Examination of the role of Taft-type steric parameters in asymmetric catalysis.

We report the use of Taft steric parameters to correlate the substituent size of a ligand with the enantiomeric ratio of a reaction. Linear free energy relationships can be constructed by plotting the log of enantiomeric ratio (er) versus the steric parameters reported by Taft and modified by Charton. Successful correlations were found for aldehyde and ketone allylation under NHK conditions using modular oxazoline ligands developed in our laboratory. Using these correlations, ligands were designed and evaluated for carbonyl allylation reactions. A break in the Charton plot results and is attributed to a global structural change in the catalyst. Additionally, several previously reported enantioselective reactions are analyzed resulting in excellent correlations for both catalysts and substrates. Finally, limitations and issues are presented with illustrative examples.

Two-photon absorption in two-dimensional conjugated quadrupolar chromophores.

We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores.

Stereochemical diversity in chiral ligand design: discovery and optimization of catalysts for the enantioselective addition of allylic halides to aldehydes.

[reaction: see text] We have identified a new set of stereochemically diverse oxazoline ligands derived from simple amino acids that promote the Cr-catalyzed enantioselective addition of allylic halides to aldehydes in up to 95% ee. The Cr-catalyzed allylation using ligand 1d is rather insensitive to the nature of the allylic bromide (crotyl, allyl, and methallyl) in that >90% ee is observed for all three bromides evaluated in the addition to benzaldehyde.

Structure-property relationships of donor/acceptor-functionalized tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)benzenes.

A series of tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)benzenes have been synthesized containing tetra-substitutions of neutral, donor, and mixed donor/acceptor groups. To ascertain the importance of substitutional and structural differences of the phenylacetylenes, the optical absorption and emission properties of each series were examined. Conjugation effectiveness, electron density, planarity, and geometry of charge-transfer pathways were found to have a pronounced effect on the overall optical and material properties. Considerable self-association behavior due to face-to-face stacking in solution was observed for donor/acceptor-functionalized macrocycles and was quantified by concentration-dependent (1)H NMR measurements. A solvent-dependent polymerization of one macrocycle regioisomer was observed and characterized. To provide further insight into the energy levels and electronic transitions present, computational studies of each system were performed.