Halogen-Exchange Fluorination of β-Chlorovinyl Aldehydes - Unexpected Cascade Transformations in the Fluorination of 4-Chloro-2H-chromene and 4-Chloro-2H-thiochromene-3-carbaldehydes

Halogen-exchange (Halex) fluorinations in vinylic systems are rare; under controlled conditions, halex fluorination of vinylic aldehydes, 1-chloro-3H-benzo[f]chromene-2-carbaldehyde (8) and 4-chloro-2H-benzo[h]chromene-3-carbaldehyde (11) successfully furnished their respective fluoro exchanged derivatives 1-fluoro-3H-benzo[f]chromene-2-carbaldehyde 9 and 4-fluoro-2H-benzo[h]chromene-3-carbaldehyde 12. However, 4-chlorobenzopyran-3-carbadehydes (3) underwent a novel cascade reaction exclusively via the fluorine exchanged intermediate to afford 2,3-dihydro-3,3 '-methylenebischromones (4). Under similar conditions, in a somewhat different manner, the thio analogous system, 4-chlorobenzothiopyran-3-carbaldehydes (1) yielded 3,3 '-methylene bisthiochromen-4-ones (2) also in a domino fashion. Sterically hindered vinylic aldehyde, 4-chloro-2,2-dimethyl-benzopyran-3-carbaldehyde (18) exhibited a totally different behavior by affording a bicyclic benzopyran annulated bisacetal (19). The mechanism of these transformations has been investigated.