Directing Selectivity to Aldehydes, Alcohols, or Esters with Diphobane Ligands in Pd-Catalyzed Alkene Carbonylations

Abstract

Phenylene-bridged diphobane ligands with different substituents (CF3, H, OMe, (OMe)2, tBu) have been synthesized and applied as ligands in palladium-catalyzed carbonylation reactions of various alkenes. The performance of these ligands in terms of selectivity in hydroformylation versus alkoxycarbonylation has been studied using 1-hexene, 1-octene, and methyl pentenoates as substrates, and the results have been compared with the ethylene-bridged diphobane ligand (BCOPE). Hydroformylation of 1-octene in the protic solvent 2-ethyl hexanol results in a competition between hydroformylation and alkoxycarbonylation, whereby the phenylene-bridged ligands, in particular, the trifluoromethylphenylene-bridged diphobane L1 with an electron-withdrawing substituent, lead to ester products via alkoxycarbonylation, whereas BCOPE gives predominantly alcohol products (n-nonanol and isomers) via reductive hydroformylation. The preference of BCOPE for reductive hydroformylation is also seen in the hydroformylation of 1-hexene in diglyme as the solvent, producing heptanol as the major product, whereas phenylene-bridged ligands show much lower activities in this case. The phenylene-bridged ligands show excellent performance in the methoxycarbonylation of 1-octene to methyl nonanoate, significantly better than BCOPE, the opposite trend seen in hydroformylation activity with these ligands. Studies on the hydroformylation of functionalized alkenes such as 4-methyl pentenoate with phenylene-bridged ligands versus BCOPE showed that also in this case, BCOPE directs product selectivity toward alcohols, while phenylene-bridge diphobane L2 favors aldehyde formation. In addition to ligand effects, product selectivities are also determined by the nature and the amount of the acid cocatalyst used, which can affect substrate and aldehyde hydrogenation as well as double bond isomerization.

Reppe‐Carbonylation of Alkenes with Carboxylic Acids: A Catalytic and Mechanistic Study

“…As a model reaction, we studied the carbonylation of 1‐octene in the presence of nonanoic acid, in order to produce the symmetric anhydride, nonanoic anhydride (Table 1). 1‐Octene is widely been used in carbonylation reactions, including hydroformylation [23–28] …”

“…1-Octene is widely been used in carbonylation reactions, including hydroformylation. [23][24][25][26][27][28] We found that an initial catalyst system consisting of Pd(OAc) 2 , PPh 3 and trifluoroacetic acid is capable of catalyzing the Reppe-carbonylation of 1-octene with nonanoic acid in the presence of 50 bar CO (Entry 1). In order to optimize the catalyst concentration, the effect of the Pd loading (0.2-1 mol %) on the yield of the nonanoic anhydride was investigated (Entries 1-3), while keeping the ratio of Pd:PPh 3 (1 : 20) and Pd:TFA (1 : 40) constant.…”

Tetramethylphosphinane as a new secondary phosphine synthon

“…Heterocyclic phosphines offer unique properties that can be tuned by ring size, substituents and functional groups 20 – 23 . In order to access more ligands incorporating bulky phosphinanes, we targeted the synthesis of the secondary phosphine synthon, TMPhos.…”

Dioxaphosphabicyclooctanes: small caged phosphines from tris(hydroxymethyl)phosphine

“…17 We have been interested in developing new routes to bulky diphosphine ligands, as some of these have proved effective at Pd-catalysed isomerising carbonylation reactions. 2,18…”