The [C6H10]•+ Hypersurface: the Parent Radical Cation Diels−Alder Reaction

Synthesis and Functionalization of Isomeric Sesquihomodiamantenes

anti- and syn-sesquihomodiamantenes (SDs) were prepared and structurally characterized. anti-SD and parent sesquihomoadamantene were CH-bond functionalized by utilizing a phase-transfer protocol. The density functional theory-computed ionization potentials of unsaturated diamondoid dimers correlate well with the experimental oxidation potentials obtained from cyclic voltammetry. Similar geometries ensue for both the reduced and ionized SD states, whose persistence is supported by the β-hydrogen's spatial sheltering. This makes SDs promising building blocks for the construction of diamond materials with high stability and carrier mobility.

Hole Catalysis of Cycloaddition Reactions: How to Activate and Control Oxidant Upconversion in Radical-Cationic Diels-Alder Reactions

In order to use holes as catalysts, the oxidized product should be able to transfer the hole to a fresh reactant. For that, the hole-catalyzed reaction must increase the oxidation potential along the reaction path, i.e., lead to "hole upconversion." If this thermodynamic requirement is satisfied, a hole injected via one-electron oxidation can persist through multiple propagation cycles and serve as a true catalyst. This work provides guidelines for the rational design of hole-catalyzed Diels-Alder (DA) reactions, the prototypical cycloaddition. After revealing the crucial role of hyperconjugation in the absence of hole upconversion in the parent DA reaction, we show how upconversion can be reactivated by proper substitution. For this purpose, we computationally evaluate the contrasting effects of substituents at the three possible positions in the two reactants. The occurrence and magnitude of hole upconversion depend strongly on the placement and nature of substituents. For example, donors at C1 in 1,3-butadiene shift the reaction to the hole-upconverted regime with an increased oxidation potential of up to 1.0 V. In contrast, hole upconversion in C2-substituted 1,3-butadienes is activated by acceptors with the oxidation potential increase up to 0.54 V. Dienophile substitution results in complex trends because the radical cation can be formed at either the dienophile or the diene. Hole upconversion is always present in the former scenario (up to 0.65 V). Finally, we report interesting stereoelectronic effects that can activate or deactivate upconversion via a conformational change.

How Ionization Catalyzes Diels-Alder Reactions

The catalytic effect of ionization on the Diels-Alder reaction between 1,3-butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels-Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels-Alder reaction to stepwise for the radical-cation Diels-Alder reaction. Our detailed activation strain and Kohn-Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels-Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π-orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical-cation dienophile to engage in an additional three-electron bonding interaction with the highest occupied molecular orbital of the diene.

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time-of-flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C6 H9 (+) , C6 H7 (+) , C5 H7 (+) , C5 H5 (+) , C4 H6 (+) , C4 H5 (+) , C3 H5 (+) and C3 H3 (+) were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X-D/6-31+g(d,p) level. The coupled cluster method, CCSD(T)/cc-pVTZ, was employed to calculate the corresponding energies with the zero-point energy corrections by the ωB97X-D/6-31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro-Cope rearrangement was found to play a crucial role in the formation of C4 H6 (+) , C4 H5 (+) and C3 H5 (+) . Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8- to 15.5-eV photon energy region.

Structure and Reactivity of Radical Ions: New Twists on Old Concepts

Electron transfer is the simplest reaction possible, yet it has a profound impact on the structure and reactivity of organic compounds. These changes allow a new look at some of the fundamental concepts that are used to explain organic chemistry, such as symmetry, aromaticity, and bonding. The results from high-level electronic structure calculations are used to analyze the mechanistic differences in the pericyclic reactions of simple hydrocarbons and their radical cation counterparts. The importance of state symmetry correlation, Jahn-Teller distortions, delocalization, and fractional bonding for the reaction pathways of hydrocarbon radical cations is discussed.

