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In this study, we applied the concept of the "contribution factor of the first kind (CFFK)" to the original electron-transfer (ET) rate theory proposed by Marcus. Mathematical derivations provided simple and convenient formulas for estimating the relative contributions of ten physical and chemical parameters involved in the Marcus ET rate formula: (1) the maximum strength of the electronic coupling energy between two molecules, (2) the exponential decay rate of the electronic coupling energy versus the distance between both molecules, (3) the distance between both molecules, (4) the equilibrium distance between both molecules, (5) the Gibbs free energy, (6) reorganization free energy in the prefactor of the Marcus ET rate equation, (7) reorganization free energy in the denominator of the exponential term, (8) reorganization free energy in the argument of the exponential term, (9) Boltzmann constant times absolute temperature in the prefactor of the rate equation, and (10) Boltzmann constant times absolute temperature in the denominator of the exponential term. We applied our theories to (i) ET reactions at bacterial photosynthesis reaction centers, PSI and PSII, and soluble ferredoxins (Fd); (ii) intraprotein ET reactions for designed azurin mutants; and (iii) ET reactions in flavodoxin (Fld). The formulas and calculations suggest that the theory behind the CFFK is useful for quantitatively identifying major and minor physical and chemical factors and corresponding trade-offs, all of which affect the magnitude of the Marcus ET rate.

Herein, we report the transition-metal-free O-arylation of alcohols and phenols with S -arylphenothiaziniums, which can be easily synthesized from boronic acids. Aryl substituents derived from arylboronic acids were selectively introduced into the hydroxy groups in alcohols and phenols, and a variety of aryl ethers were synthesized. This selectivity is supported by theoretical calculations.

Herein, we report the development of a transition-metal-free oxidative C(sp 2 )-C(sp 2 ) coupling of readily available boronic acids and organolithiums via phenothiazinium ions. Various biaryl, styrene, and diene derivatives were obtained using this reaction system. The key to this process is N -methylphenothiazine S -oxide (PTZSO), which allows efficient conversion of boronic acids to phenothiazinium ions. The mechanism of phenothiazinium formation using PTZSO was investigated using theoretical calculations and experiments, which provided insight into the unique reactivity of PTZSO.

The intermolecular dearomative cycloaddition of acidified bicyclic azaarenes with olefins was recently reported. We report here the crucial role of the acid in the dearomative photocycloaddition of quinolines to olefins. Experimental and theoretical results show that the key role of the protonation of quinolines is not to promote the energy transfer but to enhance the reactivity of the triplet state of quinolines toward olefins.

Herein we report a method for converting para -substituted pyridine rings into meta -dialkylamino-substituted benzene rings through sequential ring-opening and ring-closing reactions. The nitrogen atom in the pyridine rings was replaced with a methine group, and a dialkylamino substituent was introduced onto the original unsubstituted carbon atom in the pyridine rings. This process can be formally regarded as a hybrid of the skeletal editing and C-H amination of pyridine rings.

Enzymes involved in the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs) often have relaxed specificity profiles and are able to modify diverse substrates. When several such enzymes act together during precursor peptide maturation, a multitude of products can form, yet usually the biosynthesis converges on a single natural product. For the most part, the mechanisms controlling the integrity of RiPP assembly remain elusive. Here, we investigate the biosynthesis of lactazole A, a model thiopeptide produced by five promiscuous enzymes from a ribosomal precursor peptide. Using our in vitro thiopeptide production (FIT-Laz) system, we determine the order of biosynthetic events at the individual modification level and supplement this study with substrate scope analysis for participating enzymes. Our results reveal an unusual but well-defined assembly process where cyclodehydration, dehydroalanine formation, and azoline dehydrogenation events are intertwined due to minimal substrate recognition requirements characteristic of every lactazole enzyme. Additionally, each enzyme plays a role in directing LazBF-mediated dehydroalanine formation, which emerges as the central theme of the assembly process. Cyclodehydratase LazDE discriminates a single serine residue for azoline formation, leaving the remaining five as potential dehydratase substrates. Pyridine synthase LazC exerts kinetic control over LazBF to prevent the formation of overdehydrated thiopeptides, whereas the coupling of dehydrogenation to dehydroalanine installation impedes generation of underdehydrated products. Altogether, our results indicate that substrate-level cooperation between the biosynthetic enzymes maintains the integrity of lactazole assembly. This work advances our understanding of RiPP biosynthesis processes and facilitates thiopeptide bioengineering.

