Although the chemistry of Pd(0), Pd(I) and Pd(II) is well established, high oxidation state Pd(IV) complexes are less well-known. This situation has highly changed in recent years. Many well-defined Pd(IV) complexes has been isolated and characterized, providing evidence for a series of proposed Pd(II)/Pd(IV) catalytic reactions. A deep understanding of the behavior of Pd(IV) complexes could lead to the design and development of novel reactions that could not be accessed by traditional Pd(0)/Pd(II) chemistry. This critical review describes the stoichiometric reactions of Pd(IV) complexes and discusses their potential mechanism in catalytic reactions (137 references).

Organopalladium(IV) chemistry

Palladium-catalyzed bond-forming processes (e.g., C-C, C-X, C-Y; X ) F; Y ) NR2, OR, SR, etc.) represent essential tools for the synthetic chemist. A fascinating myriad of adventurous and unique Pd-catalyzed transformations are routinely found as key steps in target-oriented syntheses, affording complex natural products, functional advanced materials, fluorescent compounds, pharmaceutical lead compounds, and other high-value commercial products. Innovative Pd catalyst design, the identification of new synthetic methodologies, and the acquirement of detailed mechanistic insight, spanning both homogeneous and heterogeneous fields, underpin the numerous developments seen in this area over the past 40 years. Most commonly, Pd-catalyzed bond-forming processes involve Pd0/PdII complexes as intermediates. In recent times, the involvement of PdIV complexes have been implicated in many new synthetic methodologies, for which important advances have been made in the last 5 years or so. While observing the emergence of catalytic PdIV chemistry, particularly in organic synthesis, we identified the need to comprehensively review this area, which draws on aspects from both inorganic (organometallic) and organic chemistry fields. The historical background to organopalladium(IV) chemistry is therefore detailed. We have selected a wide range of diverse transformations where PdIV complexes are believed to act as key intermediates. It is clear that catalytic reaction manifolds involving PdIV intermediates offer new * Corresponding author. E-mail: ijsf1@york.ac.uk. Telephone: +44 (0)1904 434091. Fax: +44 (0)1904 432516. Chem. Rev. 2010, 110, 824–889 824

Emergence of palladium(IV) chemistry in synthesis and catalysis.

Sensing of metal ions and anions is of great importance because of their widespread distribution in environmental systems and biological processes. Colorimetric and fluorescent chemosensors based on organic molecular species have been demonstrated to be effective for the detection of various ions and possess the significant advantages of low cost, high sensitivity, and convenient implementation. Of the available classes of organic molecules, porphyrin analogues possess inherently many advantageous features, making them suitable for the design of ion chemosensors, with the targeted sensing behavior achieved and easily modulated based on their following characteristics: (1) NH moieties properly disposed for binding of anions through cooperative hydrogen-bonding interactions; (2) multiple pyrrolic N atoms or other heteroatoms for selectively chelating metal ions; (3) variability of macrocycle size and peripheral substitution for modulation of ion selectivity and sensitivity; and (4) tunable near-infrared emission and good biocompatibility. In this Review, design strategies, sensing mechanisms, and sensing performance of ion chemosensors based on porphyrin analogues are described by use of extensive examples. Ion chemosensors based on normal porphyrins and linear oligopyrroles are also briefly described. This Review provides valuable information for researchers of related areas and thus may inspire the development of more practical and effective approaches for designing high-performance ion chemosensors based on porphyrin analogues and other relevant compounds.

Development of Ion Chemosensors Based on Porphyrin Analogues

The unique reactivity of hypervalent iodine reagents with Pd0 and PdII complexes has been exploited for a variety of synthetically useful organic transformations. For example, IIII reagents have been used in place of aryl halides for diverse Pd-catalyzed C−C and C−heteroatom bond-forming cross-coupling reactions. In addition, these reagents have found application in Pd-catalyzed oxidation reactions, including the oxidative functionalization of C−H bonds and the 1,2-aminooxygenation of olefinic substrates. This review discusses both the synthetic utility and the interesting mechanistic features of these transformations.

Reactions of hypervalent iodine reagents with palladium: mechanisms and applications in organic synthesis.

Porphyrins and related families of molecules are important organic modules as has been reflected in the award of the Nobel Prizes in Chemistry in 1915, 1930, 1961, 1962, 1965, and 1988 for work on porphyrin-related biological functionalities. The porphyrin core can be synthetically modified by introduction of various functional groups and other elements, allowing creation of numerous types of porphyrin derivatives. This feature makes porphyrins extremely useful molecules especially in combination with their other interesting photonic, electronic and magnetic properties, which in turn is reflected in their diverse signal input–output functionalities based on interactions with other molecules and external stimuli. Therefore, porphyrins and related macrocycles play a preeminent role in sensing applications involving chromophores. In this review, we discuss recent developments in porphyrin-based sensing applications in conjunction with the new advanced concept of nanoarchitectonics, which creates functional nanostructures based on a profound understanding of mutual interactions between the individual nanostructures and their arbitrary arrangements. Following a brief explanation of the basics of porphyrin chemistry and physics, recent examples in the corresponding fields are discussed according to a classification based on physical modes of detection including optical detection (absorption/photoluminescence spectroscopy and energy and electron transfer processes), other spectral modes (circular dichroism, plasmon and nuclear magnetic resonance), electronic and electrochemical modes, and other sensing modes.

Porphyrin-based sensor nanoarchitectonics in diverse physical detection modes

Palladated azobenzenes and regiospecific aromatic metaloxylation

L'etude est faite sur les complexes dichloro(m-tolylazo-2 pyridine)-Pd(II) (I) et trans-bis(phenylazoacetaldoximats)-Pt(II) (2) En faisant bouillir une suspension de (1) dans l'eau en presence de Ag + dissous, on obtient un complexe Pd C 12 H 10 N 3 OCl dont on etudie la structure: le noyau phenyl a subi l'hydroxylation, avec formation d'une liaison Pd-O

Two novel examples of hydroxylation of aromatic rings in coordination chemistry

The C–Pd bond in cyclopalladated azobenzenes [(1) and (3)] is converted into the C–OPd function [(2) and (5)] in good yield by stoicheiometric reactions with m-chloroperbenzoic acid; in effect azobenzene is successively oxidised to 2-hydroxyazobenzene and 2,2′-hydroxyazobenzene.

Pinaki Bandyopadhyay

Papers

Palladated azobenzenes and regiospecific aromatic metaloxylation

Two novel examples of hydroxylation of aromatic rings in coordination chemistry

Aromatic hydroxylation: the direct oxidation of C–Pd into C–OPd

Oxygen insertion into palladium-arene bonds by iodosylbenzene

Regiospecific cyclometallation: Synthesis, spectral properties and crystal structure of peri -palladate