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The coupling of aryl iodides with sp3 C-H bonds in carboxylic acid derivatives has also been achieved using a slight modification of the above conditions (eq 33).82 In this system, Ag2CO3 was used as the AgI source in combination with 2 equiv of NaOAc and 1 equiv of K2HPO4. While the reaction yields were generally high, mixtures of mono- and diarylated products (ranging between 3:1 and 5:1) were observed in all cases. The arylation of sp3 C-H bonds with aryl iodides has been reported using PdII/0 catalysis and phosphine ligands (Scheme 31).86 Pd(OAc)2 was employed as a precatalyst in conjunction with Cy-JohnPhos and CsF to arylate an R-methyl group in N-(perfluorophenyl)acetamides. At least one additional R-substituent (either an alkyl or an aryl group) was generally required to achieve efficient catalysis in these transformations (Scheme 31). PdII/0-catalyzed sp3 C-H arylation of carboxylic acids has been achieved with arylboronic esters.82 Pd(OAc)2 was used as the catalyst with benzoquinone (BQ) as an additive along with 1.5 equiv of K2HPO4 and 1 equiv of Ag2CO3 (eq 35). Interestingly, extensive screening revealed that alternative silver oxidants (e.g., Ag2O or AgOAc) and bases (e.g., Na2CO3) afforded <5% yield of the arylated products. While excellent (>98%) selectivity for monoarylation was observed, the yields were generally modest (20-38%). Boronic acids have also been studied as coupling partners in the Pd-catalyzed ortho-arylation of N-alkyl acetanilides using Cu(OTf)2 as the oxidant and Ag2O as a key additive (eq 36).87 A number of commercially available boronic acids proved effective as arylating reagents, particularly those containing electron-donating groups. In contrast, the presence of electron-withdrawing substituents or ortho-substitution on the boronic acid resulted in lower yields. A similar reaction with boronic acids was later applied to 2-arylpyridines using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as the oxidant. 88 Arylsilylethers are another class of transmetalating reagents that can be utilized for directed C-H arylation reactions.89,90For example, the combination of catalytic Pd(OAc)2, 2 equiv of AgF, and 2 equiv of Cu(OTf)2 resulted in clean orthoarylation of acetanilide substrates with ArSi(OMe)3 (eq 37). These reactions were proposed to proceed through a PdII/0 manifold. While only trimethoxyphenylsilanes were examined with acetanilides, a variety of different aryl silicon reagents proved effective for the related Pd-catalyzed orthoarylation of enamides.90 Finally, aryl acylperoxides have been used to effect pyridine-, quinoline-, oxime ether-, and oxazoline-directed ortho-arylation (eq 38).91 A mechanism involving decarboxylation to generate aryl radicals (Ar · ) followed by reaction of Ar · with cyclopalladated intermediates was proposed. Consistent with this pathway, conducting the catalytic reactions in the presence of ascorbic acid (a potential radical scavenger) led to dramatically decreased conversions to product. |
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