Snezhana M. Bakalova

Excited state intramolecular proton transfer in 2-acetylindan-1,3-dione

Abstract Results on the absorption and luminescence properties of 2-acetylindan-1,3-dione in different solvents are reported. Evidence suggesting intramolecular proton transfer in the excited singlet state is presented. All possible tautomeric forms are studied by means of the AM1 method in both the ground and the first excited singlet state. On the basis of the results obtained the tautomers involved in the photocycle are proposed.

An Alternative Mechanism for Diels–Alder Reactions of Evans Auxiliary Derivatives

The mechanism proposed by Evans for the dialkylaluminum chloride promoted Diels-Alder reaction of cyclopentadiene with alpha,beta-unsaturated N-acyloxazolidinones has been widely used as a basis for the rationalization of the experimental selectivities observed in many different types of reactions in which oxazolidinones or imidazolidinones are used as chiral auxiliaries. In this manuscript we introduce a new and more general model based on molecular modeling and NMR spectroscopy data that avoids several ambiguous concepts raised by the Evans model and fully explains all available experimental data. While the Evans proposal relies on the formation of high-energetic ionic chelates that promote the rotation of the amide bond in the N-acyloxazolidinone molecule, our model is based on the catalysis by means of low-energetic mono- or bicomplexes at the chain and the ring carbonyl groups that are easily observed by NMR spectroscopy measurements. The observed selectivities are explained by a chirality-transfer concept, in which an achiral Lewis acid works as a bridge for the transfer of chirality between a chiral auxiliary and a prochiral reactive center. Different to the Evans proposal, this mechanism fully explains the experimental selectivities for low Lewis acid concentrations, based on the catalysis by means of concurrent monocomplexes at the chain or the ring carbonyl groups, as well as the increased reaction rates and selectivities experimentally observed for high Lewis acid concentrations. The model can be extrapolated to nonchelating and other chelating Lewis acids, thereby allowing for the rationalization of much experimental data that were never explained by the Evans proposal.

Computational probes into the conceptual basis of silver ion chromatography: I. Silver(I) ion complexes of unsaturated fatty acids and esters

Silver ion chromatography of unsaturated fatty acid derivatives can be described satisfactorily by the suggestion of Ag(I) complexation with more than a single bonding site of esters of the 18:1, 18:2, etc. series (that is, one, two, etc. methyleneinterrupted double bonds in the C18 hydrocarbon chain) and by the formation of chelate-like complexes. Ab initio RHF effective core potential CEP-31G calculations of model C6 to C18 complexes in the gas phase and in solution indicate that the stability of such complexes depends on the position of CyC double bonds in ligand molecules, while a significant part of the complexation energy is contributed by the interaction of Ag(I) and alkoxy- (or aryloxy-)carbonyl groups of fatty acid esters. Factors important to the stability of Ag(I) fatty acid chelates are the ring sizes and strain, particularly manifested in unsaturated acids with the CyC double bond closer to the carboxyl group, i.e. with smaller chelate rings, as well as conformations of larger rings and hydrocarbon chains. Calculated theoretical solvent effects on chelation are more significant for larger Ag(I) containing rings and increase with increasing ring size. q 2002 Elsevier Science B.V. All rights reserved.

Semi-empirical AM1 calculation of the solvent effect on the fluorescence spectra of some dihydroquinolinones

The COnductor-like Screening MOdel of solvent-solute interactions of Klamt and Schüürmann, COSMO, at the semiempirical AM1 level of MO calculations, augmented by limited singles and doubles configuration interaction, proves useful for the study of solvent induced shifts of fluorescence spectra. Optimization of geometry of ground S0 and excited S1 states for each solvent separately provides estimates of the changes of solvation energy accompanying the electron transition process and helps the understanding of the related solvent-solute reorganization and fluorescence mechanisms.

Lewis Acid Catalyzed Reactions of Chiral Imidazolidinones and Oxazolidinones: Insights on the Role of the Catalyst

The mechanism proposed by Evans to justify the selectivity obtained in Lewis acid catalyzed Diels-Alder reactions of cyclopentadiene with acyloxazolidinones has been generalized and used in the rationalization of selectivities obtained in many other systems. However, we recently proposed an alternative mechanism, on the basis of open-chain mono- and bicomplexes, that avoids the need for chelates and explains the selectivity obtained by Evans. In this manuscript we apply our proposal to the catalyzed conjugated addition of amines to acylimidazolidinones, reported by Cardillo, and we clearly show that aluminum chelates are not involved in the reaction, as they induce no selectivity, while Cardillo observed high experimental selectivities. Our data equally show that bicomplexes with carbonyl parallel orientation, proposed by Cardillo to justify the experimental selectivity with nonchelating Lewis acids, indeed induce the opposite selectivity and have also to be dismissed. On the other hand, our mechanistic proposal allows for the full rationalization of the data obtained by Cardillo with aluminum, boron, or zinc Lewis acids and supports our previous proposal on DA cycloadditions of dienes to Evans chiral auxiliary derivatives.

