Platinum-Catalysed Bisindolylation of Allenes: A Complementary Alternative to Gold Catalysis

One-pot sequential synthesis of unsymmetrical diarylmethanes using methylene chloride as a C1-synthon

Bisindolylmethane (BIM) and its derivatives are widely used in the pharmaceutical industry due to their significant biological activities. However, most reported synthetic methods are focused on the synthesis of symmetric BIMs, while the synthesis of unsymmetrical BIMs remains a challenge. Herein, an unprecedented two-step one-pot method to afford unsymmetrically substituted 3,3'-BIM frameworks, using methylene chloride (DCM) as the C1-synthon is reported. In this protocol, the formation of two C-C bonds can be achieved via a one-pot reaction. The utility of commercially available phenols and anilines was also demonstrated in the construction of unsymmetrical diarylmethanes. This protocol provides a straightforward approach to access diverse unsymmetrical diarylmethane derivatives under simple and mild conditions. The broad substrate compatibility and good functional group tolerance of the protocol support its practical application potential.

A Tandem Hydroformylation-Organocatalyzed Friedel-Crafts Reaction for the Synthesis of Diindolylmethanes

Herein, we present an efficient and atom-economic tandem hydroformylation organocatalyzed Friedel-Crafts reaction sequence for the synthesis of diindolylmethanes. Classic syntheses have relied on (Lewis) acid activation of aldehydes, which are often not commercially available and rather sensitive in handling. In contrast, the combination of rhodium-catalyzed hydroformylation and subsequent organocatalytic activation of the in-situ formed aldehydes allows the use of readily available and stable alkenes with various functional groups while avoiding acidic conditions to expand the range of available diindolylmethanes. A broad scope of diindolylmethanes was prepared in yields up to 85 % demonstrates the utility of the presented method.

The Bis(indolylmethyl) ethers: Design, Prototypical Synthesis, and Scope Studies

The design, prototypical synthesis, isolation, and characterization of bis(indolylmethyl) ethers from corresponding indolylcarbinols is described. This approach involves very mild conditions and exhibits good scope for indolylcarbinols (both N-electron withdrawing group and N-electron donating group). Cross etherification between two electronically different indolylcarbinols is also demonstrated for the generation of unsymmetrical ethers. For the first time, the intermediacy of the bis(indolylmethyl) ethers for the formation of bis(indolyl)methanes from indolylcarbinols is proved experimentally and by 1H NMR analysis.

Precious metal complexes of bis(pyridyl)allenes: synthesis and catalytic and medicinal applications

The incorporation of donor-type substituents on the allene core opens up the possibility of coordination complexes in which the metal is bonded to the donor groups, with or without interaction with the double bond system. Despite the challenges in the synthesis of such allene-containing metal complexes, their unique 3D environments and dual functionality (allene and metal) could facilitate catalysis and interaction with chemical and biological systems. Bis(pyridyl)allenes are presented here as robust ligands for novel Pd(II), Pt(IV) and Au(III) complexes. Their synthesis, characterisation and first application as catalysts of benchmark reactions for Pd, Pt and Au are presented with interesting reactivity and selectivities. The complexes have also been probed as antimicrobial and anticancer agents with promising activities, and the first studies on their unusual interaction with several DNA structures will open new avenues for research in the area of metallodrugs with new mechanisms of action.

Metal-free oxidative coupling of arylmethylamines with indoles: a simple, environmentally benign approach for the synthesis of 3,3'-bis(indolyl)methanes

The efficient metal-free oxidative coupling of arylmethylamines with indoles has been developed using molecular oxygen as a green oxidant. The present reaction provides a novel route towards the synthesis of 3,3'-bis(indolyl)methanes in excellent yields of up to 95% via C-C and C-N bond formation. This attractive and environmentally friendly one-pot protocol is a simple procedure that features inexpensive acetic acid as the catalyst and molecular oxygen as the sole oxidant, and it supports a wide substrate scope with the good tolerance of functional groups.

Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes

The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a SEOx process via Pt(IV)–H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed.

Different reactivity of phosphorylallenes under the action of Brønsted or Lewis acids: a crucial role of involvement of the P=O group in intra- or intermolecular interactions at the formation of cationic intermediates

3-Methylbuta-1,2-dien-1-ylphosphonic acid derivatives (phosphorylallenes) [X₂(O=)P–CR=C=CMe₂, X = Cl, OMe, NR₂, or SAr] undergo intramolecular cyclization into the corresponding 1,2-oxaphospholium ions in the Brønsted superacid TfOH. These cations have been thoroughly studied by means of NMR spectroscopy. The hydrolysis of superacidic solutions of these species afforded cyclic phosphonic acids and other phosphorus-containing compounds. Contrary to Brønsted acids, 3-methylbuta-1,2-dien-1-ylphosphonic dichloride [Cl₂(O=)P–HC=C=CMe₂] reacted with the Lewis acid AlCl₃ in an intermolecular way forming noncyclic intermediates, which were investigated by NMR spectroscopy and DFT calculations. Hydrolysis of these species resulted in the formation of phosphoryl-substituted allyl alcohols and 1,3-butadienes. A strong coordination of the oxygen of the P=O group with AlCl₃ prevented the formation of cyclic 1,2-oxaphospholium ions and played a crucial role in the different reactivity of such phosphorylallenes under the action of Brønsted or Lewis acids. Apart from that, the reaction of dichlorophosphorylallenes with arenes and AlCl₃ led to products of hydroarylation of the allene system, phosphoryl-substituted alkenes and/or indanes. This is the first example of a Lewis acid-promoted intermolecular hydroarylation of allenes bearing electron-withdrawing substituents. Plausible reaction mechanisms have been proposed on the basis of the investigated reactions, and NMR analysis and DFT studies of the intermediate cationic species.

