Catalytic Decarbonylation of Epoxyaldehydes: Applications to the Preparation of Terminal Epoxides

 

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Abstract

A catalytic decarbonylation reaction for epoxyaldehydes is reported. This reaction may be sequentially used with known asymmetric methods to access optically active mono- and disubstituted terminal epoxides, as illustrated for a key example.

References and Notes

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General procedure for the decarbonylation reaction: [Rh(cod)Cl]₂ (3.8 mg, 0.0077 mmol) and rac-BINAP (18.5 mg, 0.03 mmol) were dissolved in o-dichlorobenzene (3 mL) in a 25 mL sealed tube under an argon atmosphere. After 15 min of stirring at r.t., a homogenous dark-red solution formed. The corresponding α,β-epoxyaldehyde (0.3 mmol) was then added to the stirring mixture, the tube was evacuated and filled with argon three times and finally sealed under vacuum. The reaction mixture was then immerged in a pre-heated oil bath at the corresponding temperature (140 ˚C). After 14 h, the mixture was cooled to r.t. and directly loaded onto a column packed with silica gel. Prior elution with pure pentane to remove the o-dichloro-benzene, followed by elution with the corresponding solvent mixture (pentane-Et₂O) afforded the pure compound as an oil.
Caution: careful work-up is necessary since some of the products are highly volatile! Triisopropyl{3-methyl-2-[2-(2-methyloxiran-2-yl)ethyl]but-3-enyloxy}silane (Table 1, entry 7, A): ^¹H NMR (300 MHz, CDCl₃): δ (mixture of isomers) = 4.81 (br s, 1 H), 4.70 (br s, 1 H), 3.62-3.58 (m, 2 H), 2.60-2.57 (m, 2 H), 2.19 (m, 1 H), 1.63 (s, 3 H), 1.60-1.40 (m, 4 H), 1.30 (s, 3 H), 1.05 (m, 21 H). ^¹³C NMR (75 MHz, CDCl₃): δ (mixture of isomers) = 145.4, 112.1, 66.4, 57.0, 54.1, 53.8, 49.8, 34.4, 25.0, 21.1, 21.0, 20.3, 20.2, 18.2, 12.1. IR (neat): 2924 (m), 2865 (m), 1462 (m), 1106 (m), 882 (s), 680 (s)
cm^-¹. HRMS (ESI): m/z calcd for C₁₉H₃₈O₂SiNa⁺: 349.2534; found: 349.2534.
6-Methyl-5-[(triisopropylsilyloxy)methyl]hept-6-en-2-one (Table 1, entry 7, B): ^¹H NMR (300 MHz, CDCl₃): δ = 4.81 (s, 1 H), 4.70 (s, 1 H), 3.62-3.58 (m, 2 H), 2.43-2.38 (m, 2 H), 2.19 (m, 1 H), 2.12 (s, 3 H), 1.82 (m, 1 H), 1.66 (s, 3 H) 1.60 (m, 1 H), 1.05 (m, 21 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 209.1, 145.3, 112.3, 66.2, 49.2, 41.5, 29.8, 23.4, 19.8, 17.9, 11.9. IR (neat): 2942 (m), 2865 (m), 1719 (s), 1112 (m), 881 (s), 679 (s) cm^-¹. HRMS (ESI): m/z calcd for C₁₈H₃₆O₂SiNa⁺: 335.2377; found: 335.2376.