Interaction of Palladium(II) Complexes with Amino-Alcohols: Synthesis of New Amino-Carbonyl Complexes, Key Intermediates to Cyclic Carbamates

The interaction between carbon monoxide and aromatic or aliphatic amino-alcohols promoted by some palladium(II) tetracoordinated complexes, stabilized by aryl mono- and diphosphines [triphenylphosphine (PPh3); 1,2-bis(diphenylphosphine)ethane (dppe)], an amino-phosphine [(2-(β-diphenylphosphino)ethylpyridine) (PN)], and diamines [2,2′-dipyridine (dipy); 1,10-phenanthroline (phen)], was investigated. All tested complexes and substrates were shown to interact with CO to afford either stable carbamoyl complexes, which were isolated and studied for their reactivity, or, directly, organic products. From the reaction of PdCl2(PN) with 4-aminophenol (4-APhOH, a) the stable carbamoyl complex (PN)PdCl(CONHC6H4-OH), able to release the amino-carbonyl ligand as isocyanate HOC6H4NCO, was isolated. The aliphatic amino-alcohols H2N-R-OH [2-aminoethanol (2-AE, b); 1-amino-2-propanol (1-A2P, c); 3-aminopropanol (3-AP, d); 2-aminobutanol (2-A1B, e); 4-aminobutanol (4-AB, f); 5-aminopentanol (5-APE, g); 6-aminohexanol (6-AHX, h)] were reacted with all Pd complexes described above, but only when the triphenylphosphine ligand was used was it possible to isolate the stable carbamoyl complexes (PPh3)2PdCl(CONH-R-OH). The Pd(II) complexes with other ligands reacted in the same way, but the relevant intermediate complexes decomposed during the progress of the reaction, giving a mixture of cyclic carbamates and ureas and the relevant Pd(0) complex, “Pd-L” (L = PN, dppe, dipy, phen), which converted into Pd-black and the free ligand. Carbamoyl complexes with an aliphatic amino-alcohol bearing a primary amine are unprecedented in the literature. They were characterized by means of IR and NMR spectroscopy and studied for their reactivity. All the complexes upon simple heating or by reaction with I2 or CuCl2 release the amino-carbonyl function as cyclic carbamate and/or urea, depending on the complex and the presence or absence of the free amino-alcohol in solution.