A combined experimental and theoretical study of the thermal [3+2] cycloaddition of carbonyl ylides with activated alkenes

Iron Porphyrins and Iron Salens as Highly Enantioselective Catalysts for the Ring-Expansion Reaction of Epoxides to Tetrahydrofurans

Iron porphyrins have been found to be efficient catalysts for the ring-expansion reaction of epoxides with alkenes to give the corresponding tetrahydrofurans; very low loadings (down to 1 mol %) are sufficient to achieve high yields (up to 98%). Additionally, an iron(III) salen complex based on the axially chiral 1,1-binaphthalene backbone has been shown to catalyze the reaction with high enantioselectivity (e.g., 92% ee for calyxolane A).

Synthesis of Functionalized Five-Membered Heterocycles from Epoxides: A Hydrogen-Bond Donor Catalytic Approach

The synthesis of highly functionalized five-membered oxa- and aza-heterocycles has been reported utilizing hydrogen-bond donor (HBD) catalysis. In this method, an epoxide was taken as a substrate and reacted with functionalized arylidene/alkylidene malononitrile derivatives in the presence of a newly designed HBD catalyst. In all the cases, the products 2,5-disubstituted tetrahydrofurans (2,5-THFs) were obtained in good to excellent yields (up to 86%) with high diastereoselectivity (dr up to 99:1) as a single regioisomer. The stereochemistry at the 2- and 5-positions of the five-membered ring has been confirmed by single-crystal X-ray analysis, and cis is found to be the major product. The same strategy has been further utilized to obtain substituted oxazolidines whenever the epoxide has been reacted with isocyanate as an electrophile. In order to induce enantioselectivity, a chiral epoxide has been reacted with both the electrophiles in the presence of the same catalyst system to afford the single stereoisomer of the final products. This synthetic methodology involves a low catalyst loading and ambient reaction condition and has been generalized with various substituents present in the starting electrophiles to produce the resultant products in acceptable yields and stereoselectivity.

Understanding the mechanism and regio- and stereo selectivity of [3 + 2] cycloaddition reactions between substituted azomethine ylide and 3,3,3-trifluoro-1-nitroprop-1-ene, within the molecular electron density theory

The selectivity and the nature of the molecular mechanism of the [3 + 2] cycloaddition (32CA) reaction between 2-(dimethylamino)-1H-indene-1,3(2H)-dione (AY11) and trans(E)-3,3,3-trifluoro-1-nitroprop-1-ene(FNP10) has been studied, in which the molecular electron density theory using density functional theory methods at the MPWB1K/6-31G(d) computational level was used. Analysis of the global reactivity indices permits us to characterize FNP10 as a strong electrophile and AY11 as a strong nucleophile. Four reactive pathways associated with the ortho/meta regioselective channels and endo/exo stereoselective approaches modes have been explored and characterized in the gas phase and in the benzene solvent. The analysis of the relative energies associated with the different reaction pathways indicates that the 32CA reactions of the azomethine ylide (AY) with the nitroalkene (FNP) is meta regioselective with high endo stereoselectivity. This result is in good agreement with the experimental observations. electron localization function topological analysis of the most favored reactive pathways allows for characterizing the mechanism of this 32CA reactions as a non-concerted two-stage one-step mechanism. Finally, non-covalent interactions and quantum theory of atoms in molecule analyses at the meta/endo transition state structure indicate that the presence of different several weak interactions, namely, CF and NH contributed in favoring the formation of a meta-endo cycloadduct.

Chemo- and regioselective ring-opening of donor-acceptor oxiranes with N-heteroaromatics

The first ring-opening of D-A oxiranes with N-heteroaromatics in a chemoselective C-C bond cleavage manner was achieved. In the presence of 5 mol% of Y(OTf)3 as the catalyst, diverse N-heteroaromatics, including benzotriazoles, purines, substituted benzimidazole, imidazole and pyrazole, reacted well with various D-A oxiranes, providing acyclic nucleoside analogues containing a N-glycosidic bond in up to 97% yield and up to >95 : 5 regioselectivity. Through simple transformation, the Ganciclovir analogue could also be obtained.

Chiral ScIII-N,N'-Dioxide-Catalyzed 1,3-Dipolar Cycloaddition of Diaziridines with Chalcones

A highly enantioselective 1,3-dipolar cycloaddition of meso-diaziridines with chalcones was realized by utilizing the Sc^III-N,N'-dioxide complex as the catalyst. In this transformation, the 1,3-dipole intermediates generated from the C-N bond cleavage of diaziridine were trapped by chiral N,N'-dioxide/scandium(III) complex activated chalcones to undergo enantioselective 1,3-dipolar cycloaddition. A range of chiral 1,5-diazabicylo[3.3.0]octane derivatives were readily synthesized in good yields with high diastereo- and enantioselectivities.

A Molecular Electron Density Theory Study of the Synthesis of Spirobipyrazolines through the Domino Reaction of Nitrilimines with Allenoates

The reaction of diphenyl nitrilimine (NI) with methyl 1-methyl-allenoate yielding a spirobipyrazoline has been studied within molecular electron density theory (MEDT) at the MPWB1K/6-311G(d) computational level in dichloromethane. This reaction is a domino process that comprises two consecutive 32CA reactions with the formation of a pyrazoline intermediate. Analysis of the relative Gibbs free energies indicates that both 32CA reactions are highly regioselective, the first one being also completely chemoselective, in agreement with the experimental outcomes. The geometries of the TSs indicate that they are associated to asynchronous bond formation processes in which the shorter distance involves the C1 carbon of diphenyl NI. Despite the zwitterionic structure of diphenyl NI, the appearance of a pseudoradical structure at the beginning of the reaction path, with a very low energy cost, suggests that the 32CA reaction between diphenyl NI, a strong nucleophile, and the allenoate, a moderate electrophile, should be mechanistically considered on the borderline between pmr-type and cb-type 32CA reactions, somewhat closer to the latter.

A molecular electron density theory study on the [3+2] cycloaddition reaction of thiocarbonyl ylides with hetaryl thioketones

In situ generated TCY 2 is trapped with THK 3 over the course of a non-polar, entirely C1–C4 regioselective, pdr-type 32CA reaction passing through TS 1 in a non-concerted two-stage one-step molecular mechanism.