Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23 degrees C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, alpha,alpha,alpha-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,pi*, pi,pi*(CT) and arenoid pi,pi* lowest triplets with (triplet) reduction potentials (E(red)*) varying from about -10 to -38 kcal mol(-1). The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log k(Q) vs E(red)* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,pi* triplet ketones and those pi,pi* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the pi,pi* ketone triplets. Ketones with lowest charge-transfer pi,pi* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid pi,pi* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.

The photochemistry of a series of alkoxyacetophenone derivatives bearing remote phenolic groups has been studied using laser flash photolysis techniques. The compounds are structured with a p- or m-phenolic moiety attached via a m- or p-oxyethyl linkage to the carbonyl chromophore, and each have a lowest triplet state of π,π* configuration. The corresponding methoxy-substituted compounds have also been examined. The triplet lifetimes of the phenolic ketones vary with the positions of attachment (meta or para) of the oxyethyl spacer to the carbonyl and phenolic moieties, indicating a very strong dependence of the rate of intramolecular H-abstraction on geometric factors. For example, the para,para‘-linked phenolic ketone has a lifetime τT ≈ 12 ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta‘-derivative exhibits a lifetime τT ≥ 11.5 μs at infinite dilution and no detectable intramolecular reactivity. The presence of as little as 0.03 M water in the solvent leads to a significant increase in triplet decay rate in all cases, in contrast to its retarding effect on the rate of bimolecular phenolic H-abstraction in model compounds. Semiempirical (PM3) calculations have been carried out to determine the optimum conformation for abstraction in each molecule as a function of substitution pattern. The variation in the rate constants for intramolecular H-abstraction throughout the series is consistent with a mechanism involving coupled electron/proton transfer within a hydrogen-bonded triplet exciplex, the overall rate of which depends critically on orbital overlap factors between the aromatic rings in the ketone and the phenol.

The photochemistry of a series of alkoxyacetophenone, -benzophenone, and -indanone derivatives, which contain a remote phenolic group linked to the ketone by a para,para¢-or meta,meta¢-oxyethyl spacer, has been studied in acetonitrile and dichloromethane solutions using laser flash photolysis techniques. The corresponding methoxysubstituted compounds and, in the case of the alkoxyindanones, derivatives bearing just a remote phenyl substituent, have also been examined. The triplet lifetimes of the phenolic compounds are determined by the rates of intramolecular abstraction of the remote phenolic hydrogen, and depend on the solvent, the geometry of attachment and the configuration of the lowest triplet state. In contrast to the large (>500-fold) difference in lifetime of the para,para¢-and meta,meta¢-alkoxyacetophenone derivatives, both of which have lowest p,p* triplet states, smaller differences are observed for the alkoxyindanone (lowest charge transfer triplet,~twofold difference) and alkoxybenzophenone (lowest n,p* triplet,~18-fold difference) derivatives in acetonitrile solution. The triplet lifetimes of the acetophenone and benzophenone are significantly shorter in dichloromethane than in acetonitrile, consistent with the intermediacy of a hydrogen-bonded triplet exciplex in the reaction. This is not the case with the para,para¢-indanone derivative, sugesting that hydrogen abstraction in this compound is dominated by a mechanism involving initial charge transfer rather than hydrogen bonding. This is most likely due to orientational constraints that prevent the remote phenolic -O-H group from adopting a coplanar arrangement with the n-orbitals of the carbonyl group.Résumé : Faisant appel à des techniques de photolyse éclair au laser et opérant dans des solutions d'acétonitrile et de dichlorométhane, on a étudié la photochimie d'une série de dérivés alkoxyacétophénones, -benzophénones et -indanones comportant un groupe phénolique dans une position éloignée, mais liée à la cétone par un espaceur oxyéthyle en para,para¢-ou méta,méta¢-. On a aussi étudié les composés correspondants portant des substituants méthoxy ainsi que, dans le cas des alkoxyindanones, des dérivés ne portant qu'un substituant phényle en position éloignée. On a déterminé les temps de vie du triplet des composés phénoliques à partir des vitesses d'enlèvement intramoléculaire de l'hydrogène phénolique éloigné; ils dépendent du solvant, de la géométrie de fixation et de la configuration de l'état triplet le plus bas. Par opposition à la grande différence (>500 fois) dans les temps de vie des dérivés para,para¢-et méta,méta¢-alkoxyacétophénones qui ont tous les deux des états triplets p,p* plus faibles, on observe des différences plus faibles pour les dérivés alkoxyindanones (triplet de transfert de charge le plus faible; différence du simple au double) et alkoxybenzophénones (triplet n,p* le plus faible; différence correspondant à une réaction environ 18 fois plus rapide) dans une solution d'acétonitrile. Les temps de vie des dérivés de l'acétop...

Recently, Brown and co-workers reported the use of the bromonium ion of adamantylideneadamantane, as the triflate salt ([AdAdBr](OTf)), to transfer Br + to acceptor olefins. 1 On the other hand, work from this laboratory has shown that reaction of iodine with Os 3 (CO) 12 proceeds via the [Os 3 (CO) 12 -(I)] + cation, and it was suggested that bromination of Os 3 (CO) 12 proceeded through a similar intermediate. 2 Here we report the use of the bromonium ion reagent to prepare the [M 3 (CO) 12 -(Br)] + (M ) Ru, Os) ions and the determination of their crystal structures. These ions are the first halonium ions of metal carbonyl cluster compounds to be structurally characterized. The two cations have markedly different structures: the ruthenium derivative can be considered as an organometallic analogue of the bromonium ion of ethylene, [C 2 H 4 Br] + , whereas the osmium cation can be thought of as the corresponding ion of cyclopropane, [C 3 H 6 Br] + . Furthermore, evidence is presented that the form of [Os 3 (CO) 12 (Br)] + characterized here is not an intermediate in the bromination of Os 3 (CO) 12 .Addition of [AdAdBr]{B[C 6 H 3 (CF 3 ) 2 ] 4 } 3 to M 3 (CO) 12 (M ) Ru, Os) in CH 2 Cl 2 at or below room temperature readily afforded [M 3 (CO) 12 (Br)]{B[C 6 H 3 (CF 3 ) 2 ] 4 }, which on addition of hexane formed air-stable, red-orange (M ) Ru) or pale yellow (M ) Os) crystals. 5 The structure of the ruthenium cation (1) as found in the X-ray structure determination of [Ru 3 (CO) 12 -(Br)]{B[C 6 H 3 (CF 3 ) 2 -3,5] 4 } 6 is shown in Figure 1. It consists of a bent chain of three ruthenium atoms (Ru(1)Ru(2)Ru(3) ) 157.19(4)°), one Ru-Ru bond of which is bridged by the

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