Well-defined [Rh(NHC)(OH)] complexes enabling the conjugate addition of arylboronic acids to α,β-unsaturated ketones

Article abstract of DOI:10.1039/c1ob06112g

The synthesis and catalytic activity of three well-defined monomeric rhodium(i) hydroxide complexes bearing N-heterocyclic carbene (NHC) ligands are reported. [Rh(cod)(ICy)(OH)] promoted the 1,4-addition of arylboronic acids to cyclic enones, with TONs an

Full text of DOI:10.1039/c1ob06112g

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PAPER
Well-defined [Rh(NHC)(OH)] complexes enabling the conjugate addition of
arylboronic acids to a,b-unsaturated ketones†
Byron J. Truscott, George C. Fortman, Alexandra M. Z. Slawin and Steven P. Nolan*
Received 13th June 2011, Accepted 15th July 2011
DOI: 10.1039/c1ob06112g
The synthesis and catalytic activity of three well-defined monomeric rhodium(I) hydroxide complexes
bearing N-heterocyclic carbene (NHC) ligands are reported. [Rh(cod)(ICy)(OH)] promoted the
1,4-addition of arylboronic acids to cyclic enones, with TONs and TOFs of 100,000 and 6,600 h-1,
respectively, at 0.001 mol% catalyst loadings. Mechanistic studies permitted the isolation of a
phenylrhodium intermediate.
The ability to manipulate and assemble C–C bonds is paramount
in organic chemistry. One such transformation is the conjugate
addition of organometallic compounds to activated alkenes. In
1997, a practical method for rhodium-catalyzed addition of
organoboron reagents to a,b-unsaturated carbonyls was disclosed
by Miyaura.1 Since then, efforts have focused on tailoring rhodium
complexes to improve both activity and selectivity in this transfor-
mation, with particular attention paid to ancillary ligands.2
The generally accepted trend regarding ancillary ligand ef-
fects in Rh-catalyzed 1,4-addition is that activity increases with
electron-poor ligands,3 such as chiral dienes,2d,2g,2i,4 diphosphines,3
phosphoramidites2f,2h,5 and BINAP-type ligands.6 Recently, the ac-
tivity of the electron-poor chiral diphosphine, MeO-F12–BIPHEP
on [RhCl(C2H4)2]2 was reported by Sakai,3b showing very high
activity (TON and TOF of 320,000 and 53,000 h-1, respectively
at catalyst loading of 2.5 ¥ 10-4mol%), attributed to the superior
p-accepting capability of the ligand, thereby inducing favourable
conditions for transmetalation between boron and rhodium.3a
Mechanistic investigations by Hayashi,6b using [Rh(acac)((S)-
binap)], identified a rhodium hydroxide species (formed in situ
in an aqueous medium) as the active catalyst. Well-defined
rhodium hydroxide complexes generally show improved efficiency
compared to those prepared in situ.6b
namely IPr (2a), ICy (2b) and IDD (2c) (Scheme 1), can stabilize
the Rh–OH moiety and lead to the isolation of well-defined
monomeric rhodium(I) hydroxide complexes (3a–c, Scheme 1).10
Complexes 3a–c were synthesized in a one-pot process by reacting
[Rh(cod)Cl]2 (1) with the free NHCs 2a–c in the presence of CsOH
(Scheme 1).
Scheme 1 Preparation of [Rh(cod)(NHC)(OH)] complexes 3a–c.
Complexes 3a–c are stable under inert conditions and were
fully characterized. Single crystal X-ray analysis of 3a and 3b
unambiguously confirmed the monomeric square planar geometry
of the complexes.10 Fig. 1 presents ORTEPs of 3a and 3b. Of note,
˚
˚
the Rh–OH bonds were found to be 2.030(4) A (3a) and 2.036(2) A
(3b), which is typical of M–OH bond lengths.7b,11
Our recent success with well-defined gold7 and copper8 hy-
droxide complexes prompted us to examine whether the general
synthetic methodology developed could be extended to rhodium.
