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Vol. 26 No. 5
September-October 2004

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Recent Advances in Organometallic Chemistry Directed Towards Organic Synthesis

Pure and Applied Chemistry
Vol. 76, No. 3, pp. 453-695 (2004)

reviewed by William Tam

The development of new chemical reactions is a major focus of research activity in organic chemistry. Reactions that form multiple bonds, rings, and stereocenters are particularly important tools for efficient assembly of complex molecular structures. Many of the traditional organic reactions require the presence of polar functional groups in the substrate to facilitate the transformation. Metal catalysts and organometallic reagents provide new opportunities for highly selective reactions that are not achievable by traditional synthetic methods. For example, complexation of the metal to an olefin, diene, or acetylene significantly modifies the reactivity of this moiety, allowing for improved reactivity and novel chemistry. One of the most important consequences of metal complexation is to temporarily polarize and activate an otherwise unreactive species. In addition to rate enhancements observed in the presence of the metal catalyst, the possibility of achieving enantioselective transformations by adding chiral ligands is one of the most attractive features of this strategy. Thus, the use of metal catalysts and organometallic reagents in organic synthesis has increased dramatically in the last two decades.

The March 2004 issue of Pure and Applied Chemistry (Vol. 76, No. 3, pp. 453-695) focused on recent advances in the field of organometallic chemistry in organic synthesis. The issue contains 23 plenary and invited lectures presented at the 12th IUPAC International Symposium on Organometallic Chemistry Directed Towards Organic Synthesis (OMCOS-12), Toronto, Ontario, Canada, 6-10 July 2003. The themes of these papers include asymmetric synthesis using organometallic reagents, design of new catalysts for organic synthesis, new C-C and C-heteroatom bond-forming reactions, structural and mechanistic aspects of organometallic chemistry, organometallics in materials research, control and amplification of chirality by transition-metal catalysis, activation of alkynes by transition metals, novel carbon-carbon bond formations using transition metal catalysts, and the uses of organometallic reagents in natural products synthesis.

The work by Antonio M. Echavarren and co-workers on metal cyclopropyl carbenes in the reactions of alkynes with alkenes and furans, shed light on the involvement of metal carbenes as the key intermediates in the transition-metal catalyzed reactions of alkynes with alkenes or furans. Electrophilic transition metal complexes catalyze the reaction of enynes in the presence of water or alcohols to give hydroxy- or alkoxycyclization derivatives. The reaction proceeds by the anti addition of the alkene and the metal to the alkyne. The key intermediates in this reaction are cyclopropyl metal carbenes (Scheme 1), which are also probably involved in the metathesis-type rearrangement of enynes. A general scheme is proposed for the cyclization of enynes initiated by the coordination of the metal to the enyne by transition metals, which included 5-exo-dig and 6-endo-dig pathways. The intramolecular reaction of furans with alkynes also proceeds via cyclopropyl metal carbenes.

Tamio Hayashi reported the recent development of rhodium-catalyzed asymmetric addition of aryl and alkenylboron reagents to electron-deficient olefins. Asymmetric 1,4-arylation and -alkenylation was achieved by use of organoboronic acids or their derivatives in the presence of a rhodium catalyst coordinated with BINAP or its related ligands (Scheme 2). The scope of this asymmetric addition is very broad, with α,β-unsaturated ketones, esters, amides, 1-alkenylphosphonates, and 1-nitroalkenes efficiently converted into the corresponding 1,4-addition products with over 95% enantioselectivity. The catalytic cycle in water is proposed to involve three intermediates (aryl- or alkenyl-rhodium, (oxa-π-allyl)rhodium, and hydroxo-rhodium) by NMR studies on the rhodium intermediates.

