Phenyl isocyanate reaction

A 1,4-dipolar cycloaddition between tetrahydropyrido[l,2-a]pyrimidi-none 114 (R = Me) and 4-methyl-l, 2,4-triazoline-3,5-dione 666gave stable adduct 667 in acetonitrile or in acetic acid at room temperature for 1 hour (Scheme 44) (85CB4567). When ethyl cyanoformate was used as dienophile in boiling toluene for 20 hours, ethyl 3-methyl-4-oxo-6,7,8,9-tetrahydro-4//-pyrido[ 1,2-a]pyrimidine-2-carboxylate 669 was obtained (86CB1445). Pyrido[l,2-a]pyrimidine-2-carboxylate 669 was formed from the initial adduct 668 by elimination of phenyl isocyanate. Reaction of tetrahydropyr-ido[l,2-a]pyrimidinone 114 (R = Me) with l-(diethylamino)-l-propyne in... 

In view of the fact that the methanol-phenyl isocyanate reaction is known to be an auto-catalytic reaction due to the weakly basic character of the phenylurethane formed, one would expect an upward drift of the second-order plot if the reaction were truly of the second order. This is apparently the case in n-butyl acetate and dioxane. The straight line plot in MEK and acetonitrile and the downward curvature found in toluene and nitrobenzene were taken to indicate an order greater than 2 with respect to the reactants. It appeared that the deviations from second-order kinetics toward higher order in the sequence toluene > nitrobenzene > acetonitrile > MEK > n-butyl acetate > dioxane increased with an increase in the hydrogen bonding capacity of the solvent. 

Entelis assumes the formation of an activated alcohol-isocyanate binary complex during the catalysis of the methanol-phenyl isocyanate reaction by dibutyltin dilaurate (DBTDL) (3, 5) Activated alcohol-isocyanate-catalyst ternary complexes have also been proposed by others. However, significant differences can be noted in the structures of either the postulated one (2, 4, 6, 7) or two (8) coordination positions of the isocyanate to the metal. To explain the synergistic effects observed when tertiary amine and organometallic compounds are combined, several authors suggest the formation of an activated quaternary complex I, II or III (2, 6, 9, 10, 11, 27). 

The effect of solvents on the kinetics of the phenol-phenyl isocyanate reaction is seen in Table V. The reactivity increased in the following order dioxane < toluene < DMF (1 3.3 92). The difference in reactivity is due to the combined effects of the relative permitivity and the specific solvation. 

Figure 3.13 shows the reaction kinetics curves (conversion against time) of the phenyl isocyanate reaction with oligo-polyols having various percentages of primary hydroxyls. 

Amine pK Relative rate of butanol/ phenyl isocyanate reaction... 

Table 14.3. Catalytic activity in phenyl isocyanate reactions [5].

Similarly, withdraw samples from the polyol/phenyl isocyanate reaction solution at various other time intervals (see Table 3.21 after commencement of the reaction and immediately introduce the sample into 10 ml di-n-butylamine in order to analyse it for phenyl isocyanate content as described previously. Tabulate the titrations obtained. Titrations of the 10 cm 0.2 M di-n-butylamine blank solution carried out at the beginning and at the end of the experiment usually agree within 0.05 cm. ... 

Table 1.15 - Comparison of Results Obtained on Polypropylene-glycol by Catalyzed Acetylation, Phthalation and Phenyl Isocyanate Reaction... 

Pipette 10 ml 0.2 N di-n-butylamine solution into each of twelve dry 100 cm conical flasks and quickly replace the stoppers (15 s pipette draining time). Ensure that the same amoimt of di- -butylamine is introduced into each flask by keeping the temperature of the stock di- -butylamine solution constant during the pipetting. These flasks will be used later for the determination of the phenylisocyanate content of samples withdrawn from the polyol/phenyl isocyanate reaction at various times. 

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