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‐3 H ‐benzo[ f ]chromene‐2‐carbaldehyde ( 8 ) and 4‐chloro‐2 H ‐benzo[ h ]chromene‐3‐carbaldehyde ( 11 ) successfully furnished their respective fluoro exchanged derivatives 1‐fluoro‐3 H ‐benzo[ f ]chromene‐2‐carbaldehyde 9 and 4‐fluoro‐2 H ‐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.