A-aryl amides

A reagent composed of tetra-n-butylammonium nitrate and TFAA in methylene chloride has been used to nitrate a series of A-alkyl and A-aryl amides (40-90 %). The formation of significant amounts of A-nitrosamides was noted. Tetra-n-butylammonium nitrate and triflic anhydride in methylene chloride has been used to successfully nitrate a variety of heterocyclic amides, imides and ureas (66). ... 

Kalinski C, Umkehrer M, Ross G, Kolb J, Burdack C, Hiller W (2006) Highly substituted indol-2-ones, quinoxalin-2-ones and benzodiazepin-2, 5-diones via a new Ugi(4CR)-Pd assisted A-aryl amidation strategy. Tetrahedron Lett 47 3423-3426... 

In the alkyl aryl ketones, the aryl groups migrate preferentially, yielding A-aryl amides. 

An alternative one-step polar cyclization procedure involves the condensation of n nucleophiles with amides to efficiently produce highly substituted pyridines <07JA10096>. As shown below, acetylenes 5 or enol ethers 6 react with electron-poor and electron-rich N-vinyl and A-aryl amides 7 that are activated with triflic anhydride in the presence of 2-chloropyridine. This novel method employs mild reaction conditions and provides rapid access to highly substituted pyridines 8 with good regiocontrol. 

Aldol reaction. New amides of (5)-proline and 4-substituted prolines continue to be tested for their effectiveness in promoting enantioselective aldol reactions, mainly based on the model reaction of cyclohexanone with an ArCHO. The long list of compounds includes those of A-aryl amides 1, 2, and those bearing additional chiral elements such as 3 and 4." ... 

Enamides. The addition of A -aryl amides to alkynes affords enamides. A phosphine is present in the reaction medium. 

Spirofused (3-lactams related to 64 were prepared from 3-hydroxy-3-arylpropanamides after activation of the hydroxyl group by formation of a phosphate ester <97JOC6412>. A similarly easy cyclization occurred when the A-aryl amide 66 was treated with potassium carbonate <97TA739>. Radical induced 4-e.w-trig cyclizations of yV-vinyl-2-bromobutan-... 

More recently, the use of potassium phosphate has also been reported for the a-arylation of ketones with chloroarenes. KHMDS was the most effective base for the intermolecular arylation of A Af-dialkylamides (eq 35). Higher yields were observed with KHMDS compared to LiTMP. Use of NaOr-Bu resulted in low conversion together with a high level of undesired side products, while LDA appeared to deactivate the system. Coupling of unfunctionalized and electron-rich aryl bromides with A,A-dimethylacetainide afforded a-aryl amides in moderate to good yields when the reaction was conducted with at least 2 equiv of KHMDS base. Diarylation of acetamides as well as hydro-dehalogenation of the aryl halides were side reactions that limited this process. 

The o-keto ester 513 is formed from a bulky secondary alcohol using tricy-clohexylphosphine or triarylphosphine, but the selectivity is low[367-369]. Alkenyl bromides are less reactive than aryl halides for double carbonyla-tion[367], a-Keto amides are obtained from aryl and alkenyl bromides, but a-keto esters are not obtained by their carbonylation in alcohol[370]. A mechanism for the double carbonylation was proposed[371,372],... 

In each of the following reactions an amine or a lithium amide derivative reacts with an aryl halide Give the structure of the expected product and specify the mechanism by which it is formed... 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

The N—CO distance of 1.38 A in (58) is rather greater than that of a normal amide (ca. 1.32 A) this has been attributed to ring strain and to inhibition of normal amide resonance by interaction with the N-aryl substituent. This inhibition of resonance is more pronounced in the N-tosyl-4-thioxoazetidin-2-one (59), which exhibits very short C=0 and C=S distances as well as the unusually long C—N bonds (80TL4247). NMR investigations... 

Cyclization of A -aryl-2-(ethoxycarbonyl)-3-(2-pyridylamino)acrylamides 307 in AcOH, and in PPA, or in ethylene glycol afforded A-aryl-4-oxo-4//-pyrido[l,2-n]pyrimidine-3-carboxylic amide 308 (94KGS629, 95KFZ(5)39). 