Kinetics study and theoretical modeling of the Diels-Alder reactions of cyclopentadiene and cyclohexadiene with methyl vinyl ketone. The effects of a novel organotungsten catalyst

The Diels-Alder reaction rate constants of methyl vinyl ketone with cyclopentadiene and cyclohexadiene in the presence of a novel organotungsten catalyst, [P(2-py)(3)W(CO)(NO)(2)](2+), have been measured experimentally and modeled theoretically at several temperatures. The uncatalyzed systems were also studied for direct comparison. When 0.0022 M of catalyst is present at room temperature, the rate constants were found to be approximately 5.3 and 5300 times higher than the corresponding uncatalyzed reactions for cyclopentadiene and cyclohexadiene systems, respectively. Experimental data suggested that the catalyst reduced the activation energies by 5-10 kcal/mol. However, the preexponential factors showed reduction of more than 3 orders of magnitude upon catalysis due to the entropic effects. The energy barriers and the rate constants of the uncatalyzed systems were accurately modeled by correlated electronic structure and dual-level variational transition state theory calculation. The calculated endo selectivity is in good agreement with the observed product distribution. Theoretical calculation also suggested the catalyzed reactions proceeded in a highly asynchronous or even stepwise fashion.

Diels-Alder Reaction of phosphaethene with 1,3-dienes: an ab initio study

Computations on Diels-Alder (DA) reactions of phosphaethene with 1,3-butadiene and with isoprene reveal asynchronous transition structures. The DFT (B3LYP/6-311+G) activation energies of these reactions, 12-14 kcal/mol, are much lower than that of the parent ethene-butadiene reaction, 28 kcal/mol, even though the exothermicities of all lie in the same range, from -29 to -33 kcal/mol. The transition states (TSs) for the phosphethene-butadiene or isoprene DA reactions are earlier than the TSs of the butadiene-ethene cycloaddition. Due to the weakness of the C=P pi bond compared to the C=C pi bonds, the energies required to reach the phosphaethene TSs are much less than the carbocyclic cases. The computed (1)H NMR chemical shifts and nucleus independent chemical shifts (NICS) quantify the aromatic character of the transition states. Regioselectivities of the neutral phosphaethene-isoprene DA reactions are modest, at best. However, computations on radical cation DA reactions of phosphaethene with isoprene, which proceed stepwise with open chain intermediates, can account for the high regioselectivities that have been observed in some cases.

Isotope effects and the mechanism of an electron-transfer-catalyzed Diels-Alder reaction

The electron-transfer-catalyzed Diels-Alder reaction of indole and 1,3-cyclohexadiene was studied by a combination of experimental and theoretical methods. The (13)C kinetic isotope effects were determined at natural abundance by NMR methodology. B3LYP/6-31G* calculations allow for the first time a quantitatively accurate description of the different possible pathways and provide the basis for an analysis of the experimentally observed isotope effects. The computational results, in conjunction with experimental observations, show that the reaction has a stepwise mechanism that is initiated by attack of the diene into the 3-position of the indole. Numerical simulation of the experimentally observed isotope effects shows that the first step is rate-determining and that the electron exchange in the reactant contributes partially to the overall isotope effect. The combination of electronic structure theory, experimental isotope effects, and numerical simulation thus allows a detailed analysis of a complex reaction mechanism.

The structure of ionized 1,5 hexadiene in the gas phase

The structure of ionized 1,5-hexadiene, prepared by charge transfer between 1,5-hexadiene and CS2+*, is examined using energy-resolved collision-induced dissociation (CID). By comparing the product distributions and product appearance curves with those of authentic low-energy C6H10+* ions, it is determined that 1,5-hexadiene cation spontaneously rearranges to cyclohexene cation in the gas-phase. The proposed mechanism for formation of cyclohexene cation in the gas phase is analogous to that determined for this process under matrix isolation conditions, where it proceeds via a Cope rearrangement to the cyclohexane-1,4-diyl cation, followed by isomerization to cyclohexene cation. It is shown that electron ionization (EI) of 1,5-hexadiene gives a different molecular ion than is obtained upon chemical ionization (CI). The energy-resolved CID mass spectrum for the EI product is consistent with what would be obtained for a mixture of low energy ion isomers.

