Carbonyl oxygen donors

The overall structure found first in the (nonactin)KNCS complex552 is retained in changing the metal present. The ligand wraps itself around the cation as the seam of a tennis ball (Figure 30) and the cation is thence eight-fold coordinated by the four furanyl and four carbonyl oxygen donor atoms. The periphery of the molecule is lipophilic and the lowered lipophilicity of nonactin relative to tetranactin leads to a diminished transfer of these compounds. 

Note The cationic palladium(II) carbene complex prefers the weak donor acetonitrile over the chelate carbonyl oxygen donor. 

Step 1 The carboxylic acid is protonated on its carbonyl oxygen The proton donor shown in the equation for this step is an aUtyloxonium ion formed by proton transfer from the acid catalyst to the alcohol... 

The relative basicity of carbonyl oxygen atoms can be measured by studying strength of hydrogen bonding between the carbonyl compound and a hydrogen donor such as phenol. In carbon tetrachloride, values of for 1 1 complex formation for the compounds shown have been measured. Rationalize the observed order of basicity. 

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

Chelation Control. The stereoselectivity of reduction of carbonyl groups can be controlled by chelation when there is a nearby donor substituent. In the presence of such a group, specific complexation among the substituent, the carbonyl oxygen, and the Lewis acid can establish a preferred conformation for the reactant. Usually hydride is then delivered from the less sterically hindered face of the chelate so the hydroxy group is anti to the chelating substituent. 

Fig. 13. The binding sites of calcium in (a) parvalbumin (41a), (b) annexin (41) and (c) calmodulin (42). The drawings show two bidentate carboxylates coordinated to Ca2 in the EF-hand site of parvalbumin, and one bidentate carboxylate coordinated to Ca2 in annexin and calmodulin. All the donor atoms coordinated to the calciums are oxygen donor atoms from carboxylates of asp = aspartate, or glu = glutamate, or else peptide carbonyl oxygens from gly = glycine or met = methionine. Redrawn after Refs. (41-42).

When the steric size of the substituents increases, no adducts with bulky (Et3N) or less basic (EtzO) donors are formed with (Me3C)2Si= NSi(CMe3)3.14 The structure of the benzophenone adduct is quite remarkable.16 Instead of the expected cycloaddition product, the carbonyl oxygen atom is coordinated to the undersaturated silicon atom. 

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