Vinylcyclobutane-cyclohexene rearrangement: theoretical exploration of mechanism and relationship to the Diels-Alder potential surface

[graph: see text] The vinylcyclobutane-cyclohexene rearrangement has been studied computationally with density functional theory and complete active space SCF calculations. The rearrangement proceeds through a diradical that exists on a very flat potential energy surface. Transition structures for conformational processes, only slightly higher in energy than the minimum energy reaction path, account for the stereochemistries of products observed in the thermal rearrangements of vinylcyclobutane derivatives. The connection of this rearrangement to the Diels-Alder reaction of butadiene with ethylene is discussed.     

Dynamics of the degenerate rearrangement of bicyclo[3.1.0]hex-2-ene

Quasiclassical direct dynamics simulations are applied to a 4-fold degenerate rearrangement which yields a nonstatistical product distribution. The simulated product ratio agrees with experiment and is found to be entirely dynamically determined. Trajectory lifetimes are on the order of a low-frequency vibrational period. The interaction of reaction momentum with the geometric features of the potential surface produces selectivity despite a common energy barrier. A geometric model is described for qualitatively estimating much of the dynamically determined product ratio independently of trajectory calculations. The characteristics of this reaction are expected also to apply to others involving modestly stabilized diradical intermediates.     

Mapping the triplet potential energy surface of 1-methyl-8-nitronaphthalene

Spin-unrestricted calculations and time-dependent DFT were used to characterize structure and reactivity of 1-methyl-8-nitronaphthalene (1) in the triplet state. Four hybrid models (B3LYP, PBE0, MPW1K, BHLYP) with significantly different amount of the exact exchange were employed. The triplet potential energy surface of 1 was mapped by using the UB3LYP and UMPW1K techniques. Both hybrid models provided qualitatively consistent pictures for the potential energy landscape. Thirty-one stationary points, of which 15 were minima, were found at the UB3LYP level of theory. Three minima corresponding to the nitro form of 1 were located on the triplet surface; just one was found for the singlet ground state. Two reaction paths leading from 1 either to a nitrite-type intermediate (2) or to the aci-form (3) were characterized. For both paths, reaction products were of diradical nature. The lower activation energy was obtained for the triplet-state tautomerization affording 3. The ground state of triplet multiplicity was predicted for two isomers of the aci-form. The triplet diradical 3 is expected to react through the thermal population of a close-lying singlet excited state. The results are discussed in relation to mechanisms of photoinduced rearrangements of peri-substituted nitronaphthalenes that can be used to develop novel photolabile protecting groups.     

Tuning the Bergman cyclization by introduction of metal fragments at various positions of the enediyne. Metalla-Bergman cyclizations

We expand the scope of the Bergman cyclization by exploring computationally the rearrangement of two osmaenediynes and one rhodaenediyne. The three hypothetical metallaenediynes are constructed by substituting the 14-electron Os(PH3)3 fragment for the C fragment, or the 15-electron Os(PH3)3H or Rh(PH3)3 fragments for the sp2 CH fragment, of 3-ene-1,5-diyne. This replacement is guided by the isolobal analogy and previous metallabenzene chemistry. The rearrangement of osmaenediyne with an Os(PH3)3 fragment in place of C is exothermic by 3 kcal/mol (the parent Bergman reaction is computed to be endothermic by 5 kcal/mol) and associated with a significant decrease in the barrier to rearrangement to 13 kcal/mol (the Ea of the parent reaction computed at the same level of theory is 33 kcal/mol). The replacement of a CH by the isolobal analogue Os(PH3)3H reduces the energy of activation for the rearrangement to 23 kcal/mol and produces a corresponding metalladiradical that is 8 kcal/mol less stable that the corresponding osmaenediyne. The activation energy corresponding to the rearrangement of the rhodaenediyne is the same as that of the organic parent enediyne. Interesting polytopal rearrangements of metallaenediynes and the diradical nature of the resulting intermediates are also explored.     