A study of gas-phase reactions of radical cations of mono- and dihaloethenes with alcohols by FT-ICR spectrometry and molecular orbital calculations: substitution versus oxidation

The ion-molecule reactions of the radical cations of vinyl chloride (1), vinyl bromide (2), 1,2-dichloroethene (3), 1,2-dibromoethene (4), 1,1-dichloroethene (5), and 1,1-dibromoethene (6) with methanol (MeOH) and ethanol (EtOH) have been studied by FT-ICR spectrometry. In the case of EtOH as reactant the oxidation of the alcohol to protonated acetaldehyde by a formal hydride transfer to the haloethene radical cation is the main process if not only reaction observed with the exception of the 1,2-dibromoethene radical cation which exhibits slow substitution. In secondary reactions the protonated acetaldehyde transfers the proton to EtOH which subsequently undergoes a well known condensation reaction of EtOH to form protonated diethyl ether. With MeOH as reactant, the 1,2-dihaloethene radical cations of 3.+ and 4.+ exhibit no reaction, while the other haloethene radical cations undergo the analogous reaction sequence of oxidation yielding protonated formaldehyde. Generally, bromo derivatives of haloethene radical cations react predominantly by substitution and chloro derivatives by oxidation. This selectivity can be understood by the thermochemistry of the competing processes which favors substitution of Br while the effect of the halogen substituent on the formal hydride transfer is small. However, the bimolecular rate constants and reaction efficiencies of the total reactions of the haloethene radical cations with both alcohols exhibit distinct differences, which do not follow the exothermicity of the reactions. It is suggested that the substitution reaction as well as the oxidation by formal hydride transfer proceeds by mechanisms which include fast and reversible addition of the alcohol to the ionized double bond of the haloethene radical cation which generates a beta-distonic oxonium ion as the crucial intermediate. This intermediate is energetically excited by the exothermic addition and fragments either directly by elimination of a halogen substituent to complete the substitution process or rearranges by hydrogen migration before dissociation into the protonated aldehyde and a beta-haloethyl radical. Reversible addition and hydrogen migrations within a long lived intermediate is proven experimentally by H/D exchange accompanying the reaction of the radical cations of vinyl chloride (1) and 1,1-dichloroethene (5) with CD3OH. The suggested mechanisms are substantiated by ab initio molecular orbital calculations.

Mass spectrometric studies of organic ion/molecule reactions

In the past 25 years, a tremendous amount of work has been published on the ion/molecule reactions of organic species. This review provides an overview of the areas where gas phase ion chemistry has made a contribution to our understanding of fundamental organic reaction processes. It is clear that the gas phase work can provide insights into subtle features of reaction mechanisms that could not be addressed by conventional condensed phase methods. The study of ion/molecule reactions has already had a major impact on the way that organic chemists think about reaction mechanisms and interpret substituent effects. Moreover, it has heightened our awareness of the importance of solvation effects and how they can alter not only absolute rates but also relative rates, leading in some cases to complete reversals in reactivity patterns. A large body of work could not be included in this review due to space limitations. For example, the study of thermochemistry in the gas phase (i.e., acidities, basicities, bond strengths, binding energies, etc.) has provided a wealth of data that has been exceptionally useful in interpreting organic reaction mechanisms. This has spilled over into the study of organometallic systems, and several groups are making major headway in using mass spectrometry to probe the stability and reactivity of transition metal species. Finally, work involving chemical ionization has provided abundant information on gas phase reaction mechanisms. The future appears to be very promising for the study of gas phase organic reaction mechanisms. In particular, the emergence of new ionization techniques and more powerful mass analyzers will allow chemists to explore a wider range of species. Although still at an early stage, the gas phase study of biochemical transformations offers great promise and has been facilitated by electrospray and matrix assisted laser desorption ionization methods. In addition, these techniques provide a means for introducing important, metal-centered catalytic species into the gas phase and exploring the details of their reactivity. Finally, mass spectrometry continues to play a major role in the study of atmospheric ion chemistry and is providing important kinetic as well as mechanistic data.