DABCO base strength

Tertiary amines are catalysts for both isocyanate-hydroxyl and isocyanate-water reactions. Generally, an increase in base strength in tertiary amines increases the catalytic strength. However, in the case of triethylene diamine (DABCO) the nucleophilicity is enhanced by the steric configuration. Electron-donating substituents enhance catalytic activity. Some tertiary amines are illustrated in Figure 2.20. 

In the rearrangement of 1-protio- and I-deuterio-l,3-dimethylindene, the isotope effect tended to increase with increasing base strength of the catalyst. At 30°C, potassium / r/.-butoxide in tert,-buty alcohol, sodium methoxide in methanol and DABCO in methanol gave k lko ratios of 8.1, 6.7, and 4.8,... 

In contrast with the above findings in water, condensations in chloroform exhibit a considerable specificity of the base used as a catalyst, as shown by two model reactions ethyl nitroacetate with norbornene and benzoylnitromethane with styrene [65]. A screening of several organic bases (tertiary amines and heteroaromatic A-bases with one or two basic sites) applied to these reactions indicates that the results are not related to the base strength. In fact, the most effective bases are those with the highest H-bonding basicity [74], particularly those having two basic sites, like DABCO, NMI, etc. [75-77]. 

Bases accelerate all the isocyanate reactions and in general their catalytic effect increases with increasing strength of the base. Table 4.7 compares the action of several amine catalysts at near ambient temperature. The significant increase in urethane reaction rate is apparent but particularly so in the case of triethylene diamine (l,4-diazo-[2,2,2]-bicyclo-octane), commonly known as DABCO. The reason for this is probably the complete lack of steric hindrance, given its cage-like structure. 

Two model reactions have been investigated with different bases ethyl nitroacetate with allyl alcohol ( in water example) and ethyl nitroacetate with styrene ( on water example). In both cases the results do not depend on the kind of base however, there is evidence that bases considerably weaker than nitronate have little, if any, catalytic effect on the reaction [72]. Thus, when the dipolarophile is an unsaturated carboxylic acid, the catalytic effect of an added base (sodium hydroxide or DABCO) depends on its acid strength, as... 

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