Chelate ring large

The X-ray crystallographic analysis of the unsymmetrical BisP shows a strong distortion of the five-membered chelation ring as compared to that of symmetric BisP [32]. The large difference in the steric repulsions between the bulky substituent borne on one phosphorus atom and the neighboring atoms on the one hand and the other (different) bulky substituent borne on the other phosphorus atom and the same neighboring atoms on the other hand is believed to be responsible for better steric matching with some substrates. 

It is of interest that (69) does not lie completely flat on coordination to Ni(n) but assumes a conformation in which the aromatic rings point alternately above and below the N4-plane. The ruffling seems to be a reflection of strain in the respective chelate rings and may be associated with this 16-membered ring being slightly too large for Ni(n) when coordinated in a planar fashion. 

Until now, only two families of ligands have been realized where both P groups are attached to side chains, probably because the resulting metal complexes have relatively large chelate rings which usually are not suitable for enantioselective catalysis. A cursory inspection of the ligands depicted in Fig. 25.14 shows that, due to steric bulk of the ferrocene backbone, both diphosphines probably have sufficiently restricted flexibility so that good stereocontrol is still possible. 

In a similar manner, butadienyl phenylacetate 71, an achiral diene, is expected to approach the chiral dienophile (R)-10 from its Re-prochiral face. The two faces of the chelate ring are differentiated by the small hydrogen and large benzyl groups attached to the chiral center of (R)-10 (Scheme 1-18) the ratio of the Si attack product to the Re attack product is 1 8.88... 

The synthetic compounds of para. VII contain multiple groups which form five-membered chelate rings. Although the metal-oxygen distances, Tables 4 and 5, vary by amounts large compared with the standard deviations, it is clear that sodium-water distances are shorter than sodium-ether oxygen distances, even for the most flexible ligand, (XVIII). 

Kx for copper (II)-diamine complex is 10.36 and 9.45 for 1,2-ethanediamine and 1,3-propanediamine, respectively (-2)]. The large difference in the stabilities of the two copper (II)-diamine complexes is attributed to an unfavorable entropy effect associated with an increase in the size of the metal-chelate ring (2). Extrapolating to the / -ketoimine derivatives, it seems reasonable to expect that the stability of bisacetylacetonetrimethylenediiminocopper(II) would be less than that of the ethylenediamine analog and to suspect that the former compound is less stable than bis-(4-iminopentane-2-ono) copper (II). That this is reasonable is... 

The tris and bis complexes of acetylacetone (2,4-pentanedione) (167) with chromium(III) have been known for many years (168,169).739 The tris compound is generally prepared by the reaction of an aqueous suspension of anhydrous chromium(III) chloride with acetylacetone, in the presence of urea.740 Recently a novel, efficient synthesis of tris(acetylacetonato)chromium-(III) from Cr03 in acetylacetone has been reported.741 The crystal structure of the tris complex has been determined.744 A large anisotropic motion was observed for one of the chelate rings, attributed to thermal motion, rather than a slight disorder in the molecular packing. 

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