Halogen Bonding in the Decoration of Secondary Coordination Sphere of Zinc(II) and Cadmium(II) Complexes: Catalytic Application in Cycloaddition Reaction of CO2 with Epoxides.

Three new zinc(II) complexes [Zn(H2L3)2(H2O)3] (Zn2), [Zn(H3L2a)(H2O)3]n (Zn3) (H3L2a = 2,4-diiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl)isophthalate) and [Zn(HL4)(DMF)(H2O)]n (Zn4) were synthesized by the reaction of Zn(II) salts with 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl) isophthalic acid (H3L3), 2,4,6-triiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl) isophthalic acid (H5L2) (in the presence of NH2OH·HCl) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)-2,4,6-triiodoisophthalic acid (H3L4), respectively. According to the X-ray structural analysis, the intramolecular resonance-assisted hydrogen bond ring remains intact, with N···O distances of 2.562(5) and 2.573(5) Å in Zn2, 2.603(6) Å in Zn3, and 2.563(8) Å in Zn4. In the crystal packing of Zn3, the cooperation of I···O and I···I types of halogen bonds between tectons leads to a one-dimensional supramolecular polymer, while I···O interactions aggregate 1D chains of coordination polymer Zn4. These new complexes (Zn2, Zn3, and Zn4) and known [Zn(H3L1)(H2O)2]n (Zn1) (H3L1 = 5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene) hydrazineyl)isophthalate), {[Zn(H3L1)(H2O)3]·3H2O}n (Zn5), [Cd(H3L1)(H2O)2]n (Cd1), {[Cd(HL3)(H2O)2(DMF)]·H2O}n (Cd2), [Cd(H3L3)]n (Cd-3), {[Cd2(µ-H2O)2(µ-H2L4)2(H2L4)2]·2H2O}n (Cd4), and {[Cd(H3L1)(H2O)3]·4H2O}n (Cd5) were tested as catalysts in the cycloaddition reaction of CO2 with epoxides in the presence of tetrabutylammonium halides as the cocatalyst. The halogen-bonded catalyst Zn4 is the most efficient one in the presence of tetrabutylammonium bromide by affording a high yield (85-99%) of cyclic carbonates under solvent-free conditions after 48 h at 40 bar and 80 °C.

Protonated Porphyrins: Bifunctional Catalysts for the Metal-Free Synthesis of N-Alkyl-Oxazolidinones.

The protonation of commercially available porphyrin ligands yields a class of bifunctional catalysts able to promote the synthesis of N-alkyl oxazolidinones by CO2 cycloaddition to corresponding aziridines. The catalytic system does not require the presence of any Lewis base or additive, and shows interesting features both in terms of cost effectiveness and eco-compatibility. The metal-free methodology is active even with a low catalytic loading of 1 % mol, and the chemical stability of the protonated porphyrin allowed it to be recycled three times without any decrease in performance. In addition, a DFT study was performed in order to suggest how a simple protonated porphyrin can mediate CO2 cycloaddition to aziridines to yield oxazolidinones.

An In-Depth Computational Study of Alkene Cyclopropanation Catalyzed by Fe(porphyrin)(OCH3) Complexes. The Environmental Effects on the Energy Barriers.

