[3054-95-3] · C7H14O2 · 3,3-Diethoxy-1-propene · (MW 130.19)
(reagent for ethoxyallylation of activated carbonyl compounds;2,3 forms enol ethers with organocopper reagents;4-6 acrolein equivalent for ionic Diels-Alder reaction;7 precursor for ethoxyallyl cation;7 acrolein synthon for Heck coupling with halides8,9)
Alternate Name: acrolein diethyl acetal.
Physical Data: bp 125 °C; d 0.862 g cm-3.
Solubility: slightly sol water; very sol ethanol and ether.
Form Supplied in: clear liquid, 95% pure; widely available.
Analysis of Reagent Purity: 1H NMR (CDCl3) d 1.20 (t, 6H, J = 7 Hz), 3.5 (m, 2H), 3.6 (m, 2H), 4.85 (d, 1H, J = 2 Hz), 5.26 (d, 1H, J = 7 Hz), 5.38 (d, 1H, J = 11 Hz), 5.85 (m, 1H).
Purification: generally used as supplied; can be purified by distillation at atmospheric pressure from anhydrous sodium carbonate.
Handling, Storage, and Precautions: store under nitrogen.
The extreme reactivity of acrolein, which often results in extensive polymerization under many reaction conditions, is moderated by an acetal function in 3,3-diethoxy-1-propene (1) and thus (1) is a useful three-carbon reagent for many synthetic transformations.
Thermal reaction of active methylene compounds with (1) furnishes ethoxyallyl products (eq 1).2 In a detailed study it was found that the reaction was sluggish for the less acidic b-keto esters, but was effectively catalyzed by Nickel(II) Acetylacetonate (Ni(acac)2) (eq 2).3 The mechanism of this transformation is believed to involve a Claisen rearrangement of the initially formed mixed acetal; however, direct C-alkylation of the substrate at the g-position of the ethoxyallyl cation is also possible.
Copper(I) Bromide catalyzed additions of Grignard reagents to (1) give mostly (Z)-enol ethers, which are hydrolyzed to the corresponding aldehyde by aq HCl (eq 3).4 In a later study it was shown that the (E) isomers predominate if a Lewis acid-modified organocopper reagent is used,5 and in another report a nickel(II) phosphine catalyst (NiCl2(dppp)) gave only the (E) isomer.6
The ethoxyallyl cation produced by ionization of (1) is a very potent dienophile which reacts with a variety of dienes below 0 °C to give Diels-Alder adducts (eq 4).7 The yields of the addition products generally improved when a cyclic acetal (2-vinyl-1,3-dioxolane) was used.7
The palladium-catalyzed (Palladium(II) Acetate, Pd(o-Tol)3) coupling of (1) with aryl and vinyl halides in the presence of secondary amines such as piperidine and morpholine proceeds smoothly, in contrast to acrolein and other a,b-unsaturated aldehydes which tend to polymerize.8,9 The reaction with vinyl halides furnishes a mixture of acetals and aminoacetals which are hydrolyzed to dienals by aqueous acid (eq 5).8 However, with aryl halides the products are ethyl esters, probably arising from hydrolysis of the ketene acetal formed by elimination of the acetal hydrogen (eq 6).9 With an isolated arylpalladium complex, formation of an aldehyde and its cyclization to a quinoline derivative are also reported.10
Shrenik K. Shah
Merck Research Laboratories, Rahway, NJ, USA