Systems containing Two Identical Heteroatoms

Irradiation of the dioxolenone (192) in an argon matrix at 8K yields carbon monoxide, carbon dioxide, and a transient intermediate regarded as dichloroketen. Lithium aluminium deuteride reduces the dioxolan (193) to the alcohol (194) in 98% enantiomeric excess.  

Colonna, L. Greci, and L. Marchetti, Colloq. Int. CNRS, 1977, 278 (Radicaux Libres Org.), 

---

From the general structure of 1,2-diheterocines 1 with identical heteroatoms other than the scries with two nitrogen atoms (X — NR, which have already been discussed in Section 1.2. as they can be regarded as 1,2-dihydro-l, 2-diazocines), only one other example of such a 1,2-di-hcterocine system exists i.e. dibenzo derivative 4 containing the 1,2-dithiocin system 2.1... 

Although most of the chelate ligands you will encounter have a carbon backbone and heteroatoms like nitrogen, oxygen or sulfur as donors, this is not essential. This can best be illustrated by comparing the two pairs of chelated systems in Figure 2.19, which have identical ring sizes and shapes, yet one of each pair contains no carbon atoms at all. 

---