1,3,2‐Diazaphospholene‐Catalyzed Reductive Cyclizations of Organohalides**

Enantioselective Synthesis of Dihydrobenzofurans, Dihydrobenzosulfones, and Dihydroindoles by Merging One-pot Intramolecular Heck-Matsuda Reactions from Anilines with Redox-Relay Process

A one-pot tandem process was developed aiming at the concise and expeditious enantioselective synthesis of dihydrobenzofuran, dihydrobenzosulfone, and dihydroindole scaffolds under mild, and open-flask conditions. This process combines the in situ generation of aryldiazonium salt directly from the anilines in methanol telescoped to an intramolecular Heck-Matsuda reaction linked to a redox relay process to provide the final products as the dimethyl acetals. These Heck products were smoothly converted into the corresponding primary alcohols or esters. The robustness and the efficiency of the protocol are demonstrated by the synthesis of 24 enantioenriched dihydrobenzofurans, dihydrobenzosulfones, and dihydroindoles in overall yields up to 78 % in enantiomeric ratios up to 99 : 1 by a sequential 5-step protocol.

Hydrazone Phosphaketene as a Synthetic Platform To Obtain Three Classes of 1,2,4-Diazaphosphol Derivatives by Switchable Chemoselectivity Strategies

We introduce switchable chemoselectivity strategies based on the hydrazone phosphaketene intermediate to synthesize three classes of 1,2,4-diazaphosphol derivatives. First, the five-membered heterocyclic P and O anion intermediates acted as nucleophilic agents in the selective construction of C-P and C-O bonds. Second, the phosphinidene served as a phosphorus synthon, allowing for the formation of C-P and C-N bonds. Finally, a stepwise mechanism, supported by DFT calculations, was invoked to explain the reaction selectivity.

Activation of Aryl and Alkyl Halides Enabled by Strong Photoreduction Potentials of a Hantzsch Ester/Cs2CO3 System

We disclose herein a light-induced Hantzsch ester-initiated aryl and alkyl radical generation protocol from aryl halides (Br and Cl) and alkyl iodides. This method provides access to a wide range of benzo-fused heterocycles and C(sp3)-C(sp3) coupling products. The reductive detosylation reaction has also been demonstrated using the same reaction conditions. Initial mechanism studies provide evidence of the formation of an alkyl radical.

Photocatalytic Radical Cascade Dehalogenation/Carbo-cyclization/Sulfonylation Leading to Indole- and Benzofuran-Based Benzylic Sulfones

This study presents a convenient approach to the synthesis of indole- and benzofuran-based benzylic sulfones using unactivated alkynes containing aryl iodides and sodium sulfinates under visible light irradiation. The procedure involves a sequential series of dehalogenation, carbo-cyclization, and radical sulfonylation. Plausible insights into the reaction mechanism are derived from control experiments, leading to the proposal of a radical cascade reaction pathway.

Reductive Asymmetric Aza-Mislow-Evans Rearrangement by 1,3,2-Diazaphospholene Catalysis

1,3,2-diazaphospholene hydrides (DAP-H) enable smooth conjugate reduction of polarized double bonds. The transiently formed phosphorus-enolate provides a potential platform for reductive α-functionalizations. In this respect, asymmetric C-heteroatom bond forming processes are synthetically appealing but remain elusive. We report a 1,3,2-diazaphospholene-catalyzed three-step cascade reaction of N-sulfinyl acrylamides comprised of conjugate reduction, [2,3]-sigmatropic aza-Mislow-Evans rearrangement and subsequent S-O bond cleavage. The obtained enantio-enriched α-hydroxy amides are formed in good yields and excellent enantiospecificity. The stereo-defined P-bound N,O-ketene aminal ensures an excellent transfer of chirality from the sulfur stereocenter to α-carbon. The transformation operates under mild conditions at ambient temperature. Moreover, DAP-H is a competent reductant for the cleavage of formed sulfenate ester, eliminating the extra step in traditional Mislow-Evans processes.

Synthetic applications of NHPs: from the hydride pathway to a radical mechanism

We briefly summarized synthetic applications of N-heterocyclic phosphines in both hydridic and radical reductions with an emphasis on their recently discovered radical reactivity.