Accessing Ambiphilic Phosphine Boronates through C?H Borylation by an Unforeseen Cationic Iridium Complex

Article abstract of DOI:10.1002/anie.201812857

Ambiphilic molecules, which contain a Lewis base and Lewis acid, are of great interest based on their unique ability to activate small molecules. Phosphine boronates are one class of these substrates that have interesting catalytic activity. Direct access to these phosphine boronates is described through the iridium-catalyzed C?H borylation of phosphines. An unconventional cationic iridium catalyst was identified as optimal for a range of phosphines, providing good yields and selectivity across a diverse class of phosphine boronates (isolated as the borane-protected phosphine). A complimentary catalyst system (quinoline-based silane ligand with [(COD)IrOMe]₂) was optimal for biphenyl-based phosphines. Selective polyborylation was also shown providing bis- and tris-borylated phosphines. Deprotection of the phosphine boronate provided free ambiphilic phosphine boronates, which do not have detectable interactions between the phosphorus and boron atoms in solution or the solid state.

Full text of DOI:10.1002/anie.201812857

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Accepted Article
Title: Accessing Ambiphilic Phosphine Boronates through C–H
Borylation by an Unforeseen Cationic Iridium Complex
Authors: Shawn E. Wright, Stephanie Richardson-Solorzano, Tiffany N.
Stewart, Christopher D. Miller, Kelsey C. Morris, Christopher
J. A. Daley, and Timothy B. Clark
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812857
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10.1002/anie.201812857
Angewandte Chemie International Edition
COMMUNICATION
Accessing Ambiphilic Phosphine Boronates through C–H
Borylation by an Unforeseen Cationic Iridium Complex
Shawn E. Wright, Stephanie Richardson-Solorzano, Tiffany N. Stewart, Christopher D. Miller, Kelsey
C. Morris, Christopher J. A. Daley, and Timothy B. Clark*
Abstract: Ambiphilic molecules, which contain a Lewis base and
Lewis acid, are of great interest based on their unique ability to
activate small molecules. Phosphine boronates are one class of these
substrates that have interesting catalytic activity. Direct access to
these phosphine boronates is described through the iridium-catalyzed
C–H borylation of phosphines. An unconventional cationic iridium
catalyst was identified as optimal for a range of phosphines, providing
good yields and selectivity across a diverse class of phosphine
synthesis of a series of phosphine boronates, the role of the
substrate as ligand severely limited the phosphines that were
compatible with the reaction conditions. The proposed active
catalyst utilizes the substrate as a C,P-bound ligand in which the
X-type ligand interaction of the arene carbon with iridium served
to replace one of the boryl ligands of the known iridium(III)trisboryl
active catalyst[45] (Scheme 1), maintaining an open coordination
site for C–H activation upon phosphine coordination. We herein
report a general method to rapidly form a series of ambiphilic
phosphine boronates by phosphine-directed C–H borylation,
utilizing a unique cationic iridium complex and a known
silylquinoline that provide complimentary reactivity in this
selective C–H borylation reaction.
boronates (isolated as the borane-protected phosphine).
A
complimentary catalyst system (quinoline-based silane ligand with
[(COD)IrOMe]2) was optimal for biphenyl-based phosphines.
Selective polyborylation was also shown providing bis- and tris-
borylated phosphines. Deprotection of the phosphine boronate
provided free ambiphilic phosphine boronates, which do not have
detectable interactions between the phosphorus and boron atoms in
solution or the solid state.
Scheme 1. Proposed Active Catalyst for Phosphine-Directed C–H Borylation
with No Added Ligand
The metal-catalyzed C–H borylation reaction has garnered
profound interest in recent decades based on the synthetic utility
of the resulting C–B bond and the ability to selectively install a
boron substituent without a pre-existing functional group.[1-3] Early
work in the area of metal-catalyzed C–H borylation reactions
utilized catalysts that were sterically-controlled, avoiding ortho C–
H bonds in arenes and favoring primary C–H bonds for alkanes.[4]
Starting in 2008, new systems were developed that utilized
directing groups to achieve alternative selectivity patterns (ortho-
or meta-directed).[5-34] The approaches used to achieve directed
C–H borylation are diverse, but, the key to achieving the desired
directing effect has been to modify the catalyst in a way that the
directing interaction does not form a coordinatively saturated 18-
electron complex, which then has no open site for activation of the
C–H bond. Using these approaches, many directing groups have
been used to access functionalized arylboronates.
