Visible-Light Induced Direct Synthesis of Polysubstituted Furans from Cyclopropyl Ketones

Abstract

In this article, a photoredox protocol for the synthesis of furans via oxidative coupling of olefin generated in situ from cyclopropyl ketones with ketonic oxygen atom is presented. Moreover, bromination of furans in the presence of overstoichiometric oxidant has been achieved with high regioselectivity.

Ligand-Metal Cooperation Enables C–C Activation Cross-Coupling Reactivity of Cyclopropyl Ketones

“…17 Stronger evidence for the absence of a radical intermediate can be found in the regioselectivity observed for 1,2-substituted cyclopropanes (3ai-3ak in Figure 5); the regiochemistry is the opposite observed in known metalloradical and free-radical reactions. [12][13][14][15][16][17][18][19][20][21][22] This coordination/rearrangement sequence also helps explain the observed Z-enol-ether geometry in the product (Figure S23). Moreover, the only example of a nickel-catalyzed ring-opening difunctionalization of 1a with a trimethylaluminum nucleophile afforded the (E)-TMS-enol-ether in >95:5 selectivity, consistent with the different mechanisms proposed for the C-C activation steps.…”

“…Redox changes at both the ligand and metal center enable concerted C-C oxidative addition in a manner that is distinct from concerted mechanisms involving redox changes only at the metal center, [7][8][9][10]23 or SET/radical fragmentation. [12][13][14][15][16][17][18][19][20][21][22] In this new mechanism, upon substrate binding, charge transfer from the terpyridine ligand to the substrate p* orbital "turns on" subsequent C-C activation by weakening the C-C bond. In contrast to metalloradical ring-opening reactions, [17][18][19][20] this approach avoids a free-radical intermediate and results in a functionalized alkylnickel intermediate suitable for cross-coupling with organozinc reagents.…”

“…11 Single electron transfer (SET) approaches enable a mechanistically distinct avenue for cyclopropyl ketone activation based on radical rearrangements (Figure 1B). [12][13][14][15][16][17][18][19][20][21][22] Bond activation selectivity in these systems is dictated by the stability of the resulting ringopened, C-centered radical. Accordingly, selectivity in SETmediated pathways is often different from polar metal-catalyzed processes, which favor the most accessible bond.…”

Visible‐Light‐Mediated Ring‐Opening Reactions of Cyclopropanes

“…In 2016, Xia and coworkers reported the synthesis of tetrasubstituted furans 18/19 from cyclopropyl ketones 16 via photoredox mediated ringopening reaction of cyclopropanes (Scheme 11). [69] The reaction is catalyzed by Ru(dtbbpy) 3 (PF 6 ) 2 in acetonitrile solvent. Carbon tetrabromide is used as the oxidative quencher, which also releases the bromide anion that can act as a weak base.…”

“…Ring expansion of acyl cyclopropanes. [69] Scheme 12. Reaction mechanism for the ring opening of acyl cyclopropane to β,γ-unsaturated ketone.…”