Phosphine-directed C-H borylation reactions: Facile and selective access to ambiphilic phosphine boronate esters

Article abstract of DOI:10.1002/anie.201402868

Ambiphilic ligands have received considerable attention over the last two decades due to their unique reactivity as organocatalysts and ligands. The iridium-catalyzed C-H borylation of phosphines is described in which the phosphine is used as a directing group to provide selective formation of arylboronate esters with unique scaffolds of ambiphilic compounds. A variety of aryl and benzylic phosphines were subjected to the reaction conditions, selectively providing stable, isolable boronate esters upon protection of the phosphine as the borane complex. After purification, the phosphine-substituted boronate esters could be deprotected and isolated in pure form.

Full text of DOI:10.1002/anie.201402868

Angewandte
Chemie
DOI: 10.1002/anie.201402868
À
C H Borylation
À
Phosphine-Directed C H Borylation Reactions: Facile and Selective
Access to Ambiphilic Phosphine Boronate Esters**
Kristina M. Crawford, Timothy R. Ramseyer, Christopher J. A. Daley, and Timothy B. Clark*
Abstract: Ambiphilic ligands have received considerable
attention over the last two decades due to their unique
reactivity as organocatalysts and ligands. The iridium-cata-
groups. There have been several different approaches to
achieving these directing effects ranging from hydrogen
bonding interactions between the substrate and the boronate
ester oxygen[21] to Lewis-base-directed activation using hemi-
labile diamine ligands.[19,20] We have used this latter approach,
as have Fernꢀndez and Lassaletta, to achieve complimentary
selectivity to the typical rigid ligands of 4,4’-di-tert-butylbi-
À
lyzed C H borylation of phosphines is described in which the
phosphine is used as a directing group to provide selective
formation of arylboronate esters with unique scaffolds of
ambiphilic compounds. A variety of aryl and benzylic
phosphines were subjected to the reaction conditions, selec-
tively providing stable, isolable boronate esters upon protec-
tion of the phosphine as the borane complex. After purifica-
tion, the phosphine-substituted boronate esters could be
deprotected and isolated in pure form.
pyridine and substituted phenanthrolines, which are largely
[23–25]
À
controlled by steric congestion at the arene C H bond.
Flexible diamine ligands have been proposed to undergo
facile dissociation of one arm of the bidentate ligand to allow
À
for the required C H activation step on the otherwise
coordinatively saturated iridium complex (Scheme 1, B to
T
ertiary phosphines have played a prominent role in the field
of organometallic chemistry based on their ability to serve as
ancillary ligands in a plethora of homogeneous catalysis
reactions.[1–5] The wide variety of steric and electronic proper-
ties that are available with commercially and readily acces-
sible phosphines has facilitated their practical use.[1,6–8] New
applications of trivalent phosphines continue to be explored
in metal-catalyzed reactions and frustrated Lewis pair (FLP)
chemistry.[9,10] Recent interest in ambiphilic organocatalysts
and ligands[11] has led to a number of intriguing catalytic
systems for small-molecule activation and reaction. Phos-
phine-substituted boranes and boronate esters are particu-
larly noteworthy for their application as organocatalysts or
ligands in metal-catalyzed transformations.[12] In spite of the
promising reactivity of these compounds, a limited variety of
scaffolds have been explored due to the inability to readily
access such molecular structures under the current methods.
Based on the preliminary work by our group and others in
À
Scheme 1. Proposed mechanism of amine-directed C H borylation
with hemi-labile ligands.
the area of substrate-directed arene C H borylation,[13–21] we
became interested in expanding this methodology to phos-
phines,[22] simplifying access to valuable phosphine building
blocks and organocatalysts. Current methods to achieve
À
C).[19,20] A critical step in this type of catalysis relies on the
ability of the diamine ligand to displace the borylated
substrate to allow catalyst turnover. We reasoned that
appropriately chosen bidentate ligands could effectively
displace a phosphine-bound substrate and allow catalyst
turnover under otherwise similar reaction conditions. We
À
substrate-directed C H borylation reactions utilize
silanes,[14,17] esters,[15,16] amides,[15] ethers,[15] halides,[15] and
nitrogen-containing functional groups[13,18–21] as directing
À
herein report the directed C H borylation of tertiary
phosphines to provide phosphine-substituted arylboronate
esters in high yield and selectivity.
[*] K. M. Crawford, T. R. Ramseyer, C. J. A. Daley, T. B. Clark
University of San Diego
San Diego, CA (USA)
E-mail: clarkt@sandiego.edu
Initial reaction conditions were probed with benzyldiphe-
nylphosphine as the substrate and an array of diamine and
aminophosphine ligands [Eq. (1), Table 1]. Several diamine
[**] Partial support by NSF (CHE-1151092), USD, and Research
Corporation for Science Advancement; Dr. David S. Glueck (Dart-
mouth College) for helpful discussions and Dr. John Greaves
(University of California, Irvine) for mass spectrometry; NSF for
NMR (0417731) and X-ray (CHE-1126585) facilities at USD.
