Propargyl Cations Research Articles

Rotational excitation and de-excitation of the interstellar propargyl (H2CCCH+) cation by collisions with helium atoms

Rotational excitation and de-excitation of the interstellar propargyl (H2CCCH+) cation by collisions with helium atoms

Synthesis of vinylidenecyclopropanes via gold(I)-catalyzed cyclopropanation of vinyl arenes with γ-stannylated propargyl esters.

The reaction of γ-stannylated propargyl esters in the presence of a cationic gold(I) catalyst affords vinylidenecyclopropane derivatives as a mixture of diastereomers. The cis-geometry of the alkenes is almost entirely retained in the product. DFT calculations suggest the involvement of the gold(I)-stabilized propargyl cation as a resonance form of gold(I)-coordinated allenylidene species.

Cationic indium catalysis as a powerful tool for generating α-alkyl propargyl cations for SN1 reactions

Dehydration is an abundant and promising process in chemical, biochemical, and industrial fields. Dehydration methods can contribute to building a modern and sustainable society with minimal environmental impact. Breakthrough advances in the dehydrative SN1 reaction can be achieved through the discovery of new cationic indium catalysts. Here we show that the breakthrough advances in the dehydrative SN1 reaction can be achieved using the cationic indium catalysts. The dehydrative carbon–carbon bond formation of α-alkyl propargyl alcohols afforded a wide variety of α-aryl- and heteroaryl-propargyl compounds. Mechanistic investigations into this process revealed that the InCl3/AgClO4/Bu4NPF6/1,1’-binaphthol catalytic system generated a powerful cationic indium catalyst that could promote the dehydration of alcohols. Labile α-alkyl propargyl cations were found to self-condense, and the catalyst system efficiently regenerated propargyl cations for reaction with nucleophiles. This propargylation reaction directly proceeded from the corresponding alcohols under mild and open-air conditions and tolerated a broad scope of functional groups. Furthermore, a wide variety of nucleophiles, including aromatic and heteroaromatic compounds, phenols, alcohols, and sulfonamides, reacted with the corresponding cations to afford the propargyl compounds in good to high yields. Finally, the synthetic utility of this reaction was demonstrated by the synthesis of colchicine and allocolchicine analogues. The dehydration process could help create new compounds that were previously impossible to synthesize and is more eco-friendly and efficient than conventional methods.

One-pot silyl imidate formation-N-alkylation reactions of amides with propargyl propionates

The combination of trimethylsilyl trifluoromethanesulfonate (TMSOTf) and i-Pr2NEt promotes the N-alkylation of primary and secondary amides with propargyl propionates. The amide substrates are believed to undergo conversion to nucleophilic silyl imidates in the presence of TMSOTf and the amine base, conditions which also promote propargyl cation formation in the same flask through the loss of trimethylsilyl propionate, leading to N-propargylation of the amide. A range of amides and propargyl propionates react efficiently in this reliable one-pot process with yields ranging from 19 to 99%.

Reinterpreting the vibrational structure in the electronic spectrum of the propargyl cation (H2C3H+) using an efficient and accurate quantum model.

The B̃1A1 ← X̃1A1 absorption spectra of propargyl cations H2C3H+ and D2C3D+ were simulated by an efficient two-dimensional (2D) quantum model, which includes the C-C stretch (v5) and the C≡C stretch (v3) vibrational modes. The choice of two modes was based on a scheme that can identify the active modes quantitively by examining the normal coordinate displacements (∆Q) directly based on the ab initio equilibrium geometries and frequencies of the X̃1A1 and B̃1A1 states of H2C3H+. The spectrum calculated by the 2D model was found to be very close to those calculated by all the higher three-dimensional (3D) quantum models (including v5, v3, and another one in 12 modes of H2C3H+), which validates the 2D model. The calculated B̃1A1 ← X̃1A1 absorption spectra of both H2C3H+ and D2C3D+ are in fairly good agreement with experimental results.

