Cornelis A. Kruithof

Palladium-Based Telomerization of 1,3-Butadiene with Glycerol Using Methoxy-Functionalized Triphenylphosphine Ligands

Glycerol is considered a potential renewable building block for the synthesis of existing as well as new chemicals. A promising route is the telomerization of 1,3-butadiene with glycerol leading to C8 chain ethers of glycerol with applications in, for example, surfactant chemistry. Recently, we reported a new set of palladium-based homogeneous catalytic systems for the telomerization of 1,3-butadiene with glycerol and found that palladium complexes bearing methoxy-functionalized triphenylphosphine ligands are highly active catalysts capable of converting crude glycerol without any significant loss of activity. Herein, we present a detailed account of these investigations by reporting on the influence of the butadiene/glycerol ratio, temperature, and reaction time on product selectivity and activity allowing further optimization of catalyst performance. Maximum activity and yield were reached for high 1,3-butadiene/glycerol ratios at a temperature of 90 degrees C, whereas the selectivity for mono- and diethers of glycerol could be optimized by combining high reaction temperatures and short reaction times with low butadiene/glycerol ratios. Variation of the PdII metal precursors and the metal/ligand ratio showed that palladium precursors with halogen ligands gave unsatisfying results, in contrast to precursors with weakly coordinated ligands such as [Pd(OAc)2] and [Pd(acac)2]. [Pd(dba)2], the only Pd0 precursor tested, gave the best results in terms of activity, which illustrates the importance of the ability to form a Pd0 species in the catalytic cycle. Finally, base addition resulted in a shortening of the reaction time and most likely facilitates the formation of a Pd0 species. Based on these results, we were able to realize the first attempts towards a rational ligand design aimed at a high selectivity for mono- and diether formation.

Telomerization of 1,3-butadiene with various alcohols by Pd/TOMPP catalysts: new opportunities for catalytic biomass valorization

The telomerization of 1,3-butadiene with various alcohols has been investigated using a catalyst based on a Pd(acac)2 precursor and a phosphine ligand, TOMPP (TOMPP = tris-(o-methoxyphenyl)phosphine). We were able to demonstrate the capability of the catalyst to telomerize 1,3-butadiene with various multifunctional nucleophiles having primary and secondary alcohol functions. High yields of telomer products (>98%) were obtained in very short reaction times (<2 h). The telomerization activity and selectivity of the Pd/TOMPP complex was strongly influenced by the type of alcohol used as substrate. When diols were used, telomerization of 1,3-butadiene with 1,2-propanediol and 1,2-butanediol afforded the highest yield of mono-telomer (over 70%) and for 1,2-butanediol a turnover frequency (TOF) of 300 000 h−1 was reached, combined with a turnover number (TON) of 7800.

Solid-state structural characterization of cutinase-ECE-pincer-metal hybrids

The first crystal structures of lipases that have been covalently modified through site-selective inhibition by different organometallic phosphonate-pincer-metal complexes are described. Two ECE-pincer-type d(8)-metal complexes, that is, platinum (1) or palladium (2) with phosphonate esters (ECE = [(EtO)-(O=)P(-O-C(6)H(4)-(NO(2))-4)(-C(3)H(6)-4-(C(6)H(2)-(CH(2)E)(2))](-); E = NMe(2) or SMe) were introduced prior to crystallization and have been shown to bind selectively to the Ser(120) residue in the active site of the lipase cutinase to give cut-1 (platinum) or cut-2 (palladium) hybrids. For all five presented crystal structures, the ECE-pincer-platinum or -palladium head group sticks out of the cutinase molecule and is exposed to the solvent. Depending on the nature of the ECE-pincer-metal head group, the ECE-pincer-platinum and -palladium guests occupy different pockets in the active site of cutinase, with concomitant different stereochemistries on the phosphorous atom for the cut-1 (S(P)) and cut-2 (R(P)) structures. When cut-1 was crystallized under halide-poor conditions, a novel metal-induced dimeric structure was formed between two cutinase-bound pincer-platinum head groups, which are interconnected through a single mu-Cl bridge. This halide-bridged metal dimer shows that coordination chemistry is possible with protein-modified pincer-metal complexes. Furthermore, we could use NCN-pincer-platinum complex 1 as site-selective tool for the phasing of raw protein diffraction data, which shows the potential use of pincer-platinum complex 1 as a heavy-atom derivative in protein crystallography.

The Suzuki cross-coupling reaction: a powerful tool for the attachment of organometallic ‘NCN’-pincer units to biological scaffolds☆

Platinum(II) and palladium(II)NCN {NCN is the terdentate coordinating monoanionic ‘pincer’ ligand [C6H3(CH2NMe2)2-2,6]−} complexes have been covalently bonded via their para-position to both the α-carbon of an α-amino acid and to the γ-position of an alkyl phosphonate by means of Suzuki cross-coupling reactions. The resulting platinum(II) complexes can be used as biomarkers, while the palladium(II) analogs are active Lewis-acid catalysts. Both the pincermetal substituted α-amino acid and phosphonate can be used to introduce these organometallic units in biomolecules such as proteins or enzymes.