Search

Ministry of Science Technology [2005CB221408]; National Science Foundation of China [20571061]; PCSIRIC [IRT 1036]

In the presence of hydrochloric acid and under oxygen purging, N-nitrosodimethylamine photolytically adds to 1-arylpropenes regiospecifically to give erythro- and threo-α-amino nitrates as the primary products which can then be reduced, using lithium aluminium hydride, to give α-amino alcohols or can undergo substitution reactions under photochemical conditions. The stereochemical courses of the addition, as well as the solvolysis of the diastereoisomeric α-amino nitrates with intramolecular amino group participation, are discussed in order to explain the product distributions.

The ethylene ketal of 2-cyclopentenone and 2-cyclohexenone in acetonitrile in the presence of a trace of an acid rapidly undergoes the Diels-Alder type dimerization. The products are regiospecific but not stereoselective and are hydrolysed sequentially to afford keto-ketals and diketones whose structures are determined by extensive applications of NMB spectroscopy.

Dimeric mixed-valence oxovanadium citrate [V 2O 3(phen) 3(Hcit)].5H 2O ( 1) (H 4cit = citric acid, phen = 1,10-phenanthroline) was isolated from a weak acidic medium. It could be converted quantitatively into a tetrameric oxovanadium citrate adduct of 1,10-phenanthroline [V 2O 3(phen) 3(Hcit) 2(phen) 3O 3V 2].12H 2O ( 2). This was supported by the trace of infrared spectra and X-ray diffraction patterns. The two compounds feature a bidentate citrate group that chelates only to one vanadium center through their negatively charged alpha-alkoxy and alpha-carboxy oxygen atoms, while the other beta-carboxy and beta-carboxylic acid groups are free to participate in strong intramolecular and intermolecular hydrogen bonding [2.45(1) in 1 and 2.487(2) A in 2], respectively. This is also the case of homocitrato vanadate(V/IV) [V 2O 3(phen) 3( R, S-H 2homocit)]Cl.6H 2O ( 3) (H 4homocit = homocitric acid), which features a binding mode similar to that found in the R-homocitrato iron molybdenum cofactor of Mo-nitrogenase. Moreover, the homocitrato vanadate(V) [VO 2(phen) 2] 2[V 2O 4( R,S-H 2homocit) 2].4H 2O.2C 2H 5OH ( 4) is isolated as a molecular precursor for the formation of mixed-valence complex 3. The V-O alpha-alkoxy and V-O alpha-carboxy bond distances of homocitrate complexes 3 and 4 are 1.858(4) and 1.968(6) av and 2.085(4) and 1.937(5) A, respectively. They are shorter than those of homocitrate to FeVco (2.15 A). The gamma-carboxy groups of coordinated homocitrato complexes 3 and 4, and the free homocitrate salt Na 3(Hhomocit).H 2O ( 5), form strong hydrogen bonds with the chloride ion and the water molecule [2.982(5) in 3, 2.562(9) in 4, and 2.763(1) A in 5], respectively.

The photocycloaddition of some difluoro[(methylamino-κN)alkenonato-κO]boron complexes 1 with arylalkenes 2 is discussed. The resulting [2+2] photoaddition gave the cyclobutane and azetidine derivatives (Schemes 1, 3, and 5). Rearrangements of the cyclobutane gave 1,5-diketones derivatives (Schemes 2, 4, and 5). The yields of the photoadducts were governed by the reduction and oxidation potentials. Furthermore, the configurations of the products established high regio- and stereoselectivity, suggesting the presence of a singlet exciplex. The reactivity and the stereochemistry were rationalized by means of FMO (frontier molecular orbital) calculations.

