Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

Schiff base ligands are considered “privileged ligands” because they are easily prepared by the condensation between aldehydes and imines. Stereogenic centres or other elements of chirality (planes, axes) can be introduced in the synthetic design. Schiff base ligands are able to coordinate many different metals, and to stabilize them in various oxidation states, enabling the use of Schiff base metal complexes for a large variety of useful catalytic transformations. Practical guidelines for the preparation and use of different Schiff base metal complexes in the field of catalytic transformations are discussed in this tutorial review.

Metal–Salen Schiff base complexes in catalysis: practical aspects

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

The use of coordinating moieties as directing groups for the functionalization of aromatic CH bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of CH activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non-chelate-assisted CH activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.

Beyond directing groups: transition-metal-catalyzed C-H activation of simple arenes.

140 years ago Adolf von Baeyer proposed the structure of a heteroaromatic compound which revolutionized organic and medical chemistry: indole. After more than a century, indole itself and the complexity of naturally occurring indole derivatives continue to inspire and influence developments in synthetic chemistry. In particular, the ubiquitous presence of indole rings in pharmaceuticals, agrochemicals, and functional materials are testament to the ever increasing interest in the design of mild and efficient synthetic routes to functionalized indole derivatives. This Review emphasizes the achievements in the selective catalytic functionalization of indoles (C-C bond-forming processes) over the last four years.

Catalytic Functionalization of Indoles in a New Dimension

After more than 125 years, the Friedel-Crafts alkylation is still one of the most studied and most utilized reactions in organic synthesis. What is the secret of this astonishing success? Perhaps the great versatility in scope and applicability continues to justify its crucial role in the synthesis of more and more complex molecules. However, it has taken more than a century for asymmetric catalytic versions of this reaction to be developed and subsequently extended to a range of aromatic compounds and alkylating agents. Herein we review recent developments in the design and use of catalytic and stereoselective strategies for the alkylation of aromatic systems and synthesis of a wide range of polyfunctionalized enantiomerically enriched compounds.

New Catalytic Approaches in the Stereoselective Friedel–Crafts Alkylation Reaction

Synthetic organic chemistry has been markedly affected by the booming of gold catalysis over the past decade. The renaissance of this coinage metal allowed unprecedented transformations to be realized in a highly selective manner and rendered "old chemistry" more accessible from a practical point of view. Particularly, organic compounds containing C-C multiple bonds benefited from the high carbophilicity of gold species, that opened access to a great chemical diversity through direct and selective π-electrophilic activations. Nowadays, the complexity of naturally occurring compounds based on functionalized aromatic frameworks continues to inspire and influence developments in synthetic chemistry. Furthermore, the ubiquitous presence of arene-based systems in pharmaceuticals, agrochemicals, and functional organic materials warrants the growing demand for mild, selective and sustainable synthetic routes to their preparation. In this context, although the peculiar aptitude of gold salts/complexes for interaction with aromatic compounds (auration process) has long been known, the direct catalytic gold decoration of arenes, has risen to prominence only recently. Here, the extensive use of electrophilic activation of C-C multiple bonds by gold species deserves a prominent mention, and the great strides made in the field over the last few years are described in this tutorial review.

Gold-catalyzed decorations of arenes and heteroarenes with C-C multiple bonds.

In this Account our recent results in relation to the catalytic and stereoselective Friedel-Crafts (FC) alkylation of indoles are described. Over the last decade, remarkable efforts have been devoted towards the replacement of the primal approaches with new more efficient, reliable, and environmentally benign strategies for the functionalization of indoles. Moreover, the emerging area of catalytic asymmetric FC processes is addressed and some examples of enantioselective alkylations of indoles via 1,4-additionto α,β-un-saturated systems and asymmetric ring-opening reaction of aromatic epoxides are described.

A Journey Across Recent Advances in Catalytic and Stereoselective Alkylation of Indoles

Homogeneous gold catalysis has received growing attention over the past few years, enabling the replacement of consolidated organic reactions with more simple, selective, and chemically sustainable alternatives. The fine-tunability of the electronic as well as steric properties of gold catalysts contributed substantially to the popularity of the research field, with robust applications in total synthesis and asymmetric catalysis. In this context, the metal counterions proved of pivotal importance in impacting both kinetics and selectivity of gold-assisted transformations. Despite the intrinsic difficulties in properly rationalizing and predicting the role of anions in complex reaction machineries, nowadays, some general trends are available. This review aims at presenting some leading examples of counterion-controlled gold catalysis, with particular emphasis on their structure–activity relationship.

Marco Bandini

Papers

Catalytic Functionalization of Indoles in a New Dimension

New Catalytic Approaches in the Stereoselective Friedel–Crafts Alkylation Reaction

Gold-catalyzed decorations of arenes and heteroarenes with C-C multiple bonds.

A Journey Across Recent Advances in Catalytic and Stereoselective Alkylation of Indoles

Counterion Effects in Homogeneous Gold Catalysis