Доступ к ценным Лиганд Опоры для переходных металлов: Модифицированная, Intermediate Scale Получение 1,2,3,4,5-Pentamethylcyclopentadiene
Summary
Reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented. The revised protocol for the synthesis and purification of the ligand minimizes the need for specialized laboratory equipment while simplifying reaction workups and product purification. Use of Cp*H in the synthesis of [Cp*MCl2]2 complexes (M = Ru, Ir) is also described.
Abstract
A reliable, intermediate scale preparation of 1,2,3,4,5-pentamethylcyclopentadiene (Cp*H) is presented, based on modifications of existing protocols that derive from initial 2-bromo-2-butene lithiation followed by acid mediated dienol cyclization. The revised synthesis and purification of the ligand avoids the use of mechanical stirring while still permitting access to significant quantities (39 g) of Cp*H in good yield (58%). The procedure offers other additional benefits, including a more controlled quench of excess lithium during the production of the intermediate heptadienols and a simplified isolation of Cp*H of sufficient purity for metallation with transition metals. The ligand was subsequently used to synthesize [Cp*MCl2]2 complexes of both iridium and ruthenium to demonstrate the utility of the Cp*H prepared and purified by our method. The procedure outlined herein affords substantial quantities of a ubiquitous ancillary ligand support used in organometallic chemistry while minimizing the need for specialized laboratory equipment, thus providing a simpler and more accessible entry point into the chemistry of 1,2,3,4,5-pentamethylcyclopentadiene.
Introduction
Since the discovery and structural elucidation of ferrocene in the 1950s,1,2,3,4 cyclopentadienyl (Cp) substituted ligands have played a vital role in the development of organometallic chemistry. These ligands have served as versatile ancillary supports for a range of metals, leading to studies of unusual structure and bonding,5,6,7 the activation and functionalization of small molecules,8,9,10,11,12,13 and catalysis, including olefin polymerization.14,15
The 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) anion has proven to be a particularly valuable ligand in transition and main group metal chemistry, as the methyl groups impart greater steric protection, increased electron donation by the anionic ligand, and block potential activation of the cyclopentadienyl ring.16,17 The Cp* ligand remains relevant even today, as the anion has recently been utilized to support H/D exchange by Ir(III),18 hydride transfer by Rh,19 and conjugate aminations mediated by Ti(III).20
Our interest in the Cp* ligand stems from the desire to access reactive sources of cobalt(I) for use in small molecule activation.21 These studies have resulted in the generation of both Cp*CoI and Cp*CoIL (L = N-heterocyclic carbene) equivalents for use in sp3 and sp2 C-H bond oxidative addition.22,23,24 As access to our Cp*Co(II) starting materials necessitate significant quantities of 1,2,3,4,5-pentamethylcyclopentadiene, we desired a multigram synthesis of Cp*H, given the substantial commercial cost of the ligand.
Two major methods currently exist for the large scale preparation of Cp*H, each of which presents inherent technical challenges. A procedure developed by Marks and coworkers involves a two-step synthesis of 2,3,4,5-tetramethylcyclopent-2-enone followed by installation of the final methyl group using methyl lithium.25 The synthesis is described on a massive scale, using a 12 L reaction vessel and mechanical stirring, while also requiring sustained low temperature cooling at 0 °C for four days.
An alternative procedure originally developed by Bercaw and coworkers,26 and later adapted by Marks,27 utilizes in situ generation of an alkenyl lithium for nucleophilic attack of ethyl acetate to produce an isomeric mixture of 3,4,5-trimethyl-2,5-heptadien-4-ols followed by acid mediated cyclization to provide Cp*H. The initial reports of this method were performed on a large (3-5 L) scale and required mechanical stirring. In addition, a significant excess of lithium metal was used, complicating quenching and subsequent workup of the intermediate heptadienols. A subsequent revision of the procedure scales down the reaction and the amount of lithium,28 but safe quenching of the reaction mixture remains an issue. Reproducibility in the initiation of the alkenyl lithium, due to differences in lithium source and purity or dryness of the 2-bromo-2-butene reactant are further noted concerns. Given these issues with the commonly used procedures for preparing Cp*H, we looked to develop better access to the ligand on an intermediate scale (30-40 g) which would circumvent use of specialty laboratory glassware and equipment, improve reaction reproducibility and safety, and simplify workup and ligand purification.
