Effect of Ring Size and Migratory Groups on [1,n] Suprafacial Shift Reactions. Confirmation of Aromatic and Antiaromatic Transition-State Character by Ring-Current Analysis.

Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes, 3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studied computationally at the MP2/6-31+G* level for structures and energetics and with the ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The hydrogen shifts in cyclopentadienes have a diatropic ring current indicating aromatic, cyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-fluorocyclopentadiene which requires much more energy and has a paratropic ring current in the TS pointing to antiaromatic, cyclopentadienyl cation character.

Are copper(I) carbenes capable intermediates for cyclopropanations? The case for ylide intermediates.

A novel approach is used to synthesize a stable, ligated copper(I) carbene in the gas phase that is capable of typical metal carbenoid chemistry. However, it is shown that copper(I) carbenes generally undergo rapid unimolecular rearrangements including insertions into copper-ligand bonds and Wolff rearrangements. The results indicate that most copper(I) carbenes are inherently unstable and would not be viable intermediates in condensed-phase applications; an alternative intermediate that is less prone to rearrangements is required.

Reactivity in the nucleophilic aromatic substitution reactions of pyridinium ions.

The "element effect" in nucleophilic aromatic substitution reactions (SNAr) is characterized by the leaving group order, L = F > NO2 > Cl ≈ Br > I, in activated aryl substrates. A different leaving group order is observed in the substitution reactions of ring-substituted N-methylpyridinium compounds with piperidine in methanol: 2-CN ≥ 4-CN > 2-F ∼ 2-Cl ∼ 2-Br ∼ 2-I. The reactions are second-order in [piperidine], the mechanism involving rate determining hydrogen-bond formation between piperidine and the substrate-piperidine addition intermediate followed by deprotonation of this intermediate.

Effect of allylic groups on S(N)2 reactivity.

The activating effects of the benzyl and allyl groups on S(N)2 reactivity are well-known. 6-Chloromethyl-6-methylfulvene, also a primary, allylic halide, reacts 30 times faster with KI/acetone than does benzyl chloride at room temperature. The latter result, as well as new experimental observations, suggests that the fulvenyl group is a particularly activating allylic group in S(N)2 reactions.

Stabilities of Uracil and Pyridone-Based Carbanions: A Systematic Study in the Gas Phase and Solution and Implications for the Mechanism of Orotidine-5'-Monophosphate Decarboxylase.

The stabilities of the C6-centered carbanions derived from 1,3-dimethyluracil, N-methyl-2-pyridone, and N-methyl-4-pyridone were systematically investigated in the gas phase and in DMSO and water solutions. The stabilities of the carbanions in the gas phase and DMSO were directly measured through their reactions with carbon acids with known proton affinity or p values. The stabilities of the carbanions in DMSO were also probed through their kinetic isotope effects of protonation over deuteriation using acids with different acidity.

The element effect revisited: factors determining leaving group ability in activated nucleophilic aromatic substitution reactions.

The "element effect" in nucleophilic aromatic substitution reactions (S(N)Ar) is characterized by the leaving group order, F > NO(2) > Cl ≈ Br > I, in activated aryl halides. Multiple causes for this result have been proposed. Experimental evidence shows that the element effect order in the reaction of piperidine with 2,4-dinitrophenyl halides in methanol is governed by the differences in enthalpies of activation.

Base-catalyzed dehydration of 3-substituted benzene cis-1,2-dihydrodiols: stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation.

Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol.

Hyperaromatic stabilization of arenium ions: cyclohexa- and cycloheptadienyl cations-experimental and calculated stabilities and ring currents.

Measurements of pK(R) show that the cycloheptadienyl cation is less stable than the cyclohexadienyl (benzenium) cation by 18 kcal mol(-1). This difference is ascribed here to "hyperaromaticity" of the latter. For the cycloheptadienyl cation a value of K(R) = [ROH][H(+)]/[R(+)] is assigned by combining a rate constant for reaction of the cation with water based on the azide clock with a rate constant for the acid-catalyzed formation of the cation accompanying equilibration of cycloheptadienol with its trifluoroethyl ether in TFE-water mixtures.

Stabilities of carbenes: independent measures for singlets and triplets.

Thermodynamic stabilities of 92 carbenes, singlets and triplets, have been evaluated on the basis of hydrogenation enthalpies calculated at the G3MP2 level. The carbenes include alkyl-, aryl-, and heteroatom-substituted structures as well as cyclic 1,3-diheteroatom carbenes. Over a wide energy range, a good correlation is seen between the singlet-triplet gaps and the hydrogenation enthalpies of the singlets, but there are some clear outliers, which represent cases where the triplet has unusual stability or instability.

Hyperaromatic stabilization of arenium ions.

Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ↔ HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.

The protonation of allene and some heteroallenes, a computational study.

Protonation of allene and seven heteroallenes, X = Y = Z, at the terminal and central positions has been studied computationally at the MP2/6-311+G**, B3LYP/6-31+G**, and G3 levels. In all but one case protonation at a terminal position is preferred thermodynamically. The exception is allene, where protonation at C2 giving allyl cation prevails by about 10 kcal/mol over end-protonation, which gives the 2-propenyl cation.

Primary semiclassical kinetic hydrogen isotope effects in identity carbon-to-carbon proton- and hydride-transfer reactions, an ab initio and DFT computational study.

Enthalpies of activation, transition state (ts) geometries, and primary semiclassical (without tunneling) kinetic isotope effects (KIEs) have been calculated for eleven bimolecular identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, eleven identity hydride transfers, and two 1,2-intramolecular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311++G levels. We find the KIEs to be systematically smaller for hydride transfers than for proton transfers. This outcome is not the result of "bent" transition states, although extreme bending can lower the KIE.

Dye-sensitized photooxygenation of the C=N bond. 5. substituent effects on the cleavage of the C=N bond of C-aryl-N-aryl-N-methylhydrazones.

The title compounds are cleaved cleanly at the C=N bond by singlet oxygen ((1)O(2), (1)Delta(g)) yielding arylaldehydes and N-aryl-N-methylnitrosamines. These reactions take place more rapidly at -78 degrees C than at room temperature. The effects of substituent variation at both the C-aryl and N-aryl groups were studied using a competitive method.

Identity hydride-ion transfer from C-H donors to C acceptor sites. Enthalpies of hydride addition and enthalpies of activation. Comparison with C...H...C proton transfer. An ab initio study.

Enthalpies of addition of hydride ion to eleven carbonyl acceptors (X-CHO), two conjugate addition sites (X-CH=CH2; X = CHO, NO2), eight carbenium ion acceptors, fulvene, borane, and SiH3(+) were calculated at the MP2/6-311+G level. Correlation between calculated and experimental enthalpies of addition of hydride ion is excellent. Transition states (ts) for the identity hydride transfers between the acceptors and their corresponding hydride adducts (hydride donors) were also calculated.

Identity proton-transfer reactions from C-H, N-H, and O-H acids. An ab initio, DFT, and CPCM-B3LYP aqueous solvent model study.

Identity proton-transfer reactions between 21 acids, Y-X-H, and their conjugate bases, (-)X-Y, were studied according to the reaction scheme, Y-X-H + (-)X-Y --> (Y-X-H...