Two N(n-Pr)PPh2 groups appended to the 9,10-anthracene positions through methylene spacers as a bidentate ligand coordinate with Cu+, Ag+, and Au+ ions efficiently to afford endocyclic metal complexes with metal-η6-arene bonding, rarely observed for the group 11 metal ions. The three-center metal−NP and metal−η6-arene bonding patterns in the endocyclic complexes were confirmed using ab initio and density functional calculations. The close metal−anthracene centroid distances significantly affected absorption and fluorescence properties of the complexes in solution. Among them the Cu+ complex has the shortest metal−anthracene distance and most drastic spectral abnormality. The close distances also controlled the endocyclic-exocyclic metal transition of the complexes, wherein the equilibrium heavily favored the former on account of η6 interactions. These complexes reacted with phosphines in solution to give highly fluorescent species that were proposed to possess the exocyclic configuration with the three-co...

Acetylacetonatoboron difluoride (AABF2) and oxalate (AABO) undergo cycloaddition from their singlet excited state with cyclic and terminal olefins to give stereospecific but non-regiospecific products; the latter oxalate reacted slightly faster than the former difluoride. A conjugated diene, 1,3-cyclooctadiene, and methyl substituted benzenes also reacted similarly but with slower rates owing to electron transfer at the encounter stage. The resulting radical ion pairs may undergo radical ion type reactions and/or reverse electron transfer, which leads to the partial waste of the photoenergy. For the same reasons, the reaction occurs more efficiently in non-polar solvents, such as ether, dioxane and methylene chloride than in acetonitrile.

Photolysis of 4-diazonaphthalen-1(4H)-one (1a), 2-methyl-4-diazonaphthalen-1(4H)-one (1b) and 4-diazo-2-nitronaphthalen-1(4H)-one (1c) in neat THF and 1,4-dioxane produced a variety of products depending on the 2-substituent and the solvent. While 1a and 1c produced predominantly insoluble copolymers in both solvents, 1b produced, besides small amounts of copolymers, a novel polyether bridged naphthalene, 2,3:11,12-bis(2-methyl-1,4-naphthylene)-18-crown-6 (2b), 1,4-dioxa-3′-methyl-4′-oxo-1′,4′-dihydronaphthalene-1′-spiro-5-cycloheptane (3b), 2-methyl-4-(1,4-dioxan-2-yl)naphthol (4b) and 2-methylnaphthol (5b) in dioxane, and 3′-methyl-4′-oxo-1′,4′-dihydronaphthalene-1′-spiro-2-pyrane (3b′), 2-methyl-4-(tetrahydrofuran-2-yl)naphthol (4b′) and 5b in THF. Compound 2b is characterised by X-ray crystallography.

Photocarbonylation of alkenes was successfully carried out on a multigram scale (5–10 g) on a number of alkenes. Optimum conditions (temperature, solvent, concentration, UV source and filters) were established; it was shown that the rate of reaction depends on the concentration of the cobalt catalyst. The cobalt catalyst was stable only under a narrow range of conditions and any deviation from them either reduced the rate of photocarbonylation, or caused the cobalt to precipitate and stop the reaction. It was shown that the rate and selectivity of photocarbonylation reaction depend on a degree of substitution of alkenes. Mono and disubstituted alkenes gave the corresponding methyl esters in good yields (69–85%), while trisubstituted alkenes either did not react at all, or gave complex mixtures of products. Selectivity can be achieved in carbonylation of dienes if conversion is kept low.

