Yoshio Suzuki

On the prerequisites for the formation of solution complexes from [60]fullerene and calix[n]arenes: A novel allosteric effect between [60]fullerene and metal cations in calix[n]aryl ester complexes

Abstract In order to find calix[n]arenes which can interact with [60]fullerene in solution we have screened a number of different calix[n]arenes by spectroscopic methods. We eventually discovered such three calix[n]arenes (15·But·H, 16·But·H, and 23·But·H) which could accept [60]fullerene even in solution. They commonly possess a cone conformation and a benzene ring inclination suitable to [60]fullerene inclusion. Furthermore, it was found that calix[n]aryl ester derivatives, which cannot interact with [60]fullerene, become excellent [60]fullerene-acceptors in the presence of certain specific metal cations. This was attributed to a metal-induced conformational change to preorganize cone calix[n]aryl esters. This phenomenon is a sort of positive allosterism, which is the first example in fullerene chemistry. We believe that these findings will open a door to new fullerene-calix[n]arene conjugate chemistry.

New Chiral Sulfoxide Ligands Possessing a Phosphano or Phosphanoamino Functionality in Palladium-Catalyzed Asymmetric Allylic Nucleophilic Substitution Reactions

Abstract New chiral sulfoxide ligands possessing a phosphano or phosphanoamino functionality as an alternative coordinating element were developed, and their usefulness was demonstrated by applying them to palladium-catalyzed asymmetric allylic nucleophilic substitution reactions. The structure of the catalyst precursor coordinated by the chiral phosphino sulfoxide was determined by X-ray crystallographic analysis. The possible mechanism for the asymmetric induction using these chiral ligands was proposed on the basis of the stereochemical outcome obtained.

(S)-Proline-derived new chiral ligands with phosphino, organosulfur or organoselenenyl functionality as an enantiocontrollable coordinating element

Abstract The synthesis of ( S )-proline-derived chiral ligands bearing phosphino, organosulfur or selenenyl groups and their use as chiral ligands in palladium-catalyzed asymmetric allylic alkylations has been accomplished. In particular, the ( S )-proline-derived phosphine ligands bearing alkylsulfenyl groups provided high enantioselectivity, and the degree of asymmetric induction was dependent upon the steric bulk of the alkyl substituents in sulfenyl groups. The stereochemical results are rationalized by a plausible mechanism involving the assistance of chelates formed by the participation of the two preferred heteroatoms involved.

Chiral sulfoxide ligands bearing nitrogen atoms as stereocontrollable coordinating elements in palladium-catalyzed asymmetric allylic alkylations

Abstract Palladium-catalyzed asymmetric allylic alkylations were studied by using chiral sulfoxide ligands bearing nitrogen atoms as coordinating elements, such as chiral α-sulfinylacetamides, β or γ-amino sulfoxides, and β-sulfinyl sulfonamides. The effects of the chiral sulfinyl functions on the asymmetric induction were demonstrated. Use of ( S )-2-pyrrolidinophenyl p -tolyl sulfoxide or ( S )-2-( N -butyl- N -methylaminomethyl)phenyl p -tolyl sulfoxide as chiral ligands in the palladium-catalyzed asymmetric allylic alkylations provided the highest enantioselectivity (50 or 58% e.e., respectively) among chiral sulfoxide ligands examined by us. The participation of the sulfinyl groups in these catalytic asymmetric reactions is rationalized, and the mechanism for the asymmetric induction is proposed on the basis of the stereochemical outcome obtained.

A NEW CHIRAL IMINOPHOSPHINE LIGAND DERIVED FROM (1S, 4S)-FENCHONE IN PALLADIUM-CATALYZED ASYMMETRIC ALLYLIC ALKYLATIONS

Abstract A new chiral iminophosphine ligand derived from (1 S ,4 S )-fenchone has been developed, and its usefulness as a chiral ligand in asymmetric synthesis was demonstrated in palladium-catalyzed allylic alkylations.

