1547-69-9Relevant articles and documents
Rate Constants for Degenerate Hydrogen Atom Exchange between α-Hydroxy Radicals and Ketones
Wagner, Peter J.,Zhang, Yuanda,Puchalski, Allen E.
, p. 13368 - 13374 (1993)
When a phenyl ketone is irradiated in the presence of 1-phenylethanol, the photoproducts include acetophenone pinacol, the pinacol from the other phenyl ketone, a mixed pinacol, and acetophenone, which is formed by irreversible hydrogen atom exchange between the hemipinacol radical of acetophenone and the other ketone.Rate constants for this hydrogen atom exchange between the hemipinacol radical of acetophenone and three other ketones were determined by measuring how acetophenone yields depend on the concentration of the other ketone.Comparable measurements were made for p-chloroacetophenone formation by irradiating acetophenone in the presence of 1-(p-chlorophenyl)ethanol.As the starting ketone concentration increases, so does the amount of exchange relative to pinacol, while the pinacol content reflects decreasing amounts of the original alcohol.Exchange is measurable at ketone concentrations below 0.01 M and is complete by 0.1 M.Rate constants kex of 3.7, 6.3, 4.2, and 8.6 * 103 M-1 s-1 were deduced for hydrogen transfer to propiophenone, isobutyrophenone, p-methylacetophenone, and p-chloroacetophenone, respectively, based on a competing rate constant for radical coupling of 2 * 109 M-1 s-1.Equilibrium constants for the hydrogen transfer were determined from the product ratios obtained by irradiating a mixture of two ketones with 2-propanol; from these k-ex values of 11.5, 57, 13, and 1.8 * 103 M-1 s-1, respectively, were deduced for hydrogen transfer to acetophenone from the four other ketone hemipinacol radicals.These exchange rate constants depend more on the structure of the radical than on that of the ketone.Actual quantum yields for pinacol formation do not exceed 50 percent ; this maximum quantum efficiency rises to 71 percent for 1-phenylethanol-O-d.From this inverse isotope effect it is concluded that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants.
Bryce-Smith et al.
, p. 914 (1971)
The Competition between Cross-coupling and the Exchange Reaction in the Photoreduction of Aromatic Ketones
Lund, Torben,Lundgren, Birger,Lund, Henning
, p. 755 - 761 (2007/10/02)
Four aromatic ketones K with different reduction potentials and a series of hydrogen atom donors AH2 with different oxidation potentials of AH were irradiated in pairs and the product composition analyzed.Based on the product analysis the rate kex of the exchange reaction K + AH -> KH + A, was characterized as slow, medium fast or fast, respectively, relative to the radical-radical coupling reactions of K and AH.The results clearly point to the value of a knowledge of the oxidation potential of the AH radicals and the reduction potential of the ketones for the prediction of the outcome of the photoreduction of a ketone by an alcohol or another hydrogen atom donor.
Extent of Charge Transfer in the Photoreduction of Phenyl Ketones by Alkylbenzenes
Wagner, Peter J.,Truman, Royal J.,Puchalski, Alan E.,Wake, Ronald
, p. 7727 - 7738 (2007/10/02)
Rate constants for triplet-state reaction of various ring-substituted benzophenones (BPs), acetophenones (APs), and α,α,α-trifluoroacetophenones (TFAs) with toluene and p-xylene have been determined by a combination of flash kinetics, steady-state quenching, and quantum yield measurements.The relative amounts of primary and tertiary radicals formed by reaction of the same ketons with p-cymene have also been measured.For all three types of ketones, rate constants correlate well with triplet ketone reduction potentials.The magnitude of the kinetic isotope effects observed with toluene-d8 and p-xylene-d10 diminishes as the ketones become easier to reduce.All of the ketone triplets react with alkylbenzenes primarily by a charge-transfer mechanism, with the rate-determining step changing from complexation to hydrogen transfer as the ketones become harder to reduce.The least reactive AP triplets probably react significantly via simple hydrogen atom abstraction as well.Those ketones with n,?* lowest triplets (all BPs and some APs) react with p-cymene to give primary/tertiary radical ratios that vary no more than a factor of 2 from the 0.40 value displayed by tert-butoxy radicals; those with ?,?* lowest triplets (TFAs and some APs) give ratios that favor primary radicals and that vary by an order of magnitude with the triplet ketone reduction potential.The variation in cymene product ratios reflect different orientations for attack on cymene by n,?* and ?,?* triplets and differing degrees of partial electron transfer within the exciplexes, which are not tight radical ion pairs.The variation seen for ?,?* triplets represents a stereoelectronic effect within face-to-face exciples, as evidenced by the excerptional behavior of p-diacylbenzenes, which give the highest ratio of tertiary radicals from cymene.There is no set intrinsic ratio of reactivity for ?,?* triplets vs. n,?* triplets in these CT reactions.The two types of triplets show similar reactivity for the more easily reduced triplets, with the harder to reduce ?,?* triplets being only one-tenth as reactive as n,?* triplets of comparable triplet reduction potential.When the extent of electron transfer in the exciplex is small, hydogen transfer is rate determining and ?,?* reactivity drops.A study of two radical reactions which generate benzyl and α-hydroxy-α-methylbenzyl radicals indicates that radical disproportionation cannot explain the low quantum yields (0.10) of most ketone-toluene photoreductions, which apparently involve substantial radiation less decay by the exciplex intermediates.