A series of substituted phenanthroquinones (PQ’s) was prepared, their phosphorescence spectra and decay rate constants, cyclic voltammograms, and photoreaction in the presence of tetramethylethylene (TME) were investigated. With 450 nm irradiation their triplet excited state cyclized with TME to give the corresponding dioxenes, 2′,2′,3′,3′-tetramethyl-1′,4′-dioxa-1′,2′,3′,4′-tetrahydrotriphenylene, cleanly and much faster than that with 300 nm irradiation. The pattern of the reaction was effected weakly in random fashion by the substituent at the 3-position of PQ’s. The photocycloaddition, as exemplified by PQ itself, is so efficient that it occurred even by room light and during optical spectroscopic recordings; its limiting quantum yield in benzene is established to be unity with 450 nm irradiation. It owes the unusual efficiency to a long lifetime of the PQ triplet excited state with the n – ∗ π configuration, and to the successful cyclization in every diffusion controlled collision as implicated by TME quenching of PQ phosphorescence intensity in CCl 4 solution. The triplet excited state reaction was supported by competitive quenching of the dioxene formation by triplet quenchers as well as by oxygen. These PQ’s do not fluoresce in solution, but show phosphorescence in solid solution with lifetimes of about 10 ms and in CCl 4 solution with lifetimes in the order of 100 μs in the room temperature range. PQ’s with 3-cycano, 3-chloro, 3-methoxy, and without substitutions photocyclized to cyclohexene to give the corresponding dioxenes with small stereochemical scrambles, that was interpreted that the extent of electron transfer in the 1,6-diradical stage is small. Similar photocycloadditions to isobutene gave a 1:1 mixture of two regio-isomers, this was taken as evidence for a direct radical attack on olefins to give 1,6-diradical intermediates.

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)ex of these exciplexes, the radiative (k(ex)f) and nonradiative (k(ex)NR) rate constants were calculated. The former k(ex)f were also computed from a semi-empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(ex)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+) *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. Also, those plots from k(ex)f values (being determined directly from experiments) show better correlation than those from k(ex)NR. In contrast to the CRIP type exciplexes in the Marcus "inverted" region, these k(ex)f and k(ex)NR increase in the common trend with increasing transition energy gaps. The k(ex)NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the "normal" to "inverted" region. Both the attenuation and reversal of the k(ex)NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin-orbit coupling in highly polar exciplexes.

The singlet excited state of the title borofluoride, *DBMBF2, reacted efficiently with acyclic enones and en-esters but only feebly with the corresponding excimer in analogy to simple acyclic olefins to give cycloaddition products, in spite of substantially different electronic character of the double bonds. Cyclic enones, on the contrary, reacted with both *DBMBF2 and the excimer with increasing fluorescence quenching constants and adduct quantum yields as the rigidity of enone rings relaxes. They interacted with the excimer to cause the enone stimulated excimer dissociation which occurred most likely by the intermediary of the enone exciplexes. The interaction of 2-cyclopentenone (CP) by this process caused strong increases in *DBMBF2 fluorescence intensity, that was interpreted to arise from a fast reverse step (kx) from the exciplex as shown by increasing *DBMBF2 fluorescence intensities upon addition of CP. Apparently the exciplexes from 2-cyclohexenone, 2-cyclootenone and 2-cyclododecenone had a decreasing degree of reversibility (i.e., relatively smaller reverse reaction rates): this trend caused Φa increases and, correspondingly, lesser enhancement of *DBMBF2 fluorescence in the same order. The last enone, while mainly reacted with excimer, showed almost the same reactivity and Φa pattern of acyclic enones. In contrast to cyclic olefins, the last three enones reacted to show decreased Φa on increasing initial [DBMBF2] (i.e., higher excimer concentrations); and hidden deactivation step was implicated in the excimer reaction with these enones. The anomalous reaction patterns were assumed to be caused by differences in the geometrical orientation of an enone within exciplexes and of enone attack on the excimer.

Irradiation of borinates derived from 9-borabicyclo[3,3,1]nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4-addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.Key words: borinate photochemistry, charge transfer absorption, alkylborane addition, boronate intermediates, B—C bond homolytic scission

Ethylenediaminetetraacetate is converted into its triacetate by peroxotitanate(IV), and strong chelation of the triacetate ligand to the metal center facilitates elimination of the pendant acetoxylate group. Various species of peroxotitanium(IV) complexes in the reaction sequence are fully characterized.

The diazide of 2,4-azulenoquinone carrying 1,3-cyano and carbomethoxy substituents was synthesized and photolyzed in cyclic ethers (THF, dioxane, and tetrahydropyran) to give good yields of 2,4-pol...

