Sumito Tokuji

Facile Formation of a Benzopyrane-Fused [28]Hexaphyrin That Exhibits Distinct Möbius Aromaticity

A benzopyrane-fused [28]hexaphyrin, 2, was prepared by simple heating of [26]hexaphyrin 1 in acetic acid. Fused [28]hexaphyrin 2 features a molecular twist, a distinct diatropic ring current, a large HOMA value, a large negative NICS value, and a large two-photon absorption (TPA) cross section even at room temperature, all of which support the Möbius aromaticity of 2. To the best of our knowledge, 2 is the first macrocycle that acquires distinct Möbius aromaticity without any assistance from metal coordination, temperature control, or protonation.

Oxidative Fusion Reactions of meso‐(Diarylamino)porphyrins

In recent years, meso-aminoporphyrins have received increasing attention owing to their potential for vastly improving the economic and environmental validity of dye-sensitized solar cells (DSSC). As a prime example, Gr tzel and co-workers have recently reported a co-sensitized DSSC device incorporating a meso-aminoporphyrin dye that is capable of a power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight conditions. Beside this, meso-aminoporphyrins have been explored as scaffolds for realizing novel nitrogen-connected dimers, mixed-valent cation radicals, inter-valence charge-transfer absorption, photo-induced reductive meso–meso dimerization, and light-harvesting supramolecular aggregates. Although the amino groups of meso-(diarylamino)porphyrins raise the porphyrin HOMO level, electronic perturbation is only moderate because the diarylamino groups twist out of the bulky porphyrin plane, minimizing conjugation. 7] Our continued interest in conjugated porphyrinoids led us to envision the fusion of a meso-diarylamino group onto the periphery of porphyrins as the expected fused porphyrin products would be interesting in their own right, because they would be structurally similar to azagraphene-type porphyrinoids. To the best of our knowledge, there are no reports of oxidative fusion of meso-aminoporphyrins, which would enable effective pexpansion by coplanarization of the peripheral nitrogen atom lone pair and porphyrin p-electronic system. meso-Phenoxazinoporphyrin 2a (Scheme 1)was prepared in 93 % yield using a Buchwald–Hartwig amination 11] of meso-bromo Ni-porphyrin 1Ni with phenoxazine using PdPEPPSI-IPent as the catalyst. By following previously reported procedures, 13] oxidative fusion of 2 a was attempted by subjecting 2a to 20 equivalents of FeCl3 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) for 1.5 hours in CH2Cl2/MeNO2 at room temperature. This reaction afforded an unexpected fused porphyrin 3 a as a sole product in 48 % yield, in which the two 3,5-di-tertbutylphenyl groups had fused to the porphyrin periphery and not the meso-phenoxazine moiety. Although we examined numerous fusion reactions of 2a by changing the oxidants, solvents, and temperature, we could not find reaction conditions that gave us the desired phenoxazine-fused porphyrins. The ESI mass spectrum of 3a showed a parent ion peak at m/z 1107.5374 (calcd for C74H75N5NiO, m/z 1107.5320 [M]), supporting that four protons were removed as a result of oxidation. The H NMR spectrum of 3a in CDCl3 shows a singlet signal owing to the b-protons adjacent to the fused positions at 7.78 ppm. The signals related to the phenoxazine moiety do not show particularly pronounced changes. X-ray crystallographic analysis unambiguously determined the structure of 3a (Figure 1). Interestingly, the fusion reaction took place regioselectively at the bpositions, apart from the phenoxazine moiety. The UV/Vis absorption spectrum of 3a displays broad, red-shifted bands reaching out to around 880 nm, which is similar to that of closely related fused porphyrins that are not functionalized with diarylamino moieties (Figure 2). 14] The electrochemical properties of 2a and 3 a have been studied by cyclic voltammetry (see Supporting Information, Figures S40,S42). The fused porphyrin 3a displays characteristic negative shifts of Eox2 and positive shifts of Ered1, while Eox1 is only slightly affected. As a result, the electrochemical HOMO–LUMO gap of 3a (1.80 eV) is smaller than that of 2a (2.14 eV). Scheme 1. Formulas of porphyrins 1–6. Ar = 3,5-di-tert-butylphenyl.

Rhodium-catalyzed asymmetric conjugate alkynylation of enones with alkynylsilanols.

Asymmetric conjugate alkynylation of alpha,beta-unsaturated ketones with (triisopropylsilyl)ethynylsilanols giving beta-alkynylketones took place in high yields with high enantioselectivity in the presence of chiral rhodium catalysts.

