Conrotatory Pathways Research Articles

Thermal isomerization of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene through the (E,Z)-1,3-cycloheptadiene intermediate.

The thermal isomerization of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene was studied using ab initio methods at the multiconfiguration self-consistent field level. The lowest-energy pathway for thermolysis of both structures proceeds through the (E,Z)-1,3-cycloheptadiene intermediate. Ten transition states were located, which connect these three structures to the final product, (Z,Z)-1,3-cycloheptadiene. Three reaction channels were investigated, which included the conrotatory and disrotatory ring opening of tricyclo[4.1.0.0(2,7)]heptane and bicyclo[3.2.0]hept-6-ene and trans double bond rotation of (E,Z)-1,3-cycloheptadiene. The activation barrier for the conrotatory ring opening of tricyclo[4.1.0.0(2,7)]heptane to (E,Z)-1,3-cycloheptadiene was found to be 40 kcal mol(-1), while the disrotatory pathway to (Z,Z)-1,3-cyclohetpadiene was calculated to be 55 kcal mol(-1). The thermolysis of bicyclo[3.2.0]hept-6-ene via a conrotatory pathway to (E,Z)-1,3-cycloheptadiene had a 35 kcal mol(-1) barrier, while the disrotatory pathway to (Z,Z)-1,3-cyclohetpadiene had a barrier of 48 kcal mol(-1). The barrier for the isomerization of (E,Z)-1,3-cycloheptadiene to bicyclo[3.2.0]hept-6-ene was found to be 12 kcal mol(-1), while that directly to (Z,Z)-1,3-cycloheptadiene was 20 kcal mol(-1).

Biasing a transition state search to locate multiple reaction pathways

A variety of chemical systems exhibit multiple reaction pathways that adjoin to a common reactant state. In fact, any reaction producing side products or proceeding via a stable intermediate involves a species possessing at least two reaction pathways. Despite improvements in ab initio transition-state search algorithms it remains difficult to detect multiple reaction pathways. Typically, multiple reaction pathways can only be detected by intuitively varying the initial point in the transition-state search trajectory. This reliance on intuition limits the ability to discover new and unexpected chemistry using ab initio methods. This paper proposes a systematic and intuition-free method for biasing a transition-state search to identify multiple reaction pathways originating from a common reactant state. The method allows the successive location of transition states, with each successful search contributing to a cumulative bias potential for the following search. The method is applicable to all psuedo-Newton–Raphson-type transition-state searches. The procedure was tested for a model potential energy surface and for the thermal rearrangement of trans-1,4-dimethylcyclobutene. In the latter case, four reaction pathways were discovered: two exothermic conrotatory ring openings leading to hexadienes, an endothermic methyl migration pathway leading to a carbene, and an exothermic rearrangement leading to 3-methyl-1,4-pentadiene. In accordance with experiment, the calculations predict that the conrotatory pathway leading to trans,trans-2,4-hexadiene is the kinetically dominant pathway. The methodology was also used to compute selectivities for competitive pathways producing trans and cis triflouropentadiene products in the thermal rearrangement of 3-triflouromethyl-cyclobutene. Again, results were in accord with experimental observations.

Ring-opening of the cyclopropyl radical in the condensed phase: A combined density functional theory/molecular mechanics quasiclassical trajectory study

A combined density functional theory/molecular mechanics quasiclassical trajectory study has been performed to investigate the ring-opening stereochemistry and dynamics of the cyclopropyl radical in both argon and helium condensed phase environments. In a previous theoretical study it was found that the ring-opening of an isolated cyclopropyl radical can occur through both disrotatory and conrotatory pathways. In addition, it was discovered that subsequent rotations of the methylene groups can take place following formation of the allyl radical, altering the reaction stereochemistry. Only at high densities does the environment significantly affect the initial reaction stereochemistry, but does reduce the number of internal rotations in the allyl radical, regardless of solvent density. Analysis of the internal energy and center of mass motion of the solute radical indicate more extensive collisional deactivation in the lower mass He bath, resulting in approximately a 10% increase in the number of disrotatory reactions. This study suggests that the initial stereochemistry of the ring-opening of the cyclopropyl radical is unaffected by collisions with the surrounding gas environment at low inert gas densities.

