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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.

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. Computational work on identity S(N)2 reactions, e.g., chloride(-) displacing chloride(-) and ammonia displacing ammonia, shows that negatively charged S(N)2 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 S(N)2 transition states of the ammonia reactions are dramatically stabilized by the triafulvenyl group, which directly conjugates with a reaction center having S(N)1 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.

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. Computational results indicate that deprotonation of the H-bonded complex is probably barrier free, and is accompanied by simultaneous loss of the leaving group (E2) for L = Cl, Br, and I, but with subsequent, rapid loss of the leaving group (E1cB-like) for the poorer leaving groups, CN and F. The approximately 50-fold greater reactivity of the 2- and 4-cyano substrates is attributed to the influence of the electron withdrawing cyano group in the deprotonation step. The results provide another example of β-elimination reactions poised near the E2-E1cB mechanistic borderline.

Carbocations and carbenes, as electron-deficient species, require electron donation from the remainder of the molecule to the carbon center by whatever means available. Classical interactions include resonance, polar and polarizability effects, but neighboring group participation of several sorts can also serve as stabilizing factors. Simple carbocations with directly attached electron-withdrawing groups (EWGs), that is, EWG–CH2+ ions, comprise one group by which these interactions may be probed. This article provides computational evidence at the MP2/6-311 + G** level for variable but significant stabilizing interactions between the carbocation center and common EWGs, many via bridging (partial or symmetrical), neighboring group participation, homoconjugation or π interactions. Bridging from atoms possessing nonbonding electron pairs is a common motif. Removal of bridging by application of geometric constraints nullifies bridging stabilization, but does not eliminate the possibility of other stabilizing interactions, for example polar, polarizability and π donation from the EWG. The potential for π donation from strong EWGs has not been widely appreciated in the past. A carbocation stabilization enthalpy, CSE+, is defined as the enthalpy of the isodesmic reaction CH3+ + R–H CH4 + R+. Comparisons are made with singlet carbenes bearing EWGs for which many structural features are similar. Copyright © 2013 John Wiley & Sons, Ltd.

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 pKa 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 stabilities of the carbanions in water were determined through the rates of hydrogen–deuterium exchange reactions of the corresponding conjugate acids. The carbanions derived from the two pyridones were found to have the same stability, whereas the carbanion derived from 1,3-dimethyluracil was more stable. The order of the stability of the carbanions showed no correlation with the decarboxylation rates of their corresponding carboxylic acids. The implications of the results for the mechanism of orotidine-5′-monophosphate decarboxylase (ODCase) are discussed.

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.

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. Rather, it is a consequence of generally greater total bonding in a hydride-transfer ts than in a proton-transfer ts, most prominently manifested as a reduced contribution from the zero-point vibrational energy difference between reactant and transition states (the DeltaZPVE factor) for hydride transfers relative to proton transfers. This and other differences between proton and hydride transfers are rationalized by modeling the central .C...H...C unit of a proton-transfer ts as a 4-electron, 3-center (4-e 3-c) system and the same unit of a hydride-transfer ts as a 2-e 3-c system. Inclusion of tunneling is most likely to magnify the observed differences between proton-transfer and hydride-transfer KIEs, leaving our qualitative conclusions unchanged.

Flash photolysis of 2-diazocyclopentane-1,3-dione in aqueous solution produced 2-oxocyclobutylideneketene, which underwent hydration to the enol of 2-oxocyclobutanecarboxylic acid; the enol then isomerized to the keto form of this acid. Rates of the ketene and enol reactions were measured in acid, base, and buffer solutions across the acidity range [H+] = 10(-1)-10(-13) M, and analysis of these data, together with rates of enolization of the keto form of 2-oxocyclobutanecarboxylic acid determined by bromine scavenging, gave keto-enol equilibrium constants as well as acidity constants of the keto and enol forms. The keto-enol equilibrum constants proved to be 2 orders of magnitude less than those reported previously for the next higher homolog, 2-oxocyclopentanecarboxylic acid, reflecting the difficulty of inserting a carbon-carbon double bond into a small, strained carbocyclic ring. The acidity constant of the enol group of 2-oxocyclobutanecarboxylate ion, on the other hand, is greater, by 4 orders of magnitude, than that of the corresponding enol in the cyclopentyl system. This remarkable increase in acidity with diminishing ring size is consistent with the enhanced s character of the orbitals used to make the exocyclic bonds of the smaller cyclobutane ring.

