Angewandte
Communications
Chemie
Decarbonylation
Nickel-Catalyzed Decarbonylative Borylation of Amides: Evidence for
À
Acyl C N Bond Activation
Jiefeng Hu, Yue Zhao, Jingjing Liu, Yemin Zhang, and Zhuangzhi Shi*
Abstract: A nickel/N-heterocyclic carbene catalytic system has
been established for decarbonylative borylation of amides with
B nep by CÀN bond activation. This transformation shows
2
2
good functional-group compatibility and can serve as a power-
ful synthetic tool for late-stage borylation of amide groups in
complex compounds. More importantly, as a key intermediate,
the structure of an acyl nickel complex was first confirmed by
X-ray analysis. Furthermore, the decarbonylative process was
also observed. These findings confirm the key mechanistic
features of the acyl CÀN bond activation process.
A
rylboronic acids or arylboronates are versatile synthetic
[1]
intermediates in modern organic synthesis. These com-
pounds are usually prepared by either alkyl and aryl lithium
compounds or Grignard reagents, processes which are not
[
2]
Scheme 1. Miyaura borylation reactions. Tf=trifluoromethanesulfonyl,
TM=transition metal.
compatible with numerous functional groups. In recent
years, the development of transition-metal-catalyzed Miyaura
borylation reactions has allowed the synthesis of aryl
boronate esters under mild reaction conditions (Scheme 1a).
Numerous palladium-catalyzed methods have emerged for
the conversion of aryl iodides, bromides, chlorides, and
triflates into either the corresponding pinacol or catechol
nance stability of the amide bond. In 2015, Garg and Houk
[
21]
et al. reported an important breakthrough for the conver-
sion of amides into esters by nickel-catalyzed activation of the
amide CÀN bond. DFT calculations support a catalytic cycle
[
3]
boronate esters. Remarkably, borylation of unreactive
bonds has shown promise because it confers synthetic
which involves a rate-determining oxidative addition step,
followed by ligand exchange and reductive elimination. With
the aim of developing borylation of amides using nickel salts,
we hypothesized that the acyl nickel(II) intermediate I,
formed after nickel(0) insertion into the inert C(acyl)ÀN
[
4]
versatility of such inert functional groups. Among them,
[
5]
nickel-catalyzed cleavage of CÀO and CÀN bonds have led
to the cross-coupling of unconventional phenol- and aniline-
[6]
[7]
based electrophiles, such as aryl carbamates, ethers, N-aryl
bond, might undergo decarbonylation to give the aryl nickel
intermediate II, thereby providing access to Miyaura boryla-
tion reactions (Scheme 1b). To achieve this goal, several
difficulties need to be considered: 1) this transformation is to
cleave a stable CÀN bond while forming an easily trans-
[
8]
[9]
amides, ammonium triflates, and cleavage of CÀF bonds
[10]
have allowed the cross-coupling of various fluoroarenes.
Since Yamamoto et al. demonstrated the stoichiometric
nickel(0)-mediated decarbonylation of aryl carboxylates via
[11]
[22]
an acylmetal species in 1980,
catalytic decarbonylative
formable CÀB bond; 2) the CÀN bond scission has a high
coupling reactions of aroyl derivatives, including acyl chlor-
activation energy and its selective cleavage in the presence of
[
12]
[13]
[14]
[15]
ides, ketones, carboxylic anhydrides, carboxylates,
CÀH, CÀO, and CÀF bonds is challenging; 3) the arylboro-
[
16]
[17]
[18]
phthalimides, aldehydes, isatins, and distorted cyclic
nate products have a tendency to undergo Suzuki–Miyaura
[19]
[20]
[23]
imides, have been reported in succession. Despite the
reported advances, the decarbonylative Miyaura borylation
process has been virtually unexplored.
coupling with unconsumed amides. Herein we report our
results on the utilization of N-Boc-activated secondary
amides as the electrophilic component in a nickel-catalyzed
decarbonylative borylation reaction.
Amides are abundant in naturally occurring and artificial
chemicals, and are poor electrophiles because of the reso-
Initial studies involved the evaluation of the selective
borylation of the imide derivative 1a with B (nep) (Table 1).
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In the presence of 10 mol% [Ni(COD) ], 15 mol% PCy , and
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3
[
*] J. Hu, Dr. Y. Zhao, J. Liu, Y. Zhang, Prof. Dr. Z. Shi
State Key Laboratory of Coordination Chemistry
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210093 (China)
E-mail: shiz@nju.edu.cn
3
.0 equivalents K PO , at 1508C under an Ar atmosphere in
3 4
toluene. We indeed observed, by GC-MS, the desired product
aa in trace amounts after 24 hours (entry 1). Among various
3
NHC ligands (L1–L5), a dramatic effect of ICy·HCl (L3) was
observed with 46% yield of 3aa (entry 3). Under these
Homepage: http://hysz.nju.edu.cn/zzshi/
reaction conditions, K CO was not as effective as K PO
(Table 1, entry 4), and nearly the same yield was observed by
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3
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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