1-acyl-2-cyclopentenes and 5-acylbicyclo[2.1.0]pentanes: photochemical and thermal isomerizations

The photochemistry of 1-acyl-2-cyclopentenes varies with the nature of the acyl group. On direct irradiation the aldehyde eliminates carbon monoxide in the singlet excited state, and the aroyl compounds cleave to allyl-aroyl radical pairs both from the singlet and triplet states. In competition to α-cleavage the methyl ketones isomerize in an allylic 1,3-acetyl shift. The lowest-lying reactive triplet of these methyl ketones, characterized as a ³(π, π*) state in the case of the 3-phenyl homologue, undergo oxadi-π-methane rearrangement to a mixture of endo- and exo-5-acetylbicyclo[2.1.0]pentanes.The ground state-acetylbicyclopentanes react in two ways at elevated temperatures: endo-exo stereomutation by selective cleavage of the central cyclopropane bond and reclosure of the 1,3-cyclopentane diradical intermediate, and a rearrangement of the endo isomer to 1-acetyl-2-cyclopentenes on a separate potential energy surface involving a formal 1,2-acetyl shift. The unusually large negative entropy of activation for this latter reaction is suggestive of a concerted electrocyclic process in which the electrons of the internal cyclopropane and the C(5)-acetyl bonds participate.     

Sulfur Monoxide Release and Capture: A Computational Study

Thermolysis of thiirane oxide leads to production of highly reactive sulfur monoxide. The liberated SO can in turn be trapped with a diene scavenger forming dihydrothiophene oxide. Since the intermediate diatomic possesses a triplet ground state, the SO transfer can proceed on two spin-state surfaces. Here, we study the competition between singlet concerted and stepwise triplet diradical mechanisms utilizing the M06-2X density functional as well as CCSD(T) and MRCI+Q wavefunction theories. We find that the decomposition of thiirane oxide prefers to pass through a triplet diradical intermediate that becomes accessible from a nearby minimum energy crossing point (MECP). Hence, the thermolysis of thiirane oxide is expected to predominantly release triplet ground state sulfur monoxide in agreement with previous experimental reports. The addition of ³SO to 1,3-butadiene initially generates an allylic diradical, that ring-closes to a thiirane oxide through another MECP, and a subsequent rearrangement gives access to the final product.     

Femtosecond Dynamics of Norrish Type-II Reactions: Nonconcerted Hydrogen-Transfer and Diradical Intermediacy

Norrish type-II and McLafferty rearrangements, which both involve an intramolecular transfer of a gamma H atom, can be differentiated on the femtosecond time scale. The McLafferty rearrangement results in ion fragmentation of the parent ketone, whereas the Norrish type-II reaction leads to a diradical species, which then either cyclizes or fragments (see scheme). For Norrish type-II reactions, the reaction time for the transfer of the hydrogen atom is within 70 - 90 fs, and the lifetime of the diradical intermediate is in the range of 400 - 700 ps at the total energy studied.     

Theoretical Studies on the Mechanism of Photocycloaddition Reaction Between 6 Azauracil and Acetone

The mechanism of photocycloaddition reaction between 6-azauracll and acetone was studied by using semiemptrical SCFMO AMI method. It was found that this reaction is not a concerted one. The calculated results are as follows:(1) A T1 state exciplex is on the T1 state energy surface; (2) T exciplex as a reactant will proceed along the energy surface of T1 state to form a diradical intermediate. The energy barrier of this reaction step is 63. 6 kJ/mol; (3) The T1 state diradical intermediate happens to be close in energy to the ground state intermediate with a similar geometry. Such a situation turns out to be very favorable for an intersystem crossing (jump from the T, state to the ground state) ; (4) The final product will be formed from the ground S0 state intermediate via an energy barrier 88. 2 kJ/mol.     

Ab initio computational studies of conformationally restricted cope rearrangements. First examples of fully concerted allenyl cope rearrangements

