Chelation structure five-membered

Ketone 13 possesses the requisite structural features for an a-chelation-controlled carbonyl addition reaction.9-11 Treatment of 13 with 3-methyl-3-butenylmagnesium bromide leads, through the intermediacy of a five-membered chelate, to the formation of intermediate 12 together with a small amount of the C-12 epimer. The degree of stereoselectivity (ca. 50 1 in favor of the desired compound 12) exhibited in this substrate-stereocontrolled addition reaction is exceptional. It is instructive to note that sequential treatment of lactone 14 with 3-methyl-3-butenylmagnesium bromide and tert-butyldimethylsilyl chloride, followed by exposure of the resultant ketone to methylmagnesium bromide, produces the C-12 epimer of intermediate 12 with the same 50 1 stereoselectivity. 

However, EDTA has the widest general application in analysis because of its powerful complexing action and commercial availability. The spatial structure of its anion, which has six donor atoms, enables it to satisfy the coordination number of six frequently encountered among the metal ions and to form strainless five-membered rings on chelation. The resulting complexes have similar structures but differ from one another in the charge they carry. 

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

Complexes of bismuth chloride with 2 equiv of the AT,AT-substituted dithioxamide ligands of the general formulas [BiCl3 RNHC(S)C(S) NHR 2] (R = Me, Et, Pr, nBu, CH2Ph) likely involve five-membered chelate interactions (123) as demonstrated by the structurally characterized example [BiCl3 EtNHC(S)C(S)NHEt 2] Me2CO, which involves a seven-coordinate pentagonal bipyramidal environment for bismuth (124), with chlorine atoms in axial positions [Bi-Cl 2.635(10) and 2.717(1) A] and one equatorial [Bi-Cl 2.651(12) A] with four sulfur centers [Bi-S 2.818(13)-3.042(12) A],... 

In 50 the strain resulting from tridentate coordination of 48 is alleviated by the >NH bridge between the thiazole and one of the pyridine rings, creating one six-membered and one five-membered chelate ring. This is evident from the structure of the high spin fluoroborate salt [Fe 502](BF4)2.3H2O in... 

Structural Characterization. Compound 70a crystallized in the triclinic space group PI with two crystallographically independent molecules in the asymmetric unit, each with similar gross conformations, Fig. 12. The five-membered chelate ring on each molecule is essentially planar with the... 

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