Reports of stable, well-defined monomeric Rh(I)–OH complexes
are rare.9 As Rh–OH bearing electron-poor ancillary ligands
are highly unstable,3a the active species is usually prepared in
situ, either by reaction of [Rh(cod)(m-OH)]2 with the desired
ligand, or by transformation of the precatalyst in the presence
of a base in aqueous media. Herein, we report that a number
of strongly electron-donating N-heterocyclic carbenes (NHCs),
Fig. 1 ORTEPs of [Rh(cod)(IPr)(OH)] 3a and [Rh(cod)(ICy)(OH)] 3b,
showing 50% thermal ellipsoid probability. Selected distances (A) for 3a:
EaStCHEM School of Chemistry, University of St Andrews, St Andrews,
KY16 9ST, U.K.. E-mail: snolan@st-andrews.ac.uk
˚
Rh1–NHC = 2.046(6), Rh1–O1 = 2.030(4) and 3b: Rh1–NHC = 2.022(3),
Rh1–O1 = 2.036(2).
† Electronic supplementary information (ESI) available: Synthetic and
structural data. See DOI: 10.1039/c1ob06112g
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With complexes 3a–c in hand, we turned our attention to the
conjugate addition of arylboronic acids to a,b-unsaturated car-
bonyls. The addition of phenylboronic acid 5a to cyclohexenone
4a (eqn (1)) was selected as benchmark reaction. In a reaction
conducted at room temperature, the desired product was obtained
in 54% yield after 1 h with a catalyst loading of 0.2 mol% using
[Rh(cod)(ICy)(OH)] 3b.
obtained using these two reagents,13 further proving the versatility
of the catalyst and the method employed.
A series of stoichiometric reactions were then conducted to
shed light on the mechanism at play in this transformation.
[Rh(cod)(ICy)Ph] 7 was prepared from [Rh(cod)(ICy)(OH)] 3b
and phenylboronic acid 5a in C6H6 in a very rapid transformation
as shown in eqn (2). 1H NMR analysis confirmed complete
conversion of 3b into 7 after only 15 min at room temperature.
(1)
(2)
To rapidly afford optimized conditions and simultaneously test
the thermal stability of our complexes under catalytic conditions,
transformations were examined under microwave irradiation.
Testing the well-defined 3a–c at 0.05mol% under conditions
illustrated in eqn (1), the desired product (6a) was obtained in
59 (using 3a), 86 (using 3b) and 74% (using 3c) isolated yield.
The stronger s-donating cycloalkyl-substituted NHC complexes
3b and 3c were found to outperform the aryl-substituted analogue
3a. Among the cycloalkyl-NHCs, the complex containing the
smaller12 ICy ligand 2b resulted in the highest yields. Under these
reaction conditions, the catalysts appear very tolerant of elevated
reaction temperatures.
Having identified [Rh(cod)(ICy)(OH)] 3b as an active promoter
for the 1,4-addition, it was tested at homeopathic quantities,
achieving full conversion after 12 h under conventional heating
(100 ◦C) at 0.001 mol%, with a TON of 100,000 and TOF of 6,600
h-1. Gratifyingly, catalyst 3b proved to be exceedingly stable, as
gauged by 1H NMR even after 16 h in C6D6 solution open to air.
A scope for the transformation was next examined to demon-
strate the versatility of 3b, we examined the coupling of arylboronic
acids 5a–d, substituted with activating and deactivating groups to
three cyclic enones 4a–c of differing ring size under optimized
conditions (eqn (1)). Fig. 2 shows the products 6a–d, with isolated
yields, as representatives of the full scope, which can be found
in the SI. Cyclopentenone 4b and cycloheptenone 4c proved
more difficult substrates than cyclohexenone 4a, particularly
when reacted with the deactivated 4-chlorophenylboronic acid
5b and 4-(trifluoromethyl)phenylboronic acid 5c. The sterically
demanding, 2-methylphenylbonic acid 5d was found to give high
yields with all enones examined. Phenylboronic acid pinacol ester
and potassium phenyltrifluoroborate were also successfully used
as an aryl transfer source. Yields of 89% and 87% respectively were
When the reaction was repeated with [Rh(cod)(IPr)(OH)] 3a
only 50% conversion as evaluated by H NMR was noted after
16 h.
1
This difference in the rate of transmetalation between 3a and
3b may very well account for the lower activity of 3a observed
during catalyst screening, the ligand principal effect being steric.
