藤田 守文

The ring opening of alkylidenecyclopropanone acetal under acidic conditions produces the 1-alkylidene-2-oxyallyl cation as an intermediate, which reacts with furan to give the [3+2] and [4+3] cycloadducts as well as an electrophilic substitution product. The product distribution is controlled by the oxy substituents of the cation and by the solvent employed.

Ring opening reactions of 2-cyclohexylidene-3,3-dimethylcyclopropanone acetal (1) are readily induced by treatment of hydrogen chloride in various solvents. Bond cleavage takes place at the C1-C2 or C2-C3 bond, and the ratio of C1-C2/C2-C3 cleavages changes from > 99/1 to < 1/99 depending on the solvent. The two modes of bond cleavage must be initiated by protonations at the carbon-carbon double bond and the acetal oxygen, respectively. The regioselectivity can be rationalized by the rate-determining protonation at carbon and the equilibrium protonation at oxygen. (c) 2006 Elsevier Ltd. All rights reserved.

Alkylideneallyl cation generated from Lewis acid-mediated ring-opening reaction of alkylidenecyclopropanone acetal was employed for the reaction with siloxyalkenes to give [3 + 2] cycloaddition and acyclic addition products. All the products are the result of nucleophilic addition to the sp(2) center of the alkylideneallyl cation, and there is no sign of the nucleophilic addition to the sp center. The regioselectivity is independent of the electronic and steric effects of siloxyalkene nucleophiles, and is compatible with charge distribution of the allylic cation.

Alkenylboronic esters having an acyloxy, alkoxy, or methoxycarbonyl group were employed for the reaction with (diacetoxyiodo)benzene in the presence of BF(3)center dot OEt(2) to provide the alkenyliodonium tetrafluoroborates with inversion of configuration: (E)- and (2)-boronates give (Z)and (E)-iodonium salts, respectively. This selectivity can be reversed by the addition of ether to the dichloromethane solution. The stereoselectivity can be explained by participation of the neighboring oxy group.

[GRAPHICS] The reaction of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate, (1a) and the 4-chlorophenyl derivative (1b) with bromide ion was examined in methanol, acetonitrile, and chloroform. Products include those derived from the intermediate cyclohexenyl cation as well as 1-bromocyclohexene. Kinetic measurements show that the reaction of I is strongly retarded by the added bromide. The curved dependence of the observed rate constant on the bromide concentration is typical of a pre-equilibrium formation of the intermediate adduct with a fast bromide-independent reaction (solvolysis of the iodonium ion). The formation of the adduct, lambda(3)-bromoiodane, was also confirmed by the UV spectral change. The relative reactivity of the iodonium ion and lambda(3)-bromoiodane is evaluated to be on the order of 10(2). The bromide substitution product forms both via the S(N)1 reaction of the free iodonium ion and via the ligand coupling of the iodane.

The review discusses reaction paths of cyclohexenyliodonium salts in the presence of bases and nucleophiles, in particular those involving formation of cyclohexynes.

Cyclohexynes can be generated efficiently from 1-cyclohexenyl-iodonium salts with acetate or other bases via the E2 and El mechanism. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes conforms to the LUMO populations: the less deformed acetylenic carbon is more electrophilic. Cycloheptyne can form by the El-type elimination via 1,2-rearrangement from cyclohexylidenemethyliodonium salt under very weakly basic conditions.

DIAGRAM Reactions of cyclohexenyl and cyclopentenyl iodonium salts with cyanide ion in chloroform give cyanide substitution products of allylic and vinylic forms. Deuterium-labeling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of the iodonio group and 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate. The hydrogen shift preferentially occurs from the methylene rather than the methine beta-position of the carbene, and the selectivity is rationalized by the DFT calculations. The Michael reaction was also observed in the reaction of cyclopentenyliodonium salt with acetate ion in chloroform. The vinylic substitution products are ascribed to the ligand-coupling (via lambda(3)-iodane) and elimination-addition (via cyclohexyne) pathways.

Alkylidenecyclopropanone sityl acetals are readily available from the reaction of an alkylidenemethyliodonium salt and ketene silyl acetals in the presence of triethylamine. The three different C-C bonds of the cyclopropane ring can be selectively cleaved with HCl, Lewis acid, or fluoride. The alkylideneallyl cation formed via the cleavage of C2-C3 bond with Lewis acids shows further selectivity in reacting with a nucleophile. (C) 2004 Elsevier Ltd. All rights reserved.

The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (k(H)/k(D) > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as SO reactions (k(H)/k(D) approximate to 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile.

