Photoisomerization of allylic

In addition, a 532 (visible) or 355 (UV region) nm laser-induced photoisomerization of allylic alcohols to aldehydes catalyzed by [Fe3(CO)i2] or [Fe(CO)4PPh3] was developed by Fan [176]. In this reaction, key intermediates such as the 7i-allyl hydride species [FeH(CO)3(q -C3H3ROH)] (R = H, Me) were detected by pulsed laser FTIR absorption spectroscopy. These results strongly support the 7i-allyl mechanism of photoisomerization of allyl alcohols. 

In the triarylallyl carbanion study and in the absence of accepting stilbene, EjZ photoisomerization of the irradiated carbanion was observed and the effect of changing the substituent at position 2 was examined. Starting from the zero coefficient at carbon-2 in the non bonding MO of the allyllic system, Tolbert considers that the substituent at the 2 position does not affect the energy of the MO. If an electron transfer mechanism governs the reactivity, the substituent on this position will not modify the process in a significant way. The experimental results were very dependent on the C-2 substitution and this reaction was therefore considered as relevant of the intrinsic photochemistry of the anion [145]. This view has been confirmed in other studies on 1,3-diphenylallyl carbanion the kinetic parameters of the photoisomerizations were found to be inconsistent with an electron transfer mechanism [146, 147]. 

Photoisomerization of dimethyl 4,4,6,6-tetramethyl-2,3-diazabicyclo[3.1.0]hex-2-ene-l,5-dicarboxylate generated an allyl-substituted a-diazo ester that on prolonged irradiation yielded... 

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

This weakening of the bond between the allylic carbon and the neighboring atom can lead to bond rupture as illustrated in the photoisomerization of 2H,6//-thiin-3-one-l-oxides to 3//,7H-l,2-Oxathiepin-4-ones [14], These seven-membered ring systems undergo further thermal ring contraction to thietanones via a 2,3-sigmatropic rearrangement followed by a Pummerer reaction (Scheme 10). 

Both aikenes and alkynes quench the excited state Pt2(pop)4. Aikenes such as trans-stilbcnc that have triplet energies of less than -57.7 kcal/mol quench the A2u State of Pt2(pop)4 by energy transfer. This quenching reaction is followed by emission from the excited state of stilbene, and also by photoisomerization of the stilbene. For cycloalkenes such as cyclohexene and cyclopentene that contain allylic C-H bonds, quenching reactions with Pt2(pop) proceed by hydrogen-atom abstraction ... 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

Photoisomerization about the C=N bond of iminium salts bearing a substituted styryl group (130) has been investigated. Allyl derivatives have previously been studied in a like manner. Detailed investigations of the photoreactions of the protonated ketone (131a) and acid (131b) have been ctirried... 

The ions (216) can be prepared by protonation of the corresponding 2,3-homotropones in fluorosulphonicacid. Irradiation (A > 360 nm, — 70°C)ofthe ions leads to their isomerization. The selectivity shown in the photoisomerizations is attributed to the circumambulatory migration of the C-8 group via intermediates such as (217—219). Photoisomerization about the C-1—O and about the C-2—C-3 bond of the allyl cations (220—222) has been reported cf. the earlier preliminary report. Childs has reviewed the photochemical reactions of protonated unsaturated compounds. 

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