Inverse Hydroboration of Imines with NHC-Boranes Is Promoted by Diphenyl Disulfide and Visible Light

Article abstract of DOI:10.1021/acs.orglett.1c00230

We describe a simple and efficient procedure for nucleophilic borylation of imines in the absence of a photoredox catalyst. Visible light irradiation of an acetonitrile solution of an imine, an NHC-borane, and diphenyl disulfide (10 mol %) provides various stable α-amino NHC-boranes in good yields. The reaction proceeds via addition of a nucleophilic boryl radical to an imine, followed by hydrogen abstraction from thiophenol, which is generated from NHC-borane and diphenyl disulfide.

Full text of DOI:10.1021/acs.orglett.1c00230

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Letter
Inverse Hydroboration of Imines with NHC-Boranes Is Promoted by
Diphenyl Disulfide and Visible Light
Takuji Kawamoto,* Tsubasa Morioka, Kohki Noguchi, Dennis P. Curran, and Akio Kamimura
Cite This: Org. Lett. 2021, 23, 1825−1828
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ABSTRACT: We describe a simple and efficient procedure for
nucleophilic borylation of imines in the absence of a photoredox
catalyst. Visible light irradiation of an acetonitrile solution of an
imine, an NHC-borane, and diphenyl disulfide (10 mol %)
provides various stable α-amino NHC-boranes in good yields. The
reaction proceeds via addition of a nucleophilic boryl radical to an
imine, followed by hydrogen abstraction from thiophenol, which is generated from NHC-borane and diphenyl disulfide.
rganoboron compounds are versatile building blocks in
O
organic synthesis.1 In the past two decades, boron-
containing drugs2 and functional materials3 have attracted
attention. In particular, α-amino boronic acids play an
important role in pharmaceuticals.4 Direct C-borylation of
imines is an ideal method for the formation of α-amino
boronic acids.5,6 Standard ionic hydroboration cannot be used
for this reaction because boranes behave as hydride sources
and simple reduction to an amine typically ensues.7 However,
several valuable methods based on diborylation are known. In
2000, Baker and co-workers reported diborylation of imines
with bis(catecholato)diboron in the presence of Pt(cod)Cl2
catalyst.6a Elman developed asymmetric diborylation of chiral
reactions, a radical Y adds to an unsaturated bond followed
by hydrogen abstraction from Z−H to give a product along
with a Z radical. The Z radical regenerates a Y radical thus
keeping a radical chain.13 If we could select an appropriate
radical initiator and hydrogen donor, the α-amino boranes
could be obtained through a radical chain mechanism under
simple reaction conditions. We have now discovered that the
inverse hydroboration reaction of imines with NHC-BH3
proceeds efficiently by using PhSSPh as an additive under
compact fluorescent light (CFL) irradiation conditions (eq 2).
Initially, we chose (E)-N-phenyl-1-(p-tolyl)methanimine
(1a) as a model substrate in this study (Table 1). Curran
reported hydrodehalogenation of aryl iodides and bromides
using NHC-BH3 and a catalytic amount of disulfide with a sun
lamp.14 These conditions are known to form thiols in situ.
Thus, a reaction in acetonitrile was conducted with 1a (0.5
mmol), diMeImd-BH3 2 (1.2 equiv), and PhSSPh (0.1 equiv)
sulfinyl imines with copper catalyst,6b and Fernandez reported
́
enantioselective diborylation of tosylaldimines with chiral
phosphine.6c The atom economy in these reactions is not
ideal because one of the two boron functional groups is
ultimately sacrificed for a proton.
In recent years, carbon−boron bond-forming reactions of N-
heterocyclic carbene boranes (NHC-boranes) with alkenes and
alkynes have been discovered.8−10 These hydroboration
reactions involve additions of NHC-boryl radicals to multiple
bonds. The resulting NHC-borane adducts are stable
compounds that nonetheless have a wide variety of reaction
chemistry centering around the boron atom and its
substituents.11 In 2015, we reported radical cyanation of
NHC-boranes with dinitriles.9a This reaction proceeds via
addition of a nucleophilic boryl radical to a nitrile to give an
iminyl radical, followed by β-cleavage.
Recently, Xie/Zhu and co-workers achieved inverse hydro-
boration of NHC-boranes to imines to afford α-amino boranes
(eq 1).12 This reaction requires a photocatalyst, benzyl thiol
(0.1 equiv), sodium hydride (0.2 equiv), and light, and two
nonchain mechanisms based on photoredox catalysis were
proposed. In one, the new C−B bond is formed by radical−
radical coupling, and in the other, it is formed by addition of an
NHC-boryl radical to the imine. In traditional radical
Received: January 20, 2021
Published: February 23, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00230
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1825−1828
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Letter
Table 1. Optimization
Table 2. Substrate Scope for Inverse Hydroboration
a
Reactions of Imines
a
entry
light or initiator
CFL
CFL
CFL
thiol or disulfide
temp
yield
c
b
1
2
3
4
5
6
7
8
9
PhSSPh (0.1 equiv)
none
BnSSBn (0.1 equiv)
−
−
−
88% (78%)
0%
c
c
0%
none (under dark) PhSSPh (0.1 equiv) 80 °C
10%
97%
91%
24%
59%
0%
AIBN (0.1 equiv)
