Theoretical studies on the overall catalytic cycle of isomerizing alkoxycarbonylation reveal the steric congestion around the diphosphine coordinated Pd-center as decisive for selectivity and productivity. The energy profile of isomerization is flat with diphosphines of variable steric bulk, but the preference for the formation of the linear Pd-alkyl species is more pronounced with sterically demanding diphosphines. CO insertion is feasible and reversible for all Pd-alkyl species studied and only little affected by the diphosphine. The overall rate-limiting step associated with the highest energetic barrier is methanolysis of the Pd-acyl species. Considering methanolysis of the linear Pd-acyl species, whose energetic barrier is lowest within all the Pd-acyl species studied, the barrier is calculated to be lower for more congesting diphosphines. Calculations indicate that energy differences of methanolysis of the linear versus branched Pd-acyls are more pronounced for more bulky diphosphines, due to involvement of different numbers of methanol molecules in the transition state. Experimental studies under pressure reactor conditions showed a faster conversion of shorter chain olefin substrates, but virtually no effect of the double bond position within the substrate. Compared to higher olefins, ethylene carbonylation under identical conditions is much faster, likely due not just to the occurrence of reactive linear acyls exclusively but also to an intrinsically favorable insertion reactivity of the olefin. The alcoholysis reaction is slowed down for higher alcohols, evidenced by pressure reactor and NMR studies. Multiple unsaturated fatty acids were observed to form a terminal Pd-allyl species upon reaction with the catalytically active Pd-hydride species. This process and further carbonylation are slow compared to isomerizing methoxycarbonylation of monounsaturated fatty acids, but selective.

To study the influence of electronics on catalytic polymerization properties independent from sterics, phosphinesulfonato Pd(II) complexes bearing remotely located substituents on the nonchelating P-bound aryls [κ 2 -(P,O)-(4-R-2-anisyl) 2 PC 6 H 4 SO 2 O]Pd(Me)(dmso) (1a−edmso: 1a, R = CF 3 ; 1b, R = Cl; 1c, R = H; 1d, R = CH 3 ; 1e, R = OCH 3 ) were prepared. The electron-poor complex 1admso (4-CF 3 ) undergoes the fastest insertion of methyl acrylate (MA) and is the most active for ethylene polymerization. The polyethylene molecular weight increases by a factor of 2 for the more electron rich complex 1e-dmso (4-OCH 3 ) (M n = 17 × 10 3 vs 8 × 10 3 for 1a-dmso (4-CF 3 )). MA/ ethylene copolymerization experiments revealed that the MA incorporation ratio and copolymer molecular weights are largely independent of the electronic nature of the remote substituents. These trends were further confirmed by studies of two mixed Paryl/-alkyl complexes 1f-dmso ([κ 2 -(2,4,6-(OMe) 3 C 6 H 2 )( t Bu)PC 6 H 4 SO 2 O]Pd(Me)(dmso)) and 1g-dmso ([κ 2 -(C 6 H 5 )( t Bu) PC 6 H 4 SO 2 O]Pd(Me)(dmso)). In ethylene/MA copolymerization, 1f-dmso affords a significantly higher molecular weight polymer with reasonable MA incorporation (M n = 12 × 10 3 and 7.7 mol % MA) and activities similar to those observed for complexes 1a−e-dmso.

Isomerizing functionalization reactions that convert the internal double bonds of unsaturated fatty acids from plant or algae oils to a terminal functional group are attractive because they can generate linear long-chain α,ω-difunctional compounds that incorporate the entire length of the substrates chain. The state of the art toward this formidable synthetic challenge via different catalytic approaches, namely isomerizing borylations, silylations, and carbonylations (and for comparison, olefin metathesis) is reviewed comprehensively and analyzed with regard to underlying mechanistic principles, performance, practicability, and scope.

