James R. Keeffe

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. By use of hydrogenation enthalpies, separate carbene stabilization enthalpy scales (CSEs) have been developed for singlets and triplets, and these highlight structural features that affect the stability of each. The treatment also allows estimates of aromaticity in cyclic carbenes. In this way, imidazol-2-ylidene is estimated to have an aromatic stabilization energy of about 20 kcal/mol.

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 -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation.

Correlations between Carbene and Carbenium Stability: Ab Initio Calculations on Substituted Phenylcarbenes, Nonbenzenoid Arylcarbenes, Heteroatom-Substituted Carbenes, and the Corresponding Carbocations and Hydrogenation Products

Calculations were completed at the G3MP2 level on a large group of carbon- and heteroatom-substituted carbenes (X-CH, singlets and triplets), carbenium ions (X-CH2(+)), and their hydrogen addition products (X-CH3). One series includes 11 meta- and 12 para-substituted phenylcarbenes, X = Ar. Gas-phase enthalpies of reaction were calculated for four processes: singlet-triplet enthalpy gaps of the carbenes, DeltaH(ST); enthalpies for deprotonation of the cations yielding singlet carbenes, DeltaH(ACID); hydride ion affinities of the carbenium ions, HIA; and enthalpies of hydrogenation of the singlet carbenes, DeltaH(HYDROG). A plot of HIA vs DeltaH(HYDROG) values provides a direct comparison of substituent effects on the stabilities of the singlet carbenes and the corresponding benzylic cations. These effects are larger for the cations but are remarkably consistent over a wide range of reactivity: 166 kcal/mol in HIA. All four processes were analyzed according to the relative importance of polarizability, polar, and resonance effects. Polar and resonance effects are large and of similar magnitude for meta compounds. For the para compounds resonance effects are more dominant. Calculations were made on three nonbenzenoid arylcarbenes: Ar = cycloheptatrienyl(+), cyclopentadienyl(-), and cyclopropenyl(+). The cyclopentadienyl(-)-substituted system fits the HIA vs DeltaH(HYDROG) correlation, but the other two fall well off the line, suggesting markedly different interactions are at play. A set of heteroatom-substituted carbenes and carbocations was also examined. Points for these groups lie well above the correlation line for the HIA vs DeltaH(HYDROG) plot defined by the aryl compounds, confirming that heteroatoms stabilize the singlet carbene proportionally more than the carbocation.

The strength of a low-barrier hydrogen bond in water

Abstract There are large differences between the acidity of the enol of the acyclic diketone, 2,4-pentanedione and those of two cyclic diketones, 1,3-cyclopentanedione and 1,3-cyclohexanedione. Computational studies have demonstrated that these differences are largely due to the strength of the internal low-barrier hydrogen bond (LBHB) in the enol of 2,4-pentanedione. It is thus estimated that the lower limit of the additional free energy of formation in water for this LBHB over that of a conventional hydrogen bond is 4.1–5.3 kcal mol −1 .

Intermediates in the dye-sensitized photooxygenation of nitrones and hydrazones

Abstract In the dye-sensitized photooxygenation of C-aryl nitrones and hydrazones, short-lived peroxidic intermediates are observed. Their characterization, mechanisms leading to them, as well their decomposition pathways are discussed.

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

The activating effects of the benzyl and allyl groups on SN2 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 SN2 reactions. Computational work on identity SN2 reactions, e.g., chloride– displacing chloride– and ammonia displacing ammonia, shows that negatively charged SN2 transition states (tss) are activated by allylic groups according to the Galabov–Allen–Wu electrostatic model but with the fulvenyl group especially effective at helping to delocalize negative charge due to some cyclopentadienide character in the transition state (ts). In contrast, the triafulvenyl group is deactivating. However, the positively charged SN2 transition states of the ammonia reactions are dramatically stabilized by the triafulvenyl group, which directly conjugates with a reaction center having SN1 character in the ts. Experiments and calculations on the acidities of a variety of allylic alcohols and carboxylic acids support the special nature of the fulvenyl group in stabilizing nearby negative charge and highlight the ability of fulvene species to dramatically alter the energetics of processes even in the absence of direct conjugation.

Characterization of the indan-1-one keto–enol system

3-Hydroxyindene was generated by Norrish type II photoelimination of 2-Methoxyindan-1-one, and its subsequent rate of ketonization was measured in dilute aqueous perhloric acid, sodium hydroxide, acetic acid buffer and ammonia beffer solutions. These data, when combined with acid-catalysed rates of enolization of indan-1-one determined by both bromine and iodine scavenging, give pKE= 7.48 for the keto–enol equilibrium constant, pKaE= 9.48 for the acidity constant of the enol ionizing as an oxygen acid, and pKaK= 16.96 for the acidity constant of the ketone ionzing as a carbon acid. These results are compared with corresponding values for the acetophenone keto–enol system, using the redetermined acetophenone enol acidity constant, pKaE= 10.40, obtained here from rates of ketonization of the enol in sodium hydroxide and ammonia buffer solutions. All present equilibrium constantss refer to wholly aqueous solution at 25 °C and ionic strength = 0.10 mol dm–3.

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. Computational data suggest that ylides formed by the complexation of the carbene with solvent or other weak nucleophiles are viable intermediates in the reactions of copper(I) carbenes.