Aldehydes allylstannane reactions

Use of di-M-butylstannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.106 Reaction is also promoted by butylstannyl trichloride.107 Both SnCl4 and SnCl2 also catalyze this kind of addition. Reactions of tetraallyl-stannane with aldehydes catalyzed by SnCl4 also appear to involve a halostannane intermediate. It can be demonstrated by NMR that there is a rapid redistribution of the allyl group.108... 

The reaction of allylstannanes with aldehydes is conceptually very close to the reaction of allylsilanes with aldehydes (Sakurai reaction). Tlie major advantage of allylstannanes reagents is their high reactivity, a characteristic that has facilitated the development of the catalytic reactions. Allylstannanes are about 10,000 times more reactive than the corresponding allylsilanes, as measured by Mayr (Figure 1). The stannyl group increases the reactivity of the double bond by a factor of 10 -10 compared to H. Tributyl stannane is 10 times more nucleophilic than triphenyl stannane. 

The stereoselectivity of the boron trifluoride induced reactions was initially discussed in terms of open-chain, antiperiplanar transition states66. However studies of Lewis acid induced intramolecular allylstannane-aldehyde reactions are supportive of a synclinal process56,67. 

Analogous reactions of more complex allylstannanes have been applied to natural product synthesis72, although /(,/i-dialkylated aldehydes appear not to react, possibly due to deactivation of the aldehyde towards nucleophilic attack72. 

In contrast to this generally high preference for. tyn-products for boron trifluoride mediated reactions between 3-a//c v/-Substituted allylstannanes and aldehydes, //-products are preferred for reactions involving 3-p/ cn>7-substituted allylstannanes. This stereoselectivity was observed for a range of aldehydes, and was explained in terms of the increased propensity for the tin-allylie carbon bond to be polarized when the -substituent is able to stabilize a positive charge so favoring a cyclic transition state73. 

Good Cram selectivity is observed for Lewis acid induced reactions between allylstannanes and aldehydes with alkyl-substituted a-chiral centers66,87. This enhanced Cram selectivity may be due to the effect of the Lewis acid on the trajectory of nucleophilic attack on the aldehyde66. 

With 2-butenyl- and 3-phenyl-2-propenylstannanes coupling of this Cram selectivity with the intrinsic stereoselectivity of the Lewis acid induced allylstannane-aldehyde reaction gives useful stereocontrol at three contiguous stereogenic centers66,81. 

Effective 1,7-asymmetric induction was observed in the analogous reactions between C-hydroxy-allylstannanes and aldehydes, although in this case the free hydroxy group was required126. 

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. 

The indium-mediated allylation carried out with allylstannanes in combination with indium chloride in aqueous medium was reported by Marshall et al.113 Allylindium was proposed as the reaction intermediate. Various aldehydes can be alkylated very efficiently with 3-bromo-2-chloro-l-propene mediated by indium in water at room temperature. Subsequent treatment of the compound with ozone in methanol followed by workup with sodium sulfite provided the desired hydroxyl ester in high yield.114... 

Allylstannanes undergo diasterospecific additions to chiral a-alkoxyaldehydes, as shown in reaction 45295. Stereospecific additions to aldehydes are attained in the presence of... 

A case of the addition of an allylstannane to aldehydes has been reported by Tagliavini to proceed with appreciable enantioselectivity (Scheme 6.15) [40]. A notable feature of the Zr-catalyzed transformations is that they proceed more rapidly than the corresponding Ti-catalyzed processes reported by the same research team (see Scheme 6.16). Furthermore, C—C bond formation is significantly more efficient when the reactions are carried out in the presence of 4 A molecular sieves the mechanistic rationale for this effect is not known. It should be noted that alkylations involving aliphatic aldehydes are relatively low-yielding, presumably as the result of competitive hydride transfer and formation of the reduced primary alcohol. 

Homoallylic alcohols can readily be prepared from commercially available tetraallytin and acetals or aminoacetals in the presence of trifluoroacetic acid on silica gel this provides an alternative to the reaction of an allylstannane with an aldehyde.278 By a similar mechanism, homoallylic peroxides can be prepared from monoperoxyacetals (Equations (99) and (100)).279... 

The catalytic activity of the trityl moiety was unobjectionably adjusted in the addition reaction of the allylstannanes to aldehydes [148]. In this allylation process the trityl chloride 52, due to its disposition to partially ionic character of the halogen bonding, was employed as a catalyst in the complementary tandem with weak Lewis acid TMSCl (Scheme 57). The excess of the silyl component was necessary in order to release the trityl catalyst from the intermediate to complete the catalytic cycle. The achieved yield was 93%, when trityl chloride 52 was used. 

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-... 

Asymmetric catalysis of BINOL-Ti complexes in the reaction of aliphatic and aromatic aldehydes with an allylstannane has also been reported independently by Umani-Ronchi [54] and Keck [55]. The former group has suggested that a new complex generated by the reaction of the BINOL-Ti complex with allylstannane is the catalytic species that provides remarkably high enantioselectivity (Scheme 8C.23). It is interesting that no reaction occurs if dry MS 4A... 

Stereo specific generation and reactions of allylic alkali and alkaline earth metals have been reviewed121 and solvent-mediated allylation of carbonyl compounds with allylstannanes has been explored.122 Chiral phosphoramides derived from (5 )-proliiie have been used to catalyse asymmetric allylation of aromatic aldehydes by allylic trichlorosilanes.123... 

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. 

The enantioselective addition of allylstannanes to glyoxylates and glyoxals, and also simple aromatic and aliphatic aldehydes, catalysed by chiral (salen)Cr(III) complexes, has been studied.133 The reaction proceeded smoothly for the reactive 2-oxoaldehydes and allyltributy 1 tin in the presence of small amounts (1-2 mol%) of (salen)Cr(III)BF4 under mild, undemanding conditions. The stereochemical results have been rationalized on the basis of the proposed model. 

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]. 

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. 

Maruoka has reported that chiral bimetallic Lewis acid catalysts 9-11, prepared from (S)-BINOL, M(0-i -Pr)4 (M=Ti, Zr, Hf), and the corresponding spacer, strongly enhance the reactivity of aldehydes or ketones toward allyl transfer from allylstannanes [18-20]. For example, treatment of acetophenone (42) with tetraallyltin (41) in the presence of 30 mol% of the chiral bidentate Ti(IV) catalyst 10 provided the (S)-enriched homoallylic alcohol 43 in 95% yield with 90% ee (Scheme 2) [19]. A suggested reaction mechanism involves double activation of carbonyls owing to the simultaneous coordination of two Ti atoms to a carbonyl oxygen atom. 

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