Ruthenium-catalyzed tandem annulation/arylation for the synthesis of unsymmetrical bis(heteroaryl)methanes

A new Ru-catalyzed tandem furan annulation/arylation strategy has been developed to afford unsymmetrical bis(heteroaryl)methanes by a reaction between propargyl amines and indoles. A series of bis(heteroaryl)methanes containing furan and indole as well as indole and imidazopyridine moieties have been synthesized in high yields. The reaction possibly proceeds through furan annulation followed by nucleophilic addition of indole.

Gold(I)-Catalysed Hydroarylation of 1,3-Disubstituted Allenes with Efficient Axial-to-Point Chirality Transfer

Hydroarylation of enantioenriched 1,3-disubstituted allenes has the potential to proceed with axial-to-point chirality transfer to yield enantioenriched allylated (hetero)aryl compounds. However, the gold-catalysed intermolecular reaction was previously reported to occur with no chirality transfer owing to competing allene racemisation. Herein, we describe the development of the first intermolecular hydroarylations of allenes to proceed with efficient chirality transfer and summarise some of the key criteria for achieving high regio- and stereoselectivity.

Electrochemical Synthesis of Bisindolylmethanes from Indoles and Ethers

An electrochemical bisindolylation of ethers was developed. Carried out under ambient conditions and in the absence of any chemical oxidants, this reaction exhibits a broad substrate scope and good functional group compatibility.

Synthesis of a Novel Type of 2,3'-BIMs via Platinum-Catalysed Reaction of Indolylallenes with Indoles

Optimisation, scope and mechanism of the platinum-catalysed addition of indoles to indolylallenes is reported here to give 2,3'-BIMs with a novel core structure very relevant for pharmaceutical industry. The reaction is modulated by the electronic properties of the substituents on both indoles, with the 2,3'-BIMs favoured when electron donating groups are present. Although simple at first, a complex mechanism has been uncovered that explains the different behaviour of these systems with platinum when compared with other metals (e.g. gold). Detailed labelling studies have shown Pt-catalysed 6-endo-trig cyclisation of the indollylallene as the first step of the reaction and the involvement of two cyclic vinyl-platinum intermediates in equilibrium through a platinum carbene, as the key intermediates of the catalytic cycle towards the second nucleophilic attack and formation of the BIMs.

Visible light-induced aerobic oxidative cross-coupling of glycine derivatives with indoles: a facile access to 3,3′ bisindolylmethanes

A rapid synthesis of 3,3′-bisindolylmethanes is achieved via a photocatalyzed double Friedel–Crafts alkylation reaction between glycine derivatives and indoles.

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.

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

This detailed protocol describes the new Spin Saturation Transfer Difference Nuclear Magnetic Resonance protocol (SSTD NMR), recently developed in our group to study processes of mutual-site chemical exchange that are difficult to analyze by traditional methods. As the name suggests, this method combines the Spin Saturation Transfer method used for small molecules, with the Saturation Transfer Difference (STD) NMR method employed for the study of protein-ligand interactions, by measuring transient spin saturation transfer along increasing saturation times (build-up curves) in small organic and organometallic molecules undergoing chemical exchange. Advantages of this method over existing ones are: there is no need to reach coalescence of the exchanging signals; the method can be applied as long as one signal of the exchanging sites is isolated; there is no need to measure T1 or reach steady state saturation; rate constant values are measured directly, and T1 values are obtained in the same experiment, using only one set of experiments. To test the method, we have studied the dynamics of the hindered rotation of N,N-dimethylamides, for which much data is available for comparison. The thermodynamic parameters obtained using SSTD are very similar to the reported ones (spin-saturation transfer techniques and line-shape analysis). The method can be applied to more challenging substrates that cannot be studied by previous methods. We envisage that the simple experimental set up and the wide applicability of the method to a great variety of substrates will make this a common technique amongst organic and organometallic chemists without extensive expertise in NMR.

Chirality transfer in metal-catalysed intermolecular addition reactions involving allenes

This review covers chirality transfer in metal-catalysed intermolecular addition reactions involving allenes, including axial-to-central chirality transfer, asymmetric catalysis and racemization.

Kinetics of intramolecular chemical exchange by initial growth rates of spin saturation transfer difference experiments (SSTD NMR)

We report here the Initial Growth Rates SSTD NMR method, as a new powerful tool to obtain the kinetic parameters of intramolecular chemical exchange in challenging small organic and organometallic molecules.

Gold(i)- and platinum(iv)-catalyzed intramolecular annulations of allenes towards furans

Intramolecular annulations of allenes towards furans to give six-membered ring fused furans with gold and platinum catalysts have been developed.