Lewis Acid-Catalyzed Allylation Reaction

13 Chiral Lewis Acid-Catalyzed Allylation Reaction 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

The BINOL/BINAP Lewis acid complexes and the CAB catalyst are complementary in the following respects in general, the BINOL/BINAP-Lewis acid complexes provide excellent enantiocontrol in the reactions of aldehydes with allyltri-n-butylstannane, but poor diastereocontrol (syn anti) in the reactions of aldehydes with crotyltri- -butylstannane. In contrast, when the CAB catalyst is used to promote the reaction of aldehydes and crotylsilane or crotylstannane reagents, excellent levels of diastereo- and enantioselectivity are achieved, while in the corresponding reactions with allyltri-n-butylstannane poor levels of enantioselectivity are realized. 

Five highly selective Lewis acid catalysts containing (5)-BINOL or (S)-BINAP ligands are shown in Fig. 11-38 [289-293]. In addition to being fairly simple to 

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. 

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Lewis acid-catalyzed allylation reactions, the syn anti ratio observed here did not depend on the E Z ratio of the starting stannane.4 Although the reaction mechanism was not clear, this result would suggest that the reaction took place via transmetalla-tion from stannane to silver to generate an allylsilver species. 

Charette and Brochu have reported an alternate protocol for the Lewis acid-catalyzed cyclopropanation reaction of allylic alcohols, in which the uncatalyzed process is suppressed . The addition of Zn(CH2l)2 (1 equivalent) to an allylic alcohol (1 equivalent)... 

Substitution of sulfonesJ Organoaluminum reagents, particularly vinyl- or alkynylalanes, undergo Lewis acid catalyzed substitution reactions with sulfones. Thus the vinylalane 2 reacts with the allylic sulfone 1 in the presence of A1C13 to... 

The reactive species in Lewis acid catalyzed IMDA reactions of a,3-unsaturated ester dienophiles is formally a Lewis acid stabilized dioxolenium ion. It stands to reason that other allylic species bearing positive charges, such as dioxolenium ion (22) and allyl cations (23), should also function as highly reac-... 

Charette and Brochu have reported an alternative protocol for the Lewis acid-catalyzed cyclopropanation reaction of allylic alcohols, in which the uncatalyzed process is minimized [21]. The addition of Zn(CH2l)2 (1 equiv) to an allylic alcohol (1 equiv) produced the iodomethylzinc alkoxide (Scheme 9). Methylene transfer from these less reactive species is triggered by the addition of a Lewis acid in catalytic amounts. Several achiral Lewis acids such as TiC, SnCl4, and BBr3 were effective in inducing the cyclopropanation. 

Previously the author reported that an allylic nucleophilic species is generated readily from trimethylallylsilane by the catalysis with fluoride ions. The reaction provides a versatile method of introducing an allyl group into carbonyl compounds.However, the exclusive regiospecificity of allylation, which is characteristic of the Lewis acid-catalyzed allylation with allylsilanes, disappeared. 

Essentially all allylsilanes (M = SiR3, Section D.l.3.3.3.5.) with the exception of fluorosil-iconates11 and most of the trialkyl(allyl)stannancs (Section D.l. 3.3.3.6.), which have only very weak Lewis acidic properties, require a strong Lewis acid to trigger the reaction with a carbonyl compound by the preceding formation of an x-oxycarbenium ion, which attacks the allylic compound in an ionic open-chain pathway. These Lewis acid catalyzed carbonyl additions offer new possibilities for the control of the simple and induced diastereoselectivity12. 

As with the silanes, the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C-Sn bond and it is a weaker bond.156 The most useful reactions in terms of syntheses involve the Lewis acid-catalyzed addition of allylic stannanes to aldehydes.157 The reaction occurs with allylic transposition. 

A novel latent-active glycosylation strategy was reported. This strategy is based on a rhodium-catalyzed isomerization of substituted allyl glycosides followed by a Lewis acid-mediated glycosylation reaction (Scheme 46).76 77... 

Burger2 has shown that alkynes undergo both Lewis acid-catalyzed and thermal carbonyl-yne reactions with 3,3,3-trifluoropyruvates to give allenes. Reaction of 1 (Equation (2)) occurs to give a 1 1 mixture of diastereomeric allenyl carbinols 2. Alternatively, reaction of hexyne 1 and methyl trifluoropyruvate with MgBr2-OEt2 at low temperature afforded 2 as an 8 1 mixture of diastereomers. The thermal reaction does not suffer from allylic alcohol byproducts arising from reaction of the substrate with the Lewis acid.3... 

The Lewis acid catalyzed reactions expand the scope of aldehyde substrates with certain boronate reagents. For example, whereas cyclohexanecarboxalde-hyde is unreactive nnder thermal (nncatalyzed) conditions, it does react with allylic boronate 8 nnder Sc(OTf)3 catalysis at room temperature (Eq. 53, see also Eq. 6).26... 

Scheme 9.6 gives some other examples of Lewis acid-catalyzed reactions of allylic stannanes with carbonyl compounds. 

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