Urea-derived catalysts

Carbon dioxide reacts with amines (ArNH2) and iodoethane, under electrolysis conditions, to give the corresponding carbamate, (ArNHC02Et). Urea derivatives were obtained from amines, CO2, and an antimony catalyst. ... 

The asymmetric Strecker-type reaction developed by the Jacobsen group is suitable for both aUphatic and aromatic imines, giving high enantiomeric excesses for a wide range of substrates. In this reaction the urea derivative also acts as the catalyst (Scheme 36). 

Important by-products are urea derivatives (ArNHC(0)NHAr) and azo compounds (Ar-N=N-Ar). The reaction is highly exothermic (—128kcalmol-1) and it is surprising that still such low rates are obtained (several hundred turnovers per hour) and high temperatures are required (130 °C and 60 bar of CO) to obtain acceptable conversions.533 Up to 2002, no commercial application of the new catalysts has been announced. Therefore, it seems important to study the mechanism of this reaction in detail aiming at a catalyst that is sufficiently stable, selective, and active. Three catalysts have received a great deal of attention those based on rhodium, ruthenium, and palladium. Many excellent reviews,534"537 have appeared and for the discussion of the mechanism and the older literature the reader is referred to those. Here we concentrate on the coordination compounds identified in relation to the catalytic studies.534-539... 

TABLE 12. The asymmetric reduction of prochiral ketones under catalysis of chiral urea derivative 8173 (in all reactions 5% catalyst was used)... 

Bifunctional catalysts have proven to be very powerful in asymmetric organic transformations [3], It is proposed that these chiral catalysts possess both Brpnsted base and acid character allowing for activation of both electrophile and nucleophile for enantioselective carbon-carbon bond formation [89], Pioneers Jacobsen, Takemoto, Johnston, Li, Wang and Tsogoeva have illustrated the synthetic utility of the bifunctional catalysts in various organic transformations with a class of cyclohexane-diamine derived catalysts (Fig. 6). In general, these catalysts contain a Brpnsted basic tertiary nitrogen, which activates the substrate for asymmetric catalysis, in conjunction with a Brpnsted acid moiety, such as urea or pyridinium proton. 

Figure 6.10 Active sites of lipase (1), triflinctional (thio)urea derivatives (38 39) mimicking the acive site of serine hydrolases (2), and acetyl-catalyst intermediate of the biomimetic transesterification between vinyl trifluoroacetate methanol and 2-propanol, respectively (3).

It was found that both the replacement of the secondary amide unit with a bulkier tertiary amide and the incorporation of a thiourea moiety instead of the urea unit resulted in a significant improvement in stereoinduction (from initial 80% ee obtained with 42 to 97% ee). This led to the identification of hydrogen-bonding Schiff base thiourea catalyst 47, while the urea derivatives 43-46 gave lower ee values (Figure 6.16). 

C9-epi-122 98% conv. (99% ee) after 30h, respectively (Figure 6.40). This structure-efficiency relationship supported the results already published by the Soos group for quinine- and quinidine-derived thioureas (Figure 6.39) [278]. C9-epimeric catalysts were found to be remarkably more efficient in terms of rate acceleration and stereoinduction than the analogs of natural cinchona alkaloid stereochemistry. This trend was also observed for the corresponding (thio)ureas derived from DHQD as shown by the experimental results in Figure 6.40 [279]. 

Ricci and co-workers introduced a new class of amino- alcohol- based thiourea derivatives, which were easily accessible in a one-step coupling reaction in nearly quanitative yield from the commercially available chiral amino alcohols and 3,5-bis(trifluoromethyl)phenyl isothiocyanate or isocyanate, respectively (Figure 6.45) [307]. The screening of (thio)urea derivatives 137-140 in the enantioselective Friedel-Crafts reaction of indole with trans-P-nitrostyrene at 20 °C in toluene demonstrated (lR,2S)-cis-l-amino-2-indanol-derived thiourea 139 to be the most active catalyst regarding conversion (95% conv./60h) as well as stereoinduction (35% ee), while the canditates 137, 138, and the urea derivative 140 displayed a lower accelerating effect and poorer asymmetric induction (Figure 6.45). The uncatalyzed reference reaction performed under otherwise identical conditions showed 17% conversion in 65 h reaction time. 

