Electrophilicities of 1,2-Disubstituted Ethylenes

Reusable Supported Pyridine-Mediated Cascade Synthesis of trans-2,3-Dihydroindoles via In Situ-Generated N-Ylide

Merrifield resin-anchored pyridines were prepared and applied as reusable mediators for trans-selective cascade synthesis of 2,3-dihydroindoles. The developed approach relied on in situ N-ylide formation followed by Michael substitution reactions. The cascade reaction was also carried out efficiently with simple pyridine. The products were further transformed into synthetically valuable compounds, and supported pyridine was reused for multiple cycles. Density functional theory calculations confirmed the trans-selectivity as the lower-energy pathway.

Reactivity of electrophilic cyclopropanes

Cyclopropanes that carry an electron-accepting group react as electrophiles in polar, ring-opening reactions. Analogous reactions at cyclopropanes with additional C2 substituents allow one to access difunctionalized products. Consequently, functionalized cyclopropanes are frequently used building blocks in organic synthesis. The polarization of the C1-C2 bond in 1-acceptor-2-donor-substituted cyclopropanes not only favorably enhances reactivity toward nucleophiles but also directs the nucleophilic attack toward the already substituted C2 position. Monitoring the kinetics of non-catalytic ring-opening reactions with a series of thiophenolates and other strong nucleophiles, such as azide ions, in DMSO provided the inherent S_N2 reactivity of electrophilic cyclopropanes. The experimentally determined second-order rate constants k ₂ for cyclopropane ring-opening reactions were then compared to those of related Michael additions. Interestingly, cyclopropanes with aryl substituents at the C2 position reacted faster than their unsubstituted analogues. Variation of the electronic properties of the aryl groups at C2 gave rise to parabolic Hammett relationships.

How electrophilic are 3-nitroindoles? Mechanistic investigations and application to a reagentless (4+2) cycloaddition

The electrophilicity of 4 different 3-nitroindole derivatives has been evaluated by Mayr's linear free energy relationship (log k(20 °C) = s_N(E + N)) and reveals unexpected values for aromatic compounds, in the nitrostyrene range. 3-Nitroindoles are sufficiently electrophilic to interact with a common diene namely the Danishefsky's diene at room temperature, in the absence of any activator, to furnish smoothly the dearomatized (4+2) cycloadducts in good yields.

Iron-Catalyzed Photoinduced LMCT: a 1° C-H Abstraction Enables Skeletal Rearrangements and C(sp3)-H Alkylation

Herein we disclose an iron-catalyzed method to access skeletal rearrangement reactions akin to the Dowd-Beckwith ring expansion from unactivated C(sp³)-H bonds. Photoinduced ligand-to-metal charge transfer at the iron center generates a chlorine radical, which abstracts electron-rich C(sp³)-H bonds. The resulting unstable alkyl radicals can undergo rearrangement in the presence of suitable functionality. Addition to an electron deficient olefin, recombination with a photoreduced iron complex, and subsequent protodemetallation allows for redox-neutral alkylation of the resulting radical. Simple adjustments to the reaction conditions enable the selective synthesis of the directly alkylated or the rearranged-alkylated products. As a radical clock, these rearrangements also enable the measurement of rate constants of addition into various electron deficient olefins in the Giese reaction.

Electrophilic reactivities of cyclic enones and α,β-unsaturated lactones

The reactivities of cyclic enones and α,β-unsaturated lactones were characterized by following the kinetics of their reactions with colored carbon-centered reference nucleophiles in DMSO at 20 °C. The experimentally determined second-order rate constants k₂ were analyzed with the Mayr–Patz equation, lg k = s_N(N + E), to furnish the electrophilicity descriptors E for the Michael acceptors. Cyclic enones and lactones show different reactivity trends than their acyclic analogs. While cyclization reduces the reactivity of enones slightly, α,β-unsaturated lactones are significantly more reactive Michael acceptors than analogously substituted open-chain esters. The observed reactivity trends were rationalized through quantum-chemically calculated Gibbs energy profiles (at the SMD(DMSO)/M06-2X/6-31+G(d,p) level of theory) and distortion interaction analysis for the reactions of the cyclic Michael acceptors with a sulfonium ylide. The electrophilicities of simplified electrophilic fragments reflect the general reactivity pattern of structurally more complex terpene-derived cyclic enones and sesquiterpene lactones, such as parthenolide.

