Axially chiral P-N ligands for the copper catalyzed β-borylation of α,β-unsaturated esters

Article abstract of DOI:10.1039/b900741e

The synthesis and resolution of a new axially chiral Quinazolinap ligand are reported. The application of this and other related P-N ligands to the copper catalyzed β-borylation of α,β-unsaturated esters resulted in conversions of up to 100% and ee values

Full text of DOI:10.1039/b900741e

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Volume 7 | Number 12 | 21 June 2009 | Pages 2469–2656
ISSN 1477-0520
FULL PAPER
ChemicalScience
William J. Fleming et al.
Axially chiral P-N ligands for the
copper catalyzed β-borylation of
α, β-unsaturated esters
In this issue...
1477-0520(2009)7:12;1-F

PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Axially chiral P-N ligands for the copper catalyzed b-borylation of
a,b-unsaturated esters†
William J. Fleming,a Helge Mu¨ller-Bunz,‡a Vanesa Lillo,b Elena Ferna´ndez*b and Patrick J. Guiry*a
Received 14th January 2009, Accepted 17th March 2009
First published as an Advance Article on the web 15th April 2009
DOI: 10.1039/b900741e
The synthesis and resolution of a new axially chiral Quinazolinap ligand are reported. The application
of this and other related P-N ligands to the copper catalyzed b-borylation of a,b-unsaturated esters
resulted in conversions of up to 100% and ee values of up to 79%. A diastereomerically pure
palladacycle of the new ligand was characterised by X-ray crystallography.
Introduction
The synthetic challenges presented by structurally complex and
diverse natural products provides the impetus for the development
of new catalytic systems, which in turn offers an insight into the
fundamental understanding of how new bonds can be formed.1
Of particular interest is the challenge presented by issues of
stereochemistry. Since the outstanding success of Noyori et al.’s
BINAP ligand,2 and its subsequent applications in asymmetric
transition metal catalysis,3 the development of new atropisomeric
ligands has become an area of intense interest.4 Of particular
relevance to our work is the development of chiral heterobidentate
Fig. 1 Quinap and quinazolinap ligands.
systems due to their ability to induce asymmetry in a variety of
reactions. Of these the phosphinamine ligand class has received the
most interest due to the combination of steric and electronic effects
exerted on substrates at the coordination sphere of the transition
metal to which they are bound.5
which Quinazolinap ligands can be applied and therefore initiated
a study on the metal catalyzed b-borylation of a,b-unsaturated
compounds, which is a promising transformation towards the
stereoselective formation of a new Cb-B bond (Scheme 1).9
The first successful axially chiral phosphinamine ligand, Quinap
1, was developed by Brown et al. and incorporated a naphthalene–
isoquinoline backbone with the necessary steric requirements to
prevent free rotation about the biaryl linkage.6 Stemming from this
our group developed the Quinazolinap ligands (exemplified by 2a–
e, Fig. 1).7 This allowed for studies on the effects of altering the
basicity of the donor nitrogen, and for the facile introduction of
steric bulk at the 2-position, which is believed to be important
in the transfer of chiral information from catalyst to reaction
product.
Scheme 1
The quinazolinap ligands have proved efficacious in a va-
riety of transition metal-catalyzed transformations, including
rhodium-catalyzed hydroborations and palladium-catalyzed al-
lylic alkylations.7f–g,8 We wished to increase the scope of reactions to
b-Borylation reactions have been catalyzed by a range of
metals, most notably platinum and rhodium, yet the use of
the inexpensive metal copper has recently been shown to be
remarkably successful.10 While bidentate P,P ligands11 and N-
heterocyclic ligands (NHC)12 have recently demonstrated their
efficiency in moderate to high asymmetric induction for the
copper mediated reaction, only one bidentate P,N ligand has
been tested to date.11a Herein we wish to report the application of
Quinap 1 and Quinazolinaps 2a–e to the asymmetric b-borylation
of a,b-unsaturated esters 3a–d, incorporating the synthesis and
resolution of the new 7-substituted Quinazolinap 2c. This is the
first ligand in the Quinazolinap series that allows for the study of
electronic variation at the 7-position. It was also envisaged that
post-resolution modification at this position would provide an
attractive route to a range of electronically diverse Quinazolinap
ligands.