The photocycloaddition of dibenzoylmethanatoboron difluoride (DBMBF2) with conjugated nitriles, ketones, and esters occurs more slowly than that with simple olefins and with fair regioselectivity. For these electron-deficient olefins, the cyclic analogues react more slowly than the acyclic ones and the cyano analogues react more slowly than all others; also a β-methyl substituent reverses the regioselectivity in adduct formation. Benzoylacetonatoboron difluoride (BABF2) photocycloadds to simple cyclic olefins from the excited state isomer in which the double bond is conjugated with the phenyl group, and occurs more slowly than that of the corresponding reaction of DBMBF2. The efficiency and regiochemistry of these cycloadditions have been explained with respect to the exciplex as the immediate intermediate. Keywords: photocycloaddition, dibenzoylmethanatoboron difluoride, benzoylacetonatoboron difluoride, exciplex.

The mechanism of the interaction of singlet excited dibenzoyl(methanato)boron difluoride (*DBMBF2 = *A) with olefins and substituted benzenes (SB) was investigated by the determination of effective quenching distances (Reff) from fluorescence monitors in acetonitrile as analyzed by the Smoluchowski−Collins−Kimball equation and by solvent effects on the dichotomy of the formation of cycloadducts and [4 + 2] dimers from 1,3-cyclohexadiene (CHD) and 1,3-cyclooctadiene (1,3-COD). The correlation of Reff with directly demonstrates that the reactants have to get closer to affect electron transfer from these substrates as the driving force become more positive and coalesce on exciplexes. Both photophysical and photochemical probes converge on the partition of encounter pairs to the SSRIP (the ket pathway which leads to radical ion reactions and nonfluorescent) and exciplexes (the kex pathway which leads to cycloadducts and fluorescent species); the former pathway was promoted by low redox energies of donor−accep...

The ground-state EDA complex of (dibenzoylmethanato)boron difluoride (DBMBF2) with substituted benzenes (SB) was studied by absorption spectroscopy to determine the equilibrium constant and enthalpy of the molecular association process in acetonitrile and cyclohexane. Excitation of the EDA complexes showed the corresponding exciplex fluorescence due to excitation of DBMBF2 in the presence of SB. Integration of these data with the previously published kinetics and thermodynamics of the exciplex formation and decay showed that both types of excitations converged on the exciplex energy well and that the former excitation goes through the Franck−Condon state, *(A-D+)v, which is different in molecular distance and electron and energy distributions. In acetonitrile, the discrepancy of fluorescence intensities from two excitations is taken as proof for electron transfer at the encounter pair on the energy surface to generate radical ions. This also implies that excitation of EDA complexes in acetonitrile forms t...

A series of tetracoordinated boron complexes derived from various 1,3-diketones and bidentate ligands (oxalato, malonato, catecholato and difluoro) were prepared. Their absorption, fluorescence and phosphorescence spectra, at ambient temperature and 77 K, and cyclic voltammmetry in acetonitrile were studied. The boron complexes exhibited high molar absorptivity and fluorescence intensities as compared with the parent diketones; the phenomena are in agreement with the lowest singlet excited state possessing the ϕ, ϕ,* configuration with a rigid chelate structure. The redox potential differences correlate linearly with the lowest singlet (Es) as well as the triplet (ET) excited state energy with slopes lower than unity. These linear correlations re-state that the electron exchange and Coulombic integral also vary proportionally to the potential differences. From these correlations, it is possible to determine Es and ET of those boron complexes that do not give fluorescence and phosphorescence.

