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4- and 5-arylethynyl water-soluble Mannich bases and related quaternary ammonium salts were synthesized and investigated as a model of conjugated quinone methide precursors (QMPs) by UV-vis light activation. Preparative photohydration and trapping reactions by thiols were studied, together with the detection of both transient QMs and competing QMP lowest triplet excited states (T1), by laser flash photolysis. The efficiency of the arylethynyl derivatives as QMPs was remarkably affected by structural features (i.e., conjugating arylethynyl moieties, substituents, and leaving groups) and protic vs aprotic solvation. Our collective data clarify the dichotomy in the photoreactivity of conjugated Mannich bases and related quaternary ammonium salts as alkylating agents and singlet oxygen sensitizers.

Bromo- and/or alkylamino-substituted and hydrosoluble naphthalene diimides (NDIs) were synthesized to study their multimodal photophysical and photochemical properties. Bromine-containing NDIs (i.e., 11) behaved as both singlet oxygen ((1)O2) photosensitizers and fluorescent molecules upon irradiation at 532 nm. Among the NDIs not containing Br, only 12 exhibited photophysical properties similar to those of Br-NDIs, by irradiation above 610 nm, suggesting that for these NDIs both singlet and triplet excited-state properties are strongly affected by length, structure of the solubilizing moieties, and pH of the solution. Laser flash photolysis confirmed that the NDI lowest triplet excited state was efficiently populated, upon excitation at both 355 and 532 nm, and that free amine moieties quenched both the singlet and triplet excited states by intramolecular electron transfer, with generation of detectable radical anions. Time-resolved experiments, monitoring the 1270 nm (1)O2 phosphorescence decay generated upon laser irradiation at 532 nm, allowed a ranking of the NDIs as sensitizers, based on their (1)O2 quantum yields (ΦΔ). The tetrafunctionalized 12, exhibiting a long-lived triplet state (τ ~ 32 μs) and the most promising absorptivity for photodynamic therapy application, was tested as efficient photosensitizers in the photo-oxidations of 1,5-dihydroxynaphthalene and 9,10-anthracenedipropionic acid in acetonitrile and water.

Irradiation of 2-alkynylphenols resulted in the generation of vinylidene-quinone methides (QMs), which were detected by laser flash photolysis in organic solvents and aqueous acetonitrile. QMs' spectroscopic properties and electrophilicity were both significantly affected by β-silicon effect. The hydration of the alkynyl moiety (22 and 900 M(-1) s(-1)for QM-1 and QM-2, in aqueous acetonitrile) was an acid- and base-catalyzed process. The addition of amines was fast (9.2 × 10(3) M(-1) s(-1) < k(2) < 1.3 × 10(8) M(-1) s(-1)), yielding ketimines, with primary amines.

Photochemical activation of water-soluble 1,8-naphthalimide derivatives (NIs) as alkylating agents has been achieved by irradiation at 310 and 355 nm in aqueous acetonitrile. Reactivity in aqueous and neat acetonitrile has been extensively investigated by laser flash photolysis (LFP) at 355 nm, as well as by steady-state preparative irradiation at 310 nm in the presence of water, amines, thiols, and ethyl vinyl ether. Product distribution analysis revealed fairly efficient benzylation of the amines, hydration reaction, and 2-ethoxychromane generation, in the presence of ethyl vinyl ether, resulting from a [4 + 2] cycloaddition onto a transient quinone methide. Remarkably, we found that the reactivity was dramatically suppressed under the presence of oxygen and radical scavengers, such as thiols, which was usually associated with side product formation. In order to unravel the mechanism responsible for the photoreactivity of these NI-based molecules, a detailed LFP study has been carried out with the aim to characterize the transient species involved. LFP data suggest a photoinduced electron transfer (PET) involving the NI triplet excited state (λ(max) 470 nm) of the NI core and the tethered quinone methide precursor (QMP) generating a radical ions pair NI(•-) (λ(max) 410 nm) and QMP(•+). The latter underwent fast deprotonation to generate a detectable phenoxyl radical (λ(max) 390 and 700 nm), which was efficiently reduced by the radical anion NI(•-), generating detectable QM. The mechanism proposed has been validated through a LFP investigation at 355 nm exploiting an intermolecular reaction between the photo-oxidant N-pentylnaphthalimide (NI-P) and a quaternary ammonium salt of a Mannich base as QMP (2a), in both neat and aqueous acetonitrile. Remarkably, these experiments revealed the generation of the model o-QM (λ(max) 400 nm) as a long living transient mediated by the same reactivity pathway. Negligible QM generation has been observed under the very same conditions by irradiation of the QMP in the absence of the NI. Owing to the NIs redox and recognition properties, these results represent the first step toward new molecular devices capable of both biological target recognition and photoreleasing of QMs as alkylating species, under physiological conditions., (© 2011 American Chemical Society)

A straightforward route for the preparation of 6-substituted naphthols and 6,6'-disubstituted binols (binol = 2,2'-dihydroxy-1,1'-binaphthyl) is presented. The synthesis has been accomplished by a one-step procedure starting from 6-bromo derivatives via direct lithiation with n-BuLi, followed by the addition of several electrophiles. This C-C functionalization has been successfully achieved with iodomethane, 3-methoxybenzaldehyde, benzophenone, methyl-2-methylbenzoate, methylbenzoate, dimethyl carbonate, ethyl 2-chloro-2-oxoacetate, and 2,2-dimethyloxirane (E). This reactivity offers a useful protecting group free synthetic protocol, toward chiral disubstituted 6,6'-binols with configuration retention of the binol moiety.

