JACS: photocatalysis of anomalous selective functionalization of 1,2-dibasic acid ester

Photoredox catalysis can produce free radical species under mild conditions Alkylboron reagents, including borates and trifluoroaborates, can be prepared by single electron oxidation to form alkyl radical intermediates, which are involved in Giese addition, hydrogen atom transfer, nickel catalyzed cross coupling and radical polarity cross reaction (scheme 1a) Although there are many studies on the conversion of alkylboron by photocatalysis, the reaction of 1,2-dibasic acid ester as a free radical precursor has not been explored At present, the reported selective monofunctionalization of 1,2-dibasic borates is carried out on the primary borates with small steric hindrance, while the secondary borates with more hindrance are retained (source: J am Chem SOC.) recently, varinder K Aggarwal et al From the University of Bristol, UK, found that in the photocatalysis of 1,2-dibasic acid ester, β - primalkyl radicals produced by single debonding undergo rapid 1,2-boron transfer to form thermodynamic favorable secondary alkyl radicals, thus allowing the space hindered borate ester to undergo selective conversion The reaction is stereospecific and can be used for highly enantioselective fragmentation and cross cyclization Relevant research results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b07564) The reaction conditions were screened with 1,2-dibasic acid ester 1A as the substrate The reaction system includes: phenyllithium (used to form borate complex with 1a), Photocatalyst (4czipn), tert butyl acrylate and tert butanol Through the selection of phenyl lithium, solvent and concentration, the author determined that aryl lithium reagent B and acetonitrile with large steric hindrance were the best conditions At the concentration of 0.1M, the reaction yield could reach 90% (source: J am Chem SOC.) next, the author studied the range of dibasic borates (scheme 2a) The substrates with steric hindrance can obtain excellent yields However, the yield of 3G containing phenyl is only moderate (35%), and most of the raw materials are unreacted The authors speculate that the competitive addition of instantaneous free radical intermediates to aromatic rings may (I) lead to nonspecific degradation; (II) slow down the turnover of photocatalysts, so as to decompose borate complex intermediates over time When the homologous substrate was used, the yield of product 3H was increased significantly (66%), which indicated that the formation of 3G was inhibited near phenyl The 1,2-dibasic acid ester derivative (1o) of β - Terpinene reacts to form a single ring product 3O with a yield of 53% The compound is the product of partial ring opening of cyclobutane after the migration of 1,2-boron, which highlights the free radical process of the transformation A series of electron deficient olefins and styrene derivatives are good free radical receptors (source: J am Chem SOC.) based on the previous reports on the oxidation of borate complexes and the observed regioselectivity, the author proposed a mechanism (scheme 3): the first-order free radical is produced by the oxidation of the first-order borate complex 2 by the excited state photocatalyst, while the arylborate 7 is lost; then the second-order free radical 9, which is thermodynamically advantageous, is obtained by the migration of 1,2-boron The final product 3 was obtained by protonation of anion 11 (source: J am Chem SOC.) further, the author extended the reaction to the selective functionalization of 1,2,3-tribasic acid ester Thermodynamic control is favorable for the most stable free radical center as the reaction site A scheme 4B reaction can be carried out for the synthesis of borates from cyclooctadiene (source: J am Chem SOC.) finally, the author studied the stereochemistry of the reaction (scheme 5a) by using the non enantiomeric pure 1z derived from α - pinene The product has a DR of > 20:1, in which borate is migrated with high stereochemical fidelity The author treated allylborate 17 with Langlois reagent, and obtained clear evidence of 1,2-boron migration (scheme 5b) DFT modeling supports the simplicity of 1,2-boron migration (scheme 5C) Boron isotopic labeling studies confirmed that the formation of borate complex with aryl lithium B occurred only on scheme 5D (source: J am Chem SOC.) conclusion: varinder K Aggarwal et al Of the University of Bristol, UK, have first proved that the transfer of free radical 1,2-boron under the control of photocatalysis thermodynamics allows the unconventional selective functionalization of more hindered borates in 1,2-dibasic acid esters The reaction has a wide range of substrates.