The phenylrhodium 7 was fully characterized and single crystal X-
ray analysis (eqn (2)) unambiguously confirmed the square planar
arrangement about the Rh center.10 Bond lengths and angles
were found to agree with similar Rh-aryl complexes found in the
literature.14
Hartwig15 has shown that transmetalation of an aryl moiety
from boron to rhodium occurs via formation of a rhodium aryl-
boronate (RhOB(OH)Ar) species, followed by b-aryl elimination
to give the arylrhodium complex. The reaction with 3b is too rapid
to observe this intermediate but might be observable using a Rh-
species with a slower transmetalation rate. The phenylrhodium
complex 7 was used to catalyze the conjugate addition of phenyl-
boronic acid 5a and cyclohexenone 4a under microwave conditions
and resulted in nearly identical activity to that of 3a (98%).
Additionally, the reaction of 7 with 2 equivalents of cyclohexenone
4a in THF/H2O resulted in the formation of 6a in a yield of 72%
under catalytic conditions. These experiments strongly suggest
that the phenylrhodium 7 is an important intermediate along
the catalytic pathway. Reaction between 7 and cyclohexenone 4a
in various solvents was very slow, even at elevated temperatures,
indicating the allyl formation as the possible rate-determining step.
This is in contrast to previous studies reporting transmetalation
as the rate-determining step.16
The role played by the base in the reaction is still somewhat
unclear. Several reports have indicated that base is essential for
the in situ preparation of the hydroxorhodium species,2e,3a,17 where
precatalysts such as [RhCl(cod)]2 or [Rh(acac)(C2H4)2] are used.
Inorganic bases have been shown to accelerate transmetalation
between organoboron reagents and metal complexes,2c via attack
upon the boronic acid to form an anionic organoboronate,
Fig. 2 Conjugated products from the addition of arylboronic acids (5a–d)
to cyclohexenone 4a. Reaction conditions: 3b(0.2 mol%), KOH (0.5 equiv.),
THF/H20, mw, 100 ◦C, 200 W, 30 min. Isolated yields are average of two
runs, after flash chromatography. See ESI for details of full scope.†
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which subsequently coordinates the metal centre, promoting
transmetalation. This concept has been explored by Suzuki18 in
Pd-catalyzed cross-coupling and supported by DFT studies.19
The theory has been extended to Rh-systems2c but is not yet
supported experimentally. Miyaura and co-workers2e suggested
that the base might play a further role in facilitating hydrolysis
of the enolate intermediate. To examine the role of the base,
our present systems, by already bearing the base, may facilitate
analysis of stoichiometric reactions and shed light on its role
under catalytic conditions. Using a [Rh(cod)(NHC)(OH)] catalyst
eliminates the need for in situ preparation of a Rh-(OH) and
the swift transformation from Rh–OH 3b to Rh–Ph 7 (eqn (2))
suggests that base might not be required up to that point. However,
catalysis in the absence of a base produces the conjugated product
in less than 10% yield. Reactions performed with D2O resulted
in deuterium incorporation into the conjugated product 6a, a- to
the carbonyl (confirmed by NMR studies, see ESI†). These data
support hydrolytic cleavage of 6a by H2O from the rhodium center.
A proposed mechanism is presented in Scheme 2 which is
consistent with the one proposed by Hayashi.6b Activation of
the arylboronic acid by the Rh–OH 3b forms the Rh-aryl
complex 7, followed by enone insertion to most likely give the
unobserved transient oxa-p-allyl Rh intermediate 8. Hydrolysis of
8 completes the cycle with release of the 1,4-addition product 6a
and reformation of the Rh–OH complex.
partners in the conjugate addition, leading to high conversions.
Mechanistic studies were performed, enabling isolation of the
phenylrhodium intermediate 7. Further catalytic potential of this
novel well-defined Rh-hydroxide is currently under investigation.
