Microwave-mediated stereocontrolled annulations of diazo(aryl)methyl(diaryl)phosphine oxides with pyridinium 1,4-zwitterionic thiolates

Highly Stereoselective [2 + 4] Annulation of Phosphenes and Enones with β-Electron-Donating Groups

Enones with β-electron-donating groups can exist in their zwitterionic resonance forms, which favor nucleophilic attack to phosphenes generated from phosphoryl (aryl)diazomethanes under blue light irradiation to yield zwitterionic intermediates. The intermediates cyclize to afford highly stereoselective trans-3,4-dihydro-1,2-oxaphosphinine 2-oxides in good to excellent yields. In the cyclic transition state of the final cyclization, the stereoelectronic effect and the Coulomb force control the high stereoselectivity. The electron-donating groups of enones play double roles in both nucleophilic attack and cyclization. The reaction features readily available starting materials, high atom-economy, no catalyst, a fast reaction rate, broad functional group-tolerance and substrate scope, high yields and stereoselectivity, and mild conditions.

Transition-Metal-Free Radical Approach for the Synthesis of Isothiocyanates by Pyridinium 1,4-Zwitterionic Thiolates as a Sulfur Source

Herein, we disclose a novel, mild, transition-metal-free approach to synthesizing diversely functionalized isothiocyanates from the corresponding isocyanide precursors, achieving high to excellent yields (up to 97%). The current method sheds light on the reactivity of pyridinium 1,4-zwitterionic thiolates as an unprecedented sulfur source strikingly distinct from their previously known reactivity in ionic annulation reactions, showcasing an innovative approach to organic synthesis.

Construction of 4-Vinyl-1,2-oxaphospholane 2-Oxides from Vinyloxiranes and Phosphoryl Diazomethanes

Various 4-vinyl-1,2-oxaphospholane 2-oxides are prepared in good to excellent yields from vinyloxiranes and in situ-generated phosphenes from phosphoryl diazomethanes via the phosphene-induced ring expansion of oxiranes. The reaction is a novel and expeditious strategy for the synthesis of 1,2-oxaphospholane 2-oxide derivatives, featuring ring expansion of readily available starting materials, high atom economy, no catalyst, a fast reaction rate, broad functional group tolerance, high yields, and separable diastereomers.

Divergent Synthesis of Sulfur-Containing Bridged Cyclobutanes by Lewis Acid Catalyzed Formal Cycloadditions of Pyridinium 1,4-Zwitterionic Thiolates and Bicyclobutanes

Bridged cyclobutanes and sulfur heterocycles are currently under intense investigation as building blocks for pharmaceutical drug design. Two formal cycloaddition modes involving bicyclobutanes (BCBs) and pyridinium 1,4-zwitterionic thiolate derivatives were described to rapidly expand the chemical space of sulfur-containing bridged cyclobutanes. By using Ni(ClO4)2 as the catalyst, an uncommon higher-order (5+3) cycloaddition of BCBs with quinolinium 1,4-zwitterionic thiolate was achieved with broad substrate scope under mild reaction conditions. Furthermore, the first Lewis acid-catalyzed asymmetric polar (5+3) cycloaddition of BCB with pyridazinium 1,4-zwitterionic thiolate was accomplished. In contrast, pyridinium 1,4-zwitterionic thiolates undergo an Sc(OTf)3-catalyzed formal (3+3) reaction with BCBs to generate thia-norpinene products, which represent the initial instance of synthesizing 2-thiabicyclo[3.1.1]heptanes (thia-BCHeps) from BCBs. Moreover, we have successfully used this (3+3) protocol to rapidly prepare thia-BCHeps-substituted analogues of the bioactive molecule Pitofenone. Density functional theory (DFT) computations imply that kinetic factors govern the (5+3) cycloaddition reaction between BCB and quinolinium 1,4-zwitterionic thiolate, whereas the (3+3) reaction involving pyridinium 1,4-zwitterionic thiolates is under thermodynamic control.

Chameleonic Reactivity of Imidoyl Sulfoxonium Ylides: Access to Functionalized Pyrroles and Dihydro-pyridines

We have found a chameleonic reactivity of imidoyl sulfoxonium ylides. On the one hand, imidoyl sulfoxonium ylides react with electron-deficient reagents, such as alkynyl esters, to lead to the formation of 1,2-dihydro-pyridines. The methyl group attached to the sulfur atom acts as a methylene donor. On the other hand, imidoyl sulfoxonium ylides react with pyridinium 1,4-zwitterionic thiolates, which leads to the formation of functionalized pyrroles. Both transformations feature mild reaction conditions and good functional group tolerance.

Photoredox Catalyzed [3 + 2]-Annulation Reaction of Pyridinium 1,4-Zwitterionic Thiolates with Alkenes: Synthesis of Dihydrothiophenes

Pyridinium 1,4-zwitterionic thiolates are usually used to develop ionic annulation reactions. However, radical reactions were rare. We developed a photoredox catalyzed [3 + 2]-annulation reaction of pyridinium 1,4-zwitterionic thiolates with alkenes, disclosed the new reactivity of pyridinium 1,4-zwitterionic thiolate, and provided a new synthetic method for dihydrothiophene.