Gold-catalyzed reactions of C–H bonds

Dual gold catalysis: σ,π-propyne acetylide and hydroxyl-bridged digold complexes as easy-to-prepare and easy-to-handle precatalysts

A series of dinuclear gold σ,π-propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π-complex of gold with a gold acetylide. The air-stable and storable catalysts can be isolated as silver-free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw-away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis.

Mechanistic insights into enantioselective gold-catalyzed allylation of indoles with alcohols: the counterion effect

Enantioselective gold-catalysis is emerging as a powerful tool in organic synthesis for the stereoselective manipulation of unfunctionalized unsaturated hydrocarbons. Despite the exponential growth, the molecular complexity of common chiral gold complexes generally prevents a complete description of the mechanism steps and activation modes being documented. In this study, we present the results of a combined experimental-computational (DFT) investigation of the mechanism of the enantioselective gold-catalyzed allylic alkylation of indoles with alcohols. A stepwise S(N)2'-process (i.e. anti-auroindolination of the olefin, proton-transfer, and subsequent anti-elimination [Au]-OH) is disclosed, leading to a library of tricyclic-fused indole derivatives. The pivotal role played by the gold counterion, in terms of molecular arrangement (i.e. "folding effect") and proton-shuttling in restoring the catalytic species, is finally documented.

Mechanistic and sterochemical insights on the Pt-catalyzed rearrangement of oxiranylpropargylic esters to cyclopentenones

A mechanism for the rearrangement of oxiranylpropargylic esters to cyclopentenones catalyzed by PtCl(2) is proposed based on DFT calculations (M06/6-31++G(d,p)). Although the basic steps are coincidental with those proposed by Sarpong et al., who characterized a 2H-pyran as intermediate, calculations have revealed other intricate details of this complex rearrangement. The 2H-pyran is proposed to result from the ring-opening of a bicyclic oxonium ion that follows the nucleophilic capture by the epoxide of a platinum carbene generated by an initial Pt-mediated 1,2-propargylic rearrangement. The key steps in the evolution of this system are the electrocyclic ring-opening of the 2H-pyran to a α-methoxycarbonyl dienone and an iso-Nazarov ring closure. Prior to those, changes in hapticity and in the conformation of the dienone are required in order to produce the helical conformation needed to generate a single diastereomer of the cyclopentenone product obtained experimentally. The metal is needed well beyond the first step of the mechanism, and both electrocyclic reactions are favored by coordination to the metal when compared to their uncomplexed counterparts. Moreover, we have experimentally demonstrated that the rearrangement is stereoconvergent, a feature that is traced back to the initial configuration of the epoxide, which determines the somewhat counterthermodynamic placement of the metal syn to the methyl group of the stereogenic center in the 2H-pyran intermediate. Finally, starting from enantiopure oxiranylpropargylic ester 13, a racemate of cyclopentenone (R*,S*)-16 was obtained. Thus, the sequence does not proceed with memory of chirality, and the absolute stereochemical information is already lost at the stage of the 2H-pyran 14.

Gold vinylidene complexes: intermolecular C(sp3)-H insertions and cyclopropanations pathways

Highly reactive gold vinylidene species are used for intermolecular C(sp(3))-H insertions into unactivated alkanes (see scheme). In addition, they can be regarded as synthons for alkylidene carbenes. Initiated by cyclopropanation of the vinylidene species/alkylidene carbenoide, cyclobutene derivatives are formed in a diastereoselective fashion by a ring-enlargement cascade in only one step.

Diastereoselective total synthesis of (±)-schindilactone A, Part 1: Construction of the ABC and FGH ring systems and initial attempts to construct the CDEF ring system

First-generation synthetic strategies for the diastereoselective total synthesis of schindilactone A (1) are presented and methods for the synthesis of the ABC, FGH, and CDEF moieties are explored. We have established a method for the synthesis of the ABC moiety, which included both a Diels-Alder reaction and a ring-closing metathesis as the key steps. We have also developed a method for the synthesis of the FGH moiety, which involved the use of a Pauson-Khand reaction and a carbonylative annulation reaction as the key steps. Furthermore, we have achieved the construction of the central 7-8 bicyclic ring system by using a [3,3]-rearrangement as the key step. However, unfortunately, when this rearrangement reaction was applied to the construction of the more advanced CDEF moiety, the anticipated annulation reaction did not occur and the development of an alternative synthetic strategy would be required for the construction of this central core.

N-heterocyclic carbene gold(I) and copper(I) complexes in C-H bond activation

Environmental concerns have and will continue to have a significant role in determining how chemistry is carried out. Chemists will be challenged to develop new, efficient synthetic processes that have the fewest possible steps leading to a target molecule, the goal being to decrease the amount of waste generated and reduce energy use. Along this path, chemists will need to develop highly selective reactions with atom-economical pathways producing nontoxic byproduct. In this context, C-H bond activation and functionalization is an extremely attractive method. Indeed, for most organic transformations, the presence of a reactive functionality is required. In Total Synthesis, the "protection and deprotection" approach with such reactive groups limits the overall yield of the synthesis, involves the generation of significant chemical waste, costs energy, and in the end is not as green as one would hope. In turn, if a C-H bond functionalization were possible, instead of the use of a prefunctionalized version of the said C-H bond, the number of steps in a synthesis would obviously be reduced. In this case, the C-H bond can be viewed as a dormant functional group that can be activated when necessary during the synthetic strategy. One issue increasing the challenge of such a desired reaction is selectivity. The cleavage of a C-H bond (bond dissociation requires between 85 and 105 kcal/mol) necessitates a high-energy species, which could quickly become a drawback for the control of chemo-, regio-, and stereoselectivity. Transition metal catalysts are useful reagents for surmounting this problem; they can decrease the kinetic barrier of the reaction yet retain control over selectivity. Transition metal complexes also offer important versatility in having distinct pathways that can lead to activation of the C-H bond. An oxidative addition of the metal in the C-H bond, and a base-assisted metal-carbon bond formation in which the base can be coordinated (or not) to the metal complexes are possible. These different C-H bond activation modes provide chemists with several synthetic options. In this Account, we discuss recent discoveries involving the versatile NHC-gold(I) and NHC-copper(I) hydroxide complexes (where NHC is N-heterocyclic carbene) showing interesting Brønsted basic properties for C-H bond activation or C-H bond functionalization purposes. The simple and easy synthesis of these two complexes involves their halide-bearing relatives reacting with simple alkali metal hydroxides. These complexes can react cleanly with organic compounds bearing protons with compatible pK(a) values, producing only water as byproduct. It is a very simple protocol indeed and may be sold as a C-H bond activation, although the less flashy "metalation reaction" also accurately describes the process. The synthesis of these complexes has led us to develop new organometallic chemistry and catalysis involving C-H bond activation (metalation) and subsequent C-H bond functionalization. We further highlight applications with these reactions, in areas such as photoluminescence and biological activities of NHC-gold(I) and NHC-copper(I) complexes.

Enantioselective gold-catalyzed synthesis of polycyclic indolines

The synthesis of architecturally complex polycyclic fused indolines is achieved in a chemo-, regio-, and stereodefined manner, via an enantioselective gold-catalyzed cascade hydroindolination/iminium trapping synthetic sequence. Highly functionalized tetracyclic fused furoindolines (2) and dihydropyranylindolines (4) are synthesized in moderate to good yields and enantiomeric excesses of up to 87%.