gem-Difluorination of carbon-carbon triple bonds using Brønsted acid/Bu4NBF4 or electrogenerated acid.

gem-Difluorination of carbon-carbon triple bonds was conducted using Brønsted acids, such as Tf2NH and TfOH, combined with Bu4NBF4 as the fluorine source. The electrochemical oxidation of a Bu4NBF4/CH2Cl2 solution containing alkyne substrates could also give the corresponding gem-difluorinated compounds (in-cell method). The ex-cell electrolysis method was also applicable for gem-difluorination of alkynes.

Pheophytinization kinetics of chlorophyll c under weakly acidic conditions: effects of acrylic acid residue at the 17-position.

Pheophytinization of chlorophyll (Chl) c1, which was isolated from the diatom Chaetoceros gracilis, was kinetically analyzed under weakly acidic conditions, and was compared with that of protochlorophyllide (PChlide) a and chlorophyllide (Chlide) a. Chl c1 possessing a trans-acrylic acid residue at the 17-position exhibited slower pheophytinization kinetics than PChlide a and Chlide a, both of which possessed a propionic acid residue at the same position. The difference in pheophytinization properties between Chl c1 and (P)Chlide a was ascribable to the electronegativity of the 17-substituent in Chl c1 larger than that of (P)Chlide a due to the C17(1)-C17(2) double bond with the conjugated 17(2)-carboxy group in Chl c1. Demetalation kinetics of PChlide a was slower than that of Chlide a, which originated from the effect of the π-macrocyclic structures.

Biosynthesis of unnatural bacteriochlorophyll c derivatives esterified with α,ω-diols in the green sulfur photosynthetic bacterium Chlorobaculum tepidum.

Unnatural bacteriochlorophyll (BChl) c derivatives possessing a hydroxy group at the terminus of a hydrocarbon chain at the 17-propionate were biosynthesized in the green sulfur photosynthetic bacterium Chlorobaculum tepidum. Addition of exogenous 1,8-octanediol, 1,12-dodecanediol, and 1,16-hexadecanediol in acetone to liquid cultures resulted in accumulation of BChl c monoesterified with the corresponding diols. The relative ratios of the novel BChl c derivatives esterified with 1,8-, 1,12-, and 1,16-diols to totally producing BChl c were 8.2, 50.2, and 57.6% in the cells grown with additive α,ω-diols at concentrations of 1.5, 0.06, and 0.06 mM, respectively, at the final concentration. The homologue composition of BChl c derivatives esterified with these α,ω-diols was similar to that of original, coexisting BChl c esterified with farnesol (BChl c(F)), suggesting that esterification of α,ω-diols occurred at the last step of the BChl c biosynthetic pathway by BChl c synthase, BchK, in the same manner as in BChl c(F). Chlorosomes, which were isolated from cells grown in the presence of exogenous α,ω-diols, contained a ratio and a composition of BChl c derivatives esterified with the diols similar to those in the whole cells, indicating that these BChl c derivatives were actually present in chlorosomes. Q(y) absorption bands of C. tepidum cells containing the novel BChl c derivatives were shifted to a shorter wavelength, although their bandwidths were analogous to those of cells obtained by normal cultivation. Circular dichroism spectra of cells that had BChl c derivatives esterified with α,ω-diols exhibited S-shaped signals in the Q(y) region, whose polarities were the reverse of those of cells grown in the normal medium and by supplementation with neat acetone as a control experiment. These spectral features of C. tepidum possessing BChl c derivatives esterified with α,ω-diols imply that the novel BChl c derivatives possessing a hydroxy group at the terminus of a hydrocarbon chain affect their self-assembly in chlorosomes.

Effects of molecular structures on reduction properties of formyl groups in chlorophylls and pheophytins prepared from oxygenic photosynthetic organisms.

