Simple Diastereoselective Using Type II Allylmetal Reagents

2 Simple Diastereoselection Using Type II Allylmetal Reagents 

Denmark studied the intramolecular allylation reaction of allylstannane 15 in order to differentiate between the syn and anti S g transition states (only anti S h is shown below) as well as to differentiate between the synclinal and antiperiplanar transition states 19 and 20, which are analogous to transition states 11 and 14, respectively (Eq. (11.1)) [51]. Denmark found that the major product of the BF3 OEt2-promoted reaction of 15 was adduct 16, which must arise from the synclinal, anti S e transition state 19. The minor adduct 17 must ari.se through the antiperiplanar transition state 20. 

Denmark argued that the synclinal transition state 19 may be favored due to stabilization by stereoelectronic effects such as secondary orbital overlap or minimization of charge separation. The allylstannane HOMO and the aldehyde LUMO could participate in. secondary orbital overlap in transition state 19, with specific-interactions between the allylstannane a-carbon and the aldehyde oxygen [50, 55]. Alternatively, the preference for the synclinal transition state 19 can also be attributed to minimization of charge separation in the transition state, compared to the situation in the antiperiplanar transition state 20 [50, 56], 

In his analysis of transition states (Z)-ll and ( )-ll, Keck pointed out that in (Z)-ll, the a-carbon of the (Z)-crotyltri-n-butylstannane is farther from the aldehyde oxygen in transition state (Z)-ll than is the a-carbon of the ( )-crotyltri- -butylstannane in transition state ( -11, and thus is less able to participate in secondary orbital overlap interactions. The decrease in stereoelectronic stabilization of transition state (Z)-ll, compared to ( )-ll, allows access to other competing transition states which can lead to the diastereomeric anti homoallylic alcohol 3 (e.g. transition. states 13 or 14, Fig. 11-3, see above) in the reactions of the (Z)-crotylstannane reagent. Also, the (Z)-crotyltri-n-butylstannane in transition state (Z)-ll probably experiences increased steric interactions with the aldehyde R group relative to ( l)-crotyltri-n-buty]stannane in transition state ( )-ll. 

Throughout this review, we will generally invoke the Denmark-Keck synclinal transition state 11 rather than the Yamamoto antiperiplanar transition state 12 in our analysis of Type II allylation reactions, in spite of the fact that many research groups favor the antiperiplanar transition state 12 when rationalizing their results (see below). As previously noted by Keck, subtle changes in electronics, steric interactions of the reacting partners, and the Lewis acid that promotes the reaction ultimately determine whether one of the transition states, 11 or 12, is favored over the other [53]. 

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