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The Mechanism of the Reaction of Diphenylketene with Bases in Aqueous Solution: Nucleophilic Attack Versus General Base Catalysis of Ketene Hydration

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The Mechanism of the Reaction of Diphenylketene with Bases in Aqueous Solution: Nucleophilic Attack Versus General Base Catalysis of Ketene Hydration J. Am. Chem. SOC.1992,114, 5643-5646 5643 The Mechanism of the Reaction of Diphenylketene with Bases in Aqueous Solution: Nucleophilic Attack versus General Base Catalysis of Ketene Hydration J. Andraos and A. J. Kresge* Contribution from the Department of Chemistry, University of Toronto, Toronto, Ontario MSS lA1, Canada. Received March 2, 1992 Abstract: Diphenylketene was generated in aqueous solution by flash photolysis, and rates of its decay accelerated by 30 bases of various structure were determined. The rate constants so obtained did not show the regular dependence on basic strength expected if the bases were serving as general base catalysts assisting the attack of water on the ketene, but they did vary with polarizability and steric bulk of the base in the way expected for direct nucleophilic attack of the base on the carbonyl carbon atom of the ketene. Assignment of a direct nucleophilic role to the bases is supported by the formation of amide products in addition to diphenylacetic acid in the reaction of diphenylketene accelerated by ammonia and morpholine, and quantitative analysis of the product ratios shows that these two bases serve only as nucleophiles and that the diphenylacetic acid is formed by uncatalyzed reaction of diphenylketene with solvent water. Ketenes are interesting and useful substances whose chemistry N.. is receiving renewed attention,’ stimulated in part by the devel- ph%Ph hv_ ph& 0-0 + N2 opment of rapid, flash-photolytic techniques for studying these (3) 0 Ph reactive molecules.2 This has led to a wealth of new information, among which is the discovery that the reaction of ketenes with 2 1 wholly or partly aqueous solvents is accelerated by for bases serving as proton transfer agents, eq 1, and bases serving This acceleration could be due to general base catalysis of ketene as nucleophiles, eq 2.4 We have also made use of the fact that hydration, through a transition state in which the base assists the products of these two kinds of reaction are different (cf. eqs nucleophilic attack of a water molecule on the ketene by removing 1 and 2). one of its protons, eq 1, or it could be the result of direct nu- cleophilic attack by the base itself, eq 2. Experimental Section t Materials. Azibenzil was prepared by mercuric oxide oxidation of benzil monohydrazone: and 4-(2,2-diphenylacetyl)morpholine was ob- -BH tained by treating morpholine with diphenylacetyl chloridee6 All other - materials were best available commercial grades. Ketenes. Flash photolysis was carried out using a system of conven- tional design that has already been described.2b Rates of decay of di- 1 A phenylketene were determined by monitoring the decrease of ketene absorbance at X = 267 nm. The reaction solutions were wholly aqueous 0 and were maintained at OC; initial azibenzil concentrations BH II 25.0 0.05 - PhzCHCOH (1) in these solutions were 2 X 10” M. The photochemical conversion of -E- azibenzil to diphenylketene in our apparatus under these conditions was quite efficient, with 80-90% of the azibenzil being consumed in the single flash that was used. The rate data fit the first-order rate law well, and observed first-order rate constants were obtained by least-squares fitting to an exponential function. Product Studies. The products of reaction of diphenylketene in am- monia and morpholine buffers were determined by HPLC analysis using a Waters Novapack C18 column in a Varian Vista 5500 instrument interfaced to a Varian Polychrom 9060 diode array detector. Solvents, either 40/60 (v/v) or 60/40 (v/v) water/methanol, were passed through Millipore glass filters before use, and the instrument was operated at a flow rate of 1 mL min-’ and a pressure of 150-180 atm; injection volumes were 200 pL. In order to distinguish between these two alternatives, we have In a typical experiment, 3 mL of aqueous buffer was placed in a quartz carried out a detailed study of the reaction of a wide variety of cuvette and a sufficient quantity (3-10 pL) of stock solution of azibenzil bases with diphenylketene, 1, generated in wholly aqueous solution in acetonitrile (Aldrich, HPLC grade) was added to give a final azibenzil by flash photolysis of azibenzil, 2, eq 3.2b Our