A more general catalyst system for phosphine-directed C–H
borylation was initially sought by examining the synthesis of pre-
formed P,C-bidentate ligands that could mimic the proposed
active catalyst (Scheme 1). These efforts did not provide a viable
catalyst and were not consistent with our goal to identify readily
available pre-catalysts and ligands for this transformation. A
series of known substrate-directed C–H borylation conditions,
which were expected to be compatible with phosphines, were
examined with 3-methoxybenzyldicyclohexylphosphine (Table 1),
a substrate that was unreactive under the original reaction
conditions (entry 1).[23] By identifying optimal reaction conditions
for this problematic substrate, a more general set of reaction
conditions across a wide range of phosphines was expected.
Taking advantage of the insights gained in substrate-directed
C–H borylation reactions, our group extended the available
directing groups to phosphines. Our initial efforts in phosphine-
directed C–H borylation were reported in 2014, providing the first
example of a general phosphine-directed C–H functionalization[35-
44]
and providing access to phosphine boronate esters from
Entry[a]
Catalyst
Ligand
Conversion
valuable phosphine precursors.[23] The key to obtain the desired
reactivity in these studies was running the reaction without added
ligand. We proposed a mechanism in which the phosphine
substrate served as the ligand during the catalytic reaction
(Scheme 1). While these catalytic conditions were useful in the
1
2
3
4
5
[(COD)IrOMe]2
[(COD)IrOMe]2
[(COD)IrOMe]2
[(COD)IrOMe]2
[Ir(P,N)]PF6 (5)
none
NR
NR
2
3
NR
4
NR
none
33%
Table 1. Condition Screen for Phosphine-Directed C–H Borylation
[*]
S. E. Wright, S. Richardson-Solorzano, T. N. Stewart, C. D. Miller,
K. C. Morris, Prof. C. J. A. Daley, and Prof. T. B. Clark*
Department of Chemistry & Biochemistry
[a] Conditions: 0.83 equiv. B2pin2, 1.25 mol % [Ir], 2.5 mol % ligand, toluene,
130 °C, 24 h.
University of San Diego,
5998 Alcalá Park, San Diego, CA 92110, United States
E-mail: clarkt@sandiego.edu
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201812857
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Figure 1. Ligands and Pre-Catalysts Used in Condition Screen
°C further improved the selectivity without a subsequent loss of
conversion, providing 99% conversion and a 99:1 ratio of 1:1ʹ.
B2pin2 is the most commonly-used boron source in C–H borylation
reactions. Formation of the active catalyst from the pre-catalyst is
typically accelerated with HBpin, but the increased concentration
of hydrogen gas as the reaction progresses can retard the
reaction.[47] In this study, full conversions are observed with
HBpin, suggesting that hydrogen gas does not hinder the reaction
significantly. Further mechanistic studies are required to fully
The first set of conditions examined were those developed by
our group for amine-directed C–H borylation, which utilize a hemi-
labile ligand (N-benzylaminopyridine, 2, Figure 1),[18, 27] providing
no evidence of the formation of 1 (Table 1, entry 2). Miyaura
reported ester-directed C–H borylation using an electron-deficient
phosphine ligand (tris[3,5-bis(trifluoromethyl)phenyl]phosphine,
3);[10] No C–H borylation was observed under Miyaura’s original
conditions nor with alternative solvents and elevated temperature
(entry 3). Smith and Maleczka reported silane-based bidentate
ligands (4) that proved compatible with a range of directing
groups,[25] but were also unreactive with the test phosphine (entry
4). A cationic iridium complex with a rigid bidentate ligand was
examined with the idea that the cationic nature of the complex
could open an additional coordination site for directed C–H
borylation (vide supra). Commercially available complex 5 was
found to provide modest, but promising, conversion to boronate 1
(entry 5).
understand this effect.
Table 2. Reaction Optimization with [Ir(COD)2]BF4
Boron (equiv.)
B2pin2 (0.83)
HBpin (1.7)
HBpin (6.7)
HBpin (3.3)
HBpin (3.3)
HBpin (3.3)
T (°C)
130
t (h)
24
Conv.