The title compounds are cleaved cleanly at the CN bond by singlet oxygen (1O2, 1Δg) yielding arylaldehydes and N-aryl-N-methylnitrosamines. These reactions take place more rapidly at −78 °C than at...

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. The carbonyl and fulvene reactions have transition states with one imaginary frequency: the hydrogen transfer coordinate. The carbenium ions, borane, and SiH3(+) gave not transition states but stable compounds upon addition of the hydride donor. Computational differences between these hydride transfers and previously reported proton transfers include shorter partial C...H bonds and a tendency toward bent C...H...C angles for the hydride transfer ts and addition compound structures, particularly when a bent geometry improves interactions elsewhere in the structure. These and other differences are explained by modeling the hydride transfer ts and addition compounds as two-electron, three-center systems involving the transfer termini and the shared hydrogen but the proton transfer ts structures as four-electron, three-center systems. Charge and geometry measures suggest transition states in which these features change synchronously, again in contrast to many proton transfer reactions. For the X-CHO set, polar effects dominate enthalpies of hydride addition, with resonance effects also important for resonance donors; these preferentially stabilize the acceptor, reducing its hydride ion affinity. Activation enthalpies are dominated by resonance stabilization of the acceptors, greatly attenuated in the transition states.

Nucleophilic attack of cyanide anion at the carbonyl carbon was studied computationally at the MP2/6-311+G** level for ketene and at the MP2/6-31+G* and MP2/6-31+G*//HF/6-31+G* levels for all four title compounds. Heats of addition were computed for formation of in-plane adducts and perpendicular adducts where the terms 'in-plane' and 'perpendicular' indicate the direction of nucleophilic attack relative to the ketene plane. Heats of activation were also computed for the two modes of attack. For ketene (1) the in-plane adduct is favored by 37.6 kcal mol - 1 and the in-plane transition state by 18.8 kcal mol - 1 . For the 6-methylene compound (2) the in-plane adduct is favored by 20.1 kcal mol - 1 and the in-plane transition state by 7.3 kcal mol - 1 For the 6-oxo compound (3) the in-plane adduct is more stable by 11.2 kcal mol - 1 whereas the in-plane transition state is preferred by 8.8 kcal mol - 1 . For the 4-oxo compound (4) the in-plane adduct is favored by 19.3 kcal mol - 1 but its transition state by only 2.8 kcal mol - 1 '. The in-plane adduct for ketene is the enolate of acetyl cyanide while the perpendicular adduct is a pyramidalized carbanion attached to a cyanocarbonyl group. Calculated NPA charges support this conclusion. The perpendicular adduct is actually the transition state for rotation about the C-C bond of the ketene moiety in the in-plane adduct, and the 37.6 kcal mol - 1 enthalpy difference is closely matched by the calculated activation enthalpy for rotation about the C-C bond of acetaldehyde enolate: ΔH = 33.6 kcal mol - 1 . The smaller differences found for in-plane vs perpendicular adduct formation for the 6-methylene, 6-oxo and 4-oxo compounds are correlated with decreases in bond length alternation within the six-membered rings which occur upon addition of the nucleophile. Less alternation may be associated with an approach to aromaticity, hence an increase in stability. Both modes of attack lead to reduced bond length alternation, but the amount of alternation is lowered more for the perpendicular adducts than for the in-plane adducts. Changes in the C-O bond lengths are also diagnostic. For all four ketenes the ketenyl C=O length in the in-plane adducts is greater than in the perpendicular adducts reflecting more single bond (enolate) character in the former and more double bond (carbonyl) character in the latter. On the other hand the lengths of the 6-oxo and 4-oxo C-O bonds in their in-plane adducts are smaller than in the perpendicular adducts, consistent with increased conjugation with the oxo substituent in the latter. The greater NPA charge on the 6-oxo and 4-oxo oxygens of the perpendicular adducts supports this conclusion. Calculated enthalpies of activation are in the order ketene > 6-methyleneketene > 6-oxoketene, in qualitative agreement with experimentally determined rates of hydration.

