Stannanes, Keck allylation reaction

Keck [89a-c], Tagliavini [89d,e], and Yu [89f] have extensively studied the BINOL-Ti- or binol-Zr promoted reactions of achiral aldehydes with allylstan-nanes. The initial studies employed BINOL and either Ti(Oi-Pr)4 or TiCl2(0/-Pr)2 as the Lewis acid promoter in the reaction of achiral aldehydes with allyltributyl-stannane. The reaction affords good yields of the desired homoallylic alcohol with a high degree of enantioselectivity even with as little as 10 mol% of the chiral catalyst (Scheme 10-49) [89a]. The rate and turnover of the catalytic, asymmetric allylation reaction have also been optimized. It was found that when /-PrSSiMe3 is added to the reaction, a rate acceleration occurs, allowing as little as 1-2% of the catalyst to be used [89 fj. 

Denmark and Keck provide important evidence of the synclinal model as the preferred open transition state, and it has been proposed that attractive forces of secondary orbital interactions of the HOMO at the site of the stannyl methylene with the LUMO of the carbonyl oxygen may introduce a stabilizing condibution in this arrangement.i Further evidence of inherent advantages in the synclinal transition state is acquired from the efforts of Nishigaichi. These results demonstrate stereospecificity in the allylation reactions of (E)- and (Z)-3,3-dialkyl-substituted allyfic stannanes with a number of simple aldehydes. 

Keck and co-workers have investigated the exo cyclizations of the allylstan-nanes (Z)-158 and ( )-159 these reactions can be promoted by both thermal and Lewis acidic conditions (Table 11-10) [54]. As illustrated in Table 11-10, the (Z)-stannane 158 preferentially forms the 1,2-syn adduct 160 under both thermal promotion and catalysis by Bp3 OEt2. In contrast, the (i )-stannane 159 cyclizes to form the, 2-anti carbocycle 161 as the major adduct in the BF -OEta-catalyzed allylation, while the, 2-syn carbocycle 160 is the major adduct in the thennji] allylation process. 

The thermal intramolecular allylation of (Z)-158 occurs spontaneously at 25 C. Conversely, the thermal intramolecular allylation of the ( )-stannane 159 required much higher temperature (110°C) for reaction to occur. Keck rationalized that (Z)-158 must react preferentially through the cyclic chair-chair transition state 162 (Fig. 11-14) to form the major adduct 160, while the minor adduct, 161, must arise through the sterically disfavored chair-boat transition state 163. The major... 

Keck and Tagliavani reported within months of each other the asymmetric ally-lation reactions with allyltri-n-butylstannane and various aldehydes with BINOL-Ti(IV) catalysts 451 and 452, respectively [289, 2901. Although the two catalysts give similar yields and enantioselectivities with a range of aldehydes, the diiso-propoxide catalyst 451 has been used more extensively. Keck and co-workers have shown that a variety of aldehydes react with allyl and methallyltri-n-butyl-stannane in modest to excellent yield and with good to excellent enantioselection using (/ )-451 as the catalyst (Table 11-25) [289, 296, 2971. 

The first reports of radical addition-fragmentation processes appeared in the synthetic organic chemistry literature in the early 1970s. Well-known examples of processes that involve a reaction step with an Sh2 mechanism include allyl transfer reactions with allyl sulfides and stannanes (the Keck... 

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