Ketone Triplets

The rate constants of 1-AdS with EtsSiH and f-BuMePhSiH are relatively fast taking into consideration the endothermicity of these reactions. The sila-nethiyl radicals Ph3SiS and t-BuMePhSiS abstract hydrogen with similar rate constants, whereas the more hindered (t-BuO)3SiS and i-Pr3SiS radicals show an enhancement in their reactivity, which suggests that entropic effects may play an important role. 

The excited states of carbonyl compounds are often considered to be similar to alkoxyl radicals because of the unpaired electron on the oxygen atom. In particular, the benzophenone n—tt triplet mostly reacts in the same manner as and at a similar rate to t-BuO radicals. 

Absolute rate constants for the reaction of the benzophenone triplet with Et3SiH,n-C5HiiSiH3,PhSiH3, and ClsSiH have been measured by laser flash 

Excited triplets of other ketones have been studied to a lesser extent [23,35]. The xanthate and the / -methoxyacetophenone triplets were found to be more (6.0 X 10 s ) and less ( 8x10 s ) reactive, respectively, than the 

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Whereas bromine radicals (133) and succinimidyl radicals (134) react by the Sh2 mechanism at the tin center in tetraalkyltins, but not in alkyltin halides, alkoxyl radicals (135) and ketone triplets (136) react with alkyltin halides, but not tetraalkyltins this may reflect the conflicting, electronic demands of the radical reagents which, as electrophilic species, should be more reactive towards tetraalkyltins than alkyltin halides, but which would also tend to make use of a 5d orbital... 

The relative reactivities of alkyltin compounds towards tert-butoxyl radicals, ketone triplets, and succinimidyl radicals are dominated by the steric effect of the alkyl ligands (R" > R"), but that towards bromine atoms follows the reverse sequence (R" < R ). 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

Turro69 has reported that several alkyl ketones react with 1,2-dicyano-ethylene (12) by nucleophilic attack of the ketone (rr, n) singlet state to give oxetanes stereospecifically. Addition of diene quenchers which are known to deactivate ketone triplets do not affect the rate of oxetane formation. This... 

Piperylene has been used quite successfully in quantitative studies of the lifetimes of various ketone triplets using the Stern-Volmer technique. However, the assumption that piperylene cannot interfere in any other way is risky. Not only does piperylene react with excited... 

Reactivity in the elimination reaction is probably largely determined by the nature of the substituents (in addition to hydrogen) attached to the y-carbon atom. If the reaction is stepwise, one would expect faster reaction with either 2-hexanone or methoxyacetone than with 2-pentanone, because the biradical intermediates would be more stable in the former two cases. Such a conclusion follows from the similarity shown by various ketone triplets in intermolecular reactions333 to the known selectivity of attack by /-butoxy radicals in both inter-353 and intramolecular reactions.354... 

Since intramolecular hydrogen abstraction could not compete with decay of the two ketones having it,it lowest triplets, the reaction must have a rate constant of < 103 sec-1, the rate of their decay in solution.155,337 Consequently a factor of at least 10 exists between the reactivities of pure n,ir and -it,it ketone triplets in this particular reaction. Since the phenyl ketone triplets are about one-tenth as reactive as the triplets of aliphatic ketones, we have suggested that... 

Another physical study which used flash photolysis relates directly on the MB/amine system. Kayser and Young (36) examined a more extensive series of amines, both aromatic and aliphatic, than Steiner (33). Their results are shown in Table 3. Excellent correlation was obtained between the amine ionization potential and the rate constant for MB quenching the slope of the logarithmic plot was -1.75 eV-1- This value is relatively small compared to some oxidizing excited states (e.g. hydrocarbons, -17 eV l (37)), but it is similar to the value observed for ketone triplet quenching by amines (-1.5 eV l (38)), and does indicate that the quenching interaction becomes more facile as... 

Due to the low rate constants for y-hydrogen abstraction ( 1% of those for analogous phenyl ketone triplets), more remote hydrogen abstractions and cycloadditions to remote double bonds commonly compete and sometimes are the only reactions of pyruvates. 

Introduction of a hydroxy group at the 7-yyft-position of norbornene (138c) re-inverts the diastereoselectivity in this case the exo-adduct 139c is formed with d.e. >97%. The later effect could be due to hydrogen bonding which precomplexes the excited carbonyl species. A similar photoreaction has been reported for norbornadiene [141]. As was shown by Gorman et al., ketone triplets do not add to but interconvert norbornadiene it to... 

It has been thought for some time that the interactions of triplet ketones with olefins involve a competition between hydrogen abstraction from, energy transfer to, and cycloaddition to the double bond 126>. Cycloaddition has generally been considered to proceed via a diradical intermediate. At first, the only evidence for a diradical was that the regioselectivity of oxetane formation often — but not always — is such as would be expected from the relative stabilities of the possible diradical intermediates 126>. Moreover, n,n ketone triplets are known to act like alkoxy radicals, which add to olefins, albeit less efficiently than do most other... 

The rates at which several substituted benzenes quench triplet benzophenone have been measured 17 8). No single linear free energy relationship can be derived. For alkoxybenzenes, alkylbenzenes, benzene itself, and benzotrifluoride as quenchers, one finds a linear plot of log vs. IP with a slope similar to that found for the plot of all substituted benzenes and triplet a-trifluoroacetophenone. A given aromatic such as benzene quenches the fluorinated ketone triplet, which has an E - -E(A /A) value of only 16 kcal, some 50 times faster than it quenches triplet acetophenone or benzophenone 132>. This rate difference reflects only 20% of the full 12 kcal difference in thermodynamic redox potentials. However, the halobenzenes and benzonitrile quench triplet benzophenone faster than does benzene 178>. It seems likely that with these electron poor benzene derivatives, some alternate chemical reaction becomes dominant. Although a reverse CT process has been suggested, with the triplet ketone as donor, it is perhaps more likely that some sort of radical addition occurs with conjugating substituents on... 

Table Z Data of the photopolymerization of methyl methacrylate with ketone-amine initiator systems (IE = ionisation potential, 3k H = quenching constant of ketone triplet state with amine in benzene, AV/At = dilatometric contraction) [1]...

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