Piperylene, isomerizations

Very early in the study of photosensitization it was discovered that salicylaldehyde, 2,4-dihydroxybenzophenone, and 2-hydroxy-4-methoxy-benzophenone do not sensitize piperylene isomerization, indicating that enolization is much faster than quenching by the diene.37 Later it was shown that 2-hydroxybenzophenone would sensitize the dimerization of isoprene in concentrated solution, but that the reaction was much less efficient than when benzophenone was used as a sensitizer (Table I).36... 

The quantum yield for isomerization in the direct photolysis was found to be d> = 0.94. The reaction could also be sensitized with acetophenone ( = 1.02) and quenched with piperylene, indicating a reactive triplet species with a rate constant kr of 3 x 10l°sec-1. With a 3-(p-methoxyphenyl) derivative two products were obtained<81) ... 

That oxetane formation results from a singlet state reaction follows from the following evidence (a) Acetone fluorescence is quenched by addition of the olefin, (b) oxetane formation is relatively insensitive to piperylene, and (c) cis-trans isomerization of the olefin is quenched at high olefin concentrations but oxetane formation is not affected. Since oxetane formation was... 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

In the triplet counting technique the substance whose intersystem crossing yield is to be determined is used as a sensitizer for the photoisomerization of piperylene (1,3-pentadiene). Under conditions where essentially all of the sensitizer triplets are intercepted by diene, the quantum yield of isomerization is directly proportional to the intersystem crossing yield of the sensitizer. If the diene interferes with sensitizer singlets however, the yield of triplets will fall, resulting in an anomalously low value for 0,. 

Daubendiek et al.55 have extended this technique to demonstrate electronic energy transfer at the gas-solid interface. They irradiated photosensitizers, in polycrystalline form, in the presence of m-piperylene vapor and found that the extent of isomerization was comparable to values obtained for the same sensitizer in a solution of piperylene. Their results indicated an energy difference of about 1 kcal between triplet states of sensitizers in solution and on the surface of organic crystals. 

Carene has also been isomerized over an S-alumina catalyst to a 50 50 mixture of dipentene (15) and carvestrene (30). The crude mixture can be readily polymerized to a terpene resin or copolymerized with piperylene (63,64). 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

Optically active aryl sulfoxides undergo facile photoracemiza-tion.472 The photo-inversion is sensitized by naphthalene but only partially quenched by piperylene.473 The quenching action was originally interpreted as indicating a triplet mechanism for isomerization. 

Formation of the DVCB derivatives is complicated by the nickel-catalyzed isomerization of m-piperylene into traw.v-piperylene which proceeds through the formation of the cyclobutane (LIII). The rate of isomerization is dependent on the nature of the ligand attached to the metal, and increases in the series tri(o-phenylphenyl)phosphite < triphenyl-phosphine < tricyclohexylphosphine. In the case of the nickel-tricyclo-hexylphosphine catalyst, the rate of isomerization is faster than the cyclization reaction. 

Another factor must be considered. With increasing conversion, the rate of the nickel-catalyzed cleavage of methyl-substituted DVCB to piperyl-ene [Eq. (56)] increases. This reverse reaction is dependent on the ligand in the same sense as the isomerization of cw-piperylene. If the diene is removed as soon as it is formed (by working in a partial vacuum), no... 

Ethoxyisoindolenone (50) sensitized cis-trans isomerization of c/s-piperylene has been used in a triplet counting experiment to measure the intersystem crossing efficiency. A linear triplet counting plot is obtained which gives an intersystem crossing efficiency of 0.96 0.01 and a triplet lifetime of 1.5 x 10 7 sec. for 50 in methylene chloride solvent at ambient temperature in the absence of olefin. 

We reiterate our belief that there are still other classes of radical reactions which may produce electron polarization and which are awaiting to be discovered. For example, a possible mechanism that would produce polarized radicals is one in which the three triplet sublevels of a photoexcited molecule react at different rates. This concept was suggested by El-Sayed (51,53) in connection with some solid-state photochemical systems. Also, the observations reported by Gupta and Hammond (66) on the small change in the initial quantum yields in the benzophenone-sensi-tized isomerization of stibenes and piperylene in the presence of an external magnetic field have not been completely explained (14). 

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 (4>,, = 0.09, = 0.01) and very small quantities of di-... 

Data on the photochemical stability and stabilizing action of HB s, benzotriazoles, and related compounds have been reported on several occasions (6,9,70,71,7,82,87). In order to be a good stabilizer, it is not sufficient that a compound does not decompose if dissolved for example, in a plastic, it is equally important that it does not act as a sensitizer. Investigations on the sensitizing action of HB s compared to that of benzophenone and nonchelated derivatives are, therefore, of interest. Such studies are Included by Lamola (8), A. Heller (80), and Hammond (74). Cls-trans isomerization of piperylene (1,3-pentadiene) in benzene is easily sensitized by benzophenone via triplet transfer... 

The photochemistry of o-hydroxy-aryl ketones and aldehydes has been extensively studied by several authors. It has been found that salicyl-aldehyde, 2-hydroxybenzophenone and 2,4-dihydroxybenzophenone are different from the unsubstituted compounds and from the p-hydroxy or o-methoxy derivatives in that they are completely inactive as sensitizers for the isomerization of piperylene [130]. Moreover, 2,4-dihydroxybenzophenone gives no luminescence in a rigid hydrocarbon glass at 77°K, whereas 4-hydroxybenzophenone exhibits an intense phosphorescence in... 

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