Iridium-Catalyzed Distal Hydroboration of Aliphatic Internal Alkenes

Article abstract of DOI:10.1002/anie.201902464

The regioselective hydroboration of aliphatic internal alkenes remains a great challenge. Reported herein is an iridium-catalyzed hydroboration of aliphatic internal alkenes, providing distal-borylated products in good to excellent yields with high regioselectivity (up to 99:1). We also demonstrate that the C?B bond of the distal-borylated product can be readily converted into other functional groups. DFT calculations indicate that the reaction proceeds through an unexpected Ir^III/Ir^V cycle.

Full text of DOI:10.1002/anie.201902464

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Accepted Article
Title: Iridium-Catalyzed Distal Hydroboration of Aliphatic Internal
Alkenes
Authors: Guangzhu Wang, xinyi Liang, Lili Chen, Qian Gao, Jian-Guo
Wang, Panke Zhang, Qian Peng, and Senmiao Xu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902464
Angew. Chem. 10.1002/ange.201902464
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http://dx.doi.org/10.1002/ange.201902464

10.1002/anie.201902464
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Iridium-Catalyzed Distal Hydroboration of Aliphatic Internal
Alkenes
Guangzhu Wang,[a][c]‡, Xinyi Liang,[b]‡ Lili Chen,[c] Qian Gao,[c] Jian-Guo Wang,[b] Panke Zhang,*[a] Qian
Peng,*[b] and Senmiao Xu*[c]
Dedicated to 100th Anniversary of Nankai University
Abstract: Regioselective hydroboration of aliphatic internal alkenes
remains a great challenge. Reported here is an iridium-catalyzed
hydroboration of aliphatic internal alkenes, providing distal borylated
products in good to excellent yields with high regioselectivity (up to
99:1). We also demonstrated that the C-B bond of the distal
borylated product could be converted facilely to other functionalities.
DFT calculations indicated that the reaction undergoes an
unexpected Ir(III/V) cycle.
responsible for the observed regioselectivity.[6g] Despite these
advances, the addition of a boryl group to the distal carbon
remains a distinct challenge for aliphatic internal alkenes.[8]
Herein, we report a highly distal selective hydroboration of
aliphatic internal alkenes using readily available iridium dimer
precursor [Cp*IrCl2]2 (Cp* = pentamethylcyclopentadienyl)[9] as
the catalyst. This unusual selectivity complements reported Cu-,
Rh-, and Ir-catalyzed hydroboration. The current method could
tolerate
a variety of β,γ- and γ,-unsaturated carbonyl
compounds, affording the borylated products in good to
excellent regioselectivity (up to 99:1). DFT calculations revealed
the steric effect of Cp* and Bpin gives rise to the observed
regioselectivity. In addition, a rare Ir(III/V) mechanism was
proposed through calculations that is distinct from the generally
accepted Ir(I/III) mechanism in hydroboration of alkenes.[9c,10]
Alkylboronic acids and their derivatives have found widespread
applications in pharmaceuticals and synthetic chemistry.[1]
Accordingly, numerous methods have been developed to
prepare
these
structures.
Transition-metal-catalyzed
hydroboration of alkenes represents one of the most convenient
and straightforward methods to synthesizing these compounds
with chemo- and regioselectivity that complements those
obtained via the uncatalyzed routes.[2] These reactions were
successful in controlling regioselectivity of terminal alkenes,[3]
activated alkenes,[4] and styrenes.[3c,5] Because of the low
reactivity of aliphatic internal alkenes and the difficulty of
controlling the regioselectivity of hydroboration, only a few
examples have been developed for regioselective hydroboration
of aliphatic internal alkenes (Scheme 1, left)[6] as well as other
hydrofunctionalizations.[7] For example, Rh- or Ir-catalyzed
hydroboration of alkenes tethered directing groups proceeded in
a substrate chelation mechanism due to multiple coordination
sites available at the metal center. The regioselectivity was
governed by coordination of directing groups such as amides
and oximes to the transition-metal center, affording β-selective
borylated products in preference to γ-isomers.[6a-f,6i] The proximal
regioselectivity could also be controlled by inductive effects.[6g] In
this case, during formation of the transition states, stabilization
of the accumulating negative charge on the copper-bound
carbon atom by the nearby polar directing group was
Scheme 1. Transition-metal-catalyzed regioselective hydroboration of aliphatic
internal alkenes.
Table 1. Optimization of Reaction Conditions.
Entry[a]
Cat.
2a: 3a: 4a[b]
27: 32: 42
8: 70: 22
30: 54: 16
98: 1: 1
Yield (%)[b]
1
[IrCl(COD)]2
4
2
[RhCl(COD)]2
6
3
[Cp*RhCl2]2
trace
71
[a]
G. Wang, Prof. P. Zhang
4
[Cp*IrCl2]2
