Asymmetric allylation allyl stannane

The best procedure reported to date for the asymmetric allylation of aldehydes using tributyl(2-propenyl)stannane involves the catalyzed addition with the BINOL-TiCl2 complex as catalyst. Good yields and ee s were obtained for both aromatic and aliphatic aldehydes using 20 mol% of the catalyst127. 

Certain S- and e-oxygenated allylic stannanes have been found to transmetallate with SnCU to give chiral pentacoordinated chloro stannane intermediates which add stereos-electively to aldehydes (Scheme 31)74. These reactions proceed with net 1,5-and 1,6-asymmetric induction. 

Although there are some examples of diastereoselective addition of allylic stannanes to substituted 1,3-oxazolidi-nones (Scheme 52),141 these reactions have still not been applied to asymmetric synthesis. 

BINOL-Ti complexes (1) has been shown to serve as efficient asymmetric catalysts for the carbonyl addition reaction of allylic stannanes and silanes 152,53]. The addition reactions to glyoxylates of ( )-2-butenylsilane and -stannane proceed smoothly to afford the corresponding syn-product with high enantiomeric excess (Scheme 8C.21) [52]. 

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-... 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

In 1996 Yanagisawa, Yamamoto, and their colleagues first reported the asymmetric allylation of aldehydes with allylic stannanes catalyzed by a BINAP silver(I) complex [29]. The chiral phosphine-silver(I) catalyst can be prepared simply by stirring an equimolar mixture of BINAP and silver(I) compound in THF at room tempera-... 

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. 

Double asymmetric reactions between [7-(alkoxy)allyl]stannanes 230 and the a-benzyloxy aldehyde 55 exhibited clear matched and mismatched behavior [168]. With BF3 OEt2 catalysis, the matched double asymmetric reaction between (R)-230a and aldehyde (S)-55 generates exclusively the syn,anti adduct 425 (Eq. (11.40)). Formation of 425 can be rationalized through either the antiperipla-nar, Felkin transition state 426 (as proposed by Marshall) or the synclinal Felkin transition state 427. 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

Williams and coworkers have expanded the utility of this methodology via the application to highly functionalized substrates. transmetalation with R.R)-ot (S,S)-228 is quantitative and dependable, with a variety of allylic stannanes with C-2 substitution. Carbon chains at C-2 of the allyl component may contain additional functionality, including benzyl and allyl ethers, silyl ethers, esters, alkenes, halogens, or thioacetals, as well as stereogeificity. Asymmetric induction in the condensation with aldehydes is dominated by the chiral auxiliary. In this fashion, the allylation reaction may be designed as a convergent... 

In addition, the asymmetric allylation was utilized to address the issues of stereocontrol at C37 and C38 in the construction of the acyclic carbon chain of 234 (Scheme 5.2.53). Reactions of the sensitive /3,y-unsaturated aldehyde 241 were effected upon transmetalation of stannane 240 using (R,R)-22S,... 

Matched and mismatched characteristics have been observed in reactions with non-racemic a-benzyloxypropionaldehyde. The matched asymmetric allylation of (E)-stannane 278 with (S)-aldehyde, initiated by complexation with BFs OEta, exclusively provides the E-4.5-syn-5,6-anti compound 279 as the expected Felkin-Anh adduct (Scheme 5.2.61, top). On the other hand, the Q -chelation-controlled process can also be achieved via a matched case of double diastereoselection using the (S)-stannane 280 and pre-complexation with MgBr2 OEt2. The syn product 281 is rationalized via the antiperiplanar transition state 282 (Scheme 5.2.61, bottom). 

Scheme 5.2.61 Examples of matched asymmetric allylation reactions of chiral, nonracemic stannanes 278 and 280...

A chiral phosphine-silver(I) complex generated from BINAP and AgOTf-catalyzed asymmetric allylation of aldehydes with allylstannanes, resulting in high enantioselectivity. With 2-butenylstannane, the anti adduct was obtained preferentially irrespective of the double-bond geometry of the stannane (Scheme 12.28) [76]. 

Closely related to the previous section is the asymmetric addition of allyl stannanes to aldehydes. The catalyst is the same and the main difference is that an R3Sn group is lost instead of a proton. This has the advantage that there is no ambiguity about the position of the alkene in the product. An example is the addition of an allyl group to the functionalised aldehyde 199. Complexation of either or both oxygen atoms to the Ti is indicated.45... 

Most of the catalytic methods reported so far are the chiral metal compound-catalyzed allylation reactions using allylic silanes or allylic stannanes. Two types of catalytic mechanism, Lewis acid mechanism and transmetallation mechanism, can be considered for these reactions as illustrated in Scheme 2. If the chiral metal catalyst acts as a Lewis acid in the asymmetric allylation, an al-... 

Chiral titanium complexes 4 and 5, which were developed as chiral catalysts for asymmetric carbonyl-ene reactions with prochiral glyoxylate esters [50], were first apphed to the catalytic asymmetric allylation of carbonyl compounds by Mikami and Nakai (Scheme 5) [9]. The titanium catalysts are prepared from (S)-binaphthol and diisopropoxytitanium dihahde (X=C1 and Br) in the presence of 4 A molecular sieves. Using these catalysts, glyoxylates are enantio- and diastereoselectively allylated with allylic trimethylsilanes or allylic tributylstan-nanes. High levels of enantioselectivity and syn selectivity are observed for (E)-crotylsilane and -stannane. The syn selective allylation reaction is believed to proceed mainly through an antiperiplanar transition state. 

Tagliavini and Umani-Ronchi found that chiral BINOL-Zr complex 9 as well as the BINOL-Ti complexes can catalyze the asymmetric allylation of aldehydes with allylic stannanes (Scheme 9) [27]. The chiral Zr catalyst 9 is prepared from (S)-BINOL and commercially available Zr(0 Pr)4 Pr0H. The reaction rate of the catalytic system is high in comparison with that of the BINOL-Ti catalyst 4, however, the Zr-catalyzed allylation reaction is sometimes accompanied by an undesired Meerwein-Ponndorf-Verley type reduction of aldehydes. The Zr complex 9 is appropriate for aromatic aldehydes to obtain high enantiomeric excess, while the Ti complex 4 is favored for aUphatic aldehydes. A chiral amplification phenomenon has, to a small extent, been observed for the chiral Zr complex-catalyzed allylation reaction of benzaldehyde. 

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]. 

Non-racemic epoxide 307, prepared from (+)-epichlorohydrin, was alkeny-lated to give allyl silane 308, which was converted into allyl stannane 309. Corey s chiral boron reagent-mediated asymmetric allylation [120] to aldehyde 311 provided homoallylic alcohol 306 with a 11 1 diastereomer ratio. Intramolecular Sn2 cyclization of the corresponding 7-hydroxy-3-TsO unit derived from 306, followed by hydrolysis of the dithiane, afforded aldehyde 312 (Scheme 66). 

Asymmetric allylation of aldehydes with allyhc agents catalyzed by Lewis acid is a practical method for synthesizing optically active hranoallylic alcohols [10]. The chloro complex 1 serves as an efficient catalyst for asymmetric allylation of aldehydes with allylstannane [8, 9, 11]. In the presence of 5 mol% of the benzyl-phebox-Rh complex 1-Bn, the coupling reaction of benzaldehyde with allyltributyl-stannane in CH2CI2 at room temperature proceeded smoothly to provide the corresponding homo-aUyl alcohol 7a in 88% yield with 61% ee (Scheme 2). When methaUylstannane was subjected to the reactiOTi, enantioselectivity of the allylated product 8 significantly increased to be over 90% ee. 

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