Efficient oxidative coupling of 2,6-disubstituted phenol catalyzed by a dicopper(II) complex.

Complexation of a rigid multi-pyridine ligand bis(2-pyridyl)-1,8-naphthyridine (bpnp) with [Cu(2)(TFA)(4)] (TFA = trifluoroacetate) resulted in the formation of a dinuclear copper(II) complex, namely [Cu(2)(bpnp)(µ-OH)(TFA)(3)] (1). This complex has been characterized by X-ray crystallographic, spectroscopic and elemental analyses. Complex 1 is an efficient catalyst for the oxidative coupling of various 2,6-disubstituted phenols with molecular oxygen. Yields and selectivity depend on the reaction conditions employed, the best results being obtained in isopropanol or dioxane at 90 °C with yields of >99%. Mechanistic pathway of the catalysis is discussed.

Palladium(II) complexes based on 1,8-naphthyridine functionalized N-heterocyclic carbenes (NHC) and their catalytic activity.

Palladium complexes containing 2,7-bis(mesitylimidazolylidenyl)naphthyridine (NHC-NP) have been synthesized and characterized. Reaction of [{Ag(3)(NHC-NP)(2)}(PF(6))(3)] with [Pd(PhCN)(2)Cl(2)] provided an unusual dipalladium complex bridged by two NHC-NP units, forming a 20-membered dinuclear metallacycle [{Pd(2)(NHC-NP)(2)Cl(2)}(PF(6))] (2) in high yield. Treatment of 2 with KI in acetone yielded a neutral species [Pd(2)(NHC-NP)I(4)] (3). Meanwhile, the pyridinyl N-heterocyclic carbene (NHC-Py) precursor, 1-(2-pyridinyl)-3-mesitylimidazolium chloride, reacted with Pd(2)(dba)(3) directly to form the mononuclear palladium complex [Pd(NHC-Py)Cl(2)] (4). These complexes were characterized by elemental analyses as well as NMR spectroscopy, and the structures of 3 and 4 were further identified by X-ray diffraction analysis. The use of these palladium complexes for Suzuki-Miyaura and Kumada-Corriu coupling reactions has been examined. There is no significant difference in catalytic activities between 2 and 4 in Suzuki-Miyaura coupling reactions. However, the catalytic activity of 2 in the Kumada-Corriu coupling of ArBr with cyclohexylmagnesium bromide is quite different from that of 4. Thus complex 2 is active for the cross coupling, but complex 4 is active for the reduction of aryl halides.

Synthesis, characterization and catalytic activity of saturated and unsaturated N-heterocyclic carbene iridium(i) complexes.

Both saturated and unsaturated N-benzyl substituted heterocyclic carbene (NHC) iridum(i) complexes were synthesized. The unsaturated carbene complex [(un-NHC-Bn)Ir(CO)(2)Cl] in the cis form was prepared via the carbene transfer from the corresponding silver complex to [Ir(COD)(2)Cl](2) followed by ligand substitution with CO, whereas the saturated complex was obtained via the transfer from (sat-NHC-Bn)W(CO)(5). The treatment of phosphines with (NHC)Ir(CO)(2)Cl complexes yielded the products with the phosphine ligand trans to the carbene moiety via substitution. X-Ray structural determination shows that distances of Ir-C((carbene)) in both (un-NHC-Bn)Ir(CO)(PR(3))Cl and (un-NHC-Bn)Ir(CO)(PR(3))Cl are essentially the same. Analyses of spectroscopic and crystal structural data of iridium complexes [(NHC)Ir(CO)(PR(3))Cl] and Vaska's complex show similar corresponding data in both types of complexes, suggesting that the studied NHC ligands and phosphines have similar bonding with Ir(i) metal center. All iridium complexes studied in this work illustrated their catalytically activity on N-alkylation of amine with alcohol via hydrogen transfer reduction. It appears no dramatic difference on the catalytic activity among these iridium carbene complexes; but the saturated carbene complex (sat-NHC-Bn)Ir(CO)(PR(3))Cl appears to be slightly more active. For example, the reaction of benzyl alcohol with aniline in the presence of catalyst (1 mol%) under basic conditions at 100 degrees C provided the secondary amine (N-benzylaniline) in 96% yield.

Coordination chemistry and catalytic activity of N-heterocyclic carbene iridium(I) complexes.

Iridium complexes [(CO)2Ir(NHC-R)Cl] (R = Et-, 3a; PhCH2-, 3b; CH3OCH2CH2-, 3c; o-CH3OC6H4CH2-, 3d; NHC: N-heterocyclic carbene) are prepared via the carbene transfer from [(NHC-R)W(CO)5] to [Ir(COD)Cl]2. By using substitution with 13CO, we are able to estimate the activation energy (G) of the CO-exchange in 3a-d, which are in the range of 12-13 kcal mol-1, significantly higher than those for the phosphine analog [(CO)2Ir(PCy3)Cl]. Reactions of 3b and 3d with an equimolar amount of PPh3 result in the formation of the corresponding [(NHC-R)Ir(CO)(PPh3)Cl] with the phosphine and NHC in trans arrangement. In contrast, the analogous reaction of 3a or 3c with phosphine undergoes substitution followed by the anion metathesis to yield the corresponding di-substituted [(NHC-R)Ir(CO)(PPh3)2]BF4 (5) directly. Treatment of 3b or 3d with excess of PPh3 leads to the similar product of disubstitution 5b and 5d. The analysis for the IR data of carbonyliridium complexes provides the estimation of electron-donating power of NHCs versus phosphines. The NHC moiety on the iridium center cannot be replaced by phosphines, even 1,2-bis(diphenylphohino)ethane (dppe). All the carbene moieties on the iridium complexes are inert toward sulfur treatment, indicating a strong interaction between NHC and the iridium centers. Complexes 3a-c are active on the catalysis of the oxidative cyclization of 2-(o-aminophenyl)ethanol to yield the indole compound. The phosphine substituted complexes or analogs are less active.

Synthesis of iridium pyridinyl N-heterocyclic carbene complexes and their catalytic activities on reduction of nitroarenes.

Coordination of iridium(I) metal ions with a pyridinyl imidazol-2-ylidene ligand (pyNwedgeC-R) [R=Me, mesityl(2,4,6-trimethylphenyl)] that processes bulky substituents has been investigated. The iridium carbene complexes [(C-pyNwedgeC-R)IrCl(COD)] (COD=1,5-cyclooctadiene) are prepared via transmetalation from the corresponding silver carbene complexes. Upon the abstraction of chloride, the chelation of pyNwedgeC becomes feasible, resulting in the formation of [C,N-(pyNwedgeC-R)Ir(COD)](BF4) (4). The coordinated COD of complex 4 can be replaced by carbon monoxide to yield the corresponding carbonyl species [C,N-(pyNwedgeC-R)Ir(CO)2](BF4). The labile nature of the pyridinyl nitrogen donor is readily replaced by acetonitrile, as is evidenced by the NMR study. All iridium complexes show catalytic activity on the hydrogen-transfer reduction of carbonyl and nitro functionalities. By manipulation of the reaction conditions, the iridium-catalyzed reduction of nitroarenes can selectively provide aniline or azo compounds as the desired product.