Solvolysis of 4-methylcyclohexylidenemethyl(phenyl)iodonium tetrafluoroborate (1) and its R isomer (69% ee) was carried out in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) in the presence of bases such as acetate, pyridine, triethylamine, and alkoxide. The reaction is much faster in TFE than in HFP. Products ill TFE include solely un-rearranged (racemized) enol ether 2 together with iodobenzene, while the main product ill HFP is ring-expanded (partially racemized) 1-alkoxycycloheptene 3. Results show that 2 is formed via alpha-elimination with alkylidenecarbene as an intermediate, while the reaction in HFP to give 3 involves a cycloheptyne intermediate that is mostly derived from an intermediate cyclohept-1-enyl cation via the E1-type pathway.

Reaction of 4-substituted cyclohex-1-enyliodonium salt with cyanide in chloroform produces three isomeric cyanocyclohexenes, ipso and two cine products. Deuterium labeling experiments showed that the allylic cine product is formed via the Michael addition of cyanide, followed by elimination of the iodonio group and a 1,2-H shift.

Reactions of (R)-4-methyleyclohexylidenemethyl(phenyl)iodonium salt and its 3-trifluoromethylphenyl and 4-methoxyphenyl derivatives (1) with tetrabutylammonium mesylate and triflate were carried out in chloroform at 60 degreesC. The products include (S)-4-methylcyclohexylidenemethyl sulfonate (2) and (R)-5-methylcyclohept-1-enyl sulfonate (3) as well as iodoarene. Reactions of (S)-1 were confirmed to provide the counterpart results. The rearranged triflate (R)-3Tf formed in the reaction with triflate maintains mostly the ee (enantiomeric excess) of (R)-1, while the ee of the mesylate product 3Ms is largely lost. The C-13-labeling at the exocyclic position of 1 results in the isotopic scrambling of C-1 and C-2 of 3Ms in the mesylate reaction. The degree of the scrambling agrees well with that of the loss of ee of (R)-3Ms obtained from (R)-1, implying that the racemization is not due to the intermediate formation of achiral, primary 4-methylcyclohexylidenemethyl cation. Reaction of 1 with mesylate in the presence of CH3OD provided the 3Ms deuterated at the 2-position. When tetraphenylcyclopentadienone was added to the mesylate reaction system, the adduct of the 4-methylcycloheptyne intermediate was obtained in 24% yield, but the normal products 2Ms and 3Ms were still formed. The 3Ms obtained here in a low yield maintains the high ee of 1. These results indicate that the cycloheptyne is an intermediate responsible for the formation of racemic product 3Ms in the mesylate reaction. It is also concluded that the unrearranged products 2 are formed via the competitive pathways of in-plane and out-of-plane S(N)2 reactions.

Optically active 4-methylcyclohexylidenemethyl(aryl)iodonium tetrafluoroborate (1.BF4-) was prepared and its solvolysis was carried out at 60 degreesC in various solvents. The main product is optically active 4-methylcycloheptanone (or its enol derivative) in unbuffered solvents, accompanied by the iodoarene. The rearranged product always maintains the optical purity of the starting 1. Its stereochemistry conforms to a mechanism involving the rearrangement via the sigma-bond participation in departure of the nucleofuge, followed by trapping of the resulting chiral 5-methylcyclohept-1-enyl cation with a nucleophilic solvent. That is, the achiral, primary vinyl cation is not involved during the reaction. The unrearranged substitution product is also obtained in a minor fraction in unbuffered methanol, ethanol, and acetic acid, but not in trifluoroethanol or hexafluoro-2-propanol: the methoxy product from methanolysis is largely racemized, but the acetolysis product is obtained mainly via retention of configuration. Reactions of 1 with bromide, acetate, and trifluoroacetate in chloroform give unrearranged substitution products in different degrees of inversion. These unrearranged products are concluded to be formed via the direct nucleophilic substitutions. Added bases such as sodium acetate in methanol lead to the unrearranged methoxy products of complete racemization, which is ascribed to the alpha elimination (to give an alkylidene-carbene) followed by the solvent insertion.

The title reaction gives largely racemized 4-methylcycloheptanone, but the enantiomeric product in slight excess has a different form depending on the leaving group, ArI, of the iodonium salt. The results indicate that the photosolvolysis does not proceed via a completely achiral state. (C) 2002 Elsevier Science Ltd. All rights reserved.

