Chemistry and catalysis advances in organometallic chemistry and catalysis
, 2000. 20. (a) Bonati, F.; Burini, A.; Pietroni, B. R.; Bovio, B. J. Organomet. Chem. 1989
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10 CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL: CATALYSTS FOR REDUCTIONS Beatriz Royo Instituto de Tecnologia Qu´ımica e Biol´ogica, Universidade Nova de Lisboa, Oeiras, Portugal 10.1 INTRODUCTION N-heterocyclic carbenes (NHCs), singlet carbenes with the carbene being incorporated in a nitrogen-containing heterocyclic, have emerged as a new class of versatile ancillary ligands in organometallic chemistry [1–4]. Their powerful electron- donating properties, strong binding energies, and specific topology are key features that make NHCs very attractive ligands. The easy access to NHCs and their potential application in a large number of homogeneously catalyzed processes has led to a rapid development in the design of new NHC-containing ligands with different topologies [5–8]. Now, we can find in the literature a large number of examples in which NHCs are incorporated into chelating [9], pincer [10, 11], and chiral [12, 13] architectures. Polydentate NHC ligands in which the carbene is bound to a neutral or an anionic donor by an organic linker are under constant development, because they can offer stability and fine-tuning of the stereoelectronic properties of their metal complexes [14]. It has been demonstrated that NHC ligands are compatible with a large set of functionalities including pyridines, phophines, oxazolines, amino, ethers, alkoxo, amides, and other donor groups [14–17]. Recently, cyclopentadienyls tethered to NHC ligands (Cp-NHCs) have attracted the interest of some research groups [18–35]. The cyclopentadienyl (Cp = C 5
5 ) ligand is probably the most popular spectator ligand in organometallic chemistry. The Cp, an electron donor ligand of five electrons, binds very strongly to transition metal centers, predominantly in a η 5 manner and its steric and electronic properties can be easily modified by introducing appropriate substituents on the five-membered ring [36, 37]. Cyclopentadienyl ligands with pendant donors such as phosphines and amino groups have been the subject of intensive research [38–41]. Cp-functionalized N-heterocyclic ligands remained elusive until the work reported by Danopoulos and coworkers in 2006, in which two indenyl- and fluorenyl-NHC-functionalized ligands were reported [18]. Two years later, our group described the synthesis of the first tetramethylcyclopentadienyl tethered to an NHC (Cp*-NHC) (Cp* = η
5 -C 5 Me 4 ) [27]. The introduction of chelating Cp-NHC ligands in the coordination sphere of metal complexes has some interesting consequences because they can increase the thermal stability of their metal complexes and favor the rigidity required for the preparation of effective catalysts. The possibility to independently vary their structural components, the cyclopentadienyl ring, the spacer, and the azole unit offers an enormous synthetic flexibility and allows the fine-tuning of the steric and electronic properties of the metal center. In addition, the indirect tuning of metal-NHC bonding by the adjustment of the length of the tether may have important consequences in catalyst design. Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book, First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
134 CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE COMPLEXES OF IRON AND NICKEL M N
Y R (a) (b) M N N R
Chelation introduced through the Cp-NHC unit (a) or through a bidentate NHC-Y ligand (b). An important advantage of the Cp tethered to NHC ligands compared to the nonlinked systems is that chelation does not consume an “extra” coordination site, leaving the metal complexes with an additional site for catalysis (Fig. 10.1). As we have already pointed out, the tethered NHC unit could not only increase the stability of the half-sandwich metal–NHC complexes by chelation, but also could enhance the catalytic activity of their metal complexes by the stabilization of intermediate active species. In addition, the hemilabile dynamic behavior of the Cp-NHC ligand may allow to efficiently control the reactivity and stability of the catalytically active center. An interesting feature of these bidentate ligands is that the introduction of chirality in the linker between the Cp and the NHC units can help to control the stereochemistry of reactions taking place at the metal center and eventually can increase the stereoselection in asymmetric processes. In this line, half-sandwich complexes bearing chiral Cp-NHC ligands may offer a good opportunity to design organometallic chiral-at-metal complexes, as the linked Cp-NHC system could assists in controlling the metal configuration. This approach has already afforded some degrees of success in metal half-sandwich complexes bearing cyclopentadienyls tethered to chiral phosphines [42]. In addition, the different strength of the metal–NHC bond across the periodic table may lead to versatile coordination chemistry. 