aCentre for Synthesis and Chemical Biology, School of Chemistry and
Chemical Biology, UCD Conway Institute, University College Dublin,
Belfield, Dublin 4, Ireland. E-mail: p.guiry@ucd.ie; Fax: +353 1 716 2127
bUniversidad Rovira i Virgili, C/Marcel.l´ı Domingo s/n. 43005, Tarragona,
Spain. E-mail: mariaelena.fernandez@urv.cat; Fax: +34 977 559563;
Tel: +34 977 558046
† Electronic supplementary information (ESI) available: Synthesis and
characterisation data for 2c, 4 and 6–11, details of the resolution of 2c and
the general procedures employed for the asymmetric borylation. CCDC
reference numbers 716086 and 716087. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b900741e
‡ Correspondence concerning single-crystal X-ray data should be directed
to this author (helge.muellerbunz@ucd.ie)
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palladacycles derived from palladium amine complexes, followed
by fractional crystallisation to yield a diastereomerically pure
complex. The palladium complex (R,R)-cis-13 was used in the
resolution of Quinap 1 and previously prepared members of the
Quinazolinap family (Scheme 3). The racemic phosphane 2c was
resolved using this method via the formation of diastereomeric
palladacycles (Sa,R)-14 and (Ra, R)-14. These palladacycles were
obtained by stirring the racemic phosphane 2c in MeOH with
the chloro-bridged resolving agent (R,R)-cis-13 and subsequent
addition of KPF6 in water to form the corresponding diastere-
omers in a 1:1 ratio. Crystallisation from hot butanone/Et2O
precipitated (Sa,R)-14 whose purity was determined to be >99% by
1H and 31P NMR spectroscopy. The mother liquor was subjected
to two more crystallisations which afforded more (Sa,R)-14, with
a total yield of 40%. Further crystallisations yielded only di-
astereomerically enriched crystals, although the remaining mother
liquor was found to be diastereomerically pure (Ra,R)-14. Crystals
of (Sa,R)-14 suitable for X-ray crystallographic analysis were
grown from CDCl3 and pentane to confirm the stereochemistry
(Fig. 2). Enantiomerically pure phosphinamine (Ra)-(+)-2c was
obtained by decomplexation from the (Ra,R)-14 complex with
1,2-bis(diphenylphosphino)ethane in dichloromethane (95%).
Results and discussion
Synthesis and resolution of 2c
Racemic Quinazolinap 2c was prepared using the synthetic route
previously employed for the synthesis of other members of this
class of ligand (Scheme 2).7 The quinazolinone 6 was prepared
by a 2 step acylation–cyclocondensation between commercially
available 4-chloroanthranilamide 4 and isobutyryl chloride 5,
which proceeded in high overall yield (89% from 4). Chlorination
of 6 with POCl3/PhNEt2 in refluxing benzene gave the electrophilic
partner 7 for the Suzuki–Miyaura cross-coupling, this proceeded
with a yield of 69%. Using 3 mol% Pd(PPh3)4 as the catalyst and
CsF as the base, the coupling of 7 and 2-methoxynaphthyl boronic
acid 12 afforded the biaryl 8 in moderate yield (55%). The methyl
ether of 8 was converted to the corresponding naphthol 9 by a
BBr3-mediated demethylation. Upon treatment of 9 with Tf2O
in the presence of DMAP the aryl triflate 10 was produced in
good yield (81% from 8). The final step in the ligand synthesis
was the nickel-catalyzed cross-coupling between the triflate 10
and diphenylphosphine in the presence of NiCl2(dppe)/DABCO
following the protocol of Cai et al.13 This afforded the desired
triarylphosphane 2c in low yield (35%), which can be accounted
for by the unexpected formation of the di-coupled product 11 in
29% yield.14 The formation of 11 shows an unusually high activity
for an aryl–chloride bond which we intend to exploit for further
diversification of the Quinazolinap ligand series.
Copper(I) catalyzed b-borylation of a,b-unsaturated compounds
The copper-catalyzed enantioselective b-borylation of a,b-
unsaturated olefins is a valuable synthetic transformation. The
boron functionality can be readily converted into various
The resolution of atropisomeric P-N ligands is typically car-
ried out by the formation of diastereomers using enantiopure
Scheme 2 Synthesis of racemic ligand 2c.
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Scheme 3 Optical resolution of 2c.
