Sulfa-Michael addition, conjugate additions

Very recently two comprehensive review articles dealing with organocatalytic carbon-sulfur bond-forming reactions and asymmetric sulfa-Michael additions have been published. Historically, the first catalytic enantio-selective sulfenylations of conjugated enones in the presence of Cinchona alkaloids were published in 1977 by Pracejus where acrylamides and... 

Several derivatives of cinchona alkaloids 1—4 were prepared and used in the asymmetric sulfa-Michael addition. The first highly efficient method, based on the catalyst (DHQD)2PYR 5, was presented by the Deng group in 2002 [18]. Especially high ees were observed in the conjugated addition of 2-thionaphthol to several six-to nine-membered cyclic enones at low temperature (Scheme 14.3). Although 2-cyclopentenone reacted with moderate enantioselectivity (41% ee), the ee was increased dramatically with 4,4-dimethyl-2-cyclopentenone (92% ee). 

SCHEME 14.12. Conjugate sulfa-Michael addition to a-substituted (3-nitroacrylates promoted by thiourea 12 and further transformation into a-thio-p -amino acids. 

Optically active sulfur containing compounds play a very important role in biochemistry as well as synthetic chemistry. The asymmetric conjugate addition of sulphur nucleophiles, or sulfa-Michael addition [379], provides a direct and versatile approach toward optically active sulfur compounds. This strategy is particularly valuable, since enantioselective nucleophilic additions to a C-S double bond, unlike those to carbonyls and imines, are not synthetically feasible. 

Numerous guanidine-catalysed asymmetric Michael reactions and its related variants such as aza-Michael, oxa-Michael, phospha-Michael, sulfa-Michael have been reported in the literature. A nonexhaustive selection of conjugate addition reactions that is relevant to green chemistry will be presented. Glycine imines are commonly employed in Michael additions. They are protected a-amino acids and must he deprotected if an amino acid derivative is desired (Scheme 23.5). The large molecular mass of the imine group then makes waste generation a problem. 

Most reports on organocatalytic sulfa-Michael reactions are based on Br0nsted base catalysis, in order to activate pro-nucleophiles containing a S H or a Se—H bond. The early works, appeared in the lates 1970s, featured natural cinchona alkaloids 1-4 as basic catalysts (Figure 14.1). In their seminal works, Wynberg and co-workers employed less than 1 mol% of quinine 1 as chiral catalyst for the conjugated addition of arenethiols to 2-cyclohexen-l-ones. The enantiocontrol was unsatisfactory with benzyhnercaptan [6]. The quasi-enantiomeric catalyst quinidine 2 furnished the... 

As anticipated by the work of Wynberg [7], the presence of a protic group in the catalyst s structure, capable of activating and orienting the electrophile, had a positive impact on the enantioselectivity of the conjugate additions. On this basis, several Brpnsted acid/Brpnsted basic bifunctional catalysts has been applied in the sulfa-Michael addition. A list of them is reported in Figure 14.4. 

The sulfa-Michael reaction to a (3,(3-disubstituted Michael acceptor, affording adducts with a new quaternary stereocenter, is usually challenging, mainly due to the low reactivity and reversibility of the process. Xiao and co-workers [41] reported that thiourea 12 promoted the highly enantioselective conjugate addition of arenethiols and substituted benzyl thiols to a-substituted (3-nitroacrylates. The enantioenriched adducts could be easily transformed into a-thio-(P -amino acids, valuable building blocks for the synthesis of p- or mixed p/a-peptides (Scheme 14.12). 

Enantioselective Michael reactions of thiols provide a convenient route to diverse chiral sulfides. Chen and coworkers have demonstrated that a chiral squaramide catalyst was effective in promoting a sulfa-Michael conjugated addition of various aromatic and aliphatic thiols to a wide range of trans-chalcones (Scheme 10.19) [97]. Moderate to excellent yields and enantioselectivities were achieved. Notably, the reactions between various benzylthiol and trans-chalcones bearing electron-donating or electron-withdrawing substituents proceeded with high enantioselectivities. 

Cinchona-alkaloid-catalysed conjugate cyanation of enones has enabled the synthesis of trifluoromethyl-substituted diarylpyrroles with ee<96%P° Thiochro-manes have been formed by asymmetric domino sulfa-Michael-aldol reactions of 2-mercaptobenzaldehyde with a,/ -unsaturated A-acylpyrazoles. Asymmetric organocatalysed oxy-Michael addition to y-hydroxy a,/ -unsaturated thioesters on reaction with t-BuCHO has been used to form -hydroxy carbonyl compounds HOCH2C H(OH)CH2CO.SAr via cyclic hemiacetal intermediates. 

In a manner largely complementary to secondary amine-catalyzed asymmetric conjugate addition to enals with heteroatom nucleophiles, chiral primary amines were recently found to be the catalysts of choice for similar Michael addition to a,p-unsaturated ketones. With their previously developed cinchona-type catalyst 91, Melchiorre and coworkers achieved the asymmetric sulfa-Michael addition to a,p-unsaturated ketones with either benzyl or tert-butyl mercaptane (Scheme 5.30) [58a]. The same catalyst could be further extended to oxa-Michael addition to enones by optimizing the ratio of acidic additive and solvents (Scheme 5.30) [58b]. 

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