College of Chemistry and Molecular Engineering, Henan Institute of
Advanced Technology, Zhengzhou University, Zhengzhou 450001,
China
5
[Cp*IrCl2]2/PPh3
[Cp*IrCl2]2/PCy3
[Cp*IrCl2]2/SPhos
[Cp*IrCl2]2/RuPhos
[Cp*IrCl2]2/DavePhos
[Cp*IrCl2]2/XantPhos
[Cp*IrCl2]2/XantPhos
[Cp*IrCl2]2/XantPhos
98: 1: 1
trace
68
E-mail: pkzhang@zzu.edu.cn
6
98: 1: 1
[b]
[c]
X. Liang, Prof. J.-G. Wang, Prof. Dr. Q. Peng
State Key Laboratory of Elemento-Organic Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China
E-mail: qpeng@nankai.edu.cn
7
98: 1: 1
82
8
98: 1: 1
86
G. Wang, L. Chen, Q. Gao, Prof. S. Xu
State Key Laboratory of Oxo Synthesis and Oxidation, Center for
Excellence in Molecular Synthesis, Suzhou Research Institute,
Chinese Academy of Sciences, Lanzhou 730000, China
E-mail: senmiaoxu@licp.cas.cn
9
98: 1: 1
85
10
11[d]
12[c][d]
98: 1: 1
91
‡
These authors contribute equally to this work.
98: 1: 1
97
Supporting information for this article is given via a link at the end of
the document.
98: 1: 1
96 (96)[e]
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10.1002/anie.201902464
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With optimized reaction conditions identified, we then
investigated the substrate scope of this Ir-catalyzed distal
hydroboration as shown in Scheme 2. The reaction of most
substrates proceeded with excellent regioselectivities. The
substituent on the N-Ar group of the amide significantly affected
the reactivity while having almost no influence on γ-selectivity
(2b-2k). Reaction of substrates with aniline derived β,γ-
unsaturated amides gave corresponding borylated products in
good yields (79-96%) with uniformly excellent γ-selectivity (97-
98%) when terminal R group is an alkyl or ester-containing alkyl
substituent (2a, 2b, 2l-2n and 2r). When RuPhos was used
instead of XantPhos, the reaction is also compatible with
substrates bearing R groups of benzyl, homobenzyl, ether, imide
and (indol-3-yl)methyl, giving the corresponding products with
good to excellent γ-selectivities (2o-q, 2s and 2t). Reaction of
disubstituted terminal alkene 1u gave a single regioisomer 2u in
95% yield. In addition, the N-benzyl and tertiary amide
substrates were also tolerated, affording borylated products 2v
and 2w with excellent γ-selectivity (98-99%). The corresponding
alcohol of 2u was isolated due to its instability with column
chromatography. Interestingly, the current method was also
amendable to γ,-unsaturated amides when XantPhos was
replaced with Davephos, affording -selective borylated products
2x and 2y with moderate to good -selectivity (71-90%). This
method was also applicable for γ-borylation of ,γ-unsaturated
ester 1z, affording 2z in 45% 1H NMR yield with 96% γ-
selectivity.
[a] The reaction were carried out with [1a] = [HBpin] = 0.2 M in THF (1.0 mL)
at room temperature for 12 h. [b] The ratio and yield of 2a were determined by
GC analysis with a calibrated internal standard. [c] Dioxane as solvent. [d] 2
mol% cat. [e] Number in parentheses refers to the isolated yield.
To initiate our study, we chose ,-unsaturated amide 1a as
our pilot substrate. Initial attempt using [MCl(COD)]2 (M = Rh, Ir)
as catalysts afforded trace amount of borylated product with
poor regioselectivity (Table 1, entries 1 and 2). Fortunately,
reaction of 1a with pinacolborane (HBpin) in the presence of 5
mol% of [Cp*IrCl2]2 in tetrahydrofuran (THF) at room temperature
for 12 h furnished distal hydroboration adduct 2a with 98% γ-
selectivity albeit with moderate yield (Table 1, entry 4). We then
turned our attention to ligand effect on this reaction. When PPh3
was used, the reaction was almost shut down probably due to
the formation of a stable phosphine-Ir complex, in which there is
no vacant orbital available for the olefin moiety to bind.
Fortunately, in the presence of bulkier ligands,the reaction
facilely afforded product 2a in good to excellent yield with
consistently excellent regioselectivity (2a: (3a+4a) = 98:2) (Table
1, entries 5-10). For example, when bulky ligands such as PCy3,
SPhos, RuPhos or DavePhos were used, significantly enhanced
yields were observed (Table 1, entries 6-10). Particularly, γ-
selective product 2a could be obtained in 91% GC yield with 98%
regioselectivity when XantPhos was employed (Table 1, entry
10). Further investigation of the reaction conditions revealed that
2a could be obtained in 96% isolated yield with 98%
regioselectivity when the reaction was carried out in 1,4-dioxane
with 2 mol% catalyst loading (Table 1, entry 12)[11]
To demonstrate the utility of the current Ir-catalyzed distal
hydroboration, several transformations of the C-B bond were
conducted as depicted in Figure 1. Treatment of 2a with KHF2 in
