(reagents used as precursors to complexes with dihapto-bound aromatic molecules)
Physical Data: brown to brick-red microcrystalline solids.
Solubility: all insoluble in H2O and hexanes; somewhat soluble in most other organic solvents, with the MeIm system being the least soluble.
Purification: recrystallization from toluene or 1,2-dimethoxy-ethane (DME) and hexanes.
Handling, Storage, and Precautions: compounds can be stored at room temperature in air.
Various TpReIII dihalides may be synthesized from TpRe(O)(Br)2 via reduction by a phosphine.1-4 Reagents of the general formula TpRe(L)(Br)2 (Tp = hydrido-tris(pyrazolyl)borate; L = tBuNC, PMe3, pyridine (py), and 1-methylimidazole (MeIm)) have proven to be useful precursors to a variety of asymmetric rhenium(I) dearomatization agents, which have shown synthetic utility for organic transformations of aromatic compounds (vide infra).
The dihalides described herein may be reduced with sodium under a CO atmosphere to produce either h2-aromatic or h2-olefin complexes of the general formula TpRe(CO)(L)(Lp) (Lp = dihapto-coordinated ligand).2-5 Classified according to their respective immediate starting materials, three different methodologies have been developed for the synthesis of rhenium h2-aromatic complexes. These are tandem oxidation/ reduction of TpRe(CO)(L)(Lp), direct reduction of TpRe(L) (Br)2, and direct substitution with TpRe(CO)(L)(Lp) (
For these systems, a complexed olefin at ReI is not labile. However, oxidation to ReII (e.g., with AgOTf) decreases the electron density at the metal and lessens the back-bonding interaction with the olefin rendering it significantly more labile (
Coordination of an aromatic compound directly from the ReIII stage eliminates the need to isolate a ReI precursor. In this procedure, similar to that used for olefin complexation, the ReIII dihalide reacts with Na/Hg or Na0 and the desired ligand under a CO atmosphere at 20-40 °C (
Finally, performing a direct substitution at the ReI stage is the most straightforward method for binding aromatic ligands. By utilizing a relatively stable complex (i.e., isolable) with a single labile ligand, a substitution can be performed with an aromatic compound present in excess (
Through these three methodologies, a variety of h2-aromatic compounds may be synthesized. Although the tBuNC, PMe3, and py fragments bind naphthalenes, thiophenes, and furans, no stable complexes with benzenes or pyrroles have been isolated. The more electron-rich MeIm system, however, coordinates a wider variety of aromatic compounds including furans, thiophenes, naphthalenes, benzenes, pyrroles, and 2,6-lutidine. Furthermore, coordination of carbonyl moieties such as aldehydes, ketones, esters, imides, and amides has been achieved.6,7
The most diagnostic feature of the TpRe(CO)(L)(Lp) systems is the upfield shift of the bound carbons and the adjacent protons. Additionally, infrared and electrochemical data are useful by providing characteristic vco and II/I potentials, respectively. These compounds range in color from orange to yellow to white. Due to their neutrality, they are soluble in a wide range of solvents from methanol to diethyl ether. They are insoluble in water and, in most cases, hexanes. Almost all complexes, unlike their pentaammineosmium(II) counterparts,8 are stable to silica or alumina chromatographic separation and/or purification.
The kinetic stability of these systems is related to the electronic properties of the metal. Under a nitrogen atmosphere, the rate of dissociation of dihapto-coordinated aromatic ligands decreases as the metal fragment becomes more electron-rich. For example, the MeIm-naphthalene complex is more stable than its isonitrile analog (
The stability of these compounds to air demonstrates an inverse relationship. Whereas solutions of the tBuNC and PMe3 h2-aromatic complexes are moderately stable when exposed to air, the py and MeIm systems decompose rapidly under these conditions (
These rhenium systems have a number of synthetic applications. Unlike the pentaammineosmium system, these rhenium systems are tunable. The steric and electronic environments may be altered by variation of L (
The rhenium systems promote both 1,2 and 1,4 tandem additions to naphthalene in a stereoselective fashion (
The rhenium systems promote the tandem addition of methanol across the uncoordinated portion of the furan ring as well as ring-opening (
The MeIm system promotes 1,3-dipolarcycloaddi-tions between N-methylpyrrole and electron-deficient olefins such as fumarate (
The MeIm system promotes Diels-Alder reactions between benzenes and N-methylmaleimide (
The MeIm system can be utilized to desymmetrize imides (
The general method of decomplexation to liberate the organic is performed with an oxidant (e.g., AgOTf) and moderate heating (25-80 °C). Utilizing this process, olefins, dihydronaphthalenes, and vinyl ethers have been isolated in moderate to high yields (65-85%).9,11
University of Virginia, Charlottesville, VA, USA