A Pd-catalyzed, site-selective p-hydroxyphenyloxylation of benzylic α-C(sp)-H bonds with 1,4-benzoquinone using thioamide as a directing group is reported. 1,4-Benzoquinone is employed as the p-hydroxyphenyloxy source without extra oxidants. This method exclusively gives site selectivity at α-C(sp)-H bonds rather than the usual β-C(sp)-H bonds through C-H activation mode. The reactions proceed with high functional group tolerance in yields of 42-93%.
Novel chiral stationary phases based on peptoid combining a quinine/quinidine moiety through a C9-position carbamate groupBy connecting a quinine or quinidine moiety to the peptoid chain through the C9-position carbamate group, we synthesized two new chiral selectors. After immobilizing them onto 3-mercaptopropyl-modified silica gel, two novel chiral stationary phases were prepared. With neutral, acid, and basic chiral compounds as analytes, we evaluated these two stationary phases and compared their chromatographic performance with chiral columns based on quinine tert-butyl carbamate and the previous peptoid. From the resolution of neutral and basic analytes under normal-phase mode, it was found that the new stationary phases exhibited much better enantioselectivity than the quinine tert-butyl carbamate column; the peptoid moiety played an important role in enantiorecognition, which controlled the elution orders of enantiomers; the assisting role of the cinchona alkaloid moieties was observed in some separations. Under acid polar organic phase mode, it was proved that cinchona alkaloid moieties introduced excellent enantiorecognitions for chiral acid compounds; in some separations, the peptoid moiety affected enantioseparations as well. Overall, chiral moieties with specific enantioselectivity were demonstrated to improve the performance of peptoid chiral stationary phase efficiently.
Eight peptoid chiral stationary phases (CSPs) terminated with N′‐substituted phenyl‐L‐proline or L‐leucine amide were prepared and evaluated under normal phase mode. With 59 racemic analytes, we compared the enantiomeric separations on CSPs terminated with p‐methylphenyl, p‐chlorophenyl and unsubstituted phenyl. For short peptoid selectors containing only one S‐N‐(1‐phenylethyl) glycine (Nspe) unit, the terminal p‐methyl substituent did not affect chiral recognition abilities significantly. In L‐proline amide terminated CSPs, p‐chloro substituent resulted in obviously inferior selectivity while in L‐leucine amide terminated CSPs, it worked much better. Longer peptoid selectors containing two more Nspe units generally performed much better than the shorter ones, due to the great contributions of peptoid chain to chiral recognition. Meanwhile, the effects of the terminal substituent on selectivity were found changed on these CSPs. For CSPs terminated with L‐leucine amide, the terminal p‐chloro substituent in longer selector no longer produced the best recognition ability; the CSP with unsubstituted phenyl instead performed best. Comparison of these peptoid CSPs varied in terminal substituents and chain length was conducted to gain a better understanding of the chiral recognition mechanism of this type CSP and promote the development of more useful CSPs.
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