The application of Pd-complexes of diphenylphosphinoferrocenyloxazoline ligands to catalytic enantioselective allylic amination

Article abstract of DOI:10.1016/S0022-328X(00)00183-2

The preparation of cationic palladium (η³-C₃H₅) complexes, possessing an enantiopure diphenylphosphinoferrocenyloxazoline unit, is described. These complexes were applied to the enantioselective amination of the test substrate, ethyl (2E)-1,3-diphenylprop-2-enyl carbonate, with benzylamine in moderate to high conversions with enantioselectivities of up to 72% for the (S)-valinol derived oxazoline complex. Enantioselectivities of up to 37% were obtained for the (S)-tert-leucinol derived oxazoline complex.

Full text of DOI:10.1016/S0022-328X(00)00183-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2000) 110–115
The application of Pd-complexes of
diphenylphosphinoferrocenyloxazoline ligands to catalytic
enantioselective allylic amination
Yvonne M. Malone, Patrick J. Guiry *
Department of Chemistry, Uni6ersity College Dublin, Belfield, Dublin 4, Ireland
Received 25 January 2000; received in revised form 5 March 2000
Abstract
The preparation of cationic palladium (h3-C3H5) complexes, possessing an enantiopure diphenylphosphinoferrocenyloxazoline
unit, is described. These complexes were applied to the enantioselective amination of the test substrate, ethyl (2E)-1,3-diphenyl-
prop-2-enyl carbonate, with benzylamine in moderate to high conversions with enantioselectivities of up to 72% for the (S)-valinol
derived oxazoline complex. Enantioselectivities of up to 37% were obtained for the (S)-tert-leucinol derived oxazoline complex.
© 2000 Elsevier Science S.A. All rights reserved.
Keywords: Allylic amination; Diphenylphosphinoferrocenyloxazoline ligands; Palladium catalysis
1. Introduction
Helmchen obtained 94% ee with the phenylglycine-
derived diphenylphosphinooxazoline (2) [10] and Saigo
reported ees up to 97% with related ligands 3 [11].
Pregosin applied the binaphthalene-based diphenyl-
phosphinooxazoline (4) with ees up to 99% [12]. The
pyrazole-containing ferrocenylphosphinamine ligand 5
was reported by Togni to afford similarly excellent ees
of up to 99% [13,14] and they also noted a remarkable
anion effect with this system [15]. Recently Buono has
tested the quinolinephosphine (6) and obtained up to
93% ee [16].
Part of our research programme is devoted to the
design and application in asymmetric catalysis of biden-
tate phosphinamine ligands [17–22]. One of the ligand
classes, which ourselves, and others have prepared, is
the diphenylphosphinoferrocenyl oxazolines, repre-
sented by 7 and 8 [23–25], and these have been success-
fully applied in asymmetric catalysis by a range of
research groups [26]. We have recently used ligands 7
and 8 in the intermolecular asymmetric phenylation [27]
and cyclohexenylation [28] of 2,2-dimethyl-2,3-dihydro-
furan and obtained ees of up to 98%. We have also
applied these ligands in a synthetic and mechanistic
study in palladium-catalysed allylic alkylations in which
we obtained up to 92% ee [29]. We now report the
application of their palladium complexes to enantio-
The enantioselective alkylation of allylic acetates is a
carbon–carbon bond forming process catalysed by
transition metal complexes of palladium and has been
the subject of several recent reviews [1–5]. It is an
important transformation and has proved to be a useful
testing ground for the design of new ligands and for
gaining mechanistic insights into organopalladium
chemistry [6]. Although less well-studied, significant
progress has been made recently with the analogous
allylic amination process using palladium complexes
previously successful in allylic alkylation [7,8]. A selec-
tion of the ligands employed in allylic amination, with
the relevant ee values obtained in the standard reaction
between benzylamine and ethyl (2E)-1,3-diphenylprop-
2-enyl carbonate, is shown in Fig. 1. The ferro-
cenylphosphine ligand 1 tested by Hayashi and Ito gave
up to 97% ee [9]. Since that report numerous examples
of mixed phosphorous–nitrogen bidentate ligands have
been employed and tested with success in allylic amina-
tion. Such ligands induce asymmetry through a combi-
nation of steric and electronic interactions. Pfaltz and
* Corresponding author. Fax: +353-1-7062127.
E-mail address: p.guiry@ucd.ie (P.J. Guiry)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00183-2

