Metalloporphyrins are a class of versatile catalysts with the capability to functionalize saturated C-H bonds via several well-defined atom/group transfer processes, including oxene, nitrene, and carbene C-H insertions. The corresponding hydroxylation, amination, and alkylation reactions provide direct approaches for the catalytic conversion of abundant hydrocarbons into value-added functional molecules through C-O, C-N, and C-C bond formations, respectively. This tutorial review describes metalloporphyrin-based catalytic systems for the functionalization of different types of sp(3) C-H bonds, both inter- and intramolecularly, including challenging primary C-H bonds. Additional features of metalloporphyrin-catalyzed C-H functionalization include unusual selectivities and high turnover numbers.

The mechanism of cobalt(II) porphyrin-catalyzed benzylic C-H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides [N(3)C(O)OMe, N(3)SO(2)Ph, N(3)C(O)Ph, and N(3)P(O)(OMe)(2)] as nitrene sources was studied by means of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy. The DFT computational study revealed a stepwise radical process involving coordination of the azide to the metal center followed by elimination of dinitrogen to produce unusual "nitrene radical" intermediates (por)Co(III)-N(•)Y (4) [Y = -C(O)OMe, -SO(2)Ph, -C(O)Ph, -P(O)(OMe)(2)]. Formation of these nitrene radical ligand complexes is exothermic, predicting that the nitrene radical ligand complexes should be detectable species in the absence of other reacting substrates. In good agreement with the DFT calculations, isotropic solution EPR signals with g values characteristic of ligand-based radicals were detected experimentally from (por)Co complexes in the presence of excess organic azide in benzene. They are best described as nitrene radical anion ligand complexes (por)Co(III)-N(•)Y, which have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety. These key cobalt(III)-nitrene radical intermediates readily abstract a hydrogen atom from a benzylic position of the organic substrate to form the intermediate species 5, which are close-contact pairs of the thus-formed organic radicals R'(•) and the cobalt(III)-amido complexes (por)Co(III)-NHY ({R'(•)···(por)Co(III)-NHY}). These close-contact pairs readily collapse in a virtually barrierless fashion (via transition state TS3) to produce the cobalt(II)-amine complexes (por)Co(II)-NHYR', which dissociate to afford the desired amine products NHYR' (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R'(•)···(por)Co(III)-NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R'(•) by (por)Co(III)-NHY (via TS4) to form the corresponding olefin and (por)Co(III)-NH(2)Y, which dissociates to give Y-NH(2). This process for the formation of olefin and Y-NH(2) byproducts is also essentially barrierless and should compete with the collapse of 5 via TS3 to form the desired amine product. Alternative processes leading to the formation of side products and the influence of different porphyrin ligands with varying electronic properties on the catalytic activity of the cobalt(II) complexes have also been investigated.

To fully characterize the CoIII–‘nitrene radical’ species that are proposed as intermediates in nitrene transfer reactions mediated by cobalt(II) porphyrins, different combinations of cobalt(II) complexes of porphyrins and nitrene transfer reagents were combined, and the generated species were studied using EPR, UV–vis, IR, VCD, UHR-ESIMS, and XANES/XAFS measurements. Reactions of cobalt-(II) porphyrins 1P1 (P1 = meso-tetraphenylporphyrin (TPP)) and 1P2 (P2 = 3,5-DitBu-ChenPhyrin) with organic azides 2Ns (NsN3), 2Ts (TsN3), and 2Troc (TrocN3) led to the formation of mono-nitrene species 3P1Ns, 3P2Ts, and 3P2Troc, respectively, which are best described as [CoIII(por)(NR″•−)] nitrene radicals (imidyl radicals) resulting from single electron transfer from the cobalt(II) porphyrin to the ‘nitrene’ moiety (Ns: R″ = –SO2-p-C6H5NO2; Ts: R″ = –SO2C6H6; Troc: R″ = –C(O)OCH2CCl3). Remarkably, the reaction of 1P1 with N-nosyl iminoiodane (PhI=NNs) 4Ns led to the formation of a bis-nitrene species 5P1Ns. This species is best described as a triple-radical complex [(por•−)CoIII(NR″•−)2] containing three ligand-centered unpaired electrons: two nitrene radicals (NR″•−) and one oxidized porphyrin radical (por•−). Thus, the formation of the second nitrene radical involves another intramolecular one-electron transfer to the “nitrene” moiety, but now from the porphyrin ring instead of the metal center. Interestingly, this bis-nitrene species is observed only on reacting 4Ns with 1P1. Reaction of the more bulky 1P2 with 4Ns results again in formation of mainly mono-nitrene species 3P2Ns according to EPR and ESI-MS spectroscopic studies. The mono- and bis-nitrene species were initially expected to be five- and six-coordinate species, respectively, but XANES data revealed that both mono- and bis-nitrene species are six-coordinate Oh species. The nature of the sixth ligand bound to cobalt(III) in the mono-nitrene case remains elusive, but some plausible candidates are NH3, NH2−, NsNH−, and OH−; NsNH− being the most plausible. Conversion of mono-nitrene species 3P1Ns into bis-nitrene species 5P1Ns upon reaction with 4Ns was demonstrated. Solutions containing 3P1Ns and 5P1Ns proved to be still active in catalytic aziridination of styrene, consistent with their proposed key involvement in nitrene transfer reactions mediated by cobalt(II) porphyrins.