Solvent induced shifts of electronic spectra IV. Computational study on PRODAN fluorescence and implications to the excited state structure

Vertical S(1)-S(0) electronic transitions of the highly solvent-sensitive fluorescence label 2-propionyl-6-dimethylamino naphthalene (PRODAN) are modeled by semiempirical CISD AM1 and TD DFT calculations in a large number of solvents of different polarity and hydrogen donating ability. Calculations correctly reproduce the observed solvent induced shifts of the emission maxima. The fluorescence Frank-Condon transition energies in solvent can be predicted quantitatively at the AM1 SM5.42 OPEN(2,2) C.I.=5 CISD level. For the planar PRODAN emitting state at the latter level we obtain a regression with practically unit slope and zero intercept for aprotic solvents. The respective relationship for the O-twisted S(1) state has a slope of 0.59 and intercept of 9100 cm(-1). These results support the concept that no geometry twist in the S(1) state of PRODAN is necessary to explain the observed solvent effects on fluorescence.

Origins of Stereoselectivity in the Diels-Alder Addition of Chiral Hydroxyalkyl Vinyl Ketones to Cyclopentadiene: A Quantitative Computational Study †

Modest basis set level MP2/6-31G(d,p) calculations on the Diels-Alder addition of S-1-alkyl-1-hydroxy-but-3-en-2-ones (1-hydroxy-1-alkyl methyl vinyl ketones) to cyclopentadiene correctly reproduce the trends in known experimental endo/exo and diastereoface selectivity. B3LYP theoretical results at the same or significantly higher basis set level, on the other hand, do not satisfactorily model observed endo/exo selectivities and are thus unsuitable for quantitative studies. The same is valid also with regard to subtle effects originating from, for example, conformational distributions of reactants. The latter shortcomings are not alleviated by the fact that observed diastereoface selectivities are well-reproduced by DFT calculations. Quantitative computational studies of large cycloaddition systems would require higher basis sets and better account for electron correlation than MP2, such as, for example, CCSD. Presently, however, with 30 or more non-hydrogen atoms, these computations are hardly feasible. We present quantitatively correct stereochemical predictions using a hybrid layered ONIOM computational approach, including the chiral carbon atom and the intramolecular hydrogen bond into a higher level, MP2/6-311G(d,p) or CCSD/6-311G(d,p), layer. Significant computational economy is achieved by taking account of surrounding bulky (alkyl) residues at 6-31G(d) in a low HF theoretical level layer. We conclude that theoretical calculations based on explicit correlated MO treatment of the reaction site are sufficiently reliable for the prediction of both endo/exo and diastereoface selectivity of Diels-Alder addition reactions. This is in line with the understanding of endo/exo selectivity originating from dynamic electron correlation effects of interacting pi fragments and diastereofacial selectivity originating from steric interactions of fragments outside of the Diels-Alder reaction site.

The ring closure of ethylene phosphites is a new P(III)-insertion reaction. A computational study

The observed equilibrium interconversion between open ethylene phosphites and bicyclic spirophosphoranes is recognized as the result of a new insertion reaction of phosphite P(III) into the labile H– O bond of the hydroxyethyl ester, a process more typical to electron deficient species as carbenes or nitrenes, and akin to oxidative additions of transition metals. Energy relationships between species along the reaction path are studied by MO RHF, MP2(full), and MP4SDTQ, as well as DFT calculations, and usually predict 2-hydroxyethyl ethylene phosphites 1 more stable than strained tetraoxo-[10-P-5]spirophosphoranes 2, contrary to known experiments. The results of DFT calculations could be referred to underestimation of nondynamic electron correlation in the unfavorable four-electron three-center axial fragment of spirophosphoranes, and thereby spurious prediction of their lower stability. This interpretation of the DFT results is corroborated by CAS SCF vs. MC-QDPT2 calculations.

Electronic spectra and photochemical properties of acetyldihydroquinolinones

The absorption and luminescence properties of 6-acetyl-2,3-dihydro-4(1H)-quinolinone are described and compared to those of the parent 2,3-dihydro-4(1H)-quinolinone. The character of the electronic transitions is determined by PPP quantum chemical calculations. An N-acyl migration leading to the formation of 6-acetyl-2,3-dihydro-4(1H)-quinolinone takes place upon UV irradiation or heating with polyphosphoric acid of 1-acetyl-2,3-dihydro-4-(1H)-quinolinone. A radical mechanism of the photochemical rearrangement is proposed.

Solvent effects on the fluorescence properties of methyldihydroquinolinones

Abstract The absorption and fluorescence spectra and the fluorescence quantum yields of two methyl-2,3- dihydro-4(1H)-quinolinones in various solvents are reported. In some solvents deuteration-induced fluorescence enhancement has been observed. The solvent effects in aprotic solvents are consistent with the general type of solvent interaction. For protic solvents, solute-solvent complexation in the excited state is suggested