Synergistic Photoredox Catalysis and Organocatalysis for Inverse Hydroboration of Imines

Article abstract of DOI:10.1002/anie.201800421

The first catalytic inverse hydroboration of imines with N-heterocyclic carbene (NHC) boranes has been realized by means of cooperative organocatalysis and photocatalysis. This catalytic combination provides a promising platform for promoting NHC-boryl radical chemistry under sustainable and radical-initiator-free conditions. The highly important functional-group compatibility and possible application in late-stage hydroborations represent an important step forward to an enhanced α-amino organoboron library.

Full text of DOI:10.1002/anie.201800421

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Accepted Article
Title: Synergistic Photoredox and Organocatalysis for Inverse
Hydroboration of Imines
Authors: Jin Xie
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10.1002/anie.201800421
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Synergistic Photoredox and Organocatalysis for Inverse
Hydroboration of Imines
Nengneng Zhou, Xiang-Ai Yuan, Yue Zhao, Jin Xie* and Chengjian Zhu*
Abstract: The first catalytic inverse hydroboration of imines with N-
heterocyclic carbene boranes has been realized by means of
cooperative organocatalysis and photocatalysis. This catalytic
combination brings a promising platform to promote NHC-boryl
radical chemistry under sustainable and radical initiator free
conditions. The highly important functional group compatibility and
possible application in late-stage hydroboration represent an
important step-forward to an enhanced -amino organoboron lead
library.
highly desirable, which can afford a general and practical
strategy to one class of unprecedented -aminoboron
compounds (Scheme 1b). During the last five years, the
development of B-centered radical synthetic chemistry has
gained increasing momentum.[4],[5] Thus, we rationalized that
radical hydroboration of imines with organoboranes could
potentially realize inverse hydroboration to deliver -aminoboryl
products instead of the existing intermediates with N-B bond.
Hydroboration of C=N bonds with organoboranes or
borohydrides is a workhorse reaction for the mild reduction of
imines into amines (Scheme 1a).[1] In this fundamental
transformation, the addition of B-H bond to C=N bond typically
results in nitrogen-bound boryl intermediates. Its exclusive
regioselectivity derives from the favorable interaction of B-H
bond with positively polarized carbon atom on C=N bond. Since
1995, no catalytic inverse hydroboration has been reported.
Scheme 2. Mechanistic hypothesis.
The N-heterocyclic carbene (NHC) boranes are bench-stable,
nontoxic, easy available and convenient to handle
organoboranes.[6]
Malacria have revealed a fruitful chemical space for
NHC-boryl radicals,[7] including radical deoxygenation of
xanthates, radical reduction of halides, radical reductive
decyanation etc. More recently, Wang’s group[8] reported an
elegant radical borylation and subsequent cyclization of 1,6-
enynes with NHC-boranes in the presence of radical initiators.
Despite this great progress, the exploitation of NHC-boranes for
radical C-B bond formation is still of great importance to enrich
boron-containing libraries and currently its application in highly
polarized C=N-bond is scarcely studied owing to competing
reduction pathway. The proof-of-concept for radical inverse
hydroboration of imines was shown in Scheme 1c.
Scheme 1. Selective hydroboration of imines and our design.
-Amino organoborons are versatile building blocks in
organic synthesis and potent protease inhibitors in medicinal
chemistry.[2] Although transition-metal-catalyzed diborylation of
C=N bond has been reliable to access,[3] the limited flexibility
and moderate functional group tolerance compromise their late-
stage applications in complex bioactive molecules. To further
enhance -amino organoboron lead library, the development of
catalytic inverse hydroboration of imines with organoboranes is
[*]
N. Zhou, Dr Y. Zhao, Prof. Dr J. Xie, and Prof. Dr C. Zhu
State Key Laboratory of Coordination Chemistry, Jiangsu Key
Laboratory of Advanced Organic Materials, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing 210023 (P.
R. China)
Email: xie@nju.edu.cn or cjzhu@nju.edu.cn
Homepage: http://hysz.nju.edu.cn/xie/
http://hysz.nju.edu.cn/cjzhu/
Dr. X.-A. Yuan
Scheme 3. Calculated BDEs and SOMO energies.
School of Chemistry and Chemical Engineering, Qufu Normal
University, Qufu 273165 (P. R. China)
Prof. Dr C. Zhu
Usually, the generation of NHC-boryl radical requires the
use of potentially toxic radical initiators (e.g., 2,2'-
azoisobutyronitrile, di-tert-butyl hyponitrite and di-tert-butyl
peroxoide).[7] The recent efforts on synergetic photocatalysis
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Shanghai 200032 (P. R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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and
thiol
catalysis
demonstrated
that
photoexcited
and these variable reactivities indicate possible tunability for the
H-atom transfer step.
