Abstract
The catalytic activity of Pd/TOMPP (TOMPP: tris(2-methoxyphenylphosphine) in the telomerization of 1,3-butadiene was screened with carbohydrates and phenolics that can be potentially derived from lignocellulosic biomass. High conversions and selectivities were obtained and clear structure-activity relationships were found for all of the tested classes of substrates. These results demonstrated the
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high potential of the Pd/TOMPP catalyst for the telomerization of 1,3-butadiene with biomass-based oxygenates. In case of aldose and ketose sugars, catalyst deactivation was observed, which was related to the participation of the anomeric hydroxyl group. For phenolic substrates it was observed that the composition of the reaction medium strongly influences the product selectivity. These observations could not be easily explained using the well-known telomerization mechanism and prompted detailed mechanistic studies which emphasize the influence of the substrate properties on catalytic activity and selectivity. Analysis of reaction mixtures showed that the ligand is alkylated towards phosphonium species during catalysis and that the extent of this reaction correlates to the deactivation found for sugars. It was concluded that phosphine depletion caused the observed deactivation. Coordination of the phosphonium species to palladium complexes resulted in the formation [PdI(1-3η-octa-2,7-dien-1-yl)(TOMPP)] (F) and [Pd(1-3,7,8η-octa-2,7-dien-1-yl)(TOMPP)]BF4 (C), which are known reactive intermediates of the telomerization cycle and demonstrated that the alkylation reaction is reversible. Similarly, the reversibility of the telomer product formation reaction was studied a led to the development of a simple one-pot procedure for the efficient synthesis of complexes of type C. The influence of ligand structure on the reactivity of these complexes was studied by nucleophilic addition reactions and the reactivity trend could be rationalized by steric, electronic and electrostatic considerations. The reaction of the C with 1 eq. phosphine was studied in different solvent compositions and showed that the stability of cis-[Pd(1-3η-octa-2,7-dien-1-yl)(TOMPP)2]+ (E), another key intermediate which is known to exhibit lower regioselectivity than C, is heavily dependent on the solvent composition. Equilibration reactions of linear phenol telomers also confirmed that product formation via E is also reversible. Furthermore, the formation of 1,3,7-octatriene was also observed and a new mechanism was proposed to account for the increasing selectivity towards the Z-isomer of this byproduct. Two new heterogeneous catalyst systems were tested in combination with biomass-based substrates. First, layered double hydroxide materials were use as basic, ion-exchange material for the immobilization of Pd/TPPTS (TPPTS: 3,3’,3’’-phosphinidynetris(benzenesulfonic acid) trisodium salt) complexes. The immobilized catalysts were tested in the heterogeneous, solvent-free telomerization of 1,3-butadiene with ethylene glycol and methanol and performed comparably to the homogeneously catalyzed reaction. Next to the expected C8 products, higher telomers with C16 or C24 chains were observed and a chain-transfer mechanism was proposed which accounts for this change in selectivity. Second, a novel porous phosphine-based covalent framework, PolyPPh3 was prepared used as a support Pd(acac)2 complexes. The loaded material was tested as catalyst in the heterogeneous, solvent-free telomerization of 1,3-butadiene with phenol and glycerol and high conversions and selectivities were observed. In case of glycerol, Pd/PolyPPh3 outperformed its homogeneous analog Pd/PPh3.
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