Butyrophenone, triplet reactions

The only type of triplet reaction for which rate constants have been measured for various kinds of ketones is type II photoelimination. The rate constants for 2-pentanone and 2-hexanone have been found to be 2 x I08 and 1 x 10 sec-1, respectively.215 The corresponding rates for triplet butyrophenone and valerophenone are 8 x 10 and 1.4 x 10s sec-1, appreciably slower. Neither p-phenylbutyrophenone366 nor a-valeronaphthone273 undergo photoelimination in a readily measurable quantum yield. These results can be rationalized by a reasonable set of hypotheses such as follow. 

It cannot be overemphasized that the quantum yields for two-step triplet reactions such as those initiated by hydrogen abstraction often reflect partitioning of the intermediate biradicals more than relative rate constants for triplet reaction. This is especially true for the very rapid intramolecular hydrogen abstractions of n,7t triplets, such as recorded in Tables 52.1 and 52.3. This fact was first demonstrated by the behavior of 2-hexanone, in which y-deuteration increases the quantum yield of type II cleavage and of y-substituted butyrophenones, in which quantum yields for some type II cleavages decrease as rate constants for y-hydrogen abstraction increase, as shown in Table 52.3. ... 

The first reported application of the technique by Kessler and Wilkinson [12] in 1981, deals with various aromatic hydrocarbons chemisorbed on Y-alumina. The samples were adsorbed at less than monolayer levels, and the transient spectra observed were shown to be due to triplet-triplet absorptions. Figure 6 shows the time resolved triplet-triplet absorption spectra obtained from 3% coverage of acridine on powdered silica [14] demonstrating the sensitivity of the method and its ability to probe acid-base reactions at catalytic surfaces. Turro and his coworkers [25,26] have studied various molecules on silica surfaces. The lifetimes of the triplet states of valerophenone and diphenyl-butyrophenone adsorbed on powdered silica were determined to be 0.3 and 0.9 [is, respectively [25] which is at least two orders of magnitude greater than in homogeneous solution. Turro et al [26] also demonstrated triplet energy transfer from benzophenone to naphthalene on silica surfaces via static and dynamic pathways. 

In these systems the stabilizer absorbing the UV light is excited to triplet state Ti (tt-t ) which either produces a photochemical rearrangement or becomes relatively inert in respect to photochemical reactions. The efficient compounds for this system can be phenol ester of benzoic acid, salicylates, butyrophenones and their substitution compounds. 

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