Quenching by 1,3-pentadiene

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

Turro and McDaniel(as) have shown that this reaction probably occurs from the excited singlet state (reaction could not be quenched by 1,3-pentadiene) and is stereospecific,... 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

However, those with PEG for the reaction of 5a were very similar to crown ether. Photolyses of 5c with PEG were fairly superior to those with crown ether and could be used for the preparation of 3c. Both the conversion of 5 and formation of 3 were quenched by 1,3-pentadiene l mol for 5a and 6 for 5c 6 for 3a and... 

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. 

An expeditious route to amidoboratabenzenes and alkylboratabenzenes has been developed that starts with commercially available 1,3-pentadiene (Scheme 9).18 Deprotonation of the diene, followed by quenching with BCl(NMe2)2, affords a 2,4-pentadienylborane. Treatment with LiNEt2 then leads to ring closure, thereby generating the amidoboratabenzene. 

The rearrangement of /1-diketones is most simply explained assuming the involvement of an n,n excited reactive state. Its triplet configuration was demonstrated by the quenching effect of 1,3-pentadiene on the rearrangement 176 - 175.109 The nature of the excited state of enol lactone is not... 

The dienes, specially 1,3-pentadiene (piperylene) and hexadiene, quench the triplets of suitable sensitizers by energy transfer with unit efficiency. Hence, they are used widely in mechanistic studies of photochemical reactions, either to count the triplets or to establish the triplet energy of a sensitizer whose Et is not determinable from spectroscopic data (chemical spectroscopy). 

The formation of 1,2-dimethylcyclobutene (Formula 385) in the vapor phase irradiation of 2,3-dimethyl-l,3-butadiene (Formula 384) is not quenched by oxygen or nitric oxide (169). Addition of inert vapor (diethyl ether) increased the quantum efficiency in this reaction (169). The inert vapor presumably removes excess vibrational energy from the product cyclobutene thus stabilizing the product (169). Rate studies on the cis- and Jrans-isomers of 1,3-pentadiene in solution indicate that the iraras-isomer is the only source of 3-methylcyclobutene (169). The photoisomerization to 3-methylcyclobutene is faster than photoisomerization of trans- to m-l,3-pentadiene (169). 

The products formed upon direct irradiation of 1 include 9-phenanthrol and fluorene. The former is believed to be formed in a triplet process. This is supported by the capacity to quench phenanthrol formation with tram-1,3-pentadiene and to circumvent the rearrangement to 292 (R = H) by utilization of sensitizers such as benzophenone or triphenylene. In an attempt to confirm that the chemically significant excited state in the conversion of 1 to 292... 

Triplet-sensitized cis-trans isomerization is frequently more efficient. Thus, the direct excitation of 1,3-pentadiene (piperylene), which has been studied in great detail, results in cis-trans isomerization with very low quantum yields (< >,, = 0.09, <, <. = 0.01) and very small quantities of di-methylcyclopropene as a side product, whereas when benzophenone is used as sensitizer, 4><, = 0.55 and = 0.44. This efficient quenching of higher-energy triplet states by conjugated dienes is utilized in mechanistic studies for identifying the excited state responsible for a photochemical reaction. 

Both the conversion of 5 and formation of 3 were neither quenched with 1,3-pen tad iene ( 50 mM) nor sensitized with acetone (" 50 mM) whose E.J1 ( 78 kcal/mol)l is high enough for sensitizing 5 to their triplet state. Fluorescence of 5 was not affected by either 1,3-pentadiene or acetone. These data suggest that the cyanations proceed through comparatively shorter-lived intermediates (probably Si state). 

Quenching with l,3 Pentadiene, Solutions sampled as above with a solution of certain amounts of 1,3-pentadiene (distilled directly before use bp 42-44 C) were irradiated and the resulted mixtures were analysed by GLC, Formation of 3 was quenched when 2 was absent, but was not quenched when 2 was present. 

An argument in favor of the intermediacy of triplet states in at least some of these reactions < an be based on the observation that cycloadditions do not appear to occur when vertical excitation transfer from the triplet state of the ketone to the olefin is energetically feasible. It has been found, for example, that ketones with triplet excitation energies sufficiently greater than those of the stilbenes or the 1,3-pentadienes, among several systems, are quenched with unit efficiency by the olefins and that no adducts are formed (Hammond el at., 1964a). The occurrence of Reaction (64) implies that cycloaddition can compete with nonvertical triplet excitation transfer. 

The evidence favoring the intermediacy of triplet states in the photoconversion of santonin (XXV) to lumisantonin (XXVI) is twofold. First, the rearrangement can be induced by means of triplet excitation transfer from benzophcnone to (XXV), and second, the photoconversion is essentially completely quenched when santonin is irradiated in 1,3-pentadiene. If the quenching process can be attributed to transfer of electronic excitation, then a santonin triplet state is required since singlet excitation transfer from santonin to 1,3-pentadiene is not energetically feasible. 

Strong quenching effects were observed by 02,1,3-pentadiene, tri-t-butyl phenol and tetramethylethylene (in increasing order of effectiveness). As DP A and 1,3-diphenyl isobenzofuran act as fluorescers, but not 9,10-dibromoan-thracene, the active species appears to be in the singlet state, although quantitative results have not been reported. It was concluded from the kinetic and spectroscopic data that dioxetanedione as well as the 1,4-diradical are present not only in the... 

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