Different reactivity trends for cyclic and acyclic Michael acceptors were found within the framework of Mayr's experimental reactivity scales and analyzed through quantum-chemical studies.

Efficient synthesis of tetra- and penta-substituted benzenes via a domino annulation reaction of a pyridinium ylide and chalcone o-enolate

A very simple and highly efficient protocol for synthesizing tetra- and penta-substituted benzene derivatives has been developed.

Antimony-Oxo Porphyrins as Photocatalysts for Redox-Neutral C-H to C-C Bond Conversion

The use of high-valent antimony-oxo porphyrins as visible-light photocatalysts operating via direct hydrogen atom transfer has been demonstrated. Computational analysis indicates that the triplet excited state of these complexes shows an oxyl radical behavior, while the Sb^V center remains in a high-valent oxidation state, serving uniquely to carry the oxo moiety and activate the coordinated ligands. This porphyrin-based system has been exploited upon irradiation to catalyze C-H to C-C bond conversion via the addition of hydrogen donors (ethers and aldehydes) onto Michael acceptors in a redox-neutral fashion without the need of any external oxidant. Laser flash photolysis experiments confirmed that the triplet excited state of the photocatalyst triggers the desired C-H cleavage.

Methyl Anion Affinities of the Canonical Organic Functional Groups

Calculated methyl anion affinities are known to correlate with experimentally determined Mayr E parameters for individual organic functional group classes but not between neutral and cationic organic electrophiles. We demonstrate that methyl anion affinities calculated with a solvation model (MAA*) give a linear correlation with Mayr E parameters for a broad range of functional groups. Methyl anion affinities (MAA*), plotted on the log scale of Mayr E, provide insights into the full range of electrophilicity of organic functional groups. On the Mayr E scale, the electrophilicity toward the methyl anion spans 180 orders of magnitude.

Selective synthesis of spirooxindoles via a two-step reaction of N-phenacylpyridinium bromide, 1,3-indanedione and N-alkylisations

The three-component reaction of 1,3-indanedione and N-alkylisatins with two molecular N-phenacylpyridinium salts in dry acetonitrile in the presence of triethylamine resulted in unique functionalized spiro[indoline-3,4'-naphtho[1,2-b]furan] derivatives in good yields, which were successfully converted to spiro[indoline-3,2'-naphthalen]-4'-yl acetate derivatives by acylation with acetyl chloride in methylene dichloride and alkoxy-substituted spiro[benzo[h]chromene-5,3'-indolines] by acid-catalyzed etherification reaction in alcohol. The reaction mechanism involved the sequential cycloaddition of the in situ-generated pyridinium ylide to dipolarophilic enone, ring-opening of 1,3-indanedione, and selective annulation.

Diastereoselective synthesis of spirocyclic isoxazolo[5,4-c]pyrrolo[2,1-a]isoquinolines via cascade double [3 + 2]cycloadditions

The one-pot base-promoted cascade double [3 + 2] cycloaddition reaction of N-cyanomethylisoquinolinium chloride with (E)-3-arylideneindolin-2-ones and (E)-N-hydroxybenzimidoyl chloride afforded novel polycyclic spiro[indoline-3,8'-isoxazolo[5,4-c]pyrrolo[2,1-a]isoquinolines] in good yields and with high diastereoselectivity. When 4-arylidenepyrazol-3-ones, 2-arylidene-1,3-indanediones and arylidenemalononitriles were employed in the reaction, the corresponding spiro[isoxazolo[5,4-c]pyrrolo[2,1-a]isoquinoline-8,4'-pyrazole], spiro[indene-2,8'-isoxazolo[5,4-c]pyrrolo[2,1-a]isoquinoline], and isoxazolo[5,4-c]pyrrolo[2,1-a]isoquinoline derivatives were also prepared in good yields and with high diastereoselectivty. The relative configurations of the polycyclic spiro compounds were clearly elucidated by determination of fifteen single crystal structures.

