Catalysis by cobalt compounds of aldol and retroaldol reactions

Aldol condensation of acetone, butanone, and 2-pentanone, and retroaldol reaction of neat diacetone alcohol and 3-methyl-3-hydroxyheptan-5-one are catalyzed by (Ph₃P)₃CoCH₃ and (Ph₃P)₃CoSi(CH₃)₃. The condensations are reversible and the retroaldol reaction is favoured. Several hindered or cyclic ketones, such as higher homologues, 3-pentanone, methyl isopropyl ketone or cyclohexanone do not condense. By comparison, aliphatic aldehydes react fast and irreversibly to yield products comprised of three aldehyde units. (Ph₃P)₃CoCH₃ also catalyzes protium/deuterium scrambling between ketones and acetone-d₆. This exchange takes place preferably at the C3 carbon whereas condensation takes place at the C1 carbon. The reactions take place at 20°C or below, and of the several organometallic compounds tested only the two mentioned above were found to be active.