2,2 -Bipyrimidine catalyst

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

Both pyrimidine and quinazoline derivatives have been investigated as ligands for palladium-mediated reactions. Examples of ligands prepared include the phosphine-free bipyrimidine 1181, which gave the isolable palladium dichloride catalyst 1182 <2001JOM(634)39>, and the chiral oxazolinyl-quinazoline BINAP analogs 1183 and 1184 <20060L5109>. 

Oxidation of Methane. A variety of new catalyst systems have been disclosed, and new reagents were developed with the aim to perform selective transformation of methane to methanol, methyl esters, and formaldehyde. Much work was carried out in strongly acidic solutions, which enhances the electrofilicity of the metal ion catalyst, and the ester formed is prevented from further oxidation. An important advance in the selective oxidation of methane to methanol is Periana s 70% one-pass yield with high selectivity in sulfuric acid solution under moderate conditions.1073 The most effective catalyst is a Pt-bipyrimidine complex. Pt(II) was shown to be the most active oxidation state generating a Pt-methyl intermediate that is oxidized to yield the product methyl ester. A density functional study... 

The relatively vigorous reaction conditions required to effect vinyl substitution by heteroaryl halides under the influence of Pd-catalysis may lead to homo-coupling. The dominating reaction path in attempted alkenyl-ations of 4-iodopyrimidines was the formation of 4,4 -bipyrimidines (19) (Scheme 4 Section II.A.2). In reactions of 4-iodopyrimidines with a Pd-catalyst at 160°C, near-quantitative yields of the 4,4 -bipyrimidine were obtained (79CPB193). Also, in the reaction of 2-iodo-4-methylquinoline with Pd(OAc)2 as added catalyst, a major reaction path led to 4,4 -dimethyl-2,2 -biquinoline (Scheme 2 Section II.A.2.) (82CPB3647). 

A later variant involved incorporation of an oxidant, Pt(IV), which led to formation of functionalized species, RX, from alkane, RH. In the typical chloride-rich Shilov systems, X is commonly Cl and OH. The Pt(IV) oxidant is reduced to Pt(II) during the reaction, but it has proved hard to replace this expensive oxidant by a cheaper one while retaining activity. A remarkable system of this type discovered by Periana [109] uses cone. H2SO4 as both oxidant and solvent and a Pt(II) 2,2 -bipyrimidine complex as catalyst with the result that CH3OSO3H, a methanol derivative, is formed from methane. 

Figure 5.1 Illustration of chloride-free catalysts for the oxidative carbonylation of methanol (a) 2,2 -bipyrimidine-Cu(OAc)2 and (b) a Co"-Schiff base complex.

As a catalyst, the blue dimer has limitations due to oxidatively induced coordination of anions which slow down the catalytic cycles. Since the 0-0 bond forming step occurs at a single Ru(V) site, it has been demonstrated that simpler and more robust mononuclear Ru(lll) aquo complexes of the type [Ru(tpy)(bpm)(H20)] , where tpy is the 2,2 6 2" terpyridine and bpm is the bipyrimidine ligand, can undergo hundreds of turnovers without showing decomposition according to the cycle schematized in Scheme 2 [20]. It must be noted that the Ru(III) state appears to be a missing state due to instability toward the disproportionation to Ru(IV) and Ru(II). 

Several systems for selective catalytic reactions based on Shilov s system have been developed with oxidants more practical than platinum(IV). Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO,. One of the catalysts is a simple mercuric halide, and reactions catalyzed by this mercury compound generated methyl sulfate with turnover frequencies of 10" s" . The second system is more reactive and is based on a platinum complex containing a bipyrimidine ligand (Equation 18.7). In this case, methane is converted to methyl bisulfate with 81% selectivity, greater than 500 turnovers, and a turnover frequency of 10 s" . These reactions are selective for the functionalization of methane to this methanol derivative because the electron-withdrawing... 

A heterodinuclear iridium-ruthenium complex [Ir (Cp )(H20)-(bpm)Ru (bpy)2](S04)2 (Ir -OH2, Cp = j -pentamethyl-cyclopentadienyl, bpm = 2,2 -bipyrimidine, bpy = 2,2 -bipyridine) can also act as an effective catalyst for removal of dissolved O2 by the four-electron reduction of O2 with formic acid in water at an ambient temperature. The Ir complex reacts efficiently with O2, as shown by the spectral titration in Figure 4.24, where the absorption spectra due to Ir are changed to those of Ir -OH2. The titration... 

The failure of the mononuclear [Ru(bpy)2(py)OH2] to promote oxygen evolution followed by success of the blue dimer and the fact that the OEC itself contains a multi-metal centre, lead researchers to believe that multiple sites were required for catalytic water oxidation. However, the mechanism proposed for the activation of the blue dimer showed that in reality, only one of the metal centres is involved in the oxygen-oxygen bond formation step. This suggestion was later confirmed when Meyer and co-workers showed that single site ruthenium-aqua complexes were capable of oxidizing water in the presence of Ce(iv). One of these catalysts, [Ru(tpy)(bpm)OH2] " (tpy = terpyridine, bpm = 2,2 -bipyrimidine), is shown in Scheme 5.2. 

Ru(bpy)3] by triethanolamine which yields [Ru(bpy)3], the reductant for [Ru(bpy)2(CO)2]. Subsequent reaction with CO2 gives [Ru(bpy)2 (C0XC02)]". Similar chemistry is observed in the electrochemically driven reaction/ Complexes of the type [Ir(bpm)2Cl2] and [Rh(bpm)2Br2] (bpm = 2,2 -bipyrimidine) have also been investigated as catalysts for CO2 reduction/ ... 

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