Synthesis and Structure of an Enantiomerically Pure C2 Symmetric Ferrocenyl Carbene

Abstract

The straightforward, high-yield synthesis and X-ray structural analysis of the air-stable planar-chiral bis(ferrocenyl)carbene 1,3-bis-{(1R)-1-[(1R)-1-(trimethylsilyl)ferrocen-2-yl]ethyl}imidazol-2-ylidene (5) is reported. Compound 5 is obtained in four steps from the amine [(1R)-1-(dimethylamino)ethyl]ferrocene (1) upon diastereoselective silylation, methylation, nucleophilic substitution by imidazole, and deprotonation. The X-ray crystal structure of the free carbene shows the typical conformational features of the 1,2-disubstituted ferrocenyl units, as found in other ferrocenyl ligands derived from 1. 1. Introduction. ± Stable N-heterocyclic carbenes (NHCs) [1] [2], mostly bearing bulky substituents at the N-atoms, are becoming ubiquitous ligands in organometallic chemistry [3] and homogeneous catalysis [4 ± 11] as phosphine substitutes [12]. However, chiral enantiomerically pure derivatives are still quite rare, and only a very limited number of applications have been reported [13 ± 17]. Furthermore, chiral ferrocenyl derivatives have proven very successful in asymmetric catalysis, as ligands often displaying both central and planar chirality, because of their great synthetic versatility [18]. These two observations prompted us to develop a simple synthesis of a ferrocenebased carbene for use as a monodentate ligand in asymmetric catalysis. We reasoned that such a preparation should start from a readily available chiral ferrocene derivative that would also be set up for a modular construction of possibly a range of related compounds. Most sensibly, this starting material should be the amine 1 ([(1R)-1-(dimethylamino)ethyl]ferrocene, Scheme) because of both its typical diastereoselective ortho-lithiation chemistry and the ease of nucleophilic replacement of the amino group by a variety of nucleophiles. Very recently, Bolm et al. reported a more elaborate preparation of a monoferrocenyl carbene starting from a chiral ferrocenyl sulfoxide [19].

Solvent-controlled selective synthesis of a trans-configured benzimidazoline-2-ylidene palladium(II) complex and investigations of its Heck-type catalytic activity

“…It has been reported that carbenes derived from benzimidazolium precursors exhibit the topology of an unsaturated N-heterocyclic carbene, but show spectroscopic and structural properties and the reactivity of carbenes with a saturated N-heterocyclic ring [3]. Previously, several examples of palladium(II) carbene complexes derived from benzimidazolium precursors with achiral [4] and chiral [5] alkyl groups or even the ferrocene moiety [6] adjacent to the nitrogen atoms have been reported. However, their potential application as phosphine free precatalysts remains relatively unexplored [7].…”

Synthesis of a new bidentate ferrocenyl N-heterocyclic carbene ligand precursor and the palladium (II) complex trans-[PdCl2(C∧fc∧C)], where (C∧fc∧C)=1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene

“…This ferrocene imidazolium salt has been subsequently shown to act as an anion receptor [17,18]. Similarly, the general structure of the ferrocene substituted imidazolium salt has also been used to prepare chiral carbene ligands [19][20][21], Fig. 1(b) and (c).…”

Retro‐Brook Rearrangement of Ferrocene‐Derived Silyl Ethers

“…It has been documented that nucleophilic substitutions on the α‐carbon atom in ferrocenyl compounds proceed with retention of the configuration of the stereogenic center 27. The analysis of the 1 H NMR spectra revealed that our diastereoisomer 12 [ δ =0.29 (TMS), 1.18 (CH 3 ), 2.36 (NMe 2 ), 3.34 (C α H), 4.12 ppm (Cp)] differs from compound ( R , S p )‐ 12 [ δ =0.26 (TMS), 1.22 (CH 3 ), 2.04 (NMe 2 ), 3.82 (C α H), 4.07 ppm (Cp)] reported in the literature 26b. Signs and values of the optical rotation match literature data (see the Experimental Section for details).…”