藤田 守文

Alkenyl(phenyl)iodonium tetrafluoroborates dissolved in chloroform, or in the solid state, decompose thermally at 60 degrees C to yield fluoroalkenes and iodobenzene as major products via an S(N)1- or S(N)2-type reaction within the ion pair of the substrates. (C) 2000 Elsevier Science Ltd. All rights reserved.

Alkenyl(phenyl)iodonium tetrafluoroborates dissolved in chloroform, or in the solid state, decompose thermally at 60 degrees C to yield fluoroalkenes and iodobenzene as major products via an S(N)1- or S(N)2-type reaction within the ion pair of the substrates. (C) 2000 Elsevier Science Ltd. All rights reserved.

C-2 symmetric chiral 2,2'-bis(triisopropylsilyloxymethyl)bi(dihydropyran)s (S,S)-1 and (R,R)-1 were prepared from the corresponding glycidols and selectively reacted with 1,2-diols to give dispiroketals. The products of these reactions could be deprotected following treatment with fluoride, oxidation and reductive cleavage with samarium(II) iodide.

Glycolic acid can be converted to optically active 1,2,3,4-tetraols using a dispiroketal unit as a protecting group and chiral auxiliary. Aldol reactions of dispiroketal protected glycolate with aldehydes afford one diastereoisomer preferentially with two newly formed stereogenic centres. To extend the polyol chain, the carbonyl group of the aldol product is converted to a vinyl ether by the Tebbe reagent after protection of the free alcohol. A subsequent hydroboration-oxidation protocol affords the dispiroketal protected tetraol. The final deprotection of the tetraol occurs selectively without epimerisation or migration of the silyloxy protecting groups.

C-2 symmetric chiral 2,2'-bis(triisopropylsilyloxymethyl)bi(dihydropyran)s (S,S)-1 and (R,R)-1 were prepared from the corresponding glycidols and selectively reacted with 1,2-diols to give dispiroketals. The products of these reactions could be deprotected following treatment with fluoride, oxidation and reductive cleavage with samarium(II) iodide.

Glycolic acid can be converted to optically active 1,2,3,4-tetraols using a dispiroketal unit as a protecting group and chiral auxiliary. Aldol reactions of dispiroketal protected glycolate with aldehydes afford one diastereoisomer preferentially with two newly formed stereogenic centres. To extend the polyol chain, the carbonyl group of the aldol product is converted to a vinyl ether by the Tebbe reagent after protection of the free alcohol. A subsequent hydroboration-oxidation protocol affords the dispiroketal protected tetraol. The final deprotection of the tetraol occurs selectively without epimerisation or migration of the silyloxy protecting groups.

The Diels-Alder reaction of tetracyanoethylene (TCNE) with a l-trienol unit in the tropilidenes at the 1,4-position was a quick and reversible process, whereas the 3,6-addition only proceeded in polar solvent and was irreversible.

The presence of a zwitterionic intermediate during the oxygenation of electron-rich naphthalenes with singlet oxygen is clarified by the clean and efficient formation of hydroperoxide via nucleophilic addition of the intramolecular alcohol to the intermediate and by its stereochemical analysis using an optically active linker of the alcohol.

The Diels-Alder reaction of tetracyanoethylene (TCNE) with a l-trienol unit in the tropilidenes at the 1,4-position was a quick and reversible process, whereas the 3,6-addition only proceeded in polar solvent and was irreversible.

The presence of a zwitterionic intermediate during the oxygenation of electron-rich naphthalenes with singlet oxygen is clarified by the clean and efficient formation of hydroperoxide via nucleophilic addition of the intramolecular alcohol to the intermediate and by its stereochemical analysis using an optically active linker of the alcohol.

A novel stereochemical approach is employed in anodic oxidation of naphthalene derivatives to discriminate the intramolecular radical addition vs, intermolecular radical addition paths; the contribution of the latter is revealed to be important,judging from the stereodifferentiating addition of MeOH at C-4 during the anodic oxidation of (1'S,3'R)-1-(3'-hydroxy-1'-methylbutoxy)-4-methylnaphthalene.

A novel stereochemical approach is employed in anodic oxidation of naphthalene derivatives to discriminate the intramolecular radical addition vs, intermolecular radical addition paths; the contribution of the latter is revealed to be important,judging from the stereodifferentiating addition of MeOH at C-4 during the anodic oxidation of (1'S,3'R)-1-(3'-hydroxy-1'-methylbutoxy)-4-methylnaphthalene.

(1S, 2R, 6RS)-1,2,6-Trimethyldecyl propionate, a lower homolog of the sex pheromone of known sawflies, strongly attracted Diprion nipponica, a popular species in Japan.

