A Cu/Pd Cooperative Catalysis for Enantioselective Allylboration of Alkenes

Abstract

A cooperative Cu/Pd-catalyzed asymmetric three-component reaction of styrenes, B 2 (pin) 2 , and allyl carbonates was reported. This reaction, in the presence of chiral CuOAc/SOP and achiral Pd(dppf)Cl 2 catalysts, occurs smoothly with high enantioselectivities (up to 97% ee) . The allylboration products, which contain alkene (or diene) unite and alkylboron group, are easily functionalized. The utility of this protocol was demonstrated through the synthesis of an antipsychotic drug, (−)-preclamol.M ultifunctional and enantioenriched organoboranes are useful building blocks in the synthesis of natural products and bioactive compounds. 1 Transition metal-catalyzed asymmetric carboboration reaction is an efficient and straightforward approach to access chiral multisubstituted alkyl-or alkenylboranes. 2 For instance, Ito and co-workers developed a successful copper-catalyzed borylative cyclization to prepare optically pure β-aryl or silylated cyclopropylboronates. 3 The related asymmetric carboboration of allenes, alkynes and 1,3-enynes are also applied in the synthesis of chiral di-, tri-, and tetra-substituted alkenylboron esters by Hoveyda 1c,4 and Lin. 5 However, up to date, enantioselective carboboration of alkenes for the construction of enantioenriched and multifunctional alkylborons are less developed. Sporadic examples were reported and limited to Cu-catalyzed borylative aldol carboboration 6 and Pd-catalyzed 1,1-arylboration. 7 Very recently, Semba, Nakao 8 and Brown 9 independently reported a remarkable three-component carboboration of alkenes with bis(pinacolato)diboron (B 2 (pin) 2 ) and aryl or vinyl halides by a Cu/Pd cooperative catalysis. We envisioned using this combination catalysis 10 as a platform to develop enantioselective carboboration of simple alkenes. The proposed procedure involves two cooperative catalytic cycles (Scheme 1), a Cu-catalyzed enantioselective generation of β-borylalkylcopper 11 and a Pd-catalyzed cross-coupling transformation of this enantioenriched reagent. 1a,12 We believe that the stereospecific transmetalation of alkylcopper with XPd(L)R′ in the proposed mechanism, and the compatibility of the chiral L*Cu(I)-complex with the achiral Pd(0)-or Pd(II)-complex would be the key issues. In our previous work, 13 a stereospecific copper− tin transfer was observed in Cu(I)-catalyzed enantioselective alkene stannylboration when using chiral sulfoxide-(P-aryl)-phosphine (SOP) ligand. In this work, we found that P-alkyl sulfoxide-phosphine ligands 14 can effectively promote the aforementioned Cu−Pd transmetalation and realize a highly enantioselective allylboration of alkenes.To test the feasibility, the three-component reaction of styrene (1a), B 2 (pin) 2 , and allylic electrophiles was performed in the presence of (SOP)CuCl catalyst precursor and Pd(dppf)Cl 2 cocatalyst. After screening a series of allyl substrates with different leaving groups (e.g., halides, esters and carbonates), t-butyl allyl carbonate 2a was confirmed as the best electrophile in terms of the re...

Asymmetric Suzuki-Miyaura coupling of heterocycles via Rhodium-catalysed allylic arylation of racemates

“…To demonstrate the method can be used to synthesize valuable molecules we prepared the antipsychotic drug preclamol44 whose asymmetric synthesis has recently been reported using powerful methods4546. Using standard conditions for piperidene derivatives, 81 which had been previously converted to preclamol47, was prepared from 3-MeO-Ph boronic acid in three steps in 64% overall yield and 96% ee (Fig.…”

Versatile Homoallylic Boronates by Chemo‐, SN2′‐, Diastereo‐ and Enantioselective Catalytic Sequence of Cu−H Addition to Vinyl‐B(pin)/Allylic Substitution

“…[1] Notable examples are enantioselective boron–hydride additions to alkenes with Rh- or Ir-based complexes [2] and related diboryl additions with Pt-, Pd- or carbohydrate-derived catalysts [3] (Scheme 1a). An alternative approach entails site- and enantioselective Cu–B(pin) (pin = pinacolato) addition to an olefin followed by in situ protonation (proto-boryl addition) [4] or allylic substitution (boron–allyl addition) [5] of the Cu–C bond; these transformations are typically promoted by a chiral Cu-based complex (Scheme 1a). …”

Catalytic Enantioselective Arylboration of Alkenylarenes

“…While diastereoselective variants of these dual catalyst reactions has been achieved, enantioselective processes have been slow to emerge. [7] However, recent progress has been made by Liao et al with Pd/Cu-catalyzed enantioselective allylboration of alkenes (Scheme 2A); [8] one example of arylboration was also presented. Buchwald and coworkers recently disclosed an enantioselective hydroarylation that operates by Pd/Cu dual catalysis.…”