Reactions of (R)-4-methyleyclohexylidenemethyl(phenyl)iodonium salt and its 3-trifluoromethylphenyl and 4-methoxyphenyl derivatives (1) with tetrabutylammonium mesylate and triflate were carried out in chloroform at 60 degreesC. The products include (S)-4-methylcyclohexylidenemethyl sulfonate (2) and (R)-5-methylcyclohept-1-enyl sulfonate (3) as well as iodoarene. Reactions of (S)-1 were confirmed to provide the counterpart results. The rearranged triflate (R)-3Tf formed in the reaction with triflate maintains mostly the ee (enantiomeric excess) of (R)-1, while the ee of the mesylate product 3Ms is largely lost. The C-13-labeling at the exocyclic position of 1 results in the isotopic scrambling of C-1 and C-2 of 3Ms in the mesylate reaction. The degree of the scrambling agrees well with that of the loss of ee of (R)-3Ms obtained from (R)-1, implying that the racemization is not due to the intermediate formation of achiral, primary 4-methylcyclohexylidenemethyl cation. Reaction of 1 with mesylate in the presence of CH3OD provided the 3Ms deuterated at the 2-position. When tetraphenylcyclopentadienone was added to the mesylate reaction system, the adduct of the 4-methylcycloheptyne intermediate was obtained in 24% yield, but the normal products 2Ms and 3Ms were still formed. The 3Ms obtained here in a low yield maintains the high ee of 1. These results indicate that the cycloheptyne is an intermediate responsible for the formation of racemic product 3Ms in the mesylate reaction. It is also concluded that the unrearranged products 2 are formed via the competitive pathways of in-plane and out-of-plane S(N)2 reactions.

Optically active 4-methylcyclohexylidenemethyl(aryl)iodonium tetrafluoroborate (1.BF4-) was prepared and its solvolysis was carried out at 60 degreesC in various solvents. The main product is optically active 4-methylcycloheptanone (or its enol derivative) in unbuffered solvents, accompanied by the iodoarene. The rearranged product always maintains the optical purity of the starting 1. Its stereochemistry conforms to a mechanism involving the rearrangement via the sigma-bond participation in departure of the nucleofuge, followed by trapping of the resulting chiral 5-methylcyclohept-1-enyl cation with a nucleophilic solvent. That is, the achiral, primary vinyl cation is not involved during the reaction. The unrearranged substitution product is also obtained in a minor fraction in unbuffered methanol, ethanol, and acetic acid, but not in trifluoroethanol or hexafluoro-2-propanol: the methoxy product from methanolysis is largely racemized, but the acetolysis product is obtained mainly via retention of configuration. Reactions of 1 with bromide, acetate, and trifluoroacetate in chloroform give unrearranged substitution products in different degrees of inversion. These unrearranged products are concluded to be formed via the direct nucleophilic substitutions. Added bases such as sodium acetate in methanol lead to the unrearranged methoxy products of complete racemization, which is ascribed to the alpha elimination (to give an alkylidene-carbene) followed by the solvent insertion.

Cyclohexynes are effectively generated by treatment of cyclohex-1-enyliodonium salts with a mild base such as acetate and fluoride ion in chloroform. Regioselectivity of the nucleophilic addition of acetate ion to cyclohexynes depends on the 4-substituent.

Molecular chirality of 4-methyleyclohexyhdenemethyl iodonium salt is used to probe the chirality of intermediate state of the reaction. A possible achiral intermediate, primary vinyl cation is excluded for the reactions of the iodonium salt under any reaction conditions employed, while achiral 5-methylcycloheptyne, formed via rearranged cation, is involved in the reaction with sulfonate. The reaction is extended to generation of some small ring cycloalkynes. Vinylic S(N)2 mechanisms are also proposed.

Solvolysis of (R)-4-methylcyclohexylidenemethyl triflate (6) was examined at 140degreesC in various aqueous methanol and some other alcoholic solvents. The main product was (R)-4-methylcycloheptanone that maintains the stereochemical purity of 6, with accompanying 4-methylcyclohexanecarbaldehyde. In the presence of bromide ion, the bromide substitution product was also obtained, mostly with inversion of configuration. It is concluded that the solvolysis does not involve the formation of the primary vinyl cation but proceeds via a-bond participation to form the rearranged cycloheptenyl cation as an intermediate. Copyright (C) 2002 John Wiley Sons, Ltd.