Metal Complexes of Chiral Möbius Aromatic [28]Hexaphyrin(1.1.1.1.1.1): Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis†

The concept of M bius aromaticity, first proposed by Heilbronner in 1964, predicts the aromatic characters for [4np]annulenes with singly twisted so-called M bius topology. Importantly, this concept complements the established H ckel aromaticity that is based on normal planar pelectronic network, 2] and therefore stimulates both theoretical and experimental studies. Among these studies, the first M bius aromatic [16]annulene was reported by Herges and co-workers in 2003, and was followed by an interesting example of a di-p-benzi-hexaphyrin that exhibited a temperature dependent structural change between H ckel and M bius conformations. In the last two years, M bius aromatic systems have been efficiently prepared starting from meso-aryl-substituted expanded porphyrins through metal coordination, temperature control, protonation, and intramolecular fusion reactions. Macrocycles that have a singly twisted M bius topology are intrinsically chiral, and can be either P-twist or M-twist, as shown in Figure 1. Control of the chirality of M bius aromatic expanded porphyrins is important to understand their magnetic properties and to apply these systems to chirality sensing and asymmetric catalysts. Although the enatiomeric separation of M bius aromatic annulenes has been accomplished by Herges and co-workers, their absolute configurations have not been determined. In addition, only limited examples of the enantiomeric separations of twisted expanded porphyrins have been reported to date. We envisioned the enantiomeric separation of Group 10 metal complexes of [28]hexaphyrin(1.1.1.1.1.1) on the basis that hexaphyrins are considered to be rather robust, as indicated by their almost temperature-independent H NMR spectra. Herein, we report the enantiomeric separation of these complexes by using preparative HPLC on a chiral stationary phase, as well as the asymmetric synthesis of the palladium(II) complex. [28]Hexaphyrin(1.1.1.1.1.1) complexes of Pd (2), Ni (3), and Pt (4) were prepared according to a previously reported method that entailed the addition of PdCl2, Ni(acac)2, and PtCl2 at reflux for 12 hours, 1 hour, and 5 days, respectively (acac = acetylacetonate). 7] In the case of complex 2, we found a far more effective method that entailed treatment of [26]hexaphyrin(1.1.1.1.1.1) 1 (see Scheme 1 for structure) with [Pd2(dba)3] in the presence of sodium acetate in a mixture of CH2Cl2/MeOH (dba = 1,5-diphenyl-1,4-pentadien3-one). This method produced complex 2 in 75 % yield at room temperature in 2 hours. The enantiomeric separation of complex 2 was examined by using an analytical HPLC column (SUMICHIRAL OA-3100; f= 10 mm), which, after extensive experimentation, led to a clear separation of the enantiomers with a mixture of CH2Cl2/n-hexane (v/v = 1:1) as an eluent (See the Supporting Information). Based on this finding, the preparative chiral separation of complex 2 was performed successfully on a larger HPLC column (f= 20 mm) with a less polar eluent (CH2Cl2/n-hexane, v/v = 1:2). Under similar conditions, the enantiomeric separations of complexes 3 and 4 were also successfully accomplished. The spectra of the isolated enantiomers displayed opposite Cotton effects (Figure 2). The CD spectrum of the enantiomer that eluted first, 2A, shows the first Cotton effect as a negative–positive bisignate split signal around 639 nm, and the second signal as a positive Cotton effect at 397 nm (Figure 2a). The enantiomers of 3 and 4 that eluted first show practically the same CD spectra as those of 2 ; the bisignate split signal and the positive signal at 639 and 387 nm for 3A and at 619 and 383 nm for 4A, respectively (Figure 2b, c). The large De values ( 300m 1 cm ) for these enantiomers, Figure 1. Two enantiomers of hexaphyrin metal complexes. C6F5 groups at the meso positions were omitted for simplicity.

Solvent- and temperature-dependent conformational changes between Hückel antiaromatic and Möbius aromatic species in meso-trifluoromethyl substituted [28]hexaphyrins.

We investigated the photophysical properties of figure-eight-like meso-hexakis(trifluoromethyl) [26]- and [28]hexaphyrins(1.1.1.1.1.1) denoted as TFM26H and TFM28H, respectively, using steady-state and time-resolved spectroscopy along with theoretical calculations to explore their electronic and magnetic natures depending on their molecular aromaticity. TFM26H exhibited a well-resolved absorption feature and intense fluorescence, both of which were neither solvent- nor temperature-dependent. These optical properties were in agreement with its Hückels [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S(1)-state lifetime of ~50 ps for TFM26H in solution was shorter than those in planar aromatic hexaphyrins (>100 ps) presumably due to nonplanar figure-eight geometry of TFM26H. However, TFM28H exhibited remarkable changes in solvent- and temperature-dependent absorption spectra as well as excited-state lifetimes indicating that a dynamic equilibrium occurs between the two conformational species. With the help of quantum mechanical geometry optimization and vertical excitation energy calculations, we found that the figure-eight double-sided conformer observed in the solid-state and single-sided distorted one could be the best candidates for the two conformers, which should be Hückel antiaromatic and Möbius aromatic species, respectively, based on their optical characteristics, molecular orbital structures, and excited-state lifetimes. Conformational dynamics between these two conformers of TFM28H was scrutinized in detail by temperature-dependent (1)H NMR spectra in various solvents, which showed that the conformational equilibrium was quite sensitive to solvents and that a conformational change faster than the NMR time-scale occurs even at 173 K.

