Organocatalysts binaphthyl

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

A new class of suitable optically active organocatalyst for enantioselective alkylations has recently been developed by Maruoka and co-workers [le, 33-37]. This catalyst is not based on an alkaloid-related quaternary ammonium salt but consists of a C2 -symmetric compound of type 29 (or derivatives thereof bearing other types of substituent on the 3,3 positions of the binaphthyl unit) [33, 34], In the presence... 

In addition, chiral Schiff base catalysts, which were developed previously for the Strecker reaction, were also found to be suitable catalysts for the Mannich reaction starting from imines and enolates [36, 37]. Very recently, further efficient organocatalysts for the Mannich reaction, such as chiral pyrrolidinyltetrazole and chiral binaphthyl phosphoric acids, have been reported [38]. 

Another type of organocatalyst, which is suitable for the Mannich reaction with ketene silyl acetals, is a chiral binaphthyl phosphoric acid [38c]. Very recently, it has been reported that high enantioselectivity of up to 96% ee can be obtained with this type of catalyst [38c]. 

A polymeric binaphthyl zinc complex has been used for related epoxidation reactions <1999JOC8149>.A bimetallic samarium-based Lewis acid complex catalyzes the nucleophilic epoxidation of unsaturated carbonyl compounds very efficiently <2002JA14544>. The use of amino acids has organocatalysts for asymmetric epoxidations of enones and enals has been investigated <2005OL2579, 2005JA6964>. 

Vishnumaya and Singh developed a highly efficient water-compatible binaphthyl derived diamine organocatalyst Ih and successfully used it in asymmetric Michael reactions. The treatment of ketones with both aryl and allgrl nitro-olefins afforded the desired products with high to excellent stereoselectivities (Scheme 9.16). The Chen group used the bicyclic... 

Several chiral organocatalysts such as chiral squaramide 31, chiral phosphoric acid (S)-32a, and binaphthyl-based chiral sulfonimide (i )-33 have also been successfully applied in the AFC reaction of indoles with N-sulfonyl imines (Scheme 6.14). 

Jurczak and co-workers reported hybrid diamines derived from l,r-binaphthyl-2,2 -diamine and a-amino acids as organocatalysts for 1,3-dipolar cycloaddition of aromatic nitrones 187 to ( )-crotonaldehyde 28b, Scheme 3.62 [79]. The L-phenylalanine-based l,r-binaphthyl-2,2 -diamine (BINAM) catalyst 194, with trifluoromethanesulfonic acid as an additive, afforded the optimal results with good endo-diastereoselectivity and enantioselectivity up to 95% ee. 

Binaphthyl-derived Cyclic Amines and Their Salts as Asymmetric Organocatalysts... 

In 2005, Wang and coworkers reported a new bifunctional binaphthyl-derived amine thiourea 16 as an efficient organocatalyst for the Morita-Baylis-Hillman reaction of cyclohexenone with aliphatic, aromatic and sterically hindered aldehydes. The design of the catalyst follows Takemoto s design of a bifunctional motif. This catalytic protocol provided access to useful chiral allylic alcohol building blocks in high yields and high enan-tioselectivities (Scheme 19.21). 

Other chiral Bronsted acid organocatalysts developed more recently include chiral binaphthyl-derived disulfonic acids [162] and sulfonimides [163] and SPINOL-derived phosphoric acids [164]. 

Since methylenemalonates are known to react readily with amines, the development of a highly efficient asymmetric Michael addition of aldehydes to methylenemalonates catalyzed by amine catalysts is quite challenging. Most recently, Maruoka and co-workers [34] found that the use of less nucleophilic binaphthyl-based secondary amine 27 as organocatalyst could effectively solve the problem, affording the synthetic useful products with good to excellent yields and enantio-selectivities (Scheme 5.16). 

Ma et al. have found that the presence of molecular sieves increased the diastereo- and enantioselectivity in the aldolisation of cycKc ketones with various aromatic aldehydes using binaphthyl-based axially chiral prolinamides as the organocatalysts. The beneficial influence of the molecular sieves could be due to their water-trapping properties, preventing water-mediated background reactions. The best results are collected in Scheme 2.15. 

Few chiral secondary amine organocatalysts have been applied to the asymmetric aldol reaction, in 2008. As an example, Maruoka and Kano have designed a binaphthyl-based amino acid, which was applied to induce the asymmetric aldolisation of both acyclic and cyclic ketones with both aliphatic and aromatic aldehydes performed in DMF. ° Remarkable results were obtained in general with enantioselectivities of up to 99% ee for either cyclic or acyclic ketones, which led to the <2 n-products (Scheme 2.41). Furthermore, these authors have used a closely related binapthyl-based amino sulfonamide to promote the cross-aldol reaction between aldehydes, which yielded the corresponding syn products in moderate to high yields (71-91%) and diastereoselectivities (72-90% de) combined with excellent enantioselectivities (94-99% ee). 

Excellent enantioselectivities have also been observed by Maruoka et al. by using several other chiral secondary amines as organocatalysts for the anti-Mannich reaction between various aldehydes and A -protected hnines including a-imino esters. Therefore, the involvement of a chiral binaphthyl-based amino sulfonamide as the organocatalyst allowed a series of UM/i-Mannich products to be obtained from both iV-Boc-protected imines and A -PMP-protected a-tmino esters by reaction with aldehydes. As shown in Scheme 3.12, excellent yields and enantioselectivities (97-99% ee) were observed along with moderate to high anti diastereoselectivities of up to 90% de. 

In the same area, these authors have also investigated the efficiency of various chiral sterically congested phosphane-amide bifunctional organocatalysts with a binaphthyl scaffold in asymmetric aza-Morita-Baylis-Hillman reactions of A -sulfonated imines with activated olefins such as methyl and ethyl vinyl ketones." The corresponding aza-Morita-Baylis-Hillman adducts could be obtained in moderate to excellent yields (37-98%) and moderate to good... 

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