Allylstannanes, chiral

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. 

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

Allylstannane additions to aldehydes can be made enantioselective by use of chiral catalysts. A catalyst prepared from the chiral binaphthols R- or S-BINOL and Ti(0-i-Pr)4 achieves 85-95% enantioselectivity.187... 

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

Kobayashi and colleagues developed a catalytic enantioselective method for the allylation of imines 24 by substituted allylstannanes 25 with chiral zirconium catalysts 26 and 27 prepared from zirconium alkoxides and l,l -bi-2-naph-thol derivatives (Scheme 10) [19]. The allylation of aromatic imines 24 with 25 afforded the corresponding homoallylic amines 28 in good yields (71-85%) with high stereoselectivities (87-99% ee). 

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... 

Chiral (salen)chromium(III) complexes catalyse the asymmetric allylation of a range of aldehyde types, using allylstannane reagents.171... 

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. 

Recently, Yu et al. [88] and Keck et al. [89] reported the synthesis of enantio-enriched homoallyl alcohols 198. Alcohols 198 are prepared from allylstannane 197, by using chiral Ti(IV)-based catalysts [88, 89], with ees ranging from 90 to 96% (Scheme 13.70). 

Keywords Allylation, Allylsilanes, Allylstannanes, Carbonyl compounds, Chiral Lewis acid catalysts, Chiral Lewis base catalysts... 

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. 

The chiral titanium complex BINOL-T1CI2 also catalyzes the asymmetric carbonyl addition reaction of allylic silanes and stannanes. Thus the addition reaction of glyoxylate with (E)-2-butenylsilane and -stannane proceeds smoothly to give the syn product in high enantiomeric excess (eq 12). The syn product thus obtained can be readily converted to the lactone portion of verrucaline A. The reaction of aliphatic and aromatic aldehydes with allylstannane is also catalyzed by BINOL-TiC to give... 

In complex examples, high levels of stereodifferentiation require the consideration of the conjoined influences of a-asymmetry in the allylstannane, and chirality of the starting aldehyde, in addition to the choice of auxiliary 1 (eq 10). ... 

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. 

In 1995, Porter et al. [34] reported the first excellent results for free radical addition to an electron-deficient alkene by use of chiral zinc complexes. Reaction of the oxa-zolidinone 9 with tert-butyl iodide and allyltributylstannane 30 in the presence of Zn(OTf)2 and a chiral bis(oxazoline) ligand 12 gave the adduct 44 in 92 % yield with 90 % ee (Sch. 18). The chiral bis(oxazoline) complexes derived from ZnCl2 or Mg(OTf)2 gave racemic products. In this reaction, lower allyltin/alkene ratios gave substantially more telomeric products, and a [3 + 2] adduct 45 of the oxazolidinone 9 and the allylstannane 30 was obtained at temperatures above 0 °C. 

Umani-Ronchi et al. [56] investigated the asymmetric allylation of aldehydes with allylstannane in the presence of chiral bis(oxazoline) ligands and several metal salts (Sch. 31). Combination of zinc halides and the bis(oxazoline) ligand 70 gave the ally-lated product 71 with moderate enantioselectivity (40 % ee), while other metal salts afforded either no product or racemic products. Because the formation of an allyl-zinc-bis(oxazoline) species was excluded on the basis of the NMR experiments, the reaction was considered to proceed by a Lewis acid-mediated pathway. 

Allylstannanes can be employed as nucleophiles and they add efficiently to Lewis acid-activated aldehydes (Sch. 33) [68]. The chiral reagent 137 activates alkyl aldehydes towards allylation no enantioselectivity for the alcohol 138 was observed. 

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