Shuhei Fujinami

para-Bridged Symmetrical Pillar[5]arenes: Their Lewis Acid Catalyzed Synthesis and Host–Guest Property

Condensation of 1,4-dimethoxybenzene (DMB) with paraformaldehyde in the presence of BF3.O(C2H5)2 gave novel para-bridged pentacyclic pillar DMB (DMpillar[5]arene). Moreover, para-bridged pentacyclic hydroquinone (pillar[5]arene) was prepared. Pillar[5]arene formed 1:1 host-guest complexes with dialkyl viologen and alkyl pyridinium derivatives. However, pillar[5]arene did not form complexes with the diadamantyl viologen derivative since a bulky adamantyl group was unable to thread the cavity of pillar[5]arene.

Facile, rapid, and high-yield synthesis of pillar[5]arene from commercially available reagents and its X-ray crystal structure.

We monitored the progress of formation of dimethoxypillar[5]arene by size-exclusion chromatography. Surprisingly, the cyclization reaction completely finished in just 3 min. By improving the reaction conditions and purification process, we successfully obtained dimethoxypillar[5]arene in a short time and in high yield (71%) from commercially available reagents. By improving the deprotection reaction of the methoxy moieties, pillar[5]arene was isolated quantitatively. Single crystal X-ray analysis confirmed the structure of pillar[5]arene in the solid state.

Synthesis and Conformational Characteristics of Alkyl-Substituted Pillar[5]arenes

A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar[5]arene derivatives 1,4-bis(ethoxy)pillar[5]arene (C2), 1,4-bis(propoxy)pillar[5]arene (C3), 1,4-bis(butoxy)pillar[5]arene (C4), 1,4-bis(pentyloxy)pillar[5]arene (C5), 1,4-bis(hexyloxy)pillar[5]arene (C6), and 1,4-bis(dodecanoxy)pillar[5]arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar[5]arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar[5]arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar[5]arenes.

Clickable Di- and Tetrafunctionalized Pillar[n]arenes (n = 5, 6) by Oxidation–Reduction of Pillar[n]arene Units

We report a new route for the selective synthesis of di- and tetrafunctionalized pillararenes via oxidation and reduction of the pillararene units. Hypervalent-iodine oxidation of perethylated pillar[5]arene afforded pillar[5]arene derivatives containing one benzoquinone unit and two benzoquinones at the A,B- and A,C-units. A pillar[6]arene derivative containing one benzoquinone unit was also synthesized. Reduction of the benzoquinone units yielded position-selective di- and tetrahydroxylated pillararene derivatives. This methodology avoids the generation of many constitutional isomers and overcomes the isolation problem of numerous constitutional isomers. From these hydroxylated pillararenes, Huisgen reaction-based clickable di- and tetraalkynylated pillar[5]arenes were prepared. Because of the highly selective and reactive nature of Huisgen alkyne-azide cycloaddition, these pillar[5]arenes can serve as key compounds for a large library of di- and tetrafunctionalized pillararenes. Based on these di- and tetrafunctionalized pillar[5]arenes as key compounds, fluorescent sensors were created by the modification of di- and tetrapyrene moieties via Huisgen-type click reactions.

Aliphatic C−H Bond Activation Initiated by a (μ-η2:η2-Peroxo)dicopper(II) Complex in Comparison with Cumylperoxyl Radical

A (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, [Cu(2)(H-L)(O(2))](2+) (1-O(2)), supported by the dinucleating ligand 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]benzene (H-L) is capable of initiating C-H bond activation of a variety of external aliphatic substrates (SH(n)): 10-methyl-9,10-dihydroacridine (AcrH(2)), 1,4-cyclohexadiene (1,4-CHD), 9,10-dihydroanthracene (9,10-DHA), fluorene, tetralin, toluene, and tetrahydrofuran (THF), which have C-H bond dissociation energies (BDEs) ranging from approximately 75 kcal mol(-1) for 1,4-CHD to approximately 92 kcal mol(-1) for THF. Oxidation of SH(n) afforded a variety of oxidation products, such as dehydrogenation products (SH((n-2))), hydroxylated and further-oxidized products (SH((n-1))OH and SH((n-2))=O), dimers formed by coupling between substrates (H((n-1))S-SH((n-1))) and between substrate and H-L (H-L-SH((n-1))). Kinetic studies of the oxidation of the substrates initiated by 1-O(2) in acetone at -70 degrees C revealed that there is a linear correlation between the logarithms of the rate constants for oxidation of the C-H bonds of the substrates and their BDEs, except for THF. The combination of this correlation and the relatively large deuterium kinetic isotope effects (KIEs), k(2)(H)/k(2)(D) (13 for 9,10-DHA, approximately > 29 for toluene, and approximately 34 for THF at -70 degrees C and approximately 9 for AcrH(2) at -94 degrees C) indicates that H-atom transfer (HAT) from SH(n) (SD(n)) is the rate-determining step. Kinetic studies of the oxidation of SH(n) by cumylperoxyl radical showed a correlation similar to that observed for 1-O(2), indicating that the reactivity of 1-O(2) is similar to that of cumylperoxyl radical. Thus, 1-O(2) is capable of initiating a wide range of oxidation reactions, including oxidation of aliphatic C-H bonds having BDEs from approximately 75 to approximately 92 kcal mol(-1), hydroxylation of the m-xylyl linker of H-L, and epoxidation of styrene (Matsumoto, T.; et al. J. Am. Chem. Soc. 2006, 128, 3874).