An ab initio study on the structure and reactivity of 1,4-disilabenzene

Possible reaction pathways for the conversion of Dewar-type 1,4-disilabenzene ( 1) to 1,4-disilabenzene ( 2) are discussed by means of B3LYP/6-31G** density functional theory and multiconfigurational CASSCF(6,6)/6-31G** calculations. The activation energy for the symmetry-allowed conrotatory ring opening of 1 is 34.6 kcal mol −1 at the B3LYP level and 47.7 kcal mol −1 at the CASSCF level, and that for the symmetry-forbidden disrotatory ring opening is 36.6 kcal mol −1 and 54.3 kcal mol −1 at the same levels. In both calculations the conrotatory ring opening of 1 proceeds through a potential energy barrier that is 2.0–6.6 kcal mol −1 lower than that of the disrotatory ring opening. Thus, we conclude that the conrotatory pathway is energetically more preferred than the disrotatory pathway. The Diels–Alder reaction of 1,4-disilabenzene and acetylene is also discussed. The activation energy for the Diels–Alder addition is computed to be 4.2 kcal mol −1 at the B3LYP/6-31G** level. Thus, 1,4-disilabenzene is highly reactive, and once 1,4-disilabenzenes are produced, the Diels–Alder additions of 1,4-disilabenzenes with many kinds of dienophiles should readily take place.

Disrotatory versus conrotatory electrocyclic ring opening of Dewar benzene: the conrotatory pathway is preferred and does not involve trans-benzene

For the electrocyclic ring opening of Dewar benzene ( 2) into benzene ( 1), both a disrotatory and conrotatory pathway with distinct transition states, TS1 and TS2, respectively, were found at the CASSCF(10,10)/6-311G** level of theory. The importance of the CASSCF(10,10) active space for the proper description of TS1 and TS2 was illustrated by similar calculations using a smaller active space, viz. CASSCF(2,2), CASSCF(4,4) and CASSCF(6,6). Although TS2 represents a true saddle point, TS1 appears to be a higher order saddle point at these levels of theory. Single-point multi-reference SDCI (MRCI) calculations at the CASSCF(10,10)/6-311G** geometries and natural orbitals were performed at all stationary points to obtain more reliable total energies. In contrast to common belief, TS2 lies below TS1 (by 6.62 kcal/mol), i.e. the conrotatory process is favored. Moreover, CASSCF(10,10)/6-311G** Intrinsic Reaction Coordinate (IRC) calculations show that upon conrotatory electrocyclic ring opening 2 does not give the extraordinarily strained trans-benzene ( 3), i.e. cis, cis, trans-cyclohexa-1,3,5-triene. The conversion of 3 into 1 and vice versa is a distinct process on the C 6H 6 potential energy surface.

Theoretical Studies of the Electrocyclic Reaction Mechanisms of Hexatriene to Cyclohexadiene

The potential energy surfaces for electrocyclic reactions of hexa-1,3,5-triene were calculated by ab initio molecular orbital methods. The transition states of two electrocyclic reaction pathways (conrotatory and disrotatory) for hexa-1,3,5-triene were obtained. Both transition states are a true transition state. The transition state of the disrotatory pathway is about 11 kcal/mol lower in energy than that of the conrotatory pathway. The reaction path through cis−trans isomerization from cZc-hexa-1,3,5-triene to cyclohexa-1,3-diene was proposed. The conrotatory and the disrotatory reaction pathways were also characterized by the CiLC-IRC method.

Fluxionality and Isomerism of the Bis(dihydrogen) Complex RuH(2)(H(2))(2)(PCy(3))(2): INS, NMR, and Theoretical Studies.