Reduction of the 7-formyl groups in chlorophyll (Chl) b and its demetalated compound pheophytin (Phe) b was kinetically analyzed by using tert-butylamine-borane complex (t-BuNH(2)·BH(3)), and was compared with that of the 3-formyl groups in Chl d and Phe d. Reduction kinetics of the 7-formyl group in Chl b was similar to that in Phe b in dichloromethane containing 5mM t-BuNH(2)·BH(3). Little difference of the reduction kinetics of the 7-formyl groups between Chl b and Phe b was in sharp contrast to the reduction kinetics of the 3-formyl groups in Chl d and Phe d: the 3-formyl group in Phe d was reduced 5.3-fold faster than that in Chl d. The 7-formyl groups in Chl b and Phe b were reduced more slowly than the 3-formyl groups in Chl d and Phe d, respectively. The difference of the reactivity between the 3- and 7-formyl groups was in line with (13)C NMR measurements of chlorophyllous pigments, in which the chemical shifts of carbon atoms in the 7-formyl groups of Chl b and Phe b were high-field shifted compared with those in the 3-formyl groups of Chl d and Phe d, respectively. These indicate that the 7-formyl groups in chlorophyllous pigments were less reactive for reduction to the corresponding hydroxymethyl groups than the 3-formyl groups due to the difference in electronic states of the formyl groups in the A- and B-rings of the chlorin macrocycle.

Demetalation kinetics of natural chlorophylls purified from oxygenic photosynthetic organisms: effect of the formyl groups conjugated directly to the chlorin pi-macrocycle.

Demetalation kinetics of natural chlorophyll (Chl) d purified from Acaryochloris marina was first studied and compared with those of Chls a and b. The demetalation rate constant of Chl d, which possessed a formyl group at the 3-position, was five-fold smaller than that of Chl a possessing a vinyl group at the same position in aqueous acetone at the proton concentration of 1.2 x 10(-3) M at 25 degrees C. In contrast, the demetalation rate constant of Chl b possessing a formyl group at the 7-position was 26 times smaller than that of Chl a. The activation energy of demetalation reaction of Chl d was larger than that of Chl a, but smaller than that of Chl b. These indicate that the substitution effect of 3-formyl group on the acidic removal of central magnesium in Chls was smaller than that of 7-formyl group.

Physicochemical studies of demetalation of light-harvesting bacteriochlorophyll isomers purified from green sulfur photosynthetic bacteria.

Demetalation kinetics of bacteriochlorophylls (BChls) c, d and e from green sulfur photosynthetic bacteria were studied under weakly acidic conditions. Demetalation rate constants of BChl e possessing a formyl group at the 7-position were significantly smaller than those of BChls c and d, which had a methyl group at this position. The activation energy of demetalation of 3(1)R-8,12-diethyl([E,E])-BChl e was 1.5-times larger than that of 3(1)R-[E,E]-BChl c. 15N-labeled 3(1)R-[E,E]-BChls c and e were purified from cells of green sulfur bacteria grown in a medium containing 15NH4Cl, and their 15N NMR spectra were measured. The chemical shifts of N21, N22 and N23 atoms of 3(1)R-[E,E]-BChl e were lower-field shifted than those of 3(1)R-[E,E]-BChl c, respectively, and especially the difference in chemical shifts of N22 was significantly large. These results suggest that the electron-withdrawing formyl group at the 7-position of BChl e affected an electronic state of the chlorin macrocycle and caused BChl e to be more tolerant for removal of the central magnesium compared with BChls c and d.

Low temperature in situ Raman spectroscopy of an electro-generated arylbis(arylthio)sulfonium ion.

A low temperature in situ Raman spectroscopic method was developed for the detection of unstable intermediates in electro-organic chemistry. It was effective for monitoring the generation of ArS(ArSSAr)(+) by the electrochemical oxidation of ArSSAr (Ar = p-FC6H4) in Bu4NBF4/CH2Cl2 at 195 K. The intensity of a Raman band at 427 cm(-1), which is attributable to the S-S vibration of ArS(ArSSAr)(+), increased with an increase in the electricity until 2/3 F of the electricity was consumed, whereas decreased with a further increase in the electricity indicating the decomposition of ArS(ArSSAr)(+).

