Synlett 2004(3): 393-421  
DOI: 10.1055/s-2004-815422
ACCOUNT
© Georg Thieme Verlag Stuttgart · New York

Geometric and Electronic Control of Thermal Bergman Cyclization

Diwan S. Rawat, Jeffrey M. Zaleski*
Department of Chemistry, Indiana University, Bloomington, IN 47405
e-Mail: zaleski@indiana.edu;
Further Information

Publication History

Received 16 September 2003
Publication Date:
26 January 2004 (online)

Abstract

The geometric and electronic contributions to Bergman cyclization of simple enediynes are compared and contrasted to those of recently developed metalloenediyne analogues. The ­comparison reveals that metals can have more diverse, subtle, and in some cases, more dramatic geometric effects upon Bergman ­cyclization than simple substitution chemistry. In addition to ­geometric consequences imposed by metals, electronic control of metalloenediyne cyclization is emerging as a complementary ­parameter in complex design.

  • 1 Introduction

  • 1.1 Historical Perspective

  • 1.2 Geometric Factors Influencing Bergman Cyclization

  • 1.3 Electronic Contributions to Bergman Cyclization

  • 1.4 Steric Contributions to Bergman Cyclization

  • 2 Organometallic σ/π Enediynes: Electronic Influence of Metal Complexation

  • 3 Coordination Chemistry Approaches to Metalloenediynes: Geometric Contributions to Thermal Bergman Cyclization

  • 4 Metal Geometry Control of Bergman Cyclization: Bidentate Chelation

  • 4.1 d10 Metalloenediynes: Tetrahedral vs. Square Planar ­Geometry

  • 4.2 d9 and d10 Copper Metalloenediynes: Tetragonal vs. ­Tetrahedral Geometry

  • 4.2.1 Conformationally Flexible Bis(pyridyl) Enediyne Ligation

  • 4.2.2 Structurally Rigid Bis(pyridyl) Ligation

  • 5 Metal Geometry Control of Bergman Cyclization: ­Tetradentate Enediyne Ligation

  • 5.1 Macrocyclic Complexes of Copper and Zinc

  • 5.2 Tetradentate Complexes of Copper and Zinc: R-Group Modulation

  • 5.3 Mg2+ Modulated Ambient Temperature Bergman ­Cyclization

  • 6 Electronic Control of Metalloenediyne Cyclization

  • 7 Alternative Approaches to Bergman Cyclization and Future Perspectives

117

DSC traces are often useful as measures of Bergman cyclization reactivity once exothermic peaks can be correlated with cyclization and not side reactions. Not all exothermic peaks reflect Bergman cyclization however for those that do correlation with the temperatures for solution reactivity is high.

123

The thermal cyclization temperatures of these compounds suggest enhanced stability of the benzannulated derivatives. However without detailed kinetics information data it is impossible to differentiate between this effect and a change in the rate limiting step for the cyclization reaction (c.f. Kanecko et al. [76] and Jones et al. [81] ).

125

Chandra, T.; Allred, R. A.; Kraft, B. J.; Berreau, L. M.; Zaleski, J. M. Inorg. Chem. 2004, in press.