Convenient construction of dibenzo[b,d]furanes and 2,6-diaryl-4-(2-hydroxyphenyl)pyridines via domino reaction of pyridinium ylides with 2-aryl-3-nitrochromenes

The DBU promoted domino annulation reaction of 2-aryl-3-nitrochromenes with two molecular 4-(N,N-dimethylamino)-1-phenacylpyridinium bromides in DMF resulted in unprecedented 1-benzoyl-2,4-diaryldibenzo[b,d]furane derivatives in good yields.

Diastereoselective synthesis of dispirooxindoles via [3+2] cycloaddition of azomethine ylides to 3-phenacylideneoxindoles and evaluation of their cytotoxicity

The three-component reaction of 1,2,3,4-tetrahydroisoquinoline, isatins and 3-phenacylideneoxindoles in refluxing ethanol afforded dispiro[indoline-3,1'-pyrrolo[2,1-a]isoquinoline-3',3'-indolines] (4a-4x) in good yields via 1,3-dipolar cycloaddition of in situ generated azomethine ylide with the exocyclic double bond of 3-phenacylideneoxindoles. 1H NMR spectra and single crystal structures indicated the reaction has high regioselectivity and diastereoselectivity. Furthermore, their biological activities have been preliminarily demonstrated by in vitro evaluation against mouse breast cancer cells 4T1 and human liver cancer cells HepG2 by MTT assay. The results demonstrated that some of the compounds showed cytotoxicities to cell lines of 4T1 and HepG2, and indicated that novel spirooxindoles may become potential lead compounds for further biological screenings of their medicinal applications.

One-pot three-component synthesis and oxidation of functionalized tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles

Triethylamine-promoted cycloaddition reactions of N-phenacyl and N-alkoxycarbonylmethylbenzothiazolium bromides with aromatic aldehydes and malononitrile (ethyl cyanoacetate, pivaloylacetonitrile) in ethanol afforded functionalized tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles in good yields and various diastereoselectivity. The oxidation reaction of the functionalized tetrahydrobenzo[d]pyrrolo[2,1-b]thiazoles with DDQ in different solvents resulted in diverse benzothiazole derivatives and benzo[d]pyrrolo[2,1-b]thiazoles. The reaction mechanism and the stereochemistry of this tandem [3 [Formula: see text] 2] cycloaddition reaction and sequential oxidation reaction are illustrated based on analysis of the reactive intermediates and obtained products.

Tandem Double [3 + 2] Cycloaddition Reactions at Both C-1 and C-3 Atoms of N-Cyanomethylisoquinolinium Ylide

The base-promoted cycloaddition reaction of N-cyanomethylisoquinolinium chloride with 2-arylidene-1,3-indanediones in dry tetrahydrofuran resulted in the expected spiro[indene-2,1'-pyrrolo[2,1-a]isoquinoline] derivatives. However, the triethylamine-promoted three-component reaction of N-cyanomethylisoquinolinium chloride, aromatic aldehydes, and two molecules of 1,3-indanediones in acetonitrile afforded unique spiro[benzo[f]imidazo[5,1,2-cd]indolizine-4,2'-indene] derivatives in satisfactory yields through tandem double [3 + 2] cycloaddition reactions.

