Cationic Carbon Atom Research Articles

Intramolecular 1,2-Aroyl Migration in Spiro Donor-Acceptor Cyclopropanes: Formation of 1,4-Naphthoquinones and 1-Naphthols as Ring-Expansion Products.

Most of the known rearrangement reactions of donor-acceptor cyclopropanes (DACs) involve the migration of cationic carbon atom to anionic carbon or heteroatoms in 1,3- or 1,4-positions. In the present work, we observed that spiro DACs based on 1,3-indanedione or 1-indanone moiety undergo intramolecular 1,2-aroyl migration when treated with titanium(IV) chloride to afford 1,4-naphthoquinones and α-naphthols readily. The reactions take place through the formation of putative 1,3-dipolar intermediates, followed by cleavage and migration of the aroyl group to the adjacent carbon to afford the ring-expansion products.

Conjugative Stabilization versus Anchimeric Assistance in Carbocations.

In this study, an old concept of anchimeric assistance is viewed from a different angle. Primary cations with two different heteroatomic substituents in the α-position to the cationic carbon atom CHXY−CH2+ (X, Y = Me2N, MeO, Me3Si, Me2P, MeS, MeS, Br) can be stabilized by the migration of either the X or Y group to the cation center. In each case, the migration can be either complete, resulting in the transfer of the migrating group to the adjacent carbon atom and the formation of a secondary carbocation stabilized by the remaining heteroatom, or incomplete, leading to an anchimerically assisted iranium ion. For all combinations of the above groups, these transformations have been studied by theoretical analysis at the MP2/aug-cc-pVTZ level and were shown to occur depending on the ability of anchimeric assistance by X and Y, as well as the conformation of the starting primary carbocation. In the conformers of α-amino cations with the p-orbital, C−N bond and the nitrogen lone pair in one plane, the Me2N group migrates to the cationic center to give aziranium ions. Otherwise, the second heteroatom is shifted to give iminium ions, without or with very slight anchimeric assistance. In the α-methoxy cations, the MeO group can be shifted to the cationic center to give the O-anchimerically assisted ions as local minima, the global minima being the ions anchimerically assisted by another heteroatom. The electropositive silicon tends to migrate towards the cationic center, but with the formation of a π-complex of the Me3Si cation with the C=C bond rather than a Si-anchimerically assisted cation. The phosphorus atom can either fully migrate to the cationic center (X = P, Y = S, Se) or form anchimerically stabilized phosphiranium ions (X = P, Y = O, Si, Br). The order of the anchimeric assistance for the heaviest atoms decreases in the order Se >> S > Br.

Pd0‐Mediated Cross‐Coupling of [11C]Methyl Iodide with Carboxysilane for Synthesis of [11C]Acetic Acid and its Active Esters: 11C‐Acetylation of Small, Medium, and Large Molecules

AbstractA rapid Pd0‐mediated cross‐coupling of [11C]CH3I with carboxytriphenylsilane (1) to synthesize [2‐11C]acetic acid ([11C]2) and its corresponding imidyl esters as radiolabeling reagents for 11C‐acetylation is reported. The reaction of [11C]CH3I with 1 using a Pd2(dibenzylideneacetone)3/P(o‐tolyl)3 (1 : 4) catalyst in tetrahydrofuran/H2O (9 : 1) at 90 °C for 4 min afforded [11C]2 having satisfactory radioactivity and a good yield. The coupling of the cationic carbon atom of methyl iodide and the carboxyl group in 1 is an ionicity‐driven reversed‐polarity reaction. An injectable radiopharmaceutical formulation of [11C]2 prepared using 33 GBq of [11C]CH3I had a radioactivity of 7.4–7.8 GBq, and the decay‐corrected radiochemical yield of [11C]CH3I into [11C]2 was 63–73 %. [11C]2 was converted to [2‐11C]acetic acid phthalimidyl ester ([11C]3) or its succinimidyl ester ([11C]10), which were used for the 11C‐acetylation of 4‐aminophenol, oxytocin, and albumin. Injectable solutions of these products (0.25–2.0 GBq) are suitable for in vivo positron emission tomography studies.

Influence of temperature and alkyl chain length on physicochemical properties of trihexyl- and trioctylammonium based protic ionic liquids

A series of trialkylammonium cation and carboxylate anion based room temperature protic ionic liquids (PILs) were synthesized by varying cation alkyl chain length to hexyl, octyl and anion chain length to butanoate, hexanoate, and octanoate. The corresponding ILs are named as trihexylammonium butanoate ([THA][But]), trihexylammonium hexanoate ([THA][Hex]), trihexylammonium octonoate ([THA][Oct]), trioctylammonium butanoate ([TOA][But]), trioctylammonium hexanoate ([TOA][Hex]), and trioctylammonium octonoate ([TOA][Oct]). The thermophysical properties such as density (ρ), speed of sound (u), viscosity (η) and refractive index (nD) were measured over a wide temperature range. The calculated parameters from experimental data such as thermal expansion coefficient, isentropic compressibility decreased linearly with the number of carbon atoms in cation or anion, whereas they increased with the temperature. Thermogravimetric analysis revealed an increase in the thermal stability of the ILs with the increase in total chain length. The shear-dependent viscosity measurements suggest that all the studied PILs showed fluid nature and Newtonian behavior at 298.15 K. The effect of alkyl chain length of cation or anion and temperature on the physicochemical properties were discussed with respect to the structure of ILs and in correlation with the reported structurally similar ILs.

