DABCO catalysts

The simultaneous use of urea, or thiourea [76] and DABCO catalyst was introduced by the Connon group for the addition of methyl acrylate and benzaldehyde [39]. The study revealed that, although both ureas and thioureas accelerated the reaction relative to the uncatalyzed process, urea was superior to thiourea in terms of stability and efficiency. Chiral thiourea derivatives may offer, however, superior enantioselectivity. It was postulated, that the catalysts operate mainly via a Zimmerman-Traxler-type transition state 69 for addition of the resulting enolate anion to the aldehyde (Scheme 5.15). 

This study compares the effect of catalysts on aliphatic and aromatic isocyanates. With the exception of di-n-butyltin dithiocarbonate, all the di-n-butyltin catalysts perform similarly. The DABCO catalyst shows excellent catalysis for aromatic isocyanates and is less effective for aliphatic isocyanates. Combining this amine catalyst with DBTDL gives excellent catalytic activity for both aliphatic and aromatic isocyanates. Stannous, zirconium, and zinc octanoate show reduced activity in comparison to organotin. 

DABCO catalyst (triethylenediamine, Air Products and Chemicals, Inc.) was purified by sublimation and stored in a desiccator. 

TABLE I. SYNERGISTIC EFFECT OF DABCO CATALYST AND DIBUTYLTIN DILAURATE FOR THE CATALYSIS OF PHENYL ISOCYANATE (0.07M) REACTION WITH BUTANOL-2 (0.07H) IN DIOXANE AT 25°C... 

Evidence for this complex formation between DBTDL and DABCO catalyst was obtained from nuclear magnetic resonance studies. 

Another supporting evidence for complex formation as a prerequisite to synergism was obtained from the study of the catalysis of phenyl isocyanate-butanol reaction by soluble organic cobalt compounds in presence and absence of DABCO catalyst. The results obtained are presented in Figures 4 and 5. It is evident that the combination of DABCO catalyst with divalent cobalt compounds shows synergistic effects while the trivalent cobalt acetylacetonate shows relatively low activity. The explanation of these observations is the structure of these compounds. 

For the purpose of calculating results it is necessary to know the weight of 50 cm of the Dabco catalyst solution at 25 1 °C. This may be obtained simply by weighing 50 cm of the reagent from the thermostatted stock flask into a dry 250 cm stoppered conical flask. 

Place the two 1 litre reagent flasks containing the stocks of 1.000 0.005 M phenyl isocyanate solution and of 0.56% w/v Dabco catalyst solution in a separate water bath thermostatted at 25 1 °C, and allow these flasks to reach this temperature over a period of 30 minutes. 

When the reaction flask containing the toluene solution of polyol has reached 40 °C, pipette into this flask 50 cm of normal phenyl isocyanate stock solution (at 25 °C), allowing a 15 s pipette draining time. No reaction occurs between polyol and phenyl isocyanate in the absence of Dabco catalyst. Leave the mixture for 15 minutes to reach thermal equilibrium. Transfer, by pipette, 50 cm of stock 0.56% wiv Dabco solution from the storage vessel (thermostatted at 25 °C) into a dry 50 cm volumetric flask. Remove the B34/B24 sampling adaptor from the reaction flask assembly and insert the neck of the 50 cm volumetric flask into the B34 socket to introduce the Dabco catalyst into the reaction mixture. Allow a 15 s draining time for the Dabco flask and replace the B34/B24 sampling adaptor. Start the stopwatch at the same moment that the Dabco solution is first tipped into the reaction flask, (i.e., at the exact moment that the Dabco catalysed reaction between polyol and phenyl isocyanate commences). 

Di-n-butylamine flask number Test Time interval between adding Dabco catalyst (i.e., start of reaction) and sampling for phenyl isocyanate determination (min)... 

The small amount of Dabco catalyst present in samples taken from the reaction flask interferes slightly in these determinations of residual phenyl isocyanate. This interference is allowed for in the method of calculation. 

The hydrochloric acid titration correction (Tp cm ) required to allow for the presence of Dabco catalyst in the portion of reaction solutions W (g) withdrawn for phenyl isocyanate determination is given by ... 

Dabco is the trade name for triethylene diamine (Jacobson and Van den Burg and Co. (U.K.) Ltd, 3-5 Crutched Friars, London, E.C.3). Weigh out 6 0.1 g Dabco and transfer to an over-dried 1 litre volumetric flask. Add dry toluene (less than 15 ppm water) from the dispenser (see Figure 7.12) to make the volume up to approximately 950 cm Dissolve the Dabco by shaking and stand the flask in a water bath thermostatted at 25 + rC. When the solution has reached 25°C make up to the 1 dm mark with dry toluene and mix thoroughly. Stand the solution in a water bath, thermostatted at 25 + rC. A measured volume of this solution then contains a constant weight of Dabco catalyst. 

In 1995 Hirama and coworkers [16] reported an asymmetric Baylis-Hillman reaction catalyzed by chiral 2,3-disubstituted DABCO derivatives under increased pressure (Scheme 21.2a). Hirama observed an enhancement of both reaction rate and enantioselectivity for the reaction of 4-nitrobenzaldehyde with MVK under 5 kbar. At atmospheric pressure the reaction proceed very slowly (3 weeks) with low enantioselectivity (12-15% ee). The best results in terms of yield and enantioselectivity (up to 47%) were obtained with chiral DABCO catalyst bearing benzyl or 1-naphthyl groups. This is the first example of an organocatalytic reaction demonstrating that an increase in pressure can significantly enhance the enantioselectivity (e.g., from 12% to 47%). 

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