Results of (8,8)CASPT2/6-31G//(8,8)CASSCF/6-31G level calculations on the potential surface for the conformationally restricted allenyl Cope rearrangements of syn-5-propadienylbicylco[2.1.0]pent-2-ene (14) and syn-6-propadienylbicyclo[2.1.1]hex-2-ene (15) are reported. Both are found to proceed through concerted pathways. Also included are the results of (6,6)CASPT2/6-31G//(6,6)CASSCF/6-31G level calculations on the Cope rearrangements of syn-5-ethenylbicyclo[2.1. 0]pent-2-ene (18), syn-6-ethenylbicyclo[2.1.1]hex-2-ene (19), and syn-7-vinylnorborene (20), which are found to involve diallylic diradical intermediates 26, 30, and 36, respectively. Previous studies have shown that the allenyl Cope rearrangement of 1,2, 6-heptatriene (1) to 3-methylene-1,5-hexadiene (2) involves a single transition structure that either proceeds to the monoallylic cyclohexane-1,4-diyl derivative 3 or bypasses 3 to form 2 directly. (4) More recently, the conformationally restricted allenyl Cope rearrangement of syn-7-allenylnorbornene (7) has also been found to involve tricyclic monoallylic cyclohexane-1,4-diyl intermediate 11. (7) The rearrangements of 14 and 15 appear to represent the first reported examples of fully concerted allenyl Cope rearrangements. Concertedness in these cases is ascribed to two parallel factors: (1) the relative instability of possible tricyclic diradical intermediates 16 and 17, compared to diradical intermediates 3 and 11 formed in the rearrangements of 1 and 7, respectively; and (2) the opportunity that exists to form sp-sp(2) sigma bonds in transition structures 21 and 23 that lead, respectively, to products 22 and 24. By contrast, only weaker sp(2)-sp(2) sigma bonds could form in unobserved concerted transition structures leading to products 28 and 32, formed in the nonconcerted rearrangements of 18 and 19.     

The 1deltag dioxygen ene reaction with propene: a density functional and multireference perturbation theory mechanistic study

This study aims to determine whether a balance between concerted and non-concerted pathways exists, and in particular to ascertain the possible role of diradical/zwitterion or peroxirane intermediates. Three non-concerted pathways, via 1) diradical or 2) peroxirane intermediates, and 3) by means of hydrogen-abstraction/radical recoupling, plus one concerted pathway (4), are explored. The intermediates and transition structures (TS) are optimized at the DFT(MPW1K), DFT(B3LYP) and CASSCF levels of theory. The latter optimizations are followed by multireference perturbative CASPT2 energy calculations. (1) The polar diradical forms from the separate reactants by surmounting a barrier (deltaE(++)(MPW1K)=12, deltaE++(B3LYP)=14, and deltaE(++)(CASPT2)=16 kcal mol(-1) and can back-dissociate through the same TS, with barriers of 11 (MPW1K) and 8 kcal mol(-1) (B3LYP and CASPT2). The diradical to hydroperoxide transformation is easy at all levels (deltaE(++)(MPW1K)<4, deltaE(++)(B3LYP)=1 and deltaE(++)(CASPT2)=1 kcal mol(-1)). (2) Peroxirane is attainable only by passing through the diradical intermediate, and not directly, due to the nature of the critical points involved. It is located higher in energy than the diradical by 12 kcal mol(-1), at all theory levels. The energy barrier for the diradical to cis-peroxirane transformation (deltaE(++)=14-16 kcal mol(-1)) is much higher than that for the diradical transformation to the hydroperoxide. In addition, peroxirane can very easily back-transform to the diradical (deltaE(++)<3 kcal mol(-1)). Not only the energetics, but also the qualitative features of the energy hypersurface, prevent a pathway connecting the peroxirane to the hydroperoxide at all levels of theory. (3) The last two-step pathway (hydrogen-abstraction by (1)O(2), followed by HOO-allyl radical coupling) is not competitive with the diradical mechanism. (4) A concerted pathway is carefully investigated, and deemed an artifact of restricted DFT calculations. Finally, the possible ene/[pi2+pi2] competition is discussed.     

Ring opening in methylenecyclopropane

The indo molecular orbital calculations on methylenecyclopropane show that the ¹A₁ state of the (90,90) configuration is the most stable one at θ = 64°. The ground state of the ring-opened trimethylenemethane is predicted to be the ³A'₂ state (D_3h symmetry), however the³A″ state of non-planar (0,90) trimethylenemethane is about 10 kcal/mole higher in energy than the planar diradical species. This suggests intermediacy of the orthogonal diradical in the degenerate thermal rearrangement of substituted methylenecyclopropane. The calculated geometry, dipole moment of methylenecyclopropane, and spin densities of trimethylenemethane are in excellent agreement with the experimental values.     

