Kota Yoshida

Combined Experimental and Theoretical Investigations on Optical Activities of Möbius Aromatic and Möbius Antiaromatic Hexaphyrin Phosphorus Complexes

Intrinsically chiral Möbius aromatic [28]hexaphyrin monophosphorus(V) and Möbius antiaromatic [30]hexaphyrin bisphosphorus(V) complexes have been optically resolved and their absolute configurations (ACs) were determined by combined experimental and theoretical investigations on their circular dichroisms (CDs). First elutes in chiral HPLC exhibited strong positive Cotton effects (CEs) at the B-band, characteristic for the ML configurations in their Möbius strips. Weak CEs at the Q-band, if attainable, complemented their AC assignment. The whole CD pattern and intensity were well reproduced by time-dependent approximate coupled cluster theory using model systems that omit five outward meso-aryl substituents (inward-meso-retained model), providing a solid basis for AC assignment. The cost efficient TD-DFT method with appropriate functionals for fully substituted (nontruncated) complexes well reproduced CEs around the B-band (but less satisfactory at the Q-band), also allows the rapid AC estimation for their Möbius strips. Observed difference in CDs between aromatic and antiaromatic hexaphyrins were better interpreted by their shifts in energy levels and altered interactions of relevant molecular orbitals, rather than small differences in Möbius geometries nor aromatic/antiaromatic character, despite the correlations recently claimed in planar π-systems.

Peripherally Hexasulfanylated Subporphyrins.

Peripherally hexachlorinated meso-triphenyl subporphyrin 4 was prepared by chlorination of meso-triphenyl subporphyrin 1 with N-chlorosuccinimide and was effectively transformed to hexasulfanylated subporphyrins 5-8 via nucleophilic aromatic substitution (S(N)Ar) reactions with the corresponding thiols under basic conditions. The structures of 5-8 have been all well characterized by single-crystal X-ray analysis. (1)H NMR studies indicated that the meso-phenyl substituents undergo restricted rotation for 5-8, while the β-sulfanyl substituents are conformationally flexible in 5, 6, and 8, and are strictly regulated to an anti-conformation in 7. Judging from the absorption spectra, the oxidation and reduction potentials, and the DFT calculations, the substituent effects decrease in the order of 5>6>7>8. Subporphyrin 8 effectively captures C60 in a 1:1 manner in [D8 ]toluene solution.

Probing the rotational dynamics of meso-(2-substituted)aryl substituents in A2B-type subporphyrins.

A2 B-type B-methoxy subporphyrins 3 a-g and B-phenyl subporphyrins 7 a-c,e,g bearing meso-(2-substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable-temperature (VT) (1) H NMR spectroscopy. In these subporphyrins, the rotation of meso-aryl substituents is hindered by a rationally installed 2-substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH(≠) and ΔS(≠) in ΔG(≠). 2-Methyl and 2-ethyl groups of the meso-aryl substituents in subporphyrins 3 e, 3 f, and 7 e induce larger rotational barriers than 2-alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4-dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B-phenyl subporphyrins than for B-methoxy subporphyrins indicate a negligible contribution of SN 1-type heterolysis in the rotation of meso-aryl substituents.

β,β-(1,4-Dithiino)subporphyrin Dimers Capturing Fullerenes with Large Association Constants.

β,β-(1,4-Dithiino)subporphyrin dimers 7-syn and 7-anti were synthesized by the nucleophilic aromatic substitution reaction of 2-bromo-3-(4-methoxyphenylsulfonyl)subporphyrin 4 with 2,3-dimercaptosubporphyrin 5 under basic conditions followed by axial arylation. Additions of C60 or C70 to a dilute solution of 7-anti (ca. 10(-6)  m) in toluene did not cause appreciable UV/Vis spectral changes, while similar additions to a concentrated solution (ca. 10(-3)  m) resulted in precipitation of complexes. In contrast, dimer 7-syn captured C60 and C70 in different complexation stoichiometries in toluene; a 1:1 manner and a 2:1 manner, respectively, with large association constants; Ka =(1.9±0.2)×10(6)  m(-1) for C60 @7-syn, and K1 =(1.6±0.5)×10(6) and K2 =(1.8±0.9)×10(5)  m(-1) for C70 @(7-syn)2 . These association constants are the largest for fullerenes-capture by bowl-shaped molecules reported so far. The structures of C60 @7-anti, C70 @7-anti, C60 @7-syn, and C70 @7-syn have been determined by single-crystal X-ray diffraction analysis.

