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Urea catalysts thiourea

Abstract After an overview of chiral urea and thiourea synthetic methods, this review describes the main applications of urea and thiourea complexes in asymmetric catalysis. Some recent examples of thioureas as catalysts are also presented. Coordination chemistry of ureas and thioureas is briefly discussed. [Pg.232]

The reaction was first tested with these substances as ligands but the organic molecule, in the absence of any added metal ion, proved to be the most enantioselective catalyst (library 1 19% ee vs. less than 13% ee for the best metal catalyst). The effects of selective variations of the amino acid nature and of the salicylidene moiety on the diamine structure were investigated for urea and thiourea derivatives via HTS (library 2 48 urea compounds and... [Pg.256]

The effectivity of ureas and thioureas as catalysts and the possibility to catalyse Diels-Alder reactions leads to the assumption that thioureas also should be active catalysts for the [4 + 2]-cycloaddition. A thiourea should be a more active nitrogen-containing... [Pg.1061]

Since Curran and Kuo and Schreiner and coworkers reported that urea and thiourea derivatives act like Lewis acid catalysts, several chiral urea and thiourea catalysts have been designed by Jacobsen et al. and Takemoto et al. ... [Pg.359]

The last few years have witnessed major advances in the use of small organic molecules as organic acid catalysts in asymmetric catalysis [1], Selected examples of such organic acid catalysts include urea and thiourea [2], TADDOL [3], BINOL [4], and phosphoric acid derived from BINOL [5] (Figure 2.1). [Pg.5]

Figure 6.3 Stereoselective, chiral thiourea derivatives of achiral benchmark thiourea organocatalyst N,N -bis [3,5-(trifluoromethyl)phenyl]thiourea 9 stereoselective hydrogen-bonding thiourea organocatalysts incorporating the privileged 3,5-bis(trifluoromethylphenyl)thiourea moiety. The (thio)urea catalyst structure is the leitmotif for the chapter organization. Figure 6.3 Stereoselective, chiral thiourea derivatives of achiral benchmark thiourea organocatalyst N,N -bis [3,5-(trifluoromethyl)phenyl]thiourea 9 stereoselective hydrogen-bonding thiourea organocatalysts incorporating the privileged 3,5-bis(trifluoromethylphenyl)thiourea moiety. The (thio)urea catalyst structure is the leitmotif for the chapter organization.
The asymmetric catalytic Strecker reaction is an elegant means of synthesis of optically active a-amino acids. The Jacobsen group developed optimized organocata-lysts [21, 44-48], optically active urea or thiourea derivatives, which were found to be the most efficient type of catalyst yet for asymmetric hydrocyanation of imines (see also Section 5.1 on the hydrocyanation of imines). Because of its high efficiency, Jacobsen hydrocyanation technology has already been used commercially at Rodia ChiRex [49]. The concept of the reaction is shown in Scheme 14.7. In the presence of a catalytic amount (2 mol%) of the readily available organocatalyst... [Pg.401]

N-Acetylation.1 Amides, thioamides, ureas, and thioureas are N-acetylated at 25° by acetic acid in CH2CI2 in the presence of this reagent. It can serve as a catalyst, but stoichiometric amounts are required for high yields. [Pg.214]

The simultaneous use of urea, or thiourea [76] and DABCO catalyst was introduced by the Connon group for the addition of methyl acrylate and benzaldehyde [39]. The study revealed that, although both ureas and thioureas accelerated the reaction relative to the uncatalyzed process, urea was superior to thiourea in terms of stability and efficiency. Chiral thiourea derivatives may offer, however, superior enantioselectivity. It was postulated, that the catalysts operate mainly via a Zimmerman-Traxler-type transition state 69 for addition of the resulting enolate anion to the aldehyde (Scheme 5.15). [Pg.167]

Cyclohexanediamine-derived amine thiourea 70, which provided high enantio-selectivities for the Michael addition [77] and aza-Henry reactions [78], showed poor activity in the MBH reaction. This fact is not surprising when one considers that a chiral urea catalyst functions by fundamentally different stereoinduction mechanisms in the MBH reaction, and in the activation of related imine substrates in Mannich or Streclcer reactions [80]. In contrast, the binaph-thylamine thiourea 71 mediated the addition of dihydrocinnamaldehyde 74 to cyclohexenone 75 in high yield (83%) and enantioselectivity (71% ee) (Table 5.6, entry 2) [79]. The more bulky diethyl analogue 72 displayed similar enantioselectivity (73% ee) while affording a lower yield (56%, entry 3). Catalyst 73 showed only low catalytic activity in the MBH reaction (18%, entry 4). [Pg.167]

Copper (II) chloride proved to be a very efficient catalyst for the Biginelli reaction in the absence of solvent [27]. When ethyl acetoacetate, aldehydes and urea or thiourea were heated neat in the presence of copper (11) chloride, the Biginelli products were isolated, after recrystallization from hot ethanol, in high yields and purities (Scheme 7). [Pg.238]

The Beckmann rearrangement of ketoximes to the corresponding amides (31), the Fischer indole cyclization, isomerization of epoxides to the corresponding aldehydes, ketones, or alcohols, hydration and ammo-nolysis of epoxides, oxygen-sulfur interchange, formation of diaryl-ureas and -thioureas from condensation of aniline and carbonyl sulfide, and olefin carbonylation occur over zeolite catalysts (62). The oxo reaction over rhodium and cobalt containing zeolites recently has been claimed (22). [Pg.271]


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