Forward and reverse hydride transfer with rigorous diastereodifferentiation is attained using intramolecular Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation of a 2-acetylphenyl ether containing an optically active 3-hydroxy-1-methylbutyl group.

Forward and reverse hydride transfer with rigorous diastereodifferentiation is attained using intramolecular Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation of a 2-acetylphenyl ether containing an optically active 3-hydroxy-1-methylbutyl group.

Addition of alkylbenzenes with 10-methylacridinium ion (AcrH+) occurs efficiently under visible light irradiation in deaerated acetonitrile containing H2O to yield 9-alkyl-10-methyl-9,10-dihydroacridine selectively. On the other hand, the photochemical reaction of AcrH+ with alkylbenzenes in the presence of perchloric acid in deaerated acetonitrile yields 10-methyl-9,10-dihydroacridine, accompanied by the oxygenation of alkylbenzenes to the corresponding benzyl alcohols. The photooxygenation of alkylbenzenes occurs also in the presence of oxygen, when AcrH+ acts as an efficient photocatalyst. The studies on the quantum yields and fluorescence quenching of AcrH+ by alkylbenzenes as well as the laser flash photolysis have revealed that the photochemical reactions of AcrH+ with alkylbenzenes in both the absence and presence of oxygen proceed via photoinduced electron transfer from alkylbenzenes to the singlet excited state of AcrH+ to produce alkylbenzene radical cations and 10-methylacridinyl radical (AcrH·). The competition between the deprotonation of alkylbenzene radical cations and the back electron transfer from AcrH· to the radical cations determines the limiting quantum yields. In the absence of oxygen, the coupling of the deprotonated radicals with AcrH· yields the adducts. The photoinduced hydride reduction of AcrH+ in the presence of perchloric acid proceeds via the protonation of acridinyl radical produced by the photoinduced electron transfer from alkylbenzenes. In the presence of oxygen, however, the deprotonated radicals are trapped efficiently by oxygen to give the corresponding peroxyl radicals which are reduced by the back electron transfer from AcrH· to regenerate AcrH+, followed by the protonation to yield the corresponding hydroperoxide. The ratios of the deprotonation reactivity from different alkyl groups of alkylbenzene radical cations were determined from both the intra- and intermolecular competitions of the deprotonation from two alkyl groups of alkylbenzene radical cations. The reactivity of the deprotonation from alkylbenzene radical cations increases generally in the order methyl &lt ethyl &lt isopropyl. The strong stereoelectronic effects on the deprotonation from isopropyl group of alkylbenzene radical cations appear in the case of the o-methyl isomer.

Mechanism of Mukaiyama-Michael reaction of ketene silyl acetal has been discussed. The competition reaction employing various types of ketene silyl acetals reveals that those bearing more substituents at the beta-position react preferentially over less substituted ones. However, when ketene silyl acetals involve bulky siloxy and/or alkoxy group(s), less substituted compounds react preferentially. The Lewis acids play an important role in these reactions. Enhanced preference for the more sterically demanding Michael adducts is obtained with Bu(2)Sn(OTf)(2), SnCl4, and Et(3)-SiClO4 in the former reaction while TiCl4 gives the highest selectivity for the less sterically demanding products in the latter case. These results are interpreted in terms of alternative reaction mechanisms. The reaction of less bulky ketene silyl acetals are initiated by electron transfer from these compounds to a Lewis acid. On the other hand, bulkier ketene silyl acetals undergo a ubiquitous nucleophilic reaction. Such a mechanistic change is discussed based on a variety of experimental results as well as the semiempirical PM3 MO calculations.

Mechanism of Mukaiyama-Michael reaction of ketene silyl acetal has been discussed. The competition reaction employing various types of ketene silyl acetals reveals that those bearing more substituents at the beta-position react preferentially over less substituted ones. However, when ketene silyl acetals involve bulky siloxy and/or alkoxy group(s), less substituted compounds react preferentially. The Lewis acids play an important role in these reactions. Enhanced preference for the more sterically demanding Michael adducts is obtained with Bu(2)Sn(OTf)(2), SnCl4, and Et(3)-SiClO4 in the former reaction while TiCl4 gives the highest selectivity for the less sterically demanding products in the latter case. These results are interpreted in terms of alternative reaction mechanisms. The reaction of less bulky ketene silyl acetals are initiated by electron transfer from these compounds to a Lewis acid. On the other hand, bulkier ketene silyl acetals undergo a ubiquitous nucleophilic reaction. Such a mechanistic change is discussed based on a variety of experimental results as well as the semiempirical PM3 MO calculations.