We report the photocatalytic C-H amination of aromatics overcoming redox potential limitations. Radical cations of aromatic compounds are generated photocatalytically using Ru(phen) 3 (PF 6 ) 2 , which has a reduction potential at a high oxidation state ( E red (Ru III /Ru II ) = +1.37 V vs SCE) lower than the oxidation potentials of aromatic substrates ( E ox = +1.65 to +2.27 V vs SCE). The radical cations are trapped with pyridine to give N -arylpyridinium ions, which were converted to aromatic amines.

Two of five conceivable geometrical isomers of a hexacoordinated dihydrophosphate bearing two sets of a bidentate ligand were investigated. X-ray crystallographic analysis of both of isomers, 1a-TPP and 1b-TEA, revealed their octahedral geometries of C(2) and C(1) symmetry, respectively, which were consistent with the NMR spectra. The isomer 1b-TEA underwent both hydride reduction of an aldehyde and proton exchange with water at room temperature in DMSO without any additive. A one-pot reaction of both of the reactions of 1b-TEA with D(2)O and an aldehyde or a ketone under the above conditions proceeded successfully to give the deuterated alcohol. Thus, umpolung of a hydrogen atom of water with 1b-TEA was achieved under much milder conditions than those used in the reaction with another isomer, 1a-TEA. Quantitative isomerization of 1b-TEA to 1a-TEA occurred in methanol at room temperature. Calculations on the five conceivable geometrical isomers of the anionic part of the dihydrophosphate revealed their relative stability, which reasonably explained the isomerization, and the larger negative charge at the atoms located at the trans positions of the oxygen atoms. The smaller coupling constants of the P-H and P-C bonds located at the rear of an oxygen atom in the NMR spectra resulted in the smaller s character of these bonds. The differences in both hydride-donation and proton-exchange reactivities between 1a-TEA and 1b-TEA could be explained by the differences in the atomic charge of the hydrogen atom and the stability difference of the initially formed phosphorane intermediates, respectively.

The polarity of a chemical bond is an important factor that determines the reactivity because a reaction of a positively charged atom with a negatively charged atom easily occurs. We report here a method for reversing the polarity of a hydrogen atom of water, which meant conversion of a protic hydrogen (H(delta+)) to a hydridic hydrogen (H(delta-)), by using a hexacoordinated phosphorus species. The method enables a reductive deuteration of carbonyl compounds utilizing inexpensive and easily usable D(2)O under mild conditions. The umpolung of a hydrogen atom was achieved by utilizing tautomerization of phosphorus compounds. Classical metal hydrides never have this function, and these results demonstrated the potential utility of nonmetallic compounds for umpolung of a hydrogen atom.

N-Aryl, N-alkyl, N-alkoxy, and N-amino derivatives of 2-[bis(pentafluorophenyl)boryl]benzylideneamine were synthesized by the condensation reactions of 2-[bis(pentafluorophenyl)boryl]benzaldehyde with the corresponding amines. Their structures were investigated by NMR and X-ray crystallographic analysis. Their properties were investigated by UV-vis and fluorescence spectroscopy. The boryl-substituted N-arylimines show blue or green fluorescence in hexane at room temperature, and their fluorescence efficiency is much higher than that of N-benzylideneaniline. In particular, the boryl-substituted N-(4-dimethylaminophenyl)imine showed strong green emissions with at least 7000 times higher fluorescence quantum yield (0.73) compared with that of N-benzylideneaniline. The boryl-substituted N-(1-indolyl)- and N-(9-carbazolyl)imines showed dual emissions, one of which was assignable as arising from the lowest singlet excited state and the other from the local excited state of the substituent on the imine nitrogen. The fluorescent properties of the boryl-substituted N-butyl- and N-methoxyimines were also investigated. Reactions of the N-arylimine derivatives with cyanide ion gave the corresponding cyanide adducts and quenched the fluorescence, indicating that these 2-[bis(pentafluorophenyl)boryl]benzylideneamine derivatives have a potential as a cyanide ion sensor.

