Quantum mechanical study of the ring-closing reaction of the hexatriene radical cation

The ring-closing reaction of hexatriene radical cation 1(*)(+) to 1,3-cyclohexadiene radical cation 2(*)(+) was studied computationally at the B3LYP/6-31G* and QCISD(T)/6-311G*//QCISD/6-31G* levels of theory. Both, concerted and stepwise mechanisms were initially considered for this reaction. Upon evaluation at the B3LYP level of theory, three of the possible pathways-a concerted C(2)-symmetric via transition structure 3(*)(+) and stepwise C(1)-symmetric pathways involving three-membered ring intermediate 5(*)(+) and four-membered ring intermediate 6(*)(+)-were rejected due to high-energy stationary points along the reaction pathway. The two remaining pathways were found to be of competing energy. The first proceeds through the asymmetric, concerted transition structure 4(*)(+) with an activation barrier E(a) = 16.2 kcal/mol and an overall exothermicity of -23.8 kcal/mol. The second pathway, beginning from the cis,cis,trans rotamer of 1(*)(+), proceeds by a stepwise pathway to the cyclohexadiene product with an overall exothermicity of -18.6 kcal/mol. The activation energy for the rate-determining step in this process, the formation of the intermediate bicyclo[3.1.0]hex-2-ene via transition structure 9(*)(+), was found to be 20.4 kcal/mol. More rigorous calculations of a smaller subsection of the potential energy hypersurface at the QCISD(T)//QCISD level confirmed these findings and emphasized the importance of conformational control of the reactant.     

Enhancing reactivity of carbonyl compounds via hydrogen-bond formation. A DFT study of the hetero-Diels-Alder reaction between butadiene derivative and acetone in chloroform

To examine how hydrogen-bond (HB) formation involving chloroform solvent molecules influences the chemical reactivity of ketones, the hetero-Diels-Alder reaction of N,N-dimethyl-1-amino-3-methoxy-1,3-butadiene and acetone has been studied by using density functional theory (DFT) at the B3LYP/6-31G level. The effects of the chloroform on the activation energies have been modeled by means of discrete-continuum models. In the gas phase, the formation of specific HB between acetone and one and two chloroform molecules decreases the activation barriers from 19.3 to 13.6 and 8.5 kcal/mol, respectively. Inclusion of solvent effects by means of combined discrete and polarizable continuum models yields a change of molecular mechanism from a concerted to a two-step mechanism, and the first nucleophilic step is the rate-limiting step. The corresponding values of activation barriers in chloroform are 18.6 kcal/ mol (no HB), 13.5 kcal/mol (one HB), and 9.6 kcal/mol (two HBs). These theoretical results account for the experimental observation that chloroform accelerates the reaction more markedly than more polar aprotic solvent such as acetonitrile. A DFT analysis of the global electrophilicity power of the reagents provides a sound explanation about the catalytic effects of chloroform.     

Reactions of 1,3-cyclohexadiene with singlet oxygen. A theoretical study.

A thorough study of the reaction of singlet oxygen with 1,3-cyclohexadiene has been made at the B3LYP/6-31G(d) and CASPT2(12e,10o) levels. The initial addition reaction follows a stepwise diradical pathway to form cyclohexadiene endoperoxide with an activation barrier of 6.5 kcal/mol (standard level = CASPT2(12e,10o)/6-31G(d); geometries and zero-point corrections at B3LYP/6-31G(d)), which is consistent with an experimental value of 5.5 kcal/mol. However, as the enthalpy of the transition structure for the second step is lower than the diradical intermediate, the reaction might also be viewed as a nonsynchronous concerted reaction. In fact, the concertedness of the reaction is temperature dependent since entropy differences create a free energy barrier for the second step of 1.8 kcal/mol at 298 K. There are two ene reactions; one is a concerted mechanism (DeltaH(double dagger) = 8.8 kcal/mol) to 1-hydroperoxy-2,5-cyclohexadiene (5), while the other, which forms 1-hydroperoxy-2,4-cyclohexadiene (18), passes through the same diradical intermediate (9) as found on the pathway to endoperoxide. The major pathway from the endoperoxide is O-O bond cleavage (22.0 kcal/mol barrier) to form a 1,4-diradical (25), which is 13.9 kcal/mol less stable than the endoperoxide. From the diradical, two low-energy pathways exist, one to epoxyketone (29) and the other to the diepoxide (27), where both products are known to be formed experimentally with a product ratio sensitive to the nature of substitutents. A significantly higher activation barrier leads to C-C bond cleavage and direct formation of maleic aldehyde plus ethylene.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