Transition‐Metal‐Catalyzed Propargylic Substitution of Propargylic Alcohol Derivatives Bearing an Internal Alkyne Group

AbstractThe development of the transition‐metal‐catalyzed propargylic substitution of propargylic alcohol derivatives is one of the active research subjects in organic synthesis. In this minireview, we summarize the catalytic propargylic substitution of propargylic alcohol derivatives bearing an internal alkyne group using a transition metal as the catalyst. Although the asymmetric propargylic substitution of the substrate class still underdeveloped, some transition metal catalyses using nickel, palladium, and copper, and the cooperative catalysis enabled the challenging transformation. A various types of transition‐metal catalysts are known to promote the non‐asymmetric propargylic substitution of the internal alkyne‐substituted propargylic alcohol derivatives via a propargylic cation intermediate, realizing the carbon‐carbon and carbon‐heteroatom bond formation at the propargylic position.

Electronic Spectrum and Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH.

The B̃1A' ← X̃1A' electronic spectra of the 1-butyn-3-yl cation (H3CCHCCH+) and the H3CCHCCH+-Ne and H3CCHCCH+-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H3C4H2+) and propargyl (H2C3H+) cations. The photofragment ions are C2H3+ (major) and C4H3+ (minor), with the preference for C2H3+ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

Synthesis of Thiazinoimidazoles by Lewis Acid–Catalyzed [3+3] Cycloaddition Reactions of Propargyl Alcohols with 2‐Mercaptoimidazoles

The unique ytterbium‐catalyzed generation of sulfur‐ and selenium‐substituted propargylic and allenic cations and their reactions with 2‐mercaptoimidazole derivatives were described. The regioselective [3+3] annulation reactions proceeded to give a wide variety of S,N‐acetal‐containing bicyclic and tricyclic thiazinoimidazoles in good to high yields. Treatment of thiazinoimidazoles with LDA readily underwent ring contraction to afford thiazolobenzimidazoles. Deselenenylation and successive functionalization broaden the scope of accessible thiazinoimidazoles.

Spectroscopic and Structural Study of Some Oligosilanylalkyne Complexes of Cobalt, Molybdenum and Nickel

Metal induced stabilization of α-carbocations is well known for cobalt- and molybdenum complexed propargyl cations. The same principle also allows access to reactivity enhancement of metal coordinated halo- and hydrosilylalkynes. In a previous study, we have shown that coordination of oligosilanylalkynes to the dicobalthexacarbonyl fragment induces striking reactivity to the oligosilanyl part. The current paper extends this set of oligosilanylalkyne complexes to a number of new dicobalthexacarbonyl complexes but also to 1,2-bis(cyclopentadienyl)tetracarbonyldimolybdenum and (dippe)Ni complexes. NMR-Spectroscopic and crystallographic analysis of the obtained complexes clearly show that the dimetallic cobalt and molybdenum complexes cause rehybridization of the alkyne carbon atoms to sp3, while in the nickel complexes one π-bond of the alkyne is retained. For the dicobalt and dimolybdenum complexes, strongly deshielded 29Si NMR resonances of the attached silicon atoms indicate enhanced reactivity, whereas the 29Si NMR shifts of the respective nickel complexes are similar to that of respective vinylsilanes.

Acidic Zeolite-Catalyzed Reaction of 2,4-Diaryl-1,1,1-trifluorobut-3-yn-2-ols with Benzene. A New Synthesis of 1,3-Diaryl-1-trifluoromethylindenes

Reactions of 2,4-diaryl-1,1,1-trifluorobut-3-yn-2-ols with benzene in the presence of HUSY acidic zeolite (CBV-720) at 100°C in 1 h (glass high-pressure reactor) led to the formation of 1,3-diaryl-1-trifluoromethylindenes in 39–92% yield. A probable reaction mechanism was proposed, which involves formation of the corresponding trifluoromethyl-substituted propargyl cations.

Direct IR Absorption Spectra of Propargyl Cation Isolated in Solid Argon

The direct infrared (IR) absorption spectra of propargyl cations were recorded. These cations were generated via the electron bombardment of a propyne/Ar matrix sample during matrix deposition. Secondary photolysis with selected ultraviolet (UV) light was used for grouping the observed bands of various products. The band assignment of the propargyl cation in solid Ar was performed according by referring to the previous infrared photodissociation (IRPD) and velocity-map imaging photoelectron (VMI-PE) data, and via theoretical predictions of the anharmonic vibrational wavenumbers, band intensities, and deuterium-substituted isotopic ratios. Almost all the IR active bands with an observable intensity were recorded and the ν11 mode was reported for the first time.

