Deantiaromatization as a driving force in an electrocyclic reaction.
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Biomedical subjects
Armando Navarro-Vázquez
Publications and source records attributed to Armando Navarro-Vázquez.
Preparation of 13C4-6-methyl anthranilic ester via a Diels-Alder-type process. an experimental and theoretical study to characterize an unexpected isotope exchange.
Buta-2,3-dienoate reacts with vinyl ketenimine to give the corresponding substituted aniline through a Diels-Alder cycloaddition. Besides the expected Diels-Alder adduct 3a, the aniline 3b was also obtained in a ratio of 91:9. The observed (13)C exchange is explained on the basis of a reversible [2 + 2] cycloaddition competing with the [4 + 2] process. This is supported by B3LYP DFT computations, as a stepwise pathway lies very close in energy to the [4 + 2] concerted one. [reaction: see text]
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1,2-Didehydro[10]annulenes: structures, aromaticity, and cyclizations.
The conformational space of C(10)H(8) 1,2-didehydro[10]annulenes, along with their unimolecular conversion to isonaphthalenes (cyclic allenes), has been studied computationally using DFT (B3LYP), single-reference [CCSD(T)], and multireference (MCQDPT2) post-HF methods. The introduction of the linear alkynyl moiety releases enough angle strain to make a nearly planar "heart" aromatic form the preferred conformer by more than 6 kcal/mol [CCSD(T)] over a localized C(2) "twist" structure, as opposed to the closely related C(10)H(10) [10]annulene system. Computations also show that electrocyclic ring-opening of isonaphthalenes to the heart C(10)H(8) annulene takes place through a low barrier of 15 kcal/mol, and this should be considered the working mechanism for the reported isomerizations during dehydro Diels-Alder reactions of phenylacetylenes.
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Computational studies on the cyclizations of enediynes, enyne-allenes, and related polyunsaturated systems.
Quantum chemical studies of cyclizations of enediynes and enyne-allenes have proven to be computationally tractable thanks to the success of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals; the results can further be improved through single-point energy coupled-cluster computations [CCSD(T), BD(T)]. This made comprehensive computational studies on substituent effects and heterosubstituted systems possible. For convenience and predicting new reactions, these transformations can be grouped within larger "families". Alternative cyclization modes are predicted and await experimental realization.
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Cope reaction families: to be or not to be a biradical.
The cyclization reactions of polyunsaturated systems such as enediynes, enyne-allenes, as well as many others, can be structurally related to the Cope reaction and grouped into two branches: 6pi (e.g., Cope, allenyl-Cope) and [2sigma + 4pi] systems (e.g., Bergman, Myers-Saito). After extensive computional (BCCD(T)/cc-pVDZ//UBLYP/6-31G) examination of a large set of Cope-related reactions, we derive the following simple rule: biradical intermediates are involved along the reaction path if they are stabilized by allylic resonance or aromaticity.
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Intramolecular dehydro Diels-Alder reactions of diarylacetylenes: switching between benzo[b]- and benzo[c]fluorenones as products by controlling the rearrangement of cyclic allene intermediates.
Thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-trimethylsilylpropynones affords a mixture of benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied between 100:0 and 0:100 by introducing substituents with appropriate electronic and steric properties on the aryl rings and using an appropriate solvent.
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Cyclic allene intermediates in intramolecular dehydro Diels-Alder reactions: labeling and theoretical cycloaromatization studies.
A comprehensive theoretical and experimental investigation of dehydro Diels-Alder reactions examining the evolution of the cyclic allene intermediates under conditions for intramolecular and ionic and radical intermolecular cycloaromatization processes is reported. Theoretical calculations showed that the most favored intramolecular path for cycloaromatization of 1,2,4-cyclohexatriene 4 and its benzoannulated derivative 14, strained cyclic allenes, consists of a pair of successive [1,2] H shifts rather than a [1,5] shift. Cycloaromatization of cyclic allenes may follow both inter- and intramolecular pathways, depending on the experimental conditions (use of protic or aprotic solvents). For synthetic purposes, the best procedure is to use a protic solvent to promote the ionic intermolecular route, the fastest and highest yielding. When the reaction is carried out in CCl4, intermolecular radical addition of chlorine to the cyclic allene competes with intramolecular aromatization paths. Theoretical calculations predict a low barrier for the reaction of cyclic allenes with carbon tetrachloride, and that the cyclic allenes act as nucleophiles in this reaction.
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A study of aryl radical cyclization in enaminone esters.
Aryl radical cyclization in N-phenyl, N-benzyl, and N-phenethyl enaminone esters 1a-f was studied. N-Benzyl and N-phenethyl enaminones afforded 5-exo and 6-exo cyclization products, respectively, but radical cyclization did not occur in N-phenyl enaminones. The rate constants for the 5-exo and 6-exo cyclization processes in secondary enaminones were estimated as being on the order of 10(7) s(-1) at 353 K; since DNMR experiments showed the rate constant for rotation around the enaminone C3-N bond to be on the order of 10(4) s(-1) at this temperature, the initial enaminone configuration is maintained throughout the cyclization process. PM3 calculations suggested that the nonoccurrence of endo and 4-exo cyclizations is due to the corresponding transition structures involving significant distortion of the conjugated enaminone system.
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A graphical tool for the prediction of vicinal proton-proton 3J(HH) coupling constants.
The easy to use and free available graphical tool MestRe-J, developed for Win-32 platforms, calculates the vicinal proton-proton coupling constants 3J(HH) from the torsion angle phi between the coupled protons for the two kinds of generalized Karplus equations developed by Altona's group as well as for equations from other authors. Besides the classical Haasnoot-de Leeuw-Altona equations, including individual substituent effects that depend on their relative Huggins's electronegativities Deltachi, the program incorporates the more recent and precise Díez-Altona-Donders equations. The substituent effects in these equations, that include effects of interactions between substituents, depend on substituent parameters lambda optimized from the 3J(HH) couplings to methyl groups. Weighted time-averaged couplings can be calculated. The equations for 3J(HH) can be solved to provide the torsion angles phi.
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