Benzene derivatives give, in cyclohexane solution, a strongly emitting (Φf up to 0.8) excited complex with dibenzoylmethanatoboron difluoride (DBMBF2). The rate constant kex for the complex formation varies between 3.5 × 109 and 2.6 × 1010 M−1 s−1 depending on the benzene derivative, and the complex emission can already be detected at very low concentrations of the aromatics. This behaviour is different from that found in acetonitrile where the DBMBF2 emission disappeared completely before the new emission of the complex could be detected. The position of the complex emission and its lifetime depend upon the redox potential of the aromatics: the lower the latter, the longer the lifetime and the more the emission is red-shifted. The monotonous increase of the lifetime and of the fluorescence quantum yield of the emission with the concentration of the aromatics has been interpreted on the basis of an equilibrium between the exciplex and excited DBMBF2. The equilibrium constant varies with the electron-donating property of the aromatics, from 2.7 for benzene to 560 M−1 for isodurene. A decrease in the excited complex lifetime or in its fluorescence quantum yield is observed in some cases when the aromatics concentration is close to that of the pure solvent. Such behaviour is believed to arise from a new equilibrium resulting from the interaction of the exciplex with a second molecule of the aromatics to form triplex species. A detailed kinetic study has been undertaken in case of p-xylene, and the rate constants of all the processes involved in the double equilibrium have been determined. No basic difference is observed in the rate constant for exciplex formation when a single or double equilibrium is assumed; on the other hand, large discrepancies are found in the exciplex deactivation processes, specially for the back reaction from the exciplex to excited DBMBF2. The equilibrium constants between DBMBF2 and the exciplex, on one hand, and between the latter and the triplex are 56.6 and 0.68 M−1, respectively, when the concentration of p-xylene in cyclohexane is 3.3 M. Keywords: fluorescence, exciplex, triplex, dibenzoylmethanatoboron difluoride, kinetics.

The photolysis of dibenzoylmethanatoboron difluoride (DBMBF2) in the presence of vinyl ethers in acetonitrile gave dimers of the latter from acid catalysis or electron transfer sensitization, but, it contrast, gave cycloaddition products in an efficient reaction in less polar solvents such as tetrahydrofuran (THF) and benzene.

Bis(acetylacetonato)nickel(II), Ni(acac),, quenched triplet state aromatic ketones with high efficiency but only certain aromatic ketones with high triplet energy levels successfully sensitized the photoreduction of Ni(acac)* to give the acetylacetone radical and Ni(1) complexes. A short-lived Ni(1) species was detected by ESR and UV spectroscopy, and the acetylacetonyl radical was trapped by 2-nitroso-2-methylpropane or phenyl-tert-butylnitrone as a stable nitroxide and by H atom donors as acetylacetone. The transient Ni(1) complexes decayed by either first-order or second-order kinetics, depending on the concentrations, to give Ni(0) complexes or ferromagnetic colloidal nickel. The photoreduction of Ni(acac)* occurs only if the acetylacetonyl radical was trapped. Various evidence suggested that the quenching mechanism was not likely to occur by energy transfer and was best interpreted to involve a partial charge transfer within an excited-state complex. The acetylacetonyl radical in the complex undergoes hydrogen abstraction from solvent in competition with reverse electron transfer. The quantum efficiency of the photoreduction is attributed to the extent of charge-transfer contribution and the hydrogen atom donating ability of solvents. It is also confirmed that the ketyl radical derived from triplet state ketones is not responsible for the reduction of Ni(acac)2. In solution, transition-metal complexes and cations quench the triplet excited states of aromatic ketones and hydrocarbons with high efficiencyz-8 and have been shown to be potential "ultraviolet stabilizers" of polymers in recent years.& The mechanism of deactivation of electronically excited states has been studied ex- tensively by kinetic flash photolysis?" the investigations of which have lead to a variety of proposals. In early investigations, Hammond's group3 observed that transition-metal P-diketonates quench the triplet states of the aromatic compounds with similar rate constants in the range of IO9 M-' s-l regardless of the triplet state energy levels.3 Porter's groupZ which pioneered the inves- tigations in this area has found efficient quenching of triplet excited states by paramagnetic species such as oxygen, nitrogen oxides, and metal ions, and suggested a catalyzed intersystem crossing as the mechanism, the nature of which has been expanded along allowed electron spin exchange93l0 (eq 1) and charge-transfer2.6Jo *A + B - (A*--B?) charge-transfer reaction (1) catalyzed intersystem crossing (2)

The pattern of the photoreactions of DBMBF2 with a cyclic diene is highly dependent on solvent nature, giving mainly the dimers of dienes in polar solvents but cycloadduct in non-polar solvents. The quantum yields of the adduct (ΦA) correlate smoothly with an empirical polarity constant (ETN) in spite of exciplex formations with aromatic solvents.