We commenced to investigate the inverse hydroboration
model reaction of imine 1a and NHC-borane 2a (Table 1). The
III/II
*Ir(ppy)2(dtbbpy)PF6 (E1/2
*
= + 0.66 V vs SCE; τ= 557 ns)
could readily accept one electron from thiol to give reactive thiyl
radical.[9] As our ongoing interests in photocatalysis,[10] we
wondered if the thiyl radical generated in situ via such a
photocatalytic synergistic mode is capable of accomplishing the
inverse regioselectivity in hydroboration of imines under mild
conditions. After an insight into photocatalytic radical C-C
functionalization of imines,[11] two possible pathways for inverse
hydroboration were depicted in Scheme 2. In path a, it is likely
to undergo single electron transfer (SET) reduction of imine first
optimized conditions were determined to be
1
mol%
Ir(ppy)2(dtbbpy)PF6 as photocatalyst and 20 mol% phenyl-
methanethiol III as organocatalyst together with 20 mol% NaH
as inorganic base in MeCN under the irradiation with 33 W CFL
at room temperature (Table 1, entry 1). Replacing
Ir(ppy)2(dtbbpy)PF6 with Ru(bpy)3Cl2 or fac-Ir(ppy)3 did not give
product 4a (Table 1, entries 2 and 3). To our delight, consistent
with DFT calculation results, the use of suitable thiol was vital for
success (Table 1, entries 4-6). When thiols I and V were
employed, no desired product was detected; other aliphatic
thiols II and IV instead of thiol III delivered moderate yields
(Table 1, entries 5 and 6). Change the light source from CFL to
blue LEDs, the yield of 4a decreased to 57% with a significant
amount of byproduct 3a (Table 1, entry 7). The better yield for
CFL might be due to the thermal factor. The control experiments
demonstrated that photocatalyst, thiol and light were three
essential factors (Table 1, entries 4, 8 and 9).
with strongly reductive IrII species (E1/2 = - 1.51 V vs SCE)[9a]
III/II
followed by radical-radical C-B cross-coupling of NHC-boryl
radical 5 and -aminobenzyl radical 7. In path b, NHC-boryl
radical addition to imine 1a occurs at first followed by SET
reduction of nitrogen-centered radical 8.
Table 1: Optimization of reaction conditions.
Table 2: Scope of imines (1).
Yield:3a[a]
E y
V
Y :4a[ ]
9%
1
2
3
4
5
6
7
8
9
None
79%
0
91%
93%
91%
43%
31%
27%
92%
91%
Ru(bpy)3Cl2 as photocatalyst
fac-Ir(ppy)3 as photocatalyst
Thiol I or V instead of III or no thiol
Thiol II instead of III
Thiol IV instead of III
Blue LEDs instead of CFL
No photocatalyst
0
0
23%
49%
57%
0
No light
0
Standard conditions: Ir(ppy)2(dtbbpy)PF6 (1 mol%), thiol III (20 mol%), NaH
(20 mol%), 1a (0.2 mmol), 2a (0.4 mmol), MeCN (1.0 mL), 33 W compact
fluorescent light (CFL) bulb, 24 h, room temperature. [a] Isolated yield.
It is worthy of noting that the background reaction of imine
1a and NHC-borane 2a furnishes secondary amine 3a in almost
quantitative yield without synergistic catalytic cycle at room
temperature (Scheme 1c). To significantly suppress this
pathway, the resulting thiyl radical should have a high enough
potential to rapidly abstract one hydrogen atom from NHC-
borane 2a (BDE of B-H bond = 74.5 kcal/mol) to generate NHC-
boryl radical 5 (calculated SOMO energy = - 4.59 eV). Polar
kinetic effects aside,[7d] to some extend, the use of
thermodynamically matched thiol would be another possible
tunability for intermolecular H-atom abstraction manifestation.
Therefore, we performed density functional theory (DFT)
calculations on the BDE of S-H bond in thiols and SOMO
energy of corresponding thiyl radicals (Scheme 3). We are glad
to find that varied thiols show significant variation on both factors,
Under the optimal reaction conditions, we investigated the
imine scope with respect to aldehyde-derived moiety (Table 2).
In general, this protocol combines excellent chemo- and
regioselectivities with good functional group compatibility despite
of multiple reactive sites on imines. Only monohydroboration
products were obtained and no di- or tri-hydroboration was
observed. Aromatic rings bearing both electron- donating and
electron-withdrawing substituents could smoothly undergo
inverse hydroboration (4a-j). The position of substituents on
phenyl ring seemed to have no effect on the transformation (4e-
g). Additionally, heteroaromatic aldehyde-derived imines were
good coupling partners (4i and 4j).
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To demonstrate the practice of inverse hydroboration, we
applied this protocol to late-stage radical borylation of a series of
biologically important compounds, such as estrone, helicid and
diacetone-D-glucose derivatives (4k-m). Meanwhile, the reactive
ketone unit remained intact, underlying its synthetic advantages
in organic synthesis.