2,2'-Bis(fluorodiphenylsilyl)azobenzenes bearing two methyl and butyl groups at the 4- and 4'-positions were synthesized, and their X-ray crystallographic analyses revealed that they have tetracoordinated and pentacoordinated silicon atoms, respectively. Although the two compounds showed almost the same spectra in solution, the reflectance spectra of the solid revealed that the pentacoordinated state had a weaker absorbance in the long-wavelength region compared with that of the tetracoordinated state, which is ascribed to the shift of the n-pi* transition. This difference in the absorptions due to the n-pi* transition drastically affects the color of the azobenzenes: the color of the pentacoordinated state becomes apparently paler than that of the tetracoordinated state.

Thermolysis of a pentacoordinate 1lambda(6),2-thiazetidine, which was synthesized for the first time and characterized by X-ray crystallographic analysis, gave the corresponding aziridine and a cyclic sulfinate almost quantitatively. The potential intermediacy of a 1lambda(6),2-thiazetidine was suggested in the aza-Corey-Chaykovsky reaction.

Hydrazobenzenes 3-5 bearing a chalcogenophosphoryl group were synthesized by palladium-catalyzed cross-coupling reactions. Their X-ray crystallographic analyses and NMR and IR spectra showed the presence of intramolecular hydrogen bonds between the N-H protons and the chalcogenophosphoryl groups. The intermolecular hydrogen bonds in phosphine oxide 3 and selenide 5 were observed in the solid state. Phosphine oxide 3, sulfide 4, and selenide 5 constructed a dimeric structure, a discrete monomeric structure, and a chain structure, respectively. As the chalcogen atom changed, the crystalline structures of the 2-chalcogenophosphorylhydrazobenzenes also changed. The hydrogen bonds affected the oxidation reactions of the hydrazobenzenes, and oxidation of hydrazobenzenes bearing a lighter chalcogen atom was more difficult. For azobenzenes bearing a chalcogenophosphoryl group, X-ray crystallographic analyses and NMR spectra showed little interaction between the azo group and the chalcogenophosphoryl groups. However, in the UV-vis spectra, the red shifts of the absorption maxima due to the n --> pi transitions indicated intramolecular interactions in the excited state, in contrast to the corresponding 4-substituted azobenzenes. In addition, photoirradiation of phosphine oxide (E)-7 gave (Z)-7, whereas that of phosphine sulfide (E)-8 and phosphine selenide (E)-9 did not give (Z)-8 and (Z)-9, suggesting that heavy chalcogen atoms quench excited states by through-space interactions. Introduction of a chalcogenophosphoryl group at the 2-position had a significant effect on the structure, spectral properties, and reactivity of hydrazobenzenes and azobenzenes. Although azobenzene (E)-10 bearing a hydroxyphosphoryl group at the 2-position did not show hydrogen bonding in the crystalline state, its optical properties and photoisomerization ratio were different from those of (E)-7.

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a very high degree of p character, which results from their being both basal bonds in the SP structure and endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations, it can be considered that a sigma-pi interaction between the sigma orbital of the apical bond and the pi orbital of the C=C bond in the three-membered ring is operative in pentacoordinate phosphirenes. The sigma-pi interaction is found to lower the reactivity of the C=C bond of the three-membered ring. The reactivities of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-membered ring.

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of 2-iodoazobenzene. Their structures were determined by (1)H, (13)C, (19)F, and (29)Si NMR spectra, UV-vis spectra, and X-ray crystallographic analyses. In the UV-vis spectra, silyl groups caused red shifts of both the n-pi and pi-pi transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the Z-isomers were found to be tetracoordinate in the absence of Si-N interactions by the (29)Si NMR spectra. The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]silicate.

