Synthesis of functionalized biaryls using iminophosphorane palladacycles as Suzuki catalysts

The Suzuki-Miyaura cross coupling reaction is a very powerful tool for making of C–C bonds from organic halides and organoboron compounds [1]. This reaction, for which palladacycles have shown great catalytic activity [2] is widely employed in academic laboratories as well as in pharmaceutical industries to synthesize organic compounds or drugs such as Losartan [3], an antihypertensive medicine. Iminophosphoranes are organic compounds of general composition R3P=NR and can be used as in coordination and in organometallic chemistry [4]. The aim of this work is to study the catalytic activity of new palladium organometallic compounds derived from iminophosphorane ligands.


Introduction
Cross-coupling reactions have been widely used in organic synthesis as well as in the chemical industry.One of the most usual is the Suzuki catalysis, which allows formation of C-C bonds between two aryls by reaction of an organic halide and an organoboron compound.To perform the reaction a catalyst is needed, generally a palladium compound.In 1989, Hermann et al. showed that palladium cyclometallated compounds could behave as excellent catalysts for this reaction [2].
Iminophosphoranes are ligands in coordinated compounds as well as in cyclometallated compounds with many metals, especially Au [5], Pd [6] and Pt [7], which have luminescent and anti-cancer properties.For these reasons, we have considered important to approach the synthesis of new palladium (II) cyclometallated compounds derived from iminophosphoranes and to consider their properties as Suzuki catalysts.

Methods
2-(methylthio)aniline iminophosphoranes may behave as tridentante ligands, which may be easily obtained by the Staudinger reaction as depicted in Scheme 1.

Scheme 1: Staudinger reaction
Compound 2 may be obtained by the reaction of a palladium salt with the iminophosphorane ligand (Scheme 2).The presence of a base is not necessary but facilitates the cyclometallation reaction and prevents de formation of the [N,S] coordination compound.Scheme 2: Formation of the cyclometallated compound Compound 2 is an air-stable bright yellow solid.The metal atom shows a square-planar geometry and has two covalent bonds with nitrogen and sulfur atoms and one σ bond with the carbon atom.

Characterization
The compounds were characterized by 1 H and 31 P{ 1 H} NMR spectroscopy.
The 1 H NMR shows the formation of the iminophosphorane 1, the three phenyl rings bonded to phosphorus are chemically equivalent (Figure 1).Comparison of the 31 P{ 1 H} spectra of compounds 1 and 2 clearly shows a shift of the signal assigned to the phosphorus nucleus (Figure 3).

Catalysis
The cross-coupling reaction between 4-bromoacetophenone and phenylboronic acid was studied using a 2% M compound 2 as catalyst.The reaction was carried out in ethanol, using an excess of K3PO4 as base (Scheme 3).

Scheme 3. Catalysis reaction
The yield of the reaction was measured by 1 H NMR spectroscopy (Table 2).

Conclusions
A new palladacycle derived from an iminophosphorane ligand has been synthesized and characterized.This compound shows good catalytic activity in the Suzuki-Miyaura reaction even at relatively low temperatures.

Table 1 .
Crystal data