Anthranilamide (aam)-substituted diboron: palladium-catalyzed selective B(aam) transfer

Article abstract of DOI:10.1039/c8cc05645e

An unsymmetrical diboron bearing an anthranilamide (aam) substituent was synthesized and was found to be coupled with aryl halides through the palladium-catalyzed Miyaura-Ishiyama-type reaction. The selective transfer of aam-substituted boryl moiety led t

Full text of DOI:10.1039/c8cc05645e

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Kageyuki, I. Osaka, S. Hatano, M. Abe and H. Yoshida, Chem. Commun., 2018, DOI:
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Table 2 Ligand optimization for borylationa
configuration as judged by its dihedral angles (0.0–5.6°) with the
N1–B1–N2 bond angle of 116.3°, which indicates the boron
center has a nearly ideal sp2 hybridized orbital.14 In addition to
the B(sp2) character,15 effective overlapping between the
vacant p-orbital and lone pairs on the adjacent nitrogens results
in sufficient diminishment of the B-Lewis acidity: the shorter
B1–N2 (1.411 Å vs B1–N1 1.441 Å) bond is suggestive of more
contribution of the aniline nitrogen to the diminishment. On
the other hand, the presence of the less donative amide
nitrogen is responsible for the residual B(aam)-Lewis acidity as
compared with (pin)B–B(dan)16 having more planar six-
membered ring (dihedral angles: 0.1–2.5°) with the B–N bond
lengths of 1.420 and 1.425 Å (Fig. 2).17
View Article Online
DOI: 10.1039/C8CC05645E
Entry
1
Ligand
XPhos
Yield (%)b
quant (64)c
2
PCy3
80
62
53
8
72
0
3
PtBu3
4
5
6
CyJohnPhos
tBuXPhos
dppf
7
dppbz
8
BINAP
0
9
dppb
27
32
trace
0
10
11
12
DPEphos
Xantphos
none
Fig. 1
Solid-state structure of (pin)B–B(aam) with 50% probability ellipsoids. All of
aConditions: 4-bromotoluene (0.225 mmol), (pin)B–B(aam) (0.150 mmol)
Pd2(dba)3•CHCl3 (3.75 μmol), ligand (0.0113 mmol), KOAc (0.450 mmol), 1,4-
dioxane (0.5 mL), 60 °C, 18 h. bGC yield. c4-Bromotoluene (1.2 eq),
Pd2(dba)3•CHCl3 (1 mol%), XPhos (3 mol%), 12 h (Li’s conditions, ref. 7f).
the hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles
(deg): B1–B2 1.700, B1–N1 1.441, B1–N2 1.411, N1–C3 1.376, N2–C1 1.390, C1–C2 1.407,
C2–C3 1.468, N1–B1–N2 116.4, B1–N1–C3 124.4, B1–N2–C1 122.1, N1–C3–C2 116.3, N2–
C1–C2 120.1, C1–C2–C3 120.5.
that even electron-rich aryl chlorides underwent the B(aam)
installation to give 1a (53%) and 1d (63%), and high functional
group tolerance of the reaction was demonstrated by
employing aryl bromides having such a relatively reactive
moiety as NH2, OH, SiMe3, SMe or Ac (1g
also applicable to benzodioxole, indole and fused aromatic
compounds to furnish 1l 1p, and 3-thienylB(aam) (1q) could be
–1k). The reaction was
–
prepared in moderate yield. The versatility of the B(aam)
transfer was well illustrated by the reaction of 2-
bromopyridines, which resulted in the smooth formation of
Fig. 2
Solid-state structure of (pin)B–B(dan) with 50% probability ellipsoids. All of
the hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles
(deg): B1–B2 1.715, B1–N1 1.420, B1–N2 1.425, N1–C2 1.405, N2–C3 1.403, C1–C2 1.432,
C1–C3 1.429, N1–B1–N2 116.2, B1–N1–C2 123.8, B1–N2–C3 123.2, N1–C2–C1 117.6, N2–
C3–C1 118.1, C2–C1–C3 121.1.
diverse 2-pyridylB(aam) (1r–1x) as isolable and bench-stable
compounds, being in marked contrast to the unstable
properties of the corresponding boronic acids and
boronates.18,19 The competitive reaction by using (pin)B–
B(aam) and (pin)B–B(dan) was also conducted (Scheme 1): 1a
and 4-MeC6H4B(dan) were formed in 40 and 39% yield,
respectively, indicating similar reactivities of these
unsymmetrical diborons in the present borylation.
With the new diboron in hand, we carried out the Miyaura–
Ishiyama-type reaction with 4-bromotoluene in 1,4-dioxane
under palladium catalysis and found that selective B(aam)
transfer occurred quantitatively to give 4-MeC6H4B(aam) (1a) by
using
2-dicyclohexylphosphino-2ʹ,4ʹ,6ʹ-triisopropylbiphenyl
(XPhos), as a supporting ligand (Entry 1, Table 2). The borylation
proceeded with exclusive chemoselectivity, and thus a product
having a B(pin) moiety was not generated at all. Other electron-
rich monodentate phosphines (PCy3, PtBu3, and CyJohnPhos)
than tBuXPhos could also promote this transformation (Entries
2–5), and moreover the use of dppf only gave a comparable
yield of 1a among bidentate ligands surveyed (Entries 6–11). No
desired product was formed under ligand-free conditions,
verifying the necessity of a phosphine ligand in the B(aam)
transfer (Entry 12).
As depicted in Scheme 2, 2-pyridylB(aam) (1r) could not be
synthesized by the conventional dehydrative condensation of 2-
pyridylB(OH)2 and aam,9 probably owing to a considerable
