Flash Photolysis of 10-Diazo-9(10H)-phenanthrenone in Aqueous Solution. Hydration of Fluorenylideneketene and the Fluorene-9-carboxylic Acid Keto-Enol System

Flash photolysis of 10-diazo-9(10H)-phenanthrenone in aqueous solution was found to give two successively formed transient species and to produce fluorene-9-carboxylic acid as the major reaction product. These transients were identified, through solvent isotope effects and the form of acid-base catalysis, as fluorenylideneketene, formed by photo-Wolff reaction of the diazophenanthrenone, and fluorene-9-carboxylic acid enol, formed by hydration of this ketene. Analysis of the rate profile of the enol ketonization reaction produced the first and second ionization constants for the enol ionizing as an oxygen acid, pQa E ) 2.01 and pQ′a E ) 9.61, respectively. The rate of enolization of fluorene-9-carboxylic acid was also determined, by bromine scavenging, and that, coupled with a literature value of the acidity constant of this acid, allowed evaluation of the two keto-enol equilibrium constants (pKE ) 9.67 for interconverting un-ionized carboxylic acid and enol and pK′E ) 8.24 for interconverting singly ionized acid and enol), and it also allowed evaluation of the two carbon acid acidity constants (pQa K ) 11.67 for ionization of the un-ionized carboxylic acid as a carbon acid and pQ′a K ) 17.85 for ionization of its carboxylate ion as a carbon acid). (All acidity constants are concentration quotients applicable at ionic strength 0.10 M.) These keto-enol equilibrium constants and acid dissociation constants are large because of the enol and enolate ion stabilizing effects of the cyclopentadienyl ring of the fluorenyl group; this ring also makes fluorenylideneketene an unusually reactive substance. During the past 15 years there has been a remarkable development of methods for generating the enol isomers of simple aldehydes and ketones in solution, and as a result much is now known about the chemistry of these unstable substances.1 Much less, however, is known about enols of carboxylic acids, undoubtedly because these substances are very much less stable than the already quite labile aldehyde and ketone enols. Most of the relatively little information that is available2 has been obtained from systems in which the carboxylic acid enol is stabilized, either sterically3,4 or electronically.5-7 The cyclopentadienylidene group has proved to be especially good at stabilizing carboxylic acid enols, and this effect has enabled studies of fulvenediol, 2,5 the enol of cyclopentadienecarboxylic acid, 1, eq 1,and its benzo analog 4, the enol of indenecarboxylic acid, 3,5b,6b,d,7 eq 2. We have now completed this series by investigating the dibenzo analog 6, which is the enol of fluorene9-carboxylic acid, 5, eq 3. We generated the enol of fluorene-9-carboxylic acid by hydration of fluorenylideneketene, 8, obtained by photo-Wolff reaction of the diazoquinone 10-diazo-9(10H)-phenanthrenone, 7, eq 4. We were able to measure rates of this hydration reaction as well as rates of ketonization of the enol, and we consequently gained information about the chemistry of the ketene in addition to that of the enol. X Abstract published in AdVance ACS Abstracts, August 15, 1997. (1) See, for example: The Chemistry of Enols; Rappoport, Z., Ed.; Wiley: New York, 1990. (2) For a brief review: see Kresge, A. J. Chem. Soc. ReV. 1996, 25, 275-280. (3) O’Neill, P.; Hegarty, A. F. J. Chem. Soc., Chem. Commun. 1987, 744-745. Allen, B. M.; Hegarty, A. F.; O’Neill, P.; Nguyen, M. T. J. Chem. Soc., Perkin Trans. 2 1992, 927-934. (4) Frey, J.; Rappoport, Z. J. Am. Chem. Soc. 1995, 117, 1161-1162; 1996, 118, 5169-5181, 5182-5191. (5) (a) Urwyler, B.; Wirz, J. Angew. Chem., Int. Ed. Engl. 1990, 29, 790-792. (b) Almstead, J.-I. K.; Urwyler, B.; Wirz, J. J. Am. Chem. Soc. 1994, 116, 954-960. (6) (a) Chiang, Y.; Kresge, A. J.; Pruszynski, P.; Schepp, N. P.; Wirz, J. Angew. Chem., Int. Ed. Engl. 1990, 29, 792-794. (b) Andraos, J.; Chiang, Y.; Huang, C.-G.; Kresge, A. J.; Scaiano, J. C. J. Am. Chem. Soc. 1993, 115, 10605-10610. (c) Andraos, J.; Chiang, Y.; Kresge, A. J.; Pojarlieff, I. G.; Schepp, N. S.; Wirz, J. J. Am. Chem. Soc. 1994, 116, 73-81. (d) Andraos, J.; Kresge, A. J.; Popik, V. V. J. Am. Chem. Soc. 1994, 116, 961-967. (7) Barra, M.; Fisher, T. A.; Cernigliaro, G. J.; Sinta, R.; Scaiano, J. C. J. Am. Chem. Soc. 1992, 114, 2630-2634. 8417 J. Am. Chem. Soc. 1997, 119, 8417-8424 S0002-7863(97)01381-4 CCC: $14.00 © 1997 American Chemical Society Experimental Section Materials. 10-Diazo-9(10H)-phenanthrenone was prepared by treating 9,10-phenanthrenequinone with (p-tolylsulfonyl)hydrazine.8 Other materials were the best available commercial grades. Flash Photolysis. Flash photolysis experiments were done using two different systems: (1) a conventional apparatus with excitation flash produced by capacitor discharge through a pair of xenon flash lamps, 50 μs pulse width,9 and (2) a KrF eximer laser system, λexc ) 248 nm, 25 ns pulse width.6b Substrate concentrations were ca. 10-5 M. Enolization Rate Measurements. Rates of enolization of fluorene9-carboxylic acid were determined by using bromine to scavenge the enol as it formed. Measurements were made in sodium hydroxide solutions using the absorbance of OBrat λ ) 330 nm to monitor bromine concentration. Initial concentrations of the carboxylic acid substrate were 1.4 × 10-4 M and of bromine 1.7 × 10-4 M. The reaction is pseudo-first-order under these conditions, and the first-order rate law was obeyed accurately; observed rate constants were determined by least squares fitting of an exponential function. Further experimental details are available in the theses upon which part of this paper is based.10