To study the fluxionality of the bis(dihydrogen) complex RuH(2)(H(2))(2)(PCy(3))(2) (1), NMR spectra were recorded in Freons (mixture of CDCl(3), CDFCl(2), and CDF(2)Cl). 1 was found to remain fluxional at all temperatures, but the presence of CDCl(3) necessary for its solubilization induces its transformation into, first, RuHCl(H(2))(2)(PCy(3))(2) (3) and the new ruthenium(IV) dihydride RuH(2)Cl(2)(PCy(3))(2) (4). 4 is produced selectively in pure CDCl(3) but reacts further to give a mixture of chloro complexes. 4 was isolated from the reaction of 1 with aqueous HCl in Et(2)O and shows a fluxional process attributed to the interconversion between two symmetrical isomers. The activation parameters of this process were obtained by (1)H NMR line shape analysis, as well as those corresponding to the exchange between 3 and free dihydrogen. The fluxionality of the dihydrogen-hydride system is also evident at a much faster time scale than that of NMR studies in the inelastic neutron scattering observations of the rotation of the dihydrogen ligands. The geometries and relative energies of several isomers of complexes 1, 3, and 4 were studied using density functional theory (DFT) and MP2 methods, together with a few coupled-cluster (CCSD(T)) calculations. In contrast to what might have been expected, the two hydrides and the two H(2) units of 1 lie in the same plane, due to the attractive "cis effect" created by the hydrides. The two H(2) ligands adopt cis positions in the lowest-energy isomer. Rotation of the two dihydrogen ligands has been analyzed using DFT calculations. A slight preference for a C(2) conrotatory pathway has been found with a calculated barrier in good agreement with the experimental INS value. Two low-energy isomers of 4 have been characterized computationally, both of which have C(2)(v)() symmetry, consistent with the solution NMR spectra.

Quantum dynamics of the trimethylene biradical Stereomutation of cyclopropane and unimolecular decay

A reduced-dimensionality three degrees-of-freedom quantum dynamics calculation of the stereomutation of cyclopropane has been performed. In one set of calculations, wavepackets are initiated centered around stationary points on the conrotatory, disrotatory and single rotation (cis/trans conversion) pathways. The ratio of trans to cis isomer formed, Rtc, is shown to be highly dependent on the location, and average energy of the initial wavepacket as well as the initial orientation of the terminal methylene torsions. In a second set of calculations, wavepackets are initiated as ‘highly excited cyclopropane’, in one of the four possible isomeric configurations, designated as a trans configuration for the purposes of analysis. Here also, Rtc depends on the initial torsional orientation. Analysis of wavepacket density and the time constants for formation of product density indicate that that the contributing mechanisms range from direct, concerted conrotatorymotion to more statistical behavior. A ‘Boltzmann’ average at 695 K of theratio of ‘double’ to ‘single’ rotations, k12/k1, results in k12/k1=2Rtc=2.2. Wavepacket density is shown to orient preferentially along the conrotatory and disrotatory pathways rather than the higher energy single rotation(cis/trans conversion) pathway.

Conrotatory and disrotatory pathways in the electrocyclic isomerization of cyclobutene to cis-butadiene: The spin-coupled viewpoint

Spin-coupled theory is used to study the changes which occur in the electronic wave function as a system moves along the intrinsic reaction coordinate (IRC) for the case of conrotatory and disrotatory motion in the ground-state electrocyclic interconversion of cyclobutene to cis-butadiene. It is shown that the reaction is already well under way in the conrotatory case at the transition point (considerable cis-butadiene character), but the system still retains an overwhelming reactant (cyclobutene) character for the disrotatory motion. In both the conrotatory and disrotatory cases, the sharp changes in the weights of the two spin couplings, associated with the onset of bond breaking or bond formation, are shown to occur at points along the IRC well after the transition point.

Enhanced Conjugation in 1,4-Digermabutadiene

Due to the different strengths of the GeC and CC π bonds, 1,4-digermabutadiene H2GeHCCHGeH2 is expected to exhibit stronger conjugation through its central C−C bond than does butadiene. Ab initio calculations are used to investigate this effect. Like butadiene, 1,4-digermabutadiene has a preferred planar s-trans form, with two equivalent gauche conformers lying 3 kcal/mol higher in energy. As in butadiene, the s-cis planar form is a saddle point relating the two gauche forms, corresponding to a barrier of 1 kcal/mol. The perpendicular form associated with the rotational barrier around the central C−C bond is found to be significantly higher in the 1,4-digerma derivative than in butadiene (10 vs 6 kcal/mol). Other indicators, based on geometry or energy criteria, actually suggest that π-conjugation in 1,4-digermabutadiene is about one-and-a-half that in butadiene. The ensuing enhanced weight of valence-bond forms reflecting electron arrangements like •H2GeHC=CHGeH2• is expected to favor 1,4-couplings, such as the intramolecular cyclization into 3,4-digermacyclobutene. In its conrotatory thermally-allowed pathway, this rearrangement is found to require a weak activation barrier of about 1 kcal/mol above the starting gauche form. The strong exothermicity of such an internal ring closure, calculated at 44 kcal/mol from the gauche conformer, is due to bond energies and geometrical features in the final four-membered frame, in contrast with the endothermic butadiene-to-cyclobutene rearrangement. The 1,4-diradical character of 1,4-digermabutadiene is further attested by a weak torsional barrier around the GeC bond, estimated at less than 5 kcal/mol, and a low-lying triplet state, located at only 6 kcal/mol above the closed-shell singlet ground state.