Iron porphyrin methoxy complexes, of the general formula [Fe(porphyrin)(OCH3)], are able to catalyze the reaction of diazo compounds with alkenes to give cyclopropane products with very high efficiency and selectivity. The overall mechanism of these reactions was thoroughly investigated with the aid of a computational approach based on density functional theory calculations. The energy profile for the processes catalyzed by the oxidized [FeIII(Por)(OCH3)] (Por = porphine) as well as the reduced [FeII(Por)(OCH3)]- forms of the iron porphyrin was determined. The main reaction step is the same in both of the cases, that is, the one leading to the terminal-carbene intermediate [Fe(Por)(OCH3)(CHCO2Et)] with simultaneous dinitrogen loss; however, the reduced species performs much better than the oxidized one. Contrarily to the iron(III) profile in which the carbene intermediate is directly obtained from the starting reactant complex, the favored iron(II) process is more intricate. The initially formed reactant adduct between [FeII(Por)(OCH3)]- and ethyl diazoacetate (EDA) is converted into a closer reactant adduct, which is in turn converted into the terminal iron porphyrin carbene [Fe(Por)(OCH3)(CHCO2Et)]-. The two corresponding transition states are almost isoenergetic, thus raising the question of whether the rate-determining step corresponds to dinitrogen loss or to the previous structural and electronic rearrangement. The ethylene addition to the terminal carbene is a downhill process, which, on the open-shell singlet surface, presents a defined but probably short-living diradicaloid intermediate, though other spin-state surfaces do not show this intermediate allowing a direct access to the cyclopropane product. For the crucial stationary points, the more complex catalyst [Fe(2)(OCH3)], in which a sterically hindered chiral bulk is mounted onto the porphyrin, was investigated. The corresponding computational data disclose the very significant effect of the porphyrin skeleton on the reaction energy profile. Though the geometrical features around the reactive core of the system remain unchanged, the energy barriers become much lower, thus revealing the profound effects that can be exerted by the three-dimensional organic scaffold surrounding the reaction site.

Iron catalysts with N-ligands for carbene transfer of diazo reagents.

Transition-metal-catalyzed carbene transfer reactions, involving diazo compounds and their precursors, are powerful tools for creating new C-C bonds. Depending on the involved catalytic system, the carbene insertion can efficiently be driven towards a specific functional group for the synthesis of a wide portfolio of fine chemicals. The present report is focused on the catalytic activity of iron catalysts in promoting alkene cyclopropanations, C-H and X-H (X = N, O, S, Se, Si, Sn, Ge) functionalizations. Porphyrin, porphyrinoid and non-heme iron complexes are discussed by analyzing experimental studies and theoretical calculations performed for proposing reaction mechanisms. The catalytic activity of artificial iron biocatalysts is also briefly reported in order to underline the similarities and differences between reaction mechanisms mediated by modified biocatalysts and synthetic catalysts. This review summarizes the achievements made in this field since 2006.

Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions.

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)2 -catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C3 -substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)2 C6 H3 N3 azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)2 to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp2 C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a 'Ru(TPP)(NAr)' mono-imido active catalyst is reformed after each azide/alkyne reaction.

Correction: Resonance Raman spectroscopy as an in situ probe for monitoring catalytic events in a Ru-porphyrin mediated amination reaction.

Correction for 'Resonance Raman spectroscopy as an in situ probe for monitoring catalytic events in a Ru-porphyrin mediated amination reaction' by Paolo Zardi et al., Analyst, 2016, 141, 3050-3058.

Designing 'Totem' C2 -Symmetrical Iron Porphyrin Catalysts for Stereoselective Cyclopropanations.

The catalytic activity of the iron(III) C2 chiral porphyrin Fe(2)(OMe) in alkene cyclopropanation is herein reported. The catalyst promoted the reaction of differently substituted styrenes with diazo derivatives with trans-diastereoselectivities and enantioselectivities up to 99:1 and 87 %, respectively. In addition, high TON and TOF values (up to 10 000 and 120 000 h(-1) , respectively) were observed indicating good activity and stability of the catalyst in optimized experimental conditions. The study of the cyclopropanation reaction revealed that the porphyrin skeleton is composed of two 'totem' parts which were independently responsible for the observed enantio- and diastereoselectivities. To further our research we also investigated the catalytic role of the methoxy axial ligand coordinated to the iron atom. The molecular structure of Fe(2)(OMe) was optimized by DFT calculations which were also employed to achieve a better understanding of the mechanistic details of the carbene transfer reaction.

The ligand influence in stereoselective carbene transfer reactions promoted by chiral metal porphyrin catalysts.

The use of diazo reagents of the general formula N2C(R)(R1) as carbene sources to create new C-C bonds is of broad scientific interest due to the intrinsic sustainability of this class of reagents. In the presence of a suitable catalyst, diazo reagents react with several organic substrates with excellent stereo-control and form N2 as the only by-product. In the present report the catalytic efficiency of metal porphyrins in promoting carbene transfer reactions is reviewed with emphasis on the active role of the porphyrin skeleton in stereoselectively driving the carbene moiety to the target substrate. The catalytic performances of different metal porphyrins are discussed and have been related to the structural features of the ligand with the final aim of rationalizing the strict correlation between the three-dimensional structure of the porphyrin ligand and the stereoselectivity of carbene transfer reactions.