Quantification and Theoretical Analysis of the Electrophilicities of Michael Acceptors

In order to quantify the electrophilic reactivities of common Michael acceptors, we measured the kinetics of the reactions of monoacceptor-substituted ethylenes (H₂C═CH-Acc, 1) and styrenes (PhCH═CH-Acc, 2) with pyridinium ylides 3, sulfonium ylide 4, and sulfonyl-substituted chloromethyl anion 5. Substitution of the 57 measured second-order rate constants (log k) and the previously reported nucleophile-specific parameters N and s_N for 3-5 into the correlation log k = s_N(E + N) allowed us to calculate 15 new empirical electrophilicity parameters E for Michael acceptors 1 and 2. The use of the same parameters s_N, N, and E for these different types of reactions shows that all reactions proceed via a common rate-determining step, the nucleophilic attack of 3-5 at the Michael acceptors with formation of acyclic intermediates, which subsequently cyclize to give tetrahydroindolizines (stepwise 1,3-dipolar cycloadditions with 3) and cyclopropanes (with 4 and 5), respectively. The electrophilicity parameters E thus determined can be used to calculate the rates of the reactions of Michael acceptors 1 and 2 with any nucleophile of known N and s_N. DFT calculations were performed to confirm the suggested reaction mechanisms and to elucidate the origin of the electrophilic reactivities. While electrophilicities E correlate poorly with the LUMO energies and with Parr's electrophilicity index ω, good correlations were found between the experimentally observed electrophilic reactivities of 44 Michael acceptors and their calculated methyl anion affinities, particularly when solvation by dimethyl sulfoxide was taken into account by applying the SMD continuum solvation model. Because of the large structural variety of Michael acceptors considered for these correlations, which cover a reactivity range of 17 orders of magnitude, we consider the calculation of methyl anion affinities to be the method of choice for a rapid estimate of electrophilic reactivities.

Formation of diverse polycyclic spirooxindoles via three-component reaction of isoquinolinium salts, isatins and malononitrile

The triethylamine promoted three-component reaction of N-(4-nitrobenzyl), N-ethoxycarbonylmethylisoquinolinium bromide, isatins and malononitrile in ethanol afforded spiro[indoline-3,2′-pyrrolo[2,1-a]isoquinolines] in good yields and with high diastereoselectivity. The similar reaction of N-cyanomethylisoquinolinium chloride mainly gave complex indolo[2″,3″:2′,3′]pyrrolo[3′,4′:4,5]pyrrolo[2,1-a]isoquinoline derivatives. However, the three-component reaction of N-cyanomethylisoquinolinium chloride, isatins and ethyl cyanoacetate mainly resulted in functionalized spiro[indoline-3,8′-pyrido[2′,3′:4,5]pyrrolo[2,1-a]isoquinolines].

Diastereoselective synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes

Diastereospecific synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes.

A quantitative approach to polar organic reactivity

A method is presented which allows one to predict toxic effects which are triggered by the formation of covalent bonds between electron-deficient (electrophilic) compounds and biological electron-rich (nucleophilic) targets, as proteins or nucleic acids. It is based on our comprehensive nucleophilicity and electrophilicity scales, which we constructed as an aid for the planning of organic syntheses. For the construction of these scales, rate constants for the reactions of benzhydrylium ions (aryl2CH(+)) and structurally related quinone methides with nucleophiles have been measured and correlated by the equation lg k(20 °C) = sN(E + N), which yields absolute rate constants k (L mol(-1) s(-1)) from one parameter for electrophiles (the electrophilicity E) and two for nucleophiles (the nucleophilicity parameter N and the susceptibility sN). A freely accessible database (http://www.cup.uni-muenchen.de/oc/mayr/DBintro.html) is described, which presently comprises data for 1000 nucleophiles and 260 electrophiles and provides links to the original literature reports. The kinetic scales are complemented by a thermodynamic counterpart, which enables one to calculate association constants K (L mol(-1)) of electrophiles with nucleophiles from the empirical Lewis acidity parameters LA and Lewis basicity parameters LB by the equation lg K (20°C) = LA + LB.