Acetylacetonatoboron difluoride and benzene derivatives photolytically add to form aromatic ketone products, presumably through 2 + 2 photocycloaddition. In dioxane, acetylacetonatoboron difluoride does not show detectable emission but fluoresces in the presence of added toluene, xylenes or mesitylene within a narrow excitation window of 300–320 nm. These broad emissions (λ max of 415–435 nm) were assigned to an exciplex that had lifetimes of 1.1–4.2 ns and fluorescence quantum yields of (3–7) × 10 −3 . These exciplexes were found to be precursors of the cycloaddition products through competitive quenching experiments. From kinetic evaluations, ∗ AABF 2 was identified to be the reactive state with a lifetime of ≈ 1 ps.

In this paper, the striking acid catalysis of the photocycloaddition of methyl 2-naphthoate (2MN) to acetylacetone (AA) to give 1 and 2 was traced to the enhanced conversion of the nonemissive *(2MN–AA) exciplex to 1 and 2 by H2SO4. The partial reversibility of the exciplex in CH3CN and CH3OH was established by extended fluorescence-quenching analysis using oxygen to perturb the system, whereas time-resolved fluorescence kinetics failed to reveal the presence of this exciplex. A similar analytical method demonstrated the interaction of the nonfluorescent *(2MN–AA) exciplex and H2SO4 with the rate constants of kap = (2 ± 1) × 109 M−1 s−1 in CH3CN and (4 ± 1) × 109 M−1 s−1 in CH3OH. The quantum yield of photocycloadducts was significantly enhanced at high 2MN conversions owing to the retardation of the competing excimer *(2MN)2 formation. In the range of low conversion (2SO4] ≤ 1 mM, the quantum yield was shown to be proportional to H2SO4 concentration; the slope from such plots was analyzed to give kap values similar to those obtained from the extended fluorescence-quenching analysis. This agreement, along with the elimination of other potential acid catalytic routes, unambiguously proves that the acid catalysis originates from the interaction of the *(2MN–AA) exciplex with H2SO4.

In dioxane or ether singlet excited phenanthrene (PN) reacts with acetylacetonatoboron difluoride (AABF 2 ) irreversibly to form a fluorescent excipler with a well-defined isosbestic point in the low range of [AABF 2 ]≤0.03 M. This exciplex is shown to be the precursor to the observed product of cis-9-acetyl-9,10-dihydro-10-(2-oxopropyl)phenanthrene by quantitative quenching analysis of the product and fluorescence. In contrast, the excitation of AABF 2 in the presence of PN gives neither excipler emission nor any products. The exciplex was studied on the basis of its static and time-resolved fluorescence properties to give the dipole moment of 11.2 D and its reaction pattern

Extended fluorescence quenching analysis is demonstrated to be an efficient steady-state method to evaluate the exciplex (excimer) rate constants under certain conditions. This method is attractive as the monitor of only fluorescence of the exciplex's precursor is required for a complete analysis, regardless whether or not the exciplex. Using the interaction of methyl 2-naphlhoate in its singlet excited state with H2S04 as a kinetic model, for which all the rate constants are known, this method is described and shown to reproduce accurately these rate constants. Limitations and applications of this method are discussed.