Experimental section
General considerations
Synthesis of Rh-complexes was performed inside an MBraun
Glovebox under inert conditions. All reagents were supplied by
Aldrich and used without further purification and solvents were
distilled and dried as required. NMR data was obtained using
either a Bruker 400 MHz or 300 MHz spectrometer at 303 K in the
specified deuterated solvent. All chemical shifts are given in ppm
and coupling constants in Hz. Spectra were referenced to residual
protonated solvent signals (C6D6:1H d 7.16 ppm, 13C d 128.06
ppm). Reactions under microwave irradiation were performed in
a CEM Discover single-mode microwave apparatus, producing
controlled irradiation at 2450 MHz. Reaction times refer to hold
times at the indicated temperature and not total irradiation times
with constant cooling via propelled air flow at a set power of 200 W.
Elemental analyses were performed at the London Metropolitan
University.
Representative synthesis of [Rh(cod)(ICy)(OH)] 3a–c
[Rh(cod)Cl]2 1 (100 mg, 0.2 mmol), free carbene 2a–c (0.4 mmol)
and CsOH (0.8 mmol) were stirred in THF (5 mL) overnight. The
suspension was filtered and the eluent concentrated in vacuo. The
resultant solid was washed with hexane (3 ¥ 10 mL) and dried in
vacuo to give [Rh(NHC)(ICy)(OH)] 3a–c.
[Rh(cod)(IPr)(OH)] 3a. (190.0 mg, 75.8%) As a yellow solid;
1H NMR (300 MHz, C6D6): d 7.27–7.35 (2H, m, ArH), 7.20–7.26
(4H, m, ArH), 6.59 (2H, s, NCH), 4.29–4.42 (2H, m, cod-CH),
3.35 (4H, m, cod-CH2), 2.99 (2H, d, J = 2.5, cod-CH), 1.83–2.02
(4H, m, cod-CH2), 1.59–1.72 (2H, m, CH), 1.49 (12H, d, J = 6.6,
CH3), 1.46–1.54 (2H, m, CH), 1.06 (12H, d, J = 6.9, CH3); 13C
1
NMR (75 MHz, C6D6): d 191.8 (d, JRhC = 56.3), 147.0, 137.3,
129.9, 123.9, 92.5 (d, 1JRhC = 9.0), 63.5 (d, 1JRhC = 12.0), 33.7, 29.0,
28.9, 26.5, 23.2; Anal. Calcd for C35H49N2ORh (MW 621.08): C,
68.17, H, 8.01, N, 4.54. Found: C, 68.26; H, 7.87; N, 4.38.
[Rh(cod)(ICy)(OH)] 3b. (163 mg, 84.4%) As a yellow solid; 1H
NMR (300 MHz, C6D6) d 6.42 (2H, s, NCH), 5.73 (2H, tt, J =
12.1, 3.9, cod-CH), 5.13–5.30 (2H, m, cod-CH), 3.08 (2H, dd, J =
5.2, 2.2, NCH), 2.47–2.63 (4H, m, cod-CH2), 2.30 (2H, d, J = 12.2,
CH2), 2.12 (2H, quint, J = 6.9, CH2), 1.84–2.00 (4H, m, cod-CH2),
1.34–1.73 (10H, m, CH2), 1.27 (2H, qd, J = 12.5, 3.6, CH2), 1.10
Scheme 2 Proposed catalytic cycle for Rh-catalyzed 1,4-addition.
In conclusion, three well-defined monomeric rhodium(I) hy-
droxide complexes 3a–c bearing NHC ligands 1a–c have been
prepared and exhibit high stability. The [Rh(cod)(ICy)(OH)] 3b
complex is a very efficient catalyst in the conjugate addition of
arylboronic acids to cyclic enones, promoting full conversion
at 0.001 mol%, with TON and TOF 100,000 and 6,600 h-1,
respectively. This high activity/productivity is in contrast with
previous reports where ancillary ligands with weaker electron
donating properties were found to lead to optimal catalytic
conversions.3 Furthermore, potassium aryltrifluoroborates and
aryl boronic esters were also found to serve as viable reaction
(2H, qd, J = 12.5, 3.8, CH2), 0.94 (2H, qt, J = 12.8, 3.7, CH2); 13
C
1
NMR (75 MHz, C6D6) d 186.3 (d, JRhC = 57.0), 117.2, 97.4 (d,
1JRhC = 8.3), 66.7 (d, 1JRhC = 12.8), 60.5, 34.9, 34.5, 34.4, 29.5, 26.4,
26.0, 25.7. Anal. Calcd for C23H37N2ORh (MW 460.46): C, 59.99;
H, 8.10; N, 6.08. Found: C, 59.90, H, 8.02; N, 5.97.
