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...
Presented is the first enantioselective copper-catalyzed 1,6-conjugate addition of bis(pinacolato)diboron to para-quinone methides. The reaction proceeds with excellent yields and good to excellent enantioselectivities, and provides an attractive approach to the construction of optically active gem-diarylmehtine boronic esters. Additionally, the subsequent conversion of the derived potassium trifluoroborates into triarylmethanes with highly enantiospecificity was realized.
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