Steady state and time-resolved fluorometry were used to investigate the kinetics of the interaction of singlet excited methyl-2-naphthoate (*2MN) with H 2 SO 4 in CH 3 CN and CH 3 OH. The *2MN fluorescence intensity was quenched by H 2 SO 4 through associative interaction giving an emissive species (*2MNH + ) at [H 2 SO 4 ] ⩽ 0.1 M, in the range too low for ground state protonation. Rate constants for the association and decay of *2MN and the decay of *2MNH + were moderately larger on going from CH 3 CN to CH 3 OH, where the *2MN association rate constant was less than that of a diffusion-controlled reaction in the same solvent. However, the rate constant for *2MNH + dissociation was 36 times larger on going from CH 3 CN to CH 3 OH, which parallels the proton affinity of the solvent, i.e. the association in CH 3 OH is reversible. The *2MNH + species is quenched by H 2 SO 4 in CH 3 CN but not in CH 3 OH. In addition, *2MNH + is quenched by acetylacetone (AA) in CH 3 CN and CH 3 OH; this process is shown to contribute only a small fraction to the catalysis of the photocycloaddition between 2MN and AA.

We have established that singlet excited methyl 2-naphthoate (*2MN) forms an emissive excimer reversibly in CH3CN at concentrations above 10-2 M. The rate constant for the excimer formation was indirectly determined from extended kinetic analysis of fluorescence intensity quenching to be ked = (2.9 ± 0.7) × 1010 M-1S-1 in CH3CN at 298 K; i.e., it appears to be diffusion controlled. The dissociation rate constant was found to be k-ed = (6.0 ± 1.5) × 109 s-1 that was an order of magnitude larger than the sum of either the excimer decay rate constants (2.7 ± 0,5) × 108 s-1 or that of *2MN (1.30 ± 0.05) × 108 s-1. This method is shown to be useful for a totally reversible excimer (exciplex) formation with no new emission. The enthalpy and entropy of excimer association were determined from a Stevens-Ban plot to be -6.8 ± 0.1 kcal mol-1 and -19.6 ± 0.4 cal K-1 mol-1, respectively, where the latter signifies a rigid and ordered excimer structure. Both the *2MN monomer and excimer interacted with H2SO4 at the rate constants of (2.3 ± 0.1) × 109 M-1s-1 and (6.5 ± 1.7) × 109 M-1s-1, respectively. In contrast to the *2MN reaction, the excimer failed to react with acetylacetone as shown by no change in the excimer fluorescence relative to *2MN fluorescence and the subsequent kinetic analysis. The interaction of the *2MN excimer with H2SO4 does not lead to the emission of 2MNH+ and is also shown, by simulated calculation, not to contribute to a catalytic pathway of the 2+2 photocyloaddition between 2MN and acetylacetone.

We established that acetylacetone and acetone photolytically sensitize norbornene to undergo an efficient radical addition of solvent (ranging from hexane, cyclic ethers, haloalkanes, acetone, alcohols and acetonitrile) across the double bond. In view of its synthetic applicability, sensitized photoreactions of norbornene were reviewed and their mechanisms were compared. Photolysis of acetylacetone in the presence of norbornene in hexane induced i) acetylacetone to cycloadd to norbornene giving the expected 1,5-diketone, and ii) sensitization by triplet excited acetylacetone to generate reactive norbornene, which underwent dimerization as well as the addition of a solvent molecule by radical chain processes. In other solvents, the radical chain addition of solvent dominated the photoreaction, and superseded the cycloaddition, to give excellent to good yields of adducts to norbornene. While the excited species of acetylacetone for the sensitization was deduced to be its spectroscopic triplet excited state, that for the cycloaddition should involve a different one which may be a twisted triplet acetylacetone; sensitization experiments showed that the cycloaddition did not occur from the spectroscopic triplet state. Triplet excited acetone sensitized norbornene to undergo the same solvent addition more efficiently and cleanly than acetylacetone did. In view of various conflicts existing in the proposed energy transfer mechanism, the sensitized norbornene reactions were rationalized with electron transfer and a cation radical chain mechanism.

