Taking advantage of the structural diversity of different biomass resources, recent efforts were directed towards the synthesis of renewable monomers and polymers, either for the substitution of petroleum-based resources or for the design of novel polymers. Not only the use of biomass, but also the development of sustainable chemical approaches is a crucial aspect for the production of sustainable materials. This review discusses the recent examples of chemical modifications and polymerizations of abundant biomass resources with a clear focus on the sustainability of the described processes. Topics such as synthetic methodology, catalysis, and development of new solvent systems or greener alternative reagents are addressed. The chemistry of vegetable oil derivatives, terpenes, lignin, carbohydrates, and sugar-based platform chemicals was selected to highlight the trends in the active field of a sustainable use of renewable resources.
Herein, a novel approach is reported for the synthesis of medium- and long-chain aliphatic polyethers 2 based on the GaBr -catalysed reduction of polyesters 1 with TMDS as the reducing agent. Thus, various linear and branched aliphatic polyesters 1 were prepared and systematically investigated for this reduction strategy, demonstrating the applicability and versatility of this new polyether synthesis protocol. Medium- and long-chain chain polyethers were obtained from the respective polyesters without or with minor chain degradation, whereas short-chain polyesters, such as poly-l-lactide 1 i and poly[(R)-3-hydroxybutanoate] 1 j, showed major chain degradation. In this way, previously unavailable and uncommon polyethers were obtained and studied.
An efficient synthesis strategy for the preparation of two renewable polyesters and one renewable polyamide via catalytic oxyfunctionalization of methyl 10‐undecenoate, a castor oil derived platform chemical, is described. The keto‐fatty acid methyl ester (keto‐FAME) is synthesized applying a cocatalyst‐free Wacker oxidation process using a high‐pressure reactor system. For this purpose, catalytic amounts of palladium chloride are used in the presence of a dimethylacetamide/water mixture and molecular oxygen as sole reoxidant. The thus derived AB monomers (hydroxy‐esters, amine‐ester) are synthesized from the obtained keto‐FAME through Baeyer–Villiger oxidation and subsequent transesterification, reduction, or reductive amination, respectively. The resulting AB step‐growth monomers are then studied in homopolymerizations using 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene, DBU, and titanium(IV) isopropoxide as transesterification catalyst, yielding polymers with molecular weights (Mn) up to 15 kDa. The polyesters and the polyamide are carefully characterized by FTIR, SEC, 1H‐NMR spectroscopy, and differential scanning calorimetry analysis.
Two synthetic approaches to functionalize plant oil derived platform chemicals were investigated. For this purpose, methyl 10‐undecenoate, which can be obtained by pyrolysis of castor oil, was used in olefin cross‐metathesis under neat conditions forming an unsaturated α,ω‐acetoxy ester. A catalyst screening with 11 different ruthenium‐based metathesis catalysts was performed, revealing that well‐suited catalysts allow for full conversion and very good cross‐metathesis selectivity at a loading of only 0.5 mol%. An alternative possibility to the aforementioned synthetic method is a palladium‐catalyzed reaction of methyl 10‐undecenoate with acetic acid in the presence of dimethyl sulfoxide. Here, the formation of linear and branched unsaturated acetoxy esters as well as a ketone was observed. The conversion as well as the selectivity of this procedure was studied under different reaction conditions and compared to the cross‐metathesis results. Based on the successful functionalization of methyl 10‐undecenoate, methyl oleate was investigated in this palladium‐catalyzed CH activation reaction. Due to the lower reactivity of the internal double bond the desired acetoxy ester was only obtained in moderate conversion in this case. In summary, this study clearly shows that palladium‐catalyzed functionalization of unsaturated fatty compounds via CH activation is an attractive alternative to the well‐established olefin cross‐metathesis procedure.
