Geometric effects, carbonyl compounds

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. 

Gas phase reactivity toward allyltrimethylsilane was used to compare the reactivity of several cyclic A-acyliminium ions and related iminium ions.203 Compounds with endocyclic acyl groups were found to be more reactive than compounds with exocyclic acyl substituents. Five-membered ring compounds are somewhat more reactive than six-membered ones. The higher reactivity of the endocyclic acyl derivatives is believed to be due to geometric constraints that maximize the polar effect of the carbonyl group. 

Having reviewed the electrochemistry of homo- [10] and heterometal-sulfur clusters [11], we have directed our attention to metal-carbonyl clusters. The stereochemical effects associated with the redox behaviour of homonuclear complexes have been recently reported [13]. In this review, those of heteronu-clear compounds are discussed. Cluster compounds are classified according to their nuclearity as well as their geometrical assembly. Main-group elements at the borderline between metals and non-metals are treated as metallic elements. Occasionally, polynuclear compounds having no metal-metal bonds (and, as such, are not classifiable as clusters) are also included. 

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