[624-67-9] · C3H2O · Propargyl Aldehyde · (MW 54.05)
(conjugate acceptor; dienophile)
Alternate Names: 2-propynal; propiolaldehyde.
Physical Data: bp 59-61 °C.
Solubility: sol H2O, alcohol, ether, benzene, toluene, acetone.
Form Supplied in: not commercially available.
Handling, Storage, and Precautions: great care should be taken in handling the neat liquid as an explosion has been reported.3 The explosion was attributed to rapid polymerization, probably catalyzed by base or a radical initiator. It is recommended that handling of 2-propynal should include the use of a nonglass container and dilution with a sufficient volume of an inert solvent to absorb the heat of polymerization without boiling, e.g. <=10% in toluene. Use in a fume hood.
Grignard reagents add well to 2-propynal, as in the preparation of 1-hexyn-5-en-3-ol (eq 1).4 Aryl Grignard reagents also add successfully.5 Although most heteronucleophiles add in a conjugate fashion (see next section), aniline has been reported to form the corresponding imine.6 A pyrrole synthesis has been developed which relies on an aldol reaction of 2-propynal with a 3-silyl-2-azetidinone (eq 2).7
Conjugate additions of nucleophiles based on oxygen (eq 3),8,9 nitrogen (eq 4),10 and sulfur (eq 5)11 leading to 3-substituted acroleins have been reported.
2-Propynal reacts as a typical dienophile in Diels-Alder reactions with a variety of dienes. Nucleophilic dienes add without catalysis (eqs 6 and 7).12,13 Some dienes require Lewis-acid catalysis to react, as in the preparation of ansa-secosteroids (eq 8).14
Lanthanide salts catalyze the 1,2-cycloaddition of 2-propynal to ketene imines (eq 9).15 Sodium Azide cycloadds to 2-propynal across the triple bond, yielding a 1,2,3-triazole.16
Heating 2-propynal with 3-amino-2-cyclohexenones provides a simple route to tetrahydroquinoline derivatives as in eq 10.17,18 Tetraazaannulenes may be prepared simply by heating o-phenylenediamines with 2-propynal (eq 11).19
Monash University, Clayton, Victoria, Australia