Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

Article abstract of DOI:10.1039/c8cc01880d

Two Rh complexes of the 2-phosphinophosphinine ligand 2-PPh₂-3-Me-6-SiMe₃-C₅H₂P (1) were prepared: dinuclear trans-[{Rh(CO)(Cl)(μ-1)}₂] (2) and chelating [Rh(1)(COD)][B(Ar^F)₄] (3). Despite the widespread use of Rh catalysts for the hydroboration of alkenes, 3 is reported to be the first Rh catalyst for ketone and ketimine hydroboration, with high activity observed at 0.1 mol% loading.

Full text of DOI:10.1039/c8cc01880d

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Small bite-angle 2-phosphinophosphinine ligands
enable rhodium-catalysed hydroboration of
carbonyls†
Cite this:DOI: 10.1039/c8cc01880d
Received 8th March 2018,
Accepted 24th April 2018
a
b
a
Robert J. Newland,
Jason M. Lynam
and Stephen M. Mansell
*
DOI: 10.1039/c8cc01880d
rsc.li/chemcomm
Two Rh complexes of the 2-phosphinophosphinine ligand 2-PPh2-3-Me- by Mu¨ller and co-workers in particular.20 With regards to Rh
6-SiMe3-C5H2P (1) were prepared: dinuclear trans-[{Rh(CO)(Cl)(l-1)}2] (2) catalysis, Breit developed the use of mono- and multidentate
and chelating [Rh(1)(COD)][B(ArF)4] (3). Despite the widespread use of Rh phosphinine ligands for hydroformylation catalysis that high-
catalysts for the hydroboration of alkenes, 3 is reported to be the first Rh lighted several advantages of these unusual ligand systems in
catalyst for ketone and ketimine hydroboration, with high activity an industrially valuable reaction over classical ligands such as
observed at 0.1 mol% loading.
PPh3.21 Small bite-angle ligands have become an increasingly
popular choice in catalysis,22 and our interests lie in the develop-
The catalytic hydroboration of carbonyl substrates1 is of current ment of catalysts using small bite-angle 2-phosphinophosphinine
interest due to the importance of the controlled reduction of ligands (Scheme 1, 1) due to their unique properties.19 In particular,
carbonyls to alcohols under mild conditions and the consider- the increased s-character (ca. 61%)23 of the formally sp2 phosphorus
able safety advantages over the use of stoichiometric metal atom can lead to less-strained four-membered chelates. Evidence for
hydrides and catalytic hydrogenation.2 Transfer hydrogenation this was observed in the chelating Ru complex cis-[Ru(1)2(Cl)2] (4),24
is an alternative reaction for the reduction of ketones, however, as well as in a series of k2 group 6 tetracarbonyl complexes.25
many catalysts require forcing conditions to achieve acceptable With the increasing popularity of small bite-angle ligands in
conversion.3 There has been a great deal of interest in developing homogeneous catalysis,22 developing ligands that are less likely
catalysts for the hydroboration of carbonyl compounds, with to form bridging architectures is highly desirable.
catalysts based on s-block (e.g. Li,4 Na,5 Mg6) and p-block (e.g.
Initially, we probed the coordination properties of 1 using
B,7 Al,8 Ge, Sn9) elements, as well as first-row (e.g. Ti,10 Mn,11 Fe,12 [{Rh(CO)2(m-Cl)}2] (Scheme 1). A rapid reaction was observed
Ni13), group 6 (Mo14) and late (Re,15 Ru16) transition metals affording a deep purple solution and the expected evolution of
recently reported. Despite the prevalence of Rh catalysts for the carbon monoxide. Crystalline 2 precipitated in 63% yield and was
hydroboration of alkenes,17 to the best of our knowledge, there has characterised as a bridged dinuclear complex by multinuclear
been only one stoichiometric example of the use of a Rh complex
in the hydroboration of a carbonyl compound (benzaldehyde)
reported in the literature to date.18 ‡ Based on the wide variety
of hydroboration catalysts, and the wide-spread use of Rh catalysts
in alkene hydroboration, its absence in carbonyl hydroboration
was unexpected.
