A pseudopericyclic [3,5]-sigmatropic rearrangement of a coumarin trichloroacetimidate derivative.

Thermolysis of a coumarin trichloroacetimidate yields a single rearrangement product. The proposed mechanism is a pseudopericyclic allowed (Woodward-Hoffman forbidden) [3,5]-sigmatropic rearrangement to form the corresponding amide followed by a sigmatropic [1,5]-hydrogen migration. Transition state calculations at the B3LYP/6-31G(d,p) level support this mechanism and suggest the selectivity is influenced by the stability of the intermediates.

A Computational Study on the Addition of HONO to Alkynes toward the Synthesis of Isoxazoles; a Bifurcation, Pseudopericyclic Pathways and a Barrierless Reaction on the Potential Energy Surface.

Homopropargyl alcohols react with t-BuONO to form acyloximes which can be oxidatively cyclized to yield ioxazoles. The mechanism for the initial reaction of HONO with alkynes to form acyloximes (e.g.

Competitive Pseudopericyclic [3,3]- and [3,5]-Sigmatropic Rearrangements of Trichloroacetimidates.

The Woodward-Hoffmann rules predict whether concerted pericyclic reactions are allowed or forbidden based on the number of electrons involved and whether the cyclic orbital overlap involves suprafacial or antarafacial orbital overlap. Pseudopericyclic reactions constitute a third class of reactions in which orthogonal orbitals make them orbital symmetry allowed, regardless of the number of electrons involved in the reaction. Based on the recent report of eight-centered ester rearrangements, it is predicted that the isoelectronic eight-centered rearrangements of imidates would also be allowed.

Experimental and computational studies on the [3,3]- and [3,5]-sigmatropic rearrangements of acetoxycyclohexadienones: a non-ionic mechanism for acyl migration.

Flash vacuum pyrolysis studies of substituted 6-acetoxy-2,4-cyclohexadienones (3 and 10) from 300 to 500 °C provide strong experimental evidence that direct [3,5]-sigmatropic rearrangements in these molecules are favored over the more familiar [3,3]-sigmatropic rearrangements. The preference holds when the results are extrapolated to 0.0% conversion, indicating that this is a concerted process.

Potential energy surface for (retro-)cyclopropanation: metathesis with a cationic gold complex.

The gas-phase cyclopropanation and apparent metathesis reactivity of ligand-supported gold arylidenes with electron-rich olefins is explained by quantum-chemical calculations. A deep potential minimum corresponding to a metal-bound cyclopropane adduct is in agreement with the measured absolute energies of the cyclopropanation and metathesis channels and is also consistent with previously reported electronic effects of arylidenes and supporting phosphorus ylid ligands on the product ratios. In the gas phase, the rate-determining step for the cyclopropanation is dissociation of the Lewis-acidic metal fragment, whereas the metathesis pathway features several rate-limiting transition states that are close in energy to the final product dissociation and hence contribute to the overall reaction rate.

Multiphoton infrared initiated thermal reactions of esters: pseudopericyclic eight-centered cis-elimination.

Multiphoton infrared absorption from a focused, pulsed CO(2) laser was used to initiate gas-phase thermal reactions of cis- and trans-3-penten-2-yl acetate. By varying the helium buffer gas pressure, it was possible to deduce the product distribution from the initial unimolecular reactions, separate from secondary reactions in a thermal cascade. Thus, trans-3-penten-2-yl acetate gives 54 +/- 5% of beta-elimination to give trans-1,3-pentadiene, 40 +/- 3% of [3,3]-sigmatropic rearrangement to give cis-3-penten-2-yl acetate and 6 +/- 4% of cis-1,3-pentadiene.

A computational study of the formation and dimerization of benzothiet-2-one.

A computational B3LYP/6-31G(d,p) study of the formation of benzothiet-2-one (4) from benzothiophenedione (2) and its subsequent dimerization to 5 was performed. The proposed intermediate ketene 3 has no gas-phase barrier to ring closure to 4. Three transition structures for dimerization were located.

Photochemical dehydration of acetamide in a cryogenic matrix.

Vacuum ultraviolet (VUV) irradiation of acetamide has been monitored by Fourier transform infrared spectroscopy in argon matrix at 10 K. Several primary photoproducts, including HNCO ratio CH(4) and CO ratio CH(3)NH(2) molecular complexes, and acetimidic acid, which is reported for the first time, were characterized. The acetimidic acid identification was based on comparison between the experimental and theoretical (B3LYP) infrared spectra.

Stopped-flow kinetics of tetrazine cycloadditions; experimental and computational studies toward sequential transition states.

The Diels-Alder cycloadditions of tetrazines (1) with alkynes (2) are expected to give bicyclic adducts (3). Kinetic measurements of the cycloadditions of 1a and 1b with 2a give DeltaG(++) = 19.2 +/- 1.

A theoretical study of the [4 + 4] dimerization of thioformylketene.