Guest inclusion properties of calix[6]arene-based unimolecular cage compounds. On their high Cs+ and Ag+ selectivity and very slow metal exchange rates

Abstract The reaction of 1,3,5-tri-O-alkylated calix[6]arenes with 1,3,5-tris(bromomethyl)benzene yielded capped calix[6]arenes (2 with tert-butyl groups on the upper rim and 3 without tert-butyl groups) in unexpectedly high yields (80 – 91%). Combined studies of 2 and 3 by MM3 computation, X-ray analysis, and 1H NMR spectroscopy established that these calix[6]arenes feature a unique structure consisting of alternately-arranged three flattened mesitylene-linked phenyl units and three stand-up anisole units. Particularly, compound 2 possesses a closed ionophoric cavity: the upper hemisphere is closed by three tert-butyl groups of anisole units and the lower hemisphere is closed by a mesitylene cap and three anisole methoxy groups. The 1H NMR spectrum was scarcely changed at wide temperature range (30 ∼ 130°C), indicating that the structure is extremely rigidified. Both solvent extraction and spectroscopic studies established that this cavity shows the high selectivity toward Cs+ among alkali metal cations, the high affinity with Ag+, and the moderate affinity with RNH3+. Very surprisingly, the association-dissociation processes for 2 and cesium picrate was so slow that the rate could be followed by a conventional spectroscopic method. The thermodynamic parameters determined by kinetic studies disclosed that the major driving-force for Cs+ inclusion is the entropy term based on the desolvation.

Chiral cage molecules generated by regioselective O-alkylation of a doubly-bridged calix[8]arene

Abstract A calix[8]arene doubly-bridged at the 1,5-phenol units and 3,7-phenol units has a cage structure with D 2d symmetry. Regioselective introduction of methyl groups into the residual 2,4,6,8-phenol units affords five different cage compounds: that is, three racemic compounds, one meso compound, and one achiral compound. This paper reports systematic studies on the preparation methods of these new cage compounds and spectroscopic classification of their molecular symmetries. The results indicate that these compounds have a latent potential as a platform for designing chiral cage molecules with an ionophoric inner cavity.

Thermodynamic studies of slow metal exchange processes in ionophoric calix[n]]arenes with a capsule-like closed cavity

Abstract It was found that in a triply-capped calix[6]arene ( 1 ) and doubly-bridged calix[8]arene ( 2a and 2b ) with a closed ionophoric cavity the Cs + complexation occurs very slowly in a human time-scale and can be followed by a conventional spectroscopic method. The unusual behavior has enabled us to determine their kinetic activation parameters for both the forward and the reverse reaction. The results established that being different from the concept believed so far, the Cs + complexation is heavily governed by the entropy term but not by the enthalpy term.

Stereochemistry of transition metal catalyzed asymmetric intramolecular allyl transfer in chiral α-sulfinyl allylic esters

Abstract The stereochemistry of palladium or nickel catalyzed asymmetric intramolecular allyl transfer in chiral α-sulfinyl allylic esters was determined. The participation of the catalyst and the chiral sulfinyl functionality in these transformations, presumably by the coordination of the sulfinyl group to the catalyst, is discussed, and a novel mechanism is proposed for the rationalization of the results obtained.

Palladium-catalyzed asymmetric reactions enantiocontrolled by chiral organosulfur functionality

Abstract Participation of chiral sulfinyl functionality in palladium-catalyzed asymmetric reactions is demonstrated by using chiral sulfoxides as chiral ligands or chiral substrates. New chiral ligands, o-(phosphinoamino)phenyl and o-phosphinophenyl sulfoxides, have been developed. The structure of an intermediary palladium complex was determined by X-ray crystallographic analysis. The palladium-catalyzed asymmetric 1,3-rearrangements of (1,3-butadienyl)cyclopropanes bearing chiral sulfinyl groups to cyclopentenes are described. The participation of the chiral sulfinyl group to the palladium catalyst is described.