Journal of the American Chemical Society
Article
structure revealed that the Cr adopts an octahedral geometry
in ligation with CAAC, the imino, and an additional water
molecule in the solid state.
Scheme 1. Catalytic Addition of Pinacolborane to
Unsaturated Oxygen-Containing Motifs
The reactivity of CAAC−Cr complex 2a in the hydro-
boration of a nitro group was initially determined in the
reaction of 1-bromo-4-nitrobenzene with HBpin (Table 1). It
was pleasing to find that the reaction of 1 mol % of 2a with 10
mol % of Mg as reductant exhibited high reactivity. This
reaction allowed the addition of HBpin across a nitro group by
a deoxygenation process that proceeded efficiently at 60 °C,
leading to reduction and affording the aniline 3a in 96% yield
(entry 1). The deoxygenation tolerates a reducible bromo
substituent, without dehalogenation occurring. The formation
of bisboryloxide (O(Bpin)2) in nearly quantitative yield was
observed. O(Bpin)2 is a valuable precursor for accessing n-type
semiconducting polymers,40 orally active EP3 receptor
antagonists,41 and boryl silyl ethers.42
In the absence of a Cr complex, the deoxygenative
hydroboration with Mg did not afford an aniline, in recovery
of the nitrobenzene. The formation of reactive low-valent Cr
by the reduction of 2a with Mg could be considered for the
deoxygenative hydroboration.43 The replacement of CAAC by
the NHC of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
in complex 2a gave an inferior result (2b) (entry 2). The
CAAC scaffold in the ligand may play an important role in
promoting the reactivity of Cr in the deoxygenation of nitro
groups. Other complexes, such as CAAC−Cr−CAAC (2c),
NHC−Cr−NHC (2d), and bis(imino)Cr (2e), gave relatively
low conversions (entries 3−5). Deoxygenation using CrCl3,
CrCl2, and Cr(acac)3 gave poor results (entries 6−8). First-
row metal salts of FeCl3, CoCl2, NiCl2, and CuBr2, and PdCl2
catalyst, exhibited almost no reactivity in the reaction (entries
9−13). Of note is that the CAAC−Cr 2a exhibited extremely
high efficiency; it gave a turnover number (TON) as high as
1.8 × 106 in the gram-scale deoxygenative reduction of 4-
bromonitrobenzene.
Having in hand the efficient CAAC−Cr complex 2a, the
scope of nitro motifs for the deoxygenative reduction was
explored. We proved that the reaction allows for selectively
reducing nitro scaffolds while keeping the iodo functionality
intact, hence providing access to the substituted aniline 4b in
excellent yield (Scheme 2). The incorporation of a broad range
of polar functional groups, including chloro, fluoro, trifluor-
omethyl, trifluoromethoxy, thiomethyl, pinacol boronate ester,
hydroxyl, and amino, has no great influence on the
deoxygenative hydroboration. Diversely substituted aniline
compounds 4c−m were formed in yields of 70−99%.
Unsaturated alkynes and alkenes are usually sensitive motifs
toward hydroboration; they can undergo nucleophilic attack by
HBpin under metal and Lewis acid catalysis.44 Interestingly,
the alkyne and olefin scaffolds in nitroarenes are compatible
with the reduction system, thereby offering a strategy for the
preparation of alkynylated and alkenylated anilines 4n−q. The
primary and secondary amide groups reduced by magnesium,
lanthanide, and zinc catalysis can be retained under the present
conditions,30−32,45 affording 4-aminobenzamide (4r) and N-
(4-aminophenyl)benzamide (4s). The steric hindrance that
resulted from the ortho-methyl, -methoxy, and -phenyl
substituents did not hamper the deoxygenation (4u−w).
Through this strategy, benzene-1,2-diamines 4x and 4y that
contain two of the same, or different, amino scaffolds were
successfully derived from the commercially available nitroarene
precursors. Difluoro-, dichloro-, bromofluoro-, and amino-
selective hydroboration of nitro motifs over functional groups
of alkynes, alkenes, pyridines, and even amides that are usually
hydroborated, through a ligand-assisted umpolung process,
using CAAC as an H-shuttle.
2. RESULTS AND DISCUSSION
N-Heterocyclic carbenes (NHCs) are electron-rich σ donors
frequently used as ancillary ligands to bind to metals in the
assembly of effective catalysts.35 As compared to typical NHCs,
CAACs show strong nucleophilicity and electrophilicity.36
They have been utilized to assist metals in promoting
hydrofunctionalization, as reported by Bertrand.37 Considering
that electron-rich CAAC ligands are able to enhance the
electron density around a Cr center,37,38 facilitating the
activation of the B−H bond by σ interaction, while
incorporation of an imino anchor to the side chain of CAAC
may stabilize a reactive Cr in catalysis by bidentate
coordination, the imino-containing iminium salt 1a was
selected as precursor for the preparation of an organo-
chromium complex (Figure 1).39 Deprotonation by K[N-
(SiMe3)2] and treatment with CrCl3 in THF afforded the
CAAC−Cr complex 2a. X-ray diffraction analysis of the crystal
Figure 1. Synthesis of CAAC−Cr complex.
1619
J. Am. Chem. Soc. 2021, 143, 1618−1629