Theoretical Study on Photoisomerization of 2-Methylfuran to 3-Methylfuran

1 INTRODUCTION 2-Methylfuran belongs to the basic heteroaromatic compounds relevant to many fields of modern che- mistry, ranging from the study of natural products and biologically active substances to the develop- ment of building blocks for organic synthesis and conducting polymers[1]. Since the photochemistry ofR-furan was gradually recognized in 1960s[2～7], lots of interest has been aroused. Herein we only study one branch of photoche- mistry of R-furan: the isomerization of 2-methy…     

Effect of Lewis acid catalysts on Diels-Alder and hetero-Diels-Alder cycloadditions sharing a common transition state

The thermal and Lewis acid catalyzed cycloadditions of beta,gamma-unsaturated alpha-ketophosphonates and nitroalkenes with cyclopentadiene have been explored by using density functional theory (DFT) methods. In both cases, only a single highly asynchronous bis-pericyclic transition state yielding both Diels-Alder and hetero-Diels-Alder cycloadducts could be located. Stepwise pathways were found to be higher in energy. On the potential energy surface, the bis-pericyclic cycloaddition transition state is followed by the Claisen rearrangement transition state. No intermediates were located between these transition states. Claisen rearrangement transition states are also highly asynchronous, but bond lengths are skewed in the opposite direction compared to the bis-pericyclic transition states. The relative positions of the bis-pericyclic and Claisen rearrangement transition states may control periselectivity due to the shape of the potential energy surface and corresponding dynamical influences. Inspection of the thermal potential energy surface (PES) indicates that a majority of downhill paths after the bis-pericyclic transition state lead to the Diels-Alder cycloadducts, whereas a smaller number of downhill paths reach the hetero-Diels-Alder products with no intervening energy barrier. Lewis acid catalysts alter the shape of the surface by shifting the cycloaddition and the Claisen rearrangement transition states in opposite directions. This topographical change qualitatively affects the branching ratio after the bis-pericyclic transition state and ultimately reverses the periselectivity of the cycloaddition giving a preference for hetero-Diels-Alder cycloadducts.     

Thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one to the ketonic tautomer of phenol. Computational evidence for the formation of a diradical rather than a zwitterionic intermediate

The ground state (S(0)) and lowest-energy triplet state (T(1)) potential energy surfaces (PESs) concerning the thermal and photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-one (8) to the ketonic tautomer of phenol (11) have been extensively explored using ab initio CASSCF and CASPT2 calculations with several basis sets. State T(1) is predicted to be a triplet pipi lying 66.5 kcal/mol above the energy of the S(0) state. On the S(0) PES, the rearrangement of 8 to 11 is predicted to occur via a two-step mechanism where the internal cyclopropane C-C bond is broken first through a high energy transition structure (TS1-S(0)()), leading to a singlet intermediate (10-S(0)()) lying 25.0 kcal/mol above the ground state of 8. Subsequently, this intermediate undergoes a 1,2-hydrogen shift to yield 11 by surmounting an energy barrier of only 2.7 kcal/mol at 0 K. The rate-determining step of the global rearrangement is the opening of the three-membered ring in 8, which involves an energy barrier of 41.2 kcal/mol at 0 K. This high energy barrier is consistent with the fact that the thermal rearrangement of umbellulone to thymol is carried out by heating at 280 degrees C. Regarding the photochemical rearangement, our results suggest that the most efficient route from the T(1) state of 8 to ground state 11 is the essentially barrierless cleavage of the internal cyclopropane C-C bond followed by radiationless decay to the S(0) state PES via intersystem crossing (ISC) at a crossing point (S(0)()/T(1)()-1) located at almost the same geometry as TS1-S(0)(), leading to the formation of 10-S(0)() and the subsequent low-barrier 1,2-hydrogen shift. The computed small spin-orbit coupling between the T(1) and S(0) PESs at S(0)()/T(1)()-1 (1.2 cm(-)(1)) suggests that the ISC between these PESs is the rate-determining step of the photochemical rearrangement 8 --> 11. Finally, computational evidence indicates that singlet intermediate 10-S(0)() should not be drawn as a zwitterion, but rather as a diradical having a polarized C=O bond.     

Computational evidence for self-initiation in spontaneous high-temperature polymerization of methyl methacrylate

This paper presents computational evidence for the occurrence of diradical mechanism of self-initiation in thermal polymerization of methyl methacrylate. Two self-initiation mechanisms of interest were explored with first-principles density functional theory calculations. Singlet and triplet potential energy surfaces were constructed. The formation of two Diels-Alder adducts, cis- and trans-dimethyl 1,2-dimethylcyclobutane-1,2-dicarboxylate and dimethyl 2-methyl-5-methylidene-hexanedioate, on the singlet surface was identified. Transition states were calculated using B3LYP/6-31G* and assessed using MP2/6-31G*. The calculated energy barriers and rate constants with different levels of theory were found to show good agreement to corresponding data obtained from laboratory experiments. The presence of a diradical intermediate on the triplet surface was identified. When MCSCF/6-31G* was used, the spin-orbit coupling constant for the singlet to triplet crossover was calculated to be 2.5 cm(-1). The mechanism of monoradical generation via a hydrogen abstraction by both triplet and singlet diradicals from a third monomer was identified to be the most likely mechanism of initiation in spontaneous polymerization of methyl methacrylate.     

