Homoallylic alcohols Bismuth

Tetrahydropyrans hydroxylated at the 4-position have good synthetic value [113]. Although many synthetic methods have been reported [17-23,114,115], the search for potential alternate approaches and the development of eco-friendly and high-yielding reactions resulted in the development of a method that poses less problems for the environment. Synthesis of tetrahydropyranol derivatives can be achieved through the Prins-type cyclization reaction of homoallylic alcohols with aldehydes using bismuth triflate as catalyst in [bmim]PF6 solvent system [108] (Fig. 22). 

The cross-coupling reaction of homoallylic alcohols with aldehydes in the presence of bismuth trichloride in solvent-free conditions under microwave irradiation generated 4-chloro-2,6-disubstituted tetrahydropyrans with high c/.v-diastereos-electivity [15]. The reaction of benzaldehyde with l-phenyl-3-buten-l-ol in the presence of bismuth trichloride under microwave irradiation gave the corresponding... 

An interesting aspect of the Bi-mediated Barbier-type allylation is that the reaction can be conducted in aqueous media. When the reaction is carried out using metallic bismuth powder in water, addition of an equimolar amount of potassium fluoride improves the yield of homoallyl alcohol (Equation (40)).74 The reaction is also compatible with hydroxyl and carboxyl groups of the carbonyl substrates. When succinaldehydic acid, 4-oxopentanoic acid,... 

Aldehydes, ketones, and acetals react with allyltrimethylsilane in the presence of a catalytic amount of BiX3 (X = C1, Br, OTf) to give homoallyl alcohols or homoallyl alkyl ethers (Equation (52)).91-93 The BiX3-catalyzed allylation of aldehydes and sequential intramolecular etherification of the resulting homoallylic silyl ethers are involved in the stereoselective synthesis of polysubstituted tetrahydropyrans (Equation (53)).94,95 Similarly, these Lewis acids catalyze the cyanation of aldehydes and ketones with cyanotrimethylsilane. When a chiral bismuth(m) catalyst is used in the cyanation, cyanohydrines are obtained in up to 72% ee (Equation (54)). a-Aminonitriles are prepared directly from aldehydes, amines, and cyanotrimethysilane by the BiCl3-catalyzed Strecker-type reaction. 

Certain Lewis acids are known to induce an epoxide-aldehyde rearrangement <01TL8129>, and this chemistry has recently been combined in tandem with metal-mediated allylations. For example, epoxides react with tetraallyltin in the presence of bismuth(III) triflate to give homoallylic alcohols 116. The reaction involves an initial 1,2-shift to form an aldehyde 115, which is then attacked by the allyl tin species <03TL6501>. A similar but operationally more straightforward protocol is available by combining allyl bromide with indium metal, followed by the addition of epoxide <03TL2911>. 

The allylation and cyanation of aldehydes and ketones are mediated by BiCl3 and BiBr3 [174, 175]. When a chiral bismuth(lll) catalyst is used for cyanation, cyanohydrins are obtained in up to 72% ee (Scheme 14.85) [175]. The Bi(OTf)3-promoted intramolecular Sakurai cyclization of homoallylic alcohols is involved as a key step in the stereoselective synthesis of polysubstituted tetrahydropyrans (Scheme 14.86) [176]. In the presence of the BiCl3-xMl binary catalyst, allyltrimethylsilane [177] and silyl enolates [178] are acylated to give aUyl ketones and /l-dikelories, respectively. 

Several other metals have been used in this Barbier-type reaction. Thus, a mixture of bismuth(III) chloride/aluminum [160] or of bismuth(III)chloride/ magnesium [161] was shown to promote allylation of aldehydes under aqueous conditions yielding homoallylic alcohols in good yields. Similarly, the same reaction can be promoted with metallic lead [162], highly reactive antimony prepared by the NaBH4 reduction of SbClj [163] or a mixture of manganese and catalytic copper [164]. In the latter case, aromatic aldehydes reacted whereas ali-... 

Keywords Barbier-type allylation, bismuth, homoallyl alcohol... 

Allylation. Carbonyl compounds and imines are attacked to give homoallylic alcohols and amines by a mixture of bismuth and allyl bromide in acetonitrile. The presence of Bu4NBr is required, whereas MejSiCl and Nal are less effective. Generally, the yields range from 85 to 95%. The use of tantalum instead of bismuth works only for imines, and with lower yields. 

Reaction of several homoallyl alcohols 9 with aldehydes in the presence of bismuth trichloride under MWI for 1.5-2.5 min generated the respective 4-chlorotetrahydropyrans 12 in high yields (70-98%). Longer reaction times (4-7 h) were required at room temperature to achieve comparable yields. The reaction may proceed by the formation of hemi-acetal 10 whose cyclization gave 11 that could be attacked by the halide ion to give 12 (Scheme 2) (02SC1803). 

Stirring metallic bismuth with a mixture of an allyl halide and an aldehyde in DMF at room temperature did indeed give high yields of the expected homoallylic alcohols ... 

Moreover, it was found that nanometer-sized bismuth mediates the allylation of carbonyl compounds in water to afford the corresponding homoallylic alcohols in high yields. Nanometer-sized bismuth is much more effective than regular bismuth powder in this reaction. The regioselectivity of the reaction depends on which kind of nanometer-sized bismuth (Bi nanotubes or in situ Bi nanoparticles) is used (Equation 2) [25b]. 

A novel and rapid synthesis of tetrahydropyran derivatives through the Prins cyclization reaction of homoallyl alcohols with aldehydes in the presence of bismuth trichloride was described. The reaction proceeded rapidly under microwave irradiation in solvent-free conditions. The conversions were clean and the products were obtained in excellent yield with high diastereoselectivity (Equation 24) [43a]. 

The synthesis of 2-benzyl-4-chloro-tetrahydropyran derivatives has been reported using Bids as a reagent in the cross-cyclization between epoxides and homoallylic alcohols (Equation 41) [84]. Since the rearrangement of epoxides is known to be catalyzed by bismuth(III) salts, it is possible that a similar mechanism operates in this reaction [74]. 

Wada, M. Ohki, H. Akiba, K. (1987) Bismuth(lll) chloride-aluminium-promoted allylation of aldehydes to homoallylic alcohols in aqueous solvent, J. Chem. Soc, Chem. Commm., 708-9. 

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