A CASSCF study of photochemical cyclization of the first excited à 2B 1 state of the allyl radical

The photochemical cyclization of the first excited state of the allyl radical to the ground state of cyclopropyl radical has been studied by CASSCF calculations. The equilibrium geometries of the ground and the first excited states of the allyl radical were calculated in the C 2 v point group. Vibrational analyses of the allyl radical at its optimized geometry resulted in imaginary frequencies for the out-of plane modes in the excited state. Total energies along the CH 2 torsion and CCC bending coordinates were calculated in both conrotatory and disrotatory fashions. No barrier was found on the conrotatory pathway, and the barrier on the disrotatory pathway was estimated to be less than 0.065 eV, indicating that the photocyclization would be feasible.

Mechanistic study of the electrocyclic ring-opening reaction of thiirane

The electrocyclic ring opening of the C−C bond in thiirane is investigated in detail within the context of ab initio molecular orbital theory. The conrotatory and disrotatory reaction pathways are studied with three different basis sets ranging from double-ξ plus polarization (DZP) through triple-ξ plus double polarization (TZ2P) in conjunction with self-consistent-field (SCF), two-configuration SCF (TCSCF), and configuration interaction with single and double excitations (CISD) levels of theory

Cyclobutene photochemistry. Nonstereospecific photochemical ring opening of simple cyclobutenes

The photochemistry of bicyclo(3.2.0)hept-6-ene, bicyclo(4.2.0)oct-7-ene, and cis- and trans-3,4-dimethylcyclobutene has been investigated in hydrocarbon solution with monochromatic far-ultraviolet (185 and 193 nm) light sources. All of these simple cyclobutene derivatives undergo ring opening to yield the isomeric 1,3-dienes, and the latter three open nonstereospecifically to yield mixtures of the possible geometric isomers. The isomeric 3,4-dimethylcyclobutenes yield different mixtures of the three 2,4-hexadiene isomers, and in each case the mixtures are weighted in favor of the orbital symmetry forbidden isomer(s). Attempts have been made to analyze the relative isomeric diene yields from ring opening of bicyclo(4.2.0)octene and the isomeric 3,4-dimethylcyclobutenes within the context of the purely disrotatory, adiabatic ring-opening mechanism that recent ab initio calculations suggest should be possible. While the results for the former compounds are consistent with this mechanism, analyses of the relative yields of the isomeric 2,4-hexadienes from photolysis of the latter two compounds indicates that photochemical ring opening by the formally forbidden, conrotatory pathway may compete to some extent with disrotatory ring opening.

Orbital topology. III. Orbital mapping of unsymmetrical molecules. A survey of the thermal ring opening of isoelectronically substituted cyclobutenes and benzocyclobutenes

AbstractOrbital mapping analysis based on CNDO/2 molecular orbitals has been used to survey the thermal ring‐opening isomerizations of cyclobutenes and benzocyclobutenes. Isoelectronic substitutions within the molecular framework of cyclobutene (e.g., CH2 replaced by CH−, OH+, NH, NH2+) result in ground‐state orbital correlations via both conrotatory and disrotatory pathways in several cases, in contrast to the parent hydrocarbon conrotatory stereochemistry. The results substantiate the heteroatom effects previously revealed by orbital mapping for the disrotatory thermal isomerizations of isoelectronic Dewar benzenes. Qualitative patterns, such as nodal shifts in the butadiene π orbital, are discussed in relation to the mapping correlations. The isoelectronic benzocyclobutenes give ground‐state orbital correlations via conrotatory pathways only, which suggests that delocalization may reduce the heteroatom perturbation.