Functionalization of Sydnones with Donor‐Acceptor Cyclopropanes, Cyclobutanes, and Michael Acceptors

Nucleophilic Ring Opening of Donor-Acceptor Cyclopropanes through Umpolung Reactivity of Organochlorophosphines: Phosphine Oxide-Functionalized Boron-Pendanted Compounds

We present a novel set of frustrated Lewis pair (FLP) systems that exhibit a remarkable ability to promote the ring opening of donor-acceptor cyclopropanes (DACs). This FLP-promoted protocol offers umpolung reactivity of R1R2PCl/CN (R1, R2 = aryl, alkyl) toward DACs via nucleophilic ring-opening reactions to provide phosphinated boron-pendanted diester compounds. This novel approach exhibits the dual role of BF3·OEt2 as an activator and a reactant. The resulting compounds were found in both the keto and enol forms, with the majority being in the keto form, according to NMR analysis. The enol form was identified by single-crystal XRD analysis, and DFT calculations indicated that the keto form is more stable than the corresponding enol form.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

Diastereoselective Synthesis of Tetrahydrobenzo[b]azocines by Lu(OTf)3 Catalyzed [4 + 4] Cycloaddition of Donor–Acceptor Cyclobutanes with Anthranils

The construction of N-heterocyclic eight-membered rings remains a challenging due to unfavorable transannular strain. Herein, we report a Lu(OTf)3 catalyzed formal [4 +4] cycloaddition reaction of cyclobutane 1,1-diesters with anthranils...

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Mechanistic study of cobalt(I)-catalyzed asymmetric coupling of ethylene and enynes to functionalized cyclobutanes

Density functional theory (DFT) calculations have been performed to gain insight into the reaction mechanism of the Co(I)-catalyzed asymmetric [2 + 2] cycloaddition reaction of enyne 1a with ethylene 2 to give the functionalized cyclobutene E-4a possessing a chiral, all-carbon quaternary center in the ring framework (Science, 361, 68-72). This study reveals that the whole catalysis can be characterized via three stages: (i) oxidative dimerization followed by reductive elimination gives the intermediate IM3, (ii) the alkenyl-Co(III) metallacycloheptene IM6 formation with the addition of another equivalent ethylene via an oxidative dimerization process, (iii) β-Hydrogen elimination and reductive elimination from IM6 to result in the final product E-4a and regenerate the active speices IM1 for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible for experimental realization under mild conditions, and the formation of the alkenyl-Co(III) metallacycloheptene IM6, with a barrier of 27.2 kcal mol^-1 (i.e., IM2 → TS4), should be the rate-determining step (RDS) during the whole catalysis. In addition, the origins of enantioselectivity and regioselectivity of the product are discussed.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.

Sydnone Methides-A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N-Heterocyclic Carbenes

Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π-electron rich anionic N-heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C-C coupling reactions with 1-fluoro-4-iodobenzene under Pd(PPh3 )4 and CuBr catalysis. 77 Se NMR resonance frequencies and 1 JC4-Se as well as 1 JC4-H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N-heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13 C NMR spectroscopy at -40 °C which is extremely shifted upfield in comparison to classical N-heterocyclic carbenes.

Friedel-Crafts-Type Reactions with Electrochemically Generated Electrophiles from Donor-Acceptor Cyclopropanes and -Butanes

We describe a general electrochemical method to functionalize donor-acceptor (D-A) cyclopropanes and -butanes with arenes utilizing Friedel-Crafts-type reactivity. The catalyst-free strategy relies on the direct anodic oxidation of the strained carbocycles, which leads after C(sp³)-C(sp³) cleavage to radical cations that act as electrophiles for the arylation reaction. Broad reaction scopes in regard to cyclopropanes, cyclobutanes, and aromatic reaction partners are presented. Additionally, a plausible electrolysis mechanism is proposed.

Synthesis of substituted 1,2-dihydroisoquinolines via Ni(ii) and Cu(i)/Ag(i) catalyzed double nucleophilic addition of arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs)

A concise approach for the synthesis of substituted 1,2-dihydroisoquinolines via double nucleophilic addition of primary arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs) in the presence of a catalytic Ni(ClO₄)₂·6H₂O and CuI/AgOTf system has been developed. Further applying this protocol, some of the derived malonates were converted into the corresponding monoesters under Krapcho decarboxylation reaction conditions. Thereafter, these esters were transformed into the respective acids and alcohols. In addition, multifunctionalized 4-(2,2,2-trifluoroacetyl) 1,2-dihydroisoquinolines were also obtained with excess TFAA.

Electrophilic fluorosulfoxonium cations as hidden Brønsted acid catalysts in (n + 2) annulations of strained cycloalkanes

A fluorosulfoxonium cation and triflic acid were shown to promote (n + 2) annulations of donor–acceptor cyclopropanes or -butanes with 1,2-dipoles, with different activity and selectivity but a presumed similar role as (hidden) Brønsted acids.