The reaction of amines with aryl halides requires a catalyst in most cases to initiate the reaction. There are several approaches that result in N-aryl amines. Treatment of cyclohexylamine with p-MeC6H4B(OH)2 and Cu(OAc)2 gave the N-aryl amide in 63% yield. Aryl halides react with amines in the presence of palladium... 

Coumarincarboxylate derivatives are versatile, efficient, low molecular weight, nonpeptidic protease inhibitors. Both esters and amides behave as time-dependent inhibitors of a-chymotrypsin but the esters are clearly more efficient than the corresponding amides. The criteria for a suicide mechanism are met. The presence of a latent alkylating function at the 6-position (chloromethyl group) is required to produce to inactivation by a suicide mechanism (Scheme 11.3, pathway a). Aryl esters, in particular the meta-substituted phenyl esters are the best inhibitors. Thus, m-chlorophenyl 6-(chloromethyl)-2-oxo-27/-l-benzopyran-3-carboxylate is one of the well-known inactivator of a-chymotrypsin (kJK, = 76(),000M s 1 at pH 7.5 and 25 °C, Table 11.1). 

Such reactions are also possible in vitro, as several mild oxidizing agents are at hand nowadays. Thus, the Dess-Martin periodinane (DMP) [50] has been proven to be a versatile and powerful reagent for the mild oxidation of alcohols to the corresponding carbonyl compounds. In this way, a series of new iodine(V)-mediated reactions has been developed which go far beyond simple alcohol oxidation [51], Ni-colaou and coworkers have developed an effective DM P-mediated domino polycy-clization reaction for converting simple aryl amides, urethanes and ureas to complex phenoxazine-containing polycycles. For example, reaction of the o-hydroxy anilide 7-101 with DMP (2 equiv.) in refluxing benzene under exposure to air led to polycycle 7-103 via 7-102 in a yield of 35 % (Scheme 7.28) [52]. 

Amides and sulfonamides undergo intramolecular chemistry to form aryl amides and aryl sulfonamides (Equations (17)—(19)) in the presence of palladium catalysts ligated by arylphos-phines.35,89 Initially, complexes of P(furyl)3 and P(o-tol)3 were most effective catalysts, but complexes of Hayashi s MOP and van Leeuwen s DPEphos and xantphos have lately been shown to be more active.90 In the presence of catalysts containing one of these ligand systems, five-, six-, and seven-membered rings were formed from halogenated benzamides or from substrates containing an acetamide, an A-carbobenzyloxy, or a t-butylcarbamate substituent tethered to the aryl halide (Equations (18) and (19)) ... 

The intermolecular coupling of lactams and acyclic amides has also been reported. Reactions of carbamates with aryl halides occurred in the presence of catalysts ligated by P(/-Bu)3.78 Both carbamates and amides coupled with aryl halides in the presence of a catalyst bearing Xantphos.90 In addition, the coupling of lactams with aryl halides has been successful. A combination of Pd(OAc)2 and DPPF first formed A-aryl lactams in good yields from 7-lactams, but the arylation of amides was improved significantly by the use of Xantphos (Equations (20) and (21)).90 91 The reaction of aryl halides with vinyligous amides has also been reported 92... 

A mild and efficient a-heteroarylation of simple esters and amides via nucleophilic aromatic substitution has been described <06OL1447>. Treatment of 2-chloro-benzo[//Jthiazole 99 with tert-butyl propionate in the presence of NaHMDS under nitrogen furnishes tert-butyl 2-(benzo[c(jthiazol-2-yl)propanoate 100. When the same reaction is preformed initially under nitrogen and then exposed to air, the hydroxylation product 101 is obtained. This method offers two desirable features that are either complementary or improvements to the palladium-catalyzed a-arylation reactions. First, heteroaryl chlorides... 

Most recently, Bentz and co-workers established that the Pd-catalyzed a-arylation of esters and amides, using Reformatsky reagents and the corresponding aryl bromides, can be successfully performed under microwave conditions.417 The reported yields are somewhat lower than those reported by Hartwig 414 however, this approach does not require the use of Q-phos and can be considered as valuable alternative for a-arylation. 

Scheme 6.43 Palladium-catalyzed a-arylation of esters and amides with organozinc reagents.

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