52%-79% yields. The CN-substituted borane 2g was also a
good reaction partner. However, besides NHC-boranes, 4-
dimethylaminopyridine-boranes and triethyl amine-boranes
failed.
Scheme 5. Mechanistic studies.
Scheme 4. Scope of imines (2).
To gain insight into the mechanism, several mechanistic
experiments were carried out. The frequent detection of imine
dimerization byproduct 10 indicated that -aminobenzylic
radical 7 might be involved (Scheme 5a). Interestingly, the
NHC-boryl radical intermediate 5 was successfully trapped by
We then turned our attention to examine the effect of N-aryl
part on aldimines (Scheme 4). It was found that the aromatic
rings of N-aryl bearing either electron-rich (-Me, -tBu) or
electron-poor groups (-Cl, -Br, -Ac, -COOMe etc.) could
uniformly proceed inverse hydroboration (4n-x). Naphthyl-
amine-derived aldimines was also successful (4s, 73%). Varying
the substituents on both aromatic rings simultaneously was also
viable and 4t-x were obtained in satisfying yields. A broad range
of versatile functional groups, such as ester group (4q), acyl
group (4v), alkynyl group (4w) and alkenyl group (4x) were well
compatible, thus complementing the known protocols because
the metal-catalyzed diborylation of these unsaturated bonds are
known.[3f]
2,2,6,6-tetramethyl-1-piperidyloxy (TEMPO).
A
competing
radical-clock experiment with (1-cyclopropylvinyl)benzene was
further designed to validate the involvement of NHC-boryl
radical (Scheme 5b). Replacing NHC-BH3 2a with NHC-BD3, the
desired inverse hydroboration product was obtained in 75%
yield with a ratio of 13-D:13-H = 2:1 (Scheme 5c).[12] The DFT
calculation results demonstrated that the total Gibbs' free-energy
g (ΔG) p p S 2
comparable (See Supporting Information for details). At present,
although both radical intermediates (5 and 7) have been
detected (Scheme 5a and 5b), both the radical-radical C-B
cross-coupling (path a) driven by persistent radical effect[13] and
the B-centered radical addition to imines (path b) are possible.
However, a radical chain pathway is less likely as the quantum
yield of model reaction was determined as 0.35.
Table 3: Reaction scope with respect to NHC-boranes.
As N-aryl -amino acid serves as one core motif in a great
number of bioactive compounds,[14] the isoelectronic N-aryl -
aminoboronic acid potentially hold promising bioactivities.
However, no general access to N-aryl -aminoboronic acids was
available.[15] To our delight, the inverse hydroboration products
are good precursors to N-aryl-aminoboronates after hydrolysis
(Scheme 6).
Next, the scope with respect to NHC-boranes was explored
(Table 3). Several kinds of NHC-boranes 2a-f could participate
in the inverse hydroboration to afford the desired products in
Scheme 6. Conversion to N-aryl -aminoboronates.
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H. Wadepohl, D. W. Stephan, A. S. K. Hashmi, J. Am. Chem. Soc.
2015, 137, 15469.
In conclusion, we have developed an unprecedented inverse
hydroboration transformation of imines with organoboranes by
means of photoredox and phenylmethanethiol cooperative
catalysis. The experimental and DFT calculation results
demonstrate that matched thiol organocatalyst is one key factor,
as it not only expedites the generation of boryl radical but also
significantly depresses the classical ionic reduction pathway.
Several impressive features, such as mild reaction conditions,
radical initiator-free, good functional group tolerance and late-
stage hydroboration, make this protocol practical to access
medically important N-aryl -aminoboron compounds. A novel
radical-radical C-B coupling is likely based on mechanistic
studies. The exploration of inverse hydroboration strategy for
the synthesis of enantiopure N-aryl -aminoborons is currently
ongoing in our lab.
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We gratefully acknowledge the National Natural Science
Foundation of China (21702098, 21732003, 21703118 and
21672099), “1000-Y P ” and Shandong Provincial
Natural Science Foundation (No. ZR2017MB038) for financial
support.
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Conflict of interest
The authors declare no conflict of interest.
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W. C. MacMillan, Chem. Rev. 2013, 113, 5322; b) J. Xuan, W.-J. Xiao,
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Keywords: imines • hydroboration • visible-light photoredox
catalysis • radical • C-B coupling
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
N. Zhou, X.-A. Yuan, Y. Zhao, J. Xie*
and C. Zhu*
Page No. – Page No.
Synergistic Photoredox and
Organocatalysis for Inverse
Hydroboration of Imines
Text for Table of Contents
Unprecedented Inversion: The first catalytic inverse hydroboration of imines with N-heterocyclic carbene boranes has been realized
by means of cooperative organocatalysis and photocatalysis. The nature of thiol is a vital choice for inverse hydroboration. This
protocol represents an important step-forward to enhanced -amino organoboron lead libraries.
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