0%
1:1ʹ
NA
130
24
72%
99%
98%
99%
79%
81:19
87:13
96:4
130
24
130
18
110
18
99:1
>99:1
The reactivity of cationic pre-catalyst 5 was surprising
considering the general consensus that the active iridium complex
should be electron rich.[46-47] This cationic complex, however, was
the only reactive catalyst for the studied phosphine. The reactivity
of the cationic complex is proposed to result from obtaining the
required two open coordination sites by replacing one of the –Bpin
ligands of the known iridium(III)trisboryl complex with a non-
coordinating ligand. The resulting iridium(III)bisboryl complex
(Figure 2, A) has the ability to access two open coordination sites
for the Lewis basic phosphine and the adjacent C–H bond. Based
on this realization, a more effective bidentate ligand was sought
to increase the generality of the reaction. 3,4,7,8-Tetramethyl-
1,10-phenanthroline (TMPHEN), a commonly used ligand for non-
directed C–H borylation reactions,[48-50] was expected to form a
more active cationic complex (B) while maintaining an open
coordination site for directed C–H borylation.
100
24
The optimized reaction conditions were utilized for a series of
benzylic phosphines to examine steric and electronic influences
on the reaction efficiency. To simplify isolation and purification of
these electron rich phosphines, the crude reaction mixture was
treated with BH3·THF to form the borane complex.[23, 51] High yield
and selectivity for the ortho monoborylation product was observed
in most cases (Table 3). Diphenyl- and dicyclohexyl-substituted
benzylphosphines worked well under the reaction conditions to
provide the corresponding boronate esters (6 and 7, entries 1,2).
3- and 4-substituted benzylic phosphines provided good yields of
8–11 (entries 3–5), demonstrating compatibility with chlorine or
methoxy substituents. The ortho-substituted benzylic phosphines,
however, were prone to additional reactions that resulted in low
yields. The 2-chloro-substituted arene, for example, provided both
C–H and C–Cl borylation (71:29).[52]
Table 3. Scope of Benzylic Phosphines in Directed C–H Borylation
Figure 2. Proposed Bis-boryl Cationic Iridium Complexes
Entry
R
H
Rʹ, Rʹ
Ph, Ph
Cy, Cy
Cy, Cy
Cy, Cy
Cy, Cy
Cy, Cy
Yield
87%
81%
76%
73%
74%
77%
Product
Initial examination of TMPHEN with commercially-available
cationic pre-catalysts [Ir(COD)2]BF4 resulted in no reactivity
(Table 2). Switching the boron source from bis(pinacolato)diboron
(B2pin2) to pinacolborane (HBpin), however, drastically increased
the reactivity. With 1.7 equivalents of pinacolborane, a 72%
conversion was observed after 24 h at 130 °C. Increasing the
equivalents of pinacolborane to 6.7 provided full conversion, but
the ratio of monoborylation product (1, Table 2) to bisborylation
product (1ʹ) was modest (87:13). Reducing the equivalents of
pinacolborane to 3.3 improved the selectivity for 1 to 96% while
maintaining a high conversion. Lowering the temperature to 110
1
2
3
4
5
6
6
7
H
3-Me
3-Cl
3-OMe
4-Me
8
9
10
11
Encouraged by the generality of the C–H borylation with
benzylic phosphines, commonly used phosphine ligands were
examined under the reaction conditions. Indole-based
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10.1002/anie.201812857
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phosphines were readily borylated, providing 12–14 in high yield
and selectivity for monoborylation products (Figure 3). Biphenyl-
based phosphines, however, were less reactive under the
reaction conditions using the cationic iridium system. Treatment
of these phosphines with the silylquinoline ligand conditions
reported by Smith[25] (4, Table 1, entry 4) provided high
conversions to the desired borylation products when B2pin2 was
used as the boron source. Therefore, borylation of the commonly
used ligands CyJohnPhos, DavePhos, and MePhos,[53] provided
the corresponding boronate esters (15–17) in good yields upon
protection. In the case of 15, selectivity was reduced to 83:17
mono:bis (15:15ʹ). To verify that the C–H borylation of DavePhos
(to form 16) was directed by the phosphine, and not the amine,
the structure of 16 was confirmed by X-Ray crystallography (see
Figure 4). Borylation of diphenylphosphinoferrocene (FcPPh2)
was also examined; under both sets of conditions, competitive
bisborylation was observed. Isolation of 18, however, was
achieved in 43% yield from a 50:25:25 mixture of 18:18ʹ:FcPPh2.
