General stereochemical considerations

The book is arranged in 14 chapters. After discussing general aspects, separation of hydrocarbons from natural sources and synthesis from Ci precursors with the most recent developments for possible future applications, each chapter deals with a specific type of transformation of hydrocarbons. Involved fundamental chemistry, including reactivity and selectivity, as well as stereochemical considerations and mechanistic aspects are discussed, as are practical applications. In view of the immense literature, the coverage cannot be comprehensive and is therefore selective, reflecting the authors own experience in the field. It was attempted nevertheless to cover all major aspects with references generally until the early 1994. 

Despite mechanistic complications, however, it appears very likely that most, if not all, of the facile and synthetically attractive carbometallation reactions involve, at a critical moment, concerted addition of carbon-metal bonds where the synergistic HOMO-LUMO interactions shown in Scheme 4.3, akin to those for the concerted hydrometallation reactions, provide a plausible common mechanism. This mechanism requires the ready availability of a metal empty orbital. It also requires that addition of carbon-metal bonds be strictly syn, as has generally been observed. Perhaps more important in the present discussion is that concerted syn carbometallation must proceed via a transition state in which a carbon-metal bond and a carbon-carbon bond become coplanar. Under such constraints, one can readily see how chirally discriminated carbon-metal bonds can select either re or si face of alkenes. In principle, the mechanistic and stereochemical considerations presented above are essentially the same as for related concerted syn hydrometalla-tion. In reality, however, carbometallation is generally less facile than the corresponding hydrometallation, which may be largely attributable to more demanding steric and... 

While the mechanism of the reaction has been clarified in accordance with the scheme outlined in Eq. 89, its stereochemistry is not known in every respect. The sulfoxonium ylides are more stable and behave as better leaving groups than the sulfonium ylides. Choice of the reagent is governed by stereochemical considerations, because their stereoselectivities differ. In general, 77 attacks from... 

Generally the Wittig reaction yields a mixture of cis and trans geometrical isomers of an alkene. The stereochemical considerations are discussed elsewhere. 

Some general considerations governing the nature of selective enantiomeric interactions for both gas and liquid chromatographic phases (at least of the bonded monomeric ligand type) have been forthcoming [721,742,754,756,781,782,790). It is generally assumed that three points of simultaneous interaction at least one of which must be stereochemically controlled, are required to distinguish the chirality of a molecule. These... 

Thus solvolysis of (+)C6HsCHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (+)C6H13CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of ( + )C6H5CHMeCl in 80 % acetone/20 % water leads to 98 % racemisation (above), but in the more nucleophilic water alone to only 80% racemisation. The same general considerations apply to nucleophilic displacement reactions by Nu as to solvolysis, except that R may persist a little further along the sequence because part at least of the solvent envelope has to be stripped away before Nu can get at R . It is important to notice that racemisation is clearly very much less of a stereochemical requirement for S l reactions than inversion was for SN2. 

The major obstacle confronting the implementation of such biomimetic syntheses has been associated with the stereochemical aspects of the aldol process. Over the past few years considerable progress has been made in the development of stereoregulated aldol condensations. This chapter attempts to survey this aspect of the topic. For a more general treatment of the subject the reader is referred to several other excellent reviews (1). 

The successful mechanism for a reaction is a theory that correlates the many facts which have been discovered and is fruitful for the prediction of new experiments (1). One approach to mechanism is the study of stereochemistry which seeks information concerning the geometrical relationships between the reactants at the critical stages in the reaction. Information is gleaned from the examination of the products, if several isomers differing only in configuration may be formed, or from a study of the reactivity of closely related substances whose molecular shapes are varied in a specific manner. Occasionally a stereochemical fact places a considerable restraint upon the allowable mechanistic postulates, but the most effective employment of stereochemistry generally depends upon its detailed correlation with other experimental methods. 

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