Organic functionalization of the sidewalls of carbon nanotubes by diels-alder reactions: a theoretical prediction

[structure: see text] The viability of the Diels-Alder (DA) cycloaddition of conjugated dienes onto the sidewalls of single-wall carbon nanotubes is assessed by means of a two-layered ONIOM(B3LYP/6-31G:AM1) approach. Whereas the DA reaction of 1,3-butadiene on the sidewall of an armchair (5,5) nanotube is found to be unfavorable, the cycloaddition of quinodimethane is predicted to be viable due to the aromaticity stabilization at the corresponding transition states and products.     

Activation energies of pericyclic reactions: performance of DFT, MP2, and CBS-QB3 methods for the prediction of activation barriers and reaction energetics of 1,3-dipolar cycloadditions, and revised activation enthalpies for a standard set of hydrocarbon pericyclic reactions

Activation barriers and reaction energetics for the three main classes of 1,3-dipolar cycloadditions, including nine different reactions, were evaluated with the MPW1K and B3LYP density functional methods, MP2, and the multicomponent CBS-QB3 method. The CBS-QB3 values were used as standards for 1,3-dipolar cycloaddition activation barriers and reaction energetics, and the density functional theory (DFT) and MP2 methods were benchmarked against these values. The MPW1K/6-31G* method and basis set performs best for activation barriers, with a mean absolute deviation (MAD) value of 1.1 kcal/mol. The B3LYP/6-31G* method and basis set performs best for reaction enthalpies, with a MAD value of 2.4 kcal/mol, while the MPW1K method shows large errors for reaction energetics. The MP2 method gives the expected systematic underestimation of barriers. Concerted and nearly synchronous transition structures are predicted by all DFT and MP2 methods. Also reported are revised estimated 0 K experimental activation enthalpies for a standard set of hydrocarbon pericyclic reactions and updated comparisons to experiment for DFT, ab initio, and multicomponent methods. B3LYP and MPW1K methods with MAD values of 1.5 and 2.1 kcal/mol, respectively, fortuitously outperform the multicomponent CBS-QB3 method, which has a MAD value of 2.3. The MAD value of the O3LYP functional improves to 2.4 kcal/mol from the previously reported 3.0 kcal/mol.     

Heat of hydrogenation of 1,5-dehydroquadricyclane. A computational and experimental study of a highly pyramidalized alkene

The radical anion of the highly pyramidalized alkene 1,5-dehydroquadricyclane (1) was generated in the gas phase from the Squires reaction of 1,5-bis(trimethylsilyl)quadricyclane with F-/F2. The electron binding energy and proton affinity of 1*- were determined by bracketing experiments to be 0.6 +/- 0.1 eV and 386 +/- 5 kcal/mol, respectively. These values are in good agreement with values predicted by density functional theory (B3LYP/6-31+G*) and ab initio (CASPT2/6-31+G*) calculations. The experimental heat of hydrogenation of 1, obtained from a thermochemical cycle, was found to be 91 +/- 9 kcal/mol. This value of deltaH(H2) leads to values of 67 +/- 9 kcal/mol for the olefin strain energy (OSE) of 1, 172 +/- 9 kcal/mol for its heat of formation, and 23 +/- 9 kcal/mol for its pi bond dissociation enthalpy. Since the retro-Diels-Alder reaction of neutral 1 is computed to be highly exothermic, the finding that 1*- apparently does not undergo a retro-Diels-Alder reaction is of particular interest. The B3LYP/6-31+G* optimized geometry of 1 suggests that the bonding in this alkene is partially delocalized, presumably because the highly pyramidalized double bond in 1 interacts with the distal cyclopropane bonds in a manner that eventually leads to a retro-Diels-Alder reaction. The good agreement of the B3LYP and (2/2)CASPT2 values for the heat of hydrogenation and OSE of 1 with the experimentally derived values provides indirect evidence for the correctness of the B3LYP prediction that the equilibrium geometry of 1 lies part way along the reaction coordinate to the transition structure for the retro-Diels-Alder reaction.     

