Sulfonamide-based catalysts

Results obtained in the acylation of aromatic sulfonamides with acetic acid, in the presence of SnOTf based catalysts are presented in Table 48.4. The rate of the sulfonamide acylation follows the seqnence benzenesnlfonamide > p-nitrobenzenesulfonamide > />-methoxybenzenesnlfonamide, and is very sensitive towards the nature of the aromatic hydrogen substituent (the selectivity in acylated />-methoxybenzenesnlfonamide did not exceed 7% irrespective of the catalyst nature this corresponds to an approximate relative yield... 

Surprisingly few studies have been directed towards the development of noncinchona alkaloid-based catalysts for the alcoholative ASD of meso-anhydrides, or indeed any of the enantioselective alcoholysis processes. Uozumi has reported a series of (2S, 4R)-4-hydroxyproline-derived 2-aryl-6-hydroxyhexahydro-lfi-pyr-rolo[l,2-c] imidazolones which mediate the methanolytic ASD of ds-hexa-hydrophthalic anhydride in up to 89% ee when employed at the 10 mol% level for 20 h at —25 °C in toluene [186]. Additionally, Nagao has described the use of a bifunctional chiral sulfonamide for the thiolytic ASD of meso-cyclic anhydrides in up to 98% ee when employed at the 5 mol% level for 20 h at rt in ether [187]. 

Finally, the methanolytic desymmetrisation of a variety of cyclic anhydrides was also achieved by using a thermally robust sulfonamide-based bifunctional cinchona alkaloid as the organocatalyst. Under these conditions, an unprecedented catalytic activity combined with an excellent level of enantioselectivity of up to 98% ee were obtained at a catalyst loading of 5 mol %, as shown in Scheme 9.5. No appreciable effects of the concentration and temperature on the reactivity and enantioselectivity were observed with this catalyst. 

An interesting oxidative cyclization reaction mediated by Cu(OAc)2 has been reported in which, unlike previous cases, the cyclic product was formed in absence of a Pd based catalyst (eq 29). Both, radical and ionic, mechanisms have been elaborated the first begins with one electron oxidation of sulfonamidic nitrogen while the second one includes formation of a nitrogen-copper bond, followed by migratory insertion to the double bond. 

The enantioselective addition of ketones to nitrostyrenes was developed by using the bifunctional sulfonamide primary amine catalyst 93 (Figure 24.32). No reaction took place when water was used as reaction medium. The best solvent was CHCI3. However, addition of water increased the yield and enantioselectivity, probably because it increased catalyst turnover due to facilitating the release of the primary amine catalyst from the imine. Moreover, the authors reported an explanation of the role of water in the enhancement of enantioselectivity [114], Three different primary thiourea-based catalysts (94—96) were also used with very good results (Figure 24.32) [115-117]. 

Although hydroamination of allenes can be easily achieved with group 4 and group 5 metal catalysts, the stereoselectivity of these systems is rather limited. Several attempts to perform asymmetric hydroamination/cyclization of aminoallenes employing chiral aminoalcohols [260, 261] and sulfonamide alcohols [262] as chiral proligands for titanium- and tantalum-based catalyst systems have produced vinyl pyrrolidines with low selectivities only. While the titanium catalysts were... 

Me3Si)2NH, Me3SiCl, Pyr, 20°, 5 min, 100% yield. ROH is a carbohydrate. Hexamethyldisilazane (HMDS) is one of the most common silylat-ing agents and readily silylates alcohols, acids, amines, thiols, phenols, hydroxamic acids, amides, thioamides, sulfonamides, phosphoric amides, phosphites, hydrazines, and enolizable ketones. It works best in the presence of a catalyst such as X-NH-Y, where at least one of the groups X or Y is electron withdrawing." Yttrium-based Lewis acids also serve as catalysts. ... 

Sulfonic esters are most frequently prepared by treatment of the corresponding halides with alcohols in the presence of a base. The method is much used for the conversion of alcohols to tosylates, brosylates, and similar sulfonic esters. Both R and R may be alkyl or aryl. The base is often pyridine, which functions as a nucleophilic catalyst, as in the similar alcoholysis of carboxylic acyl halides (10-21). Primary alcohols react the most rapidly, and it is often possible to sulfonate selectively a primary OH group in a molecule that also contains secondary or tertiary OH groups. The reaction with sulfonamides has been much less frequently used and is limited to N,N-disubstituted sulfonamides that is, R" may not be hydrogen. However, within these limits it is a useful reaction. The nucleophile in this case is actually R 0 . However, R" may be hydrogen (as well as alkyl) if the nucleophile is a phenol, so that the product is RS020Ar. Acidic catalysts are used in this case. Sulfonic acids have been converted directly to sulfonates by treatment with triethyl or trimethyl orthoformate HC(OR)3, without catalyst or solvent and with a trialkyl phosphite P(OR)3. ... 

Gold-based catalysis has attracted considerable attention in recent years. A gold-catalyzed aziridination reaction has recently been reported <06JOC5876>. A series of gold catalysts were examined for their ability to catalyze the aziridination of styrene with p-nitrophenylsulfonamide (NsNH2). Styrene and phenyl-substituted styrenes provided the N-nosyl aziridines in good to excellent yields. Cinnamate however provided the aziridine product in only 25% yield. The use of other sulfonamides (e.g. tosyl, trichloroethyl) gave much lower yields of the aziridine product. 

The use of metal-catalyzed aziridination methods with chiral ligands has also been reported. The copper-based system paired with ligand 56 provides the expected cinnamyl aziridine in good yield and excellent ee <06MI4568>. It is interesting to note that the /-butyl ester is obtained with 99% ee while the smaller methyl ester is obtained in only 88% ee. The binaphthyl ruthenium catalyst 57 has been found to aziridinate a number of olefins with moderate enantioselectivity <06TL1571>. Both p-nitrophenyl (Ns) and trimethylsilyloxy (SES) sulfonamides work well with this catalytic system. As is usually seen, the aziridination of aliphatic olefins proceeds in only 32% yield and 56% ee. 

The C-H insertion of alkyl sulfonamides using hypervalent iodine reagents in the presence of a transition metal catalyst was initially disclosed by Dauban and Dodd <20000L2327>. In this report, sulfonamide 204 was treated with PhI(OAc)2 and base to form an intermediate iminoiodinane 205 (Scheme 28). The material 205 was first... 

In other diethylzinc studies, a neural network modelling approach has been used to predict the utility of new enantioselective catalysts,222 norephedrine-derived ligands with three stereogenic centres catalyse enantioselective addition to aldehydes and to chalcones,223 and a chiral sulfonamide ligand based on tartaric acid gives good ees in addition to both aldehydes and ketones.224... 

Diastereoselective Mannich-type reactions between ketene silyl acetals and chiral sulfinimines using simple metal-free Lewis bases such as tetraalkylammonium car-boxylates have been reported. The sulfinimine can even be generated in situ (from aldehyde and a chiral sulfonamide), using cesium carbonate, followed by addition of ketene silyl acetal at -78 °C, and as little as 1 mol% of catalyst.32... 

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