Photooxygenation of allylic alcohols: kinetic comparison of unfunctionalized alkenes with prenol-type allylic alcohols, ethers and acetates

Photooxygenation in Polystyrene Beads with Covalently and Non‐Covalently Bound Tetraarylporphyrin Sensitizers

“…The photooxygenation of 9a ± c (Table 1) yielded the syn-hydroxy allylic hydroperoxide as the major diastereoisomers referring to the hydroxy-directing effect of 1 O 2 in the conformationally fixed (by 1,3 A-strain) substrates. [35] Both the polymer-bound as well the free sensitizer systems gave similar diastereoselectivity (with the polymer-bound sensitizer somewhat higher), however considerably lower than in nonpolar solvents (photooxygenation of 9a in CCl 4 proceeds with a diastereoselectivity of 93%). Intermolecular hydrogen-bonding between the highly concentrated substrates molecules in both microcontainer systems accounts for the decrease in diastereoselectivity; this assumption was further supported by the fact that photooxygenation of 9a in a rose bengal/cellulose acetate film (showing additional intermolecular hydrogen-bonding between the matrix and the substrates) results in even lower diastereoselectivity (dr: syn:anti 70 : 30) in comparison with the photooxygenation carried out either in the sensitizer-bound or free sensitizer PS-DVB matrices.…”

“…We investigated several additional examples for the ene-reaction and [4 2]-cycloaddition and found that in many cases the reactivity and selectivity behavior are identical to those of solution photochemistry: the highly reactive a-pinene 3 gave the allylic hydroperoxide 4 in excellent yields and the singlet oxygen [4 2]-cycloaddition with sorbinol (5) [34] proceeded with high yields and negligible substrate loss in the polystyrene matrix. The regiochemistry of the singlet oxygen addition to mesitylol (9a) was unaffected by the polymer support, however, the diastereoselectivity of this hydroxy-directed ene reaction [35] showed to be strongly dependent on the environment: the syn:anti-ratio changed from 1.5 : 1 in ethanol, 3 : 1 in polystyrene to 10 : 1 in tetrachloromethane, indicating strong intermolecular hydrogen-bonding between substrate (9a) molecules in the unpolar matrix. The chiral diene alcohol 11 gave the endoperoxides synand anti-11 [36] in acceptable yields but low diastereoselectivity.…”

Photooxygenation Catalysis with a Polyol‐Decorated Disc‐Shaped Porphyrin Sensitizer: Shell‐Recognition Effects

“…[19] In the solution phase (benzene), photooxygenation of the polarity sensor 2,4-dimethylbut-2-en-4-ol (5, mesitylol) gives a 2:1 dr with the syn-diastereoisomer syn-6 favored due to hydrogen-bonding interaction with the reacting singlet oxygen molecule (Scheme 2). [20] Not much changed when this sensitizer was bound in a polystyrene matrix (Table 1 and Table 2). Subsequently, the porphyrine 1 was treated with five equivalents of glycidol in the presence of triethylamine.…”

Tetraphenylporphyrin‐Catalyzed Tandem Photooxygenation of Polyenes and 1,4‐Dienes: Multiple and Diverse Oxyfunctionalizations

“…In order to improve ene reactivity, alkylation of the respective double bond increases the rate constant by a factor of 10 and the cis-effect additionally by a factor of 3-4. [13,14] The gem-effect describes the preferential abstraction of allylic hydrogens from geminal positions with respect to acceptor groups. [15] We have recently demonstrated that the latter effect can also direct into vinylogous positions which allows the regioselective activation of allylic hydrogens from remote positions.…”