The use of phosphinine (the P-analogue of pyridine) ligands
in catalysis is a growing field19 with many recent contributions
a Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
E-mail: s.mansell@hw.ac.uk; Web: www.mansellresearch.org.uk
b Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
† Electronic supplementary information (ESI) available: Additional synthetic
details, analytical data and crystallographic data in CIF format. Additional
research data supporting this publication are available from Heriot-Watt Uni-
versity’s research data repository at DOI: 10.17861/14c23fe6-bc65-4806-ba5e-
63642a6ad3e9. CCDC 1822415 and 1822416. For ESI and crystallographic data
Scheme 1 Synthesis of Rh complexes of 1, and the structure of 4.
in CIF or other electronic format see DOI: 10.1039/c8cc01880d
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NMR spectroscopy, high-resolution mass spectrometry, IR spectro-
Monodentate phosphinine ligands have been structurally char-
scopy, X-ray diffraction and elemental analysis. 31P{1H}-NMR acterised binding to Rh in trans-[Rh(CO)(L)2Cl],21d [Rh(L)2(COD)]+,27
spectroscopy revealed the formation of a single product with two and homoleptic [Rh(L)4]+ complexes (see ESI†).21c Rh complexes
apparent doublet-of-triplets resonances (Fig. S2 and S3, ESI†) with tri-28 and tetraphosphinine29 ligands have also been structu-
observed at d = 250.6 ppm (phosphinine P) and 25.5 ppm (PPh2) rally characterised (see ESI†).
that were successfully simulated (ESI†). Although a chelating
In order to develop a mononuclear complex with a bidentate
complex was not achieved, 2 did facilitate comparisons to dppm phosphinophosphinine ligand, a chelating co-ligand was utilised.
and other diphosphines using the resulting carbonyl stretching Typically, sterically bulky ligands, such as dcpm (bis(dicyclo-
frequency.26 FTIR spectroscopy revealed a single carbonyl stretch hexylphosphino)methane),30 are required to stabilise small bite-
at n = 1977 cmꢀ1 which correlates with 1 being a more p-accepting angle cationic [Rh(diphosphine)(COD)]+ complexes31 due to the
ligand than dppm (n = 1968 cmꢀ1 for the analogous complex).26 Thorpe–Ingold effect,22 and no chelating [Rh(dppm)(COD)]+
The molecular structure of 2 (Fig. 1) displayed two bridging ligands complexes have been structurally characterised. Our initial efforts
with the CO and Cl ligands disordered across two positions. focused on reaction of 1 with [{Rh(COD)(m-Cl)}2] in the presence of a
The Rh2(PCP)2 unit is non-planar, and has a dihedral angle for silver salt (AgBF4, AgSbF6) in CH2Cl2.32 However, we observed
P(1)–Rh(1)–Rh(2)–P(2) of 16.88(4)1. Whilst stable in the solid multiple products, even with slow addition of 1 using dilute condi-
state, 2 appears to be unstable in solution after prolonged tions. We were inspired by the use of a bis(cyclooctadiene) complex
periods or when heated (Fig. S11, ESI†).
of Rh using the weakly coordinating anion B(ArF)4 (tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate),33 and observed a rapid reaction
of this precursor with 1, forming a single air-stable product in
an 80% yield (Scheme 1). Complex 3 was characterised by X-ray
diffraction, multinuclear NMR spectroscopy, high-resolution
mass spectrometry and elemental analysis. 31P{1H}-NMR spectro-
scopy revealed two sets of apparent doublets-of-doublets at
d = 189.4 ppm and ꢀ6.8 ppm. The clean formation of 3 and
its stability is noteworthy with such an acute P–Rh–P bite angle
of 70.64(3)1 despite the minimal steric bulk on both donors.§
With two complexes in hand, and as there were no previous
reports of Rh-catalysts for carbonyl hydroboration in the litera-
ture, a catalyst screen of Rh complexes and common phosphine
ligands was conducted using 40-bromoacetophenone and
catecholborane (Table 1).
An initial test using 0.1 mol% 3 (run D) gave rapid conver-
sion to the boronate ester, with a 94% yield observed within ten
minutes and the reaction essentially complete after 30 minutes.
