Kinetics and mechanisms of homogeneous catalytic reactions. Part 14. Hydroformylation of 1-hexene with formaldehyde catalyzed by a cationic bis(diphosphine)rhodium complex

Reactions of alkenes and alkynes with formaldehyde catalyzed by rhodium systems containing phosphine ligands

“…Although the unmodified complex Rh(acac)(CO) 2 was catalytically inactive for the hydroformylation of allyl alcohol with formaldehyde, the addition of PPh 3 , dppe and triphos to this complex did induce the formation of the corresponding aldehydes (TON varying from 24 to 195) at 130ºC. The highest activity (with the lowest percentage of isomerization-tautomerization to propanal) was obtained when two equivalents of dppe were added to Rh(acac)(CO) 2 , which generates the cationic complex [Rh(k 2 -P,P-dppe) 2 ] + , as reported by Rosales et al [13,14,24]. The catalytic activity of this system was four times higher than the activity generated by addition of 1 eq dppe, twice higher than those generated by addition of 2 or 3 eq.…”

“…The higher activity of the Rh/ nPPh 3 systems for the hydroformylation of allyl alcohol compared with simple alkenes may be explained by the coordination of the OH group of the substrate to the metal center, which should hinder the decarbonylation of the formyl group; the coordination of OH group of the substrate to the metal center has been reported by Seok et al [11] for the hydroformylation of allyl alcohol and acrolein catalyzed by RhH(CO)(PPh 3 ) 3 . Although the hydroformylation of alkenes with formaldehyde catalyzed by Rh/2dppe proceeds via hydroacylation mechanism [24], the reaction catalyzed by Rh/nPPh 3 and Rh/1dppe systems probably proceeds through a classic mechanisms in view of experimental evidences obtained in the coordination chemistry reported in the present work (vide infra). In fact, the Rh/3PPh 3 system is a more active precatalyst than Rh/dppe one for the alkene hydroformylation under syngas conditions.…”

“…More recently, some of us reported a detailed kinetic and mechanistic study of the hydroformylation of 1-hexene with formaldehyde (Eq. 1) catalyzed by [Rh(k 2 -P,Pdppe) 2 ]acac; experimental results and theoretical DFT calculations proportioned evidences on favor of a hydroacylation mechanism [24].…”

“…In previous works, we have reported that reaction of the dicarbonylrhodium complex with 2 equivalents of dppe in 1,4-dioxane solution under reflux produces the cationic complex [Rh(k 2 -P,P-dppe) 2 ] + (1), as its acetylacetonate (acac) salt [13,14]; coordination chemistry and theoretical DFT studies followed us to propose that complex 1 is the active catalytic species. When the reaction of this complex with an excess of formaldehyde ([CH 2 O]/[Rh] = 10) was carried out, it was impossible to detect any other species different than the cationic bis(dppe)rhodium complex [24]. We considered that the cationic bis(dppe)rhodium complex reacted reversibly with formaldehyde to produce [RhH(CHO) (k 2 -P,P-dppe) 2 ] + (2), which in absence of an unsaturated substrate (alkene or alkyne) come back to 1.…”

Controlling the Depolymerization of Paraformaldehyde with Pd–Phosphine Complexes

“…The mechanism of carbonylation reactions is well understood when CO gas is used. [18] In contrast to this there are very few reports that investigate the reaction mechanism when aC Os urrogate is used insteado fC Og as [19][20][21] and unfortunately even less is knowna bout the depolymerization of paraformaldehyde [22,23] in solution and the influence of homogeneous catalysts on the depolymerization process.…”