Equilibrium constants are reported for the ionization of benzylic alcohols to carbocations stabilized by cyclic or acyclic o-alkyl or o-oxygen substituents. The measurements were stimulated by the observation of small or inverse effects of replacement of an o-CH2 group by O or S in the cyclopentyl ring of indanol (kO/kCH2 = 1.2) or in the cyclohexyl ring of tetralol (kO/kCH2 = 0.6, kS/kCH2 = 0.3) on rates of carbocation formation. Values of pKR (KR = [ROH][H+]/[R+]) have been obtained by combining rate constants, kH, for the acid-catalyzed ionization of the alcohols with kH2O for attack of water on the carbocation measured by the azide clock method. For carbocations derived from the following alcohols, values of pKR are as indicated: 1-indanol, -11.7; 2,3-dihydro-3-hydroxybenzofuran (benzofuran hydrate), -9.3; 1-tetralol, -12.2; 4-chromanol, -12.0; 4-thiochromanol, -12.3; o-methyl-1-phenylethanol, -13.8; o-methoxy-1-phenylethanol, -11.7. The measurements show that, in contrast to its small kinetic effect, the equilibrium effect of replacing the o-CH2 group by O in the cyclopentyl ring of indanol is 250-fold, whereas the effect of the same replacement in the cyclohexyl ring of tetralol is only 1.6. It is concluded (a) that the efficiency of conjugation of annular o-oxygen substituents to a benzylic carbocation center is sensitive to conformational restrictions arising from ring strain and (b) that, in the case of indanol, the kinetic effect of the same oxygen atom is subject to an imbalance of favorable resonance and unfavorable inductive effects at the transition state.

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...(-)X-Y)(cx) --[Y-X...H...X-Y](ts) --(Y-X..H-X-Y)(cx) --Y-X(-) + H-X-Y, where cx indicates an ion-molecule complex and ts indicates the proton-transfer transition state. All species were optimized at the MP2/6-311+G level, and these geometries were used for single-point calculations by other methods: coupled-cluster, DFT (gas phase), and a polarizable continuum aqueous solvent model (COSMO). All methods gave enthalpies of deprotonation which correlate well with experimental measurements of deltaH(ACID) (gas) or pK(a) (aq). Calculated gas-phase enthalpies of deprotonation (deltaH(ACID)) and enthalpies of activation (deltaH(#)) are poorly correlated except for small, carefully selected sets. This result stands in contrast to the many aqueous phase Brönsted correlations of kinetic and equilibrium acid strength. On the other hand, gas-phase enthalpies of complexation and deltaH(#) are well correlated, indicating that factors which stabilize the transition state are at work in the bimolecular ion-molecule complex although to a smaller degree. We infer that intermoiety electrostatic and other interactions, similar within the complex and the transition state, but absent in the separated reactants (products), cause the lack of correlation between deltaH(ACID) and the other two quantities. Such differences are strongly attenuated in water because reactants and products do interact with polar/polarizable matter (the solvent) if not with each other. Charge distributions (NPA) were computed, allowing calculation of Bernasconi's "transition state imbalance parameter". Such measures provide intuitively satisfactory trends, but only if the reaction termini, X, are kept the same. As X is made more electronegative, the magnitude of the apparent imbalance increases, a result of greater negative charge on X in the transition state. This result gives additional support for the importance of the ion-triplet structure, [YX(-)...H(+)...(-)XY], to the stability of the transition state. Additional qualitative support for this conclusion is provided by the inverse relationship between the activation barrier and the charge on the in-flight hydrogen in the transition state, and by the dominance of polar over resonance substituent effects on the stability of the transition state. Calculations also show that the "nitroalkane anomaly", well established in solution, does not exist in the gas phase. The COSMO model partly reproduces this anomaly and performs adequately except when strong, specific intermolecular forces such as hydrogen bonding between solvent and anions are important.

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 .

Equilibrium and rate constants have been determined for the acid-catalyzed heterolysis of two alcohols, 9-xanthydrol and p-anisyldiphenylmethanol, and two sulfides, (9-xanthyl) methyl sulfide and (7-tropyl) methyl sulfide. These data together with literature information are compared with rate constants for acid-catalyzed C-C heterolysis of several (9-xanthyl) compounds, (7-tropyl) compounds, a set of 3-arylcyclobutanones, and two 2-arylnitrocyclopropanes, all of which fragment to carbocations plus a carbon-centered nucleofuge. The fragmentation mechanisms are shown to be A1 or A1(ion pair) except for the 2-arylnitrocyclopropanes which cleave in trifluoroacetic acid by a concerted mechanism. Rate comparisons among several unstrained substrate sets indicate that O-centered nucleofuges undergo acid-catalyzed heterolysis ca. 103-104 faster than S-centered nucleofuges and ca. 109-1014 faster than the C-centered nucleofuges used here. Factors assisting C-C heterolysis (and their effectiveness) include the acidity of the medium (strong); the basicity and nucleofugality of the nucleofuge (moderate); the stability of the electrofugic carbocation (strong); and relief of ring strain (enormous). Compared with acyclic cleavages, rate accelerations worth ca. 15 kcal/mol (for cyclobutanones) and ca. 27 kcal/mol (for nitrocyclopropanes) are found. These effects are discussed in terms of transition-state structure, aided by computational evidence.Key words: C-C heterolysis, fragmentation, acid catalysis, carbocation, ring strain.

