Chalcogen-containing oxazolines in the palladium-catalyzed asymmetric allylic alkylation

Abstract

Nesse trabalho descreve-se, pela primeira vez, um estudo comparativo sobre a atuação de oxazolinas quirais contendo calcogênio em suas estruturas em reação de alquilação alílica assimétrica, catalisada por paládio, do acetato de 1,3-difenil-2-propenila com malonato de dimetila. Diferenças no desempenho entre os análogos de enxofre, selênio e telúrio são observadas.A comparative study about the ability of chiral chalcogen-containing oxazolines to act as chiral ligands in the palladium-catalyzed allylic alkylation of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate is reported. Differences in the catalytic performance are observed with sulfur, selenium and tellurium analogues.Keywords: selenium, oxazolines, palladium, asymmetric allylic alkylation, tellurium IntroductionThe search for new ligands in asymmetric catalysis is a field of continuing interest. To facilitate practical applications, new ligands should be easy to prepare from simple and easily available starting materials.In this context, the use of chiral nitrogen compounds bearing an organochalcogen moiety and their metal complexes have gained increased importance in the field of asymmetric catalysis due to their novel and unprecedented properties. For example, they have been reported to be efficient chiral catalysts 1 in several different enantioselective reactions, such as the asymmetric hydrosilylation 2 and hydrogenation 3 of ketones, enantioselective diethylzinc 4 ' 5 and alkynylzinc 6 addition to aldehydes, and conjugate addition of organocopper reagents to to enones. 7 Among the transition metal-catalyzed reactions known to form carbon-carbon and carbon-heteroatom bonds, the palladium-catalyzed allylic substitution stands out as one of the most valuable synthetic tools available. 8 Many sulfur-containing ligands have been described to catalyze this transformation in an asymmetric fashion. 9 On the other hand, the parent selenium-containing ligands for asymmetric allylations have been largely overlooked. 10 Furthermore, to the best of our knowledge, telluriumcontaining analogues have never been tested in such transformation.In this paper we report about the examination on the behavior of a series of sulfur-selenium-and telluriumcontaining oxazolines as chiral catalysts for the asymmetric alkylation of racemic 1,3-diphenyl-2-propenyl acetate with dimethylmalonate, catalyzed by palladium. 11 Results and DiscussionThe chiral chalcogeno-oxazolines were synthesized in two different approaches. The first one starts from amino acids S-benzyl-L-Cysteine and L-methionine. Initially, both amino acids were cleanly reduced with the NaBH 4 /I 2 method, 12 to afford thioether amino alcohol 1 and 3 in good yields. The amino alcohol moiety was then cyclized in good yields to their corresponding oxazolines 2 and 4 by reaction with ethyl chlorobenzimidate in boiling 1,2-dichloroethane, with K 2 CO 3 as base (Scheme 1).The second approach employs L-aspartic acid as the chiral source. Ligands 8a-c were obtained by esterification of both carboxyl groups of L-...

Synthesis of seleno-carbohydrates derived from d-galactose

“…The developed protocol allowed the synthesis of arylseleno-carbohydrates with chloro (3b) and methoxy (3c) substituents, as well as with the 2-thienyl (3d) and benzyl (3e) groups. The reaction is also efficient with a sulfur nucleophile, 45 which results in the thio-galactopyranoside 7 in very high yield (entry 6).…”

Selenium reagents as catalysts

“…97 Some selenium-based ligands have received a wide range of applications in Pd-catalysed stereoselective allylic alkylations. [98][99][100][101][102][103][104][105] Additionally, few other organoselenium reagents have been identified as successful catalysts for different stereoselective reactions such as Aldol reaction, 106 Darzen reaction 107 and Baeyer-Villiger oxidations. 108 High selectivities were obtained in case of Aldol and Darzen reactions while Baeyer-Villiger oxidations could only be achieved in low yields.…”

Catálise assimétrica no Brasil: desenvolvimento e potencialidades para o avanço da indústria química brasileira

“…Utilizando essa estratégia sintética foi possível preparar diversas gerações de ligantes derivados de aminoácidos e de outros blocos sintéticos, cujas estruturas representativas [32][33][34][35][36][37] Esses ligantes foram aplicados em diferentes reações enantiosseletivas, dentre as quais podem ser destacadas a adição 1,4, adição de organozinco em aldeídos e substituição alílica, como mostrado no Esquema 8. 38 A importância dessa estratégia sintética na busca por catalisadores privilegiados vem se mostrando altamente eficiente, e foi recentemente utilizada no desenvolvimento de novos ligantes/catalisadores promíscuos contendo selênio obtidos a partir da efedrina, 39 em poucas etapas sintéticas (Esquema 9).…”