Dicarboxylic acids are compounds of high value,\ud but to date long-chain α,ω-dicarboxylic acids have been\ud difficult to access in a direct way. Unsaturated fatty acids are\ud ideal starting materials with their molecular structure of long\ud methylene sequences and a carboxylate functionality, in\ud addition to a double bond that offers itself for functionaliza-\ud tion. Within this paper, we established a direct access to α,ω-\ud dicarboxylic acids by combining isomerization and selective\ud terminal carbonylation of the internal double bond with water\ud as a nucleophile on unsaturated fatty acids. We identified the\ud key elements of this reaction: a homogeneous reaction mixture\ud ensuring sufficient contact between all reactants and a catalyst\ud system allowing for activation of the Pd precursor under\ud aqueous conditions. Experiments under pressure reactor\ud conditions with [(dtbpx)Pd(OTf)2] as catalyst precursor revealed the importance of nucleophile and reactant concentrations and the addition of the diprotonated diphosphine ligand (dtbpxH2)(OTf)2 to achieve turnover numbers >120. A variety of unsaturated fatty acids, including a triglyceride, were converted to valuable long-chain dicarboxylic acids with high turnover numbers and selectivities for the linear product of >90%. We unraveled the activation pathway of the PdII precursor, which proceeds via a reductive elimination step forming a Pd0 species and oxidative addition of the diprotonated diphosphine ligand, resulting in the formation of the catalytically active Pd hydride species. Theoretical calculations identified the hydrolysis as the rate-determining step. A low nucleophile concentration in the reaction mixture in combination with this high energetic barrier limits the potential of this reaction. In conclusion, water can be utilized as a nucleophile in isomerizing functionalization reactions and gives access to long-chain dicarboxylic acids from a variety of unsaturated substrates. The activity of the catalytic system of hydroxycarbonylation ranks as one of the highest achieved for isomerizing functionalizations in combination with a high selectivity for the linear product

Catalysis by soluble metal complexes often encompasses the occurrence of nanoparticles and aggregated inactive states, but the role of the different species is unclear. For the generation of highly active catalysts, it is crucial to know the relations between active and inactive states and the reversibility of such interconversions. This is further complicated by the question of the true nature of the active species, in particular for reactions that use dispersed nanoparticles as a catalyst (precursor), which can interconvert to soluble species. We show that a molecular catalyst can interconnect completely up to the step of bulk metal in an isomerizing methoxycarbonylation converting an unsaturated fatty acid to a linear diester with a very characteristic reactivity. The active species, a diphosphine-coordinated Pd hydride, decomposes to the diprotonated diphosphine ligand and a Pd0 species that agglomerates and precipitates as Pd black. This reaction is completely reversible, as shown for several examples of Pd0 precursors. Precise Pd nanocrystals enabled imaging of the dissolution process with the diprotonated diphosphine ligand via transmission electron microscopy. The characteristic selectivity of the isomerizing methoxycarbonylation is a clear indicator of the molecular nature of the active species. This was further demonstrated by NMR spectroscopy via capture of the active Pd hydride formed from Pd nanocrystals. CO, often a problematic reductant, acts as a stabilizer of the molecular oxidized catalyst species. The activation of Pd0 was also realized from macroscopically separated bulk species, like Pd black, sponge, and wire, evidencing that even highly agglomerated states of Pd can be converted to the molecular catalytically active species.

A dual catalysis approach enables selective functionalization of unconventional feedstocks composed of complex fatty acid mixtures with highly unsaturated portions like eicosapentaenoate (20:5) along with monounsaturated compounds. The degree of unsaturation is unified by selective heterogeneous hydrogenation on Pd/γ-AlO, complemented by effective activation to a homogeneous carbonylation catalyst [(dtbpx)PdH(L)] by addition of diprotonated diphosphine (dtbpxH)(OTf). By this one-pot approach, neat 20:5 as a model substrate is hydrogenated to up to 80% to the monounsaturated analogue (20:1), this is functionalized to the desired C α,ω-diester building block with a linear selectivity of over 90%. This catalytic approach is demonstrated to be suitable for crude microalgae oil from Phaeodactylum tricornutum genetically engineered for this purpose, as well as tall oil, an abundant waste material. Both substrates were fully converted with an overall selectivity to the linear α,ω-diester of up to 75%.

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