Seminar

Seminar

[세미나공지] 6월 4일(목)

  • POSTED DATE : 2015-05-29
  • WRITER : 관리자
  • HIT : 2783
  • DATE : 2015-06-04
  • PLACE : 화학관 330226호

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다음주 목요일(6월 4일) 정규세미나 진행됩니다.               

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제  목 : Catalytic C−H Functionalization Reactions of Pyrazoles


연  사 : 주정민 교수 (부산대학교)


일  시 : 2015년 6월 4일 (목) 오후 4시 30분


장  소 : 화학관 세미나실 (330226호실)

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Catalytic C−H Functionalization Reactions of Pyrazoles



Pyrazoles have been found in a growing number of new pharmaceuticals, agrochemicals, and ligands to transition metals. Thus, the development of methods for the synthesis of structurally diverse pyrazoles represents an important objective in organic synthesis. While catalytic CH functionalization reactions have been dramatically expanded, applications to pyrazoles have been limited largely to arylation reactions along with a single example of alkynylation. Given that installation of alkyl groups to the pyrazole ring provides conformational flexibility, displaying improvement of physical and biological properties, a method for the C-alkylation of pyrazoles that does not depend on the use of a strong base would be highly desirable.

We envisioned that the electronic effect rendered by the electron-withdrawing group should not only reduce the Lewis basicity of the N2 atom but also enhance the acidity of the CH bond of pyrazoles, thus enabling a CH alkylation using activated alkyl halides or pseudohalides. We have developed the first catalytic CH allylation and benzylation of pyrazoles that occurs with high regioselectivity by taking advantage of synthetically useful electron-withdrawing groups, such as nitro, ester, and chloro groups. Readily available allyl acetates and benzyl chlorides are employed in this reaction as the alkyl donors to provide C5-allylated and benzylated pyrazole compounds in high yields. The CH allylation and benzylation reactions by the Pd complex derived from Pd(OAc)2 and PPh3 can be carried out under practical laboratory conditions without operational difficulties involving the handling of air and moisture sensitive reagents and special equipment to procure cryogenic conditions. This approach, not requiring strong base-promoted reaction conditions, will be useful to provide rapid access to allylated and benzylated pyrazoles. Efforts to expand the substrate scope of alkyl halides will be described as well.


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References

 [1] Bae, S.; Jang, H.-L.; Jung, H.; Joo, J. M. J. Org. Chem. 2015, 80, 690.