Imidazolium cyclophanes

The two carbene units can be embedded in a (macro)cyclic ring system known as a cyclophane. A standard procedure for the synthesis of such a system starts with a,a -dibromoxylene and potassium imidazolide [368]. Cychsation can be achieved by reacting the bis-imidazole compound with a second equivalent of a,a -dibromoxylene (see Figure 3.116). The cyclic bis-imidazolium cyclophane can then be reacted with paUadium(II) acetate to form the palladium complex [369,370]. The silver(I) and gold(I) complexes are accessible from the reaction with silver(I) oxide [371] and the usual carbene transfer reaction to gold(I) [372]. 

Reaction of the bis-imidazolium cyclophane with [M(cod)Cl]j (M = Rh, Ir) in the presence of KOBu yields the corresponding cationic metal bis-carbene complex. Subsequent... 

Figure 3.116 Synthesis of a bis-imidazolium cyclophane on a xylene backbone and its...

Corrole and sapphyrin analogs are exanqples of how the pocket sizes of imidazolium cyclophanes can be altered to accommodate different sized metals into the pockets. TTie corrole analog 16 would be best suited for first row transition metals because of its decreased size. The pocket cavity of the sapphyrin analog 17 is larger and would be potentially better suited for coordinating both second and third row transition metals. The synthesis of 16 and 17 is outlined below (72). Thermal ellipsoid plots of conq)ounds 16 and 17 are shown in Figure 6 and 7, respectively. 

These cyclophane ligands - featuring two carbene and two pyridine donor groups - do act as tetradentate ligands towards transition metals. Baker et al. [67] reported the respective nickel(II) complex after reaction of the free ligand (bis-imidazolium salt) with nickel(II) bromide in the presence of sodium acetate as base. 

Imidazolium-linked ortbo-cyclophane reacts with nickel(II) and palladium(II) salts in the presence of acetate base to afford complexes where a metal centre is bound by a pair of heterocyclic carbenes, which themselves are part of a cyclophane skeleton. These cyclophane-metal complexes have been characterized by NMR spectroscopy and X-ray diffraction studies. They are highly active as promoters of Suzuki couplings [69]. 

Also, Fiirstner et al. demonstrated that oxidative addition of C2-C1 imidazolium ions to Pd(PPti3)4] generated mixed phosphine-NHC complexes. Baker described the interesting C-C oxidative addition reaction of diimida-zolium ion 8 to [Pd(PPh3)4] yielding the bis-NHC cyclophane complex 9 (Equation (3.2)). ... 

The examples of planar CILs, cyclophane-type imidazolium salts, were reported by Saigo et al. [51, 98]. Its potential use in chiral recognition was demonstrated by NMR studies. Besides the Mosher s acid salt which is widely used, enan-tiopure camphorsulfonate was also used to investigate chiral recognition between a racemic CIL and the enantiopure analyte. As an initial study on the chiral recognition ability of the cyclophane-type ionic liquid 18, the diastereomeric interaction... 

Matsuoka, Y., Ishida, Y., Sasaki, D., and Saigo, K. (2008) Cyclophane-type imidazolium salts with planar chirality as a new class of N-heterocyclic carbene precursors. Chem. Eur. J., 14, 9215-9222. 

Simons RS, Garrison JC, Kofron WG et al (2002) Synthesis and structural characterization of two bis-imidazolium-linked cyclophanes precursors toward carbeneporphyrinoid ligands. Tetrahedron Lett 43 3423-3425... 

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