Resonance Raman spectroscopy as an in situ probe for monitoring catalytic events in a Ru-porphyrin mediated amination reaction.

Resonance Raman microspectroscopy has been widely used to study the structure and dynamics of porphyrins and metal complexes containing the porphyrin ligand. Here, we have demonstrated that the same technique can be adapted to examine the mechanism of a homogeneously-catalysed reaction mediated by a transition-metal-porphyrin complex. Previously it has been challenging to study this type of reaction using in situ spectroscopic monitoring due to the low stability of the reaction intermediates and elevated-temperature conditions. We have made a straightforward modification to the sample stage on a microscope for time-lapsed Raman microspectroscopy from reaction mixtures in these media. The allylic amination of unsaturated hydrocarbons by aryl azides, which can be catalysed by a ruthenium-porphyrin complex, has been used as an illustrative example of the methodology. The mechanism of this particular reaction has been studied previously using density-functional theory and kinetic approaches. The Raman measurements support the mechanism proposed in the earlier publications by providing the first experimental verification of a precursor reaction complex between the aryl azide and the ruthenium metal ion, and evidence for the formation of a mono-imido intermediate complex under conditions of high concentration of the reactant olefin.

[Silver(I)(pyridine-containing ligand)] complexes as unusual catalysts for A(3)-coupling reactions.

Two original macrocyclic silver(I)(pyridine-containing ligand) complexes [Ag(I)(Pc-L)] were synthesized and characterized. Their ability to catalyze the coupling among aldehydes, terminal alkynes and amines (A(3)-coupling) was demonstrated. The reaction could be performed under conventional as well as dielectric heating. The catalysts were effective in both cases, but dielectric heating allowed a lower catalyst loading and reduced ratio among reaction partners in shorter reaction times. The reaction scope was broad, including aryl/alkyl aldehydes, aryl/alkyl acetylenes and secondary aliphatic amines. Some unprecedented propargylamines have been prepared. The new catalytic system was also tested with more challenging coupling partners such as aniline and ketones.

Organic azides: "energetic reagents" for the intermolecular amination of C-H bonds.

This feature article provides an overview of the application of organic azides for the intermolecular amination of sp(3) and sp(2) C-H bonds. The catalytic activity of several metal complexes was reviewed underlining both synthetic and mechanistic aspects of the C-H amination. The majority of the aminated compounds reported in literature have been collected in this paper to provide a compendium of published procedures. In addition, the discussion of involved mechanisms has been included to assist the reader to envisage the future potential of organic azides in the synthesis of aza-derivatives.

Highly diastereoselective cyclopropanation of α-methylstyrene catalysed by a C2-symmetrical chiral iron porphyrin complex.

A new chiral iron porphyrin-based catalyst performed α-methylstyrene stereoselective cyclopropanation with excellent yields (up to 99%), enantio- and diastereoselectivities (ee(trans) up to 87%, trans/cis ratios up to 99 : 1) and outstanding TON and TOF values (up to 20,000 and 120,000 h(-1) respectively).

Asymmetric cyclopropanation of olefins catalysed by Cu(I) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*).

The synthesis and characterisation of copper(I) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*) and their use as catalysts in asymmetric cyclopropanation reactions are reported. All ligands and metal complexes were fully characterised, including crystal structures of some species determined by X-ray diffraction on single crystals. This allowed characterising the very different conformations of the macrocycles which could be induced by different substituents or by metal complexation. The strategy adopted for the ligand synthesis is very flexible allowing several structural modifications. A small library of macrocyclic ligands possessing the same donor properties but with either C(1) or C(2) symmetry was synthesized. Cyclopropane products with both aromatic and aliphatic olefins were obtained in good yields and enantiomeric excesses up to 99%.

[Ru(TPP)CO]-catalysed intramolecular benzylic C-H bond amination, affording phenanthridine and dihydrophenanthridine derivatives.

