DABCO mechanisms

A prepolymer is made first by charging Pluracol E2000 [1000.0 g, 1.0 eq., poly(ethylene oxide), 56 OH, BASF] to a suitable container equipped with a mechanical stirrer and a nitrogen gas inlet. Flush the container with dry nitrogen and add Desmodur W (264.0 g, 2.0 eq., 4,4 -methylene-bis(cyclohexyl isocyanate), 31.8% NCO, Bayer). While maintaining a positive N2 pressure on the reaction mixture, stir and heat at 80°C for 2 h. Cool the product to room temperature and check the NCO content (theory = 3.32 %). It might be necessary to warm the highly viscous prepolymer to take samples for titration. To a portion of this prepolymer (250.0 g, 0.2 eq.), add Dabco T-12 (0.25 g, dibutyltin dilaurate,... 

Support for this mechanism has been obtained from the study of the effect of twelve hydrogen-bond acceptors on the reactions of 1-chloro- and l-fluoro-2,4-dinitrobenzenes with morpholine in benzene162. The reaction of l-chloro-2,4-dinitrobenzene is not catalysed by either morpholine or DABCO, i.e. kA = h the first stage of the reaction is rate-determining and the various additives have no effect on the rate constant. On the other hand, Table 22 shows that the reaction of the fluoro-substrate is highly sensitive to the presence of the various additives and it is base-catalysed while for ten additives there was a linear dependence of kA on either their concentration, [P], or on the square... 

Catalysis by DABCO in the reactions of FDNB with piperidine, r-butylamine, aniline, p-anisidine and m-anisidine (usually interpreted as base catalysis as in Section B) was also assumed to occur by the formation of a complex between DABCO and the substrate14913. The high (negative) p-value of —4.88 was deemed inappropriate for the usually accepted mechanism of the base-catalysed step (reaction 1). For the reactions with p-chloroaniline, m- and p-anisidines and toluidines in benzene in the presence of DABCO a p-value of —2.86 was found for the observed catalysis by DABCO (fc3DABC0). The results were taken to imply that the transition state of the step catalysed by DABCO and that of the step catalysed by the nucleophile have similar requirements, and in both the nucleophilic (or basicity) power of the nucleophile is involved. This conclusion is in disagreement with the usual interpretation of the base-catalysed step. 

In the proposed mechanism (Scheme 9), the rate-determining step is the reaction between aldehyde and enolate. In the absence of a solvent, a major issue with this reaction is the typical low rate and the need for a high concentration of catalyst (usually DABCO). It was reported recently that, under basic conditions, the ionic liquid [BDMIM][PF6] is inert and that the Baylis Hillman reaction in [BDMIMjPFg proceeds smoothly with better yields than in [BMIMjPFg (163). 

Figure 5. Experimental (shorter) lifetimes of DABCO in the presence of a DC field (in addition to a stray field of 0.1 V/cm) compared to a computation using classical mechanics [1] for an electron revolving about a quadrupolar anisotropic core. The smooth line is a fit to an exponential dependence on Vfield (see Ref. 1 for more details).

Similar procedures can be used to prepare AAbu (both E- and Z-isomers) from Thr derivatives. Srinivasan et al/891 found that (3-elimination of the Thr derivative, Ac-(2R,35)-Thr-OMe 37 (threo type), gave only the stable Z-isomer 38 upon O-tosylation and subsequent elimination by DABCO as a base (Scheme 14). The underlying mechanism for this reaction may be a traits E2-elimination. (25,3R)-2-Acetamido-3-chlorobutanoic add methyl ester (erythro) 39, derived from the Thr threo form by chlorination with inversion of configuration at the (3-carbon, yields predominantly the E-isomer 38 by brief treatment with DBU as a base. However, a prolonged reaction time and use of DABCO as a base causes a significant amount of isomerization to the Z-isomer. 

It is evident that, for the reaction sequences proposed, the quantitative analysis of product B as a function of added DABCO cannot be used as a decisive argument in favour of or against a singlet oxygen mechanism. Moreover, the increase of the rate of phenothiazinone formation with the concentration of protic solvent where singlet oxygen is quenched efficiently supports the given interpretation. 

Significant mechanistic insights into the DABCO-catalysed isomerization of y-hydroxy-o ,/3-alkynyl esters to y-oxo-a,p-trans-alkenyl esters have been reported.33 The reaction mechanism involves cumulene formation, protonation with the conjugate acid of the amine, and protonation of the resulting allenol with water. 

A series of A - / - n i trobe nzenesul fony 1 imincs have been reported to undergo asymmetric aza-Morita-Baylis-Hillman reactions with methyl acrylate mediated by DABCO in the presence of chiral thiourea organocatalysts with unprecedented levels of enantioselectivity (87-99% ee), albeit only in modest yields (25 19%). Isolation of a DABCO-acrylate-imine adduct as a key intermediate, kinetic investigation, and isotopic labelling, have been employed to determine the mechanism.177... 

For aryl aldehydes under polar, nonpolar, and protic conditions, it has been determined that the rate-determining step is second-order in aldehyde and first-order in DABCO and acrylate. On the basis of this reaction rate data, Tyler McQuade recently proposed the following mechanism involving the formation of a hemiacetal intermediate ... 

IV-Substituted imidazoles have been synthesized quickly in high yields and with high stereoselectivity from the adducts formed from Baylis-Hillman acetates134 and DABCO in 20% aqueous THF at room temperature.135 The substitution reaction with imidazole appears to proceed via an 5N2,-5N2/ mechanism (Scheme 18). 

An unusual transformation occurred when triethylamine reacted with disulfur dichloride and 1,4-diazabicy-clo[2.2.2]octane (DABCO) to form heptathiocane 103 (mp 72-73 °C) and thienopentathiepine 104 (Scheme 7) <20030L1939>. The proposed mechanism involved the adduct 102 and oxidation of the intermediate complex 105, followed by the formation of enamines 106 and 107. The intermediate 106 outlined a pathway to the extended polysulfur chain, such as in 107, which cyclized into heptathiocane 103. Incorporation of only one carbon into the heterocyclic ring from the ethyl group rather than both was presumably controlled by the reactivity of the enamine 107. For the mechanism of formation of thienopentathiepine 104, Chapter 13.17, CHEC-3 should be consulted. The rare heptathiocane ring stmcture was proved by X-ray crystallography (Section 14.09.3.3). 

SSIP) is formed first this subsequently collapses into a contact ion pair (CIP). This mechanism was based on the observation of a blue shift of the absorption maximum of the ketyl radical anion from 715 to 690 nm which occurs with a half-life of 200 + 50 ps. The SSIP, being more solvated than the CIP, is expected to have its absorption spectrum red-shifted compared with that of CIP. Subsequent studies by Devadoss and Fessenden on the benzophenone-DABCO system indicated, however, that the spectrum of initial transient has an absorption maximum at 700 nm, which shifts to the red (720 nm) in the picosecond time domain [157, 158], These results seem to suggest that the initial species to be formed is the CIP which eventually separates to yield the SSIP and proton transfer occurs in the SSIP. 

This mechanism is also proposed for the reactions between DNF and 2- or 4-methoxy-aniline (ArNH2) in the presence of pyridine or Dabco. 

This mechanism was based on the kinetics of the reaction of phenyl isocyanate and n-butanol conducted at various absolute and relative concentrations of DBTDL and l,4-diazabicyclo[2.2.2]octane (DABCO). 

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