1448901-30-1Relevant articles and documents
Reductive α-borylation of α,β-unsaturated esters using NHC-BH3 activated by I2 as a metal-free route to α-boryl esters
Radcliffe, James E.,Fasano, Valerio,Adams, Ralph W.,You, Peiran,Ingleson, Michael J.
, p. 1434 - 1441 (2019)
Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH3 (NHC = N-heterocyclic carbene) and catalytic I2. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC-BD3/I2 indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH2(NHC) unit can be transformed into electrophilic BX2(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C-B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.
Regioselective Radical Borylation of α,β-Unsaturated Esters and Related Compounds by Visible Light Irradiation with an Organic Photocatalyst
Li, Guosong,Huang, Guanwang,Sun, Ruixia,Curran, Dennis P.,Dai, Wen
, p. 4353 - 4357 (2021)
Radical hydroboration reactions have only recently been reported and are still rare. Here we describe a photoredox radical hydroboration of α,β-unsaturated esters, amides, ketones, and nitriles with NHC-boranes that uses only an organocatalyst and visible light. The conditions are mild, the substrate scope is broad, and the α/β regioselectivity is high. The reaction requires only the organocatalyst; there is no costly metal, and there are no other additives (base, cocatalyst, initiator).
N-Heterocyclic Carbene Boryl Iodides Catalyze Insertion Reactions of N-Heterocyclic Carbene Boranes and Diazoesters
Allen, Thomas H.,Kawamoto, Takuji,Gardner, Sean,Geib, Steven J.,Curran, Dennis P.
supporting information, p. 3680 - 3683 (2017/07/15)
Boron-hydrogen bond insertion reactions of N-heterocyclic carbene (NHC) boranes and diazoesters can be catalyzed by NHC-boryl iodides and produce stable α-NHC-boryl esters. The conditions of the reaction resemble the previous rhodium-catalyzed transformations (only the catalyst is different); however, the mechanisms of the two reactions are probably very different. The new boryl iodide catalyzed method is adept at producing α-substituted-α-NHC-boryl esters, and this has led to a family of NHC-boryl esters with amino acid and amino-acid-like side chains.
Insertion of reactive rhodium carbenes into boron-hydrogen bonds of stable N-heterocyclic carbene boranes
Li, Xiben,Curran, Dennis P.
supporting information, p. 12076 - 12081 (2013/09/02)
Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR′). Stable α-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR′) are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.
