Electrochemical and spectroscopic properties of twisted dibenzo[g,p]chrysene derivatives.

Dibenzo[g,p]chrysene (DBC), which consists of a twisted naphthalene core with four fused benzene rings, is a promising framework for organic electronic materials. Therefore, the research for structure-property relationships is important for the design of DBC-based materials. Here, the electrochemical and spectroscopic properties of DBC derivatives were investigated, and the effects of substituents and torsion of the naphthalene moiety were examined based on density functional theory (DFT) calculations. All the substituted DBC derivatives showed higher oxidation potentials than that for DBC-H, even for compounds that contained an electron-donating group such as DBC-Me and DBC-SMe. DFT calculations clearly indicate that these higher oxidation potentials are due to the ineffective conjugation of the MeO group, which is oriented perpendicular to the benzene ring because of the steric repulsion of substituents on both sides. More specifically, the inductive effect of the MeO group is dominant rather than the mesomeric effect when the substituent is located at both sides of the MeO group. Concerning the torsion of the naphthalene moiety, the twisting results in a slight increase in the HOMO and a slight lowering of the LUMO. The twisting effect is much smaller than the conjugation effect of the MeO group. Absorption spectra of all the substituted DBC derivatives also showed a red-shift as compared to that for DBC-H. Concerning the luminescence, a strong photoluminescence was observed for DBC-H and DBC-Si.

Pb, Sr and Ba calix[6]arene hexacarboxylic acid octahedral complexation: a dramatic effect of dealkylation.

Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene hexacarboxylic acid counterparts. With either receptor an octahedral cage is formed around the metal. The transformation from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host.

Recognition and organocatalysis with a synthetic cavitand receptor.

The cyclization reaction of an epoxyalcohol is catalyzed by a synthetic cavitand receptor with an inwardly directed carboxylic acid function. The receptor features a hydrophobic pocket in which the substrate is bound and positioned to react in a regioselective manner. The nature of this substrate-catalyst complex and its dynamic properties were investigated by NMR methods and with the aid of a model compound lacking the epoxide function. The kinetic parameters of the cyclization reaction were also studied. A catalytic cycle is proposed and diverse inhibition mechanisms are identified that parallel those encounterd in enzymology.

Thermal cyclotrimerization of tetraphenyl[5]cumulene (tetraphenylhexapentaene) to a tricyclodecadiene derivative.

Tetraphenyl[5]cumulene (tetraphenylhexapentaene) underwent cyclotrimerization in refluxing toluene for 10-15 min to give a tricyclodecadiene derivative in 68% yield.

Organocatalysis in a synthetic receptor with an inwardly directed carboxylic acid.

A cavitand functionalized with a Kemp's triacid derivative was used to catalyze the epoxide ring-opening cyclizations of 1,5-epoxyalcohols. A deep, cylindrical cavity containing electron-rich aromatic walls and an inwardly directed carboxylic acid displayed the necessary characteristics to bind different 1,5-epoxyalcohols and initiate their cyclization reactions. The reactions inside this synthetic receptor occurred in a catalytic and regioselective manner. These results highlight that the arrangement of functionality and unique solvation provided by the structured interiors of natural enzymes can be incorporated into synthetic systems having useful physical and chemical properties.

Detection of reactive tetrahedral intermediates in a deep cavitand with an introverted functionality.

Labile hemiaminal intermediates are stabilized by binding in a deep cavitand with an introverted aldehyde functionality. The aldehyde is attached to the cavitand via an anthracene spacer that rotates rapidly about the cavitand rim. The half-lives of these hemiaminals vary from 30 min to over 100 h at ambient temperature, due to hydrogen bonding with the organized peptide-like framework at the cavitand rim. The intermediates are sufficiently long-lived to allow study by 2D NMR techniques requiring many hours of acquisition time. Mechanistic analysis of the dehydration step shows first-order kinetics. The analogous "extroverted" reaction was also performed, where the addition took place outside the cavitand, displaying standard steady-state kinetics; no hemiaminal was observed. The cavitand shows strong selectivity based not on binding affinity but upon the rate of the product-forming step. A 10:1 ratio of product imines was obtained, while the initial binding ratio was 1:1. The cavitand acts as a mimic of enzymes in that it uses weak binding forces to stabilize reactive intermediates and isolates them from the medium. The synthetic environment allows direct detection and analysis of the intermediates, as opposed to natural systems that must be analyzed indirectly.

Cavitands with introverted functionality stabilize tetrahedral intermediates.

The synthesis and characterization of two deepened cavitand hosts with introverted functionality--functional groups directed into the cavity--is described. Two functions can be introverted, alcohol and aldehyde, and they show the formation of hemiacetals and hemiketals on binding small guests with complementary functional groups. The structures of the bound hemiacetals are determined by 1D and 2D NMR studies. The arrangements of the guests in the cavitands enhance the equilibrium constants of carbonyl additions, K/K(ctrl), between 13- and 10(5)-fold, compared to their counterparts in solution. The stabilization of the addition products is due to the prior complexation of the guests and the organized solvation provided to the tetrahedral intermediates by a network of secondary amides at the cavitand rim.

