Metal-containing Chiral Macrocycles

Stang and coworkers have also reported the synthesis of the related mixed iodonium-transition metal squares 189-192 [74]. Unlike the previous examples, the pyridyl ligands used possess rotational symmetry and thus, the squares 189-192 are chiral solely due to the presence of the chiral (R)-BINAP ligands. 

Changing the position of the pyridyl component on the binaphthalene core provided yet another geometric variation of this theme, and Lin and coworkers have shown how such a strategy can lead to a range of self-assembled macrocycles of variable size and substitution such as 225-230 [80]. All macrocycles could be produced with either Pd(ii) or Pt(ii) linkages in good to excellent yield ( 74%, in all cases) and all are highly luminescent in solution. 

X-ray crystallographic analysis of racemic 235 confirmed the helical structure and showed a large dihedral angle of 100° between the planes of the naphthyl rings (Fig. 6.10 (a)). The metal-free analog 236 was produced by treating 235 with benzoic acid, and it was also characterized crystallographically (Fig. 6.10 (b)). 

One of the most common methods used to install transition metal centers within carbon-rich structures takes advantage of the easily constructed metal-acetylide bond. In addition to their facile preparation, insertion of a metal center into an unsaturated material can introduce a range of exciting properties not pres- 

Using analogous synthetic conditions, Lin and coworkers have also reported the synthesis of dimeric metallacyclophanes 248-250 in which the acetylenes are installed at the 6,6 -positions (Fig. 6.11) [87]. As with compound 243, the free hydroxy-containing macrocycle 250 was not directly accessible, but was instead formed via the hydrolysis of compound 249. Macrocycle 250 was evaluated as a ligand for the asymmetric addition of diethylzinc to aromatic aldehydes. It was 

---

In more recent syntheses of chiral macrocycles containing pyridine units like 48, which are of interest in their reduced dihydropyridine-form as model compounds for the redox system NADH/NAD , the yield increasing effect of added cesium salts was also used [52]. The yield of 48 was doubled in the cyclization of bis-L-valine amide 47 with l,5-dibromoi ntane in DMF from 41 % (Na2C03) to 80% by application of CS2CO3. The results of cyclizations with several other alkali metal carbonates [52] are shown below. 

Despite the overall decline in output, the year has produced ftirther interesting developments in the fteld of hypervalent phosphorus chemistry. These include phosphoranes containing acetylenic links, the synthesis and enantiomeric separation of a bicyclic phosphorane with chirality only at phosphorus, and a wide range of phosphoranes incorporated within macrocyclic ring systems (Houlla et al). In addition Lattman has reported on phosphoranes bound to transition metals or enclosed within calixarene skeletons and Holmes and his co-workers have made further substantial contributions on the X-ray crystal structures of phosphoranes containing eight-membered rings. 

---