Synthesis and Characterization of Novel 7-6-7 Ring-Based PCP Pincer Rhodium Complexes

Abstract

A series of Rh-PCP pincer complexes (^iPrPCP)Rh(L) bearing a novel alkyl–aryl mixed “7-6-7” ring skeleton has been synthesized and fully characterized. The 7-6-7 ring skeleton in the hydrido-chloro and carbonyl species was found to assume the cis and trans conformations, respectively, in solid state as determined by single crystal X-ray diffraction analysis. This suggested a flexibility of the backbone compared with the similar but strictly fixed anthracene backbone. Electron density on the central rhodium atom was investigated by IR experiments and by DFT computations. NMR monitoring of the reaction of the hydrido-chloro complex with base followed by application of H₂ showed the presence of what are likely to be the active 14e^? species and the dihydride derivative. Especially, noteworthy is the former that was found to be relatively stable, in contrast to the corresponding iridium complex.

[Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon for the following free supplemental files: Additional text, tables, and figures.]     

Facile conversion of CF3-containing propargylic alcohol derivatives via the corresponding allenes

Empirical information on the acidity of the propargylic proton from our previous work allowed us to develop novel synthetic transformations of readily available terminally trifluoromethylated propargylic alcohols 1 into the corresponding allenyl tosylates 3a, 1-tosyloxy- or 1-acyloxy-4,4,4-trifluorobutan-2-ones 4, and 2-(2,2,2-trifluoroethyl)prop-2-en-1-ones 5, which was enabled by such common bases as NaOH and tertiary amines for affecting ready abstraction of this proton.     

Experimental determination of the nN --> sigma*P-O interaction energy of O-equatorial C-apical phosphoranes bearing a primary amino group

The reaction of a chlorophosphorane (9-Cl) with primary amines produced anti-apicophilic spirophosphoranes (5, O-equatorial phosphoranes), which violate the apicophilicity concept, having an apical carbon-equatorial oxygen configuration, along with the ordinarily expected O-apical stereoisomers (6) with the apical oxygen-equatorial carbon configuration. Although the amino group is electronegative in nature, the O-equatorial phosphoranes were found to be stable at room temperature and could still be converted to their more stable O-apical pseudorotamers (6) when they were heated in solution. X-ray analysis implied that this remarkable stability of the O-equatorial isomers could be attributed to the orbital interaction between the lone-pair electrons of the nitrogen atom (n(N)) and the antibonding sigma(P-O) orbital in the equatorial plane. A kinetic study of the isomerization of 5 to 6 and that between diastereomeric O-apical phosphoranes 13b-exo and 13b-endo revealed that 5b bearing an n-propylamino substituent at the central phosphorus atom was found to be less stable than the corresponding isomeric 6b by ca. 7.5 kcal mol(-1). This value was smaller than the difference in energy (11.9 kcal mol(-1)) between the O-equatorial (1b) and the O-apical n-butylphosphorane (2b) by 4.4 kcal mol(-1). This value of 4.4 kcal mol(-1) can be regarded as the stabilization energy induced by the n(N) --> sigma(P-O) interaction. The experimentally determined value was in excellent agreement with that derived from density functional theory (DFT) calculations at the B3PW91 level (4.0 kcal mol(-1)) between the nonsubstituted aminophosphoranes (5g is less stable than 6g by 10.1 kcal mol(-1)) and their P-methyl-substituted counterparts (1a is less stable than 2a by 14.1 kcal mol(-1)).     

Enantio‐ and Diastereoselective Total Synthesis of (+)‐Panepophenanthrin,a Ubiquitin‐Activating Enzyme Inhibitor,and Biological Properties of Its New Derivatives

The asymmetric total synthesis of (+)‐panepophenanthrin, an inhibitor of ubiquitin‐activating enzyme (E1), has been accomplished using catalytic asymmetric α aminoxylation of 1,4‐cyclohexanedione monoethylene ketal as a key step, followed by several diastereoselective reactions. The biomimetic Diels–Alder reaction of a monomer precursor was found to proceed efficiently in water. The investigation of the biological properties of new derivatives of (+)‐panepophenanthrin enabled us to develop new cell‐permeable E1 inhibitors, RKTS‐80, ‐81, and ‐82.     

