Mechanism of Anionic [3 + 2] Cycloadditions. An ab Initio Computational Study on the Cycloaddition of Allyl-, 2-Borylallyl-, and 2-Azaallyllithium to Ethylene

Synthesis of gem-Difluorohomoallyl Amines via a Transition-Metal-Free Defluorinative Alkylation of Benzyl Amines with Trifluoromethyl Alkenes

A mild and transition-metal-free defluorinative alkylation of benzyl amines with trifluoromethyl alkenes is reported. The features of this protocol are easy-to-obtain starting materials, a wide range of substrates, and functional group tolerance as well as high atom economy, thus offering a strategy to access a variety of gem-difluorohomoallyl amines, which are extensively distributed in pharmaceuticals and bioactive agents, with excellent chemoselectivity. The primary products can be further transformed to a diversity of 2-fluorinated pyrroline compounds.

Quantum-Chemical Study of the Assembly Mechanism of 1-Pyrrolines from N-Benzylaldimines and Arylacetylenes in KOtBu/DMSO Superbasic Medium

By using a quantum-chemical approach, B2PLYP-D2/6-311+G**//B3LYP/6-31+G*, we have carried out a detailed study of the assembly of 1-pyrrolines from N-benzyl-1-phenylmethanimine and phenylacetylene in the superbasic medium KO^tBu/dimethyl sulfoxide (DMSO). In this way, we have considered, both theoretically and experimentally, the mechanisms of the assembly through a concerted and stepwise nucleophilic cycloaddition and have addressed the side processes accompanying the assembly. It is found that the assembly via the concerted cycloaddition is kinetically more favorable than that via the stepwise cycloaddition. At the same time, the reaction of C-vinylation of aldimine with phenylacetylene occurs with a similar activation energy as the concerted cycloaddition and leads to the formation of 2-aza-1,4-pentadiene. The anion of 2-aza-1,4-pentadiene is an intermediate for the side processes leading to the formation of triarylpyridines and 1,3-diarylpropan-1-ones. Triarylpyridines are formed through the concerted cycloaddition of the next phenylacetylene molecule to 2-aza-1,4-pentadiene, while 1,3-diarylpropan-1-ones are formed as a result of the hydrolysis of 2-aza-1,4-pentadienes. It is found out that the mild conditions for the assembly of 1-pyrrolines (60 °C, 15 min) relate to the formation of complexes in the KO^tBu/DMSO superbasic medium, where the anion is readily accessible for the nucleophilic attack by the phenylacetylene molecule.

Semistabilized Diazatrienyl Anions from Pyridine Imines and Acetylenes: An Access to (Z)-Stilbene/Imidazopyridine Ensembles, Benzyl Imidazopyridines, and Beyond

Semistabilized diazatrienyl anions are generated by the reaction of 2-pyridylarylimines with arylacetylenes in superbase systems MO^tBu (M = Li, Na, K)/DMSO at ambient temperature for 15 min. The initial intermediate N-centered propargyl-1,3-diaza-1,3,5-trienyl anions undergo intermolecular cyclization to benzyl imidazopyridine anions (formally [3 + 2] cycloaddition), further intercepting a second molecule of the starting pyridylimines or a proton of medium to afford (Z)-stilbene/imidazopyridine ensembles and benzyl imidazopyridines. The charge distribution in all intermediate anions and their synthetic evolution are consistent with quantum-chemical analysis (B2PLYPD/6-311+G**//B3LYP/6-31+G*).