Table 1 Copper(I) catalyzed borylation of a,b-unsaturated compoundsa
Entry
R =
Ligand
Conversion (%)b
Ee (%)c,d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
(S)-1
100
95
65
100
100
71
100
100
75
50 (S)
20 (S)
51 (R)
40 (R)
25 (S)
13 (S)
72 (S)
34 (S)
40 (R)
38 (R)
12 (S)
15 (S)
79 (S)
35 (S)
42 (R)
48 (R)
20 (S)
20 (S)
72e (S)
42e (S)
43e (R)
(S)-2a
(R)-2b
(R)-2c
(S)-2d
(S)-2e
(S)-1
Fig. 2 Single-crystal X-ray structure of PdII complex (Sa,R)-14.17 Hy-
drogen atoms and counter-ion omitted for clarity. The unit cell contains
two independent cations, only one of which is shown. Bond lengths:
(S)-2a
(R)-2b
(R)-2c
(S)-2d
(S)-2e
(S)-1
(S)-2a
(R)-2b
(R)-2c
(S)-2d
(S)-2e
(S)-1
˚
˚
˚
˚
Pd-P 2.274 A, Pd-N1 2.215 A, Pd-N3 2.135 A and C-Cl 1.738 A. Bond
angles: N1-Pd-P 81.97◦, N1-Pd-N2 39.33◦ and N3-Pd-P1 10.08◦. Unit cell
98
99
82
◦
◦
˚
˚
dimensions: a = 9.8195(9) A, a= 90 ; b = 29.059(3) A, b= 95.039(2) ; c =
◦
˚
15.0113(14) A. g = 90 .
Et
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
i-Bu
t-Bu
t-Bu
t-Bu
100
100
100
23
100
26
100
100
100
other functional groups.15 This transformation typically em-
ploys a chiral catalyst and an achiral borane source such as
bis(pinacolato)diboron. We now wish to report the application
of Quinazolinap ligands to this copper mediated transformation.
For each catalytic reaction the catalyst system was generated
by stirring copper(I) chloride with ligand (2 equiv.)16 and NaOtBu
in THF to form a putative bidentate complex, which is the active
catalytic species. Bis(pinacolato)diboron was added followed by
the a,b-unsaturated compound, and then MeOH (2 equiv. relative
to substrate) to guarantee total conversion.11b
In the b-borylation of methyl crotonate 3a (Table 1, entries
1–6) the conversions were optimised to give good to complete
conversion, with the best ee values being obtained for the
Quinazolinap ligand 2b (51%, entry 3) and Quinap 1 (50%,
entry 1). In the case of ethyl crotonate 3b (Table 1, entries 7–12)
the conversions again were good to excellent. The ee values for the
(S)-2a
(R)-2c
a Standard conditions: 2% CuCl, 3% NaOtBu, 4% Ligand, 1.1 eq. B2Pin2,
2.0 eq. MeOH, THF, RT, 6 h. b Determined by 1H NMR. c Determined by
chiral GC of product after oxidation and conversion to the corresponding
acylated derivative. 15a b-CD, 30 m, 80 ◦C, 11.4 psi, RT = 17.4 min (R),
19.7 min (S). 15b b-CD, 30 m, 80 ◦C, 27.1 psi, RT = 13.8 min (R), 14.5 min
(S). 15c b-CD, 30 m, 100 ◦C, 14.5 psi, RT = 17.9 min (R), 20.1 min (S).
d Configuration of products was assigned by comparison to the known
optical rotation values obtained for the oxidized derivatives of 15b (see
supporting information of reference 11a). e Determined by chiral HPLC
of product after oxidation and conversion to the corresponding phenoxy
derivative. 15d Chiracel OD column, hexane:IPA (85:15), 1 mL/min.
220 nm. RT = 20.7 min (S), 31.8 min (R).
2522 | Org. Biomol. Chem., 2009, 7, 2520–2524
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ligands 2a–e were similar to those obtained in the methyl
crotonate case, although Quinap 1 showed an increase in ee to
72% (entry 7). A further increase in the size of the ester moiety
of the substrate to i-Butyl 3c (entries 13–18) did not alter the
enantioselectivity by any appreciable amount for ligands 2a–e. As
for Quinap 1, the general trend of an increased ee (79%, entry 13)
with an increase in the bulk of the substrate ester continued.
For the final set of reactions the size of the substrate ester was
increased further to a t-butyl group 3d (entries 19–21). In each of
these reactions complete conversion was observed, although the
enantioselectivity induced by Quinap 1 dropped slightly (72%)
relative to the i-butyl containing substrate.
For the b-borylation of the various substrates 3a–d the copper
complex modified with the Quinazolinap ligands 2a–e gave up
to quantitative conversion along with ee values of up to 51%
(entry 3). Of all the ligands tested, Quinap 1 gave the best and
most consistent results, with 100% conversion in all cases and ee
values ranging from 50–79%.
