A DFT-Based Theoretical Investigation of the Mechanism of the PtCl2-Mediated Cycloisomerization of Allenynes

Nucleophile dependent formation of 6- and 7-membered N-heterocycles by platinum-catalysed cyclisation of 1,5-bisallenes

An unprecedented Pt-catalysed cyclisation of N-tethered 1,5-bisallenes in the presence of oxygen nucleophiles is reported, where formation of 6- or 7-membered rings is driven by the choice of nucleophile and the mechanism dictated by the nucleophile and the electronic properties of the bisallene. The reaction in the presence of alcohols gives preferentially vinyltetrahydropyridines with an extra alkoxy group and Pt-H as the active species in the catalytic cycle, while formation of di- and tetrahydroazepines with an extra hydroxyl group is favoured when water is used as nucleophile, via nucleophilic attack/carbocyclization as the favoured pathway. The products obtained are frequently found in the core of natural products with important biological activities, so understanding this complex mechanistic behaviour and exploiting this new methodology will have a big impact in organic synthesis and organometallic chemistry.

Recent application of calculations of metal complexes based on density functional theory

Recent application of density functional theory (DFT) for metal complexes is reviewed to show the achievements of DFT and the challenges for it, as well as the methods for selecting proper functionals.

Mechanistic insights on the cycloisomerization of polyunsaturated precursors catalyzed by platinum and gold complexes

Organometallic chemistry provides powerful tools for the stereocontrolled synthesis of heterocycles and carbocycles. The electrophilic transition metals Pt(II) and Au(I, III) are efficient catalysts in these transitions and promote a variety of organic transformations of unsaturated precursors. These reactions produce functionalized cyclic and acyclic scaffolds for the synthesis of natural and non-natural products efficiently, under mild conditions, and with excellent chemoselectivity. Because these transformations are strongly substrate-dependent, they are versatile and may yield diverse molecular scaffolds. Therefore, synthetic chemists need a mechanistic interpretation to optimize this reaction process and design a new generation of catalysts. However, so far, no intermediate species has been isolated or characterized, so the formulated mechanistic hypotheses have been primarily based on labeling studies or trapping reactions. Recently, theoretical DFT studies have become a useful tool in our research, giving us insights into the key intermediates and into a variety of plausible reaction pathways. In this Account, we present a comprehensive mechanistic overview of transformations promoted by Pt and Au in a non-nucleophilic medium based on quantum-mechanical studies. The calculations are consistent with the experimental observations and provide fundamental insights into the versatility of these reaction processes. The reactivity of these metals results from their peculiar Lewis acid properties: the alkynophilic character of these soft metals and the pi-acid activation of unsaturated groups promotes the intra- or intermolecular attack of a nucleophile. 1,n-Enynes (n = 3-8) are particularly important precursors, and their transformation may yield a variety of cycloadducts depending on the molecular structure. However, the calculations suggest that these different cyclizations would have closely related reaction mechanisms, and we propose a unified mechanistic picture. The intramolecular nucleophilic attack of the double bond on the activated alkyne takes place by an endo-dig or exo-dig pathway to afford a cyclopropyl-metallocarbenoid. Through divergent routes, the cyclopropyl intermediate formed by exo-cyclopropanation could yield the metathesis adduct or bicyclic compounds. The endo-cyclization may be followed by a [1,2]-migration of the propargyl moiety to the internal acetylenic position to afford bicyclic [n.1.0] derivatives. This reaction mechanism is applicable for functional groups ranging from H to carboxylate propargyl substituents (Rautenstrauch reaction). In intramolecular reactions in which a shorter enyne bears a propargyl ester or in intermolecular reactions of an ester with an alkene, the ester preferentially attacks the activated alkyne because of enthalpic (ring strain) and entropic effects. Our calculations can predict the correct stereochemical outcome, which may aid the rational design of further stereoselective syntheses. The alkynes activated by electrophilic species can also react with other nucleophiles, such as aromatic rings. The calculations account for the high endo-selectivity observed and suggest that this transformation takes place through a Friedel-Crafts-type alkenylation mechanism, where the endo-dig cyclization promoted by PtCl(2) may involve a cyclopropylmetallacarbene as intermediate before the formation of the expected Wheland-type intermediate. These comparisons of the computational approach with experiment demonstrate the value of theory in the development of a solid mechanistic understanding of these reaction processes.

Gold-catalyzed cycloisomerization of 1,5-allenynes via dual activation of an ene reaction

Tris(triphenylphosphinegold) oxonium tetrafluoroborate, [(Ph3PAu)3O]BF4, catalyzes the rearrangement of 1,5-allenynes to produce cross-conjugated trienes. Experimental and computational evidence shows that the ene reaction proceeds through a unique nucleophilic addition of an allene double bond to a cationic phosphinegold(I)-complexed phosphinegold(I) acetylide, followed by a 1,5-hydrogen shift.

Catalytic carbophilic activation: catalysis by platinum and gold pi acids

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 pi acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric pi 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.

A Gold-Catalyzed Domino Process to the Steroid Framework

The facile formation of the B and C ring of rac-desoxyequilenin and of a chrysenone derivative in just one preparative step is demonstrated, applying a gold-catalyzed domino process, which involves a benzopyrylium cation as the key intermediate and an intramolecular [3 + 2] cycloaddition as the key step.

A platinum-catalyzed annulation reaction leading to medium-sized rings

A platinum-catalyzed domino process with intermediate benzopyrylium cations reaches its optimum utility in the formation of 7- and 8-membered rings. With iron(III) chloride, a tetracyclic product is isolated, derived from an oxidative transformation of a metal-carbene intermediate.

Theoretical Analysis of the High Versatility in PtCl2-Mediated Cycloisomerization of Enynes on a Common Mechanistic Basis

[Reaction: see text]. Transformations of enynes upon treatment with electrophilic transition-metal complexes, such as PtCl2, are strongly substrate-dependent processes and may yield a wide variety of cyclic compounds. Despite the high versatility, many of these processes could be closely related from a mechanistic point of view. Theoretical analyses of the plausible reaction mechanisms provide support for a unified mechanistic picture based on the electrophilic activation of the triple bond by the catalyst, which triggers the nucleophilic alkene attack through an endo- or exo-cyclization mode, to form the cyclopropylcarbene species. Then, these common key intermediates may evolve through alternative paths to afford a range of cyclic compounds. The preference for each path and the evolution of these intermediates are governed by the nature of the starting enyne. The effects induced by different structural motifs, such as the role played by a heteroatom directly attached to the triple bond, the tether length, the substitution on the acetylenic position, and the gem-dialkyl substitution on the tether, among others, are discussed. The proposed common mechanistic scheme can rationalize and account for the experimental observations accumulated.