beta,beta-Dimethyl-substituted ketene silyl acetal (1a) reduces p-chloranil and other activated quinones with electron-withdrawing substituents to produce the carbon-oxygen adduct, the hydrolysis of which yields the corresponding hydroquinone ether. The structure of the hydroquinone ether has been determined by the X-ray crystal analysis. The reactions are significantly slowed down in benzene where the charge-transfer spectra of electron donor-acceptor complexes formed between la and the activated quinones are observed. The comparison of the observed rate constant with that predicted for the electron transfer process from la to p-chloranil indicates that the addition of 1a to p-chloranil proceeds via the electron transfer from 1a to p-chloranil. Although no reaction takes place between 1a and p-benzoquinone, the electron affinity of which is significantly smaller than that of p-chloranil, the reduction of p-benzoquinone by 1a occurs efficiently in the presence of magnesium ion. The kinetic expression of the Mg2+ catalysis changes from the first-order to second-order in [Mg2+] under the conditions that Mg2+ forms the 2 : 1 complexes with the corresponding radical anions. The catalytic effects of Mg2+ are approximately the same as those observed for the electron transfer reduction of these oxidants, demonstrating the important contribution of the Mg2+-catalyzed electron transfer process in the addition of 1a to p-benzoquinone. On the other hand, the reaction of a nonsubstituted ketene silyl acetal (1d) with p-fluoranil yields the carbon-carbon adduct rather than the carbon-oxygen adduct. The much larger rate constants of 1d than those of 1a despite the higher oxidation potential of 1d suggest that the 1,2-addition to p-fluoranil occurs via the nucleophilic attack of 1d, which is much less sterically hindered than 1a, to the positively charged carbonyl carbon of p-fluoranil rather than an alternative electron transfer pathway.

beta,beta-Dimethyl-substituted ketene silyl acetal (1a) reduces p-chloranil and other activated quinones with electron-withdrawing substituents to produce the carbon-oxygen adduct, the hydrolysis of which yields the corresponding hydroquinone ether. The structure of the hydroquinone ether has been determined by the X-ray crystal analysis. The reactions are significantly slowed down in benzene where the charge-transfer spectra of electron donor-acceptor complexes formed between la and the activated quinones are observed. The comparison of the observed rate constant with that predicted for the electron transfer process from la to p-chloranil indicates that the addition of 1a to p-chloranil proceeds via the electron transfer from 1a to p-chloranil. Although no reaction takes place between 1a and p-benzoquinone, the electron affinity of which is significantly smaller than that of p-chloranil, the reduction of p-benzoquinone by 1a occurs efficiently in the presence of magnesium ion. The kinetic expression of the Mg2+ catalysis changes from the first-order to second-order in [Mg2+] under the conditions that Mg2+ forms the 2 : 1 complexes with the corresponding radical anions. The catalytic effects of Mg2+ are approximately the same as those observed for the electron transfer reduction of these oxidants, demonstrating the important contribution of the Mg2+-catalyzed electron transfer process in the addition of 1a to p-benzoquinone. On the other hand, the reaction of a nonsubstituted ketene silyl acetal (1d) with p-fluoranil yields the carbon-carbon adduct rather than the carbon-oxygen adduct. The much larger rate constants of 1d than those of 1a despite the higher oxidation potential of 1d suggest that the 1,2-addition to p-fluoranil occurs via the nucleophilic attack of 1d, which is much less sterically hindered than 1a, to the positively charged carbonyl carbon of p-fluoranil rather than an alternative electron transfer pathway.

Photooxygenation of 1,1-diarylethylene occurs efficiently using 10-methylacridinium ion as a photocatalyst to yield the 1,2-dioxane and/or the diaryl ketone depending on the substituents on the aryl groups. The reaction mechanism is revealed based on the dependence of the quantum yields on the concentrations of the alkene and oxygen, the fluorescence quenching of 10-methylacridinium ion by the alkene, and the direct detection of reactive intermediates by applying laser flash spectroscopy as well as pulse radiolysis. The photooxygenation proceeds via photoinduced electron transfer from the alkene to the singlet excited state of 10-methylacridinium ion. The alkene radical cation formed by the photoinduced electron transfer reacts with alkene to give the 1,4-dimer radical cation, which then reacts with oxygen to produce the oxygenated 1,6-radical cation. The subsequent one-electron reduction of the 1,6-radical cation results in formation of the 1,6-biradical which cyclizes to yield 1,2-dioxane derivative or fragmentates to yield diaryl ketone. When the 1,6-biradical is reduced by the alkene itself, the alkene radical cation is regenerated to repeat the radical chain process.