Electroreductive Synthesis of Polysilanes, Polygermanes, and Related Polymers with Magnesium Electrodes(1).

The electroreduction of alkylaryldichlorosilane carried out with Mg cathode and anode in a single compartment cell gave high molecular weight poly(alkylarylsilane) (M(n) = 5200-31000, M(w)/M(n) = 1.4-1.8) in 5-79% yield. The effects of electrode material, monomer concentration, amount of supplied electricity, and ultrasound were investigated. This electroreductive method was also successfully applied to the synthesis of polygermanes, silane-geramane copolymers, and also poly[p-(disilanylene)phenylenes].

Simple and convenient synthesis of esters from carboxylic acids and alkyl halides using tetrabutylammonium fluoride.

A simple and convenient method has been developed for the synthesis of esters from the corresponding carboxylic acids and alkyl halides by using a stoichiometric amount of tetrabutylammonium fluoride (Bu4NF) as the base. The reaction of carboxylic acids and a Bu4NF/THF solution in DMF or THF as the solvent generates carboxylate ions in situ. The carboxylate ions thus generated and accumulated are highly reactive. They are then allowed to react with alkyl halides as the electrophiles, and afford the corresponding esters in moderate to good yields. The reaction without Bu4NF does not afford any product; therefore, Bu4NF seems to play an important role as the base in these reactions. A bulky countercation such as the tetrabutylammonium cation seems to increase the reactivity of the carboxylate ions in the solution phase.

Demetalation kinetics of chlorophyll derivatives possessing different substituents at the 7-position under acidic conditions.

Conversion of the formyl group at the 7-position in chlorophyll (Chl) b to the methyl group via the hydroxymethyl group is biologically important in Chl b degradation. To clarify the effects of the 7-substituents on demetalation properties of chlorophyllous pigments in the early process of Chl b degradation, we report demetalation kinetics of the zinc Chl derivative possessing a 7-hydroxymethyl group, which is a good model compound of the intermediate molecule in the early process of Chl b degradation, under acidic conditions, and compare its properties with those of zinc Chl derivatives possessing a methyl and a formyl group, which are model compounds of Chls a and b, respectively. Demetalation rate constants of 7-hydroxymethyl zinc chlorin were much larger than those of 7-formyl zinc chlorin, but were slightly smaller than those of 7-methyl zinc chlorin. The activation energy for demetalation reaction of 7-formyl zinc chlorin was larger than those of other derivatives. Demetalation rate constants of 7-deformyl-7-hydroxymethyl Chl b were also larger than those of Chl b, and were similar to those of Chl a. These indicate that the 7-hydroxymethyl group in the chlorin macrocycle has a smaller effect on demetalation compared with the 7-formyl group.

Substitution effects in the A- and B-rings of the chlorin macrocycle on demetalation properties of zinc chlorophyll derivatives.

Effects of peripheral substituents in the A- and B-rings of the chlorin macrocycle on demetalation of zinc chlorophyll derivatives were examined. The demetalation rate constants of zinc 8-formyl-chlorin (1) were comparable to those of zinc 3-formyl-chlorin (2). Both 1 and 2 exhibited significantly slower demetalation kinetics than the corresponding zinc chlorin 3, which had no formyl group. Comparison of demetalation kinetics between zinc 3-ethyl-chlorin (3) and zinc 3-vinyl-chlorin (4) indicated that the substitution at the 3-position of the chlorin macrocycle from a vinyl to an ethyl group accelerated removal of the central zinc. Activation energies of demetalation of zinc chlorins 1, 2, and 5 possessing a formyl group in the A- or B-ring of the chlorin macrocycle were larger than those of zinc chlorins 3 and 4 lacking a formyl group. These results indicate that the formyl groups in the A- or B-ring provide tolerance to demetalation of chlorin molecules due to their electron-withdrawing effects.