Urea-based catalysts

Over the past half-dozen years, many laboratories have focused their efforts on the development of chiral hydrogen bond donors that function as catalysts for enantioselective organic reactions. One of the earliest successes in this area came from Jacobsen and co-workers, who reported the use of peptide-like chiral urea-based catalysts for the hydrocyanation of aldimines and ketoimines [40, 41]. Several other laboratories have also reported highly enantioselective transformations catalyzed by a chiral hydrogen bond donor. The following sections provide a summary of the many developments in hydrogen bond-catalyzed enantioselective reactions, along with a discussion of mechanisms and selectivity models. 

Scheme 3.18 Strecker reaction catalyzed by urea-based catalyst 1.

Catalysts. The alkanolamines continue to find use as blocked catalysts for textile resins, coatings resins, adhesives, etc. Of particular utifity in curing durable-press textiles is AMP-HCl. Other salts, such as those of the benzoin tosylate or A-toluenesulfonic acid, find utifity in melamine- or urea-based coatings (18) (see Amino resins and plastics). 

Thermosetting Reactive Polymers. Materials used as thermosetting polymers include reactive monomers such as urea—formaldehyde, phenoHcs, polyesters, epoxides, and vinyls, which form a polymerized material when mixed with a catalyst. The treated waste forms a sponge-like material which traps the soHd particles, but not the Hquid fraction the waste must usually be dried and placed in containers for disposal. Because the urea—formaldehyde catalysts are strongly acidic, urea-based materials are generally not suitable for metals that can leach in the untrapped Hquid fractions. Thermosetting processes have greater utiHty for radioactive materials and acid wastes. 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

Other polymerizations such as those of urea, melamine, or phenol with formaldehyde (see Table 1-1) require an externally added acid or base catalyst to achieve the desired rates of reaction. 

On the basis of the observed stereoinduction trend, the addition of HCN took place over the diaminocyclohexane portion of the catalyst away from the amino acid and amide unit. The last hypothesis led to the prediction that a more sterically demanding amino acid or amide unit (Figure 6.14) could additionally favor the cyanide attack compared to the less bulky diaminocyclohexane unit and thus making the Schiff base catalyst more enantioselective in Strecker reactions of aldimines and ketimines. To evaluate this perspechve, the authors performed a model-(mechanism-) driven systematic structure optimizations by stepwise modification of the amide, the amino acid, and the (thio)urea unit of catalyst 42 and examined these derivatives of 42 (lmol% loading ) in the model Strecker reaction (toluene ... 

Based on the modular structure of Schiff base catalysts such as first-generation Strecker urea catalyst 42 [196, 198] (Figure 6.15 Schemes 6.41 and 6.45), Yoon... 

Urea 198 was prepared from enantiomerically pure polyfunctLonal glucoseamine hydrochloride, which is readily accessible from chitin as a component of the natural chiral pool it appeared to be an alternative backbone structure supplanting the trans-l,2-diamocyclohexane of Schiff base catalyst 42 (Figure 6.58). 

Many noticeable examples of chiral Lewis base catalyzed allylation of carbonyl compounds have also appeared. Iseki and coworkers published a full paper on enantioselective addition of allyl- and crotyltrichlorosilanes to aliphatic aldehydes catalyzed by a chiral formamide 28 in the presence of HMPA as an additive [41]. This method was further applied to asymmetric allenylation of aliphatic aldehydes with propargyltrichlorosilane [40]. Nakajima and Hashi-moto have demonstrated the effectiveness of (S)-3,3 -dimethyl-2,2 -biquinoline N,AT-dioxide (29) as a chiral Lewis base catalyst for the allylation of aldehydes [42]. In the reaction of (fs)-enriched crotyltrichlorosilane (54 , E Z=97 3) with benzaldehyde (48), y-allylated anfi-homoallylic alcohol 55 was obtained exclusively with high ee while the corresponding syn-adduct was formed from its Z isomer 54Z (fs Z= 1 99) (Scheme 6). Catalytic amounts of chiral urea 30 also promote the asymmetric reaction in the presence of a silver(I) salt, although the enantioselectivity is low [43]. 

A broad choice of heterocondensed uracils are easily and generally accessible from heterocyclic /3-enamino esters and isocyanates (01LA200 03LA341). The mixed urea intermediate is smoothly cyclized with 5% aq. NaOH the whole procedure can be carried out in a one-step reaction, when pyridine serves as solvent and base catalyst for the ring closure (68CB3377 85AHC299) (Scheme 11). 

Studies with Au-based catalysts have focused on SCR of NO by propene [61,96,489,492,493], carbon monoxide [78,489,494] and hydrogen [4,5,83,489]. Urea [97], methane [495] and other hydrocarbons [489,496,497] have also been used. SCR using the first three reagents will be discussed here. 

Allylation. Silver tosylate is used in combination with urea, a Lewis base catalyst, to promote allylation of aldehydes with allyltrichlorosilane. 

Tan et al. [44] succeeded in developing a new urea-sulfmimide catalyst that promotes the indium mediated allylation of acyUiydrazones (Scheme 9.9). Incorporation of Lewis base functionality in proximity to the urea moiety is designed to promote the addition of organometallic reagents to the C=N bond of acylhydrazones through dual activation. 

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