The selective eletron-transfer quenching of the radical anions of dicyanoanthracene, phenazine, and anthraquinones in the excited state by a quencher such as fumaronitrile or dicyanobenzene is investigated in N,N-dimethylformamide solution at room temperature using the pulse radiolysis-laser flash photolysis combined method. The radical anions generated by pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the excited state decay into the ground state within the laser flash (5 ns). Lifetimes of approximately 4 ns are estimated for three radical anions in the excited state assuming a diffusion-controlled rate constant for the electron-transfer quenching. The shorter lifetimes of 1.0-1.4 ns for methyl and chloro substituents on anthraquinone are discussed in terms of internal conversion from the excited to the ground state of the radical anions accelerated by rotation of the substituents. The energy gap between the excited and ground states of the radical anions is a significant factor for the rate of the internal conversion. The quencher radical anion-neutral molecule pair is suggested as an intermediate in the electron-transfer quenching of the radical anions during the excited state by the quencher and is discussed with respect to separation and back electron transfer in the pair.

Photooxygenation of 1,1-diarylethylene occurs efficiently using 10-methylacridinium ion as a photocatalyst to yield the 1,2-dioxane and/or the diaryl ketone depending on the substituents on the aryl groups. The reaction mechanism is revealed based on the dependence of the quantum yields on the concentrations of the alkene and oxygen, the fluorescence quenching of 10-methylacridinium ion by the alkene, and the direct detection of reactive intermediates by applying laser flash spectroscopy as well as pulse radiolysis. The photooxygenation proceeds via photoinduced electron transfer from the alkene to the singlet excited state of 10-methylacridinium ion. The alkene radical cation formed by the photoinduced electron transfer reacts with alkene to give the 1,4-dimer radical cation, which then reacts with oxygen to produce the oxygenated 1,6-radical cation. The subsequent one-electron reduction of the 1,6-radical cation results in formation of the 1,6-biradical which cyclizes to yield 1,2-dioxane derivative or fragmentates to yield diaryl ketone. When the 1,6-biradical is reduced by the alkene itself, the alkene radical cation is regenerated to repeat the radical chain process.

The selective eletron-transfer quenching of the radical anions of dicyanoanthracene, phenazine, and anthraquinones in the excited state by a quencher such as fumaronitrile or dicyanobenzene is investigated in N,N-dimethylformamide solution at room temperature using the pulse radiolysis-laser flash photolysis combined method. The radical anions generated by pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the excited state decay into the ground state within the laser flash (5 ns). Lifetimes of approximately 4 ns are estimated for three radical anions in the excited state assuming a diffusion-controlled rate constant for the electron-transfer quenching. The shorter lifetimes of 1.0-1.4 ns for methyl and chloro substituents on anthraquinone are discussed in terms of internal conversion from the excited to the ground state of the radical anions accelerated by rotation of the substituents. The energy gap between the excited and ground states of the radical anions is a significant factor for the rate of the internal conversion. The quencher radical anion-neutral molecule pair is suggested as an intermediate in the electron-transfer quenching of the radical anions during the excited state by the quencher and is discussed with respect to separation and back electron transfer in the pair.

Photoinduced hydride reduction of 10-methylacridinium ion (AcrH(+)) by alkylbenzene occurs to yield 10-methyl-9,10-dihydroacridine in the presence of perchloric acid, while photoaddition occurs to yield 9-alkyl-10-methyl-9,10-dihydroacridine in the absence of perchloric acid. The hydride reduction of AcrH(+) in the presence of perchloric acid proceeds via protonation of acridinyl radical produced by photoinduced electron transfer from alkylbenzene.

Various NAD+ analogues have been reduced regioselectively by the tetramethylammonium salt of 2-nitropropane anion in acetonitrile at 298 K to yield the corresponding 4-alkylated NADH analogues. The one-electron oxidation potential of the tetramethylammonium salt of 2-nitropropane anion has been determined as 0.10 V (vs. SCE) by using second harmonic ac voltammetry as well as by analysing the cyclic voltammograms at various sweep rates. The rate constants for the reduction of NAD+ analogues by 2-nitropropane anion (>1 x 10(6) dm3 mol-1 s-1) are much larger than those estimated for outer-sphere electron transfer from 2-nitropropane anion to NAD+ analogues based on the one-electron oxidation potential of 2-nitropropane anion and the one-electron reduction potentials of NAD+ analogues. The origin of the regioselectivity is discussed in terms of the HSAB (hard and soft acids and bases) principle.