Regio- and stereoisomers of 1,2,omega-trimethyldecyl propionate (omega = 5-9) were prepared from stereochemically pure chiral building blocks as sex pheromone candidates of a pine sawfly; Diprion nipponica. Among the synthesized candidates, (1S,2R,8S)-1,2,8-trimethyldecyl propionate was found to be the sex pheromone of D. nipponica, based on compatibility of its GC-MS data with that of the extract of females, and its significantly high pheromone activity in a field bioassay. The field bioassay of the synthesized compounds also revealed that (1S,2R,8R)-1,2,8-trimethyldecyl propionate, (1S,2R,7S)-1,2,7-trimethyldecyl propionate, and (1S,2R,6S)-1,2,6-trimethyldecyl propionate could attract mate sawflies to some extent as pheromone mimics.

Solvolysis of (R)-4-methylcyclohexylidenemethyl triflate in aqueous methanol at 140 degreesC gave stereospecifically (R)-4-methyl-cycloheptanone to definitively rule out intermediate formation of the achiral primary vinyl cation. The rearrangement must occur via concerted a-bond participation.

4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) reacts with 1 -methoxy-4-methylnaphthalene to give the 1,4-addition product in the presence of methanol; the reaction does not proceed in the absence of the alcohol. Methoxy exchange (with CD3OD) was also observed during the reaction. Reactions of PTAD with 1-(3-hydroxypropoxy)-4-methylnaphthalene and related naphthalenes afford stereoselectively 1,4-adducts (70-98% of the major diastereomer). The stereoface-selective addition of PTAD at C-4 with concurrent formation of an acetal at C-1 takes place in a syn manner, which is induced by the hydrogen-bonding interaction between PTAD and the hydroxy group. The alpha -methyl substitution on the propoxy side chain strongly enhances the stereodifferentiation (90% de) and accelerates the cyclization by the Thorpe-Ingold effect. The alkoxy moiety of the adduct is easily removed to give 4-methyl-4-amino-4H-naphthalen-1-one with high enantiomeric excess. The gamma -methyl group of the side chain also affects the stereodifferentiation. Thus, the two stereogenic centers of the (1S,3R)-3-hydroxy-1-methylbutoxy side chain work together to achieve the high stereodifferentiating 1,4-addition from the Si-Re face (up to 96% ee). Epimerization of the cyclic acetal of a minor adduct was observed during the reaction of 1-(3-hydroxybutoxy)-4-methylnaphthalene, indicating that the minor component of the final products is derived from the initial minor syn adduct formed from the opposite face. The syn selectivity of the addition is achieved completely in the initial stage of formation of both the major and the minor adducts.

Irradiation of the absorption band of the 10-methylacridinium ion (AcrH(+)) in acetonitrile containing allylic silanes and stannanes results in the efficient and selective reduction of the 10-methylacridinium ion to yield the allylated dihydroacridines. In the photochemical reactions of AcrH(+) with unsymmetric allylsilanes, the allylic groups are introduced selectively at the or position. likewise, the reactions with unsymmetric allylstannanes afforded the ct adducts predominantly, but the y adducts were also obtained as minor products; in contrast to this, the thermal reduction of AcrH(+) and the 1-methylquinolinium ion (QuH(+)) by unsymmetric allylstannanes gave only the y adducts. The thermal reduction of QuH(+) by tributyltin hydride or hydrosilanes in the presence of a fluoride anion also occurs to yield 1-methyl-1,2-dihydroquinoline selectively. On the other hand, the photoreduction of QuH(+) derivatives by tributyltin hydride and tris(trimethylsilyl)silane yields the corresponding 1,4-dihydroquinolines exclusively. The difference in the mechanisms for the regioreversed thermal and photochemical reduction of AcrH(+) and QuH(+) is discussed in terms of nucleophilic vs electrontransfer pathways. The photochemical reactions proceed via photoinduced electron transfer from organosilanes and organostannanes to the singlet excited states of AcrH(+) and QuH(+), followed by the radical coupling of the resulting radical pair in competition with the back electrontransfer to the ground state. The rate constants of photoinduced electron transfer obtained from the fluorescence quenching of AcrH(+) and QuH(+) by organosilane and organostannane donors agree with those obtained from the dependence of the quantum yields on the donor concentrations for: the photochemical reactions. The electron-transfer rate constants are well analyzed in light of the Marcus theory of adiabatic outer-sphere electron transfer, leading to the evaluation of the reorganization energy (lambda = 0.90 eV) of the electron-transfer reactions. The transient spectra of the radical pair produced by the photoinduced electron transfer from organosilanes to the singlet excited state of AcrH(+) have been successfully detected in laser-flash photolysis of the AcrH(+)-organosilane systems. The rate constants of back electron transfer to the ground state have been determined, leading to the evaluation of the reorganization energy for the back electron transfer, which agrees with the value for the forward electron transfer.

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.

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.

(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.

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.

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.