Stabilization of labile carbonyl addition intermediates by a synthetic receptor.

Products of unfavorable chemical equilibria are not readily observed because their high energy and increased reactivity result in low concentrations. Biological macromolecules use binding forces to access unfavorable equilibria and stabilize reactive intermediates by isolating them from the medium. In a similar vein, we describe here a synthetic receptor that allows direct observation of labile tetrahedral intermediates: hemiaminals formed in the reaction of an aldehyde carbonyl group with amines. The receptor encapsulates alkyl-substituted primary amines, then orients them toward a covalently tethered aldehyde function. The hemiaminal intermediates appear at high concentration, confined from the bulk solution and observable at ambient temperature by conventional nuclear magnetic resonance spectroscopy.

A bowl-shaped phosphine as a ligand in palladium-catalyzed Suzuki-Miyaura coupling of aryl chlorides: effect of the depth of the bowl.

[reaction: see text] Bowl-shaped phosphine ligands were found to be highly effective in Suzuki-Miyaura coupling of unactivated aryl chlorides, in which the depth of the bowl affected the catalytic activity considerably.

Reaction of an Introverted Carboxylic Acid with Carbodiimide.

The reaction of carboxylic acids with carbodiimides is reviewed, and an "introverted" carboxylic acid is proposed as a means of trapping reactive intermediates along the reaction pathway. The introverted acid is a cavitand with the carboxylic function directed toward the floor of the cavity. Its reaction with diisopropyl carboodiimide gives a covalent adduct that is either the elusive O-acylisourea or the commonly encountered N-acylurea.

A reversible reaction inside a self-assembled capsule.

Reversible encapsulation allows the direct observation of the isolated molecules under ambient conditions, at equilibrium and in the liquid phase. Here we show that capsules can amplify and stabilize molecules that are present in only trace concentrations in solution. Evidence is given that reversible chemical reactions take place within the capsule. Stabilization of reaction intermediates is a characteristic property of enzymes and is widely regarded as an essential feature of catalytic activity. Reactive molecules can also be stabilized by encapsulation, a process that involves completely surrounding the reactive species within synthetic receptors. Here, we show that self-assembled capsules can isolate and stabilize molecules that are present in only trace amounts in solution. The system amplifies the concentrations of high-energy species with reduced entropies.

Experimental and computational probes of a self-assembled capsule.

[reaction: see text] This research was undertaken to explore the interior surface of a synthetic receptor 1.1 with arylpyridines as guests. The interior surface differentiates the guests through the recognition of their nitrogen atoms. Experimental and computational analyses revealed that there is a delicate balance of attractions and repulsions between the host and the lone pairs of guests.

Experimental and computational probes of the space in a self-assembled capsule.

Self-assembled capsules are hosts that recognize and surround smaller molecule guests of appropriate size, shape, and chemical surfaces. The space available inside is a cage of fixed solvent molecules, many of which are aromatic. These aromatics provide anisotropic shielding to guests, and a map of induced magnetic shielding for the inner space can be obtained through nucleus-independent chemical shift calculations. Experimental values of the magnetic environment can be determined by NMR spectra of the guests inside. We describe here the environment in a cylindrical capsule with tapered ends. A series of terminal acetylenes -- the narrowest of organic structures -- was synthesized and used to probe the magnetic shielding of the capsule's ends. Their NMR spectra showed that the acetylenic hydrogen experiences deshielding as it is forced deeper into the tapered end of the capsule where four benzene rings converge. Modeling and density functional theory calculations provided excellent agreement with the experimental values and established a molecular ruler to explore steric and magnetic environments inside the capsule.

MALDI TOF mass study on oligomerization of Pd(OAc)2(L)2 (L = pyridine derivatives): relevance to Pd black formation in Pd-catalyzed air oxidation of alcohols.

[reaction: see text] Oligomerization of Pd(OAc)2(L)2 (L = pyridine derivatives), a catalyst for the air oxidation of alcohols, is studied with MALDI TOF mass, using dithranol as the matrix. The degree of the Pd oligomerization is influenced by the pyridine ligands, and this ligand effect is similar to one observed for Pd black formation in the catalysis.

Kinetic resolution of axially chiral 2,2'-dihydroxy-1,1'-biaryls by palladium-catalyzed alcoholysis.

Palladium-diamine complexes catalyzed kinetic resolution of axially chiral 2,2'-dihydroxy-1,1'-biaryls by alcoholysis of vinyl ethers. The reaction proceeded with high selectivity for various kinds of biaryls. This process is applicable to not only binaphthols but also biphenols, which have been considered to be difficult for the enantioselective synthesis by known catalytic methods.

Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols.

In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 degrees C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields.

Lithium amide assisted asymmetric Mannich-type reactions of menthyl acetate with PMP-aldimines.

A lithium enolate of menthyl acetate added to PMP-imines, in the presence of an equimolar amount of lithium diisopropylamide, affords the Mannich-type addition products in high stereoselectivity. [reaction--see text]