Substituent Effects on Site Selectivity (C=C vs C=N) in Heterocumulene-Heterodiene [4 + 2] Cycloadditions: Density Functional and Semiempirical AM1 Molecular Orbital Calculations

The effect of substituents on the site selectivity (C=C vs C=N) in the [4 + 2] cycloaddition between heterocumulenes (ketene imines) 2a-g with heterodienes (acroleines 9a-n and 4-acylfuran-2,3-diones 1a-d) is treated by semiempirical AM1 molecular orbital and density functional calculations using Becke's three-parameter hybrid method (B3LYP/6-31G). For some reactions calculations were also done at the B3LYP/6-31+G level of theory. For reaction of the oxa 1,3-dienes with ketene imines unsubstituted at the terminal carbon invariably addition across the C=C heterocumulene double bond has a lower activation energy than addition across the C=N double bond. Substitution of methyl or especially phenyl groups at the ketene imine C-terminus leads to a reversal of the respective activation energies. Incorporation of the oxa 1,3-diene system into the heterocyclic dione 1 substantially enhances the reactivity ( approximately 10 kcal mol(-1) lower activation energies) as compared to similarly substituted acroleins. At the DFT level of theory all reactions are found to proceed via a concerted asynchronous mechanism.     

Molecule-induced alkane homolysis with dioxiranes.

The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2.     

Novel substituent effects on the mechanism of the thermal denitrogenation of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives, stepwise versus concerted

The substituent effect on the thermal denitrogenation mechanism of 7,7-disubstituted 2,3-diazabicyclo[2.2.1]hept-2-enes, concerted versus stepwise, has been investigated in detail. Unrestricted DFT calculations at the B3LYP/6-31G(d) level of theory suggest that azoalkanes that possess electron-withdrawing substituents at C(7) prefer to expel the nitrogen molecule in a stepwise manner. The activation energy is calculated to be ca. 36 kcal/mol for the dihydroxy-substituted azoalkane. In contrast, the preferred mechanism of the concerted denitrogenation is predicted for azoalkanes that possess electron-donating substituents at C(7). The activation energy is computed to be ca. 28 kcal/mol for the silyl-substituted azoalkane. The theoretical prediction of the substituent effects on the mechanistic change is supported by analyzing the activation parameters of the azoalkane decompositions. The activation enthalpy for the decomposition of the 7,7-diethoxy-substituted azoalkane is determined to be 39.1 kcal/mol, which is 13.1 kcal/mol higher in energy for the denitrogenation of the 7-silyl-substituted azoalkane. These dramatic substituent effects can be reasonably explained by the preferred electronic configuration of the lowest singlet state of the cyclopentane-1,3-diyls produced during the denitrogenation of the azoalkanes.     

C59XH(X=N,B)与1,3-丁二烯Diels-Alder环加成反应的区域选择性

应用半经验的AM1和密度泛函B3LYP/6-31G*方法对1,3-丁二烯与C59XH(X=N, B) Diels-Alder环加成反应的区域选择性进行理论研究, 选择一些有代表性的C59XH(X=N, B)的6—6键探讨环加成反应的机理. 1,3-丁二烯与C59NH进行的Diels-Alder反应, 随着加成位置远离C59NH的N原子, 活化能越来越低, 但都比1,3-丁二烯与C60相应反应的活化能高. 与此相反, 对于1,3-丁二烯与C59BH进行的环加成反应, 加成位置最靠近B原子的2,12/r-和2,12/f-过渡态的势垒最低, 并且比1,3-丁二烯与C60进行环加成反应的活化能约低18 kJ·mol-1, 其产物也是热力学最稳定的. 与C60相应的反应相比, C59NH和C59BH中N和B原子不同的电子性质对其邻位双键进行Diels-Alder环加成反应的活性产生了不同影响, 前者使反应活性降低, 后者使反应活性增强.     

