Hydration reactivity of ketenes generated by flash photolysis

Abstract

. Can. J. Chem. 65, 1719 (1987). The ketenes n-BuCH=C=O, PhCH=C=O, and Ph2C=C=0 have been generated by flash photolysis and their reactivities toward water, including acid and base catalysis, have been measured in wholly aqueous media by ultraviolet spectrophotometry. The phenyl group electronically enhances the reactivity of ketenes relative to n-butyl toward water and hydroxide, but retards reaction with protic acids. The presence of bulky groups, including phenyls, on each side in the ketene plane sterically retards nucleophilic attack. The reactions are interpreted as involving in-plane attack by nucleophilic water or hydroxide in the neutral and base-induced reactions, and electrophilic protonation perpendicular to the ketene plane for the acid-catalyzed reaction. ANNETTE D. ALLEN, A. JERRY KRESGE, NORMAN P. SCHEPP et THOMAS T. TIDWELL. Can. J. Chem. 65, 17 19 (1987). Faisant appel 2 la photolyse Cclair, on a prCparC les ctttnes n-BuCH=C=O, PhCH=C=O et Ph2C=C=O; utilisant la spectrophotomCtrie ultraviolette et opCrant dans des milieux aqueux, on a mesure leurs rCactivitCs vis-2-vis de l'eau, tant sous I'influence de catalyseurs acides que basiques. Par comparaison avec le groupement n-butyle, le groupement phCnyle augmente la rtactivitC des cCt&nes vis-a-vis de l'eau et de I'ion hydroxyde; toutefois, il retarde la reaction avec les acides protiques. La prtsence de groupements encombrants, y compris les groupements phCnyles, de chaque cBtC du plan du ctttne retarde l'attaque nuclCophile. Pour expliquer les rkactions qui se produisent en milieux neutre ou basique, on fait appel i un mecanisme impliquant une attaque, par l'eau du l'hydroxyde nuclCophiles, qui se produirait dans le plan; par ailleurs, les rkactions catalysCes par les acides se produiraient par le biais d'une protonation Clectrophile qui se ferait d'une faqon perpendiculaire au plan du cCttne.[Traduit par la revue]The kinetic study of the hydration of ketenes in wholly At least partly because of the diversity of methods and conaqueous media is complicated not only by the strong tendency ditions that have been used for these studies of ketene hydration of the ketenes to dimerize but also by their high reactivity with there are also widely divergent views of the mechanisms of water. Previously, only by using in situ generation of ketenes these processes, and these have been recently reviewed (10). by flash photolysis of diazoketones to give arylketenes (1) or of We have advocated (5, 6) the processes shown in eqs.[3]-[5] cyclobutanone to give ketene (2), together with fast measure-A+ ment techniques, have such studies been accomplished (eqs.

Laser flash photolysis study of the photochemistry of ortho-benzoylbenzaldehyde

“…The isotope effect observed in the hydration reaction (H/D -1.5) is in line with those observed by other groups for other ketene reactions with water, which values frequently fluctuate between 1.5 and 2.5 (44,(54)(55)(56)(57) , , , , , , , , , ,,,, = 300 nm) of 3 in methanol. …”

“…Naturally, in the case of 4 the -OH group is not suitably oriented to assist water addition or at least not to do this with comparable efficiency. In addition, the fact that 5 is more planar than 4 may facilitate the nucleophilic attachment by water by delocalizing more easily the negative charge generated, in a manner similar to that proposed for phenylketene (55).…”

A study of the vacuum pyrolysis of 4-diazoisothiochroman-3-one with Hel ultraviolet photoelectron spectroscopy

“…To expand our research on transient species, we used this instrument to study the vacuum pyrolysis of 4-diazoisothiochroman-3-one (la). We anticipated that this study would add new information to the limited amount of data that are available on the vacuum pyrolysis of a-diazocarbonyl compounds in contrast to the large body of work that has been accumulated on the photochemistry of this class of compounds (4)(5)(6)(7)(8)(9)(10). Our goals were to compare the chemistries of l a and l b and establish whether two new transients, thiaketene 3a and benzocyclobutenthione (6a), that are potential products of the pyrolysis of l a could be characterized with pes.…”

Flash photolytic generation and study of ketenes: acid-catalysed hydration in aqueous solution

“…Flash photolysis was carried out using a system of conventional design described previously [5]. Measurements were recorded at 25.0~0.05 "C on wholly aqueous solutions, using the decrease in absorbance which accompanies ketene hydration to monitor the reaction; the wavelengths employed were A = 210-215 nm for the aliphatic ketenes and A = 260 nm for the aromatic derivative.…”