The substituted indole nucleus [indole is the acronym from indigo (the natural dye) and oleum (used for the isolation)] is a structural component of a vast number of biologically active natural and unnatural compounds. The synthesis and functionalization of indoles has been the object of research for over 100 years, and a variety of well-established classical methods are now available, to name a few of them, the Fisher indole synthesis, the Gassman synthesis of indoles from N-halo-anilines, the Madelung cyclization of N-acyl-o-toluidines, the Bischler indole synthesis, the Batcho-Leimgruber synthesis of indoles from o-nitrotoluenes and dimethylformamide acetals, and the reductive cyclization of o-nitrobenzyl ketones.1 In the last 40 years or so, however, palladiumcatalyzed reactions, generally tolerant of a wide range of functionalities and therefore applicable to complex molecules, have achieved an important place in the arsenal of the practicing organic chemist. Since the invention of an industrial process for the palladium-catalyzed production of acetaldehyde from ethylene in the presence of PdCl2 and CuCl2, an everincreasing number of organic transformations have been based on palladium catalysis. Almost every area of the organic synthesis has been deeply influenced by the profound potential of this versatile transition metal, modifying the way organic chemists design and realize synthetic processes.2,3 Because of its catalytic nature, palladium-catalyzed synthesis can provide access to fine chemicals, agrochemical and pharmaceutical intermediates, and active ingredients in fewer steps and with less waste than classical † In memory of Prof. Bianca Rosa Pietroni, a colleague and very close friend. * To whom correspondence should be addressed. Phone: + 39 (06) 4991-2785. Fax: + 30 (06) 4991-2780. E-mail: sandro.cacchi@ uniroma1.it. 2873 Chem. Rev. 2005, 105, 2873−2920

Synthesis and functionalization of indoles through palladium-catalyzed reactions

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

2.3. Hydrocarbon Coordination 2475 2.4. σ Complexes of Alkanes and Arenes 2475 2.5. Principle of Microscopic Reversibility 2476 2.6. Kinetic Isotope Effects 2477 3. Shilov-type Chemistry 2478 3.1. C−H Activation by Pt(II) 2479 3.2. Mechanism of Multiple H/D Exchange 2480 3.3. H/D Exchanges1,2 Shift in Ethane 2481 3.4. Computational Studies of the Shilov System 2481 3.5. C−H Activation by Pt(IV) 2482 4. “Catalytica” System for Catalytic Methane Functionalization 2482

http://www.uwindsor.ca/users/j/jlichaa/reference.nsf/0/10ff8b04ff3a317885256d88005720f6/$FILE/cr-2005-105-2471%20tilset%20rev%20ch%20activ.pdf

Mechanistic Aspects of C¿H Activation by Pt Complexes

Update 1 of: Synthesis and Functionalization of Indoles Through Palladium-Catalyzed Reactions

Reactions of (π-1,5-cyclooctadiene) organoplatinum(II) compounds and the synthesis of perfluoroalkylplatinum complexes

Synthesis and reactivity of novel palladium(II)-isocyanide complexes

Solution Behavior and X-ray Structure of Cationic Allylpalladium(II) Complexes with Iminophosphine Ligands. Kinetics and Mechanism of Allyl Amination by Secondary Amines

Reactions of pyridine-2-carbaldimines with chloro-bridged palladium(II) and platinum(II) 2-methylallyl dimers. Solution behaviour of the cationic complexes [M(n3-2-MeC3H4)(py-2-CHNR)]+

I.r. spectra of palladium(II) halogeno-bridged complexes of the type [Pd(N–C)X]2= azobenzene-2-C,N; NN-dimethylbenzylamine-2-C,N; and 2-methoxy-3-NN-dimethylaminopropyl; X = Cl or (Br) have been recorded in the range (600–120 cm.–1). The bridging-halogen stretching frequencies are discussed on the basis of a planar asymmetric Pd[graphic omitted]Pd unit.Bridge-splitting reactions with mono- and bi-dentate ligands bearing nitrogen or phosphorous as donor atoms are reported and the structures of the resulting products are interpretated on the basis of their far-i.r. spectra. We have also established that the v(Pd–Cl) frequency in the products in the range 300–280 cm.–1 with chloride in the trans-position to the phenyl or alkyl groups.

Bruno Crociani

Papers

Synthesis and reactivity of novel palladium(II)-isocyanide complexes

Solution Behavior and X-ray Structure of Cationic Allylpalladium(II) Complexes with Iminophosphine Ligands. Kinetics and Mechanism of Allyl Amination by Secondary Amines

Reactions of pyridine-2-carbaldimines with chloro-bridged palladium(II) and platinum(II) 2-methylallyl dimers. Solution behaviour of the cationic complexes [M(n3-2-MeC3H4)(py-2-CHNR)]+

Far-infrared study of palladium(II)–halogen complexes with chelating ligands containing nitrogen and σ carbon as donor atoms

Kinetics of nucleophilic attacks on palladium(II) isocyanide complexes