Chelation steric effects

Much of what is currently known about substitution reactions of square planar complexes came from a lar e number of careful studies executed in the I960s and I970S.3 You should not conclude, however, that details of the mechanisms of these I eactions are of historical intei est only. Work in this area continues unabated as studies focus on chelation, steric effects, biological i eactions. and homogeneous catalysts. For example, the mechanism for the Wacker process (Chapter 15), which utilizes squai e planar [PdCl ] as a homogeneous catalyst for the industrial conversion of ethylene to acetaldehyde, is still a subject of investigation. The overall reaction for the process is ... 

For meta- and para-substituted aromatic carbonyl compounds, a linear relationship exists between the earbonyl absorption frequency and the Hammett reactivity constant. - - - A relationship between the carbonyl stretching vibration frequency of aromatic carbonyl compound and the pAT " of the corresponding aromatic carboxylic acid has been demonstrated. Correlations with other parameters, such as electronegativities,"" ionization potentials, Taft a values, half-wave potentials, etc., have also been made." For aromatic compounds with ortho- substituents, a combination of factors may be important, such as chelation, steric effects, and field effects (dipole interactions through space). 

R. Colton Coord. Chem. Rev. 6, Steric effects in substituted halo- 16 Complexes with chelating bis-... 

The important features of this transition structure are (1) the chelation of the methoxy group with the lithium ion, which establishes a rigid structure (2) the interaction of the lithium ion with the bromide leaving group, and (3) the steric effect of the benzyl group, which makes the underside the preferred direction of approach for the alkylating agent. 

Thus we see that steric effects, chelation, and the polar effects of a- and (3-substituents can influence the facial selectivity in aldol additions to aldehydes. These relationships provide a starting point for prediction and analysis of stereoselectivity... 

The effect of the steric bulk of the hydride reducing agent has been examined in the case of 3-benzyloxy-2-butanone.135 The ratio of chelation-controlled product increased with the steric bulk of the reductant. This is presumably due to amplification of the steric effect of the methyl group in the chelated TS as the reductant becomes more sterically demanding. In these reactions, the degree of chelation control was also enhanced by use of CH2C12 as a cosolvent. 

The results suggest that the five-membered peptide chelate ring, in which the atoms are nearly coplanar, may be more subject to steric effects than is the case for the gauche conformation of chelate rings of ethylenediamine and its C-methyl derivatives. However, the corresponding nucleophilic displacement reactions have not been studied and it is too early to draw... 

Recently, the improved chiral ethyl ketone (5)-141, derived in three steps from (5)-mandelic acid, has been evaluated in the aldol process (115). Representative condensations of the derived (Z)-boron enolates (5)-142 with aldehydes are summarized in Table 34b, It is evident from the data that the nature of the boron ligand L plays a significant role in enolate diastereoface selection in this system. It is also noteworthy that the sense of asymmetric induction noted for the boron enolate (5)-142 is opposite to that observed for the lithium enolate (5)-139a and (5>139b derived from (S)-atrolactic acid (3) and the related lithium enolate 139. A detailed interpretation of these observations in terms of transition state steric effects (cf. Scheme 20) and chelation phenomena appears to be premature at this time. Further applications of (S )- 41 and (/ )-141 as chiral propionate enolate synthons for the aldol process have appeared in a 6-deoxyerythronolide B synthesis recently disclosed by Masamune (115b). 

Conformational Analysis of Six-membered Rings (KeUie and Riddell). Conformational Analysis and Steric Effects in Metal Chelates 8 225... 

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