A dramatic decrease in yield was observed when either the
free ligand 1 (run B)¶ or complexes 2 and 4 were used (runs C
and E), although 2 still proved to be more active than other Rh
precursor/ligand combinations that were tested, including
Wilkinson’s catalyst (run F), which is a standard catalyst for
alkene hydroboration.17b, f Only mixtures of [Rh(COD)2] [B(ArF)4]
or [{Rh(COD)Cl}2] and PCy3 (tricyclohexylphosphine, runs G and
H respectively) gave higher than a 10% yield. Tests using
[Rh(COD)2][B(ArF)4] with less s-donating PPh3 and dppm ligands
in a 1 : 2 or 1 : 1 ratio respectively (runs I and J) gave similar, low
yields. Finally, to test if the p-accepting properties of 1 were the
source of the increased activity of 3, P(OPh)3 (run K) was tested,
however, a similar low yield was obtained.
Having established the catalytic activity of 3, a screen of readily
available acetophenone derivatives was carried out (Table 2). With
the substrates tested, clean formation of the desired boronate ester
was observed, with the exception of 40-methoxyacetophenone,
which produced multiple unidentified products (see Fig. S51,
ESI†). Complex 3 also acted as a catalyst for the hydroboration
of benzaldehyde, although the uncatalysed reaction also proceeds
readily. Ketimines were then tested as more challenging sub-
strates, and although room temperature reactions proceeded
Fig. 1 Thermal ellipsoid plots (50%) of the molecular structures of 2 (top)
and 3 (bottom, B(ArF)4 anion excluded for clarity). Selected bond distances
(Å) and angles (1) for 2: P(1)–Rh(1) 2.2845(9), P(2)–Rh(2) 2.3228(10), P(1)–
C(1) 1.734(4), C(1)–C(2) 1.411(5), C(2)–C(3) 1.402(5), C(3)–C(4) 1.386(5),
C(4)–C(5) 1.394(5), C(5)–P(1) 1.725(3), C(1)–P(2) 1.844(3), C(1)–P(1)–C(5)
107.6(2), P(1)–C(1)–P(2) 114.9(2); For 3: P(1)–Rh(1) 2.2932(8), P(2)–Rh(1)
2.2941(7), P(1)–C(1) 1.732(3), C(1)–C(2) 1.397(4), C(2)–C(3) 1.397(5), C(3)–
C(4) 1.385(5), C(4)–C(5) 1.409(4), C(5)–P(1) 1.723(3), C(1)–P(2) 1.801(3),
C(1)–P(1)–C(5) 106.9(2), P(1)–C(1)–P(2) 97.3(1), P(1)–Rh(1)–P(2) 70.64(3).
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Table 1 Catalyst screen for Rh-catalysed hydroboration of a ketone
shown to be active in the catalytic hydrogenation of styrene
and cyclohexene (see ESI† for details).
In conclusion, we have synthesised and characterised the first
two Rh complexes of a 2-phosphinophosphinine. Both were tested
in the catalytic hydroboration of 40-bromoacetophenone as well
as a previously reported ruthenium phosphinophosphinine
complex, Wilkinson’s catalyst and a series of commonly used
phosphine ligands. The results clearly demonstrated that signifi-
cant activity is only observed for the chelating complex 3, with
high catalytic activity observed for several acetophenone deriva-
tives at 0.1 mol% catalytic loading. Hydroboration of the more
challenging N-phenyl ketimine substrates was also achieved, with
good conversion in 1 hour at 1 mol% loading. Control reactions
showed that simple electronic or bite-angle effects, as shown by
the poor activity of different conventional monophosphine
ligands and dppm, do not explain this catalytic activity. Future
work will look to identify whether metal–ligand cooperativity or
the hybrid nature of the ligand is playing a key role in generating
highly active Rh catalysts.
The authors would like to acknowledge the EPSRC UK
National Mass Spectrometry Facility at Swansea University,
and Fluorochem for providing samples of the N-heterocycles.
