All chemicals were purchased as reagent grade from commercial suppliers and were used without further purification unless otherwise noted. Phosphazane derivatives (L_1-3) were prepared according to the literature procedures published by our group ^{[15, 16]}. Solvents were dried by passing over Al₂O₃ and/or by storing over molecular sieves unless otherwise noted. The ¹H NMR and ¹³C-NMR spectra were recorded on a Bruker FT-NMR 500 MHz spectrometer.

All Suzuki reactions were carried out without an inert atmosphere. A mixture of aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K₂CO₃ (2 mmol), Pd(OAc)₂ (0.5 mol%) and phosphazane ligand (L_1-3) (1.0mol%) in H₂O (3 mL) was allowed to react in a sealed tube at 90 ℃. The reaction mixtures were added to brine (15 mL) and extracted three times with diethyl ether (3 × 15 mL). The further purification of the product was achieved by flash chromatography on a silica gel column using hexane/ethyl acetate (4:1).

Characterization data of ligands (L_1-3) and coupling products are listed in Supporting information.

Phosphorus trichloride and PhNH₂, react readily to form a series of PhNH/PhN-substituted P(Ⅲ) phosphazanes. According to our previous literature ^{[15, 16]}, the nearly quantitative product L₁ is obtained at 110 ℃ in toluene, with a PCl₃:PhNH₂ molar ratio of 1:4.8. Compound L₂ is formed in greater than 80% yield by reaction of PCl₃ with PhNH₂ (1:6 molar ratio, 2 h, 0 ℃) in methylene chloride. Also at r.t.with a PCl₃:PhNH₂ molar ratio of 1:5, themain product L₃ is produced. To investigate the reactivity of L_1-3 as ligand, we first employed 4-acetylbromobenzene and phenylboronic acid as substrates to optimize the reaction conditions. Their coupling reactions were carried out under a variety of palladium sources, bases and solvents. The results are summarized in Table 1.

Table 1

Table 1 The optimized reaction condition for the Pd-catalyzed Suzuki reaction.a

Table 1 The optimized reaction condition for the Pd-catalyzed Suzuki reaction.a

The catalytic applicability of the phosphazane ligand L₁ was explored with a range of aryl halides and arylboronic acids with the results summarized in Table 2. All the reactions were carried out at 90 ℃ in air using H₂O as a reaction solvent.

As shown in Table 2, the Suzuki coupling of various phenylboronic acids with aryl iodide and bromide provided good to excellent yields of the products (entries 1-16). The reactions between aryl iodides and various phenylboronic acids were quantitative and afforded biphenyl derivatives in 91%-97% yields within 2 h with the use of 0.5 mol% of Pd(OAc)₂ in H₂O. It is worth noting that the most electron-deficient aryl bromides, 4- acetyl and 4-nitro bromobenzene, quantitatively gave the coupling products (entries 12 and 16). In order to test the feasibility of this protocol for challenging substrates, we also conducted the Suzuki reactionswith aryl chlorides and observed moderate to good product yields (entries 17-21). High catalytic activity was observed in the coupling of phenylboronic acid and 4-methoxyphenylboronic acid with aryl iodides and bromides, and moderate to good activity with chlorides. 2-Methoxyphenylboronic acid was less reactive in comparison to both phenylboronic acid and 4-methoxy phenylboronic acid (entries 2, 10 and 22).

Recently, Iranpoor and co-workers reported the use of 1,3,2,4- diazadiphosphetidine compounds (L_1-3) as ligands for Pd(Ⅱ)- catalyzed Suzuki coupling reactions of aryl iodides, bromides, and chlorides under base-free conditions in water ^[17]. In fact, they used high amounts of phosphazen ligands (0.3 mmol) as both a ligand and a base. According to high cost and toxicity of these ligands, use of a high amount of them is not good idea and therefore in this work we use only 1.0 mol% of phosphazane derivatives as ligand (0.01 mmol) for Suzuki coupling reaction. In contrast to similar, previously reported systems, the catalytic system presented in this paper does not suffer from the harsh reaction conditions, such as using large amounts of hazardous phosphine-based ligands, long reaction time and high reaction temperature, but both two systems are almost equally effective Suzuki coupling catalysts.

In conclusion, we have developed an efficient and environmentally friendly water-soluble phosphazane oligomer as ligand which is readily accessible from inexpensive and commercial materials. The in situ generated catalysts from Pd(OAc)₂ and the phosphazane ligand L₁ demonstrated good catalytic activities towards the Suzuki reaction of aryl halides and various phenylboronic acids in presence of water, as the green solvent. Under aerobic conditions, good to excellent yields of the coupled products were obtained.

M. Amini and A. Tarassoli thank respectively the Research Council of University of Maragheh and Shahid Chamran University for funding of this work.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2013.10.002.