Ketone excited state basicity

Kasha s rule, 253, 310 Kelene, 380. 386 acetal. 326-27, 469 Ketoiminoelher, 411 Ketone. See also Carbonyl compounds aldol reaction. 427 excited state basicity, 52 Keiyl radical. 397-98. 467 Kinetics of photophysical processes, 250. 297-301... 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

While phenols and amines are usually more acidic in the singlet excited state than in the ground state, some substances, e.g., aromatic ketones, may become more basic in the photoexcited state. 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

Neutral carbon of organic compounds is inherently a very weak base due to the lack of nonbonding electrons. However, the 7r-system of alkenes, alkynes, and allenes are potential basic sites due to the availability of 7t-electrons. Wooldridge and Roberts [61] first reported the facile photohydration of arylacetylenes to the corresponding ketones (in an overall Markownikoff sense) in dilute aqueous acid solutions. The facile protonation of the excited state was attributed to the enhanced basicity of the alkyne moiety in the excited state, more specifically as a result of extensive charge migration from the aryl group to the acetylene moiety. The photoaddition of methanol [62] and acetic acid to 7c-electron systems such as alkenes, alkynes, and allenes may be rationalized in the same way. 

In molecular systems where the phenol or hydroxyarene moiety is in close proximity to a functional group (typically, aromatic ketone or the nitrogen of a heterocycle) that experiences an enhancement in basicity on excitation to the well-studied phenomenon of excited-state intramolecular proton transfer (ESIPT) is observed. The vast majority of these ESIPT is reversible, some with exceptionally fast rates ... 

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