In an attempt to further understand the process under investi-
gation, a crystal structure of a complex of ligand 2c with copper
was obtained. The complex was formed by stirring copper and
2c in an equal volume mixture of IMS and Et2O (IMS = 85%
EtOH/15% MeOH). The complex formed under these conditions
is a chlorine-bridged dimer lying about an inversion centre and is
monodentate, Fig. 3. However, the high levels of enantioselectivity
induced by these ligands suggests that the catalytic complex must
have the ligands bound in a bidentate manner.
Experimental details
General experimental
All reactions were performed under anhydrous conditions and
an inert atmosphere of nitrogen in oven-dried glassware with
magnetic stirring. Yields refer to chromatographically and spec-
troscopically (1H NMR) homogenous materials, unless otherwise
indicated. Reagents were used as obtained from commercial
sources or purified according to the guidelines of Perrin and
Armarego.20 Evaporation in vacuo refers to the removal of volatiles
on a Bu¨chi rotary evaporator with an integrated vacuum pump.
Flash chromatography was carried out using Merck Kiesegel 60
F254 (230–400 mesh) silica gel following the method of Still et al.21
Thin-layer chromatography (TLC) was performed on Merck DC-
Alufolien plates pre-coated with silica gel 60 F254. They were
visualized either by quenching of ultraviolet fluorescence, or by
charring with an acidic vanillin soln. (vanillin, H2SO4 and acetic
acid in MeOH). The Microanalytical Laboratory, University
College Dublin, performed elemental analyses. Electrospray mass
spectra were recorded on a Micromass Quattro with electrospray
probe. Exact mass ESI mass spectra (HRMS) were measured on
a micromass LCT orthogonal time of flight mass spectrometer
with leucine enkephalin (Tyr-Gly-Phe-Leu) as an internal lock
1
mass. H NMR spectra were recorded on a 300 MHz Varian-
Unity spectrometer, a 400 MHz Varian-Unity spectrometer or a
500 MHz Varian-Unity spectrometer. Chemical shifts are quoted
in ppm relative to tetramethylsilane and coupling constants (J)
are quoted in Hz and are uncorrected. CDCl3 was used as the
solvent for all NMR spectra unless otherwise stated. 75.4 MHz 13
C
spectra were recorded on a 300 MHz Varian-Unity spectrometer,
101 MHz 13C spectra on a 400 MHz Varian-Unity spectrometer
and 125 MHz 13C spectra on a 500 MHz Varian-Unity spec-
trometer. Tetramethylsilane was used as the internal standard in
all 13C spectra recorded. 121.4 MHz 31P spectra were recorded
on a 300 MHz Varian-Unity spectrometer and 162 MHz 31P
spectra on a 400 MHz Varian-Unity spectrometer. 31P Chemical
shifts are reported relative to 85% aqueous phosphoric acid
(0.0 ppm). All reaction solvents were distilled before use, unless
otherwise indicated. Anhydrous solvents were obtained from
a PureSolv-300-3-MD dry solvent dispenser and used without
further purification unless otherwise stated. Melting points (mp)
are quoted to the nearest 0.5 ◦C. GC and HPLC analysis was
Fig. 3 Single-crystal X-ray structure CuI complex of 2c.18 Hydrogen
atoms omitted for clarity. Duplicate atom labels are generated by the
operation (1-x, 2-y, 2-z).
R
carried out using a Supelco 2–4304 beta-Dexꢀ 120 (30 m ¥
0.25 mm, 0.25 mm film) and a Chiralcel OD column (0.46 cm
I.D. ¥ 25 cm) respectively. Optical rotation values were measured
on a Perkin Elmer 241 Polarimeter. [a]D values are given in
10-1 deg cm2 g-1.
Conclusion
General procedure for the borylation of a,b-unsaturated esters
A new axially chiral 7-substituted Quinazolinap ligand has
been synthesised and resolved. This ligand, along with other
known Quinazolinap ligands and Quinap have been tested in
the asymmetric b-borylation of a,b-unsaturated compounds.19
Conversions of up to 100% and ee values of up to 79% were
obtained. Work on further expanding the range of Quinazolinap
ligands and on expanding the substrate scope in the b-borylation
of a,b-unsaturated compounds is ongoing and will be reported in
due course from these laboratories.
