Ether group chelation

C. Chiral Lithium Amides with Chelating Ether Groups. 388... 

The samarium and ytterbium" species have been characterized crystallographically and were found to contain discrete anions and cations, whereas in the closely related unfunctionalized ytterbium complex Cp 2Yb(OC)Co(CO)3(THF) an Yb-OC-Co interaction (i.e., an isocarbonyl bridge) is observed. This reflects nicely the coordinative saturation effected by the intramolecular coordination in the ether-functionalized complexes. Both cations adopt Cz symmetry (approximate for Sm. crystallographically exact for Yb) and contain two chelating ether groups [Sm-0 249.6(10) and 249.9(10) pm. O-Sm-0 150.5-... 

The solid state structure of complex 7b is shown in Figure 25.1. Similar to parent chelating ether complex [9b], the solid-state structure of 7b shows a distorted square-pyramidal structure with the benzylidene moiety at the apical position. The N-aryl ring is located above the benzylidene moiety resulting in the relatively close contact of the benzylidene proton with the 7r-aromatic system of the mesityl group. 

The formation of the merocyanine form 119 can be induced by addition of heavy metal cations (Pb, La, Eu, Tb ) to a solution of a spirooxazine 118 containing a crown ether group in the B-ring (Equation 1). The chelation occurs first to the crown ether and then to the negatively charged oxygen. In contrast, 118 does not react upon addition of alkaline earth metal cations (Mg, Ca, Ba ) <2005JP0504>. 

Methoxyacetophenone chelates SnCU in much the same way. There are, however, two distinct conformations about the ether oxygen in these crystals. In one conformer, the ether group is planar with sjP -like donation to the Lewis acid, while in the second conformer the ether oxygen is better described as jp -hybridized and pseudo-tetrahedral. 

The Cram chelation model (sec. 4.7.B) is an example where the chelation effects of the heteroatom influence the rotamer population and, thereby, the selectivity of the reduction. Zinc borohydride [Zn(BH4)2], effectively chelates the carbonyl oxygen and alcohol oxygen atoms in the reduction of 42 and leads to intermediate 43. Transfer of hydride to the carbonyl gave primarily the anti diastereomer, 45 (4 96, 44/45). When the chelating hydroxyl group was blocked as a tert-butyldiphenylsilyl ether (in 46 - sec. 7.3.A.i), reduction with Red-Al (sec. 4.3) led to a reversal in selectivity (96 4, 47/48).The ability to chelate a heteroatom varies with the reagent used. Lithium aluminum hydride shows less selectivity, due in part to poorer coordination with the heteroatom and reduction of 42 gave a 27 73 mixture of 44 and 45,... 

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