Laser flash photolysis study of the photochemistry of ortho-benzoylbenzaldehyde

Abstract

. Ketene-enols 4 and 5 have been generated by laser flash photolysis of or-tho-benzoylbenzaldehytle (3) and kinetically and spectroscopically characterized. In benzene or acetonitrile, the E ketene-enol, 4, shows absorption at 340 and 400 nm and a lifetime in excess of 1 ms, whereas the Z ketene-enol, 5 , shows maxima at 360 and 430 nrn and a lifeti~iie of only 1.5 ks. At shorter time scales we observed a weak absorption (A,,,;,, = 580 nm) tentatively assigned to biradical 6 with a lifetime of 140 ns. The E ketene-enol is readily quenched by oxygen, dienophiles, methanol, and water, with quenching rate constants ranging from 3.6 x I O M~. ' s ' (for methanol as a quencher) to 2.2 x 10' M ' s-.' (for diethyl ketomalonate). At high water concentrations (typically > 10 M) a new species, 7 , was detected showing maximum absorption at 5 10 nm and a growth lifetime of 7 ks. In deuterated water and using the same concentration as before we observed a formation lifetime for 7 of 10 ks, which results in an isotope effect of -1.5. It is proposed that 5 is the main precursor for 7 . Steady-state irradiation of 3 in deaerated methanol leads to the formation of dihydroanthraquinone (9). a strongly colored and fluorescent (A,,,,, = 475 nm. T , = 29 ns) species, whereas 3-phenylphthalide (2, R = Ph) is the main product when the irradiation is performed in benzene. Steady-state quenching of product formation by diethyl ketomalonate gives a Stern-Volmer constant of 380 M-' from which we conclude that 5 is the ketene-enol responsible for product formation, in agreement with the laser flash photolysis results. . L'irradiation a I'dtat stationnaire du compose 3 dans le methanol desaCr6 conduit a la formation de la dihydroanthraquinone (9), un composk fortement color6 et fluorescent (A,,,:,, = 475 nm, 7" = 20 ns) tandis que le phthalide de phCnyl-3 (2. R = Ph) est le produit majoritaire lorsque I'irradiation a lieu dans le benzene. Le blocage i I'etat stationnaire des produits obtenus B partir du cdtornalonate d'dthyle donne une constante de Stern-Volmer de 380 M-' qui nous porte B conclure que le composd 5 est le cdtkne-en01 responsable de la for~iiation des produits en accord avec les resultats de la photolyse flash au laser.[Traduit par la rCdaction]

Photochemistry of 2-Alkoxymethyl-5-methylphenacyl Chloride and Benzoate

“…The rate constant for quenching of the E-ketene-enol (from 2-benzoylbenzaldehyde) by water has been found by Netto-Ferreira and Scaiano to be 1.1 × 10 3 M -1 s -1 in acetonitrile. 50 In conclusion, Scheme 6 shows a complete mechanistic proposal of the photochemical transformation of 1a into the major products 3a and 2. It includes short-lived transients detected by LFP, productive or nonproductive isolated as well as hypothetical intermediates, and byproducts, such as methanol, methyl chloride, or the acetophenone derivative 5a that is produced in traces, apparently by homolytic C-Cl bond cleavage.…”

“…52,53 For 2-benzoylbenzaldehyde in benzene or acetonitrile intramolecular hydrogen transfer leads to the formation of two ketene-enols: E (τ ) 1 ms), showing absorption at 340 and 400 nm, and Z (τ ) 1.5 µs) with λ max ∼ 360 and 430 nm. 50 By analogy, we assign the transient with λ max ) 400 nm to Z-9 and that with λ max ) 370 nm to E-9 (Table 4; Scheme 6). The lifetime of E-9 is reduced in aerated solutions, which may indicate oxidation by O 2 , as observed in the study on 2-phthalaldehyde.…”

The photoformation of a phthalide: a ketene intermediate traced by FSRS

“…17 It was observed that addition of water shortens the lifetime of the longlived transient without leading to additional photoproducts. Water molecules may for example act as catalysts for certain reaction steps.…”

“…6 A ketene intermediate was indeed observed via IR spectroscopy in a matrix isolation experiment. Time resolved spectroscopy on oPA and related compounds 15,17,18 lacked the temporal resolution to trace the ketene formation. Time resolved spectroscopy on oPA and related compounds 15,17,18 lacked the temporal resolution to trace the ketene formation.…”

Laser flash photolysis study of the photoenols generated from ortho-benzylbenzophenone in different solvents

“…Suitably substituted aromatic ketones can form the corresponding benzocyclobutenols in fairly high quantum yields, suggesting that intramolecular [2+2] cycloaddition may be a more common process in photoen01 chemistry than had been previously assumed (6)(7)(8)(9). Other examples include chloride elimination from a-chloro acetophenones with the concomitant formation of 1-indanones (10)(11)(12), the formation of 3-phenylphthalide from the photolysis of ortho-benzoylbenzaldehyde (13), and, finally, the synthesis of 2-hydroxy-2-alkyl(or ary1)-1-indanones from ortho-alkyl substituted aromatic a-diketones (14)(15)(16)(17)(18) ysis and product studies on the photoenols in different solvents, generated by irradiation of ortho-benzylbenzophenone (1).…”