Development of a flow photochemical process for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence: in situ-generated 2-benzopyrylium as photoredox catalyst and reactive intermediate.

A flow photochemical reaction system for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence was developed, which utilizes in situ-generated 2-benzopyrylium intermediates as the photoredox catalyst and electrophilic substrates. The key 2-benzopyrylium intermediates were generated in the flow reaction system through the intramolecular cyclization of -carbonyl alkynylbenzene derivatives by the π-Lewis acidic metal catalyst AgNTf and the subsequent proto-demetalation with trifluoroacetic acid. The 2-benzopyrylium intermediates underwent further photoreactions with benzyltrimethylsilane derivatives as the donor molecule in the flow photoreactor to provide 1-isochromene derivatives in higher yields in most cases than the batch reaction system.

Consecutive π-Lewis acidic metal-catalysed cyclisation/photochemical radical addition promoted by generated 2-benzopyrylium as the photoredox catalyst.

A π-Lewis acidic metal-catalysed cyclisation/photochemical radical addition sequence was developed, which utilises generated 2-benzopyrylium cation intermediates as photoredox catalysts and electrophilic substrates to form 1-isochromene derivatives in good yields in most cases. The key 2-benzopyrylium intermediates were generated through the activation of the alkyne moiety of -carbonyl alkynylbenzene derivatives by such π-Lewis acidic metal catalysts as AgNTf and Cu(NTf), and the subsequent intramolecular cyclisation and proto-demetalation using trifluoroacetic acid. Further photo-excitation of the 2-benzopyrylium intermediates facilitated single-electron transfer from a benzyltrimethylsilane derivative as a donor molecule to promote the radical addition of arylmethyl radicals to the 2-benzopyrylium intermediates.