Gold Catalysts Can Generate Nitrone Intermediates from a Nitrosoarene/Alkene Mixture, Enabling Two Distinct Catalytic Reactions: A Nitroso-Activated Cycloheptatriene/Benzylidene Rearrangement

Abstract

Gold-catalyzed reactions of cycloheptatrienes with nitrosoarenes yield nitrone derivatives efficiently. This reaction sequence enables us to develop gold-catalyzed aerobic oxidations of cycloheptatrienes to afford benzaldehyde derivatives using CuCl and nitrosoarenes as co-catalysts (10−30 mol %). Our density functional theory calculations support a novel nitroso-activated rearrangement, tropylium → benzylidene. With the same nitrosoarenes, we developed their gold-catalyzed [2 + 2 + 1]annulations between nitrosobenzene and two enol ethers to yield 5-alkoxyisoxazolidines using 1,4-cyclohexadienes as hydrogen donors.

Photoinitiated Rearrangement of Aromatic Azides to 2-Aminonicotinates

“…1 H NMR (CDCl 3 , 400 MHz): δ 10.65 (s, 1H), 8.11 (dd, 1H, J = 7.9, 1.8 Hz), 7.61 (ddd, 1H, J = 9.0, 7.8, 1.7 Hz), 7.32−7.18 (m, 2H). 13 General Procedure for the Synthesis of Ethers 2-Azidobenzoic Acid. The starting azides 1a−d were obtained according to method 1.…”

“…1 H NMR (CDCl 3 , 400 MHz): δ 7.85 (dd, 1H, J = 7.8, 1.4 Hz), 7.51 (td, 1H, J = 8.1, 1.4 Hz), 7.26−7.21 (m, 1H), 7.21−7.14 (m, 1H), 4.37 (q, 2H, J = 7.1 Hz), 1.39 (t, 3H, J = 7.1 Hz). 13 i-Propyl 2-Azidobenzoate (1c). 1c was prepared as a yellow oil in 71% yield (2.18 g).…”

Chiral Gold Complex Catalyzed Cycloisomerization/Regio- and Enantioselective Nitroso-Diels–Alder Reaction of 1,6-Diyne Esters with Nitrosobenzenes

“…22 Subsequent coordination of two molecules of the nitrosoarene 2 to each of the gold(I) centers of the chiral dinuclear metal complex might then produce the organogold species II. 18 As a result, this may guide the 1,3,5triene adduct to approach the gold-activated nitrosoarene motif in II from its least sterically hindered face that is located on the opposite side to the 4,4′-bibenzo [d][1,3]dioxolyl motif of the chiral phosphine ligand. It might also direct the 1,3,5-triene intermediate to sit in a conformation that leads its si face to preferentially approach the organogold species and the predicted more nucleophilic C 4 position in the former to be aligned with the more electrophilic oxygen atom in the latter.…”

“…17c On introducing the nitrosoarene, the group 11 metal might then participate in coordination with two molecules of the dienophile to give the organogold species II. 18 With the 2π cycloaddition partner now situated within the intimate chiral pocket of the metal complex, this may usher the more stereoelectronically favored conformer of the 1,3,5-triene adduct to approach the organogold species from its most open face. In doing so, we reasoned this might lead to the setting up of a welldefined chiral environment in the ensuing transition-state structure TS that undergoes the NDA reaction to deliver the N,O-heterocyclic product 3 in a regio-and enantioselective manner.…”

Ring Contraction of Tropylium Ions into Benzenoid Derivatives

“…Such a method is highly feasible as tropylium ion can be easily coupled with a broad range of nucleophilic substrates such as electron-rich arenes or organometallic reagents. 13 Furthermore, the resulting cycloheptatriene derivatives or substituted tropylium ions are prone to oxidative ring-contraction reactions, as demonstrated by recent works by Tamoto et al 15 and More et al 16 The cleavage of one carbon from the seven-membered ring, to retain a phenyl group on the original organic framework, is directly opposite but complementary to the approach by the Echavarren group, in which they use transition-metal catalysis to eliminate a benzene ring from cycloheptatriene derivatives to produce organometallic carbenoid complexes for cycloaddition or insertion reactions (Scheme 1b). 17 We began our investigation by screening the reaction conditions to ring contract phenyl tropylium tetrafluoroborate 1a into biphenyl 2a in an oxidizing environment, as the tropylium moiety is electrophilic.…”