The synthesis and photochromism of a 2,2-diaryl-6-styryl-2H-[1]benzopyran: Unexpected palladium-mediated ring-contraction of a 6-bromo-2,2-diaryl-2H-[1]benzopyran

The ligand-free Pd-mediated reaction between styrene and a 6-bromo-2,2-diaryl-2H-[1]benzopyran proceeded anomalously to give a 2-(diarylmethyl)-5-styrylbenzofuran via a tandem Heck coupling – ring-contraction process; none of the styryl substituted 2,2-diaryl-2H-[1]benzopyran was observed. A 2,2-diaryl-6-styryl-2H-[1]benzopyran resulted from the condensation between 4-hydroxystilbene and a 1,1-diarylpropynol and which exhibited photochromism through the reversible electrocyclisation of the pyran unit; no isomerisation – electrocyclisation of the stilbene moiety was detected.

► Tandem palladium-catalysed, ligand-free Heck reaction — ring-contraction.
► Synthesis of photochromic 2,2-diaryl-2H-[1]benzopyrans.
► 2,2-Diaryl-6-styryl-2H-[1]benzopyrans display a relatively weak photochromic response.
► X-Ray crystal structure of a 5-styrylbenzofuran.