[reaction and structure: see text] A theoretical study (B3LYP and G3MP2B3) of the dimerization of thioformylketene (1) was performed. Four pathways-two [4 + 2] pathways with thioformylketene (1), one [4 + 4] pathway with 1, and one [4 + 2] pathway involving 1 and thietone (11)-were considered. Interestingly, the [4 + 4] pathway with 1 had the lowest barrier (3.

Experimental support for planar pseudopericyclic transition states in thermal cheletropic decarbonylations.

Low-temperature crystal structures of three pyrrolediones (3-5) and a furandione (9) were obtained and compared to structurally related compounds that cannot undergo decarbonylation. Systematic trends in bond lengths and angles are consistent with distortions along the reaction coordinate, in accord with the structure correlation principle of Dunitz. Since the pyrroledione and furandione rings in 3-5 and 9 are planar, these ground-state geometries prefigure the calculated planar, pseudopericyclic transition states.

alpha,5-didehydro-3-picoline diradicals from skipped azaenediynes: computational and trapping studies of an aza-Myers-Saito cyclization.

[reaction: see text] On the basis of density functional calculations, the isomerization of skipped azaenediynes (C-alkynyl-N-propargylimines) to azaenyne allenes and subsequent rapid aza-Myers-Saito cyclization to alpha,5-didehydro-3-picoline were predicted. We prepared the N-propargylimine of 1-phenyl-3-tri(isopropyl)silylprop-2-yn-1-one, which undergoes proto-desilylation and isomerization to an azaenyne allene when treated with tetrabutylammonium fluoride. In the presence of 1,4-cyclohexadiene, this azaenyne allene affords 6-phenyl-3-picoline and other products corresponding to the trapping of an alpha,5-didehydro-3-picoline diradical.

Nitrosation of amides involves a pseudopericyclic 1,3-sigmatropic rearrangement.

Two possible pathways for the nitrosation of formamide and N-methyl formamide by nitrosonium ion (NO(+)) have been investigated at the B3LYP/6-31G(d,p) level. The key steps are pseudopericyclic 1,3-sigmatropic rearrangements to give the observed N-nitrosamides. The transition structures (8a and 8b) are close to planar on the amide moiety and have remarkably low barriers of only 6.

Experimental and theoretical studies of the dimerizations of imidoylketenes.

Reaction conditions are presented that, for the first time, allow the generation and dimerization of N-alkylimidoylketenes, e.g.,1d, while avoiding the intramolecular rearrangements observed under conventional conditions.

The stereochemistry of the thermal cheletropic decarbonylation of 3-cyclopentenone as determined by multiphoton infrared photolysis/thermolysis.

There are two allowed pathways for the thermal cheletropic decarbonylation of 3-cyclopentenone. The stereochemistry of decarbonylation of an unconstrained derivative (trans,trans-2,5-dimethyl-3-cyclopentenone, 4) has been determined for the first time. Under conventional pyrolysis conditions, thermal rearrangements of the initial product (trans,trans-2,4-hexadiene, 5) occur at the high temperatures required for the decarbonylation.

Sequential transition States and the valley-ridge inflection point in the formation of a semibullvalene.

[structure: see text] Two sequential transition states are calculated at the B3LYP/6-31G(d,p) level for the deazetization of 3b. The intrinsic reaction coordinate for loss of nitrogen passes through a transition state and then a valley-ridge inflection point and ultimately leads to the transition state for the Cope rearrangement of semibullvalene 4b. The energetic and geometrical consequences are discussed.

A density functional theory study clarifying the reactions of conjugated ketenes with formaldimine. A plethora of pericyclic and pseudopericyclic pathways.

The reactions of vinylketene (1a), imidoylketene (1b), and formylketene (1c) with formaldimine (2) were studied at the B3LYP/6-31G* level. For the cycloadditions of these conjugated ketenes with 2, several possible pathways to both [4 + 2] and [2 + 2] products were examined. The lowest energy [2 + 2] pathways are, in most cases, calculated to be stepwise, forming the products via rate-determining conrotatory electrocyclization of zwitterionic intermediates.

Chemoselectivity in the Reactions of Acetylketene and Acetimidoylketene: Confirmation of Theoretical Predictions.

Acetylketene (1) was generated by flash pyrolysis of 2,2,6-trimethyl-4H-1,3-dioxin-4-one (6). The selectivities of 1 toward a number of representative functional groups were measured for the first time in a series of competitive trapping reactions. The trend in reactivities toward 1 follows the general order amines > alcohols >> aldehydes approximately ketones and can be rationalized by considering both the nucleophilicity and the electrophilicity of the reacting species.

Imidoylketene: An ab Initio Study of Its Conformations and Reactions.

MP4(SDQ)/6-31G//MP2/6-31G calculations on the conformations of imidoylketene (1) as well as transition structures for several of its reactions are reported. Anti-Z-1 and anti-E-1 are of equal energy, while syn-Z-1 and syn-E-1 are respectively 2.0 and 0.