One-Pot Enol Silane Formation-Alkylation of Ketones with Propargyl Carboxylates Promoted by Trimethylsilyl Trifluoromethanesulfonate.

Ketones readily undergo conversion to enol silanes in the presence of a trialkylamine base and trimethylsilyl trifluoromethanesulfonate (TMSOTf) and add to propargyl cations to yield β-alkynyl ketones. The propargyl cations are generated in the same reaction flask through the TMSOTf-promoted ionization of propargyl acetates or propargyl propionates. A range of enol silane precursors and propargyl carboxylates reacts efficiently (20 examples, up to 99% yield). Cyclization of a representative product in the presence of TMSOTf provided 61% yield of the trisubstituted furan.

Synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products.

The present work deals with the synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products like physalins, with potential as antitumor agents. This synthetic methodology involves several key reactions: (a) synthesis of polyfunctionalized cycloheptenones by [4 + 3] cycloaddition reactions of furan precursors with oxyallyl cations; (b) Nicholas reaction with propargyl cations stabilized as dicobalt hexacarbonyl complexes; (c) demetallation and hydration of the resulting acetylenes; (d) stereoconvergent aldol cyclization to generate a key oxatricyclic intermediate and (e) a β-fragmentation process that affords, under hypoiodite photolysis, the desired product with moderate to good yield. The final compounds are the result of a radicalary β-fragmentation at the level of C2-C6 with respect to the tertiary hydroxyl group on C6, with an unexpected contraction from a ten- to a nine-membered ring system, via a radical addition to the carbonyl group on C4. The synthetic methodology has been scaled up to multigram level with good overall yield. Further biological, biochemical and biophysical studies are being carried out in our laboratory on these 1,7-epoxycyclononane derivatives to determine the potential of this kind of oxabicyclic compound as future hits and/or leads for the development of new anticancer drugs. The preliminary evaluation of the anticancer activity of the representative synthesized compounds, against the leukaemia cancer cell lines K-562 and SR, shows a promising activity with a GI50 = 0.01 μM and a LC50 = 7.4 μM for a conveniently functionalized 10-oxabicyclo[5.2.1]decane.

Organocatalytic synthesis of chiral tetrasubstituted allenes from racemic propargylic alcohols

Although chiral allene preparation via formal SN2’ nucleophilic substitutions of enantioenriched propargylic derivatives or metal-catalyzed reactions of racemic propargylic derivatives has attracted considerable attention and found applications in many areas of research, direct use of propargylic alcohols instead of propargylic derivatives for catalytic asymmetric allene synthesis is unknown. Here, we show that a highly enantioselective synthesis of tetrasubstituted allenes from racemic propargylic alcohols has been realized by organocatalysis with good efficiency (up to 96% yield and 97% ee). The intermolecular C–C and C–S bond formation was achieved efficiently with simultaneous stereocontrol over the axial chirality. Furthermore, an adjacent quaternary stereocenter could also be constructed. Mechanistically, the reaction may involve efficient stereocontrol on the propargylic cation by its chiral counter anion or 1,8-conjugate addition of para-quinone methides. In sharp contrast to previous central chirality construction, this process employs quinone methides for axial chirality construction.

Co 2 (CO) 6 -propargyl cation mediates glycosylation reaction by using thioglycoside

We discovered that the cobalt-propargyl cation can mediate the glycosylation reaction by activating the thioglycoside donor. The glyco-oxacarbenium cation was formed by transferring the thio-aglycone to the cobalt-propargyl cation that was generated from the cobalt-propargylated acceptor in situ via the activating with Lewis acid. The reactivity of the donor (Armed or dis-armed) and the amount of the Lewis acid control the releasing rate of the cobalt-propargyl group.

Electronic spectrum and photodissociation chemistry of the linear methyl propargyl cation H2C4H3.