Equilibrium and rate constants have been determined for the acid-catalyzed heterolysis of two alcohols, 9-xanthydrol and p-anisyldiphenylmethanol, and two sulfides, (9-xanthyl) methyl sulfide and (7-tropyl) methyl sulfide. These data together with literature information are compared with rate constants for acid-catalyzed C-C heterolysis of several (9-xanthyl) compounds, (7-tropyl) compounds, a set of 3-arylcyclobutanones, and two 2-arylnitrocyclopropanes, all of which fragment to carbocations plus a carbon-centered nucleofuge. The fragmentation mechanisms are shown to be A1 or A1(ion pair) except for the 2-arylnitrocyclopropanes which cleave in trifluoroacetic acid by a concerted mechanism. Rate comparisons among several unstrained substrate sets indicate that O-centered nucleofuges undergo acid-catalyzed heterolysis ca. 10 3 -10 4 faster than S-centered nucleofuges and ca. 10 9 -10 14 faster than the C-centered nucleofuges used here. Factors assisting C-C heterolysis (and their effectiveness) include the acidity of the medium (strong); the basicity and nucleofugality of the nucleofuge (moderate); the stability of the electrofugic carbocation (strong); and relief of ring strain (enormous). Compared with acyclic cleavages, rate accelerations worth ca. 15 kcal/mol (for cyclobutanones) and ca. 27 kcal/mol (for nitrocyclopropanes) are found. These effects are discussed in terms of transition-state structure, aided by computational evidence.

The identity carbon-to-carbon proton transfers between nitromethane and nitromethide anion and between oxygen protonated nitromethane and aci-nitromethane have been studied by ab initio methods. Group charges calculated by Mulliken and NPA methods as well as geometrical parameters such as pyramidal angles and CN bond lengths indicate that the transition states of these reactions are strongly imbalanced. Further evidence for the imbalance comes from a consideration of the relative energies of the various corners representing hypothetical intermediates on More O'Ferrall−Jencks diagrams. Our results for the CH3NO2/CH2NO2- system, in conjunction with previous findings on other CH3Y/CH2Y- systems, indicate an increase in the imbalance in the order CN ≪ CHO ≤ CHCH2 ≤ NO2 consistent with the notion that imbalances increase with π-acceptor strength of Y. However, when comparing the CH3N+O2H/CH2NO2H system with the CH3NO2/CH2NO2- system, the results are somewhat ambiguous as to whether the stronger π-acceptor (N+O...

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.

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. Computational studies of the reaction of piperidine and dimethylamine with the same aryl halides using the polarizable continuum model (PCM) for solvation indicate that polar, polarizability, solvation, and negative hyperconjugative effects are all of some importance in producing the element effect in methanol. In addition, a reversal of polarity of the C-X bond from reactant to transition state in the case of ArCl and ArBr compared to ArF also contributes to their differences in reactivity. The polarity reversal and hyperconjugative influences have received little or no attention in the past. Nor has differential solvation of the different transition states been strongly emphasized. An anionic nucleophile, thiolate, gives very early transition states and negative activation enthalpies with activated aryl halides. The element effect is not established for these reactions. We suggest that the leaving group order in the gas phase will be dependent on the exact combination of nucleophile, leaving group, and substrate framework. The geometry of the S(N)Ar transition state permits useful, qualitative conceptual distinctions to be made between this reaction and other modes of nucleophilic attack.

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. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by values with rho = 3.2. Solvent isotope effects for the reactions are k(H2O)/k(D2O) = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of similar to 10(11) s(-1), corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 10(11)-10(12) s(-1). From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK(a) of 30.8 +/- 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion.

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. Comparison of pK(R) = -16.1 with pK(R) = -2.6 for the cyclohexadienyl cation yields the difference in stabilities of the two ions. Interpretation of this difference in terms of hyperconjugative aromaticity is supported by the effect of benzannelation in reducing pK(R) for the benzenium ion: from -2.6 down to -3.5 for the 1H-naphthalenium and -6.0 for the 9H-anthracenium ions, respectively. MP2/6-311+G** and G3MP2 calculations of hydride ion affinities of benzenium ions show an order of stabilities for substituents at the methylene group consistent with their hyperconjugative abilities, i.e., (H(3)Si)(2)cyclopropylH(2)Me(2)(HO)(2)F(2). Calculations of ring currents show a similar ordering. No conventional ring current is seen for the cycloheptadienyl cation, whereas currents in the F(2)-substituted benzenium ion are consistent with antiaromaticity. Arenium ions where the methylene group is substituted with a single OH group show characteristic energy differences between conformations, with C-H or C-OH bonds respectively occupying or constrained to axial positions favorable to hyperconjugation. The differences were found to be 8.8, 6.3, 2.4, and 0.4 kcal mol(-1) for benzenium, naphthalenium, phenanthrenium, and cyclohexenyl cations, respectively.