AIBN (0.1 equiv)
AIBN (0.1 equiv)
AIBN (0.1 equiv)
AIBN (0.1 equiv)
PhSH (0.1 equiv)
PhSH (0.2 equiv)
BnSH (0.1 equiv)
BnSH (0.2 equiv)
none
80 °C
80 °C
80 °C
80 °C
80 °C
a
Determined by 1H NMR using CHCl2CHCl2 as an internal
b
standard. Isolated yield after silica gel column chromatography.
c
The reaction was conducted at room temperature. However, the
solution temp was increased using CFL to around 33 °C.
under irradiation from a 12 W white CFL (compact
fluorescence lamp) for 14 h. The target product 3a was
formed in 88% NMR yield and was isolated in 78% yield after
column chromatography on silica gel (entry 1). No borylation
product was observed in the absence of disulfide (entry 2).
Importantly, BnSSBn is not an effective additive (entry 3).
To understand why PhSSPh succeeds and BnSSBn fails, we
conducted control experiments that omitted the imine. CFL
irradiation of PhSSPh and 2 gave diMeImd-BH2(SPh) and
diMeImd-BH(SPh)2.15 In contrast, the reaction of BnSSBn
with 2 afforded only trace amounts of NHC-boryl sulfides.
These results suggest that PhSSPh is absorbing a little light
from CFL to form an initial thiyl radical.
a
1 (0.5 mmol), 2 (0.6 mmol), PhSSPh (0.05 mmol), CFL (12 W),
b
MeCN (2.5 mL), 14 h. Determined by 1H NMR spectroscopy using
1,1,2,2-tetrachloroethame as an internal standard. Yield of product,
isolated after flash column chromatography on SiO2, is given in
c
We next explored thermal reactions at 80 °C in the dark. In
these, we used thiols in place of disulfides. Heating of 1, 2a,
and 0.1 equiv of PhSSPh gave 3a in only 10% yield (entry 4).
Thermal initiation of PhSSPh is not effective.15 However,
when the traditional initiator AIBN was added, 3a was formed
in good yields (entries 5 and 6). The thermal reaction with
BnSH gave poor results (entries 7 and 8). This difference in
the reactivity may be based on the bond dissociation energies
(BDEBnS−H = 86.9 kcal mol−1 vs BDEPhS−H = 79.1 kcal
mol−1).16 This is suggested that the hydrogen abstraction by
the iminyl radical from BnS−H would not proceed efficiently.
No product was formed when AIBN was added and the thiol
was omitted (entry 9). Taken together, these experiments
support the notion that a radical chain is operating in both
thermal and photoreactions and that PhSSPh is a precursor of
PhSH in the photoreactions.
parentheses.
forded α-amino boranes 3h−3j in 95%, 82%, and 69% yields,
respectively (entries 8−10). The reaction of imines 1k and 1l,
which are prepared from 4-bromoaniline, also worked well
(entries 11 and 12). The imines, which were prepared from the
combination of aliphatic aldehydes with aliphatic or aromatic
amines and aromatic aldehydes with aliphatic amines, were not
suitable for this inverse hydroboration reaction in optimized
conditions (Table 1, entry 1).
Finally, we conducted a gram-scale reaction of 1h (4.34
mmol) under the standard conditions (Scheme 1). This
furnished 3h in 79% yield (1.17 g) after evaporation and flash
chromatography.
Next we explored the generality of this simple radical
borylation of imines (Table 2). The reaction of 1b gave
corresponding product in 87% yield (entry 2). Substrates
having halogen atoms at para, meta, and ortho positions, 1c−
1g, gave the corresponding products 3c−3g in good yields
(entries 3−7). The selectivity with aryl bromides (entries 5−7)
is significant; no dehydrobromination products14 were
observed in these reactions. The reaction of imines 1h−1j,
which were prepared from aniline with 2-naphthaldehyde, 3-
pyridinecarboxaldehyde, and thiophene-3-carbaldehyde, af-
Scheme 1. Gram Scale Reaction
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https://dx.doi.org/10.1021/acs.orglett.1c00230
Org. Lett. 2021, 23, 1825−1828

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Letter
A chain mechanism for this inverse hydroboration of imines
is proposed in Figure 1. In the initiation step, a thiyl radical A
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00230
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by a JSPS Grant-in-Aid for
Young Scientists (B) (18K14222), The Naito Foundation,
TheYazaki Memorial Foundation for Science and Technology,
YU project for formation of the core research center
(Yamaguchi University), and The Yamaguchi University
Foundation.
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Figure 1. Possible mechanism.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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AUTHOR INFORMATION
Corresponding Author
■
Takuji Kawamoto − Department of Applied Chemistry,
Yamaguchi University, Ube, Yamaguchi 755-8611, Japan;
orcid.org/0000-0002-1845-4700; Email: tak102@
yamaguchi-u.ac.jp
Authors
Tsubasa Morioka − Department of Applied Chemistry,
Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
Kohki Noguchi − Department of Applied Chemistry,
Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
Dennis P. Curran − Department of Chemistry, University of
Pittsburgh, Pittsburgh, Pennsylvania 15260, United States;
orcid.org/0000-0001-9644-7728
Akio Kamimura − Department of Applied Chemistry,
Yamaguchi University, Ube, Yamaguchi 755-8611, Japan;
orcid.org/0000-0002-3060-4265
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