Allylation of aldehydes with allyltributyltin

A chiral dinuclear Ti(IV) oxide 20 has been successfully designed by Maruoka and coworkers and can be used for the strong activation of aldehydes, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin (Scheme 12.18). ° The chiral catalyst 20 can be readily prepared either by treatment of bis(triisopropoxy)titanium oxide [(/-Pr0)3Ti-0-Ti(0/-Pr)3] with (S)-BINOL or by the reaction of ((5)-binaphthoxy)isopropoxytitanium chloride with silver(I) oxide. The reaction of 3-phenylpropanal with allyltributyltin (1.1 equiv) under the influence of 20 (10 mol%) gives l-phenyl-5-hexen-3-ol... 

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]. 

In the Lewis acid-promoted allylation of aldehydes with allyltributyltin, virtually complete chemoselective functionalization of aldehyde carbonyls is also observable and the opposite chemoselectivity is easily accessible by use of TiCU as promoter, as shown in Sch. 148 [106],... 

Chiral bis(oxazoline) is an excellent chiral hgand as is BINAP for asymmetric reactions catalyzed by chiral metal complexes and has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (33) by Cozzi, Um-ani-Ronchi and their colleagues [34]. Among various combinations of chiral bis(oxazohnes) and metal salts, they found that the zinc complex 13 is the most effective and the ee values are in the range of 40 to 46%. 

Greeves e( al. reported that lanthanide triflates were effective for allylation of carbonyl compounds (20). Five mole percent of Yb(OTf)3 catalyzed the allylation of a series of aldehydes in good yield (Scheme 13.9) [20a]. Later they found a significant rate of acceleration by the Bronsted add in allylation of aldehydes with allyltributyltin catalyzed by ytterbium triflate (Table 13.9) [20b]. Recently, the concept of combined Lewis acid-Bronsted catalysis has attracted much attention from the chemical community because it often gives rise to significant improvement... 

An extension of the asymmetric condensation of organometallics onto aldehydes is the enantioselective silver-promoted allylation reaction of aldehydes with allyltributyltin, which has recently been performed by Shi et al. in the presence of chiral diphenylthiophosphoramide ligands and more efficiently, with binaphthylthiophosphoramide ligands. According to the nature of the ligand substituents, the corresponding allylation products were obtained in enantioselectivities of up to 98% ee, as depicted in Scheme 3.70. 

Platinum-catalyzed allylation of aldehydes with allyltin reagents was first reported in 1995.4S7 457b,483 483a Ar0matiC) aliphatic, a,/3-unsaturated aldehydes and even cyclohexanone undergo allylation with allyltributyltins in the presence of PtClgtPP 113)2 >n THF at room temperature or higher temperature (Equations (123) and (124)). Allylplatinum species are considered to be the active intermediates on the basis of related mechanistic studies on palladium catalysis. 

Table 1. Enantioselective allylation reaction of aldehydes with allyltributyltin (68) catalyzed by 5 mol % (S)-BINAP-AgOTtcomplex in THF at-20 °C.

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]. 

Allylation of aldehydes. The (R,R)-allylborane 6, obtained by reaction of the (R,R)-bromoborane 4 with allyltributyltin, adds to aldehydes to provide homoallylic... 

In 1996, Yamamoto and Yanagisawa reported the allylation reaction of aldehydes with allytributyltin in the presence of a chiral silver catalyst.2 They found that the combination of silver and a phosphine ligand accelerates the allylation reaction between aldehydes and allyltributyltin. After this discovery, they screened several chiral phosphine ligands and found that chiral silver-diphosphine catalysts can effect the reaction in an enantioselective fashion (Table 9.1).2 For example, when benz-aldehyde and allyltributyltin were mixed in the presence of 5 mol% of AgOTf and (S)-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP), the corresponding homoallyl alcohol was obtained with 96% ee and 88% yield (Table 9.1). Generally, the reaction with aromatic aldehydes afforded the corresponding homoallyl alcohols in excellent... 

The advantage of aluminum reagents over other metal reagents was also seen in the Lewis acid-promoted allylation of fluoro carbonyl compounds. For instance, MesAl-promoted selective allylation of an equimolar mixture of 2- and 4-fluorobenz-aldehydes with allyltributyltin afforded the homoallylic alcohol 53 almost exclusively. Unsatisfactory chemical yield and/or selectivity were obtained with Ti, Mg, Li, Sn, and Si Lewis acids (Sch. 38). 

Scheme 1-12. Chemoselective allylation of aldehyde carbonyls with (C6Fs)3B and allyltributyltin.

Through the use of Inis, Baba et al. had managed to dispense with TMSCI to regenerate the catalyst [30]. Only 10 to 20 mol% of Inis was required for the allylation of aldehydes and aromatic ketones with allyltributyltin, and stoichiometric amounts of Inis was found to inhibit the reaction. Reaction was expected to proceed via an entirely transmetallation pathway owing to the lower Lewis acidity of Inis. Thus, for y-substituted allylic tin compounds, typical anti preference for the addition product was observed. A chelation-controlled addition was used to explain the rate acceleration and higher yields of carbonyl compounds with vicinal alkoxy group, and the predominance of the syn addition product for such compounds (Figure 8.12). 

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-... 

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. 

An interesting allylation of carhonyl compounds has been introduced using a catalytic BINAP.AgOTf complex a mixture of allyltributyltin and various aldehydes gives good yields of alcohols with high enantiomeric excess (eqs 17 and 18). ... 

In 2000, Loh and coworkers reported the enantioselective allylation reaction in the presence of AgN03-Tol-BINAP in an aqueous medium (Scheme 9.2).6 They surveyed the reaction media and found that a 1 9 mixture of water ethanol was optimal. The reaction of allyltributyltin and aldehyde in the presence of AgN03-Tol-BINAP in water and ethanol gave the corresponding homoallyl alcohols with moderate to good enantioselectivities. 

Shi and Wang reported that the catalyst generated from AgOTf and a chiral diphenylthiophosphoramide ligand, which was prepared from chiral 1,1 -binaphthyl-2, 2 -diamine, could promote the allylation reaction of allyltributyltin and aromatic aldehydes (Table 9.4).8 Thus, the reaction of allyltributyltin and aldehyde in the presence of AgOTf and chiral diphenylthiophosphoramide gave the corresponding adducts with up to 98% ee. 

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]. 

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