Visible Light Promoted Brominative Dearomatization of Biaryl Ynones to Spirocycles

Abstract

Bromine induced spiro cyclization of biaryl ynones facilitated the synthesis of spiro [5,5]trienones suitable for extended functionality at the C(3′) position. Herein, a step-economic photo-oxidative brominative carbannulation of biaryl ynones employing ammonium bromide and riboflavin tetraacetate (RFTA) has been developed. The reactivity between distal phenyl C−H activated orthoannulation and dearomative ipso-annulation is well exemplified. The eminent features of the methodology include metal-free, external additive free, low-loading photocatalyst (0.1 mol %), and use of a simple precursor.

Visible‐light Catalysed Trifluoromethylthiolation and Related Dearomative Spirocyclizations

“…We then hypothesized that introducing photocatalyst in the process may trigger the transformation. [16] Thus, we screened several photocatalysts with varying redox and photochemical characteristics. With Riboflavin tetraacetate (RFTA) [E 1/2 (P*/ PC *À ) = + 1.67 V], the expected product was obtained in 92% yield in 9 h (Table 2, entry 1-3, details in SI).…”

“…In this context, with our continued interest in dearomative spirocyclizations, [15] our group has recently achieved an efficient CÀ Br bond forming photo-redox cascade cyclisation of biaryl ynones. [16] The unexpected discovery of 3'-bromo-2'-aryl-4'H-spiro[cyclohexa [2,5]diene-1,1'-naphthlene]-4,4'-dione in 38% yield without any photocatalyst has drawn our attention to explore the fact in mechanistic point of view. Thus, we explored the present methodology by examining the reaction of biaryl ynone 1 a with several thiyl precursors.…”