The mechanism of the cyclopropanation of alkenes with diazocompounds catalyzed by ClFeIII–porphyrin, [ClFeII–porphyrin]−, and FeII–porphyrin has been investigated by density functional theory calculations. The obtained results indicate that the only viable catalyst is the FeII–porphyrin, whose catalytic cycle involves a stepwise multistate (singlet, triplet, and quintet spin states) mechanism. The triplet FeII–porphyrin interacts with diazomethane leading to the formation of the axial and bridged FeII–carbene complexes. The former type is favored at the singlet state, the latter is the most stable at higher states, and both conformations are accessible at the experimental conditions. The second key step of the reaction consists of the (2 + 1) cycloaddition of the axial FeII–carbene complex to yield the corresponding cyclopropane. Additionally, we have explored the possibility of catalyzing the reaction via a double incorporation of the carbene moiety into the metal coordinating sphere. The obtained results indicate that the formation of the biscarbene FeII–porphyrin intermediates and generation of the (2 + 1) cycloadduct present small energetic barriers. Consequently, both complexes, monocarbene and biscarbene, result to be crucial for a complete understanding of the mechanism and kinetics in the cyclopropanation of olefins catalyzed by FeII–porphyrin. Several minimum energy crossing points ensure the kinetic and thermodynamic feasibility of the reaction. Additionally, the reported mechanism is compatible with the observed trans selectivity.

The outcome of the cycloaddition between activated ketenes and alpha,beta-unsaturated imines has been investigated both experimentally and theoretically. Our results indicate that activated monosubstituted ketenes yield exclusively [2 + 2] cycloadducts. Disubstituted activated ketenes yield [2 + 2] and/or [4 + 2] cycloadducts. In one case, an unexpected piperidin-2-one has been obtained, although its relative abundance with respect to the corresponding [2 + 2] or [4 + 2] cycloadducts can be minimized by the proper choice of experimental conditions. The ability of different ab initio and semiempirical methods to account for these results has been tested. The best agreement between theory and experiment is achieved at the MP2/6-31G level of theory, with solvent effects taken into account. The semiempirical hamiltonian AM1, at the RHF level, tends to overestimate the stability of the transition structures leading to six-membered cycloadducts, whereas 3 x 3CI-HE/AM1 and CASSCF(2,2)/6-31G methods tend to overestimate the stability and the biradical character of the transition structures leading to [2 + 2] cycloadducts.

N, cations in the presence of [(AcO) 2 (imidazole) 2 (H 2 O)Fe=O] complexh ave been studied by density functional theory.These transformations are suitable modelsfor the N-demethylation of tri-, di-, and monomethylatedl ysine residues of histones in the presence of Jumonji-Cc ontaining histoned emethylases. It has been found that the N-demethylation reactioni ss tepwise and occurs on triplet and quintet potential energy hypersurfaces. Both spin states are nearly degenerated and the quantum jump from one state to another has at ransitionp robability close to one. The preferred intrinsic mechanism depends upon the methylation degree. For trimethylated residues the mechanism consists of ap roton abstraction from am ethyl group followed by af ormation of ah ydroxymethylaminium intermediate. This mechanism also occurs when dimethylated residues are ablet oo rientate one methyl group towards the Fe=Og roup of the catalytic site. In contrast, when aN ÀH groupo ft he substrate is close enough to the Fe=Og roup, the intrinsically preferred N-demethylation reaction leads to the formation of an iminium intermediate that can be hydrolyzed to form the correspondingN-demethylated product.Scheme1.Catalytic methylation and demethylation of lysine residues (Km) of histones (Hn).Scheme2.Generalcatalytic cycle for Jumonji-C domain-containing histone demethylases.The details of the demethylation process (highlighted in grey) are gathered in Scheme 3.Scheme3.Demethylation steps associatedwith the reaction mechanism of Jumonji-C domain-containing histone demethylases.

The (3+2) cycloaddition between azomethine ylides and alkenes is an efficient, convergent and stereocontrolled method for the synthesis of unnatural pyrrolidine and proline scaffolds. In this review, the application of this reaction to the synthesis of enantiopure organometallic ligands for asymmetric catalysis is presented first. These new EhuPhos ligands can participate in a second generation of 1,3‐dipolar reactions that generate an offspring of unnatural proline derivatives that behave as efficient organocatalysts. These densely substituted unnatural l‐proline derivatives exhibit distinct features, different to those described for natural l‐proline and its derivatives. Finally, several examples of biologically active proline derivatives obtained by means of (3+2) cycloadditions involving azomethine ylides are presented. These applications show the character of privileged structures of these polysubstituted pyrrolidine rings.