Vilsmeier-Haack adduct

There are several schemes for the synthesis of cellulose formates (slow) reaction of the polymer with formic acid faster reaction in the presence of a mineral acid catalyst, e.g., sulfuric or phosphoric acid. The latter route is usually associated with extensive degradation of the polymer chain. Reaction of SOCI2 with DMF produces the Vilsmeier-Haack adduct (HC(Cl) = N (CH3)2C1 ) [145]. In the presence of base, cellulose reacts with this adduct to form the unstable intermediate (Cell - O - CH = N" (CH3)2C1 ) from which cellulose formate is obtained by hydrolysis. The DS ranges from 1.2 to 2.5 and the order of reactivity is 5 > C2 > C3 [140-143,146]. 

A comment on the properties of the base employed in reactions that involve the formation of the Vilsmeier-Haack adduct is in order, because several derivatives of cellulose are obtained by this route. Preparation of Cell-Tos has been attempted in LiCl/DMAc, by reacting the polymer with TosCl/base. Whereas the desired product was obtained by employing triethy-lamine, use of pyridine (Py) resulted in the formation of chlorodeoxycellu-lose. In order to explain these results, the following reaction pathways have been suggested [147] ... 

That is, the difference between the mechanisms of action of the two bases lies in the ability of EtsN to add to the Vilsmeier-Haack adduct to form a tetrahedral intermediate, susceptible to Sn2 attack by (at least partially de-protonated) cellulose. This leads to formation of the desired Cell - Tos. 

Well known compounds of this type are Vilsmeier-Haack adducts, imino esters, amidines, carboxylic orthoesters, orthoamides etc., which have proven their synthetic utility in thousands of reactions. As a consequence of the synthetic importance of these compounds, more or less extensive reviews are available for all of them. 

The Vilsmeier-Haack type adduct, formed by the reaction of oxalyl chloride with DMF can be also be employed for the activation of carboxylic acids, as shown in Fig. 8 [200]. 

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