Stereoselectivity chair-like transition structures

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

The reaction of highly strained allylsilacyclobutanes with aldehydes has recently been developed to produce homoallylic alcohols with a high degree of re-gio- and stereoselectivity (Scheme 10-36) [63]. These species are structurally akin to the allyltrialkylsilanes, but are more mechanistically aligned with the allyltri-halosilanes. The -2-butenylsilacyclobutane upon reaction with an aldehyde at elevated temperature will produce almost exclusively the anti homoallylic alcohol. When the Z-2-butenylsilacyclobutane is used instead, the syn homoallylic alcohol is obtained. The mechanism proposed for the reaction involves the association of aldehyde and allylsilacyclobutane to form an activated pentacoordinate silicon complex. A closed, chair-like transition structure is proposed to account for the observed stereoselectivity in the reaction (Scheme 10-36k A theoretical examina-... 

For an explanation of the stereoselectivity, the chair-like transition structures (52A and 52B) in which the mercury is chelated by the enolate o tygen and the aldehydic o tygen have been proposed by Machado et al. and Perrier and Middleton fScheme 12.141. This transition model would account for the observed stereoselectivity concerning the newly formed hydroxy group at C-5. In the transformation of 14 15a, while the transition structure 52A giving 15a has no severe steric repulsion, that of 52B giving the 5-epimer of 15a suffers 1,3-diaxial interactions. In the reaction of 53 45b, the favored transition structure would be 54B, giving 3,5-trans-isomer 45b as the major product... 

Chiral aldehydes react with the allylborane reagent, affording homoallylic aldehydes in high stereoselectivity, via a putative chair-like transition structure. Substituted allyl groups, including 2-haloallyl groups, can also be used to produce a wide array of products. 

In each case, stereoselectivity can be explained by assuming that the reaction proceeds through a chair-like transition state, in which there are interactions between the two ends of the C-Li and C=C bonds, and in which the substituents all occupy pseudo-equatorial positions (structures 338, 339 and 340). The same transition state model suffices to explain the stereoselectivities of the furan and pyrrolidine forming reactions above. Stereoselectivity in the cyclisations of the selenide-derived tertiary organolithiums would arise from a conformation with a precedented pseudo-axial phenyl ring. 

Rearrangement from the other chair-like conformation available to 21, 23 and 25 (not shown in CJH-3) results in formation of the minor products (Z-geometrical isomers). Provide 3-dimensional structures of these transition geometries (as in CJH-3) and indicate the steric interaction that is responsible for the increased stereoselectivity in the reactions of 23 and 25 relative to 21. (CJH-3)... 

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