Preparative and1H NMR Investigation on Regioselective Silylation of Starch Dissolved in Dimethyl Sulfoxide

Abstract

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.