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The Science of Synthesis Editorial Board,together with the volume editors and authors, is constantly reviewing the whole field of synthetic organic chemistry as presented in Science of Synthesis and evaluating significant developments in synthetic methodology. Four annual volumes updating content across all categories ensure that you always have access to state-of-the-art synthetic methodology.
Content of this volume: Organometallic Complexes of Scandium, Yttrium, and the Lanthanides, Metallocene Complexes with Bis(trimethylsilyl)acetylene, Titanocene-Bis(trimethylsilyl)acetylene Complexes, Zirconocene-Bis(trimethylsilyl)acetylene Complexes, Hafnocene Bis(trimethylsilyl)acetylene Complexes, Boron Compounds, Aluminum Alkoxides and Phenoxides, Aluminum Amides, Dearomatization Reactions Using Organolithiums, Carbolithiation of Carbon-Carbon Multiple Bonds, Pyrazines, Six-Membered Hetarenes with More than Three Heteroatoms, Nitriles, Oximes.
P. Dissanayake, D. J. Averill, and M. J. Allen
This chapter summarizes the use of lanthanide-containing catalysts for Mukaiyama aldol reactions since 1987. In this chapter, reactions are categorized as follows: (1) non-enantio-selective formation of β-hydroxycarbonyls (▶ Section 2.12.15.1.1), (2) enantioselective formation of β-hydroxycarbonyls in an organic solvent (▶ Section 2.12.15.1.2.1), and (3) enantioselective formation of β-hydroxycarbonyls in an aqueous solvent (▶ Section 2.12.15.1.2.2).
Lanthanide-catalyzed Mukaiyama aldol reactions between aldehydes 1 and the methyl trimethylsilyl acetal 2, to obtain Mukaiyama aldol products 3, were first reported using lanthanide(III) chlorides (▶ Scheme 1).[1] Furthermore, the reactions also proceed smoothly at room temperature when lanthanide(III) bromides are used as catalysts.[2] In addition to lanthanides in the +3 oxidation state, samarium(II) iodide can also be used as an efficient catalyst for this reaction, and the samarium(II) iodide precatalyst is stable enough to be stored under argon without oxidation (▶ Scheme 1).[3]
▶ Scheme 1 Mukaiyama Aldol Reactions Catalyzed by Lanthanide Catalysts[1]
Another variation of the lanthanide-catalyzed Mukaiyama aldol reaction is carried out in aqueous media using a catalytic amount of ytterbium(III) trifluoromethanesulfonate. These aqueous reactions between formaldehyde and silyl enol ethers 5 yield hydroxymethylated adducts 6 as shown in ▶ Scheme 2.[4]
▶ Scheme 2 Mukaiyama Aldol Reactions Catalyzed by Ytterbium(III) Trifluoromethanesulfonate under Aqueous Conditions[4]
In addition to using cosolvents with water, lanthanide Lewis acid–surfactant combined precatalysts are used for Mukaiyama aldol reactions in water (▶ Schemes 3 and 4).[5,6] The reaction between benzaldehyde and silyl enol ether 7 to yield aldol adduct 8 (▶ Scheme 3) suggests that the amount of surfactant, sodium dodecyl sulfate, influences the reaction yield. The aqueous Mukaiyama aldol reactions of α,β-epoxyaldehydes 9 with silyl enol ether 10 to yield adducts 11 have also been reported using sodium dodecyl sulfate (▶ Scheme 4).[6]
▶ Scheme 3 Mukaiyama Aldol Reaction Catalyzed by an Ytterbium(III) Trifluoromethanesulfonate–Surfactant Combined Precatalyst[5]
▶ Scheme 4 Mukaiyama Aldol Reactions Catalyzed by Lanthanide(III) Trifluoromethanesulfonate–Surfactant Combined Precatalysts[6]
Aldehyde 1 and silyl enol ether 2 were added to a suspension of LnX3 (0.05 or 0.10 equiv) in CH2Cl2 under argon at ambient temperature. After the reaction was complete the solvent was removed under reduced pressure. The crude material obtained was purified by flash chromatography (silica gel).
Methods A and B allow the preparation of β-hydroxycarbonyls 4 using SmI2. Method B is preferred when silyl ethers 3 are desired.
Method A: A 0.10 M soln of SmI2 in THF (1.0 mL) was concentrated under reduced pressure to give SmI2 (THF)2 as a blue powder. Alternatively, SmI2 (THF)2 (55 mg, 0.10 mmol) was weighed in a glovebox. This precatalyst, or SmI3 (THF)3 if desired, was suspended in CH2Cl2 (5 mL), and silyl acetal 2 (2.2–3.0 mmol) was added followed by the aldehyde 1 (2.0 mmol). The resulting yellow soln was stirred under argon. The mixture was hydrolyzed with 0.1 M HCl (5 mL) and extracted with Et2O. The extracts were washed with H2O and dried (MgSO4). After removal of the solvent, the product was purified by flash chromatography (silica gel).
Method B: Method A was followed, but instead of adding HCl the reaction was stopped by the addition of hexane (50 mL), which precipitated samarium salts. After filtration through Celite, the solvents were removed under reduced pressure, and purification by flash chromatography (silica gel) afforded the desired product.
3-Hydroxycarbonyl Compounds 6; General Procedure:[4]
CAUTION:
Formaldehyde is a probable human carcinogen, a severe eye, skin, and respiratory tract irritant, and a skin sensitizer.
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