X. Peter Zhang

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.

Experimental evidence for cobalt(III)-carbene radicals: key intermediates in cobalt(II)-based metalloradical cyclopropanation

New and conclusive evidence has been obtained for the existence of cobalt(III)-carbene radicals that have been previously proposed as the key intermediates in the underlying mechanism of metalloradical cyclopropanation by cobalt(II) complexes of porphyrins. In the absence of olefin substrates, reaction of [Co(TPP)] with ethyl styryldiazoacetate was found to generate the corresponding cobalt(III)-vinylcarbene radical that subsequently dimerizes via its γ-radical allylic resonance form to afford a dinuclear cobalt(III) porphyrin complex. X-ray structural analysis reveals a highly compact dimeric structure wherein the two metalloporphyrin units are arranged in a face-to-face fashion through a tetrasubstituted 1,5-hexadiene C(6)-bridge between the two Co(III) centers. The γ-radical allylic resonance form of the cobalt(III)-vinylcarbene radical intermediate could be effectively trapped by TEMPO via C-O bond formation to give a mononuclear cobalt(III) complex instead of the dimeric product. The allylic radical nature and related reactivity profile of the cobalt(III)-carbene radical, including its inability to abstract hydrogen atoms from toluene solvent, were established by DFT calculations.

Crystal Engineering of a Microporous, Catalytically Active fcu Topology MOF Using a Custom‐Designed Metalloporphyrin Linker

A major driving force behind the recent surge of interest in metal–organic frameworks (MOFs) lies with their amenability to design using crystal engineering strategies. In particular, MOFs with specific composition and topology can be targeted by judicious selection of organic linkers and metal-based molecular building blocks (MBBs) that serve as nodes. Furthermore, their modular nature means that prototypal MOFs can serve as blueprints or platforms for a plethora of derivatives with controlled pore size and surface area, as exemplified by the practice of “reticular synthesis”. Such features make MOFs stand out over traditional porous materials, and afford them with potential for use in gas storage, separation, CO2 capture, [6] sensor, catalysis, and other areas. High-symmetry MOFs based upon high-connectivity polyhedral cage MBBs that are in effect supermolecular building blocks (SBBs) can provide exquisite control over structure because of their high connectivity and also afford the features of confined nanospace and extra-large surface area. Such MOFs have afforded superior performance in the context of gas storage for hydrogen, methane, CO2, and other gas molecules. 11,12] The nature of the nanospace in SBB-based MOFs is such that they can encapsulate catalytically active species, for example, organometallics, polyoxometallates, metalloporphyrins (porph@MOMs), and enzymes. Along with encapsulation of catalysts, it is possible to generate porphyrin-walled MOFs by customdesigning metalloporphyrin moieties so that they can serve as vertices and/or edges and/or faces. In principle, the metal clusters residing on the vertices could also contribute as active sites to achieve an even higher density of active sites than that of porph@MOMs. Polyhedral MOFs are therefore attractive targets as catalyst supports for heterogeneous catalysis. However, although metallosalens and metalloporphyrins have been utilized as linkers for catalytically active MOFs and polyhedral cage-containing metal–metalloporphyrin frameworks exist, MOFs sustained by catalytically active metalloporphyrin linkers and catalytically active MBBs remain unexplored. Herein, we report such a MOF that based upon a previously reported fcu topology net built from 12-connected cubohemioctahedral SBBs of formula [Co2(m2-H2O)(H2O)4]6(bdc)12 and benzoimidephenanthroline tetracarboxylate (bipa-tc) linkers (Scheme 1a), fcu-MOF-1. The new com-

Cobalt-Catalyzed Asymmetric Cyclopropanation with Diazosulfones: Rigidification and Polarization of Ligand Chiral Environment via Hydrogen Bonding and Cyclization

A new D2-symmetric chiral porphyrin P6 (2,6-DiMeO-ZhuPhyrin) with enhanced chiral rigidity and polarity was designed and synthesized through incorporation of hydrogen bonding and cyclic structure. Its cobalt(II) complex [Co(P6)] is a highly active and selective catalyst for asymmetric cyclopropanation of alkenes with diazosulfones. The [Co(P6)]-based catalytic system is suitable for various aromatic olefins as well as electron-deficient olefins, including alpha,beta-unsaturated esters, ketones, and nitriles, forming the corresponding cyclopropyl sulfones under mild conditions in high yields and high selectivities. In most cases, both excellent diastereo- and enantioselectivities were achieved.

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.

Cobalt(II)-Catalyzed Intramolecular C−H Amination with Phosphoryl Azides: Formation of 6- and 7-Membered Cyclophosphoramidates

A highly effective Co(II)-based system has been developed for catalytic intramolecular C-H amination with phosphoryl azides without the need of terminal oxidant or other additives, resulting in the high-yielding production of cyclophosphoramidates with nitrogen gas as the byproduct. Additional features of this new catalytic system include the amination of primary C-H bonds and formation of 7-membered-ring structures.

A highly effective cobalt catalyst for olefin aziridination with azides: hydrogen bonding guided catalyst design.

[Co(P1)], which was designed on the basis of potential hydrogen-bonding interactions in the metal-nitrene intermediate, is a highly active aziridination catalyst with azides. [Co(P1)] can effectively aziridinate various aromatic olefins with arylsulfonyl azides under mild conditions, forming sulfonylated aziridines in excellent yields. The Co-based system enjoys several attributes associated with the relatively low cost of cobalt and the wide accessibility of arylsulfonyl azides. Furthermore, it generates stable dinitrogen as the only byproduct.

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-catalyzed asymmetric olefin aziridination with diphenylphosphoryl azide.

The cobalt(II) complexes of D2-symmetric chiral porphyrins, such as 3,5-Di(t)Bu-ChenPhyrin P5, can catalyze asymmetric olefin aziridination with diphenylphosphoryl azide (DPPA) as a nitrene source. Acceptable asymmetric inductions were observed for the [Co(P5)]-based catalytic system, forming the desired N-phosphorus-substituted aziridines in moderate to high yields and good enantioselectivities.