[reaction: see text] Pentacoordinate 1,2-oxastibetanes 14a-d, which are formal [2 + 2]-cycloadducts of the reactions of stibonium ylides with carbonyl compounds, were successfully synthesized by the reactions of the corresponding bromo-2-hydroxyalkylstiboranes with NaH. The crystal structures of 14a and 14c were established by X-ray crystallographic analyses, showing their distorted trigonal bipyramidal structures and smaller C-Sb-O angles of the four-membered ring around antimony than the C-P-O angle of pentacoordinate 1,2-oxaphosphetane 3. The 1H, 13C, and 19F NMR spectra of 14a-d are consistent with the trigonal bipyramidal structure in the solution state. Although 14a did not decompose at all at 220 degrees C in o-xylene-d(10), the thermolyses of 3-phenyl-1,2-oxastibetane 14c were carried out at 220 degrees C in o-xylene-d(10) and at 140 degrees C in acetonitrile-d(3) to give the corresponding oxirane 28 with retention of configuration and cyclic stibinite 25. The formation of 28 is explained by apical-equatorial ligand coupling around antimony via a polar transition state, which is more favorable than olefin formation. In contrast, the thermolyses of 14c in the presence of LiBr and LiBPh4 gave oxirane 29 with inversion of configuration and the olefin 30, respectively. The formation of 29 and 30 is considered to proceed via an anti-betaine-type intermediate and hexacoordinate 1,2-oxastibetanide 36, respectively. Selective formation of 28, 29, and 30 in the thermolyses of 14c, which is regarded as an intermediate in the reaction of an alpha-phenyl-substituted stibonium ylide with a carbonyl compound, showed that the change of the reaction conditions controls the reactivity of a 1,2-oxastibetane compound.

Photoisomerization of a catecholborane bearing a 2-(phenylazo)phenyl group with an N-B dative bond caused photoswitching of the coordination number of boron between 3 and 4. The Lewis acidity of the catecholborane was switched by photoirradiation, and the complexation ability of the (E)- and the (Z)-isomers of the catecholborane with pyridine differs by more than a factor of 300. [reaction: see text]

The reversible conversion between a phosphine and a phosphonium salt has been achieved by external stimuli of light and heat. Two 2-phosphinoazobenzenes were successfully synthesized by desulfurization of the corresponding phosphine sulfides. One of the phosphines bearing an azo group was in equilibrium with an inner phosphonium salt and showed thermochromism, which is derived from the change of the equilibrium constant depending on the temperature. While the 2-phosphinoazobenzene reacted as a usual triarylphosphine, its reaction with water gave phosphine oxide bearing a hydrazine moiety via a mechanism similar to the Mitsunobu reaction. The 2-phosphinoazobenzene bearing a methyl group at the 4'-position of azobenzene was isomerized to the Z-isomer by irradiation. The Z-isomer was neither in equilibrium with an inner phosphonium salt nor hydrolyzed, in contrast to the E-isomer, because its geometry is difficult for an intramolecular nucleophilic attack. Photoisomerization caused the switching of the unique reactivity toward water. Such phosphines in equilibrium with the inner phosphonium salts are expected to be useful to control organic reactions by taking advantage of the photoisomerization of the azobenzene moiety.

Photoswitching of the coordination number of silicon between four and five in allyldifluoro[2-(phenylazo)phenyl]silane, which was confirmed by X-ray analysis and multinuclear NMR spectroscopy, caused multistep reactions to proceed or stop, yielding tetrafluoro[2-(1-allyl-2-phenylhydrazino)phenyl]silicate without altering other reaction conditions.

Isomeric pentacoordinate 1,2-oxastibetanes bearing the Martin ligand have been synthesized by the reactions of bromo(2-hydroxyalkyl)stiboranes with sodium hydride of which one of the isomers was characterized by X-ray crystallographic analysis and NMR spectroscopy. Thermolysis of the isomer shown (CD3CN, 140 degrees C) afforded the oxirane with retention of configuration, while its thermolyses in the presence of lithium bromide and LiBPh4.3DME provided the oxirane with inversion of configuration and the olefin. The addition of the salts in the thermolyses of this pentacoordinate 1,2-oxastibetane controls both the stereochemistry of the oxirane formed and the ratios of the three main products.

A 1,2sigma5-selenaphosphirane bearing the Martin ligand was synthesized. Its structure in both the solid and the solution state was established by X-ray crystallographic analysis and NMR measurements, respectively. The selenaphosphirane has a polar P-Se bond, and the selenium atom is negatively charged. In the solution state, the degree of polarization of the P-Se bond depends on the acceptor number of the solvents.

[structure: see text]. The first tetracoordinate 1lambda4,2-selenazetidines were synthesized by taking advantage of the Martin ligand and characterized by X-ray crystallographic analysis. The selenazetidines gave the corresponding aziridine and the cyclic selenenate on their thermolysis, which indicates the possibility that a highly coordinate 1,2-heterachalcogenetane may provide the corresponding heteracyclopropane regardless of the heteroatoms.