tetrahedron Lett.

J. Organomet. Chem.

J. Organomet. Chem.

J. Organomet. Chem.

J. Mol. Catal.

Russ. Chem. Rev. (Engl. Transl.)

J. Org. Chem.

J. Org. Chem.

Synthesis

Carbonylation: Direct Synthesis of Carbonyl Compounds

Chemicals from Synthesis Gas, Reidel, Dordrecht