A Palladium Complex as an Asymmetric π-Lewis Base Catalyst for Activating 1,3-Dienes

Abstract

Here we report that palladium(0) complexes can coordinate in a η2 fashion to 1,3-dienes and significantly raise the energy of their highest occupied molecular orbital (HOMO) by donating the electrons from the d-orbitals to the empty antibonding molecular orbitals of double bonds (π*) via back-bonding. Thus, the uncoordinated double bond, as a more reactive partner on the basis of the principle of vinylogy, can directly attack imines, furnishing a formal hydrodienylation reaction enantioselectively. A chemoselective cascade vinylogous addition/allylic alkylation difunctionalization process between 1,3-dienes and imines with a nucleophilic group is also compatible, by trapping in situ formed π-allylpalladium species after initial ene addition. This π-Lewis base catalytic mode, featuring simple η2coordination, vinylogous activation, and compatibility with both conjugated neutral polyenes and electron-deficient polyenes, is elucidated by control experiments and density functional theory (DFT) calculations.

Palladium-Catalyzed Modular and Enantioselective cis-Difunctionalization of 1,3-Enynes with Imines and Boronic Reagents

“…Very recently, our group disclosed that Pd(0) could serve as a π-Lewis base catalyst to activate 1,3-dienes by forming η 2 -coordinated complexes I , which enabled enantioselective addition to N -sulfonylimines in a Friedel–Crafts reaction pattern (Scheme e) . Similar to the above proposed outersphere activation mode, we envisioned that the triple bond of 1,3-enynes also would be HOMO-raised by chemoselectively forming similar η 2 -coordinated complexes II , thus facilitating the unprecedented Pd(0)-catalyzed intermolecular nucleophilic addition to imines, in a potentially enantioselective pattern.…”

“…As outlined in Scheme a, we could not realize the hydroarylation reaction with 1,3-enyne 1a and boronic acid 3b under Pd(0) catalysis, via a possible hydropalladation/Suzuki coupling process . In addition, the direct regioselective hydrodienylation of imine 2a with previously prepared 1,3-diene substrate 6 also was not successful . As mentioned in Table , we further explored more catalytic conditions with alkyne 1a′ , imine 2a , and boronic acid 3a , but without detecting possible coupling product 7 (Scheme b), indicating the potential oxidative cyclization mechanism for alkynes and imines, like that of Ni or Rh catalysis, would not be suitable for Pd(0) catalysis.…”

“…41 In addition, the direct regioselective hydrodienylation of imine 2a with previously prepared 1,3-diene substrate 6 also was not successful. 40 As mentioned in Table 1, we further explored more catalytic conditions with alkyne 1a′, imine 2a, and boronic acid 3a, but without detecting possible coupling product 7 (Scheme 3b), indicating the potential oxidative cyclization mechanism for alkynes and imines, like that of Ni or Rh catalysis, would not be suitable for Pd(0) catalysis. It also demonstrated that Tsukamoto's anti-Wacker mode might not be applicable to intermolecular coupling reaction of alkynes.…”

π‐Lewis‐Base‐Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone

“…[15] According to ad ensity functional theory (DFT) computational study (see Section 5i nt he Supporting Information), intermediate II,a sap-allylpalladium complex-based zwitterion, [16] could not be identified as the stable species,w hich would easily isomerize to an intriguing h 2 -Pd 0 -coordinated [17] cyclopentadienone complex III in an electronically neutral pattern. Remarkably,similar to common 1,3-dienes, [10] the energy (À5.26 eV) of its highest occupied molecular orbital (HOMO) is significantly raised by the back-bonding of the Pd 0 complex [for L = (PPh 3 ) 2 ], in comparison with the calculated one of the parent system (À7.03 eV). [18] As ar esult, it would be envisaged that the uncoordinated double bond of complex III might serve as an electronically enriched partner in diverse reactions with suitable electrophilic reagents based on the principle of vinylogy,thus enabling the direct manipulation of avariety of unprecedented asymmetric umpolung transformations of labile cyclopentadienones.I na ddition, such aP d 0 -complexation process would be helpful for prohibiting the rapid dimerization of free cyclopentadienone (Scheme 1c).…”

“…Recently,wereported an ew concept that aP d 0 complex could form simple h 2complexes with neutral 1,3-diene substrates,a nd act as a p-Lewis base catalyst to facilitate the asymmetric Friedel-Crafts reaction with imines,i nav inylogous activation pattern (Scheme 1b,2). [10] Such as uccess inspired us to consider the potential Pd 0 -based p-Lewis base activation mode for more challenging electron-deficient diene-type substrates.…”