Yuan Han

Pd-carbene catalyzed carbonylation reactions of aryl iodides†

A series of carbene complexes [PdBr(2)((i)Pr(2)-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions.

Syntheses and catalytic activities of pseudo-pincer and CSC pincer-type Pd(II) complexes derived from benzannulated N-heterocyclic carbenes

Three new dibenzimidazolium salts bearing thioether (B.2HBr, B.2HNO3) and sulfoxide (C.2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B.2HBr and C.2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-kappa2C)] (1) and cis-[PdBr2(C-kappa2C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B.2HNO3 in the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex [PdBr(B-kappa3CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki-Heck reaction have been evaluated as well.

Pyrazole‐Derived Remote Dicarbenes: Versatile Ligands for Di‐ and Tetranuclear Complexes

The spectacular success of N-heterocyclic carbenes (NHCs) as a new class of ligands in organometallic chemistry and their application to catalytic systems has been primarily attributed to their strong electron-donating nature. The extension to yet stronger donors has recently lead to the design of non-conventional NHCs including remote NHCs (rNHCs), cyclic (alkyl)aminocarbenes (CAACs) and “carbodicarbenes” (CDCs), which can also be regarded as zerovalent carbon complexes. In relation to this development, we have recently introduced complexes of pyrazole-based remote and abnormal NHCs, which are isomeric to imidazolin-2-ylidenes and thus were termed pyrazolin-4ylidenes. However, their true nature is currently still under debate, and as depicted in Scheme 1, these ligands can be represented by a zwitterion (Ia), an abnormal rNHC (Ib) or a neutral cyclic bent allene (Ic). Although more detailed investigations are required to reveal their true identity, there is no doubt that they form a new class of strongly donating ligands (L) as shown by an even lower CO stretching frequency in [RhCl(L)(CO)2] compared to those in classical NHCs or phosphine analogues. Owing to the substantial carbenoid ligand character in their metal complexes we will refer to them as abnormal rNHCs in this work. The coordination chemistry of such abnormal rNHCs is poorly understood and only monodentate examples have been reported. Bior multidentate ligands of this type capable of constructing macrocyclic entities through metal-directed self-assembly, an area of great interest, remain unknown. Among such architectures, molecular squares built from four metal-containing corners linked by four spacers have been widely studied. Commonly-used spacers contain nitrogen donor groups, whereas spacers utilizing carbon donors are very rare. Only recently, Bielawski reported linear dior polynuclear complexes incorporating Janus-type NHCs as spacers. Employing these spacers, a Ni rectangle with 4,4’-bipyridine as a second linker was reported by Hahn. Herein, we report on the coordination chemistry of unprecedented remote and abnormal dicarbenes and their use as a new class of organometallic spacers for the formation of diand tetranuclear Pd complexes. The suitable ligand precursors with bridges of different lengths (2 a–c) were obtained through alkylation of 4,4’-diACHTUNGTRENNUNGiododipyrazoles[15] 1 a–c with Me3OBF4 as white solids in moderate to high yields of 86 %, 93 % and 62 %, respectively (Scheme 1). The topology of 2 a–c ensures the exclusive formation of dior polynuclear complexes and prevents chelation of the resulting dicarbenes. Dipyrazolium salt 2 c was reacted with a pre-mixed solution of [Pd2ACHTUNGTRENNUNG(dba)3]/PPh3 (1:4 ratio, dba=dibenzylideneacetone) in CH2Cl2 (Scheme 2) in an attempt to synthesize a dinuclear Pd–dicarbene complex. Owing to the long propylene chain, the two pyrazolium moieties are not likely to interfere with each other to a significant extent, and oxidative addition is expected to proceed in a similar manner as that [a] Dr. Y. Han, L. J. Lee, Dr. H. V. Huynh Department of Chemistry, Faculty of science National University of Singapore 3 Science Drive 3, Singapore 117543 (Singapore) Fax: (+65) 6779-1691 E-mail : chmhhv@nus.edu.sg Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902737. Scheme 1. Selected resonance structures of “pyrazolin-4-ylidenes” (Ia–c) and synthesis of di(pyrazolium) salts 2a–c.

Preparation and characterization of the first pyrazole-based remote N-heterocyclic carbene complexes of palladium(II).

The first pyrazolin-4-ylidene complexes of palladium(II) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd(2)(dba)3/PPh(3) and were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.