The reaction of alkenes (allyl alcohol, styrene and C6 alkenes) with formaldehyde was efficiently performed by using Rh precatalysts formed <em>in situ</em> by the addition of triphenylphosphine (PPh₃), 1,2-bis(diphenylphosphino)ethane (dppe) or 1,1,1-tris(diphenylphosphinomethyl) ethane (triphos) to the complex Rh(acac)(CO)₂ at 130ºC in 1,4-dioxane, yielding their corresponding aldehydes; the best catalytic system was Rh(acac)(CO)₂/2dppe, which generates the cationic complex [Rh(k²-P,P-dppe)₂]⁺. However, the reaction of phenylacetylene with formaldehyde under the same reaction conditions generated styrene, which was found to be the product of transfer hydrogenation from formaldehyde.

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