Aryl anion equivalent

An enantioselective Stetter employing a chiral NHC and Hunig s base (EtNPr-j ) in methanol at 0°C has its yield massively boosted by addition of 1 equiv of catechol (1,2-dihydroxybenzene), whereas other phenols such as isomers of catechol or its monomethyl ether have negligible effect. Studies of deuterium kinetic isotope effects suggest that initial proton transfer from catechol (97) to form the aryl anion equivalent (98) is turnover limiting. Intramolecular examples are also described, requiring subpercentage amounts of NHC, base, and catechol. 

Based on the general mechanism for Pd-catalyzed carbon-carbon bond formation (see p. 418), the boronic acid must deliver an aryl anion equivalent to Pd at some stage in the reaction. 

The base-catalysed addition of thiols to Jt-electron-deficient alkenes is an important aspect of synthetic organic chemistry. Particular use of Triton-B, in place of inorganic bases, has been made in the reaction of both aryl and alkyl thiols with 1-acyloxy-l-cyanoethene, which behaves as a formyl anion equivalent in the reaction [1], Tetra-n-butylammonium and benzyltriethylammonium fluoride also catalyse the Michael-type addition of thiols to a,P-unsaturated carbonyl compounds [2], The reaction is usually conducted under homogeneous conditions in telrahydrofuran, 1,2-dimethoxyethane, acetone, or acetonitrile, to produce the thioethers in almost quantitative yields (Table 4.22). Use has also been made of polymer-supported qua-... 

In 2008, the same group employed chiral dicarboxylic acid (R)-5 (5 mol%, R = 4- Bu-2,6-Me2-CgHj) as the catalyst in the asymmetric addition of aldehyde N,N-dialkylhydrazones 81 to aromatic iV-Boc-imines 11 in the presence of 4 A molecular sieves to provide a-amino hydrazones 176, valuable precursors of a-amino ketones, in good yields with excellent enantioselectivities (35-89%, 84-99% ee) (Scheme 74) [93], Aldehyde hydrazones are known as a class of acyl anion equivalents due to their aza-enamine structure. Their application in the field of asymmetric catalysis has been limited to the use of formaldehyde hydrazones (Scheme 30). Remarkably, the dicarboxylic acid-catalyzed method applied not only to formaldehyde hydrazone 81a (R = H) but also allowed for the use of various aryl-aldehyde hydrazones 81b (R = Ar) under shghtly modified conditions. Prior to this... 

An anionic equivalent of the Friedel-Crafts cyclization reaction has been developed for the formation of the C /C-5 bond of the 1,2-benzothiazine structure (Equation 35 Table 5) <1997SL1079>. In this reaction, directed metalation of sulfonamide-substituted aromatic systems 233 with an excess of LDA affords aryl lithium species 234 in a regiocontrolled fashion. This intermediate then reacts in situ with a proximal amide to form l,2-benzothiazine-4-one 1,1-dioxides 235. The yields of this transformation appear to be highly dependent upon the substitution pattern in 233. The authors attribute the low yield when = methyl and = H to a-deprotonation of the amide moiety. 

Rather interestingly, an aryl acetal such as (324) bearing a p-oxazolinyl substituent can be metallated with LDA or rc-BuLi to provide a stable anion (325) which reacts with electrophiles to provide a high yield of new substituted aromatics (326 Scheme 69) (79TL4155). The 2-aryl acetal thus functions as an effective nucleophile and an acyl anion equivalent. 

Several aromatic and heterocyclic acyl trimethylsilanes have been used as acyl anion equivalents by treatment with fluoride ion (Scheme 81, path a)23 133 154b160191192. Provided that the acyl substituent is electron-withdrawing, and that there are no aryl substituents on the silicon atom, acyl anions can be trapped by various electrophiles in moderate to good yields indeed, acyl anions and pentacoordinate silicon anionic species have both been detected in gas-phase reactions of acyl silanes with fluoride ion193. 

