Stduy On Mechanism Of Cobalt Catalyzed Hydroformylation Of α-olefins

Hydroformylation enables olefins converting to aldehydes.For the Co catalyzed a-olefin hydroformylation,using the density functional theory,the reaction mechanism,chem-/regio-selectivity,spatial effect of phosphine ligands and enantioselectivity were theoretically studied.Regioselectivity for HCo(CO)4 catalyzed a-hexene hydroformylation depends on the addition modes of C=C in a-hexene and Co-H bond.Energy barrier(6.69 kcal/mol)for the transition state B-TS1 in the branched reaction path is 0.14 kcal/mol higher than that(6.55 kcal/mol)for the transition state L-TS1 in the linear reaction path,indicating the poor regioselectivity.There are two reaction routes(pathl and path2)for the aldehyde formation form H2 by means of HCo(CO)4.Path2 requires lower temperature(25℃)and the reaction rate is inversely proportional to CO pressure.Pathl is the main reaction route since higher hydroformylation temperature favors adsorption-activation of H2 and H2 is dense.Increasing CO concentration can inhibit the chemical selectivity of hydroformylation.The ddition mode between C=O bond in heptanal and Co-H bond is different.n-Heptanal hydrogenation to n-heptanol takes two reaction paths A and B.Energy barriers(29.83 kcal/mol,37.67 kcal/mol)of rate-determining steps for the H2 oxidation addition are too high.Higher temperature and pressure are needed for the n-heptanal hydrogenation to n-heptanol.Energy barrier of rate-determining step for by-product heptyl formate formation is 20.01 kcal/mol.The reaction occurs easily.For Co(II)-catalyzed hydroformylation of a-hexene,the C=C bond is coordinated in parallel mode with the Co-H bond(intermediates:INT1L and INT1B),attributing to the stereoscopic effect of coordinated Co(II)catalytic center.Similarly,difference in the addition mode between C=C and Co-H bonds is the only reason for regioselectivity.Selectivity for the linear aldehyde product is poor.Phosphine ligands of strong electron attractivity raise the activity and stability of HCo(CO)2L.PPh3 with large steric hindrance suppresses the addition reaction between the C=C of α-hexene and the Co-H of HCo(CO)2L.The transition state activation energy difference(ΔΔE)between B-TS1 for branched chain alkyl Co intermediate and L-TS1 for linear chain one is 2.73 kcal/mol,suggesting that formation of the former take place difficultly whereas the addition via the linear chain pathway takes thermodynamic advantage.The electron and the steric hindrance of phosphine ligands both affect the addition reaction mode and favor addition in the linear chain pathway,with linear chain aldehydes as main products.Strong stereoscopic effect of binaphos liganded Rh catalytic center is conducive to that the C=C bond and the Rh-H bond act in parallel conformation(INT19R1ax and INT19S2ax),promoting the catalytic asymmetric hydroformylation,that is,via two pathways of R1 and S2.The transition state energy barrier for R1,however,is merely 0.95 kcal/mol higher than that for S2.Selectivity for the enantiomer product is good.