The electronic spectrum of the methyl propargyl cation (2-butyn-1-yl cation, H2C4H3+) is measured over the 230-270 nm range by photodissociating the bare cation and its Ar and N2 tagged complexes in a tandem mass spectrometer. The observed A'1←A'1 band system has an origin at 37 753 cm-1 for H2C4H3+, 37738 cm-1 for H2C4H3+-Ar, and 37 658 cm-1 for H2C4H3+-N2. The methyl propargyl cation photodissociates to produce either C2H3++C2H2 (protonated acetylene + acetylene) or H2C4H++H2 (protonated diacetylene + dihydrogen). Photodissociation spectra of H2C4H3+, H2C4H3+-Ar, and H2C4H3+-N2 exhibit similar vibronic structure, with a strong progression of spacing 630 cm-1 corresponding to excitation of the C-C stretch mode. Interpretation of the spectra is aided by ground and excited state calculations using time dependent density functional theory at the ωB97X-D/aug-cc-pVDZ level of theory. Ab initio calculations and master equation simulations were used to interpret the dissociation of H2C4H3+ on the ground state manifold. These calculations support the experimentally observed product branching ratios in which acetylene elimination dominates and also suggests that channel switching occurs at higher energies to favor H2 elimination.

Gold‐Catalyzed 1,4‐Carbooxygenation of 3‐En‐1‐ynamides with Allylic and Propargylic Alcohols via Non‐Claisen Pathways

AbstractGold‐catalyzed 1,4‐carbooxygenations of 3‐en‐1‐ynamides with allylic alcohols and propargylic alcohols yield α,β‐unsaturated amides through non‐Claisen pathways; the mechanisms involve ionization of the initial gold enol ethers to form C‐bound gold dienolates that capture allylic or propargylic cations to yield the observed products. Our 18O‐labeling experiments exclude a direct gold‐catalyzed allylation or propargylation on these 3‐en‐1‐ynamides.magnified image

Acquiring a Prognostic Power in Co2(CO)6-Mediated, Cobaltocene-Induced Radical Dimerizations of Propargyl Triflates

Cobalt-complexed propargyl triflates can be generated in situ from methyl propargyl ethers and triflic anhydride and then reduced with cobaltocene to topologically and functionally diverse 1,5-alkadiynes. The electronic effect of an α-substituent is shown to attenuate the ionic nature of an α-C–OTf bond and thus its reducibility with cobaltocene. The powerful π-donors, such as phenyl, naphthyl, alkenyl, alkynyl, and alkoxy groups, provide the ionicity of α-C–OTf bonds and make them suitable recipients for a single-electron delivery from cobaltocene. σ-Donors (alkyl groups), a H atom, and σ/π-acceptors (ester groups) do not sufficiently stabilize propargyl cations, maintaining the covalent nature of α-C–OTf bonds and making them resistant toward the reducing agent. A newly acquired ability to differentiate between the α-C–OTf bonds is used in polyethers for the regioselective reduction and radical dimerization in select propargylic positions, thus paving the way for a long sought after radical-ionic α,α′-f...

Acquiring and Exploiting Persistency of Propargyl Radicals: Novel Paradigms

Three complementary methods for altering an intrinsic nature of propargyl radicals, from transient to persistent, were developed by fine-tuning the bulkiness and degree of substitution around the secondary and tertiary propargylic carbons, as well as by sterically enhancing a π-bonded Co2(CO)6 metal core. The latter was employed, as a mechanistic tool, for precluding an acetylene–allene rearrangement, stabilizing propargyl cations, and creating a steric hindrance that could provide for said transition from transient to persistent propargyl radicals. A window of opportunities was identified wherein the steric bulkiness in propargyl radicals remains below the “persistency threshold”, providing good to excellent stereoselectivities in radical dimerization reactions (d,l 62–100%). Along with the persistency threshold for tertiary propargyl radicals (278.2 A3), two different thresholds for persistency in secondary propargyl radicals were established—306.5 and 576.0 A3—dependent upon the molecular architecture ...

ChemInform Abstract: Complementary Iron(II)‐Catalyzed Oxidative Transformations of Allenes with Different Oxidants.

AbstractReactions of allenes (I) with DDQ and tBuOOH give rise to indenes (III) and indenones (V), whilst silylated allene (VI) reacts differently to afford 1,3‐enyne (VII) and propargylic peroxyether (VIII).