[Rh(cod)(IDD)(OH)] 3c. (180.0 mg, 70.9%) As a yellow solid;
1H NMR (300 MHz, C6D6) d 6.45 (2H, s, NCH), 5.83–5.98 (2H,
m, cod-CH), 5.20 (2H, m, cod-CH), 3.04–3.15 (2H, m, NCH),
2.53–2.72 (4H, m, cod-CH2), 2.01–2.13 (4 H, m, CH2), 1.87–1.97
(4H, m, cod-CH2), 1.68–1.84 (10H, m, CH2), 1.15–1.54 (30H, m,
7040 | Org. Biomol. Chem., 2011, 9, 7038–7041
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CH2); 13C NMR (75 MHz, C6D6): d 187.1(d, 1JRhC = 57.0), 117.2,
Oshita, D. Piao, Y. Yamamoto and N. Miyaura, J. Organomet. Chem.,
2007, 692, 428; (i) K. Okamoto, T. Hayashi and V. H. Rawal, Org. Lett.,
2008, 10, 4387; (j) Y. Ma, C. Song, C. Ma, Z. Sun, Q. Chai and M. B.
Andrus, Angew. Chem., Int. Ed., 2003, 42, 5871.
3 (a) T. Korenaga, R. Maenishi, K. Hayashi and T. Sakai, Adv. Synth.
Catal., 2010, 352, 3247; (b) T. Korenaga, K. Osaki, R. Maenishi and T.
Sakai, Org. Lett., 2009, 11, 2325; (c) F. Berhal, O. Esseiva, C. Martin,
H. Tone, J. Genet, T. Ayad and V. V. Ratovelomanana-Vidal, Org. Lett.,
2011, 13, 2806.
4 T. Hayashi, K. Ueyama, N. Tokunaga and K. Yoshida, J. Am. Chem.
Soc., 2003, 125, 11508.
5 J. Boiteau, R. Imbos, A. J. Minnaard and B. L. Feringa, Org. Lett.,
2003, 5, 681.
6 (a) Y. Takaya, M. Ogasawara, T. Hayashi, M. Sakai and N. Miyaura,
J. Am. Chem. Soc., 1998, 120, 5579; (b) T. Hayashi, M. Takahashi, Y.
Takaya and M. Ogasawara, J. Am. Chem. Soc., 2002, 124, 5052; (c) K.
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3904; (e) W. Yuan, L. Cun, A. Mi, Y. Jiang and L. Gong, Tetrahedron,
2009, 65, 4130; (f) W. Yuan, L. Cun, L. Gong, A. Mi and Y. Jiang,
Tetrahedron Lett., 2005, 46, 509.
7 (a) I. I. F. Boogaerts and S. P. Nolan, J. Am. Chem. Soc., 2010, 132,
8858; (b) S. Gaillard, A. M. Z. Slawin and S. P. Nolan, Chem. Commun.,
2010, 46, 2742.
8 (a) I. I. F. Boogaerts, G. C. Fortman, M. R. L. Furst, C. S. J. Cazin and
S. P. Nolan, Angew. Chem., Int. Ed., 2010, 49, 8674; (b) G. C. Fortman,
A. M. Z. Slawin and S. P. Nolan, Organometallics, 2010, 29, 3966.
9 For a review on Transition Metal-OH complexes, see: (a) H. Roesky, S.
Singh, K. K. M. Yusuff, J. A. Maguire and N. S. Hosmane, Chem. Rev.,
2006, 106, 3813; (b) S. M. Kloek, M. D. Heinekey and K. I. Goldberg,
Angew. Chem., Int. Ed., 2007, 46, 4736.
1
1
96.0 (d, JRhC = 8.3), 62.4 (d, JRhC = 12.8), 57.4, 34.4, 32.2, 31.7,
29.4, 25.0, 24.7, 24.3, 24.1, 24.0, 23.7, 23.2, 22.7, 22.5; Anal. Calcd
for C35H61N2ORh (MW 628.78): C, 66.86; H, 9.78; 4.46. Found:
C, 66.88; H, 9.65; N, 4.46.