tendency to be protodeborylated,18 which demonstrates the
superiority of the present reaction. The synthetic utility of 2-
pyridylB(aam)
(1t) has preliminarily been illustrated by
deprotection-free cross-coupling with 4-bromotoluene under
aqueous conditions (Scheme 3).20
The present reaction would be commenced by oxidative
addition of an aryl halide to a palladium(0) catalyst, that gives
an Ar–Pd–X (2a) complex, as is the case of the conventional
Miyaura–Ishiyama borylation (Scheme 4).2,7f Subsequent
counter anion exchange between 2a and potassium acetate
A variety of aryl bromides were found to be directly
convertible into the respective Ar–B(aam) under the optimum
conditions: the reaction of bromotoluenes or bromoanisoles
afforded 1a–1f in high yield, regardless of the positions of the
methyl or methoxy substituent (Table 3). It should be noted
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Table 3 Substrate scope on aryl halidesa
View Article Online
DOI: 10.1039/C8CC05645E
Pd(OAc)2 (5 mol%)
Br
Pd2(dba)3·CHCl3 (2.5 mol%)
XPhos (7.5 mol%)
KOAc (3 eq)
B(aam)
SPhos (10 mol%)
K3PO4 (7.5 eq)
N
+
Ar Br
1.5
(pin)B B(aam)
1
Ar B(aam)
+
:
1,4-dioxane/H2O (5:1)
MW, 140 °C, 0.5 h
1,4-dioxane, 60 °C, 18 h
N
F
F
1t
1.5
83%
Scheme 3
Deprotection-free cross-coupling with 2-pyridylB(aam)
provides Ar–Pd–OAc (2b), which is then exclusively converted
into an Ar–Pd–B(aam) (2c) through σ-bond metathesis with
(pin)B–B(aam). The sole formation of 2c can be rationally
explained by selective interaction of the acetoxy moiety with
the Lewis acidic B(pin) moiety in a similar manner to the
cheomselective σ-bond metathesis between a copper alkoxide
complex and (pin)B–B(dan), operative in the Cu-catalyzed
B(dan)-installing reactions.7b,c,g-i Finally, an Ar–B(aam) is
produced through reductive elimination with regeneration of a
palladium(0) complex. Although an alternative pathway that
involves initial formation of Ar–B(pin)21 followed by pin–aam
exchange might be possible, this possibility could be excluded
by the reaction of 4-(pin)BC6H4Br with (pin)B–B(aam), where 4-
(pin)BC6H4B(aam) (1y) was solely produced with the B(pin)
moiety remained intact (Scheme 5).
aIsolated yield. Conditions: aryl halide (0.225 mmol), (pin)B–B(aam) (0.150 mmol),
Pd2(dba)3•CHCl3 (3.75 μmol), XPhos (0.0113 mmol), KOAc (0.450 mmol), 1,4-
dioxane (0.5 mL), 60 °C, 18 h. bfrom aryl chloride. c100 °C. ddppf was used instead
of XPhos.
Br
B(dan)
Pd2(dba)3·CHCl3 (2.5 mol%)
XPhos (7.5 mol%)
KOAc (3 eq)
(pin)B B(aam) (0.5 eq)
(pin)B B(dan) (0.5 eq)
Scheme 4
Catalytic cycle for B(aam)-installing borylation
+
1a +
1,4-dioxane, 60 °C, 18 h
Br
B(aam)
Pd2(dba)3·CHCl3 (2.5 mol%)
XPhos (7.5 mol%)
KOAc (3 eq)
1.5
40%
39%
Scheme 1
Competitive reaction with different diborons
+
(pin)B B(aam)
1,4-dioxane, 60 °C, 18 h
B(pin)
B(pin)
1y: 90%
1.5
1
Reaction with 4-(pin)BC6H4Br
Scheme 5
In conclusion, we have developed the new unsymmetrical
diboron having contrasting boron-Lewis acidities, (pin)B–
B(aam), that can be utilized for the palladium-catalyzed B(aam)
installation via the chemoselective Miyaura–Ishiyama-type
coupling with aryl halides, leading to the direct and convenient
access to diverse Ar–B(aam). Further studies on catalytic
Scheme 2
Condensation of 2-pyridylboronic acid with aam
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H. Asakawa, K. H. Lee, Z. Lin and M. Yamashita, Nat. Commun.,
borylation reactions with (pin)B–B(aam) via chemoselective
B(aam) transfer are in progress.
This work was financially supported by JSPS KAKENHI Grant
Numbers JP16H01031 in Precisely Designed Catalysts with
Customized Scaffolding and JP17K05864.
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Conflicts of interest
There are no conflicts to declare.
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4
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For our reports on copper-catalyzed B(pin)-installing reactions, 19 2-PyridylB(aam) (1r–1x) can stably be isolated by Florisil
see: (a) Y. Takemoto, H. Yoshida and K. Takaki, Chem. Eur. J., column chromatography in an open atmosphere.
2012, 18, 14841; (b) H. Yoshida, I. Kageyuki and K. Takaki, Org. 20 This result is contrary to the previous cross-coupling with Ar–
crystallographic data for this paper. These data are provided
free of charge by The Cambridge Crystallographic Data Centre.
18 E. Tyrrell and P. Brookes, Synthesis, 2003, 4, 469.
5
6
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Details of the deprotection-free cross-coupling will be
reported in due course.
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reaction described in Table 3.
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