Shedding light on azides: [Ru(TPP)CO] (TPP=tetraphenyl porphyrin dianion), white light and O(2) were found to be a suitable catalyst combination to perform the annulation of several biaryl azides. The high chemoselectivity of the process allows the synthesis of phenanthridines and dihydrophenanthridines in good yield and purity.

Unexpected isomerism in "[Pd(2,9-dimethylphenanthroline)X2]" (X = Cl, Br, I) complexes: a neutral and an ionic form exist.

Complexes of composition "[Pd(2,9-dimethylphenanthroline)X(2)]" (X = Cl, Br, I) have long been known and they are used as precursors for the synthesis of other derivatives or as catalysts. In the previous literature, they have invariably been described as neutral square planar complexes, but we have found that a second ionic isomer also exists, having composition [Pd(Neoc)(2)X](2)[Pd(2)X(6)], and that the formation of this isomer occurs under a wider range of conditions than that of the neutral one. Retrospectively, the ionic isomer had surely been obtained in most previous reports even if formation of the neutral one was claimed.

The key intermediate in the amination of saturated C-H bonds: synthesis, X-ray characterization and catalytic activity of Ru(TPP)(NAr)(2) (Ar = 3,5-(CF(3))(2)C(6)H(3)).

The complex Ru(TPP)(NAr)(2) inserts a nitrene group into allylic and benzylic C-H bonds and is the key intermediate in the ruthenium porphyrin-catalyzed amination of hydrocarbons by aryl azides.

Nitrene transfer reactions mediated by metallo-porphyrin complexes.

Nitrene transfer reactions represent a useful methodology to synthesize in a few steps high added-value compounds used as organic intermediates. Herein, we describe the catalytic activity of metal porphyrin complexes in a wide range of reactions such as C-H hydrocarbon amination and olefin aziridination to synthesize nitrogen containing molecules. All the most important nitrene sources have been reviewed stressing the potentiality and limits of each one in the particular class of chemical transformation.

Rearrangement of N-aryl-2-vinylaziridines to benzoazepines and dihydropyrroles: a synthetic and theoretical study.

Herein we report the one-pot synthesis of several N-heterocyclic compounds by rearrangement reactions of N-aryl-2-vinylaziridines. The optimization of the synthetic methodology employed allowed us to obtain differently substituted 2,5-dihydro-1H-benzo[b]azepines in good yields and purities. The relationship between the nature of the starting N-aryl-2-vinylaziridine and the obtained N-heterocycle was also investigated. Finally, to rationalize all the experimental results reported in this paper a theoretical study was performed that casts light on the reaction mechanism.

Designing new ligands: asymmetric cyclopropanation by Cu(i) complexes based on functionalised pyridine-containing macrocyclic ligands.

The synthesis and characterisation of copper(i) complexes, including two crystal structures of the new chiral pyridine-containing macrocyclic ligands (PC-type), and their use as catalysts in asymmetric cyclopropanation reactions are reported.

Structural determination of ruthenium-porphyrin complexes relevant to catalytic epoxidation of olefins.

A reproducible synthesis of a competent epoxidation catalyst, [Ru(VI)(TPP)(O)2)] (TPP = tetraphenylporphyrin dianion), starting from [Ru(II)(TPP)(CO)L] (L = none or CH3OH), is described. The molecular structure of the complex was determined by using ab initio X-ray powder diffraction (XRPD) methods, and its solution behavior was in detail investigated by NMR techniques such as PGSE (pulsed field gradient spin-echo) measurements. [Ru(IV)(TPP)(OH)]2O, a reported byproduct in the synthesis of [Ru(VI)(TPP)(O)2], was synthesized in a pure form by oxidation of [Ru(II)(TPP)(CO)L] or by a coproportionation reaction of [Ru(VI)(TPP)(O)2] and [Ru(II)(TPP)(CO)L], and its molecular structure was then determined by XRPD analysis. [Ru(VI)(TPP)(O)2] can be reduced by dimethyl sulfoxide or by carbon monoxide to yield [Ru(II)(TPP)(S-DMSO)2] or [Ru(II)(TPP)(CO)(H2O)], respectively. These two species were characterized by conventional single-crystal X-ray diffraction analysis.