- Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3)?H/C(sp)?H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands
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The intermolecular asymmetric radical oxidative C(sp3)?C(sp) cross-coupling of C(sp3)?H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp3)?C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp3)?C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C?H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp3)?H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.
- Gu, Qiang-Shuai,Guo, Kai-Xin,Li, Zhong-Liang,Liu, Lin,Liu, Xin-Yuan,Tian, Yu,Yang, Chang-Jiang,Ye, Liu
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supporting information
p. 26710 - 26717
(2021/11/18)
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- Anodic Oxidation of α-Substituted p-Xylenes. Electronic and Stereoelectronic Effects of α-Substituents in the Deprotonation of Alkylaromatic Radical Cations
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The effect of α-substituents on the deprotonation rate from the benzylic position of alkylaromatic radical cations, k(CH2Z)/k(CH3), has been investigated by determining the intramolecular selectivity in the anodic oxidation in acetic acid of a α-Z-substituted p-xylenes 1 (Z = H, OMe, OH, Me, tert-butyl, OAc, COOMe, CN), 5,6-dimethylindan 4 (R = H), and 2,2,5,6-tetramethylindan 4 (R = Me).Some oxidations induced by CAN have also been carried out.It has been found that, with the exception of when Z = tert-butyl, the deprotonation rate of 1.+ is always faster from the CH2Z group than from the CH3 group, independently of the electron-donating or electron-withdrawing nature of Z.The electron-donating groups (OH, OMe, Me), however, exert a larger effect than the electron-withdrawing ones (COOMe, CN).The negligible deprotonation rate from CH2-t-Bu has been ascribed to stereoelectronic effects (the bulky tert-butyl group does not allow the C-H bonds to be collinear with the ?-system), the suggestion being nicely confirmed by the observation that the deprotonation rate from the position 1(3), relative to that from the 5(6)-methyl group, is almost identical in the radical cations of 4 (R = H and Me).The effect of the other α-substituents is mainly of electronic nature and has been rationalized on the basis of a variable transition-state structure for the deprotonation process.It is suggested that with +R groups most of the charge, in the transition state, has been transferred to the Cα-H bond where it can be stabilized by the α-substituent.With electron-withdrawing groups less charge transfer has taken place and the rate-enhancing effects of these groups is ascribed to their capability to significantly decrease the strength of the Cα-H bond.
- Baciocchi, Enrico,Mattioli, Mario,Romano, Roberta,Ruzziconi, Renzo
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p. 7154 - 7160
(2007/10/02)
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- Oxidation of Alkylbenzenes by S2O82--CuII in Acetic Acid and Acetonitrile
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Oxidation of a series of toluenes by K2S2O8 or (NH4)2S2O8 + Cu(OAc)2 in acetic acid or acetonitrile is little affected by added water or acetate and gives benzyl acetates or benzaldehydes, respectively, in good yields.Data are consistent with initial formation of aromatic radical cations, proton loss to give benzyl radicals, and oxidation to final products by CuII.Benzyl alcohols, but not acetates, are selectively oxidized, suggesting partial equilibration of radical cations, with rates of proton loss determining product distributions.Oxidation of cumene gives chiefly α-methylstyrene which is oxidized futher to 2-phenylpropanal.Products from p-ethyltoluene and p-cumene indicate that, on a statistical basis, loss of secondary and tertiary protons is more rapid than loss of primary protons, contrary to some previous reports.These systems appear promising for studying fragmentation patterns of aromatic radical cations in general.
- Walling, Cheves,Zhao, Chengxue,El-Taliawi, Gamil M.
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p. 4910 - 4914
(2007/10/02)
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- ipso Nitration. XXIII. Reactions of cyclohexadiene adducts from nitration of 4-ethyltoluene in acetic anhydride
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The diastereoisomers of 4-ethyl-1-methyl-4-nitrocyclohexa-2,5-dienyl acetate (1) and 1-ethyl-4-methyl-4-nitrocyclohexa-2,5-dienyl acetate (2) are stereospecifically reduced to the corresponding nitrocyclohexadienols with aluminum hydride.Each dienol is stereospecifically methylated to the corresponding methyl ether with methyl iodide, silver oxide, and potassium hydroxide.Acid-catalysed solvolysis of the acetates 1 and 2 results in the substitution of the acetate moiety by other nucleophiles and these reactions are not stereospecific.The products of rearomatization of dienyl acetates, dienols and dienyl methyl ethers depend on the acidity and ionizing power of the solvents and are readily explained in terms of reactions involving a nitrocyclohexadienyl cation or acetoxy- (hydroxy-, methoxy-)cyclohexadienyl cation as key intermediates.In the 4-acetoxy-4-alkylcyclohexadienyl cation 1,2-migration of the acetoxyl group is more rapid then alkyl migration, but 1,2-alkyl migration is faster then migration of the hydroxyl or methoxyl groups in the corresponding cations. 1-Ethyl-4-methoxy-4-methylcyclohexa-2,5-dien-1-ol and 4-ethyl-3-nitrotoluene are significant minor products in the solvolysis of 1-ethyl-4-methyl-4-nitrocyclohexa-2,5-dien-1-ol in aqueous methanol.Nitration of p-ethyltoluene in the presence of sulfuric acid or in trifluoracetic anhydride gives a mixture of 4-ethyl-2-nitro- and 4-ethyl-3-nitrotoluene in a 2:1 ratio.
- Fischer, Alfred,Henderson, George N.
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p. 2314 - 2327
(2007/10/02)
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