Synthesis of Boron(III)‐Coordinated Subchlorophins and Their Peripheral Modifications

A pyrrole-cleaving modification to transform boron(III) meso-triphenylsubporphyrin into boron(III) meso-triphenylsubchlorophin has been developed. Boron(III) subchlorophins thus synthesized show absorption and fluorescence spectra that are roughly similar to those of boron(III) subchlorins, but B-methoxy boron(III) subchlorophin showed considerably intensified fluorescence and a small Stokes shift. Peripheral modification reactions of B-phenyl boron(III) subchlorophin such as regioselective nitration with Cu(NO3 )2 ⋅3 H2 O, ipso-substitution reactions of boron(III) α-nitrosubchlorophin with CsF and CsCl, and Pd-catalyzed cross-coupling reactions of boron(III) α-chlorosubchlorophin with arylacetylenes, have been also explored to tune the optical properties of subchlorophins.

Boron Arylations of Subporphyrins with Aryl Zinc Reagents.

Boron arylations of B-(methoxo)triphenylsubporphyrin have been developed with a combined use of ArZnI⋅LiCl and trimethylsilyl chloride. Aryl zinc reagents bearing bromo, cyano, amide, and ester groups can be employed for the B-arylation reaction to provide the corresponding B-arylated subporphyrins in moderate yields. Postmodifications of B-arylated subporphyrins have been demonstrated without loss of the B-C bond. These modifications include conversion of the cyano group into a benzoyl group with PhMgBr, hydrolysis of the ester group to give B-(4-carboxyphenyl)subporphyrin, and Pd-catalyzed Suzuki-Miyaura coupling of the 4-bromophenyl group to give a 1,4-phenylene-bridged subporphyrin-ZnII porphyrin hybrid that displays intramolecular excitation energy transfer from the subporphyrin to the porphyrin. The newly synthesized B-arylated subporphyrins have been fully characterized by NMR, UV/Vis absorption and fluorescence spectroscopies, mass spectrometry, electrochemical measurements, and X-ray diffraction analysis.

Stable NiII Porphyrin meso-Oxy Radical with a Quartet Ground State

10,15,20-Tris(pentafluorophenyl)-substituted NiII -porphyrin meso-oxy radical bearing two coordinating pyridines was synthesized as a stable radical with a quartet ground state (S=3/2). X-ray structural analysis revealed that the NiII porphyrin moiety is fairly planar and the Ni-N bond lengths are considerably longer, indicating the high-spin state of the NiII center. The radical exhibited a quartet ground state, indicating the ferromagnetic interaction between the high-spin NiII center (S=1) and the porphyrin meso-oxy radical (S=1/2).

Observation of Diastereomeric Interconversions of β‐Sulfinylsubporphyrins as Evidence for Bowl Inversion

B-Methoxy β-(4-methoxyphenylsulfinyl)subporphyrin and B-phenyl β-(4-methoxyphenylsulfinyl)subporphyrin were synthesized by oxidation of the corresponding β-sulfanylsubporphyrins with m-chloroperbenzoic acid and were separated into diastereomers, respectively. B-Methoxy subporphyrin diastereomers were interconverted to each other in methanol or ethanol, whereas such interconversion was not observed for B-phenyl subporphyrin diastereomers even at high temperature. Diastereomeric interconversions of B-methoxy subporphyrins were dramatically accelerated by addition of trifluoroacetic acid. These results suggest that the diastereomeric interconversions of B-methoxy subporphyrins, namely, their bowl inversions, proceed via a mechanism involving protonation-induced generation of subporphyrin borenium cations followed by nucleophilic attacks by alcohols.

BIII 5-Arylsubporphyrins and BIII Subporphine

Boron(III) 5-arylsubporphyrins and BIII subporphine are promising precursors for functional BIII subporphyrins bearing asymmetric meso-substituents. Herein, we report the first synthesis of these molecules. Among many aryl acid chlorides examined, 4-nitrobenzoyl chloride gave BIII 5-(4-nitrophenyl)subporphyrin in 10 % yield in condensation with triethylamine-tri-N-tripyrromethene-borane. The nitro group of this BIII subporphyrin was reduced with NaBH4 to prepare BIII 5-(aminophenyl)subporphyrin, which was converted into BIII 5-phenylsubporphyrin via the corresponding diazonium salt. BIII subporphine was synthesized by condensation of triethyl orthoformate with triethylamine-tri-N-tripyrromethene-borane. Progressive removal of meso-phenyl substituents leads to continuous changes in the optical properties, whereas the BIII subporphine deviates from this trend in some properties.