Visible-Light-Promoted Tandem Skeletal Rearrangement/Dearomatization of Heteroaryl Enallenes

Abstract

Access to complex three-dimensional molecular architectures via dearomatization of ubiquitous aryl rings is a powerful synthetic tool, which faces, however, an inherent challenge to overcome energetic costs due to the loss of aromatic stabilization energy. Photochemical methods that allow one to populate high-energy states can thus be an ideal strategy to accomplish otherwise prohibitive reaction pathways. We present an original dearomative rearrangement of heteroaryl acryloylallenamides that leads to complex fused tricycles. The visible-light-promoted method occurs under mild conditions and tolerates a variety of functional groups. According to DFT modeling used to rationalize the outcome of the cascade, the reaction involves a sequential [2+2] allene-alkene photocycloaddition, which is followed by a selective retro- [2+2] step that paves the way for the dearomatization of the heteroaryl partner. This scenario is original with respect to the reported photochemical reactivity of similar substrates and thus holds promise for ample future developments.

Keywords: allenes; cycloadditions; dearomatization; energy transfer; visible light.

Scheme 1

(a) Regiochemical issues in allene–alkene [2+2] photocycloadditions [20,21]; (b) literature precedents for tandem [2+2]/retro- [2+2] cascades involving enynes [22]; (c) our previous work on aryl-substituted acryloylallenamides [23]; (d) complementary reactivity with heteroaryl-substituted derivatives; PC = photocatalyst.

Figure 1

Schematic presentation for the synthesis of substrates.

Figure 2

Scope of the cascade and isolated yields.

Figure 3

Proposed mechanistic rationale, DFT modeling results, and selected spin densities/molecular orbitals of key intermediates.