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.

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.

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.

The CoII complex of a D2‐symmetric chiral porphyrin ([Co(D2‐Por*)], see scheme) is a highly effective catalyst for the enantioselective aziridination of alkenes with fluoroaryl azides. The reaction can be performed at RT with low catalyst loading, and the olefin is the limiting reagent. Furthermore, the reaction is tolerant toward different combinations of aromatic olefins and fluoroaryl azides.

Amino groups exist ubiquitously in natural products and synthetic molecules, and play key roles in a wide range of important applications. Consequently, immense effort has been devoted to the development of efficient and selective processes for the preparation of amines.[1] Among different approaches, the catalytic amination of abundant C À H bonds on the basis of a metal-mediated nitrene-insertion pathway is one of the most general and direct methods for installing nitrogen functionalities.[2] The promise of this approach as a synthetically useful methodology has been demonstrated with a number of intramolecular C À H amination processes through the combined use of Rh II 2 -based catalysts and iminoiodane nitrene sources. [2,3] Notably, Du Bois and coworkers elegantly demonstrated that N-Boc-protected sulfamides could be selectively converted into cyclic sulfamides by [Rh 2 (esp) 2 ] in combination with PhI(OAc) 2 and MgO to provide access to synthetically useful 1,3-diamines (esp = a,a,a',a'-tetramethyl-1,3-benzenedipropionate).[4] The Rh II 2 -based intramolecular amination was shown to be effective for both secondary and tertiary CÀH bonds with stereospecificity and high diastereoselectivity. However, the amination of strong primary CÀH bonds had yet to be demonstrated. [5] Moreover, the catalytic system was unsuitable for simple Nalkyl sulfamides, which were oxidatively degraded by the stoichiometric oxidant, PhI(OAc) 2 . [4] As stable metalloradicals, cobalt(II) complexes of porphyrins, [Co(Por)], have emerged as a new class of catalysts for CÀH amination.[6] The cobalt(II)-based metalloradical amination (MRAm) is different from the commonly studied Rh 2 system, as it can operate effectively with various azide substrates without the need for terminal oxidants and other additives. [7][8][9][10][11] To further validate the utility of C À H amination methodology based on a cobalt(II) catalyst and azides, we envisioned a general strategy for the synthesis of 1,3-diamines from monoamines through the key step of the intramolecular C À H amination of sulfamoyl azides with [Co(Por)] (Scheme 1). We report herein a cobalt(II)-based catalytic system that is highly effective for the intramolecular 1,6-CÀH amination of sulfamoyl azides to furnish six-membered cyclic sulfamides. Not only excellent regioselectivity, but also high diastereoselectivity and stereospecificity were observed with the catalytic system. The cobalt(II)-catalyzed amination is operationally simple, as it proceeds under neutral and nonoxidative conditions without the need for other reagents, and N 2 is the only byproduct. Consequently, the degree of functional-group tolerance is high, and the reaction can be applied to substrates with various substituents, such as oxidizable amide and sulfide groups. An important feature of this catalytic system is the effective amination of strong primary C À H bonds, as well as secondary and tertiary C À H bonds.A wide range of sulfamoyl azides 2 were conveniently prepared from the corresponding amines 1 on the bas...