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.

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.

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.

Aqueous solution free energies of ketonization have been combined with enthalpies of reduction of the keto forms and other thermochemical data to yield estimates of the heats of formation of the corresponding enols. A double-bond stabilization term, DOH = 5.4 ± 0.5 kcal mol−1 (1 kcal = 4.184 kJ), is obtained for the OH group. This quantity is close to Hine's double-bond stabilization parameter for the methoxyl group (4.9 ± 0.2 kcal mol−1). Comparison with available experimental gas-phase enthalpies of formation suggest that, relative to the keto form, enols are more stable in the gas phase than in water. The differences are unlikely to be due entirely to entropic effects. A wholly enthalpic, gas-phase double-bond stabilization term, DOH = 8.1 ± 0.6 kcal mol−1, may be calculated from the available experimental data. This result indicates that the hydroxyl group more effectively stabilizes the CC bond in the gas phase than in aqueous solution. An important limitation is that the results reported bere apply only to simple enols, that is, monofunctional enols not stabilized by extra resonance or hydrogen-bonding effects.

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.

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. In the heteroallenes, protonation at a terminal carbon is strongly favored, activated by electron donation from the other terminal atom. Transition states for identity proton-transfer reactions were found for 10 of the "end-to-end" proton transfers. When the transfer termini are heteroatoms these processes are barrier free. We found no first-order saddle point structures for "center-to-center" proton transfers. An estimate of DeltaH++ for an identity center-to-center proton transfer could be made only for the reaction between the allyl cation and allene; it is approximately 22 kcal/mol higher than DeltaH++ for the end-to-end proton transfer between the 2-propenyl cation and allene. First-order saddle points for the proton transfer from H3S+ to both C1 and C2 of allene were found. The difference in activation enthalpies is 9.9 kcal/mol favoring protonation at C1 in spite of the thermodynamic disadvantage. We infer that protonation of X = Y = Z at central atoms passes through transition states much like primary carbenium (nitrenium, oxenium) cations, poorly conjugated with the attached vinylic or heterovinylic group. Several other processes following upon center protonation were studied and are discussed in the text, special attention being given to comparison of open and cyclic isomers.

A new synthetic method is described here to obtain α, β-unsaturated ketones from alkenes through a sequence involving: (1) cycloaddition of alkenes with dichloroketenc; (2) dechlorination of adducts; (3) acidpromoted ring-opening isomerization of cyclobutanones in CF3 CO2H.

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. Good correlations of the resulting rate ratios with substituent constants (sigma(-) or sigma(+)) were obtained yielding small to very small rho values indicative of small to very small changes in charge distribution between the reactant and the rate determining transition state. Electron withdrawing groups on the C-aryl moiety retard reaction somewhat by preferential stabilization of the hydrazone. Electron donors on the other hand slightly stabilize the rate determining transition state. Substituents on the N-aryl group have almost no effect. Inhibition by 3,5-di-tert-butylphenol was not observed showing that free (uncaged) radical intermediates are not involved in the mechanism. We postulate a mechanism in which the initial event is exothermic electron transfer from the hydrazone to (1)O(2) leading to an ion-radical caged pair. Subsequent covalent bond formation between the hydrazone carbon and an oxygen atom is rate controlling. The transition state for this step also has a lower enthalpy than the starting reactants, but the free energy of activation is dominated by a large negative TDeltaS++term leading to the negative temperature dependence. Direct formation of a C-O bond in the initial step is not unambiguously ruled out. Subsequent steps lead to C-N cleavage.

In this new edition, (optional) molecular mechanics calculations remain the workhorse method. These are now used in conjunction with 20 experiments, and are supplemented in some cases by suggested semiempirical computations.

This paper reports a large, inverse solvent deuterium isotope effect on the initial rates of base-induced p-nitrostyrene formation (..beta..-(p-nitrophenyl)ethyl)ammonium ions in aqueous acetohydroxamate buffers.