CH3CN afforded potassium trifluoborate 5 in 78% yield.[12] In
addition, subsequent transformations of C-B bond of 2b into C-O,
C-F, C-C bonds under various reaction conditions furnished
corresponding products 6-10 in 55-99% yields.[8i,13]
Table 2. Reaction Generality of Ir-Catalyzed Distal Hydroboration.
Figure 1. Synthetic utility of 2b.
Deuterium labeling experiments were performed to
investigate the reaction mechanism.[14] First we investigated the
reaction between 1a and Dbpin under standard reaction
conditions. The product 2a-d showed that deuterium was
incorporated at both - and β-positions (eq 1). Next we
investigated the reaction between d2-1a’ and HBpin. In this case
deuterium scrambling was also observed at at both-and β-
positions (eq 2). The olefin insertion to the Ir-H bond is not likely
because deuterium did not scrambled extensively throughout the
whole alkyl chain.[15] Thus, it is more likely that the current
reaction proceeds by initial olefin insertion in an Ir-B bond.[5i,10]
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10.1002/anie.201902464
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Figure 2. Proposed Mechanism by DFT calculations with the major pathway (black arrow) and minor pathways (light grey arrow) including key energy barriers.
that this reaction proceeds via an unexpected Ir(III/V) cycle.
Further efforts to other regioselective hydroboration and
enantioselective variants are currently underway in our
laboratory.
Acknowledgements
We thank National Natural Science Foundation of China
(21573262, 21502175, 21702109, 21801246, 21890722,
To further understand the reaction pathways and the origin
of regioselectivity, DFT calculations were performed at ω-
11811530637), Natural Science Foundation of Jiangsu Province
b97xd/SMD-def2TZVPP//6-31G*/Lanl2DZ level of theory.[16] In
(BK20161259, BK20170422), the Natural Science Foundation of
computed results, the initial Ir-B insertion is rate-determining
Tianjin City (18JCYBJC21400), a Start-up Grant from Lanzhou
step (RDS) compared to the following β-H elimination, Ir-H
Institute of Chemical Physics, Outstanding Young Talent
insertion, oxidative addition (OA) and reductive elimination (RE)
Research Found of Zhengzhou University (NO.1621316005)
steps (Figure 2). And the regioselectivity of γ-C is more favored
and the Fundamental Research Funds for the Central
with low free energy barrier +25.7 kcal/mol than that of β-C with
Universities (Nankai University: Grant No.63191515, 63191523,
+33.0 kcal/mol, which could be rationalized by the less steric
63191321) for generous financial support.
repulsion of methyl groups between Cp* and Bpin in TS1a.[17a]
Moreover, TS1a is a relatively early transition state with 2.70 Å
Keywords: hydroboration
• iridium• alkylboronic acids •
Ir-B bond comparing to TS1a’ with 2.87 Å Ir-B bond, which can
explain the observed regioselectivity of Ir-B insertion as well.[17b]
The RE step via Ir(V) intermediate is calculated as a favorably
terminal step forming the product followed by the step of ligand
exchange through Ir-complex INT8a.[18] All the calculated RE
steps via Ir(III/I) pathways encounter extremely high energy
barriers over 30 kcal/mol that are not be accessible at room
temperature of the experimental condition, suggesting the weak
reductive elimination capacity of Cp*-Ir(III) intermediates in this
reaction. The -H elimination and Ir-H insertion steps are
reversible and well in agreement with our experimental
observations for deuterium scrambling between α- and β-
positions (eq. 1 and eq.2).
In conclusion, we have developed an unprecedented Ir-
catalyzed hydroboration of aliphatic internal alkenes with distal
regioselectivity. A vast array of γ-, and -selective borylated
products were obtained with excellent regioselectivity under mild
reaction conditions starting from readily available β,γ- and γ,-
unsaturated carbonyl compounds. DFT calculations revealed
synthetic methods • aliphatic internal alkenes
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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A Distal Regioselectivity: An iridium-
catalyzed hydroboration of
Guangzhu Wang, Xinyi Liang, Lili Chen,
Qian Gao, Jian-Guo Wang, Panke
Zhang,* Qian Peng,* and Senmiao Xu*
unactivated internal alkenes has been
developed using readily available
catalyst [Cp*IrCl2]2 under mild reaction
conditions. The reaction has broad
functional group compatibility,
delivering a vast array of distal
borylated products with up to 99%
regioselectivity. DFT calculations
revealed that the reaction proceeds
via an Ir(III/V) cycle.
Page No. – Page No.
Iridium-Catalyzed Distal
Hydroboration of Aliphatic Internal
Alkenes
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