Detailed dynamics of the photodissociation of cyclobutane

Semiclassical electron-radiation-ion dynamics simulations are reported for the photodissociation of cyclobutane into two molecules of ethylene. The results clearly show the formation of the tetramethylene intermediate diradical, with dissociation completed in approximately 400 fs. In addition, the potential energy surfaces of the electronic ground state and lowest excited-state were calculated at the complete-active-space self-consistent-field/multireference second-order perturbation theory (CASSCF/MRPT2) level with 6-31G* basis sets, along the reaction path determined by the dynamics simulations. There are well-defined energy minima and maxima in the intermediate state region. It is found that both C-C-C bond bending and rotation of the molecule (around the central C-C bond) have important roles in determining the features of the potential energy surfaces for the intermediate species. Finally, the simulations and potential energy surface calculations are applied together in a discussion of the full mechanism for cyclobutane photodissociation.     

Model studies of hydrogen atom addition and abstraction processes involving ortho-, meta-, and para-benzynes.

H-atom addition and abstraction processes involving ortho-, meta-, and para-benzyne have been investigated by multiconfigurational self-consistent field methods. The H(A) + H(B)...H(C) reaction (where r(BC) is adjusted to mimic the appropriate singlet-triplet energy gap) is shown to effectively model H-atom addition to benzyne. The doublet multiconfiguration wave functions are shown to mix the "singlet" and "triplet" valence bond structures of H(B)...H(C) along the reaction coordinate; however, the extent of mixing is dependent on the singlet-triplet energy gap (DeltaE(ST)) of the H(B)...H(C) diradical. Early in the reaction, the ground-state wave function is essentially the "singlet" VB function, yet it gains significant "triplet" VB character along the reaction coordinate that allows H(A)-H(B) bond formation. Conversely, the wave function of the first excited state is predominantly the "triplet" VB configuration early in the reaction coordinate, but gains "singlet" VB character when the H-atom is close to a radical center. As a result, the potential energy surface (PES) for H-atom addition to triplet H(B)...H(C) diradical is repulsive! The H3 model predicts, in agreement with the actual calculations on benzyne, that the singlet diradical electrons are not coupled strongly enough to give rise to an activation barrier associated with C-H bond formation. Moreover, this model predicts that the PES for H-atom addition to triplet benzyne will be characterized by a repulsive curve early in the reaction coordinate, followed by a potential avoided crossing with the (pi)1(sigma*)1 state of the phenyl radical. In contrast to H-atom addition, large activation barriers characterize the abstraction process in both the singlet ground state and first triplet state. In the ground state, this barrier results from the weakly avoided crossing of the dominant VB configurations in the ground-state singlet (S0) and first excited singlet (S1) because of the large energy gap between S0 and S1 early in the reaction coordinate. Because the S1 state is best described as the combination of the triplet X-H bond and the triplet H(B)...H(C) spin couplings, the activation barrier along the S0 abstraction PES will have much less dependence on the DeltaE(ST) of H(B)...H(C) than previously speculated. For similar reasons, the T1 potential surface is quite comparable to the S0 PES.     

Thermal isomerizations of vinylcyclopropanes to cyclopentenes

The thermal isomerization of vinylcyclopropane to cyclopentene was discovered in 1960 and soon recognized as the simplest known example of a [1,3] sigmatropic shift. Experimental observations for the parent rearrangement and for isomerizations shown by substituted systems suggest that diradical transition structures are involved; recent theoretical treatments of the reaction find no minima corresponding to diradical intermediates. The common dichotomy opposing concerted versus diradical and thus necessarily stepwise mechanisms appears inappropriate. The reaction of vinylcyclopropane may involve four energetically concerted paths traversed by different conformational forms of nearly isoenergetic diradical species leading through four isometric diradical transition structures to cyclopentene. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 222–231, 1998