THF (3 mL) was added to CuCl (0.010 mmol, 1 mg), NaOtBu
(0.015 mmol, 1.4 mg) and ligand (0.020 mmol) in a dry Schlenk
tube under nitrogen. The mixture was stirred for 30 min at room
temperature, at this point bis(pinacolato)diboron (0.550 mmol,
139.7 g) was added and stirred for a further 10 min. The a,b-
unsaturated ester (0.500 mmol) was then added, followed by
MeOH (1 mmol, 0.040 mL). Stirring was continued for 6 h
1
and a sample removed for H NMR analysis. Conversions were
determined by 1H NMR, ee values were determined by chiral GC
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of the acylated products or chiral HPLC of the phenoxy derivative
of the product (see ESI†).
9 (a) H. Ito, H. Yamanaka, J. Tateiwa and A. Hosomi, Tetrahedron
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Acknowledgements
WJF acknowledges financial support from the Centre for Syn-
thesis and Chemical Biology (CSCB), UCD, which is funded by
the Higher Education Authority’s Programme for Research in
Third-Level Institutions. We thank MEC for funding (CTQ 2007–
60442BQU and Consolider-Ingenio 2001 CSD-0003). VL thanks
MEC for FPJ–Grant. We also acknowledge the facilities provided
by the CSCB. We are grateful to Dr Jimmy Muldoon and Dr Dilip
Rai of the CSCB for NMR spectrometry and Mass Spectroscopy,
respectively.
14 All attempts at optical resolution of 11 have so far been unsuccessful.
15 (a) P. J. Guiry, A. M. Carroll and T. P. O’Sullivan, Adv. Synth. Catal.,
2005, 347, 609; (b) C. M. Crudden and D. Edwards, Eur. J. Org.
Chem., 2003, 4695; (c) A. G. Coyne, and P. J. Guiry, Modern Reduction
Methods, ed. P. G. Andersson, and I. Munslow, Wiley-VHC, 2008, ch.
3, pp. 65–84.
Notes and References
1 J. T. Mohr, M. R. Krout and B. M. Stolz, Nature, 2008, 455, 323.
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R. Noyori, J. Am. Chem. Soc., 1980, 102, 7932.
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Salvadori, Synthesis, 1992, 503.
4 M. McCarthy and P. J. Guiry, Tetrahedron, 2001, 57, 3809.
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D. J. Connolly, Curr. Org. Chem., 2000, 4, 821; (b) C. P. Saunders and
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16 An excess of ligand is required to reduce the possibility of the formation
of a catalytic species for the non-selective reaction. This is discussed
further in reference 11a.
17 Crystal data and structure refinement for PdII complex (Sa,R)-
14. Empirical formula: C47H42N3F6P2ClPd. Formula weight 966.63.
˚
Temperature 100(2) K. Wavelength, 0.71073 A. Crystal, system:
˚
Monoclinic. Space group P 21. Unit cell dimensions a = 9.8195(9) A,
◦
◦
◦
˚
˚
a = 90 . b = 29.059(3) A, b = 95.039(2) . c = 15.0113(14) A. g = 90 .
3.
˚
Volume 4266.8(7) A Z = 4. Reflections collected 34128. Independent
reflections 19110 [R(int) = 0.0269]. Final R indices [I>2sigma(I)] R1 =
0.0408, wR2 = 0.1005.
18 Crystal data and structure refinement for CuI complex of 2c. Empirical
formula C66H52N4P2Cl4Cu2. Formula weight 1231.94. Temperature
293(2) K. Crystal, system: Triclinic. Space group P-1 (#2).Unit cell
◦
˚
˚
dimensions a = 10.7093(9) A a = 114.885(2) . b = 12.0684(11) A b =
◦
◦
3.
˚
˚
100.794(2) . c = 13.4541(12) A g = 96.372(2) . Volume 1514.0(2) A
Z = 1. Reflections collected 9554. Independent reflections 3953
[R(int) = 0.0228]. Final R indices [I>2sigma(I)] R1 = 0.0535, wR2 =
0.1572 R indices (all data) R1 = 0.0666, wR2 = 0.1572.
19 After acceptance of this paper we became aware of a further important
contribution to the enantioselective b-borylation of acyclic enones by
Yun and co-workers: H.-S. Sim, X. Feng and J. Yun, Eur. J. Org. Chem.,
2009, 15, 1939.
20 D. D. Perrin, and W. L. F. Armarego, Purification of Laboratory
Chemicals, Pergamon Press, New York, 1998.
21 W. C. Still, M. Hahn and M. Mitra, J. Org. Chem., 1978, 43, 2923.
8 M. McCarthy, M. W. Hooper and P. J. Guiry, Chem. Commun., 2000,
1333.
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