Fluorescence of polycyclic aromatic hydrocarbons such as rubrene (Ru), naphthacene (Naph), perylene (Per), and 9,10-diphenylanthracene (DPA) is chemienergized by the dimethyl -peroxylactone. Two modes of chemienergization operate concurrently, namely, the classical energy transfer process, involving rate-determining unimolecular decomposition of the -peroxylactone, and the more efficient (ca. 50-fold) novel electron exchange luminescence process, involving a rate-determining bimolecular reaction between the fluorescers and -peroxylactone. In the latter case the fluorescers enhance the chemiluminescence by catalyzing the decomposition of the -peroxylactone. The catalysis is in the order of the oxidation potentials of the fluorescers, i.e., Ru > Naph > Per > DPA, as evidenced by a linear dependence of the catalytic rate constants (kcat) and activation energies (£a) on the fluorescer oxidation potentials (£ox). The /3-peroxylactones are ineffective as electron exchange substrates.
Die durch Zerfall des ,,Dimethyl‐oz‐peroxylactons" (I) angeregte Fluoreszenz von Rubren, Naphthacen, Perylen und 9,10‐Diphenylanthracen beruht auf zwei konkurrierenden Prozessen, nämlich de‐m klassischen Energietransfer sowie dem 50fach effizienteren Elektronenaustausch‐Lumineszenzprozeß.
Communications to the Editor 2587 conic Acid Violet": UV (H20) 534 (e 89 200), 447 (sh, e 13 000), 314 (€ 5900), 281 (sh, e 8200), 254 (e 17 800), 238 (sh, 13 100), 228 (sh, t 10 800) nm. Aqueous solutions of either the salt or the acid 4 absorb at the same region in the visible spectrum. Cyclic voltammetry (water) of 4 gave an irreversible oxidation potential at +0.3 V and an irreversible reduction
Direct a-lithiation of aromatic acetic acids by n-butyllithium in THF at -40 O C affords essentially quantitatively lithium a-lithiocarboxylates. The method can be employed as a convenient titration of alkyllithiums. a-Deuteration and bistrimethylsilylation of the a-lithiocarboxylates takes place essentially quantitatively. These are reliable electrophiles for the quantitative determination of the a-lithiocarboxylates. Direct oxygenation with molecular oxygen at room temperature affords good yields of the respectiye a-hydroxy acids, while reaction with molecular oxygen at dry ice temperature by inverse addition is an excellent method for the preparation of a-hydroperoxy acids derived from arylacetic acids. a-Hydroperoxy acids 1, which on cyclodehydration serve as precursors to a-peroxylactones 2 (eq 1),3 are extremelybase-and acid-sensitive compounds in view of the facile Grob-type fragmentations4 depicted in the respective transition states 3a and 3b. Since these decarboxylations efficiently destroy the a-hydroperoxy acids 1, it was essential to circumvent this problem by working under neutral conditions. We took advantage of the oxygenophilic propensity of the trimethylsilyl group and prepared a number of a-hydroperoxy acids 1 by singlet oxygenation of ketene bis(trimethylsily1) acetals 4 and subsequent desilylation of the peroxy ester 5 with methanol (eq 2). Analogous silatropic shifts have been ob-0 0 1 served in the singlet oxygenation of trimethylsilyl enol ether^.^ This novel a-hydroperoxylation lacks unfortunately generality since the classical prototropics shifts (ene reaction) compete with the silatropic shift when R is a nontertiary alkyl group6 in the ketene acetal 4. When R is an aromatic group, the classical [2 + 41 cycloaddition of the styryl unit with singlet oxygen competes.7Recently, we and others8 have shown that a-lithio enolates can be directly oxygenated to afford a-hydroperoxy acids 1 after acidification (eq 3). Unfortunately, this most direct a-0 0 6 1 oxygenation is limited to nonaromatic substrates since an attempt to a-oxygenate a-lithio-a-phenylacetate gave only decomposition and reduction products.8b To suppress the Grob fragmentation process, since benzaldehyde was the major product, we did not use hexamethylphosphoramide (HMPA) and pumped off the diisopropylamine prior to aoxygenation. While this process gave significantly improved results for the lower a-alkyl-and a,a-dialkylacetic acids, only poor yields of impure a-hydroperoxy acids could be realized for phenylacetic acid.9 It was clear to us that even traces of amines exerted detrimental effects in the preparation of ahydroperoxy acids derived from arylacetic acids via a-lithiation with lithium diisopropylamide (LDA). Although aliphatic carboxylates form ketones on treatment with alkyllithiums,1° Ivanov and colleagues demonstrated" that arylacetates can be a-lithiated with alkyllithiums directly; however, generally the yields were poor (19-35%). It was not clear whether the metalation process worked poorly or whether the titrati...
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