Ternary Liquid–Liquid Equilibrium of Azeotropes (Ester + Alcohol) with Different Ionic Liquids at T = 298.15 K

The liquid–liquid equilibria (LLE) for ternary systems of n-propyl acetate (PAC) + 1-propanol (NPA) + 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) or 1-hexyl-3-methylimidazolium hexafluorophosphate ([Hmim][PF6]) and n-butyl acetate (NBAC) + n-butanol (NBA) + [Bmim][PF6] or [Hmim][PF6] were measured at 298.15 K and 101.325 kPa. All the studied systems containing ionic liquids (ILs) show Treybal type I behavior of the LLE. The solute distribution coefficient (β) and the selectivity (S) were used to investigate the effect of the number of carbon atoms of cations, esters, and alcohols of ionic liquids on the liquid–liquid phase behavior for ester-alcohol-ILs. The measured data were correlated using both the NRTL model and the UNIQUAC model. Results indicate that all these systems can be correlated with reasonable accuracy by using both the NRTL model and the UNIQUAC model.

Lewis Acid Chemistry of a Cationic [CCH] Subunit in a Bisdisphenoidal Eight-Atom Tetrairon–Tetracarbon Cluster

Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)) with AgPF6 in acetonitrile gave [(η5-C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2). The X-ray diffraction study revealed that the cationic [CCH] subunit is stabilized through coordination of the acetonitrile molecule to the cationic carbon atom. As a synthon for a donor-free [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+, [(η5-C5H4Me)4Fe4(HCCH)(HCCL)](PF6)2 ([6](PF6)2, L = pyrazine) was synthesized by the reaction of [2](PF6) with AgPF6 in the presence of pyrazine. Treatment of [6](PF6)2 with tertiary amines in acetonitrile led to deprotonation of acetonitrile to form [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)](PF6) ([11](PF6)). Treatment of [6](PF6)2 with maleimide in the presence of tertiary amine in acetonitrile allowed the functionalization of the coordinated acetonitrile through the nucleophilic attack of maleimide at the nitrile carbon atom.

C−H Bond Activation by Hyperconjugation with Al−C Bonds and by Chelating Coordination of the Hydride Ion

On treating di(tert-butyl)butadiyne with dimethylaluminum hydride under different reaction conditions two unprecedented organoelement compounds, containing cationic carbon atoms stable in solution at room temperature, were obtained. A vinyl cation (2) in which the cationic carbon atom is part of a C=C double bond was produced from 3 equiv of the hydride, whereas a large excess of the hydride yielded an aliphatic carbocation (3) by complete hydroalumination of all C-C multiple bonds. Each compound is zwitterionic with the hydride counterion effectively coordinated in a chelating manner by two strongly Lewis acidic aluminum atoms. In agreement with quantum-chemical calculations the C-H bond activation and the stabilization of the cationic species are further supported by a strong hyperconjugation with Al-C single bonds. This considerably diminishes the effective positive charge at the respective cationic carbon atoms.

Facile Formation of a Persistent Butadien‐2‐yl Cation Stabilized by Hyperconjugation with an AlC Bond

An aluminating discovery: Hydroalumination of 1,4-di(tert-butyl)butadiyne with di(tert-butyl)aluminum hydride affords a persistent butadien-2-yl cation (red in scheme). The stabilization of this compound is achieved in particular by hyperconjugation with an AlC bond, and the effective chelating coordination by the aluminum atoms prevents the migration of the hydride ion to the electrophilic cationic carbon atom.

Oxidation of Alkyl Ions, CnH2n+1+ (n = 1−5), in Reactions with O2 and O3 in the Gas Phase

Rate constants and product ion branching fractions are reported for the reactions of CH3+, C2H5+, s-C3H7+, s-C4H9+, t-C4H9+, and t-C5H11+ with O2 and O3 at 300 K in a variable-temperature selected-ion flow tube (VT-SIFT). The reaction rate constant for CH3+ with O3 is large and approximately equal to the thermal energy capture rate constant given by the Su−Chesnavich equation. The C2H5+, s-C3H7+, and s-C4H9+ ions are somewhat less reactive, reacting at approximately 7−46% of the thermal capture rate. The HCO+ and C2H3O+ ions are the major products in these reactions. The t-C4H9+ and t-C5H11+ ions are found to be unreactive, with rate constants <5 × 10-12 cm3 s-1, which is the present detection limit of our apparatus using this ozone source. Ozone is a singlet in its ground state, and ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that reactant complexes can be formed, decreasing in stability with the size of alkyl chains attached to the cationic carbon atom. The decreasing reactivit...

Vinyl ether hydrolysis XXVIII. The mechanism of reaction of methyl α-(2,6-dimethoxyphenyl)vinyl ether

The acid-catalyzed hydrolysis of methyl α-(2,6-dimethoxyphenyl)vinyl ether in aqueous solution at 25 °C occurs with the hydronium ion catalytic coefficient [Formula: see text] and gives the solvent isotope effect [Formula: see text] this indicates that reaction occurs by rate-determining proton transfer from the catalyst to the substrate to generate an alkoxycarbocation intermediate. An oxygen-18 tracer study shows further that, despite the steric hindrance provided by its two ortho substituents, this cation then reacts by addition of water to the cationic carbon atom to generate a hemiacetal, and not by nucleophilic attack of water on the methyl group remote from the carbocationic center:[Formula: see text]

INDO‐FPT calculations of the dihedral angle dependence of 1J(CH) in groups adjacent to a cationic carbon atom

AbstractINDO‐FPT calculations of 1J(CH) in the hypothetical ethyl cation are presented in support of a recently derived empirical relationship which describes the effect of a cationic carbon on 1J(CH) in an adjacent CHn group.