An imidazole-based P–N bridging ligand and its binuclear copper(I), silver(I) and palladium(I) complexes: synthesis, characterizations and X-ray structures

Abstract Reaction of 1-methylimidazole with n-BuLi in tetrahydrofuran at −78°C followed by addition of chlorodiphenylphosphine afforded a P–N bridging ligand, 2-(diphenylphosphino)-1-methylimidazole (dpim) in good yield. The structure of the ligand was determined by single crystal X-ray analysis. Reaction of a stoichiometric amount of the ligand with [Cu(MeCN)4]ClO4 in acetonitrile led to the binuclear complex [Cu2(μ-dpim)3(MeCN)](ClO4)2·2CH3CN (1·2CH3CN), whose structure was determined by single crystal X-ray analysis. The solid state structure shows that the two copper ions are held by three dpim ligands and the coordination geometry around each metal atom is different, one being three- and the other being four-coordinated. One of the copper ions forms a distorted tetrahedral array with two P and one N atoms from the dpim ligand and an additional N atom from an acetonitrile ligand, whereas the other copper ion is three coordinated with two N and one P atoms from the dpim ligand. The nitrate-bridged polymeric silver(I) complex, {[Ag2(μ-dpim)2(NO3)](NO3)}n (2) was prepared by the reaction of equimolar amounts of the ligand and silver nitrate in methanol and was characterized by IR, NMR and X-ray analysis. The dimeric unit, [Ag2(μ-dpim)2], possesses an eight-membered annular core structure with two ligands bridging the two metal ions in a head-to-tail configuration. Two types of nitrate ions are present in 2: one makes an infinite one-dimensional chain by joining the dimeric units through its two oxygen atoms in an anti–anti bridging mode whereas the other is disordered with a half occupancy and is located at the left and right sides of the chain. The dynamic behavior of 2 in acetone was also studied by variable temperature 31P NMR. The binuclear palladium(I) complex [Pd2Cl2(dpim)2] (4) was prepared by a conproportionation reaction between complex [PdCl2(dpim)2] (3) and [Pd(dba)2] (dba=dibenzylideneaceton). Only one geometric isomer, probably head-to-tail (HT), was formed in this reaction. Complex 3 was prepared by treating [PdCl2(PhCN)2] with two moles of ligand in dichloromethane solution. In this complex the ligand acts as a P-monodentate. In solution it exists as a mixture of cis and trans isomers.

Synthesis and X-ray crystal structure of a difunctionalized pillar[5]arene at A1/B2 positions by in situ cyclization and deprotection.

The reaction of 1,4-dimethoxybenzene and paraformaldehyde using AlBr(3) yields multiple-deprotected pillar[5]arenes. A1/B2 di-deprotected pillar[5]arene can be isolated by silica gel chromatography and washing procedures. The X-ray structure and polymerization of the A1/B2 di-deprotected pillar[5]arene are reported.

Hydrolysis of phosphodiester with hydroxo- or carboxylate-bridged dinuclear Ni(II) and Cu(II) complexes

A hydroxo- or carboxylate-bridged dinuclear Ni(II) complexes with N,N,N′,N′-tetrakis{(6-methyl-2-pyridyl)methyl }-1,3-diaminopropan-2-ol has been synthesized as models for Ni(II)-substituted phosphotriesterase, which are more active catalysts for hydrolysis of phosphodiester than the corresponding dinuclear Cu(II) and Zn(II) complexes.

Oxidation Reactivity of Bis(μ‐oxo) Dinickel(III) Complexes: Arene Hydroxylation of the Supporting Ligand

In the nick(el) of time: Bis(mu-oxo) dinickel(III) complexes 2 (see scheme), generated in the reaction of 1 with H(2)O(2), are capable of hydroxylating the xylyl linker of the supporting ligand to give 3. Kinetic studies reveal that hydroxylation proceeds by electrophilic aromatic substitution. The lower reactivity than the corresponding mu-eta(2):eta(2)-peroxo dicopper(II) complexes can be attributed to unfavorable entropy effects.

One‐Pot Stereoselective Synthesis of 2‐Acylaziridines and 2‐Acylpyrrolidines from N‐(Propargylic)hydroxylamines

The stereoselective direct transformation of N-(propargylic)hydroxylamines into cis-2-acylaziridines was achieved by the combined use of AgBF4 and CuCl. Copper salts were found to promote the transformation of the intermediary 4-isoxazolines into 2-acylaziridines and both 3-aryl- and 3-alkyl-substituted 2-acylaziridines could be prepared by using this method. Furthermore, subsequent 1,3-dipolar cycloaddition of azomethine ylides that were generated in situ from the intermediary 2-acylaziridines with maleimides was achieved in a stereoselective one-pot procedure to afford the corresponding 2-acylpyrrolidines, which consisted of an octahydropyrrolo[3,4-c]pyrrole skeleton.