Allylstannanes reactions with aldehydes

The reaction is explained by the in situ formation of bis-7r-allylpalladium (172), which is amphiphilic and reacts with 177 [88]. It is known that the reaction of the allylstannane 176 with aldehydes proceeds under rather severe conditions. Facile formation of the homoallyl alcohol 179 at room temperature in the presence of a Pt or Pd catalyst is explained by the nucleophilic nature of bis-7c-allylplatinum or -palladium generated in situ as an intermediate [89]. 

Mechanistically, the first step in the reaction is a transmetallation of allylstannane 486 with SnCU to give an intermediate allyltin trichloride 490. Reaction with aldehyde 464 proceeds... 

As with other noncatalyzed reactions of allylstannanes with aldehydes (see Section 1.3.3.3.6.1.3.2.) these reactions are believed to proceed via a chair-like six-membered ring transition state in which the a-substituent, in this case methoxymethoxy, adopts the axial position103. 

Use of di-( -butyl)stannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.1728,172 Reaction is also promoted by butylstannyl trichloride.173 Both SnCl4 and SnCl2 also catalyze this kind of addition. 

The titanium(IV) chloride catalyzed reaction of allylstannane 22 and aldehyde 23 to give a 3.5 1 mixture of adducts 24 and 25, which is related to example . Here again, two stereogenic centers (a and b) are created and the relative disposition of the substituents at positions a and b (syn or anti), and with respect to the configuration of the starting materials, had to be determined (see pp465 and 474)119. 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Regio- and stereo-selective allylation of sulfonylimines has been carried out with trifluoro(allyl)borates and allylstannanes, using palladium-pincer complexes as catalysts.47 Syn products predominate, in contrast to the corresponding reaction of aldehyde electrophiles DFT calculations have been employed to probe the mechanistic differences. 

Catalytic asymmetric allylations of aldehydes or ketones are roughly classified into two methods, namely, those using chiral Lewis acid catalysts and those using chiral Lewis base catalysts. The former method uses less reactive allylsilanes or allylstannanes as the allyl source. The latter method requires allyltrichlorosi-lane or more reactive allylmetals. Both processes are applicable to the reactions with substituted allylmetal compounds or propargylation. 

Carbonyl Allylation and Propargylation. Boron complex (8), derived from the bis(tosylamide) compound (3), transmeta-lates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78°C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent asymmetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. 

The Lewis acid-mediated addition of allyl silanes and allyl stannanes to carbonyl compounds has been a topic of interest for many years [103,104]. Stereochemical studies of reactions of allylsilanes and allylstannanes with aldehydes have included FeCl3-mediated intramolecular reactions (Sch. 20) [105],... 

Allylsilanes and allylstannanes are known to undergo the allylation reactions of aldehydes and ketones in the presence of Lewis acids (the most commonly used are TiCU and SnCU). Zirconium Lewis acids have often been compared with the titanium analogs. For most allylation reactions using allylsilanes the best results were obtained when TiCU or SnCU was used. For allylation reactions using allylstannanes, as illustrated in Eq. (15), however, ZrCU was used as the Lewis acid to obtain the bicyclic compound 36 via the intramolecular allylation of stannylaldehyde (35) [16]. 

Yamamoto, Y, Kobayashi, K, Okano, H, Kadota, I, Asymmetric synthesis of syn-l,2-diols via the reaction of aldehydes with chiral "y-(tetrahydropyranyloxy)allylstannanes, J. Org. Chem., 57, 7003-7005, 1992. 

Epoxides. Allylstannanes react under the influence of Pbl -HMPA with carbonyl compounds such as a,P-epoxy ketones and a-bromoketones give epoxy products. On the other hand, the reaction of l-chloro-3-tributylstannyl-l-propene with aldehydes leads to vinyl-epoxides. 

The transmetallation or the metal-metal exchange reaction of an allylic tin species with an electrophile was first observed in 1970 [77]. The possibility that transmetallation may play a role in the Lewis acid-promoted reaction of allylstannanes with aldehydes was initially discussed by Tagliavini [78], Keck [79], Yamamoto [71b, 80], and Maruyama [81]. It is believed that upon transmetallation with either SnCU or TiCl4, the addition of an allylstannane and aldehyde will occur via a cyclic, six-membered transition structure. The reaction occurs by coordination of the aldehyde carbonyl with the Lewis acidic trichlorotin or trichlorotitanium reagent, thus affording the anti homoallylic alcohol (Scheme 10-42). 

The synthesis of bis-pyran 340 was initiated by the allylation reaction of aldehyde 336b with the allylboron reagent derived from allylstannane (S)-335 and... 

Marshall s chiral allenylmetal reagents have been utilized in double asymmetric reactions with chiral aldehydes for the synthesis of polypropionate natural products. All four dipropionate diastereomers are accessible from the reactions of chiral allenylmetal reagents with a-chiraI-y5-alkoxy aldehydes 97 (153, 158, 276, 277]. The BF3-OEt2-catalyzed addition of allylstannane (l )-218a to aldehyde 97a occurs in high yield and diastereoselectivity to give the xyn.syn-dipropionate 395, presumably through either the synclinal or antiperiplanar Felkin transition states 396 and 397 (Eq. (11.31)). 

The reaction of allylstannanes with aldehydes, in the presence of a Lewis acid catalyst, to give homoallylic alcohols can be regarded as a special group of ene reactions, which have mechanistic features different from those shown in Scheme 9-2. These reactions are important in organic synthesis and have an extensive literature 75 a number of reviews are available.2 37 76-81... 

Other chiral allylstannanes 2.81 can be prepared by BF3 Et20-mediated isomerization of 2.80 (R.1 = H, R" = MeOCH2). The reactions of these allylstannanes with aldehydes can give valuable selectivities [569]. 

The additions of allylsilanes and -stannanes to carbonyl groups are usually catalyzed by a Lewis add. Therefore, chirality has been introduced either on the Lewis add [786, 816] or on the reagent [569, 698, 737, 739, 740, 1213-1217]. The use of a chiral Lewis add has been proposed by Yamamoto and coworkers [777a, 786, 791], Acyloxyboranes (R,R)- or (S,S)-3.9 (R = /-Pr, R = H or 3,5-(CF3)2C5H3) catalyze the asymmetric addition of allylsilanes and -stannanes to aldehydes at -78°C. Provided that R" H, the chemical yields and the selectivities of the reactions of allylsilanes are good only with PhCHO or unsaturated aldehydes [791], Better results are obtained from allylstannanes and any aldehyde [816] (Figure 6.53). 

The reactions of aldehydes with y-alkoxystannanes 2.81 are catalyzed by BF3 Et20, and they are more selective toward syn E-homoallyl alcohols 6.62 than the previous examples [1214, 1217] (Figure 6.56). This holds for the reactions with prochiral or a-chiral aldehydes provided that the reagents are matched [569], Transition-state model Aj accounts for this selectivity. This method has been applied by Yamamoto and coworkers [1223] to the reaction of aromatic aldehydes with y-(tetrahydropyranyk>xy)-allylstannanes under AICI3 catalysis. 

Finally, Thomas has reported that allylstannanes (247) and (248) possessing stereocenters at C-4 undergo moderately diastereoselective reactions with p-nitrobenzaldehyde (Scheme 46). The origin of asymmetric induction in these cases is probably related to that presented in Section 1.1.3.1 for the reactions of achiral allyl metal compounds and chiral aldehydes (e.g. Scheme 27). 

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