The functional equivalent of an enolate dianion (103) was prepared by Stork et al. by treatment of en-amine (104) with r-butyllithium. ° This anion crystallized as the symmetrical dimer (105) with the car-banionic carbon nearly symmetrically bridging two lithium atoms as shown in (105). Doubly bridging carbons represent a characteristic feature of these lithiated vinylic anions and this structural feature is normally expected in these compounds as well as in aryl anions (see ref. 11). 

Sibi, M. P., Snieckus, V. The directed ortho lithiation of O-aryl carbamates. An anionic equivalent of the Fries rearrangement. J. Org. Chem. 1983,48, 1935-1937. 

Deprotonation. After iV,iV-diethyl-l-lithiovinylcarbamate, which is obtained by a deprotonation process (with r-BuLi as base), is converted into the organozinc reagent, it can be used to couple with aryl halides (e.g., bromides) and triflates in the presence of (dppf)PdClj to afford the enol carbamates of the acetophenones. The overall transformation can be considered an anionic equivalent of Friedel-Crafts acylation. ... 

Azomethinium salts are very versatile intermediates since they present an extremely electrophilic carbon for nucleophilic attack. High yields of these salts can be obtained in one step from a-amino acids via their tert-dcsmno derivatives Ethyl N-Benzylidene-glycinate has been used for the synthesis of a-amino acids and as an acyl carbanion equivalent A new reactive formaldehyde anion equivalent is lithium methylthioformaldine, (4,5-dihydro-5-methyl-l,3,5-dithiazin-2-yllithium A convenient synthesis of 2-alkyl- and 2-aryl-l,3-benzo-dithioles, which are precursors of acyl carbanion equivalents, has recently been published . 

The optically pure secondary phosphine boranes are easily deprotonated, giving extremely nucleophilic phosphide borane anions, which preserve the absolute configuration at the P atom at low temperatures. This fact prompted Danjo, Imamoto and co-workers to couple the phosphide boranes derived from 9 with aryl cation equivalents. In particular, they found that fluoroarene tricarbonylchromium complexes such as 17 were excellent substrates for the SNAr process due to the high electron deficiency of the arene ring and significant electrophilicity of the ipso carbon (Scheme 5.9). 

Keto-esters.—Further examples of alkylations of the acyl anion equivalent (105) have been reported (c/. 2, 226 3, 16). Only in some cases can good yields of a-keto-ester dimethyl acetals be realized. In an extension of earlier work, N-aryl-triflamides have been found to be useful reagents for the overall oxidation of a-bromo-esters to a-keto-esters. Benzoylformates are accessible by carboxylation of anions derived from 2-aryl-l,3-dithians this approach seems to be an improvement on existing methods. 

Various versions of the acyl anion equivalent (32) are described by Stork etal. (use of Et2NCH2CN), Krief and co-workers (1,3-dithianes in regioselective Michael addition), Gokel et al. (use of sulphur heterocycles with thermal deblocking), Cohen and Weisenfeld" (conversion of acids into vinyl sulphides), Meyers and Campbell " (acetals derived from aryl aldehydes), and Hunig and co-workers (cyanohydrin derivatives of aryl aldehydes). 

Ketimines are also the products of the reaction between t-butyl-lithium aldi-mine, an acyl anion equivalent derived from 1,1,3,3-tetramethylbutyl iso< anide and t-butyl-lithium, and aryl, vinyl, and acetylenic halides, which upon hydrolysis yield the corresponding ketones (Scheme 47). ... 

N-heterocyclic carbene catalysis has become one of the major categories in orga-nocatalysis. Azolium salts are ready deprotonated by weak bases to generate a carbene, which then adds to an aldehyde to form an acyl anion equivalent, generally called the Breslow intermediate. The reactive acyl anion attacks an electrophile to promote the various transformations such as benzoin, Stetter, and redox reactions [107]. Recently, an interesting approach for NHC-catalyzed generation of an enol/enolate intermediate was reported. Enantio-enriched (i-amino acid derivatives (217) are formed by the reaction between the a-aryloxyaldehyde 214 and N-tosyl-imines (215) in the presence of phenyalanine-derived azoUum salt 216 as a pre-catalyst and aryl phenoxide as a base (Scheme 28.28) [108]. 

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