Various NAD+ analogues have been reduced regioselectively by the tetramethylammonium salt of 2-nitropropane anion in acetonitrile at 298 K to yield the corresponding 4-alkylated NADH analogues. The one-electron oxidation potential of the tetramethylammonium salt of 2-nitropropane anion has been determined as 0.10 V (vs. SCE) by using second harmonic ac voltammetry as well as by analysing the cyclic voltammograms at various sweep rates. The rate constants for the reduction of NAD+ analogues by 2-nitropropane anion (>1 x 10(6) dm3 mol-1 s-1) are much larger than those estimated for outer-sphere electron transfer from 2-nitropropane anion to NAD+ analogues based on the one-electron oxidation potential of 2-nitropropane anion and the one-electron reduction potentials of NAD+ analogues. The origin of the regioselectivity is discussed in terms of the HSAB (hard and soft acids and bases) principle.

The efficient oxygenation of alkylbenzenes is initiated by photoinduced electron transfer from alkylbenzenes to the singlet excited state of 10-methylacridinium ion under visible light irradiation, followed by the deprotonation of alkylbenzene radical cations and the facile addition of oxygen to the deprotonated radicals, leading to the final oxygenated products. © 1994.

An excellent linear relationship is obtained between the relative reactivities of the carbonyl compounds in the triethylsilyl perchlorate- and perchloric acid-catalyzed reduction by triethylsilane demonstrating that identical catalytic mechanisms are operative in both the Bronsted acid (perchloric acid) and the Lewis acid (triethylsilyl perchlorate) systems in solution.

The isopropyl group has no significant stereoelectronic effect on inter- and intra-molecular competition in deprotonation from alkylbenzene radical cations in the photoaddition of alkylbenzene derivatives with 10-methylacridinium ion via photoinduced electron transfer from alkylbenzene derivatives to the singlet excited state of 10-methylacridinium ion, followed by the deprotonation of alkylbenzene radical cations.

Photoaddition of beta,beta-dimethyl-substituted ketene silyl acetals to 10-methylacridone occurs efficiently under irradiation of the visible light in benzene as well as acetonitrile to yield the siloxy adduct. The carbon-oxygen bond of the adduct is readily cleaved by an acid to yield the corresponding 9-alkylated 10-methylacridinium ion. The comparison of the observed rate constants determined from the dependence of the quantum yields on the concentrations of ketene silyl acetals as well as the fluorescence quenching by ketene silyl acetals with those predicted for the electron transfer processes indicates that the photoaddition proceeds via the photoinduced electron transfer from beta,beta-dimethyl-substituted ketene silyl acetals to the singlet and triplet excited states. No photoaddition of nonsubstituted ketene silyl acetal to 10-methylacridone occurs, since the electron donor ability is too weak to transfer an electron to the excited states of 10-methylacridone as expected from the higher oxidation potential compared with the beta,beta-dimethyl-substituted analogues.

A beta,beta-dimethyl-substituted ketene silyl acetal reduces p-chloranil to produce the carbon-oxygen adduct, the hydrolysis of which yields the corresponding hydro-quinone ether (3). The structure of 3 has been determined by the X-ray crystal analysis. On the other hand, the reaction of a nonsubstituted ketene silyl acetal with p-fluoranil yields the carbon-carbon adduct selectively. The mechanistic difference between the C-O and C-C bond formation is elucidated based on the kinetic study.

A beta,beta-dimethyl-substituted ketene silyl acetal reduces p-chloranil to produce the carbon-oxygen adduct, the hydrolysis of which yields the corresponding hydro-quinone ether (3). The structure of 3 has been determined by the X-ray crystal analysis. On the other hand, the reaction of a nonsubstituted ketene silyl acetal with p-fluoranil yields the carbon-carbon adduct selectively. The mechanistic difference between the C-O and C-C bond formation is elucidated based on the kinetic study.

Various NAD+ analogues are reduced regioselectively by ketene silyl acetals to afford the corresponding 2-, 6-, or 4-alkylated NADH analogues. The regioselectivity of the 1,2- (or 1,6-) vs. 1,4- reduction is finely controlled by the beta-methyl-substitution of ketene silyl acetals, the position of methyl-substitution of NAD+ analogues, and the addition of Et4NCl or Bu4NF.

Thermal reduction of 10-methylacridinium ion by allylic stannanes occurs via a polar mechanism to yield the dihydroacridines allylated at the γ-position exclusively, while the photoallylation of 10-methylacridinium ion proceeds via the photoinduced electron transfer from allylic silanes and stannanes to the singlet excited state of 10-methylacridinium ion to yield mainly the α-adducts.