Changes in the photochemistry of tropone or 2-methoxytropone on complexation with acids

Abstract

On addition of either BF3.Et20 or H2S04 the valence photoisomerization of 2-methoxytropone (1) in CH3CN is diverted from 1 -methoxy-(2) to 3-methoxybicyclo[3.2.0] hepta-3,6-dien-2-one (3) as products whereas with tropone there is a change from photodimerization to valence isomerization to give cleanly bicyclo[3.2.0] hepta-3,6-dien-2-one (5).

New Syntheses and Chemistry of Hexafluorotropone

“…23 With boron trifluoride in acetonitrile, photocyclization to bicyclo[3.2.0]hepta-3,6-dien-2-one (17) occurs. 24 Hexachlorotropone (5) cyclizes analogously to 18 in acetonitrile with 254 nm light, and the reaction is reversible. 25 We have found that the fluorinated tropone 3 also gives the corresponding bicyclic ketone (19) reversibly when irradiated with a medium-pressure mercury arc.…”

“…UV irradiation in acetonitrile yields a mixture of [6 + 4], [6 + 2], and [4 + 2] dimers, and in dilute sulfuric acid a [6 + 6] dimer . With boron trifluoride in acetonitrile, photocyclization to bicyclo[3.2.0]hepta-3,6-dien-2-one ( 17 ) occurs . Hexachlorotropone ( 5 ) cyclizes analogously to 18 in acetonitrile with 254 nm light, and the reaction is reversible .…”

4‐π‐Photocyclization: Scope and Synthetic Applications

“…The photocyclization was very slow, with only around 33 % conversion of 9 to 12 after 1 week of irradiation, although the authors suggested that longer irradiation times should lead to higher conversions. Similarly, Cavazza, Zandomeneghi and Pietra reported that complexation of tropone to Lewis acids also allowed selective 4‐π‐photocyclization rather than dimerization . Thus, irradiation of BF 3 ·9 in acetonitrile for 30 minutes resulted in 80 % conversion, and the photocycloadduct 12 was isolated in 59 % yield (Scheme ) .…”

“…Similarly, Cavazza, Zandomeneghi and Pietra reported that complexation of tropone to Lewis acids also allowed selective 4-π-photocyclization rather than dimerization. [20] Thus, irradiation of BF 3 ·9 in acetonitrile for 30 minutes resulted in 80 % conversion, and the photocycloadduct 12 was isolated in 59 % yield (Scheme 6). [21] Similar results were also observed through adding one equivalent of BF 3 ·Et 2 O (or excess sulfuric acid) to a solution of tropone in acetonitrile, followed by irradiation under the above conditions.…”

Photochemical reactions of 2-bromotropone and 2,7-dibromotropone with 9,10-dicyanoanthracene

“…In an acidic medium, 3 gave a 3-substituted bicyclo[3.2.0]heptadienone (6) and 1 afforded bicyclo[3.2.0]heptadienone (7). 3,4 The reaction mode of 3 is dependent on the medium. Ab initio calculations on 1 showed that 1 has the n,π* lowest excited state in neutral conditions and the π,π* lowest excited state in an acidic medium.…”