Cyclophane-palladium complex

Tschierske synthesized the butterfly mesogens 132-134 (Scheme 72, Table 9). These compounds are based on a macrocyclic crown-like para-cyclophane ring containing two 2-phenylpyrimidine units. Of/to-palladation and subsequent treatment with substituted half-disks like (5-diketones gave the desired dinuclear palladium complexes with 4 (132), 8 (133), and 12 (134) peripheral alkyl chains [127]. 

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]. 

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]. 

A donor-acceptor-type Jt-stacked polymer, in which a donor )t-electron system and an acceptor jt-electron system were alternately stacked, was synthesized (Scheme 9) [74]. The treatment of cyclophane-containing monomer 33 with 34 in the presence of a catalytic amount of palladium complex and a bulky phosphine [75] afforded the corresponding donor-acceptor jt-stacked polymer 35 in good isolated yield with the of 17,000 (PDI=2.9). The structure of polymer 35 comprised compounds 36 and 37, in which flnorene and benzothiadiazole existed as donor and acceptor units, respectively. 

High-dilution coupling of l,3,5-tris(mercaptomethyl)benzene with tris[(3-bromomethyl)phenyl] phosphine oxide afforded the thiacyclophane (171) in 17% yield <88TL5I0I>. Compound (I7I) was converted into the C3 phospha[2.2.2]cyclophane (172) in three steps, but, again, diastereomeric complexes were not observed when the latter was reacted with a homochiral palladium(II) complex. 

A palladium(0)/monophosphine complex is suitable for the synthesis of para-cyclophanes in the formal [2 - - 2 - - 2] cycloaddition of bis-enynes. 

Compound 180 obtained unexpectedly along with phosphatripyrranes 175 gave corresponding cyclophanes 181 with 2,3-dihydrophosphole unit (Scheme 12.64). The interconversion between the in and out type conformers of 181 was sufficiently slow to isolate each of them. In the in-in type Pd(II) complex 182, two asymmetric cyclophane ligands coordinated to one palladium center have the same geometry and adopt partial cone conformations. The coordination mode of 181 is similar to those of 177 [158]. 

Since the phosphorus atoms of these macrocycles are all included in phosphole rings, they readily invert close to room temperature. The macrocycles 195-197 can therefore adopt their conformations to the stereochemical requirements of the complexed metals. Macrocycles 195 and 196 can chelate either one (198) (M = Mo(CO)4 [166], PdCb [167]) or two (199-200) (M = Mo(CO)4 [166]) metal-containing units via their diagonal phosphorus atoms. The structure of the cage complex 200 shows a Mo-Mo distance of 5.883 A [166]. Life-times of the palladium catalyst based on the cyclophane ligand 195 in Stille cross-coupling and Heck reactions demonstrate extraordinary resistance of the catalyst towards degradation [167]. 

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