mechanistic concentration comparable to that used for kinetics. In order to minimize argument is based upon the different reactivity patterns expected photochemical degradation of the azibenzil in this solution by room light, the cuvette was wrapped in aluminum foil. A chromatogram and UV spectrum of a sample of this solution were recorded, and the foil was then (1) See, for example: Tidwell, T. T. Acc. Chem. Res. 1990, 23, 273-279. removed and the cuvette and its contents were subjected to one flash of (2) (a) Bothe, E.;Meier, H.; Schulte-Frohlinde, D.; von Sonntag, C. An- gew. Chem., In!. Ed. Engl. 1976, 15, 380-381. Bothe, E.;Dessouki, A. M.; ca. 50 ps duration in our conventional flash-photolysis apparatus. The Schulte-Frohlinde,D. J. Phys. Chem. 1980,84, 3270-3272. (b) Allen, A. D.; solution in the cuvette was then acidified by adding a few drops of Kresge, A. J.; Schepp, N. P.; Tidwell, T. T. Can. J. Chem. 1987, 65, 1719-1723. (c) Andraos, J.; Kresge, A. J. J. Photochem. Photobiol. A: ~ ~ ~~ Chem. 1991, 57, 165-173. (d) Allen, A. D.; Andraos, J.; Kresge, A. J.; (4) Jones, R. A. Y. Physical and Mechanistic Organic Chemistry, 2nd ad.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc. 1992, 114, 1878-1879. Cambridge University Press: New York, 1984; pp 279-284. Jencks, W. P. Jones, J., Jr.; Kresge, A. J. J. Org. Chem. 1992, 57, in press. Catalyisis in Chemistry and Enzymology; McGraw-Hill: New York, 1969; (3) (a) Allen, A. D.; Tidwell, T. T. J. Am. Chem. SOC. 1987, 109, pp 78-111, 170-182. 2774-2780. (b) Allen, A. D.; Stevenson, A,; Tidwell, T. T. J. Org. Chem. (5) Nenitzescu, C. D.; Solomonica, E. Organic Synthesis; Wiley: New 1989,54, 2843-2848. Allen, A. D.; Tidwell, T. T. Tetrahedron Lett. 1991, York, 1943; Collect. Vol. 11, pp 496-497. 32. 847-850. (6) Rossi, R. A,; Alonso, R. A. J. Org. Chem. 1980, 45, 1239-1241. 0002-7863/92/1514-5643$03.00/0 0 1992 American Chemical Society 5644 J. Am. Chem. SOC.,Vol. 114, No. 14, 1992 Andraos and Kresge I I I I Table I. Rate Constants for the Reaction of Diphenylketene with A Bases in Aqueous Solution at 25 Oca 0 0 base PKABH) ke (M-ls-') 0 0 9.25b 3.53 x 104 0 0 @e O0 00 e 5.34c 1.76 x 105 0 e 5.59d 6.98 x 104 0 5.95e 5.03 x 105 X 0 7.86 1.51 IO5 A A 7.979 1.82 X lo6 8.07h 6.48 x 103 A* 9.40d 2.38 x 105 A* 9.50' 1.83 X lo5 9.92d 3.92 x 105 2' 2 ; ; b Ib 1'1 Ih 1:' 4 4 10.601 3.52 x 105 -fog ('1Ko/p) 10.62k 1.94 x 105 Figure 1. Bronsted-type plot for the reaction of diphenylketene with bases in wholly aqueous solution at 25 OC, ionic strength = 0.10 M: 0, NH,; 8.49' 2.22 x 105" 0, RNH2; 0,RZNH; 0,R3N; A, RO-; A, RS-; V, N3-, CN-; V, HPOt-. PNH 8.84" 1.84 x 105 concentrated perchloric acid, and a chromatogram of the acidified solu- W tion and UV spectra of all detectable products were recorded; wave- n 9.730 6.89 x 105 lengths in the range X = 234-263 nm were used for detection. Flashed NH NH reaction mixtures were acidified in order to convert the diphenylacetic I acid product into its un-ionized form; if this were not done, the acid in 1.08P 4.06 x 104"' its carboxylate form was eluted at the same time as buffer components 1 and quantitative analysis could not be performed. Products were iden- tified by spiking with authentic samples, and product absorbance ratios were converted into concentration ratios using extinction coefficients 11.12q 1.03 x 105"' determined here. Further details of experimental techniques are available in the thesis upon which this report is based.' 1 1.27' 1.91 x 105 Results 7.41' 4.57 x 102"' Rates of reaction of diphenylketene were measured in wholly 8.82' 1.07 X lo3"' aqueous buffer solutions of 30 bases. Series of buffer solutions of constant stoichiometric buffer ratio and constant ionic strength but varying total buffer concentration were used. Buffer con- 10.19" 4.01 X IO2 centrations were varied by factors ranging from 3 to 100, and (H3 replicate measurements (2-17) were made at each buffer con- 11.15" 3.39 x 103 centration. These data are summarized in ref 7. all cases, observed first-order rate constants proved to be In 6.65" X linear functions of buffer concentration,* and measurements made 9.39y 3.37 x 10" at different buffer ratios showed that the buffer base was the 9.93y 3.35 x 102 reactive buffer component. The data were therefore fitted to the 12.4W 3.94 x 103" rate law given in eq 4. Least-squares analysis gave the bimolecular 4.6Y 1.17 x 105 7.20"" 1.04 x 103 (4) 9.22bb 1.96 x 104 HOCH2CHZS- 9.72Cc 2.43 X lo6"' rate constants, kB, listed in Table I and provided values of k, in 'Ionic strength = 0.10 M (NaC104).
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