Allylic and homoallylic substituents

For alkenylmetals bearing two stereocenters at both the allylic and homoallylic positions, two situations have to be distinguished depending on their relative influence (matched or mismatched) with respect to the stereochemical outcome of the allylzincation. Not surprisingly, in the matched case, as illustrated for substrate 232, the diastereoselectivity was excellent and 233 was obtained as a single diastereomer. The allyl moiety was delivered anti to both the allylic and homoallylic substituents in the chelated allyl alkenylzinc species (equation 113). 

This transition state model also accounted well for the observed diastereoselectivities when the allyl and homoallyl substituents were both part of a six-membered ring153. 

Analyze the data below concerning the effect of allylic and homoallylic benzyloxy substituents on the regio- and stereoselectivity of hydroboration-oxidation. Propose a TS that is consistent with the results. 

Chiral dirhodium(II) carboxamidates are preferred for intramolecular cyclopropanation of allylic and homoallylic diazoacetates (Eq. 2). The catalyst of choice is Rh2(MEPY)4 when R " and R are H, but Rh2(MPPIM)4 gives the highest selectivities when these substituents are alkyl or aryl. Representative examples of the applications of these catalysts are listed in Scheme 15.1 according to the cyclopropane synthesized. Use of the catalyst with mirror image chirality produces the enantiomeric cyclopropane with the same enantiomeric excess [33]. Enantioselectivities fall off to a level of 40-70% ee when n is increased beyond 2 and up to 8 (Eq. 2) [32], and in these cases the use of the chiral bisoxazoline-copper complexes is advantageous. 

A Lewis-basic substituent in the alkene can also promote addition of a Grignard reagent to a double or triple bond. Allyl, benzyl, and t-butyl Grignard reagents add readily to allylic and homoallylic alcohols and alkynols (equation 49). A magnesium alkoxide, formed initially, apparently assists intramolecularly in the addition. There appear to be multiple mechanistic pathways, with different stereochemistries. OR and NR2 groups also activate addition. 

The cases we have considered involve aryl rings as the governing structural feature for enantioselectivity. The AD systems also show excellent enantioselectivity toward functionalized alkenes, especially allylic and homoallylic systems with oxygen substituents. In these systems, another important structural variable comes into play, that is, the conformation of the allylic substituent and its possible interaction with the reaction site. ... 

Transpositional displacements. Based on the displacement of allylic carbamates alkenes containing both allylic and homoallylic silyl substituents are accessible. ... 

Ti reagents bearing heteroatom-substituted allyl residues are conveniently used in analogy with their nonsubstituted analogs to prepare homoallylic alcohols with sulfur, silicon, - phosphorus, and other -substituents. These groups can be subsequently exploited to introduce further functions. As the topic will be extensively covered in Part 1 of Volume 2, only a few salient aspects of chemo-, regio-and diastereo-selectivity of such reagents will be considered in this section. 

Using Simmons-Smith type reagents, hydroxy and alkoxy groups in the allylic or homoallylic position exhibit a very strong syn-directing effect on methylene transfer, which usually overrides the steric influence of further substituents in an alkene2,5 (see Table 2 for the transformation of simple cyclic alkenes)29 32. 

Rearrangement of dioxolanes ( )- or (Z)-34 afford, as the only observed products, the corresponding tetrahydrofurans 35, in spite of the fact that both starting dioxolanes are present as a complex mixture of diastereomers. a result which suggests that the reaction stereochemistry is independent of configuration at the allylic and/or ketal stereocenters and reflects only the initial configuration at the homoallylic center (bearing the R2 substituent)37. 

Some mechanistic experiments support the reaction mechanism shown in Scheme 5.35. Oxidative addition of Boc-protected cinnamyl alcohol is accompanied by decarboxylation to yield the (3t-cinnamyl)(tert-butoxy)nickel intermediate 28. Alkoxide exchange between 28 and homoallylic alcohol and subsequent retro-allylation would initially generate (o-allyl)(it-dnnamyl)nickel 29. The configuration of 29 would be fluxional to yield a mixture of several diallylnickel species and to lose regiospecificity. Reductive elimination would take place under steric control by the substituents or under electronic control by the phenyl group. 

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