Abstract
Dearomative functionalization reactions represent an important strategy for the synthesis of valuable three-dimensional molecules from simple planar aromatics. Naphthalene is a challenging arene towards transition-metal-catalyzed dearomative difunctionalization reactions. Reported herein is an application of naphthalene as a masked conjugated diene in a palladium-catalyzed dearomative 1,4-diarylation or 1,4-vinylarylation reaction via tandem Heck/Suzuki sequence. Three types of 1,4-dihydronaphthalene-based spirocyclic compounds are achieved in excellent regio- and diastereoselectivities. Key to this transformation is the inhibition of a few competitive side reactions, including intramolecular naphthalenyl C-H arylation, intermolecular Suzuki cross-coupling, dearomative 1,2-difunctionalization, and dearomative reductive-Heck reaction. Density functional theory (DFT) calculations imply that the facile exergonic dearomative insertion of a naphthalene double bond disrupts the sequence of direct Suzuki coupling, leading to the tandem Heck/Suzuki coupling reaction. The observed regioselectivity towards 1,4-difunctionalization is due to the steric repulsions between the introduced aryl group and the spiro-scaffold in 1,2-difunctionalization.
Highlights
Dearomative functionalization reactions represent an important strategy for the synthesis of valuable three-dimensional molecules from simple planar aromatics
As disclosed recently by the groups of Lautens[17,18,19,20], Liang[21], Yin[22,23], Zhou[24], Wu25, and us[26,27,28,29,30], the application of endocyclic C=C bonds of heteroarenes as non-classic olefins has enabled a number of efficient dearomative difunctionalization reactions of indoles and furans with Pd- or Ni-catalyst[31]. This dearomatizing strategy undoubtedly expands the scope of olefin difunctionalization reaction to arenes and constitutes an important method for the synthesis of valuable three-dimensional molecules from simple planar aromatics[32,33,34,35,36,37,38]
As our interest of dearomatization reactions[26,27,28,29,30,51,52,53,54], we report a dearomative 1,4-difunctionalization of naphthalenes via intramolecular dearomative Heck arylation or vinylation followed by intermolecular cross-coupling with the π-allyl-palladium species (Fig. 1c)
Summary
Dearomative functionalization reactions represent an important strategy for the synthesis of valuable three-dimensional molecules from simple planar aromatics. The presence of 5 mol% Pd(dba)[2, 10] mol% PPh3, and 2.5 equiv K2CO3 in toluene led to by-product 4 in 90% NMR (Nuclear Magnetic Resonance) yield via an intermolecular Suzuki crosscoupling (see entry 1). To further improve the product yield, influence of the base and solvent were investigated in the presence of ligand L5 (entries 9–13 and 15–18).
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