Carbonyl compounds conformational stability

The predominant conformation of a,P-unsaturated carbonyl compounds (aldehydes, ketones, esters) can vary when Lewis adds are coordinated to the carbonyl group, ft has been experimentally shown that the s-trans conformer of acrolein is more stable than the s-ds conformer, in the absence of any Lewis add. The two conformers are of almost equal stability in the case of acrylic acid or methyl acrylate 83] (Figure L21). The barriers between these conformers are low (4-9 kcal mol ). Interconversion is rapid, and the Curtin-Hammett principle ( 1.3) can be applied. 

The stereochemistry of the reactions of chiral carbonyl compounds with nucleophiles has been a topic of considerable theoretical and synthetic interest since the pioneering study by Cram appeared in 1952. The available predictive models focus entirely on the conformational and stereoelectronic demands of the chiral carbonyl substrate, the implicit assumption being that the relative stabilities of the competing transition states are determined only by stereoelectronics and the minimization of nonbonded interactions between the substituents on the chiral center and the nucleophile. These models totally ignore the possibility, however, that the geometric requirements of the nucleophile may also have an effect on reaction diastereoselectivity. Considerable evidence is now available, particularly in the reactions of Type I (Z)-crotylboronates and Z(0)-metal enolates, that the stereochemistry of the nucleophile is indeed an important issue that must be considered when assessing reaction diastereoselectivity. 

The excited states of other saturated carbonyl compounds are similar. Alkyl groups somewhat stabilize the excited state and reduce the excitation energy. In acetaldehyde, the conformation of the state changes from the H-O eclipsed structure in the... 

The expanding application of computational chemistry is reflected by amplified discussion of this area, especially density function theory (DFT) calculations in Chapter 1. Examples of computational studies are included in subsequent chapters that deal with specific structures, reactions and properties. Chapter 2 discusses the principles of both configuration and conformation, which were previously treated in two separate chapters. The current emphasis on enantioselectivity, including development of many enantioselective catalysts, prompted the expansion of the section on stereoselective reactions to include examples of enantioselective reactions. Chapter 3, which covers the application of thermodynamics and kinetics to organic chemistry, has been reorganized to place emphasis on structural effects on stability and reactivity. This chapter lays the groundwork for later chapters by considering stability effects on carbocations, carbanions, radicals, and carbonyl compounds. 

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