Here we report that synthesis of 1,2,3,4,5-pentamethylcyclopentadiene, based on modifications of the existing procedure developed by Bercaw and coworkers. The revised synthesis and purification of the ligand accomplishes the major goals outlined above, while permitting access to substantial amounts (39 g) of Cp*H in good yield (58%). The procedure offers other additional benefits, including a more controlled quench of excess lithium during the production of the intermediate heptadienols and a simplified isolation of Cp*H of adequate purity for subsequent metallation with transition metals. To demonstrate the utility of the prepared ligand, it was used to synthesize two [Cp*MCl2]2 (M = Ir, Ru) complexes. The revised protocol outlined below complements existing procedures and provides a simpler and more accessible entry point into the chemistry of a ubiquitous ancillary ligand support in organometallic chemistry.
Protocol
Representative Results
Discussion
During preparation of the heptadienol mixture, it is important to clean the lithium prior to initiating the reaction with the 2-bromo-2-butene. This is accomplished by wiping off residual mineral oil used for storage on paper towels, to the point that the oil appears fully removed from the surface, and by dissolving any remaining oil in the beaker of hexanes. The hexanes were used as received and not further dried before use in the procedure. Because of both the large scale of the reaction and an excess of lithium used, …
Declarações
The authors have nothing to disclose.
Acknowledgements
We are grateful to the National Science Foundation (CHE-1300508) and Mount St. Mary’s University (Startup and Summer Faculty Development) for generous support of this work. Ben Rupert (University of Delaware, Mass Spectrometry Facility) is acknowledged for LIFDI mass spectral analyses.
Materials
| Materials | |||
| Lithium wire (in mineral oil) | Aldrich | 278327-100G | >98% |
| 2-bromo-2-butene (mixture of cis/trans isomers) | Acros | 200016-364 | 98%, dried over molecular sieves from an oven overnight before use |
| Hexanes | Millipore | HX0299-3 | GR ACS, used as received |
| Ethyl actetate | Millipore | EX0240-3 | GR ACS, dried over molecular sieves from an oven overnight before use |
| Ammonium chloride | Aldrich | 213330-2.5kg | ACS Reagent |
| Diethyl ether | Millipore | EX0190-5 | GR ACS, collected from a solvent purification system before use |
| Magnesium sulfate | Aldrich | 793612-500g | Anhydrous, reagent grade |
| p-toluene sulfonic acid monohydrate | Fisher | A320-500 | ACS Certified |
| Sodium bicarbonate | Fisher | 5233-500 | ACS Certified |
| Sodium carbonate | Amresco | 0585-500g | |
| Ruthenium (III) chloride trihydrate | Pressure Chemical | 4750 | 40% Metal |
| Iridium (III) chloride hydrate | Pressure Chemical | 5730 | 53% Metal |
| Methanol | Avantor | 3016-22 | AR ACS, distilled from Mg before use |
| Pentane | J. T. Baker | T007-09 | >98%, dried with a solvent purification system before use |
| Chloroform-d | Aldrich | 151823-150G | 99.8 atom % D |
| Molecular sieves 4Å | Aldrich | 208590-1KG | dried in an oven at 140 °C before use |
| Celite 545 | Acros | AC34967-0025 | dried in an oven at 140 °C before use |
| Name | Company | Catalog Number | Comments |
| Equipment | |||
| Schlenk line, with vacuum and inert gas manifolds | Custom | NA | Used in Preps 1-4 |
| Solvent transfer manifold | Chemglass | AF-0558-01 | Used in 2.2 |
| Airfree filter funnel | Chemglass | AF-0542-22 | Used in 3.1.3 |
| Glovebox | Vacuum Atmospheres | OMNI | Used in 3.2.2 |
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