Dichlorocarbene addition to cyclopropenes: a computational study

Reaction paths for addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were calculated using hybrid density functional theory (B3LYP/6-31G) in the gas phase and in the presence of a continuum solvation model corresponding to acetonitrile. In both the gas phase and acetonitrile, :CCl2-cyclopropene addition follows an asymmetric, non-least-motion approach. Barriers to addition range from 0 to 2 kcal/mol. The reactions proceed in concerted fashion in both the gas phase and solution to yield 1,3-dienes or bicyclobutanes. The reaction pathway on this complex potential energy surface of this reaction appears to bifurcate, and the product distribution is believed to be controlled by reaction dynamics. At the present level of theory, there appears to be no minimum on the potential energy surface corresponding to a dipolar intermediate.     

A theoretical study of the mechanisms and regiochemistry of the reactions of 5-alkoxyoxazole with thioaldehydes,nitroso compounds,and aldehydes

A theoretical study based on B3LYP/6-31G calculations has been applied to the mechanisms and regiochemistry of reactions of 5-alkoxyoxazole with thioaldehydes, nitroso compounds, and aldehydes. All three reactions adopt similar mechanisms, which start with Diels-Alder (DA) reactions, followed by either a novel, concerted ring-opening-ring-closing (RORC) step to transfer the DA adduct to 2-alkoxycarbonyl-3-thiazoline and 2-alkoxycarbonyl-3-oxazoline for thioaldehydes and aldehydes, respectively, or stepwise ring-opening and ring-closing steps to generate 1,2,4-oxadiazoline for nitroso compounds. The reactions of 5-alkoxyoxazole with thioaldehydes and nitroso compounds can be conducted under thermal reaction conditions due to the 10 kcal/mol activation barriers for their rate-determining DA reactions. By contrast, the reaction of 5-alkoxyoxazole with aldehydes cannot take place under thermal conditions, since this bimolecular reaction has the rate-determining RORC transition state higher than the reactants by 30.5 kcal/mol.     

C59XH（X=N,B）与1,3-丁二烯Diels-Alder环加成反应的区域选择性

应用半经验的AMI和密度泛函B3LYP/6-31G*方法对1,3-丁二烯与C59XH(X=N,B)Diels-Alder环加成反应的区域选择性进行理论研究,选择一些有代表性的C59XH(X=N,B)的6-16键探讨环加成反应的机理.1,3-丁二烯与C59NH进行的Diels-Alder反应,随着加成位置远离C59NH的N原子,活化能越来越低,但都比1,3-丁二烯与C60相应反应的活化能高.与此相反,对于1,3-丁二烯与C59BH进行的环加成反应.加成位置最靠近B原子的2,12/r-和2,12/f-过渡态的势垒最低,并且比1,3-丁二烯与C60进行环加成反应的活化能约低18 kJ·mol-1,其产物也是热力学最稳定的.与C60相应的反应相比,C59NH和C59BH中N和B原子不同的电子性质对其邻位双键进行Diels-Alder环加成反应的活性产生了不同影响,前者使反应活性降低,后者使反应活性增强.     

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.     

Computational methods to calculate accurate activation and reaction energies of 1,3-dipolar cycloadditions of 24 1,3-dipoles

Theoretical calculations were performed on the 1,3-dipolar cycloaddition reactions of 24 1,3-dipoles with ethylene and acetylene. The 24 1,3-dipoles are of the formula X≡Y(+)-Z(-) (where X is HC or N, Y is N, and Z is CH(2), NH, or O) or X═Y(+)-Z(-) (where X and Z are CH(2), NH, or O and Y is NH, O, or S). The high-accuracy G3B3 method was employed as the reference. CBS-QB3, CCSD(T)//B3LYP, SCS-MP2//B3LYP, B3LYP, M06-2X, and B97-D methods were benchmarked to assess their accuracies and to determine an accurate method that is practical for large systems. Several basis sets were also evaluated. Compared to the G3B3 method, CBS-QB3 and CCSD(T)/maug-cc-pV(T+d)Z//B3LYP methods give similar results for both activation and reaction enthalpies (mean average deviation, MAD, < 1.5 kcal/mol). SCS-MP2//B3LYP and M06-2X give small errors for the activation enthalpies (MAD < 1.5 kcal/mol), while B3LYP has MAD = 2.3 kcal/mol. SCS-MP2//B3LYP and B3LYP give the reasonable reaction enthalpies (MAD < 5.0 kcal/mol). The B3LYP functional also gives good results for most 1,3-dipoles (MAD = 1.9 kcal/mol for 17 common 1,3-dipoles), but the activation and reaction enthalpies for ozone and sulfur dioxide are difficult to calculate by any of the density functional methods.     

