To elucidate the reaction mechanism, radical-trapping experiments were performed (
Scheme 5A). When 3.0 equiv. of radical scavengers, such as TEMPO, BHT, or 1,1-diphenylethylene (DPE) was added to the reaction, the product yields of
3a and
4a were decreased. The corresponding radical-quenched products were also detected using electrospray ionisation-time-of-flight-mass spectrometry (ESI-TOF-MS) (see Supporting information for details), suggesting that the reaction likely involves a radical mechanism. Furthermore, product
5 was isolated in the copper-catalysed spirolactonisation of 2-benzylideneindane-1,3-diones with the addition of TEMPO (
Scheme 5A, a(i)). Based on the aforementioned mechanistic studies [
53] and experimental results, a postulated mechanism is illustrated in
Scheme 5B. Initially, radical intermediate
A is generated through the Cu(I)/TBHP oxidation system (
Scheme 5B, for
Scheme 2 pathway). Alternatively, the reaction is suggested to begin with the decomposition of H
2O
2 catalysed by TBAI to generate hydroxyl radical and I
2, which capture an α-hydrogen atom from Meldrum's acid
2 to form radical intermediate
A (
Scheme 5B, for
Scheme 3 pathway). Species
A then generates radical
B via a C=C bond addition step. The resulting
B reacts with copper(II) species, followed by reductive elimination to afford product
5 [
54]. Subsequently, radical
B undergoes a radical addition to the C=O bond to give the geometry-favored five-membered ring radical
C. The radical
C is likely thermally-unstable and can be easily converted to radical
D with an expulsion of acetone. With the help of
t-BuO
·, the radical trapping reaction occurs with
D, resulting in the formation of product
3a. Otherwise, in the absence of trapping species
t-BuO
· (
i.e.,
Scheme 3 pathway), the intermediate
D thus undergoes a decarbonylation pathway and subsequent proton abstraction to give product
4a. It is of note that all the proposed intermediates
A, B, C, D and
E were successfully captured by radical scavengers, and the resulting intermediates were detected by ESI-TOF-MS (see Supporting information for details).