A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2, a suitable model for catCl2, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.

Intermolecular Hydroalkoxylation and Hydrocarboxylation of 2-Azadienes with High Efficiency

Described here is a method for intermolecular hydroalkoxylation and hydrocarboxylation of 2-azadienes through cobalt-catalyzed hydrogen atom transfer and oxidation. This protocol provides a source of 2-azaallyl cation equivalents under mild conditions, is chemoselective in the presence of other C═C double bonds, and requires no excess amount of added alcohol or oxidant. Mechanistic studies suggest that the selectivity arises from lowering the transition state that leads to the highly stabilized 2-azaallyl radical.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

α-Aminoazoles/azines: key reaction partners for multicomponent reactions

Aromatic α-aminoazaheterocycles are the focus of significant investigations and exploration by researchers owing to their key role in diverse biological and physiological processes. The existence of their derivatives in numerous drugs and alkaloids is due to their heterocyclic nitrogenous nature. Therefore, the synthesis of a structurally diverse range of their derivatives through simple and convenient methods represents a vital field of synthetic organic chemistry. Multicomponent reactions (MCRs) provide a platform to introduce desirable structure diversity and complexity into a molecule in a single operation with a significant reduction in the use of harmful organic waste, and hence have attracted particular attention as an excellent tool to access these derivatives. This review covers the advances made from 2010 to the beginning of 2020 in terms of the utilization of α-aminoazaheterocycles as synthetic precursors in MCRs.

Physical organic studies and dynamic covalent chemistry of picolyl heterocyclic amino aminals

A dynamic covalent system of the picolyl heterocyclic amino aminals has been studied. The aminals are characterized as a metastable species and easily switch to other forms via external stimuli. The solvent, temperature, acid-base and substituent effects have been examined to evaluate the dynamic covalent system. The results reveal that a more polar solvent, a lower temperature, basic conditions and an electron-withdrawing moiety contribute to the stabilities of aminals. The existence of the n → π* interaction between acetonitrile and the C[double bond, length as m-dash]N moiety makes the N-pyrimidyl imine (4c and 4d) yield higher in CD3CN. In a similar fashion, all aminals tend to convert to the corresponding hemiaminal ethers in a methanol environment. According to these findings, we successfully synthesized the following species: (a) N-2-picolylpyrimidin-2-amine 6c obtained by reduction using acetonitrile as the specific solvent; (b) a picolyl aromatic amino aminal 3e prepared from 2-pyridinecarboxaldehyde and the electron withdrawing 2-methoxy-5-nitroaniline.

Studies on the preparation of aminobipyridines and bipyridine sultams via an intramolecular free radical pathway

A variety of aminated bipyridines and bipyridine sultams are prepared by intramolecular radical [1,5]-ipso and [1,6]-ortho substitutions, using a sulfonamide as a linker to connect the pyridyl radical to the pyridine under attack. For the cases studied, different regiochemistries are observed depending on the initial position of the sulfonamide linker.

Results show that intramolecular radical [1,5]-ipso and [1,6]-ortho substitutions offer a method for the preparation of aminobipyridines or bipyridine sultams.

A facile approach to 2-alkoxyindolin-3-one and its application to the synthesis of N-benzyl matemone

2-Alkoxycarbonylindolin-3-one is synthesized from a methoxyglycine derivative via a 1,2-aza-Brook rearrangement followed by cyclization with bis(trimethylsilyl)aluminum chloride. A short-step synthesis of N-benzyl matemone is successfully carried out using the present indolin-3-one synthesis.

A short-step synthesis of N-benzyl matemone has been successfully carried out using a newly discovered 2-ethoxycarbonylindolin-3-one synthesis.