A Study Of Iron-Catalyzed Reductive Cyclizations Of 1,6- Dienes, 1,6-Enynes, 1,6-Diynes: Reactions That Proceed Via Metallocyclic Intermediates.

Abstract

A series of cyclization reactions catalyzed by (RPDI)M(N2)2 (M = Fe, Co; R PDI = 2,6-(2',6'-R2-C6H3N=CMe)2C5H3N; R= iPr, Me; tripsPDI = 2,6-(2',4',6'-iPr3- C6H3N=CMe)2C5H3N) was presented. The first reaction was a novel iron-catalyzed, hydrogen-mediated cyclization of 1,6-enynes and 1,6-diynes. This methodology employed mild conditions (room temperature and four atmospheres of hydrogen) and exhibited broad substrate scope. The reaction mechanism was studied through stoichiometric reactions and various deuterium labeling experiments, which revealed that transfer hydrogenation from the ligand isopropyl group was involved in the rate limiting step. This presents a rare example of bis(imino)pyridine ligands as both redox-active and chemically-active. The second reaction studied was an extension of the unprecedented iron-catalyzed [2!+2!] cyclization of unactivated 1,6-dienes to prepare [3.2.0]bicycloheptanes. The substrate scope was expanded and novel reactivity was observed. When a less sterically demanding ligand framework, MePDI, and a less reducing catalyst, (iPrPDI)Co(N2), were used, a new product identified as an exomethylene-cyclopentane was observed. Higher temperatures favored the expected bicyclic product. The mechanism of the [2!+2!] cyclization was studied through a series of deuterium labeling experiments. It was determined through extensive deuterium scrambling that the reaction may be best described as a controlled radical reaction. The final study involved the preparation of relevant intermediates along the enyne and diyne cyclization. These iron-metallocyclopentanes were studied by preparation of (RPDI)Fe(biphenyl) and (RPDI)Fe(diyne) (R = iPr, Me; (biphenyl = 2,2'-C12H8); diyne = bis(2-butynyl)tosylamine) complexes, which were analyzed by single crystal X-ray diffraction and Mössbauer spectroscopy. The electronic structure of these intermediates suggest that cycloaddition of dienes, eynes, or diynes results in a one-electron reduction of the bis(imino)pyridine ligand and one electron oxidation of the iron-center. In the case of the (RPDI)Fe(biphenyl) and (RPDI)Fe(diyne) complexes, these ferric centers have been assigned as low spin. However, for the intermediates of the related diene and enyne cyclizations, the ferric center could have a different spin state, thus accounting for their inability to be isolated.