DABCO formation

Hydroxy group containing tertiary amines are also used because they become incorporated into the polymer stmcture, which eliminates odor formation ia the foam (3). Delayed-action or heat-activated catalysts are of particular interest ia molded foam appHcations. These catalysts show low activity at room temperature but become active when the exotherm builds up. In addition to the phenol salt of DBU (4), benzoic acid salts of Dabco are also used (5). 

Normally the reaction Is useful for the conversion of alkyl halides to primary amines without concomitant formation of secondary amines.29 Treatment of polymer 17 with hexamethylenetetramine in a mixture of ethanol/THF afforded an insoluble resin. Using diazabicyclooctane (DABCO), we demonstrated that the reaction could be limited to attack by a single nitrogen in a multifunctional amine, so we did not anticipate crosslinking via bis-quat salt formation. Hydrolysis of 2 with anhydrous HC1 in ethanol generated free amino groups as evidenced by a positive ninhydrin test, but quantitative hydrolysis could not be achieved and the product remained insoluble. One would have expected a simple bis-quat to hydrolyse and open the crosslinked structure. 

A detailed study of the role of the base in the formation of 2-isoxazolines by condensation of primary nitro compounds with alkenes in the presence of the tertiary diamine 1,4-diazabicyclo[2.2.2]octane (DABCO) was published <06EJO4852 06EJ03016>. 

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. 

The reaction of ethyl 2,4,6-trinitrophenyl ether with aniline in dimethyl sulfoxide (DMSO) in the presence of Dabco occurs in two stages via the intermediate (6). Kinetic studies show that proton transfer is rate-limiting both in the formation of the intermediate and in the subsequent acid-catalysed decomposition to give 2,4,6-trinitrodiphenylamine. Phenoxide is a considerably better leaving group than ethoxide so that substitutions of phenyl 2,4,6-trinitrophenyl ethers and phenyl 2,4-dinitronaphthyl ether with aniline occur without the accumulation of intermediates. Both uncatalysed and base-catalysed pathways are involved. ... 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

The aldehyde can be replaced by an imine and the reaction is then called the aza-Baylis-Hillman reaction [87, 88]. (3-Amino-a-methylene structures obtained in this way could further be converted to a range of biologically important molecules, such as p-amino acids [89]. First reaction of this kind was published in 1984 [90]. Tosylimines and ethylacrylate reacted in the presence of DABCO as catalyst to give p-aminoesters. First three-component aza-Baylis-Hillman reaction was published in 1989 by Bertenshaw and Kahn [91], with imine formation in situ from an aldehyde and an amine. In the presence of triphenylphosphine as catalyst, the reaction with methylacrylate led to the formation of the p-amino-ot-methylene esters and ketones in good yields (Scheme 38). 

Dichloro-bis-dithiolium salt 85 obtained from M-ethyldiisopropylamine, S2CI2 and DABCO in chloroform at room temperature reacted with arenesulfonamides and their Af,N-dichloro derivatives with the formation of Ahhf -bis(arylsulfonyl)-dithiolothiazine diimines 90 in modest yields (2001JCS(P1)2409 Scheme 42). 

The reaction of diisopropyl sulfide with sulfur monochloride and DABCO afforded l,2-dithiolo-3-thiones 98 and 99 (1999JOC4376). Dithiole ring formation had been assumed to be similar to that produced from tertiary diisopropyl-amines. However, in the case of diisopropyl sulfide only one isopropyl group was... 

Because many of them are nearly inert, ionic liquids have been used to stabilize highly polar or ionic transition states. Ionic liquids provide favorable media for the formation and stabilization of intermediates in reactions that proceed through charged intermediates. An example is the Baylis-Hillman reaction catalyzed by 1,4-diazabicyclo (222). octane (DABCO) (Scheme 8) (162). 

Recently Diederich et al. tried to overcome the solubility problems by using modifications where the linear stearyl side chains are exchanged by branched ones [76]. Dionium salt 53 turned out to be an effective transport catalyst at physiological pH for all investigated compounds [AMP, CTP, 2, 3 -dideoxy-TTP (ddTTP), and 3 -azido-dTTP (AZTTP)] and significantly improved the rates achieved with 52. A chloroform solution of 53 extracts half an equivalent of ATP -, which indicates the formation of a neutral 2 1 complex. Analogously, bis(DABCO) tetracation 54 binds to one ATP molecule. However, the transport acceleration is about one order of magnitude smaller than that of 53 [77]. 

The rate of the isomerization of ketimine 33 to 34 can be accelerated by heating and by addition of catalytic amounts of a stronger base, e. g. DBU or l,4-diazabieyclo[2.2.2]octane (DABCO). but the yield is reduced due to formation of byproducts.26... 

Six-Membered Heterocycle Ring Formation. Heterocycle formation involving diketene usually involves acetoacetylation of a substrate, followed by intramolecular condensation. Diketene itself readily dimerizes through self-condensation forming mainly dehydro acetic acid [771-03-9] (DHA) (13). Dehydroacetic acid and sodium dehydro acetate [4418-26-2] are used as preservatives for foods and cosmetics. DHA is found as an unwanted by-product in many diketene reactions, but can be obtained intentionally by dimerizing diketene in the presence of pyridine [110-86-1] in benzene, diazabicyclo[2.2.2]octane [280-57-9] (DABCO), and other basic catalysts. 

Closer investigations of pseudoazulenes 26, 27, 39, and 4()50,214 revealed that irradiation of these systems in the presence of 302 produced singlet oxygen. It was possible to demonstrate the formation of 02 by quenching experiments with DABCO and 2,5-dimethylfuran. In a thermal reaction 02 adds to the pseudoazulenes, whereby dioxetane derivatives (126) are... 

The formation of rings with more than seven atoms has unfavorable rates because the addition step is often too slow to allow it to compete successfully with other pathways open to the radical intermediate. In stannane based chemistry for example, premature hydrogen abstraction from the organotin hydride is difficult to avoid. However, Baylis-Hillman adducts 111 derived from enantiopure 1-alkenyl (or alkynyl)-4-azetidinone-2-carbaldehydes are used for the stereoselective and divergent preparation of highly functionalized bicycles 112 and 113 fused to medium-sized heterocycles (Scheme 38) [80, 81]. The Baylis-Hillman reaction using nonracemic protected a-amino aldehydes has been attempted with limited success due to partial racemization of the chiral aldehyde by DABCO after... 

Some bridgehead amines [l,4-diazabicyclo-[2,2,2]octane (102), quinuclidine (103) and quinuclidine-3-ol] form 1 1 molecular complexes with quinones 104. Formation of 2 1 (amine/quinone) complexes was observed in solutions of DABCO (102) and chloranil (104, R = Cl). These tertiary amines are able to form complexes, while non-bridgehead amines (triethylamine, piperidine) cannot because of steric hindrance or nitrogen inversion183. Stable complexes may be predicted (by CNDO/2 calculations) for... 

In the dihydroxylation of cyclohexene by Me3N+—O-, catalysed by OsC>4, aromatic amines and aliphatic chelating (TMEDA) or bridging (DABCO, hexamine) amines were found to retard the oxidation, owing to the formation of amine adducts of the dioxomonoglycolatoosmium(VI) ester intermediates, which are more resistant to the further oxidation required for product formation.98 Alkenes derived from Gamer s aldehyde, A-Boc-/V,0-acctonide of the aldehyde of L-serine, may be dihydroxylated by OsC>4 with excellent selectivities that may be explained by A1,3 strain.99... 

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. 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

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. 

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