Direct arylation of meso-formyl porphyrin.

Palladium-catalyzed direct arylation of meso-formyl Ni(II) porphyrin with aryl bromides provided β-monoarylated meso-formyl porphyrins. In spite of the existence of the meso-formyl group, the reactions proceeded regioselectively at the β-position adjacent to the formyl group. β-Arylated formyl porphyrin was converted to a tetraline-fused porphyrin by reduction and subsequent acid-catalyzed cyclization and to a meso-meso vinylene-bridged diporphyrin by McMurry coupling (see scheme).

Homoconjugation in diporphyrins: excitonic behaviors in singly and doubly linked Zn(II)porphyrin dimers

We have comparatively investigated excitonic features between singly and doubly alkyl bridged Zn(II)porphyrin dimers (SLZn and DLZn, respectively) with Zn(II)tetraphenylporphyrin (ZnTPP) as a reference by using various time-resolved anisotropy measurements. Time-resolved fluorescence anisotropy decay of the B-state of ZnTPP in toluene showed ultrafast dephasing times of 83 and 185 fs with an initial anisotropy value (r0) of ∼0.7 being consistent with the theoretically proposed one. On the other hand, the Q-state of ZnTPP exhibited a depolarization time of 120 fs with a smaller r0 of 0.25 than that of the B-state because of probing photo-induced absorption in transient absorption anisotropy measurements. Spectroscopic features in absorption, fluorescence, and excitation anisotropy spectra of SLZn are similar to those of ZnTPP indicating a relatively weak exciton coupling in the Q-state. As a result, the observed depolarization time with a time-constant of 1.4 ps can be explained by a Forster-type incoherent energy transfer process between two constituent subunits. When compared to ZnTPP and SLZn however, DLZn exhibited very different optical characteristics such as more red-shifted absorption and emission spectra, three times larger fluorescence excitation anisotropy values, and ultrafast sign inversion in transient absorption anisotropy. According to these results, it can be inferred that the overall π-electron densities of the electronic excited states in DLZn are fully delocalized in a whole dimer and that DLZn can be regarded as a coherently coupled single quantum system not as separate individual chromophores. Frontier π-molecular orbital structures as well as electron density difference maps between occupied and unoccupied molecular orbitals involved in the electron promotions for underlying excited states clearly represent weakly and strongly exciton-coupled natures of SLZn and DLZn, respectively. Accordingly, the extended conjugation of π-electrons over the whole DLZn molecule demonstrates an example for homoconjugation between two aromatic porphyrins.

Palladium-Catalyzed Tetraarylation of 5,15-Dialkylporphyrins with Aryl Bromides

Nickel complexes of 5,15-dialkylporphyrins are subjected to palladium-catalyzed direct arylation under the modified Fagnou conditions. The arylation takes place still exclusively at the four less hindered ! positions although the meso-nonyl, hexyl, and propyl groups are considered to impose less steric hindrance than the meso-3,5-di-tert-butylphenyl group in the previous report. INTRODUCTION Due to the important roles that porphyrins adopt in a variety of pivotal biological processes, chemists have devoted much time to the design and synthesis of new artificial porphyrins that can be utilized in advanced functional materials. Peripheral functionalizations of a porphyrin core is an effective strategy that allows for the systematic construction of porphyrin-based architechtures. Palladium-catalyzed cross-coupling reactions can be used to successfully introduce a direct carbon–carbon bond at the periphery of a porphyrin. However, these reactions generally take place in moderate yield. Unlike benzene-based building blocks, which are cheap and readily available, prefunctionalized porphyrins such as bromoporphyrins and borylporphyrins are far more precious. It is therefore essential that extensive efforts be made to develop much more efficient, scalable, and reliable synthetic methodologies for achieving these highly desirable molecules. A recent dramatic growth in reports of transition-metal-catalyzed direct C–H arylations has changed the landscape of biaryl synthesis. Direct arylation does not require either an aryl metal reagent or an aryl halide and therefore represents the ideal arylation. We have been interested in the modification of functional aromatics by direct arylation and we have recently developed conditions for palladiumThis paper is dedicated to Professor Victor Snieckus on the occasion of his 77th birthday. HETEROCYCLES, Vol. 88, No. 1, 2014 223