The photochemistry of 6,6'-dibromo-BINOL (BINOL = 2,2'-dihydroxy-1,1'-binaphthyl) under mild basic conditions and its methyl and triisopropylsilyl ethers has been investigated in neat and aqueous acetonitrile through product distribution analysis and laser flash photolysis. Arylation and alkylation have been successfully achieved in the presence of allyltrimethylsilane, ethyl vinyl ether, pyrrole, pyridine, thiophene, benzene, and indole. Such a photoreactivity offers a metal and protecting group free synthetic protocol toward mono- and disubstituted 6-aryl/alkyl BINOLs, since the BINOL chirality is preserved in the photoactivation process.

Naphthalene bisimides (NDIs) with a heterocyclic 1,4-dihydro-2,3-pyrazinedione moiety have been synthesized from both 2,6-dibromonaphthalene and 2,3,6,7-tetrabromonaphthalene bisanhydrides by means of a stepwise protocol including imidization, nucleophilic displacement of the bromine atoms by ethane-1,2-diamine, in situ reductive dehalogenation, and further oxidation. These heterocycles (R = n-pentyl, cyclohexyl) are yellow dyes with green emission in organic solvent, where the acid form dominates. The orange nonfluorescent conjugate base can be generated quantitatively by CH(3)COONBu(4) addition in DMSO, where it exhibits a pK(a) = 7.63. The conjugate base becomes the only detectable species (by UV-vis spectroscopy), in water solution, even under acid conditions (pH 1). In aqueous DMSO the acid/base equilibrium is a function of the DMSO/water ratio. The unexpected acidity of these heterocyclic NDIs, which justifies the reactivity with CH(2)N(2), has been rationalized by DFT computational means [PBE0/6-31+G(d,p)] in aqueous solvent (PCM models) as a result of a strong specific solvation effect, modeled by the inclusion of three water molecules.

The photoreactivity of several 3-substituted-1,6-dibromo-2-naphthols has been investigated in neat acetonitrile in the presence of diluted Et3N and in aqueous buffered acetonitrile (pH 8, phosphate buffered), using visible light (450 nm). Hydrobromic acid loss in the presence of the base, for the unsubstituted naphthol, or heterolytic C-Br cleavage directly from the naphtholates, for the more acid 3-substutited naphthols (R = COOCH3, CONH2, CONMe2), generates electrophilic carbene intermediates, which have been successfully trapped by molecular oxygen, pyrrole, acrylonitrile, ethyl vinyl ether, and allyltrimethylsilane. Product distribution analysis reveals three types of products arising from (i) arylation, (ii) alkenylation, and (iii) cyclization reactions. The generation and the reactivity of alpha-ketocarbene intermediates, as electrophilc diradicals, has been supported by laser flash photolysis, with the detection of both the carbene (lambda(max) 510 nm) and 1,2-naphthoquinone-O-oxide (R = CONMe2, lambda(max) 600 nm) in the presence of O2.

The photochemistry of 6-bromo-2-naphthols has been studied in acetonitrile, aqueous acetonitrile, and isopropyl alcohol in the absence and in the presence of triethylamine by product distribution analysis, laser flash photolysis (LFP), fluorescence, phosphorescence, electrochemical measurements, and DFT calculations. Hydrobromic acid loss in the presence of Et(3)N occurs from the triplet state of 6-bromo-2-naphthol, generating an electrophilic carbene intermediate, which has been successfully trapped by oxygen, allyltrimethylsilane, 2,3-dimethylbut-2-ene, pyrrole, acrylonitrile, 1,4-dimethoxybenzene, and also pyridine. The generation and the reactivity of a triplet carbene intermediate has been supported by LFP, with the detection of 2,6-naphthoquinone-O-oxide (530 < lambda < 650 nm) in the presence of O(2). The electrophilic diradical character of the carbene has been supported by DFT calculations, using the B3LYP, PBE0, and MPWB1K functionals, with the 6-31+G(d,p) basis set and PCM solvation model.

Mild activation of water-soluble naphthalene diimides (NDIs) as bisalkylating agents has been achieved by base catalysis and by chemical and electrochemical reductions. NDI activation by a single electron reduction represents a novelty in the field of activatable electrophiles. Under mild reduction, induced by S2O4(2-) in aqueous solution, the resulting NDI radical anion (NDI*-) undergoes a monomolecular fragmentation to yield a new transient species, where the NDI radical anion is tethered to a quinone methide moiety. The latter still retains electrophilic properties, reacting with amines, thiols, and ethyl vinyl ether. Owing to the NDI recognition properties, these results represent the first step toward selective and bioactivatable cross-linking agents.

The photoinduced synthesis of chiral 3,3'-CH2X-disubstituted BINOL ligands (X = NR2, SR, OH) has been achieved with excellent ee by UV-visible activation of BINOLAMs bearing L-proline ester arms. Quinone methides, detected by laser flash photolysis, are the key intermediates involved in such a synthetic protocol, which undergo reversible nucleophilic conjugate additions by a great variety of nitrogen nucleophiles (amines and alpha-amino acid derivatives) with complete configuration retention of the BINOL moiety.