Role of Water in the Reaction Mechanism and endo/exo Selectivity of 1,3-Dipolar Cycloadditions Elucidated by Quantum Chemistry and Machine Learning

Asymmetric 1,3-dipolar cycloadditions of azomethine ylides with activated olefins are among the most important and versatile methods for the synthesis of enantioenriched pyrroline and pyrrolidine derivatives. Despite both theoretical and practical importance, the role of water molecules in the reactivity and endo/exo selectivity remains unclear. To explore how water accelerates the reactions and improves the endo/exo selectivity of the cycloadditions of 1,3-dipole phthalazinium-2-dicyanomethanide (1) and two dipolarophiles, an ab initio-quality neural network potential that overcomes the computational bottleneck of explicitly considering water molecules was used. It is demonstrated that not only the nature of both the dipolarophile and the 1,3-dipole, but also the solvent medium, can perturb or even alter the reaction mechanism. An extreme case was found for the reaction of 1,3-dipole 1 with methyl vinyl ketone, in which the reaction mechanism changes from a concerted to a stepwise mode on going from MeCN to H₂ O as solvent, with formation of a zwitterionic intermediate that is a very shallow minimum on the energy surface. Thus, high stereocontrol can still be expected despite the stepwise nature of the mechanism. The results indicate that water can induce global polarization along the reaction coordinate and highlight the role of microsolvation effects and bulk-phase effects in reproducing the experimentally observed aqueous acceleration and enhanced endo/exo selectivity.

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

This review covers the use of 2-azaallyl anions, 2-azaallyl cations, and 2-azaallyl radicals in organic synthesis up through June 2018. Particular attention is paid to both foundational studies and recent advances over the past decade involving semistabilized and nonstabilized 2-azaallyl anions as key intermediates in various carbon-carbon and carbon-heteroatom bond-forming processes. Both transition-metal-catalyzed and transition-metal-free transformations are covered. Azomethine ylides, which have received significant attention elsewhere, are discussed briefly with the primary focus on critical comparisons with 2-azaallyl anions in regard to generation and use.

New mechanistic interpretations for nitrone reactivity

The reactivity of nitrones in cycloadditions and related reactions is revisited by introducing a topological perspective.

Revealing Stepwise Mechanisms in Dipolar Cycloaddition Reactions: Computational Study of the Reaction between Nitrones and Isocyanates

The mechanism of cycloaddition reactions of nitrones with isocyanates has been studied using density functional theory (DFT) methods at the M06-2X/cc-pVTZ level of theory. The exploration of the potential energy surfaces associated with two reactive channels leading to 1,2,4-oxadiazolidin-5-ones and 1,4,2-dioxazolidines revealed that the cycloaddition reaction takes place through a concerted mechanism in gas phase and in apolar solvents but a stepwise mechanism in polar solvents. In stepwise mechanisms, the first step of the reaction is a rare case in which the nitrone oxygen acts as a nucleophile by attacking the central carbon atom of the isocyanate (interacting with the π-system of the C═O bond) to give an intermediate. The corresponding transition structure is stabilized by an attractive electrostatic interaction favored in a polar medium. The second step of the reaction is the rate-limiting one in which the formation of 1,2,4-oxadiazolidin-5-ones or 1,4,2-dioxazolidines is decided. Calculations indicate that formation of 1,2,4-oxadiazolidin-5-ones is favored both kinetically and thermodynamically independently of the solvent, in agreement with experimental observations. Noncovalent interactions (NCI) and topological analysis of the gradient field of electron localization function (ELF) bonding confirmed the observed interactions.

Chiral silver amides as effective catalysts for enantioselective [3+2] cycloaddition reactions

Asymmetric [3+2] cycloaddition of α-aminoester Schiff bases with substituted olefins is one of the most efficient methods for the preparation of chiral pyrrolidine derivatives in optically pure form. In spite of its potential utility, applicable substrates for this method have been limited to Schiff bases that bear relatively acidic α-hydrogen atoms. Here we report a chiral silver amide complex for asymmetric [3+2] cycloaddition reactions. A silver complex prepared from silver bis(trimethylsilyl)amide (AgHMDS) and (R)-DTBM-SEGPHOS worked well in asymmetric [3+2] cycloaddition reactions of α-aminoester Schiff bases with several olefins to afford the corresponding pyrrolidine derivatives in high yields with remarkable exo- and enantioselectivities. Furthermore, α-aminophosphonate Schiff bases, which have less acidic α-hydrogen atoms, also reacted with olefins with high exo- and enantioselectivities. The stereoselectivities of the [3+2] cycloadditions with maleate and fumarate suggested that the reaction proceeded by means of a concerted mechanism. An NMR spectroscopic study indicated that complexation of AgHMDS with the bisphosphine ligand was not complete, and that free AgHMDS, which did not show any significant catalytic activity, existed in the catalyst solution. This means that significant ligand acceleration occurred in the current reaction system.