standard cationic iridium conditions (3.3 equiv. HBpin at 130 °C)
provided bisborylation (18ʹ) as the sole product. Likewise,
benzyldiphenylphosphine could be pushed to bisborylation
product 6ʹ (67% selective for 6ʹ). Encouraged by these results,
tribenzylphosphine was examined under a variety of borylation
conditions, ultimately providing moderate to good yields of mono-
(19), bis- (20), and tris-borylated (21) products upon purification
by column chromatography. The synthesis of polyborylated
products is expected to provide intriguing polyfunctionalized
phosphines and useful comparisons with monoborylated products
as organocatalysts[54-61] and ambiphilic catalysts and ligands[62-63]
upon removal of the borane protecting group.
Scheme 2. Phosphine-Directed Polyborylation Reactions
Figure 3. Substrate Scope of Non-Benzylic Phosphines
Removal of the borane protecting group was examined to
demonstrate the facile access to a variety of ambiphilic phosphine
boronates with steric, electronic, and geometrical variations.
Monoborylated phosphines 22–24 and polyborylated phosphines
25 and 26 were readily accessed upon heating in diethylamine
with yields ranging from 60–95% (Scheme 5).
Figure 4. X-ray Crystal Structure of 16 Confirming Structure. Ellipsoids are
shown at 50% probability. Hydrogen atoms are omitted for clarity.
The increased reactivity of certain substrates (especially
CyJohnPhos
and
diphenylphosphinoferrocene)
toward
bisborylation led to the examination of conditions that could
selectively provide polyborylated products. Treatment of
CyJohnPhos with excess B2pin2 (2 equiv.) at 130 °C provided
bisborylated product 15ʹ (>90% selective for 15ʹ over 15) in 61%
yield (Scheme 2). Subjecting diphenylphosphinoferrocene to the
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Scheme 5. Deprotection of Phosphine Boronates
In summary, efficient methods for phosphine-directed C–H
borylation have been reported. These catalysts were active in a
wide range of benzylic and aryl phosphines including indole- and
ferrocene-based substrates. Polyborylation of
a series of
phosphines was also possible. Deprotection of the phosphine
boronates with diethylamine provided a series of ambiphilic
phosphine boronates, none of which showed interactions
between the free phosphorus and the boron atoms.
Acknowledgements
Acknowledgement is made to the Donors of the American
Chemical Society Petroleum Research Fund (54825-UR3), the
Henry Dreyfus Foundation, and that National Science Foundation
(CHE-1764307) for support of the research. National Science
Foundation is acknowledged for NMR (0417731) and X-ray (CHE-
1126585). S.E.W. thanks the Doheny Foundation and the Farrell
Family for summer research fellowships.
Upon deprotection of the phosphine, an interaction between the
Lewis basic phosphorus atom and the Lewis acidic boron atom
(of –Bpin) becomes possible.[16] To examine if such an interaction
is taking place, the 11B NMR spectra were compared to the
corresponding protected phosphines. If an interaction was taking
place, an upfield chemical shift would be expected. As shown in
Table 4, no significant chemical shift changes were observed,
consistent with no interaction in the solution state. The lack of
interaction was probed in the solid state by X-ray crystallography
of 23 (Figure 5). As expected from the spectral data, no interaction
was observed for 23 in the solid state.
Keywords: homogeneous catalysis • C–H functionalization •
ambiphilic • borylation • phosphorus
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10.1002/anie.201812857
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Shawn E. Wright, Stephanie
Richardson-Solorzano, Tiffany N.
Stewart, Christopher D. Miller, Kelsey C.
Morris, Prof. Christopher J. A. Daley,
and Prof. Timothy B. Clark*
Page No. – Page No.
Accessing Ambiphilic Phosphine
Boronates by C–H Borylation by an
Unforseen Cationic Iridium Complex
Phosphine-Directed C–H Borylation: A series of commonly used phosphines undergo iridium-catalyzed C–H
borylation, providing a series of ambiphilic phosphine boronates. Unexpected cationic iridium catalyst overcomes
limited reactivity allowing a large range of phosphines to be selectively functionalized, including selective
polyborylation of several phosphines.
This article is protected by copyright. All rights reserved.