À
ligands provided an appreciable amount of desired C H
borylation product 1a (entries 1–3), but the low conversions
were not significantly improved by standard ligand modifica-
tions. Aminophosphine ligands also provided modest con-
version (entries 4, 5). Upon examination of the reaction
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402868.
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Table 1: Ligand screen of phosphine-directed borylation.
Entry
Ligand
Conversion [%][a]
1a:1b[b]
1
39
96:4
2
3
4
5
37
39
33
43
92:8
93:7
80:20
98:2
Figure 1. X-ray crystallographic structure of 2. Ellipsoids are shown at
50% probability. Hydrogen atoms are omitted for clarity.
6
none
none
96
87
71:29
96:4
7[c]
[a] The conversion was determined by 1H NMR spectroscopy using a 5 s
relaxation delay to insure integral integrity. All conversions are based on
the arene substrate. [b] Ratio of 1a/1b was determined by 1H NMR
spectroscopy. [c] Conditions: toluene, 1108C, 16 h.
Scheme 2. Effect of phosphine substituent on borylation reactivity and
selectivity.
without added ligand, a significant boost in conversion was
observed, leading to nearly full conversion to 1a/1b, albeit
with poor selectivity of 1a/1b (71:29). After a brief solvent
screen, toluene was found to provide an 87% conversion to
the borylated products with a 96:4 selectivity of 1a/1b
(entry 7);[26] meta and para isomers were undetectable by
1H NMR spectroscopy under all of the reaction conditions
examined. The ability to use toluene as a solvent also
demonstrates the significant reactivity increase imparted by
benzylphosphines (Scheme 2). Benzyldicyclohexylphosphine
required increased reaction times (44 h) compared to the
diphenyl analogue but resulted in > 97% conversion to
borylated product 3a; improved selectivity for the monosub-
stituted product (> 97:3 3a/3b) was also observed. Benzyl-
diadamantylphosphine also demonstrated improvement in
selectivity (> 97:3 4a/4b) and conversion (> 97% conv.) with
a 16 h reaction time. The increased reaction rate for the more
sterically hindered diadamantyl substrate likely results from
an increased rate of dissociation of the sterically hindered
phosphine of 4 from the iridium catalyst upon borylation.
Both 3a and 4a were isolated in moderate to good yields as
the corresponding borane complexes 5 and 6. In spite of high
À
the phosphine directing group which leads to selective C H
functionalization over that of toluene, which is present in
a much higher concentration.[20]
To facilitate isolation and purification of the resulting air-
sensitive boronate ester, the phosphine was protected as the
borane complex 2,[27] providing an air-stable crystalline
product [Eq. (2)]. Purification of 2 by flash column chroma-
tography provided analytically pure product in 75% yield and
the identity of the major borylation product was confirmed by
X-ray crystallography (Figure 1).
À
conversions in the C H borylation reaction and clean
formation of the borane complex, a moderate yield is
obtained in some cases due to challenges in the isolation of
the resulting protected boronate esters.
To expand the scope of substrates that could be used
under these reaction conditions, several arylphosphines were
explored. Triphenylphosphine was unreactive under the
conditions, likely due to the inability to form the requisite
À
4-membered metallocycle upon directed C H activation (or
the related s-bond metathesis transition state).[28–30] In con-
trast, 2-(dicyclohexylphosphino)biphenyl provided the
desired borylation product (7a, precursor to 8a prior to
borane protection) in good conversion and selectivity
The effect of the phosphine substituents on substrate
reactivity and selectivity (for mono-ortho borylation) was
investigated with dicyclohexyl- and diadamantyl-substituted
2
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(Scheme 3, > 97:3 7a/7b). Borane complex 8a was obtained
in an 81% yield of the isolated product.
During the screening of reaction solvents and temper-
atures with 2-(dicyclohexylphosphino)biphenyl the level of
À
Scheme 3. Directed C H borylation of 2-(dicyclohexylphosphino)bi-
phenyl.
À
Scheme 4. C H borylation of pyrrole-containing phosphines.
selectivity for 7a over 7b varied significantly. With an interest
in accessing the bisborylated product, two equivalents of
bis(pinacolato)diboron were added to the phosphine in
xylenes and heated to 1508C for 3 days, resulting in > 97%
conversion and a 40:60 ratio of 7a/7b. The borane complex of
7b was obtained in modest yield (25%) after flash column
chromatography and recrystallization to provide 8b in pure
form (Scheme 3). Although the yield of 8b was low, this
bisborylated product has intriguing potential for ligand
synthesis and will be the subject of further study.
Unlike the consistent reactivity observed with sterically
encumbered benzylphosphines (see above), biphenyl-derived
phosphines were much more sensitive to sterically encum-
bered phosphine substituents. 2-(Di-tert-butylphosphino)bi-
phenyl, for example, was found to be unreactive using several
reaction solvents and increased reaction temperatures (< 5%
borylation product formed). The increased steric congestion,
in this case, is believed to block formation of the active
catalyst (see below).