Y.M. Malone, P.J. Guiry / Journal of Organometallic Chemistry 603 (2000) 110–115
111
Fig. 1.
selective allylic amination of ethyl (2E)-1,3-diphenyl-
prop-2-enyl carbonate.
palladium] tetrafluoroborate (9), prepared from the re-
action of di-m-chloro-bis(p-allyl)dipalladium, 1-[4-(S)-
iso-propyl-2-oxazolin-2-yl]-2-(S)-(diphenylphosphino)-
ferrocene (7) and sodium tetrafluoroborate, was con-
1
firmed by spectroscopy. The high field H-NMR spec-
trum was broad and showed the presence of only one
unsubstituted cyclopentadienyl ring at 3.83 ppm indi-
cating the existence of one of the two possible
diastereomeric endo- and exo-intermediates in solution
[nomenclature note: the exo-isomer is defined as that
allyl configuration in which the central allyl proton
points in the same direction as the C-4 substituent of
the oxazoline ring]. 31P-NMR showed a single peak at
17.5 ppm, which confirmed the presence of a single
diastereomer.
The tert-butyl-substituted analogue, [{1-[4-(S)-tert-
butyl-2-oxazolin-2-yl]-2-(S)-(diphenylphosphino) ferro-
cene}-[p-allyl]palladium] tetrafluoroborate (10), was
prepared in a similar manner. However, in this case the
1H-NMR spectrum indicated the presence of both the
exo- and endo-intermediates in a 12:1 ratio. As with
2. Results and discussion
In performing allylic substitutions it is possible to use
as the catalyst either pre-formed palladium (h3-allyl)
complexes of the ligand or to assume such complexes
are formed when a slight excess of ligand is added to
either Pd2(dba)3 or a h3-allyl palladium chloride dimer.
We prefer to use the pre-formed catalyst and hence we
prepared the air-stable h3-allyl palladium tetrafluoro-
borate salts (9 and 10), in 91 and 90% yields, respec-
tively (Scheme 1).
The structure of [{1-[4-(S)-iso-propyl-2-oxazolin-
2 - yl] - 2 - (S) - (diphenylphosphino)ferrocene} - [p - allyl]-
Scheme 1.

112
Y.M. Malone, P.J. Guiry / Journal of Organometallic Chemistry 603 (2000) 110–115
Scheme 2.
Table 1
Application of Pd-complexes 9 and 10 to the asymmetric allylic amination of carbonate 11
c
Entry
Catalyst (mol%)
Solvent
Time (d)
Temperature (°C)
% Yield a
% ee b (configuration
)
1
2
3
4
5
6
7
8
9
9 (2)
9 (2)
9 (3)
9 (3)
9 (3)
10 (2)
10 (2)
10 (3)
10 (3)
10 (2)
CH3CN
CH3CN
THF
THF
THF
CH3CN
CH3CN
THF
THF
THF
5.5
5.5
5.5
5.5
8
25
0
0
25
0
0
40
25
40
60
90
70
75
83
94
50
90
87
89
80
16 (R)
18 (R)
72 (R)
50 (R)
63 (R)
18 (R)
11 (R)
37 (R)
27 (R)
22 (R)
3
4.5
5.5
4.5
8
10
a Isolated yields.
b Enantiomeric excesses were determined by HPLC on a Daicel Chiracel OJ column: n-hexane–2-propanol 87:13; T=25°C; v/v: 0.5 ml min−1
(tR=18.3 (S) and 20.9 (R) min) for amine 12.
c Assignment is based on the sign of the optical rotation and comparison with Togni’s work [13].
complex 9 we are unable to unambiguously assign the
allyl cofiguration in this mixture although in a related
mechanistic study we have successfully assigned the
allyl configurations of the corresponding 1,3-dipheny-
lallyl complexes [29]. The unsubstituted cyclopenta-
dienyl ring of the major diastereomer appeared as a
singlet at 3.79 ppm whilst the corresponding peak oc-
curred at 3.85 ppm in the minor diastereomer. Further
evidence for the existence of two diastereomers was
obtained from 31P-NMR which showed peaks at 17.1
and 19.0 ppm.
Once prepared, we wished to test the asymmetry-in-
ducing ability of ligands 7 and 8 in what has become
one of the standard test reactions in allylic amination,
namely that between benzylamine and ethyl (2E)-1,3-
diphenylprop-2-enyl carbonate (11) (Scheme 2). The use
of diastereomeric allyl mixtures is acceptable in allylic
substitutions as the initial allyl is removed from the
metal by nucleophilic attack and subsequent olefin dis-
sociation. This generates a Pd(0) species which then
associates and oxidatively adds racemic allylic carbon-
ate (11), giving diastereomeric 1,3-diphenylallyl inter-
mediates which readily interconvert, thus allowing
racemic allylic carbonate the possibility to afford enan-
tiomerically enriched product. The results of our allylic
amination studies are given in Table 1.
the best solvent for the corresponding alkylation [29].
At ambient temperature the chemical yield of the
product amine (12) was high (90%) but the ee a disap-
pointingly low 16%, favouring the (R)-enantiomer. This
was increased slightly by lowering the temperature but
a lower yield of 70% was obtained (entry 2). Changing
the solvent to THF, the solvent of choice in previous
amination studies [10,13] gave similarly good chemical
yields but improved ees. Our optimal ee obtained was
72% when the reaction was carried out at 0°C in THF
(entry 3). Carrying out the reaction at ambient temper-
ature gave a higher yield but lowered ee of 50% (entry
4). Increasing the reaction time at 0°C gave 12 in 63%
ee and 94% yield. The use of acetonitrile as solvent with
the bulkier tert-butyl containing palladium complex 10
gave poor ees of 8–11% in moderate to good yield
(entries 6, 7). A change of solvent again to THF proved
beneficial in terms of reactivity with yields of 80–89%
being observed. However, the best ee obtained with
complex 10 in THF was 37% (entry 8). The palladium
complexes of the analogous diphenylphosphinooxazoli-
nes (2) [R=i-Pr, t-Bu] gave higher ees of 87 and 88%,
respectively, again favouring the (R)-enantiomer [10].
A model for the reaction transition-state to explain
the preferred formation of the (R)-enantiomer in our
work can be proposed. The model is based on the
mechanistic studies of Brown [30], Pfaltz [31], Togni
[32] and Helmchen [33] on the related allylic alkylation
The iso-propyl substituted diphenylphosphinoferro-
cenyloxazoline complex 9 was first tested in acetonitrile,