The synthetic applications of planar chiral cationic η3-allylmolybdenum complexes were described by Philip Kocienski and co-workers. A wide range of planar chiral η3-allylMo(CO)2Cp complexes are readily available by the reaction of allyl benzoates with either Mo(CO)4(py)2 or Mo(CO)4(THF)2 in refluxing THF. This combination of reagents has been successful in the synthesis of trisubstituted, and functionalized complexes previously inaccessible with the standard reagents. The authors have shown that chiral α-heteroalkylcopper(I) reagents are especially effective nucleophilic partners that react with cationic η3-allylmolybdenum complexes with clean retention of configuration (Scheme 3). The reactions can be used to forge bonds with excellent stereocontrol in hindered environments. Finally, they have shown that the regiochemistry of the addition is governed by many factors, including steric and electronic effects in the cationic η3-allylmolybdenum complexes, the nature of the nucleophile, temperature, and solvent.

Günter Helmchen and co-workers examined the application of allylic substitutions in natural products synthesis. Synthetic routes are described allowing all known jasmonoids as well as their enantiomers to be synthesized in enantiomerically and diastereomerically pure form. The routes are based on a set of closely related lactones containing an electrophilic allylic moiety, which are accessible via asymmetric Pd-catalyzed allylic alkylation. Regio- and diastereoselective SN2’-anti-reactions of the electrophilic lactones with organocopper compounds furnished 2,3-cis-disubstituted cyclopentenones, which were further transformed into the target compounds, 12-oxophytodienoic acid (12-OPDA) in excellent overall yields. The methodology also allowed iridoids and isoprostanes to be prepared. The configuration of an Archaea membrane lipid constituent containing cyclopentane rings was determined by total synthesis of a diol that was also obtained by degradation of the natural product.

John F. Hartwig described the development of palladium, nickel, and rhodium catalysts for the hydroamination of dienes and vinylarenes (Scheme 4). Enantioselective catalysts based on palladium have been developed for the addition of arylamines to dienes and for Markovnikov addition of arylamines to vinylarenes. In addition, nickel catalysts for the addition of aliphatic amines to dienes have been developed and rhodium catalysts for the first transition metal-catalyzed aminations of vinylarenes that generate terminal amines as the major product are described. Mechanistic data on the hydroamination of vinylarenes with palladium and rhodium is also provided.

A novel regio-, stereo-, diastereo-, and enantioselective one-pot reactions mediated by organometallic derivatives was described by Ilan Marek and co-workers. A one-pot preparation of chiral homoallylic alcohol and amine derivatives by a copper-catalyzed four-component reaction was developed (Scheme 5). In this process, 3 new carbon-carbon bonds as well as a quaternary and a tertiary chiral center are created with excellent regio- and diastereoselectivities. When the reaction was performed without adding external electrophiles, a β-elimination reaction took place to give polysubstituted allenes in good overall yields. This strategy of zinc-homologation followed by a b-elimination reaction was also synthetically used for the transformation of sp3 sulfoxides into olefins with potential application in asymmetric synthesis. They described the stereoselective preparation of metalated dienes in only two chemical steps from commercially available products. This new strategy is based on a tandem allylic C-H bond activation of a remote ω-double bond followed by an elimination reaction.

Koichi Mikami and co-workers reported the negative nonlinear effect in aquo palladium catalysts depending on tropos biphenylphosphine ligand chirality controlled by chiral diaminobinaphthyl activator. Asymmetric activation of aquo Pd catalysts with tropos biphenylphosphine (BIPHEP) ligands by a chiral diaminobinaphtyl (DABN) activator exhibits a remarkable negative non-linear effect ((—)-NLE) with higher catalytic efficiency than that achieved by enantiopure atropos BINAP- or tropos BIPHEP-Pd catalysts without DABN activator (see table).

Along with the above papers written by the plenary lecturers, this issue of PAC featured papers written by the OMCOS award winner, Kyoto Nozaki, and the following invited lecturers: Pher Andersson, Geoffrey Coates, Anny Jutand, Chul-Ho Jun, Guy Lloyd-Jones, Andrei Yudin, Warren Piers, Shane Krska, Walter Leitner, SonBinh Nguyen, Shengming Ma, Claudio Trombini, Ross Widenhoefer, Marc Lemaire, and Peter Kündig.

William Tam <> was conference editor for OMCOS-12; he is at the Department of Chemistry, University of Guelph, Guelph, Ontario, Canada.

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