Geometric and solvent effects on intramolecular phenolic hydrogen abstraction by carbonyl n,π* and π,π* triplets

Abstract

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...

Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets†

“…Figure 3 shows Hammett plots of the rate constants for quenching of the triplet states of the substituted benzophenones and acetophenones by 4MP in MeCN solution. Of the 14 benzophenones included in the plot for that series of compounds, data for nine of them were already available from earlier studies (16, 22, 24). The five new compounds studied in the present work were chosen with specific reasons in mind.…”

“…Although phenolic hydrogen abstraction by aromatic carbonyl compounds has continued to be of interest in recent years (18–29), there have been no studies reported that probe the trends in the kinetics of the reaction in nonaqueous solvents as a function of the structural and electronic properties of the ketone in an extensive and systematic way. Accordingly, in the present paper we report the results of a survey of the absolute rate constants for reaction of the lowest triplet states of nearly 40 aromatic ketones with a single phenol ( para ‐methylphenol) in acetonitrile (MeCN) solution at room temperature.…”

“…Accordingly, in the present paper we report the results of a survey of the absolute rate constants for reaction of the lowest triplet states of nearly 40 aromatic ketones with a single phenol ( para ‐methylphenol) in acetonitrile (MeCN) solution at room temperature. The specific ketones studied in this work are shown in Scheme 1; the list includes several for which rate constants have been reported previously (16, 22, 24), but most are reported here for the first time. The list includes various substituted acetophenones, benzophenones, and several bicyclic and tricyclic ketones, with lowest triplet states of n,π*, π,π* and π,π*(CT) configuration, and with triplet state reduction potentials that vary between about –38 and –10 kcal mol ‐1 .…”

Photoinduced oxidation of sea salt halides by aromatic ketones: a source of halogenated radicals

“…The implications of the presence of such light-absorbing organics and the possible photochemical reactions are not yet well understood in the sea surface microlayer. But clearly, the presence of such photoactive compounds at the air/sea interfaces could give rise to photosensitized processes (Becker et al, 1995;Canonica et al, 2006Canonica et al, , 2000Canonica et al, , 2005Das et al, 1981;Ivanov et al, 2005;Lathioor and Leigh, 2001;Lathioor et al, 1999;Leigh et al, 1996) which may also affect chemistry in the marine boundary layer (MBL) (Reeser et al, 2008). Previously, we showed that chlorophyll may be a photochemical source of halogenated radicals or the driving force for ozone deposition at low wind speed (Clifford et al, 2008).…”

Laser flash photolysis study of the phenolic hydrogen abstraction by 1,2-aceanthrylenedione triplet

“…In the latter case the reaction occurs by a hydrogen atomtransfer mechanism, whereas, in the former, the mechanism passes through a hydrogen-bonded complex and involvement of the relatively basic * triplet state has been suggested. 9 In this work we report the results of a systematic study of the reactivity of 1,2-aceanthrylenedione (1) in the presence of substituted phenols employing the laser flash photolysis technique.…”