À
Directed C H borylation of N-arylpyrrole phosphines
from the cataCXium class of ligands was pursued next to
provide intriguing analogues of these valuable ligands.[31]
À
Iridium-catalyzed C H borylation of N-phenyl-2-(dicyclo-
hexylphosphino)pyrrole [cataCXium PCy] in m-xylenes as
solvent (at 1108C for 44 h) provided the best combination of
conversion (86%) and selectivity of monoborylation product
9a (84:16 9a/9b, Scheme 4). Borane complex 10 was obtained
in a 65% yield of isolated product as a mixture of mono- and
À
Scheme 5. Proposed catalytic cycle of C H borylation.
bis-borylated products. C H borylation of 2-(dicyclohexyl-
ates.[32–35] The proposed catalytically active complex is a bis-
boryl iridium(III) complex (F), rather than the generally
accepted trisboryl complex due to the bidentate ligand
serving as an LX-type ligand.[36] Since one of the boryl
substituents is replaced by a carbon-based X ligand, the
coordination of a second phosphine does not lead to
a coordinatively saturated iridium complex. 16-electron
complex F, therefore, does not require partial dissociation
of one arm of the bidentate ligand, which is the proposed key
À
phosphino)-1-(2-methoxyphenyl)-1H-pyrrole
[cataCXium
POMeCy] was found to give optimal conversion (> 97%) to
11 in isopropylacetate at 1208C for 20 h, providing 12 as the
borane complex in 57% isolated yield.
Building from the generally accepted mechanism[28–30] for
À
arene C H borylation a plausible catalytic cycle is outlined in
Scheme 5. Formation of the active catalyst is the most distinct
À
aspect of this mechanism (Scheme 5, bottom), requiring C H
À
activation of the substrate, which subsequently serves as
a ligand, and is anchored to the iridium complex. Notably,
step of nitrogen-directed C H borylation with hemi-labile
ligands.
À
there are numerous examples of phosphine-directed C H
activation of analogous substrates by iridium complexes in the
literature to support the feasibility of these intermedi-
The major difference between this mechanism and one
requiring hemi-labile ligands[19,20] is the role of the phosphine
as both substrate and ligand. Since the mechanism requires
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À
facile C H activation of the substrate/ligand in order to enter
Experimental SectionÀ
into the catalytic cycle, the nature of the substrate is going to
play a key role in whether a catalytically active species will
form. In the case of 2-(di-tert-butylphosphino)biphenyl, the
added steric congestion of the tert-butyl substituents causes
a significant increase in the transition state energy of the
General procedure for C H borylation/borane protection: To
a 50 mL PTFE-valved reaction tube, equipped with a stir bar and
charged with [Ir(m-OMe)(cod)]2 (0.004 g, 0.015 mmol), was added
B2pin2 (0.102 g, 0.400 mmol), the phosphine (0.400 mmol), and
toluene (1.0 mL). After heating (110–1508C), the reaction mixture
was transferred to a scintillation vial in an inert atmosphere glovebox
and the volatiles were removed in vacuo. The crude reaction mixture
was removed from the glovebox and subjected to positive nitrogen
pressure. Degassed THF (2 mL) was added and the reaction vessel
was placed in a 08C bath. A solution of BH3·THF (0.8 mL, 1.0m
solution in THF, 0.800 mmol) was added dropwise. The reaction
vessel was removed from the bath and warmed to ambient temper-
ature. After 2 h the volatiles were removed in vacuo. Purification by
silica gel flash column chromatography provided the corresponding
phosphine-substituted arylboronate ester.
À
initial C H activation step to form the catalytically active
species. Since the active catalyst is inaccessible, the starting
material is left unreacted rather than showing slow formation
of product. A full study of the reaction mechanism will be
initiated to understand the details of the reaction and
ultimately expand the substrate scope.
Numerous potential transformations are envisioned with
the borylated phosphines described above utilizing the
carbon–boron bond.[37] To demonstrate an example of the
utility of these synthetic intermediates, the oxidation of 12
was achieved [Eq. (3)], providing alcohol 13 in 69% isolated
yield.[38,39] Additional transformations involving the versatile
Received: February 27, 2014
Revised: April 23, 2014
Published online: && &&, &&&&
À
C B bond that tolerate the protected phosphine to access
complex phosphines are currently underway.
À
Keywords: ambiphilic · C H borylation ·
homogeneous catalysis · phosphine boronate ·
phosphine-directed reactions
.
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À
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À
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À
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4
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Communications
À
C H Borylation
K. M. Crawford, T. R. Ramseyer,
C. J. A. Daley, T. B. Clark*
&&&& — &&&&
À
Phosphine-Directed C H Borylation
Reactions: Facile and Selective Access to
Ambiphilic Phosphine Boronate Esters
À
Directed borylation: Ambiphilic phos-
phine boronate esters are accessed by
directs the borylation to the adjacent C
H bond, providing access to intriguing
ligand and catalyst scaffolds.
À
a phosphine-directed C H borylation
reaction. The Lewis basic phosphine
6
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