Dr Gary Nichol (University of Edinburgh) is acknowledged for
collecting X-ray diffraction data for 3. Financial support is
gratefully acknowledged from the EPSRC (DTP studentship to
RJN), the Royal Society (Research grant: RG130436) and Heriot-
Watt University as well as the UK Catalysis Hub Consortium (funded
by EPSRC grants EP/K014706/2, EP/K014668/1, EP/K014854/1,
EP/K014714/1 and EP/M013219/1) for providing travel funding to
SMM and RJN.
Run
Catalyst
Yieldc,d (%)
A
B
C
D
E
F
G
H
I
—
1
2
3
1
20
35
96
12
3
22
18
7
4
[Rh(PPh3)3Cl]
[Rh(COD)2][B(ArF)4] + 2 PCy3
a
b
0.5 [{Rh(COD)Cl}2] + 2 PCy3
[Rh(COD)2][B(ArF)4] + 2 PPh3
a
J
K
[Rh(COD)2][B(ArF)4] + dppma
7
6
a
[Rh(COD)2][B(ArF)4] + 2 P(OPh)3
a
b
Premixed in THF (B0.1 ml) for 10 min before reaction. Premixed in
c
C6D6 (B0.1 ml) for 10 min before reaction. Yield measured against
1,3,5-trimethoxybenzene internal standard. Complete 1H-NMR data
d
for all catalytic runs available (ESI). Cat = catecholate: 1,2-(O)2C6H4.
Table 2 Hydroboration of acetophenone derivatives, benzaldehyde and
imines
Yieldd (%)
E
R1
R2
R3
10 min 30 min 60 min
Conflicts of interest
Oa
CH3
CH3
CH3
CH3
CH3
CH3 OCH3
H
H
H
H
H
H
H
Br
F
91
94
97
97
96
499
98
90
99
75
499
—
—
—
—
—
92
—
86
—
86
86
85
16
Oa
There are no conflicts of interest to declare.
Oa
Oa
NO2 95
CH3 66
H
H
H
H
F
CH3
H
Oa
Notes and references
Oa
97
52
95
—
—
—
—
Ob
H
H
H
H
H
H
‡ No catalysts for the hydroboration of ketones were mentioned in
several key reviews1,17g,34 or in additional thorough searches of the
Oa
H
N-Phc
N-Phc
N-Phc
CH3
CH3
CH3
¨
¨
literature. Results by Mannig & Noth indicate that hydroboration of
aliphatic ketones proceeds selectively over alkene hydroboration with-
out a catalyst,17a however, this is not the case for aryl ketones at 25 1C
(run A, Table 1). Evans & Hoveyda demonstrated that hydroboration of
b-hydroxyketones in the presence of catalytic amounts (5 mol%) of
Wilkinson’s catalyst provides some measure of increased diastereocon-
trol, however, no improvements in reaction rate or conversion were
observed.35 Westcott et al. demonstrated the Rh catalysed hydrobora-
tion of aldimines with HBCat,36 but it was previously reported that
bulkier N-phenyl aldimines can react rapidly with HBCat without the
need for a catalyst.37
—
—
—
N-(p-NO2-C6H4)c CH3
Conditions: 3, substrate (0.43 mmol), catecholborane (0.47 mmol, 1.1
eq.), C6D6d(0.6 cm3). a 0.1 mol% 3, 25 1C. No catalyst, 25 1C. 1 mol%
3, 50 1C. Yields measured against 1,3,5-trimethoxybenzene internal
standard. Cat = 1,2-(O)2C6H4.
b
c
§ For a histogram of Rh(k2-PEP) bite angles of entries found in the CSD
(E = C, N, O), see Fig. S24 (ESI†).
¶ Thus confirming that the high yield observed for 3 was not a result of
the ligand dissociating during the catalytic run. With 0.1 mol% 1, the
reaction gave a 95% yield after 14 hours. Upon mixing 1 and HBCat in
C6D6, no Lewis adduct was observed.
slowly, heating to 50 1C achieved acceptable yields within
1 hour. However, heating the reaction further gave no observable
increase in yield. In contrast to the carbonyl substrates, installa-
tion of an electron-donating or withdrawing substituent on the
C–Ar ring made little difference to the obtained yield, whereas
the presence of a nitro-group on the N–Ar substituent severely
hindered the reaction.
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