Dibenzoylmethanatoboron difluoride (DBMBF2), the BF2 complex of dibenzoylmethane, reacted from its singlet excited state with α,β-unsaturated ketones and esters to give 1,5-diketones by a [2+2] cycloaddition and ring-opening sequence in an analogous pathway to that observed in the photocycloaddition to olefins and dienes. The present photoreaction is unexpected since conjugated enones and en-esters are poor electron donors to comply with the previously proposed charge transfer requirement in DBMBF2 photoreactions. The photocycloaddition to these substrates was highly regioselective and stereoselective, giving the head-to-head orientation, which could be enhanced by α-methyl substitution; β-methyl substitution increased the alternate head-to-tail orientation. The photolysis of a mixture of DBMBF2 and a cyclic enone also caused the latter to dimerize to give head-to-head and head-to-tail dimers in significant yields. These photodimerizations were apparently caused by DBMBF2 sensitization. The mechanism of the sensitization is discussed.

Dibenzoylmethanatoboron difluoride (DBMBF2) and allied BF2 complexes interact from their singlet excited state with trans-anethole (t-A), quadricyclene (QC), and norbornadiene (NBD) by electron tra...

In acetonitrile and methanol, methyl 1- and methyl 2-naphthoate (1- and 2-NpCO2CH3) underwent [2 + 2] photocycloaddition with acetylacetone (acacH) from a reactive species derived from the lowest spectroscopic singlet excited state to give cyclobutane derivatives such as 9 that ring-opened to afford addition products of acetyl and acetonyl moieties at the 7,8-position for 1-NpCO2CH3 (2 and 3) and the 1,2-position for 2-NpCO2CH3 (7). Further irradiation of 3 and 7 caused a deep-seated rearrangement of 3 to give 4 and an intramolecular addition of 7 to give the caged product 8. The photocycloaddition quantum yields were increased remarkably in the presence of a trace amount ( 0.5 under comparable conditions in the presence of [H2SO4] = 0.001 M, approaching the maximum asymptotically at about this concentration; the increases of the quantum yield could not be correlated...

The lowest singlet excited state of dibenzoylmethanatoboron difluoride DBMBF2, a model compound of the BF2 complexes of 1,3-diketones, reacted with various simple olefins to give regiospecific and ...

Dibenzoylmethanatoboron difluoride (DBMBF2), the BF2 complex of dibenzoylmethane, reacted from its singlet excited state with α,β-unsaturated ketones and esters to give 1,5-diketones by a [2+2] cyc...

The diazide of 2,4-azulenoquinone carrying 1,3-cyano and carbomethoxy substituents was synthesized and photolyzed in cyclic ethers (THF, dioxane, and tetrahydropyran) to give good yields of 2,4-pol...

Irradiation of borinates derived from 9-borabicyclo(3,3,1)nonane and 1,3-diketones at their π-π* transition band caused a rearrangement from their triplet excited state to give boronate intermediates that could be hydrolyzed to the corresponding aldols. The primary photolysis involves the scission of one of the B—C bonds followed by a 1,3- or 1,5-migration and the formation of a more stable B—O bond by radical pathways. The triplet state reaction was established by quenching and heavy atom effect experiments. The overall reaction pattern is an irreversible 1,2- or 1,4- addition of an alkyl-boron group to an α,β-conjugated ketone system by radical processes. Evidence showed that the non-isolated boronates also underwent secondary photoreactions followed by hydrolysis to afford enones corresponding to dehydration products of aldols; mechanisms are suggested for these secondary steps. The photoreaction was more facile in nonpolar solvents than in polar solvents; different solvents also gave different product compositions. In the presence of dilute acetic acid, the photolysis gave a high yield of aldols, suggesting a rapid acetolysis of the B—C bond in boronates to acetylborates. Under oxygen, the photolysis was complicated by radical oxidation to give totally different products.