Hexadienes from photolysis

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

In some cases the nature of the products resulting from photolysis of dienes and trienes depends on whether the reaction occurs from the excited singlet state or the triplet state. The triplet states can be populated exclusively by suitable sensitizers. For example, 3-methylene-1, 5-hexadiene gives a cyclobutene derivative (A) from the singlet state and tricyclo ring compound (B) from triplet state ... 

The reductive dehalogenation of aryl chlorides in the presence of electron transfer sensitizers is thought to proceed through radical ion pairs. For example, the formation of mono-chloroanthracene from the photolysis of 9,10-dichloroanthracene in the presence of 2,5-dimethyl-2,4-hexadiene, eq. 59 (183),... 

Proof that fulvene is the absorbing volatile product of benzene photolysis has been given by Ward et and by Kaplan and Wilzbach , who studied the photolysis at 1600-2000 and at 1849 A respectively. The product has been identified as fulvene on the basis of ultraviolet, infrared, nmr and mass spectra. Kaplan and Wilzbach ° point out, however, that although fulvene is the major product there is an additional absorption which they suggest may come from l,3-hexadiene-5-yne they conclude that no benzvalene is produced. 

Pentacarbonyliron catalyzes the [4 -F 1]-cycloaddition of CO to allenyl ketones and allenyl aldehydes in high yields and with good stereoselectivity. This reaction was examined in 2-methyltetrahydrofuran (2-MeTHF) matrices where the substituted complex Fe(CO)4(2-MeTHF) was found to be produced upon photolysis of Fe(CO)s at 190-230 K. When Fe(CO)s was photolyzed at 230 K in 2-MeTHF in the presence of 5-methyl-3,4-hexadien-2-one and CO, the corresponding a-alkylidene butenolide was obtained (Scheme 4). The reaction proceeded via a dissociative process from Fe(CO)4(2-MeTHF). ... 

In some case, the photolysis products of dienes and trienes depends on the excited state in which photochemical cyclisation occurs (singlet or triplet). To populate triplet state, suitable triplet sensitizer are used if required. The. 3-Methylene-l, 5-hexadiene gives a cyclobutadiene derivative through singlet excited state and tricyclic ring compound from triplet excited state. This difference of photochemical process is because of life-time of the excited states. Higher the life-time of reacting intermediate more will be possibility to form more stable radicals. 

UV irradiation of [Fe3(]i-H)2(p3-PBu )(CO)9] in the presence of 1,3-cyc-lohexadiene afforded four products. The main product was identified as [Fe3(p3-PBA)(CO)g(ri -l,3-cyclohexadiene)] (45), obtained in around 20% yield, along with minor amounts of [Fe3(p3-PBA)(CO)g(2,5-q-2,4-hexadiene)] (46) and [Fe3(ii3-PBA)(CO)g(l,4-q-l,3-hexadiene)] (47). A fourth complex, proposed to be [Fe3(ii-H)2( i3-PBA)(CO)7(ii-l,3-ri-4,5-hexadienyl)] (48) based on spectroscopic data, was obtained in only 2% yield. Cluster 45 could be obtained in a slightly lower yield from thermolysis of [Fe3(p3-PBA)(CO)io] with the diene. Continued photolysis of 46 and 47 resulted in complete conversion of these clusters to 48, demonstrating the possibility of C-H bond activation using first row transition metals. UV irradiation of 45 in benzene or heptane afforded [Fe3(p3-PBU)( i3-CO)(CO)6(ri-C6H6)] (49), containing apically-coordinated qC enzene, albeit in low yield. ... 

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