10.2 PREPARATION OF CYCLOPENTADIENYL-FUNCTIONALIZED N-HETEROCYCLIC CARBENE LIGANDS Several synthetic pathways are used for the synthesis of NHCs tethered to cyclopentadienyl ligands. Each of the available procedures presents both advantages and limitations. A convenient and flexible entry to tetramethylcyclopentadienyl- functionalized NHC ligands is by deprotonation at the methylene group of benzylimidazole with n-BuLi, followed by the reaction with 1,2,3,4-tetramethylfulvene, and treatment with methanol. Subsequent reaction with iodomethane affords the corresponding tetramethylcyclopentadienyl-functionalized imidazolium salts as a mixture of three isomers resulting from the different position of the double bonds in the cyclopentadienyl ring (Fig. 10.2). However, this fact does not interfere with the coordination to a metal center [27]. An advantage of this pathway is that it permits the introduction of different substituents on the cyclopentadienyl ring and at the carbon adjacent to the Cp unit by choosing the appropriate fulvenes. Following this synthetic procedure, differently substituted cyclopentadienyl-functionalized NHCs, Cp
-NHCs [Cp x = Cp (η
5 -C 5 H 4 ); Cp* ( η 5 -C 5 Me 4 ); Cp Bz ( η 5 -C 5 (CH
2 Ph)
4 ] can be prepared (Fig. 10.3) [29]. In addition, this synthetic approach introduces a stereogenic center at the linker between the Cp and the NHC units, affording a chiral ligand, although racemic mixtures of the two possible enantiomers of the final pro-ligands are isolated. However, it does not allow preparing pure chiral ligands. Starting from enantiomerically pure S-1-(phenylethyl)imidazole and following the synthetic procedure previously described, the final imidazolium pro-ligand is obtained as a racemate, probably deprotonation of S-1-(phenylethyl)imidazole with n-BuLi produces racemization of the imidazolyl intermediate [28]. (i) n-BuLi N N
N N H N N H Me I MeI mixture of 3 isomers Figure 10.2 Synthesis of tetramethylcyclopentadienyl-functionalized imidazolium salts. PREPARATION OF CYCLOPENTADIENYL-FUNCTIONALIZED N -HETEROCYCLIC CARBENE LIGANDS 135 N N Me I H N N Me (a) (b)
(c) I H N N Me I H Ph Ph Ph Ph Cp * Cp Cp Bz
Unsubstituted and substituted cyclopentadienyl-functionalized imidazolium pro-ligands. NH 2 H O H O O H H N N Me (i) (NH
4 ) 2 CO 3 ,MeOH (ii) MeI I
One pot synthesis of tetramethylcyclopentadienyl-functionalized imidazolium iodide. An alternative method for the preparation of tetramethylcyclopentadiene imidazolium pro-ligand is the one pot synthesis route, starting from the easily accessible 2-(2,3,4-tetramethylcyclopentadienyl)-ethylamine [43] by condensation with glyoxal and formaldehyde. Further quaternization with iodomethane yields the corresponding imidazolium iodide (Fig. 10.4) [28]. Isomerization of the exocyclic double bond of the linker chain to give the endocyclic double bond isomers has to be addressed before coordination to a metal center. However, it has been noticed that late transition metals mediate isomerization of the exocyclic bond forming the η 5 -cyclopentadienyl fragment in the presence of acetic acid [28]. The advantage of this protocol as compared to the synthetic approach described earlier is that the one pot reaction can be performed in large scale, and the final product is purified by a simple crystallization without further need of column chromatography, which is required in the purification of the 1-alkylimidazoles described earlier. This synthesis is restricted to the preparation of tetramethylcyclopentadienyl-functionalized imidazolium salts. The previously described procedures are limited to the introduction of a primary alkyl on the nitrogen atom of the imidazole ring. When substituents other than primary alkyls are required on the nitrogen atom, the appropriate secondary and tertiary 1-alkylimidazoles have to be reacted with the Cp ligand carrying an alkyl halide linker group. This synthetic approach has been used to synthesize indenyl- and fluorenyl-functionalized NHCs (Fig. 10.5) [18]. The length of the linker can be easily modified using this synthetic approach, and in fact the corresponding three carbon side chain was introduced by the reaction of 3-bromo-propylindene with the corresponding 1-alkyl imidazole [19, 22]. However, this synthetic method cannot be applied for the synthesis of substituted cyclopentadienyl-functionalized imidazolium salts. Direct alkylation of 1,2,3,4-tetraalkylcyclopentadienides with alkyl halides or alkyl toluenesulfonates has no use in the preparation of 5-alkyl- 1,2,3,4-tetraalkylcyclopentadienides because of problems with regioselectivity [44]. An inseparable mixture of nongeminal (Fig. 10.6a) and geminal-substituted cyclopentadienides (Fig. 10.6b and c) is often obtained. Enantiomerically pure Cp-functionalized NHC ligands are synthesized starting from readily available, enantiomerically pure amino acids, such as l-valinol. The corresponding imidazole carrying a pendant alcohol is generated following Br N N R Br N N R (i) n-BuLi (ii)
Br Br
Synthesis of indenyl- and fluorenyl-functionalized imidazolium pro-ligands.
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