Effect of geminal substitution at silicon on 1-sila- and 1,3-disilacyclobutanes' strain energies, their 2+2 cycloreversion enthalpies, and Si=C pi-bond energies in silenes.

To evaluate the effect of geminal substitution at silicon on 1-sila- and 1,3-disilacyclobutanes' strain energies, their 2+2 cycloreversion enthalpies, and Si=C pi-bond energies in silenes, an ab initio MO study of silenes, R2Si=CH2 (1), 1-silacyclobutanes, cyclo-R2Si(CH2)3 (2), and 1,3-disilacyclobutanes, cyclo-(R2SiCH2)2 (3), was performed using the level of theory denoted MP4/TZ(d)//MP2/6-31G(d) (TZ means the 6-311G(d) basis set for elements of the second period and hydrogen, and the McLean-Chandler (12s,9p)/[6s,5p](d) basis set for the third period elements). In the series R = H, CH3, SiH3, CH3O, NH2, Cl, F, the growth of the reaction enthalpies and strain energies is proportional to the substituents' electronegativities. 2+2 cycloreversion of 2 is endothermic by 40.6-63.1 kcal/mol, whereas that of 3 is endothermic by 72.7-114.2 kcal/mol. On going from a silicon to a fluorine substituent at the sp2-hybridized silicon atom, the pi-bond energy in 1 weakens by 11.3 kcal/mol, and the Si=C bond length shortens by 0.053 A. The effect of substituents' electronegativities at the double-bonded silicon atom in silenes is formulated as follows: the higher electronegativity, the shorter and the weaker the Si=C pi-bond. The latter is rationalized in terms of more strained geometry resulting from the energetic cost for planarizing the R2SiC moiety. The enthalpies of the ring-opening reaction are 68.0-80.1 kcal/mol (a cleavage of the Si-C bond in 3), 65.0-76.4 kcal/mol (a cleavage of the Si-C bond in 2), and 58.0-64.9 kcal/mol (a cleavage of the C-C bond in 2). The pronounced difference in the enthalpies of 2+2 cycloreversion of 1-sila- and 1,3-disilacyclobutanes is mainly due to the difference in the enthalpies of diradicals' decomposition. The decomposition of diradicals resulting from a cleavage of C-C and Si-C bonds in 2 is exothermic by 24.3-3.3 kcal/mol (apart from the difluoro derivative which is endothermic by 5.1 kcal/mol) and 27.0-13.3 kcal/mol, respectively. The decomposition of a 1,4-diradical resulting from ring opening of 3, apart from the disilyl derivative, is the endothermic process for which the enthalpy varies from 10.6 to 40.4 kcal/mol.     

Origin of the stereo- and regioselectivities in the Diels-Alder reactions of azaphospholes: a DFT investigation

Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol⁻¹, are much smaller than the value (32.57 kcal mol⁻¹) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.     

Understanding the mechanism of non-polar Diels-Alder reactions. A comparative ELF analysis of concerted and stepwise diradical mechanisms

The electron-reorganization along the concerted and stepwise pathways associated with the non-polar Diels-Alder reaction between cyclopentadiene (Cp, 1) and ethylene (2) has been studied using the topological analysis of the electron localization function (ELF) at the B3LYP/6-31G(d) level of theory. ELF results for the concerted mechanism stresses that the electron-reorganization demanded on the diene and ethylene reagents to reach two pseudo-diradical structures is responsible for the high activation energy. A comparative ELF analysis of some relevant points of the non-polar Diels-Alder reaction between Cp and styrene (10) suggests that these concerted mechanisms do not have a pericyclic electron-reorganization.     

Concerted vs stepwise mechanisms in dehydro-Diels-Alder reactions

The Diels-Alder reaction is not limited to 1,3-dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro"-Diels-Alder cycloadditions are one class of dehydropericyclic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-1,3-cyclobutadiene. This potential energy surface appears to be flat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cycloreversions will proceed by a stepwise mechanism.