Using Light to Modify the Selectivity of Transition Metal Catalysed Transformations

Light has a remarkable and often unique ability to promote chemical reactions. In combination with transition metal catalysis, it offers exciting opportunities to modify catalyst function in a non-invasive manner, most frequently being reported to switch on or accelerate reactions that do not occur in the dark. However, the ability to completely change reactivity or selectivity between two different reaction outcomes is considerably less common. In this Minireview we bring together examples of this concept and highlight their mechanistically distinct approaches. Our overview demonstrates how these non-natural, photo-switchable systems provide key fundamental mechanistic insights, enhancing our understanding and stimulating development of new catalytic activity, and how this might lead to tangible applications, impacting fields such as polymer chemistry.

Gold-Catalyzed One-Pot Synthesis of Polyfluoroalkylated Oxazoles from N-Propargylamides Under Visible-Light Irradiation

A gold-catalyzed synthesis of polyfluoroalkylated oxazoles from N-propargylamides under visible-light irradiation has been developed. These reactions display excellent compatibility of radicals and gold catalysts under visible-light irradiation. Mechanistic experiments indicate that polyfluoroalkyl iodides play a dual role in enhanced compatibility of radicals and gold catalysts through assisted protodeauration of vinyl gold and reactivated the gold catalyst. In addition, PPh₃ AuNTf₂ not only activates N-propargylamide to generate vinyl gold intermediate, but also greatly promotes homolysis of polyfluoroalkyl iodides under blue light irradiation.

Base-Free Cross-Couplings of Aryl Diazonium Salts in Methanol: PdII -Alkoxy as Reactivity-Controlling Intermediate

Pd-catalyzed cross-coupling reactions of aryl diazonium salts are generally assumed to proceed via cationic Pd^II intermediates which in turn would be highly reactive in the subsequent transmetalation step. Contrary to this belief, we herein report our observation and rationalization of opposing reactivities of ArN₂ ⁺ in Suzuki (=effective) and Stille (=ineffective) cross-couplings in MeOH. Our systematic experimental and computational studies on the roles of transmetalating agent, solvent, base and the likely involvement of in situ formed diazoether derivatives challenge the currently accepted mechanism. Our data suggest that the observed solvent dichotomy is primarily due to Pd^II -methoxy intermediates being formed, which are unreactive with arylstannanes, but highly reactive with arylboronic acids, complementing the Suzuki "Pd-oxy" mechanism with the direct demonstration of transmetalation of a Pd^II -alkoxy complex. Lewis acids were found to circumvent this reactivity divergence, promoting efficient couplings regardless of the employed conditions or coupling partners.

Stereoselective Formation of Facial Tris-Cyclometalated PtIV Complexes: Dual Phosphorescence from Heteroleptic Derivatives

A stereoselective synthetic route to homo- and heteroleptic facial tris-cyclometalated Pt^IV complexes is reported, involving the oxidative addition of 2-(2-pyridyl)- or 2-(1-isoquinolinyl)benzenediazonium salts to cis-[Pt(C^N)₂ ] precursors, with C^N=cyclometalated 2-(p-tolyl)pyridine (tpy), 2-phenylquinoline (pq), 2-(2-thienyl)pyridine or 1-phenylisoquinoline (piq), to produce labile diazenide intermediates that undergo photochemical or thermal elimination of N₂ . The method allows the preparation of derivatives bearing cyclometalated ligands of low π-π* transition energies. The new complexes exhibit phosphorescence in fluid solution at room temperature arising from triplet ligand-centered (³ LC) excited states, which, in the cases of the heteroleptic derivatives, involve the ligand with the lowest π-π* gap. The heteroleptic piq derivatives exhibit fluorescence and dual phosphorescence from different ligand-centered excited states in rigid media, demonstrating the potential of cyclometalated Pt^IV complexes as multi-emissive materials.

Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C-H Aminations

While direct nitrene insertions into C-H bonds have become an important tool for building C-N bonds in modern organic chemistry, the generation of nitrene intermediates always requires transition metals, high temperatures, ultraviolet or laser light. We report a mild synthesis of carbazoles and related building blocks through a visible light-induced intramolecular C-H amination reaction. A striking advantage of this new method is the use of more reactive aryl sulfilimines instead of the corresponding hazardous azides. Different catalysts and divergent light sources were tested. The reaction scope is broad and the product yield is generally high. An efficient gram-scale synthesis of Clausine C demonstrates the applicability and scalability of this new method.

Gold Redox Catalysis with a Selenium Cation as a Mild Oxidant

Gold-catalyzed alkyne and allene diselenations were developed. Excellent regioselectivity (trans) and good to excellent yields were achieved (up to 98 % with 2 % catalyst loading) with a wide range of substrates. Mechanistic investigation revealed the formation of a vinyl gold(I) intermediate followed by an intermolecular selenium cation migration, suggesting that a gold(I/III) redox process was successfully implemented under mild conditions.

Mild and Efficient Synthesis of Diverse Organo-AuI -L Complexes in Green Solvents

An exceptionally mild and efficient method was developed for the preparation of (hetero)aryl-Au^I -L complexes using ethanol or water as the reaction medium at room temperature and Ar-B(triol)K boronates as the transmetalation partner. The reaction does not need an exogeneous base or other additives, and quantitative yields can be achieved through a simple filtration as the only required purification method, which obviates considerable waste associated with alternative workup methods. A broad reaction scope was demonstrated with respect to both the L and (hetero)aryl ligands on product Au complexes. Despite the polar reaction medium, large polycyclic aromatic hydrocarbon units can be incorporated on the Au complexes in very good to excellent yields. The approach was demonstrated for the chemoselective manipulation of orthogonally protected aryl boronates to afford a new class of N-heterocyclic carbene-Au-aryl complexes. A mechanistic rationale was proposed.

Synthesis of Fulvene Vinyl Ethers by Gold Catalysis

Gold-catalyzed cyclization of 1,5-diynes with ketones as reagents and solvent provides diversely substituted vinyl ethers under mild conditions. The regioselectivity of such gold-catalyzed cyclizations is usually controlled by the scaffold of the diyne. Herein, we report the first solvent-controlled switching of regioselectivity from a 6-endo-dig- to 5-endo-dig-cyclization in these transformations, providing fulvene derivatives. With respect to the functional-group tolerance, aryl fluorides, chlorides, bromides, and ethers are tolerated. Furthermore, the mechanism and selectivity are put to scrutiny by experimental studies and a thermodynamic analysis of the product. Additionally, 6-(vinyloxy)fulvenes are a hitherto unknown class of compounds. Their reactivity is briefly evaluated, to give insights into their potential applications.

Mercury-Free Synthesis of Pincer [C^N^C]AuIII Complexes by an Oxidative Addition/CH Activation Cascade

Starting from the commercially available dimethyl sulfide-gold(I) chloride complex (DMSAuCl) and diazonium salts in the presence of 2,6-di-tert-butyl-4-methylpyridine as base, symmetric and unsymmetric [C^N^C]Au^III Cl complexes were synthesized in a selective, photosensitizer-free, photochemical reaction using blue LED light. This new protocol provides the first mercury-free synthesis of these types of pincer-complexes in moderate-to-excellent yields, starting from a readily available gold(I) precursor. Owing to the extraordinary properties of the target compounds, like excellent luminescence and high anticancer activities, the synthesis of such complexes is a highly active field of research, which might make its way to an industrial application. Owing to the disadvantages of the known protocols, especially the toxicity and the selectivity issues in the case of unsymmetric complexes, avoiding the use of mercury, should further accelerate this ongoing development.