Thiourea novel bifunctional

In 2005, Sons and coworkers introduced a novel bifunctional thiourea organocatalyst based on a Cinchona alkaloid for the asymmetric conjugate addition of nitromethane to tra s-chalcones. Using 10 mol% of thiourea 17, various electron-rich and electron-poor chalcones were well tolerated providing the desired adducts in high yields (80-94%) and very high enan-tioselectivities (95-96%) (Scheme 19.24). It has to be highlighted that... 

Takemoto et al developed a novel bifunctional thiourea catalyst bearing amine moiety (9), which promoted the Michael reaction of malonate with nitroalkenes to give corresponding adducts with high enantioselectivity (Scheme 2.34) [85]. The bifunctional thiourea catalyst is effective for the 1,4-addition of the anion of malonitrile to a,P-unsaturated imide (Scheme 2.35) [86]. 

A novel bifunctional thiourea catalyst (11), bearing binaphthyl backbone, was synthesized by Wang et al They reported enantioselective Morita-Baylis-Hillman reaction of cyclohexenone with a range of aldehydes by means of (11) (Scheme 2.45) [98]. 

Tsogoeva et al developed a novel bifunctional thiourea catalyst (12), derived from 1,2-diaminocyclohexane, and applied it to the Strecker synthesis and nitro Michael reactions (Scheme 2.47) [100]. 

Nagasawa et d. designed a novel bifunctional catalyst bearing both guanidine and thiourea functional groups (19), which effectively promoted the Henry reactions with aliphatic cyclic aldehydes and branched aldehydes in the presence of K1 as an additive to provide -nitroamine with high enantioselectivity. Use of a-substituted aldehyde improved the enantioselectivity to 95-99% (Scheme 2.60) [115]. 

In a preliminary report, Chen and co-workers [30] obtained moderate enantioselectivity in the addition of thiophenol to acyclic benzimides, catalyzed by 11 (55-75% ee). The Deng group [35] successfully exploited a,(3-unsaturated A-acyloxazolidin-2-ones as acceptors. After a careful screening of bifunctional cinchona alkaloid derivatives, they identified the novel thiourea 14 as the most... 

Figure 5.45. Diverse applicability of novel N-heterocyclic carbene, bifunctional thiourea amine, and superbase catalysts for the polymerization of strained heterocyclics. Reprint courtesy of International Business Machines Corporation, copyright 2009 International Business Machines Corporation.

In 2013, Li et al. reported a novel synthesis of chiral six-memhered spir-ocyclic oxindoles bearing five consecutive stereocentres on the basis of an enantioselective three-component domino Michael-Michael-aldol reaction catalysed hy a combination of a chiral diphenylprolinol silyl ether and a chiral bifunctional quinine thiourea. " As shown in Scheme 2.26, the reaction between iV-substituted oxindoles, nitrostyrenes, and enals led to the corresponding highly substituted spirocyclic oxindoles in high yields, moderate diastereoselectivities of up to 44% de, and enantioselectivities of up to >99% ee. A large variety of substrates could be used well in this... 

Ricci et al studied a series of thiourea catalysts for the Friedel-Crafts alkylation of aromatic and heteroaromatic compounds with nitroalkenes. They have succeeded in developing the Friedel-Crafts alkylation of indoles with nitroalkenes for the first time by means of a novel thiourea catalyst (13) (Scheme 2.48) [101]. The authors proposed the bifunctional nature of the thiourea catalyst, where thiourea activates the nitro group and at the same time the free alcoholic function interacts with the indole proton through a weak hydrogen bond, directing the attack of the incoming nucleophile on the Si face of the nitroalkene (Figure 2.18). 

The concept of bifunctional catalysis as advanced for the natural cinchona alkaloids and cuprei(di)nes has resulted in the design and synthesis of a range of new cinchona derivatives. The major part of these novel organocatalysts are urea and thiourea cinchona derivatives together with cinchona alkaloids modified with, for example, a sulfonamide, squaramide, or guanidine group (Figure 6.8). 

---