A Promising New Dearomatization Agent:  Crystal Structure, Synthesis, and Exchange Reactions of the Versatile Complex TpRe(CO)(1-methylimidazole)(η²-benzene) (Tp = Hydridotris(pyrazolyl)borate)

Abstract

The π-basic metal fragment {TpRe(CO)-(MeIm)} (MeIm ) 1-methylimidazole; Tp ) hydridotris-(pyrazolyl)borate) binds a variety of aromatic molecules, including benzene, 2,6-lutidine, and 1-methylpyrrole, in an η 2 fashion. Although TpRe(CO)(MeIm)(η 2 -benzene) as a solid shows no decomposition over several months at 25 °C under a nitrogen atmosphere, the complex has proven to be a valuable precursor to a variety of other aromatic complexes through ligand exchange reactions in solution.

Highly Functionalized Piperidine Generation via Pyridine Repolarization and Hyper-Distorted Allyl Complexes of {TpW(NO)(PMe₃)}

“…Providing support for this notion is the nitrosyl stretching frequency that shifts from 1554 cm -1 for typical dihydropyridine complexes to 1608, 1585, or 1608 cm -1 , for 2, 3, or 4 respectively. 3 Meanwhile, the W(I/0) reduction potential for 2-4 is nearly 0.5 V more positive than that of dihydropyridine complexes.…”

“…3 With tungsten coordinating C3 and C4, the acetylated pyridine ring of 1 readily undergoes regio-and stereoselective nucleophilic addition at C2, providing a facile route to a broad range of  2 -(1,2-dihydropyridine) complexes. Presumably, the formation of the cyanine complex 2 is driven in part by the π-basic nature of the metal fragment.…”

“…Given the prominent role of π-allyl intermediates in  2 -aromatic chemistry, 3,4 and their potential as synthons for  2 -dienes (vide supra), we sought to understand better the structural features of the TpW(NO)(PMe 3 )(π-allyl) complexes. X-ray data were obtained from single crystals of allyl complexes 3 and 5.…”

“…In this manner,  2 -arene,  2 -pyrrole, and  2 -furan complexes have been utilized in novel organic syntheses. [1][2][3] However, the development of parallel chemistry for pyridines, diazines, diazoles, and other basic aromatic heterocycles has been hampered by the thermodynamic preference of the transition metal to coordinate at nitrogen (Scheme 1, Path A). While such coordination for pyridines can be avoided by strategic placement of substituents (e.g., 2,6-disubstituted pyridines), 4 we desired a more general method for the preparation of dihapto-coordinated complexes of basic heterocycles.…”

Development of 2nd Row Molybdenum Dearomatization Agents

“…The syntheses of complexes incorporating thiazole, pyrazole, imidazole, and amine based ancillary ligands provided access to more electron-rich rhenium complexes (11-17, Table 2). The increased electron density with the incorporation of strong σ-donor ancillary ligands is also demonstrated by comparing νCO of entries 1-10 with 11-17 in Table 2 65 Despite the limited coordination scope of {TpRe(L)(CO)} fragments compared to {Os(NH3)5} 2+ and {TpRe(MeIm)(CO)}, the ability to modulate the chemical environment within a given aromatic ligand (e.g., naphthalene, furan) with changes to L ({TpRe(L)(CO}) allows possible access to range of reactivity for each aromatic ligand.…”