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.

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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.

In the past three decades, the discovery and application of effective asymmetric catalysts has flourished in both academic and industrial settings.[1] This is especially true within the recent decade where the field of asymmetric catalysis has witnessed an incredible expansion, and the benchmarks for acceptable asymmetric induction have significantly risen. However, the approach to catalyst identification remains mainly empirical, wherein evaluation of a considerable number of catalyst structures is required to develop a mature chiral catalyst. While this process has been expedited by technological advancements, [2] the understanding of the relationship between catalyst structure and the enantioselectivity of the catalyzed reaction is generally precarious. This can be attributed to the small energy differences in the activation barrier leading to the enantiomeric products and/or the complicated preequilibria in most catalytic reactions. [3] An approach to developing greater predictive power in asymmetric catalysis is based on the use of modular ligands [4] in lieu of a detailed understanding of the microscopic processes involved in asymmetric induction. We believe the ability to make systematic changes to the ligand structure not only leads to enhanced product enantiomeric excess, [5] but allows for the elucidation of structure-enantioselectivity relationships. [6] Herein, we report the use of a modular ligand set to systematically study the effect of ligand size and the first observation of a linear free energy relationship between steric parameters and enantiomeric ratio. Additionally, our analysis of several previously reported enantioselective reactions reveals similar linear free energy relationships.We have recently disclosed the use of modular oxazoline ligands [7] in enantioselective Nozaki-Hiyama-Kishi reactions of allylic halides [8,9] with both aldehydes [5a] and ketones [5c] ( Figure 1). A key observation from these studies is that the facial selectivity of the reaction is directly correlated to the absolute configuration of the proline module of the ligand. For example, in comparing ligands 1 a and 1 b, which differ only in the configuration of the proline module, a considerable difference in enantiomeric ratio is observed for allylation of both aldehydes and ketones. Most notably, the allylation of benzaldehyde is promoted in nearly 90 % ee (96:4 e.r. and 94.5:5.5 e.r.) using both ligands but with different senses of asymmetric induction. A similar effect is also found for the allylation of acetophenone. Systematic changes to the configuration of the other chiral centers in the ligand structure had only a subtle impact. These observations prompted us to explore the role of the proline module in greater detail. In order to initiate the investigation, we decided to make systematic changes to the proline module and probe the effect on the enantioselective outcome of the reaction. This was reasoned to be most easily accomplished through modification of the proline nitrogen, as proline carbamates...

Competition is good: The reported “Pd” homocoupling procedure is now a viable alternative to the traditional Cu‐mediated reaction for the formation of diacetylenic macrocycles. From the same starting material, use of Cu(OAc)2 or [PdCl2(dppe)]/I2 (dppe=1,2‐bis(diphenylphosphanyl)ethane) leads to selective formation of either the bis[15]‐ or bis[14]annulenes (see scheme).