High-performance electric double-layer capacitors using mass-produced multi-walled carbon nanotubes

Mass-produced multi-walled carbon nanotubes (MWNTs, which have the trademark VGNF®) have been investigated for their potential for use in electric double-layer capacitors (EDLCs). The variation aspects of these MWNTs by KOH activation showed quite interesting features. The gravimetric capacitance enhancement and specific surface area on KOH activation increased linearly. However, the capacitance per unit surface area has a maximum at 200 wt.?% of KOH addition. The VGNF-KOH 500 sample exhibits a capacitance enhancement as much as 13 times greater (28.3 F/g) than that of the as-grown materials (2.2 F/g), under the conditions of charging up to 3.5 V and discharging at a current density of 10 mA/cm². Interestingly, for this MWNT (VGNF®), selective attack on its amorphous carbon impurity has also been observed, as demonstrated from both scanning electron microscopy observations and Raman spectra. Consequently, the results of this study will provide insight into the potentiality of using MWNTs for EDLC electrodes, which would enable the cheapest production cost among the various types of carbon nanotubes.     

Synthesis of (S,S)-1,2-Bis(3,5-DI-Tert-Butylphenyl)Ethane-1,2-Diol

The synthesis of (S,S)-1,2-bis(3,5-di-tert-butylphenyl)ethane-1,2-diol (98.6% ee), which should be useful as a chiral ligand of catalysts in asymmetric inductions, is described.     

In-vivo fluorescence imaging technique using colloid solution of multiple quantum dots/silica/poly(ethylene glycol) nanoparticles

This paper describes a method for producing silica particles containing multiple quantum dots (QD/SiO₂), a method for surface-modifying the particles with poly(ethylene glycol) (QD/SiO₂/PEG), and an in vivo fluorescence imaging technique using colloid solution of the QD/SiO₂/PEG particles. The QDs used were ZnS-coated CdSe_xTe_1?x nanoparticles surface-modified with carboxyl groups, and had an average size of 10.3 ± 2.1 nm. The QD/SiO₂ particles were fabricated by performing sol–gel reaction of tetraethyl orthosilicate using NaOH as a catalyst in the presence of the QDs. The produced particles formed core–shell structure composed of multiple QDs and silica shell, and had an average size of 50.2 ± 17.9 nm. Surface-modification of the QD/SiO₂ particles with PEG, or PEGylation of the particle surface, was performed by using methoxy polyethylene glycol silane. Fluorescence of QD colloid solution was not quenched even through the silica-coating and the PEGylation. Tissues of a mouse could be imaged by injecting the concentrated colloid solution into it and measuring fluorescence intensity emitted from the tissues.     

Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7–6–7 Ring‐Based Pincer Iridium Complexes

A series of Ir–PCP pincer precatalysts [(7–6–7‐^RPCP)Ir(H)(Cl)] and [(7–6–7‐^ArPCP)Ir(H)(Cl)(MeCN)] bearing a novel “7–6–7” fused‐ring skeleton have been synthesized based upon the postulate that the catalytic species would have durability due to their rather rigid structure and high activity owing to the low but sufficient flexibility of their backbones, which are not completely fixed. Treatment of these precatalysts with NaOtBu gave rise to the active 14 electron (14e) species [(7–6–7‐^iPrPCP)Ir] and [(7–6–7‐^PhPCP)Ir], which can trap hydrogen and were spectroscopically characterized as the tetrahydride complexes. Both [(7–6–7‐^iPrPCP)Ir] and [(7–6–7‐^PhPCP)Ir] were found to be highly effective in the transfer dehydrogenation of cyclooctane with tert‐butylethylene as the hydrogen acceptor, the initial reaction rate at high temperature (230 °C) being higher for [(7–6–7‐^iPrPCP)Ir] than [(7–6–7‐^PhPCP)Ir], and the turnover number (TON) of the overall hydrogen transfer being higher for the latter. Nonetheless, the estimated TONs were as high as 4600 and 4820 for the two complexes at this temperature, respectively, which are unprecedented absolute values. In terms of durability, the [(7–6–7‐^PhPCP)Ir] complex is the catalyst of choice for this reaction. Structural analysis and computational studies support the importance of the low flexibility of the ligand core.