Driving force of metallo (Mg-H and Mg-Cl)-ene reaction mechanisms

The potential-energy surfaces of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were studied using ab initio molecular-orbital (MO) methods. The concerted path and the stepwise path of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene were identified and it was determined that the energy barriers on concerted paths of the metallo-ene reactions of allyl-MgH and allyl-MgCl with ethylene are lower than those on the stepwise paths. Furthermore, the concerted path of the metallo-ene reaction of allyl-MgCl with ethylene is more favorable than that of the allyl-MgH reaction system. The reaction mechanisms were analyzed using a CiLC method on the basis of CASSCF MOs. The driving force for the concerted path reactions arises from the migration process of the metal. The difference between the reactivity of allyl-MgH and allyl-MgCl can be explained with the reaction mechanism on the basis of the driving force.

Theoretical studies of metallo (Li and Na)-ene reaction mechanisms

The reaction mechanisms of allyl-lithium and allyl-sodium with ethylene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by a CiLC-IRC method on the basis of ab initio CASSCF MOs. The ene reaction pathways of allyl-Li and allyl-Na with ethylene were located. The complex between allyl-metal and ethylene for both systems is found in the first step of the reaction, and then the metal migration and new C-C bond formation occur synchronously through the transition state. The complexation energies are -13.2 and -9.6 kcal/mol for Li and Na systems, respectively. The activation energy barriers from the reactants are 3.5 kcal/mol for the Li system and 2.0 kcal/mol for the Na system at the MRMP2 calculation level. These barriers are significantly lower than that of the ene reaction of propene with ethylene as the parent reaction. The CiLC-IRC analysis shows that the reaction of allyl-metal with ethylene is a concerted ene reaction mechanism, not a metal catalysis and/or a stepwise reaction.

Three- and four-membered rings from cycloadditions of 1,3-thiazolium-4-olates and aldehydes

2-Aminothioisomünchnones, a well-known family of masked dipoles, react with aromatic aldehydes in a domino cascade reaction that produces episulfides (thiiranes) or beta-lactams (2-azetidinones). This sequence is initiated by a [3+2] dipolar cycloaddition followed by ring opening of cycloadducts and intramolecular rearrangement to afford these unusual ring contractions. The nature of the reaction products depends on the structural characteristics of the starting dipole and the experimental conditions. Episulfides are obtained selectively as cis isomers with respect to both aryl groups, whereas beta-lactams are produced as cis/trans mixtures. These structural features were determined unequivocally by X-ray crystallographic analysis. The beta-lactams still possessed a flexible acyclic chain containing sulfur, a salient lead modification of the bioactive cyclic penems and cephems. The preferential production of exo transition structures was rationalized with the aid of computational calculations at the B3LYP/6-31G* level.

Modification of regioselectivity in cycloadditions to C70 under microwave irradiation

The regioselectivity of the cycloaddition of N-methylazomethine ylide to C70 can be modified by using microwave irradiation as the source of energy. Under microwave irradiation and by choosing the appropriate solvent and irradiation power, the 5-6 isomer is the major product, a situation that is in contrast to conventional heating where the 1-2 isomer predominates. Moreover, isomer 7-21, which represents 13% of monoadducts under classical heating, is not formed under microwave irradiation and with ODCB as solvent. Theoretical calculations predict an asynchronous mechanism and suggest that the modification of the regiochemical outcome is related to the relative energies and hardnesses of the transition structures involved.