e Xu

A General and Efficient Cobalt(II)-Based Catalytic System for Highly Stereoselective Cyclopropanation of Alkenes with α-Cyanodiazoacetates

[Co(P1)], the cobalt(II) complex of the D(2)-symmetric chiral porphyrin 3,5-Di(t)Bu-ChenPhyrin, is an effective catalyst for catalyzing asymmetric olefin cyclopropanation with the acceptor/acceptor-type diazo reagent α-cyanodiazoacetates. The [Co(P1)]-catalyzed reaction is versatile and suitable for both aromatic and aliphatic olefins with varied electronic properties, including electron-rich and -poor olefins. The Co(II)-based catalytic system can be operated in a one-time protocol under mild conditions, affording the desired cyclopropane products in high yields with both high diastereo- and enantioselectivity. The resulting enantiomerically enriched 1,1-cyclopropanenitrile esters provide convenient access to a number of densely functionalized chiral cyclopropane derivatives, including α-cyclopropyl-β-amino acids.

Regioselective Synthesis of Multisubstituted Furans via Metalloradical Cyclization of Alkynes with α-Diazocarbonyls: Construction of Functionalized α-Oligofurans

Co(III)-carbene radicals generated from activation of α-diazocarbonyls by Co(II)-porphyrin complexes have been shown to undergo a new type of tandem radical addition reaction with alkynes that affords five-membered furan structures. The Co(II) complex of 3,5-Di(t)Bu-IbuPhyrin, [Co(P1)], is effective in catalyzing the metalloradical cyclization reaction under neutral and mild conditions. The [Co(P1)]-catalyzed process tolerates a wide range of α-diazocarbonyls and terminal alkynes with varied steric and electronic properties, producing polyfunctionalized furans with complete regioselectivity. The catalytic synthesis features a high degree of functional group tolerance and can be applied iteratively to construct functionalized α-oligofurans.

Highly Asymmetric Intramolecular Cyclopropanation of Acceptor-Substituted Diazoacetates by Co(II)-Based Metalloradical Catalysis: Iterative Approach for Development of New-Generation Catalysts

3,5-Di(t)Bu-QingPhyrin, a new D(2)-symmetric chiral porphyrin derived from a chiral cyclopropanecarboxamide containing two contiguous stereocenters, has been developed using an iterative approach based on Co(II)-catalyzed asymmetric cyclopropanation of alkenes. The Co(II) complex of 3,5-Di(t)Bu-QingPhyrin, [Co(P2)], has proved to be a general and effective catalyst for asymmetric intramolecular cyclopropanation of various allylic diazoacetates (especially including those with α-acceptor substituents) in high yields with excellent stereoselectivities. The [Co(P2)]-based intramolecular metalloradical cyclopropanation provides convenient access to densely functionalized 3-oxabicyclo[3.1.0]hexan-2-one derivatives bearing three contiguous quaternary and tertiary chiral centers with high enantiomeric purity.

Enantioselective Cyclopropenation of Alkynes with Acceptor/Acceptor-Substituted Diazo Reagents via Co(II)-Based Metalloradical Catalysis

The cobalt(II) complex of a new D(2)-symmetric chiral porphyrin 3,5-diMes-ChenPhyrin, [Co(P2)], has been shown to be a highly effective chiral metalloradical catalyst for enantioselective cyclopropenation of alkynes with acceptor/acceptor-substituted diazo reagents, such as α-cyanodiazoacetamides and α-cyanodiazoacetates. The [Co(P2)]-mediated metalloradical cyclopropenation is suitable to a wide range of terminal aromatic and related conjugated alkynes with varied steric and electronic properties, providing the corresponding trisubstituted cyclopropenes in high yields with excellent enantiocontrol of the all-carbon quaternary stereogenic centers. In addition to mild reaction conditions, the Co(II)-based metalloradical catalysis for cyclopropenation features a high degree of functional group tolerance.

Cobalt(II)‐Catalyzed Asymmetric Olefin Cyclopropanation with α‐Ketodiazoacetates

Cyclopropanes are a unique class of structure elements found in a number of biologically important compounds and have been demonstrated for a wide range of fundamental and practical applications.[1] One particularly attractive strategy for enantioselective synthesis of chiral cyclopropanes is based on transition metal-catalyzed asymmetric olefin cyclopropanation with diazo reagents.[2] In principle, chiral cyclopropane derivatives with all substitution patterns may be accessible in enantioenriched form by asymmetric cyclopropanation due to the diverse availability of both alkenes and diazo reagents. Among different classes of diazo reagents with various combinations of α-substituents, many acceptor- and donor/acceptor-substituted diazo reagents have been successfully employed as effective carbene sources for metal-catalyzed asymmetric cyclopropanation.[2] In contrast, the capacity of catalytic asymmetric cyclopropanation has not been fully explored with acceptor/acceptor-substituted diazo reagents, which would afford synthetically useful cyclopropane compounds bearing geminal electron-withdrawing functionalities.[3,4] Although there have been some recent successes in this area,[5–9] several important types of acceptor/acceptor-substituted diazo reagents remain challenging for asymmetric olefin cyclopropanation.

