Abstract:Photochemical decomposition of 2,6-diarylcyclohexanones 1 a -d yields 1,2-diarylcyclopentannes 4 and 1,5-diaryl-l -pentenes 5 by intramolecular reaction of the intermediate 1,5-diaryl-1,5-pentadiyls 3. The two stereoisomers cis4 and trans-4 are formed in equal amounts. There hence exists a 1 : 1 equilibrium between the two conformers of 3 which lead to cis-and trms-4, respectively; the intramolecular combination step itself is not stereoselective. However, the product ratios of 4:5, i. e. combination: dispropo… Show more
“…On the other hand triplet biradicals survive long enough for easy detection. Interestingly, the ratio of cis- and trans- 1,2-diphenylcyclopentanes ( 12 ) is essentially the same reported for the photolysis of 13 …”
Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the final products through typical free radical reactions. No cyclopentanes are formed under low intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable with nanosecond techniques.
“…On the other hand triplet biradicals survive long enough for easy detection. Interestingly, the ratio of cis- and trans- 1,2-diphenylcyclopentanes ( 12 ) is essentially the same reported for the photolysis of 13 …”
Low intensity (lamp) photolysis of 1,5-dichloro-1,5-diphenylpentane (1) leads to the formation of the 1-chloro-1,5-diphenylpentyl radical (7) through C-Cl bond cleavage. Radical 7 leads to the final products through typical free radical reactions. No cyclopentanes are formed under low intensity conditions. In contrast, high intensity laser irradiation leads to C-Cl photocleavage of radical 7 to yield the 1,5-diphenylpentanediyl biradical (11), which results in the formation of isomeric cis- and trans-1,2-diphenylcyclopentanes; the behavior of these biradicals agrees well with that observed when their precursor is 2,6-diphenylcyclohexanone. Two-color two-laser experiments suggest that both singlet and triplet biradicals are formed, even if only the latter are detectable with nanosecond techniques.
“…Another minor pathway is C5−C(Ar) coupling, with formation of 4a (equivalent to the photo-Claisen rearrangement of 1a ). This behavior is analogous to that of the 1,5-diphenyl-1,5-pentanediyl biradical, although in the all-carbon case, the extent of disproportionation is much higher, 6a,7a and no C5-aryl coupling with formation of a seven-membered ring is observed. In regard to the 1,4-derivatives, the behavior of the 1,4-diphenyl-1,4-butanediyl biradical is very similar to that of its 1-aza analogue; the predominating process is C2−C3 cleavage (detected through the formation of styrene), although C1−C4 (or N1−C4) coupling to give the four-membered rings also takes place to a significant extent.…”
Section: Discussionmentioning
confidence: 57%
“…Biradical intermediates play an important role in many thermal and photochemical processes. 1, n -Diaryl-1, n -alkanediyl biradicals (as for example, IIa and IIb in Chart ) have been proposed as intermediates in a wide range of reactions such as the geometric photoisomerization of cycloalkanes, the dimerization of styrenes, and the photoextrusion of CO 2 , N 2 , SO 2 , or CO in lactones, cyclic azoalkanes, sulfones, or ketones, ,7a,b respectively. They have also been generated via two-photon two-laser photolysis of 1, n -dichloro-1, n -diphenylalkanes. , 1 Structures of 1, n -Azabiradicals Ia and Ib and Their All-Carbon Analogues IIa and IIb …”
The 1,5-diphenyl-1,5-azapentanediyl biradical Ia was generated by photolysis of 1,2-diphenylazacyclopentane (pyrrolidine 1a). Among the reaction pathways followed by Ia, C-N bond reformation with ring closure was found to be the predominating process, as determined by separate irradiation of either of the pure enantiomers of 1a. Disproportionation was a minor process and took place only via H abstraction by the C5 benzylic radical. Another minor pathway was C5-aryl coupling, with formation of 5-phenyl-2,3,4,5-tetrahydro-1H-benzo[b]azepine (4a), which is equivalent to photo-Claisen rearrangement of 1a. Likewise, the 1,4-diphenyl-1,4-azabutanediyl biradical Ib was generated by photolysis of 1,2-diphenylazacyclobutane (azetidine 1b). This species underwent predominating C2-C3 cleavage, as indicated by the extensive styrene formation. Although N1-C4 bond reformation also took place, this is not the major pathway occurring from Ib. Besides, C4-aryl coupling to give 4-phenyl-1,2,3,4-tetrahydroquinoline (4b) was also observed. All the possible reaction pathways were theoretically studied at the UB3LYP/6-31G computational level; the results were found to be in good agreement with the experimental observations.
ChemInform Abstract The two stereoisomers cis-and trans-(VI) are formed in equal amounts. There hence exists a 1:1 equilibrium between the two conformers of diradical (V); the intramolecular combination step itself is not stereoselective. However, the product ratios of (VI):(VII), i.e. combination : disproportionation, depend on the substituents. This regioselectivity is strongly affected by temp. and solvent.
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