Two efficient strategies for a direct catalytic and regioselective acetoxylation of terpenes are described.Acetoxylated limonene derivatives were synthesized via palladium-catalyzed C-H activation utilizing para-benzoquinone (BQ) as reoxdidation agent and acetic acid as solvent and reactant. Addition of dimethyl sulfoxide (DMSO) to the catalytic system led to highly selective functionalization of the exocyclic double bond of limonene. This catalytic acetoxylation of limonene was further optimized with regard to a more sustainable and environmentally-friendly procedure. On the other hand, the use of an aerobic tandem catalytic system using iron(II) phthalocyanine (Fe(Pc)) as co-catalyst, which acts as electron transfer mediator (ETM), enabled a highly selective acetoxylation of the endocyclic double bond of limonene with high conversions. Moreover, diacetoxylated products were prepared by a reaction sequence applying the aforementioned catalytic systems.
A novel and versatile route toward dimer fatty acid methyl esters (dimer FAMEs) via catalytic oxidation and reductive amination is described. The oxyfunctionalization of mono-unsaturated FAMEs bearing different chain lengths (C11, C18, C22) is accomplished by a co-catalyst-free Wacker Oxidation process in a high pressure reactor. The applied catalytic system of palladium(II) chloride in a dimethylacetamide/water mixture enabled the formation of ketoFAMEs in the presence of molecular oxygen as sole re-oxidant. In a first attempt, partially renewable dimer FAMEs are synthesized by reductive amination of keto-FAME (C18) in the presence of various aliphatic and aromatic diamines and sodium triacetoxyborohydride as selective reducing agent. In another approach, the keto-FAMEs directly underwent reductive amination using Raney-Nickel in order to obtain the corresponding aminoFAMEs. Subsequently, the keto-and amino-FAMEs are used for the synthesis of fully renewable dimer FAMEs via reductive amination with sodium triacetoxyborohydride as reducing agent. In order to demonstrate a possible application for these new dimer FAMEs, three out of the thirteen synthesized dimer FAMEs are selected and studied in a polycondensation with renewable 1,10-diaminodecane using TBD as catalyst. The polyamides are obtained in molecular weights (M n ) of up to 33 kDa and are carefully characterized by 1 H-NMR spectroscopy, FTIR, SEC, and DSC analysis. Practical Applications: The described catalytic route to defined dimer fatty acids has potential applications in lubricants, detergents, polymeric materials, and others. Compared to commercially available dimer fatty acids, a molecularly highly defined mixture of regioisomers is obtained, without contamination of monofunctional or trifunctional fatty acid derivatives, which is advantegous for most of the mentioned applications. Especially for polycondensations, the known stoichiometry of exactly two is a considerable benefit.
Highly efficient synthesis processes for the selective catalytic oxidation of unsaturated fatty acid methyl esters (FAMEs) are described, leading to valuable renewable platform chemicals. Here keto-FAMEs derived from methyl oleate and methyl erucate were synthesized via a cocatalyst-free Wacker oxidation process using a high pressure reactor system. The catalytic system of palladium(II) chloride in dimethylacetamide/water enabled the oxidation of monounsaturated FAMEs in the presence of molecular oxygen as sole reoxidant with reduced amounts of solvent in shorter reaction time compared to previous reports. The high product selectivity is confirmed by two different synthesis approaches and the characterization of thereof derived products using mass spectrometry measurements (GC-MS, ESI-MS-MS). The obtained keto-FAMEs are used in a subsequent Baeyer–Villiger oxidations with m-CPBA, enabling the synthesis of diesters, thus allowing access to various platform chemicals (e.g., hydroxy-esters, fatty alcohols). Moreover, an enone derivative, which can be obtained through selective photo peroxidation of methyl oleate, is studied in the cocatalyst-free Wacker oxidation process leading to a 1,3-diketone. Additionally, the same enone is used in the Baeyer–Villiger oxidation with Oxone and m-CPBA as oxidant, allowing the highly selective synthesis of a vinyl acetate and an epoxy diester derivative, respectively.
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