Preparation of [Rh(cod)(ICy)Ph] 7. [Rh(cod)(ICy)(OH)] 3b
(500 mg, 1.08 mmol) and phenylboronic acid 5a (130.0 mg,
1.08 mmol) were stirred in C6H6 (5 mL) for 1 h. The solution
was filtered and reduced in vacuo to a yellow solid. The solid was
dissolved in hexane (20 mL) and filtered before being dried in
vacuo to give [Rh(cod)(ICy)Ph] 7 (488.3 mg, 87.4%) as an orange
1
solid; H NMR (400 MHz, C6D6): d 7.81 (2H, d, J = 7.7, ArH),
7.23 (2H, t, J = 7.4, ArH), 6.99 (1H, t, J = 7.3, ArH), 6.21 (2H,
s, NCH), 5.47 (2H, tt, J = 12.1, 3.5, cod-CH), 4.74 (2H, d, J =
2.7, cod-CH), 4.08 (2H, br.s., NCH), 2.45 (4H, m, cod-CH2), 2.15
(4H, d, J = 8.9, CH2), 1.99 (2H, d, J = 12.4, CH2), 1.92 (2H, d, J =
12.1, CH2),1.66 (4H, t, J = 12.7, CH2), 1.55 (2H, d, J = 13.5, CH2),
1.41–1.52 (4H, m, CH2), 1.22 (2H, qd, J = 12.5, 3.6, CH2), 1.06
(2H, qd, J = 12.5, 3.6, CH2), 0.90–0.99 (2H, m, CH2); 13C NMR
(75 MHz, C6D6): d 189.9 (d, 1JRhC = 60.2), 177.4 (d, 1JRhC = 36.0),
1
1
138.2, 126.2, 121.0, 116.5, 87.5 (d, JRhC = 9.1), 81.7 (d, JRhC
=
7.0), 60.4, 35.2, 34.7, 32.0, 31.9, 26.4, 25.9, 25.7; Anal. Calcd for
C29H41N2Rh (MW 521.56): C, 66.91; H, 7.94; N, 5.38. Found: C,
66.81, H, 7.99 N, 5.31.
10 CCDC-823666 (3a), CCDC-823667 (3b) and CCDC-823668 (7) contain
the supplementary crystallographic data for this contribution. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
11 R. S. Ramo´n, S. Gaillard, A. Poater, L. Cavallo, A. M. Z. Slawin and
S. P. Nolan, Chem.–Eur. J., 2011, 17, 1238.
12 For a discussion of NHC steric parameters see: (a) H. Clavier and S. P.
Nolan, Chem. Commun., 2010, 46, 841; (b) P. de Fre´mont, N. Marion
and S. P. Nolan, Coord. Chem. Rev., 2009, 253, 862For a review on Late
Transition Metal-NHC complexes in catalysis: (c) S. D´ıez-Gonza´lez,
N. Marion and S. P. Nolan, Chem. Rev., 2009, 109, 3612.
13 Yields determined by 1H NMR and GC. Reaction conditions were not
optimized. See SI for details.
General procedure for catalysis. Arylboronic acid 5a–d
(0.6 mmol), [Rh(cod)(ICy)(OH)] 3b (0.46 mg, 1.0 mmol,
0.20 mol%) and KOH (14 mg, 0.25 mmol) were charged to a 10 mL
microwave vial with THF (1 mL) inside a glove box. The vial was
sealed and removed to air, where it was charged with cyclic enone
4a–c (0.50 mmol) and H2O (100 mL) before being irradiated in the
microwave at 100 ◦C, 200 W for 30 min. The resultant mixture
was flash chromatographed (hexane-ethyl acetate) to afford the
addition product 6a–l.
The ERC (Advanced Investigator Award-FUNCAT to SPN)
and Sasol (BJT) are gratefully acknowledged for support. Umicore
AG is thanked for their generous gift of materials.
14 (a) C. Krug and J. F. Hartwig, J. Am. Chem. Soc., 2004, 126, 2694;
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Organometallics, 1993, 12, 1720.
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16 A. Kina, Y. Yasuhara, T. Nishimura, H. Iwamura and T. Hayashi,
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17 S. Sakuma and N. Miyaura, J. Org. Chem., 2001, 66, 8944.
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