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

Abstract

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, pQ a 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 (pK E ) 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 (pQ a 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 available 2 has been obtained from systems in which the carboxylic acid enol is stabilized, either sterically 3,4 or electronically. [5][6][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 fluorene-9-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.

The Photoelectric Response of Gold‐Nanoparticle Monolayers

“…Recent studies on the preparation of these enols, especially of amides, involve two approaches. (1) Addition of nucleophiles (e.g., H 2 O, RR′NH) to ketenes;4–14 this mostly gives short‐lived enols,4–14 although several of them are kinetically stabilized by bulky aryl groups15–23 and are sufficiently long‐lived to be detected by 1 H and 13 C NMR spectroscopies 15–23. (2) Addition of organic isocyanates RNCO to CH 2 YY′ (where Y and Y′ are electron‐withdrawing groups (EWGs)) gives the amides 2 , and/or their enols 1 , X = NRR′ or 1 / 2 mixtures 24–32.…”

1,1‐Ethenediol: The Long Elusive Enol of Acetic Acid

“…More generally speaking, most enols are short-lived reactive species and they readily isomerize to their ketoforms; enols can be stabilized through resonance, [23] steric factors, [1] and electronic effects. [2,24] Experimental methods such as time-resolved spectroscopy, [5,25] neutralization-reionization mass spectrometry, [26,27] NMR as well as IR spectroscopy, [28] and other spectroscopic techniques [29] along with computational methods [30][31][32][33] have played a significant role in understanding the structures and reactivities of enols. The simplest enol, ethenol (vinyl alcohol), was generated in the gas phase through the pyrolysis of ethylene glycol.…”

“…Subsequent photorearrangement to acetic acid (3) and ketene(4). Biologically highly relevant enols(5)(6)(7).…”

The problem of distinguishability of rate constants in the two-step consecutive sequence A → B → C

“…Note that the subscript labels for the rate constants, 1 and 2, refer to step 1 and step 2, respectively, in the sequence. [1] A B C → → k k 1 2 This particular kinetic system is well known with several examples cited in the literature (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). [2a]…”

“…More importantly, the less well-documented experimental problem of assigning calculated rate constants, k 1 and k 2 , obtained from nonlinear regression analysis to the appropriate steps in the reaction sequence is addressed. This problem has been documented by a few authors, and essentially two methods have been described which deal with the ambiguity of rate constant assignments (7)(8)(9)(10)(11)(12)(13)(14)(15). These methods rely either on inclusion of additional chemical knowledge or on analysis of the variation of absorption coefficients to remove the ambiguity.…”