Effective Synthesis of Chiral N‐Fluoroaryl Aziridines through Enantioselective Aziridination of Alkenes with Fluoroaryl Azides

Catalytic aziridination of alkenes with nitrene sources via “C2+N1” addition represents a general approach for the direct synthesis of aziridines, the smallest three-membered N-heterocycles.[1] The enantioselective version of this catalytic process allows for efficient access to chiral nonracemic aziridines, which are versatile synthetic intermediates in asymmetric synthesis.[1-2] Several classes of transition metal-based chiral catalysts, such as Mn, Fe, Cu, Rh and Co complexes, have been found effective in catalyzing asymmetric olefin aziridination with different nitrene sources.[1,3] Besides iminoiodanes and haloamines, organic azides have been actively pursued as alternative nitrene sources for metal-catalyzed aziridination as they enjoy several advantages.[3] While sulfonyl and phosphoryl azides have been effectively employed for metal-catalyzed asymmetric aziridination,[4] catalytic processes based on the use of other types of azides, such as aryl azides, have been less developed. In addition to generating environmentally friendly nitrogen gas as the only byproduct, asymmetric catalytic aziridination with readily available aryl azides provides an attractive approach for the synthesis of valuable N-aryl aziridines through direct introduction of N-aryl groups concurrently with the ring formation.[5,6] This would avoid two additional steps of deprotection and N-arylation when other types of nitrene sources are used for preparing N-aryl aziridines.[7] However, few catalytic systems have been shown to be effective for asymmetric olefin aziridination with aryl azides.[4d,6]

Synthesis of diporphyrins via palladium-catalyzed C-O bond formation: effective access to chiral diporphyrins.

Diporphyrins can be efficiently synthesized from bromoporphyrin precursors via palladium-catalyzed C-O bond formation. The synthetic methodology is general and can be applied to various diols, forming a series of homo-diporphyrins containing different types of spacers in high to excellent yields. Chiral diporphyrins can be readily constructed through the use of optically active diols. A similar strategy allows access to hetero-diporphyrins and triporphyrins, including free-base and metalloporphyrin hetero dimers.

Stereoselective intramolecular cyclopropanation of α-diazoacetates via Co(II)-based metalloradical catalysis

Co(II) complexes of D2-symmetric chiral porphyrins have been proven to be effective metalloradical catalysts for the asymmetric intramolecular cyclopropanation of allyl α-diazoacetates. 4-(Dimethylamino)pyridine (DMAP), through positive trans effect, plays an important role in the enhancement of the asymmetric induction for the intramolecular cyclopropanation process. This metalloradical catalytic system is suitable for cyclopropanation of allyl α-diazoacetates with varied functional groups and substitution patterns, producing bicyclic products with complete diastereocontrol and good enantiocontrol.

Spectroscopic Investigation of the Noncovalent Association of the Nerve Agent Simulant Diisopropyl Methylphosphonate (DIMP) with Zinc(II) Porphyrins

Organophosphonates pose a significant threat as chemical warfare agents, as well as environmental toxins in the form of pesticides. Thus, methodologies to sense and decontaminate these agents are of significant interest. Porphyrins and metalloporphyrins offer an excellent platform to develop chemical threat sensors and photochemical degradation systems. These highly conjugated planar molecules exhibit relatively long-lived singlet and triplet states with high quantum yields and also form self-associated complexes with a wide variety of molecules. A significant aspect of porphyrins is the ability to functionalize the peripheral ring system either directly to the pyrrole rings or to the bridging methine carbons. In this report, steady-state absorption and fluorescence are utilized to probe binding affinities of a series of symmetric and asymmetric zinc(II) metalloporphyrins for the nerve agent simulant diisopropyl methylphosphonate (DIMP) in hexane. The red shifts in the absorption and emission spectra observed for all of the metalloporphyrins probed are discussed in the frame of Goutermans four orbital model and a common binding motif involving coordination between the metalloporphyrin and DIMP via interaction between the zinc metal center of the porphyrin and phosphoryl oxygen of DIMP (Zn-O═P) is proposed.

Synthesis of C-Unsubstituted 1,2-Diazetidines and Their Ring-Opening Reactions via Selective N–N Bond Cleavage

C-Unsubstituted 1,2-diazetidines, a rarely studied type of four-membered heterocyclic compounds, were synthesized through an operationally simple intermolecular vicinal disubstitution reaction. 1,2-Diazetidine derivatives bearing various N-arylsulfonyl groups were readily accessed and studied by experimental and computed Raman spectra. The ring-opening reaction of the diazetidine was explored and resulted in the identification of a selective N-N bond cleavage with thiols as nucleophiles, which stereoselectively produced a new class of N-sulfenylimine derivatives with C-aminomethyl groups.