Browsing by Author "Xu, Qing-Long"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Practical Organocatalytic Synthesis of Functionalized Non-C2-Symmetrical Atropisomeric Biaryls
    (Wiley, 2016) Gao, Hongyin; Xu, Qing-Long; Keene, Craig; Yousufuddin, Muhammed; Ess, Daniel H.; Kürti, László
    An organic acid catalyzed direct arylation of aromatic C(sp2)H bonds in phenols and naphthols for the preparation of 1,1′-linked functionalized biaryls was developed. The products are non-C2-symmetrical, atropoisomeric, and represent previously untapped chemical space. Overall this transformation is operationally simple, does not require an external oxidant, is readily scaled up (up to 98 mmol), and the structurally diverse 2,2′-dihydroxy biaryl (i.e., BINOL-type), as well as 2-amino-2′-hydroxy products (i.e., NOBIN-type) are formed with complete regioselectivity. Density-functional calculations suggest that the quinone and imino-quinone monoacetal coupling partners are exclusively arylated at their α-position by an asynchronous [3,3]-sigmatropic rearrangement of a mixed acetal species which is formed in situ under the reaction conditions.
  • Thumbnail Image
    Item
    Symmetry in Cascade Chirality-Transfer Processes: A Catalytic Atroposelective Direct Arylation Approach to BINOL Derivatives
    (American Chemical Society, 2016) Wang, Jin-Zheng; Zhou, Jin; Xu, Chang; Sun, Hongbin; Kürti, László; Xu, Qing-Long
    Herein we disclose a scalable organocatalytic direct arylation approach for the regio- and atroposelective synthesis of non-C2-symmetric 2,2′-dihydroxy-1,1′-binaphthalenes (BINOLs). In the presence of catalytic amounts of axially chiral phosphoric acids, phenols and naphthols are coupled with iminoquinones via a cascade process that involves sequential aminal formation, sigmatropic rearrangement, and rearomatization to afford enantiomerically enriched BINOL derivatives in good to excellent yields. Our studies suggest that the (local) symmetry of the initially formed aminal intermediate has a dramatic impact on the level of enantioinduction in the final product. Aminals with a plane of symmetry give rise to BINOL derivatives with significantly lower enantiomeric excess than unsymmetrical ones featuring a stereogenic center. Presumably asymmetric induction in the sigmatropic rearrangement step is significantly more challenging than during aminal formation. Sigmatropic rearrangement of the enantiomerically enriched aminal and subsequent rearomatization transfers the central chirality into axial chirality with high fidelity.
  • Thumbnail Image
    Item
    Transition metal-free direct dehydrogenative arylation of activated C(sp3)–H bonds: synthetic ambit and DFT reactivity predictions
    (Royal Society of Chemistry, 2018) Lovato, Kaitlyn; Guo, Lirong; Xu, Qing-Long; Liu, Fengting; Yousufuddin, Muhammed; Ess, Daniel H.; Kürti, László; Gao, Hongyin
    A transition metal-free dehydrogenative method for the direct mono-arylation of a wide range of activated C(sp3)-H bonds has been developed. This operationally simple and environmentally friendly aerobic arylation uses tert-BuOK as the base and nitroarenes as electrophiles to prepare up to gram quantities of structurally diverse sets (>60 examples) of α-arylated esters, amides, nitriles, sulfones and triaryl methanes. DFT calculations provided a predictive model, which states that substrates containing a C(sp3)-H bond with a sufficiently low pK a value should readily undergo arylation. The DFT prediction was confirmed through experimental testing of nearly a dozen substrates containing activated C(sp3)-H bonds. This arylation method was also used in a one-pot protocol to synthesize over twenty compounds containing all-carbon quaternary centers.