Palladium-catalyzed regioselective Heck arylation of the electron-rich olefins, vinyl ethers 1a-d, enamides 1e-g, and allyltrimethylsilane 1h, has been accomplished in imidazolium ionic liquids with a wide range of aryl bromides and iodides instead of the commonly used, but commercially unavailable and expensive, aryl triflates. The reaction proceeded with high efficiency and remarkable regioselectivity without the need for costly or toxic halide scavengers, leading exclusively to substitution by aryl groups of diverse electronic and steric properties at the olefinic carbon alpha to the heteroatom of 1a-g and beta to the heteroatom of 1h. In contrast, the arylation reaction in molecular solvents led to mixtures of regioisomers under similar conditions. Several lines of evidence point to the unique regiocontrol stemming from the ionic environment provided by the ionic liquid that alters the reaction pathway. The chemistry provides a simple, effective method for preparing branched, arylated olefins and contributes to the extension of Heck reaction to a wider range of substrates.

Ionic Liquid-Promoted, Highly Regioselective Heck Arylation of Electron-Rich Olefins by Aryl Halides

Controlling the equilibria between backbone cis- and trans-amides in peptoids, or N-substituted glycine oligomers, constitutes a significant challenge in the construction of discretely folded peptoid structures. Through the analysis of a set of monomeric peptoid model systems, we have developed new and general strategies for controlling peptoid conformation that utilize local noncovalent interactions to regulate backbone amide rotameric equilibria, including n→π*, steric, and hydrogen bonding interactions. The chemical functionalities required to implement these strategies are typically confined to the peptoid side chains, preserve chirality at the side chain N-α-carbon known to engender peptoid structure, and are fully compatible with standard peptoid synthesis techniques. Our examinations of peptoid model systems have also elucidated how solvents affect various side chain−backbone interactions, revealing fundamental aspects of these noncovalent interactions in peptoids that were largely uncharacterized ...

/pdf/new-strategies-for-the-design-of-folded-peptoids-revealed-by-4d0dygl0la.pdf

New Strategies for the Design of Folded Peptoids Revealed by a Survey of Noncovalent Interactions in Model Systems

A new synthetic route for carbazole dendrimers was discovered using the copper-catalyzed N-arylation reaction. This synthetic route allowed synthesizing the fourth generation carbazole dendrimer and several derivatives for the first time. The crystal structure, Mark−Houwink−Sakurada plots, and UV−vis and fluorescence studies showed that the dendritic carbazole backbone has a rigid and highly twisted structure. From the measurement of the redox potential of the ferrocene derivatives, the IR spectra of the benzophenone derivatives, and complexation behavior of the phenylazomethine derivatives, the inductive electron-withdrawing effect of the carbazole dendron was revealed. This suggested that the summation of this electron withdrawal from each layer may produce a potential gradient such that the outer layer is electron-rich and the inner layer is electron-poor in the carbazole dendron. By assignment of the 1H and 13C NMR spectra of the dendron, the existence of this kind of potential gradient was proved. Ov...

Dendritic structure having a potential gradient: new synthesis and properties of carbazole dendrimers.

Energetics of an n → π* Interaction that Impacts Protein Structure

(13)C NMR chemical shifts and nu(C[double bond]O) frequencies have been measured for several series of phenyl- or acyl-substituted phenyl acetates and for acyl-substituted methyl acetates to investigate the substituent-induced changes in the electrophilic character of the carbonyl carbon. Charge density, bond order, and energy calculations have also been performed. The spectroscopic and charge density results indicate that opposite to the conventional thinking, electron-withdrawing substituents do not increase the electrophilicity of the carbonyl carbon but instead decrease it. On the other hand, reaction energies of the isodesmic reactions designed show that electron-withdrawing substituents destabilize the carbonyl derivatives investigated. So, a significant ground-state destabilization of carboxylic acid esters, and carbonyl compounds in general, due to the decreased resonance stabilization, is proposed as a novel concept to explain both the increase in their reactivity and the changes in the chemical shifts and carbonyl frequencies induced by electron-withdrawing substituents.

Electron-withdrawing substituents decrease the electrophilicity of the carbonyl carbon. An investigation with the aid of (13)C NMR chemical shifts, nu(C[double bond]O) frequency values, charge densities, and isodesmic reactions to interpret substituent effects on reactivity.

13 C Substituent chemical shifts of the carbonyl sites in the side-chains of meta- and para-substituted benzenes of the type XC6H4COZ have been measured. Analysis of this data using the dual substituent parameter method shows that inductive effects are predominant. The reverse inductive contribution observed is explained in terms of a π-polarization mechanism. Critical support for this mechanism is obtained from additional series where the carbonyl is complexed with Lewis acids. The concepts of ‘extened’ and ‘localized’π-polarization are discussed.

Jeffrey E. Rowe

Papers

Carbon-13 substituent chemical shifts in the side-chain carbons of aromatic systems: the importance of π-polarization in determining chemical shifts