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yangliqiang

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虫号: 485825
注册: 2007-12-27
性别: GG
专业: 功能与智能高分子

[交流] 翻译文献

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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