A screening of (R)-bis-N-tosyl-BlNAM 151 and axially chiral (thio)urea derivatives 152-162 (10mol% loading 0.36 M catalyst concentration incorporating the N-aryl(alkyl) structural motif was performed at various reaction temperatures in di-chloroform using the asymmetric FC addition of N-methylindole to trans-Ji-nitrostyrene as model reachon (product 1 Scheme 6.158). The structure of bis(3,5-bistrifluoromethyl) phenyl functionalized binaphthyl bisthiourea 158 was identified... 

This section considers the applications of bifunctional hydrogen-bonding (thio) urea derivatives that have been designed and utilized for asymmetric organocataly-sis, but cannot clearly be assigned to one of the structural classifications mentioned above or are the catalysts of choice in only one publication that can mark the basis of further research efforts. 

In the absence of basic catalysts, propargylamines react with isocyanates to give ureas. However, in the presence of basic catalysts, 4-methylene-2-oxazoli-dinones and 4-methylene-2-imidazolidinones are obtained directly or through the urea derivative [27] (Eq. 18). 

Several organocatalysts have been recycled efficiently (selected examples are shown in Scheme 14.2). For example, the Jacobsen group has reported results from an impressive study of the recycling of the immobilized urea derivative 6, a highly efficient organocatalyst for asymmetric hydrocyanation of imines (Scheme 14.2) [11]. It was discovered that the catalyst can be recycled and re-used very efficiently - over ten reaction cycles the product was obtained with similar yield and enantioselectivity (96-98% yield, 92-93% ee). 

As an abundant, nontoxic, non-flammable, easily available, and renewable carbon resource, C02 is very attractive as an environmentally friendly feedstock for making commodity chemicals, fuels, and materials [1-7]. In this respect, PEGs-functionalized catalysts have been developed for efficient transformation of C02 into value-added chemicals or fuels such as cyclic carbonates, dimethyl carbonate (DMC), oxazolidinones, organic carbamates and urea derivatives. 

Using this protocol, primary aliphatic amines, secondary aliphatic amines, and diamines could be converted into the corresponding urea derivatives in moderate yields. Additionally, catalytic efficiency of cations derived from various bases decreases in the order of > diamines > primary amines > secondary amines > aniline, probably being due to the steric effect and basicity. The catalyst could also be recovered after a simple separation procedure, and reused over five times with retention of high activity. This process presented here could show much potential application in industry due to its simplicity and ease of catalyst recycling. 

If the latter reaction proceeds through a closed transition state (e.g., 5 in Scheme 7.2), good diastereocontrol can be expected in the case of trans- and cis-CrotylSiCl3 (2b/2c) [14, 15]. Here, the anh-diastereoisomer 3b should be obtained from trans-crotyl derivative 2b, whereas the syn-isomer 3c should result from the reaction of the cis-isomer 2c (Scheme 7.2). Furthermore, this mechanism creates an opportunity for transferring the chiral information if the Lewis base employed is chiral. Provided that the Lewis base dissociates from the silicon in the intermediate 6 at a sufficient rate, it can act as a catalyst (rather than as a stoichiometric reagent). Typical Lewis bases that promote the allylation reaction are the common dipolar aprotic solvents, such as dimethylformamide (DMF) [8,12], dimethyl sulfoxide (DMSO) [8, 9], and hexamethylphosphoramide (HMPA) [9, 16], in addition to other substances that possess a strongly Lewis basic oxygen, such as various formamides [17] (in a solution or on a solid support [7, 8, 18]), urea derivatives [19], and catecholates [10] (and their chiral modifications [5c], [20]). It should be noted that, upon coordination to a Lewis base, the silicon atom becomes more Lewis acidic (vide infra), which facilitates its coordination to the carbonyl in the cyclic transition state 5. 

Chiral Lewis-basic catalysts (Figs. 7.1 and 7.2), in particular phosphoramides 8-12 [9, 14c, 15c, 22-24], formamide 13 [17], pyridine N,N -bisoxides 17 and 18 [25-27], N-monoxides (19-26) [27-32], and N,N N"-trisoxides (27) [33] exhibit good to high enantioselectivities for the allylation of aromatic, heteroaromatic, and cinnamyl-type aldehydes (1) with allyl, trans- and ds-crotyl, and prenyl trichlorosilanes (2a-d). Chiral formamides (with the exception of 13, as discussed below) [17], pyridine-oxazolines [34], urea derivatives [19] and sulfoxides [35] are effective only in stoichiometric quantities (or in excess) and, as a rule, exhibit lower enantioselectivities. 

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