Allyltributyltin, enantioselective addition aldehydes

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

A similar chiral silver(I) catalyst 21 was applied to the asymmetric addition of allyltributyltin to various aldehydes in an aqueous medium [28]. Shi et al. have shown that chiral silver complex 22, prepared from chiral bidentate phospho-ramide and AgOTf, is also an effective chiral catalyst for the allylation [29]. Chiral bis(oxazoline) ligands have found widespread use in asymmetric reactions catalyzed by chiral metal complexes, and C2-symmetric chiral bis(oxazoline)-Zn(OTf)2 complex 23 has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (44) however, satisfactory enantioselectivity was not observed [30]. 

Keck almost simultaneously reported two procedures using chiral titanium catalysts 6A and 6B for the enantioselective addition of allyltributyltin to aldehydes [11]. In the first procedure, the catalyst 6A is prepared from a 1 1 mixture of (R)-binaphthol and titanium tetraisopropoxide. The second procedure for the preparation of 6B, in contrast, requires a 2 1 mixture of BINOL, Ti(0 Pr)4, and a catalytic amount of CF3SO3H or CF3CO2H. Using 10 mol % of the catalyst 6A or 6B, a variety of aromatic, aliphatic, and a,P-unsaturated aldehydes are efficiently transformed into the corresponding optically active homoallylic alcohols with high enantioselectivity. An improved procedure was later published for the catalytic asymmetric allylation reactions using the 2 1 BINOL/Ti catalytic system [12]. 

We found that a BINAP silver(I) complex also catalyzes the asymmetric al-lylation of aldehydes with allylic stannanes, and high y-, anti-, and enantioselec-tivities are obtained by this method [31,32,33]. The chiral phosphine-silver(I) catalyst can be prepared simply by stirring an equimolar mixture of chiral phosphine and silver(I) compound in THF at room temperature. Scheme 10 shows the results obtained by the reaction of a variety of aldehydes with allyltributyltin (33) under the influence of 5 to 20 mol % of (S)-BINAP silver(I) triflate (enf-12) in THF at -20 °C [31]. The reaction furnishes high yields and remarkable enan-tioselectivities not only with aromatic aldehydes but also with a,P-unsaturated aldehydes, with the exception of an aliphatic aldehyde which gives a lower chemical yield. In the reaction with a,P-unsatuxated aldehydes, 1,2-addition takes place exclusively. Enantioselective addition of methallyltributylstaimane to aldehydes can also be achieved using this method [31,32]. 

A spectacular activation of the chiral zirconium-BINOL Lewis acid complex was achieved by the addition of the (achiral ) r-butyl-calix[4]arene. Less than 2% of the catalyst were sufficient in the enantioselective allylation of various aldehydes by allyltributyltin to reach enantiomeric excesses of more than 90%, see Casolari, S. Cozzi, P. G. Orioli, P. Tagliavini, E. Umani-Ronchi, A. Chem. Commun. 1997, 2123-2124. 

Very recently, a limited survey of the CAB and Keck BINOL methodology with crotyltributyltin was conducted by Marshall and Palovich (Table 3) [50b]. A modified CAB, prepared from the 2,6-dimethoxybenzoic ester of (l ,jR)-tartaric acid, and 1.5 equiv. BH3-THF was used in the addition of crotyltributyltin and allyltributyltin to representative achiral aldehydes in the presence of 2 equiv. (Cp3C0)20. Addition to crotyltin proceeded with good to excellent diastereoselectivity and enantioselectivity to give syn adducts in 70-93 % ee as major products (78 22-92 8). The addition of allylstannane to cyclohexanecarboxaldehyde afforded the (R) adduct in 55 % ee. In contrast, the use of Keck s BINOL catalyst gave an allyl adduct in 87 % ee. Addition of crotylstannane to cyclohexanecarboxaldehyde with this catalyst led, however, to a 65 35 mixture of syn and anti adducts 43 (R = Me) and 44 (R = Me) in 95 % and 49 % ee. 

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