Katrin Petzold

Silylation of Cellulose and Starch – Selectivity, Structure Analysis, and Subsequent Reactions

The silylation of cellulose and starch under different starting conditions is reviewed. The control of the degree of substitution (DS) and regioselectivity in dependence of the reaction pathway are discussed in detail. The synthesis of trimethylsilyl cellulose (TMSC) in the system hexamethyldisilazane (HMDS)/ammonia leads to partially and completely silylated products controlled by the amount of the components. Hydrolytic desilylation of TMSC in tetrahydrofuran (THF)/ammonia gives the partially desilylated products. The desilylation proceeds statistically along the polymer chains. The reaction of cellulose dissolved in N,N-dimethylacetamide (DMA)/LiCl with bulky thexyldimethylchlorosilane (TDSCl) in the presence of imidazole leads to 2,6-di-O-TDS cellulose. The silylation of starch dissolved in dimethylsulfoxide (DMSO) with TDSCl/pyridine results in the formation of regioselectively 2-O and 6-O functionalized silyl ethers with DS values up to 1.8. 6-O Silyl ethers of cellulose and starch were synthesized with TDSCl highly activated in the reaction system N-methylpyrrolidone (NMP)/ammonia. Two- dimensional NMR techniques after subsequent modifications of the remaining OH groups have been established as important methods for the characterization of the substitution pattern of the described silyl ethers. In the case of starch, the distribution of the substituents could be detected not only in the anhydroglucose units (AGU) but also in the non-reducing end groups (NEG).

Chapter 16 – Cellulose Chemistry: Novel Products and Synthesis Paths

Publisher Summary Novel paths for homogeneous and regioselective functionalization of cellulose are discussed. The acylation of cellulose can be efficiently carried out by homogeneous phase chemistry applying solvents based on polar aprotic media or in ionic liquids and in situ activation of the carboxylic acid. Some unconventional cellulose derivatives are described. The regioselective derivatization of protected cellulosics leading to 3-O-, 2,3-O-, and 6-O-functionalized products is of recent interest showing remarkable differences in properties compared with common cellulose derivatives. The nucleophilic displacement reactions with cellulose tosylates provide a further tool for the design of biopolymer-based structures and properties.

Preparative and 1H NMR Investigation on Regioselective Silylation of Starch Dissolved in Dimethyl Sulfoxide

Reaction of starch 1 dissolved in dimethyl sulfoxide (DMSO) with bulky thexyldimethylchlorosilane (TDSCl) in the presence of pyridine leads to regioselectively functionalized silyl ethers with a degree of substitution (DS) up to 1.8. The control of the DSSi, of the regioselectivity, and of the reaction pathway is described in detail. The reaction proceeds homogeneously up to DSSi of 0.6. With ongoing silylation the polymers form a separate phase incorporating the silylating agent to form TDS-starches with DSSi values higher than 1.0. After peracetylation of the silyl starches, the substitution pattern has been characterized not only in the anhydroglucose repeating units (AGU) but also in the non-reducing terminal end groups (TEG) by means of two-dimensional 1H NMR techniques. Up to DSSi 1.0, a very high regioselective functionalization of the primary 6-OH groups in the AGU as well as in the TEG is detectable. With increasing silylation (DSSi > 1.0), the subsequent silylation takes place at the 2-OH groups of the AGU and at the 3-OH groups of the TEG. These results are compared with our own investigations on the silylation of starch in the reaction system N-methylpyrrolidone (NMP)/ammonia and on the silylation of cellulose in N,N-dimethylacetamide (DMA)/LiCl/pyridine solution.

Synthesis and NMR characterization of regiocontrolled starch alkyl ethers

Selective polysaccharide functionalization is of importance in molecular and supramolecular structure design and the development of new products and properties. As a typical partially branched polysaccharide, starch (72% amylopectin) was selectively and completely alkylated in the secondary positions. Starting from 6-O-thexyldimethylsilyl starch (6-O-TDS starch) prepared regio-controlled, an effective alkylation takes place by using methyl iodide and benzyl bromide, respectively, and sodium hydride as a base in THF solution. Under these conditions, the silyl ether linkages are stable. The subsequent desilylation proceeded with tetra-n-butylammonium fluoride (TBAF) under mild conditions to form the alkyl starches without cleavage of the alkyl ether linkage. The pattern of functionalization could be analyzed by using the silyl alkyl ethers or subsequently 6-O-acetylated derivatives and multidimensional NMR techniques. In the case of the methyl ethers, the substituent distribution could be detected not only in the anhydroglucose units (AGUs), but also in the non-reducing end-groups (NEGs). As a result, the starch ethers are alkylated regio-controlled in 2,3-di-O (AGU) and 2,3,4-tri-O (NEG) positions.