Y.M. Malone, P.J. Guiry / Journal of Organometallic Chemistry 603 (2000) 110–115
113
3. Experimental
employing palladium phosphinamine complexes. They
suggest that the ground state of of the 1,3-diphenylallyl
complex contains an allyl which has reoriented itself
into a product-like geometry, thus facilitating nucle-
ophilic attack on that allyl terminal carbon trans to the
phosphorus donor atom. The labilty of the 1,3-dipheny-
lallyl complexes is influenced by such electronic factors
and also by intra-complex steric clashes between
the ligand and the allyl. Attack trans to phosphorus
of the endo-diastereomer 13a engenders unwanted steric
interactions between the oxazoline C-4 substituent
and the phenyl group of the diphenylallyl as it rolls
into a product-like geometry to afford (S)-12 after
olefin dissoaciation, Scheme 3. Attack trans to phos-
phorus of the exo-diastereomer (13b), formed from 13a
via a p–s–p mechanism, is not as sterically disfa-
voured thus leading to the enantiomer formed in excess,
(R)-12.
It is clear from our amination results that these
ligands do not afford enantioselectivities as high as
those observed in the analogous alkylations. In both
amination and alkylation we assume that the ligand
electronic disparity favours attack of the nucleophile at
the allylic carbon trans to phosphorus. Therefore, the
different enantioselectivities must be due to the differ-
ent reactivities of the nucleophiles involved with the
diastereomeric intermediates (13a and 13b), with benzyl-
amine clearly being less selective.
Solvents were dried immediately before use by distil-
lation from standard drying agents and subjected to
degassing by three freeze–thaw cycles. Benzylamine
was commercially available (Aldrich Chemical Co.) and
was used as purchased. Pd salts were obtained on loan
from Johnson Matthey. Di-m-chloro-bis(p-allyl)dipalla-
dium [34], ligands 7 and 8 [24,25], ethyl-(E)-1,3-
diphenyl-2-propenyl-carbonate [9] were prepared by lit-
erature procedures. Separation by column chroma-
tography was performed using Merck Kieselgel 60 (Art.
7734).
NMR spectra were recorded on a Jeol 270 MHz or a
Varian Unity 500 MHz spectrometer. 1H chemical
shifts are reported in l ppm relative to CHCl3 (7.27
ppm), 13C chemical shifts are reported relative to the
central peak of CDCl3 (77.0 ppm), and 31P chemical
shifts are reported relative to 85% aqueous phosphoric
acid (0.0 ppm). Elemental microanalyses were carried
out in house using a Carlo Erba 1106 elemental analy-
ser. Electrospray mass spectra were recorded on a VG
(Micromass) Quattro with electrospray probe. IR spec-
tra were recorded on a Perkin–Elmer Paragon 1000 FT
spectrometer. Optical rotations were recorded on a
Perkin–Elmer 241 polarimeter. Melting points were
recorded on a Gallenkamp melting point apparatus and
are uncorrected.
In conclusion, we have prepared new palladium com-
plexes of diphenylphosphinoferrocenyloxazoline ligands
7 and 8 and applied them with modest enantioselectivi-
ties in a test allylic amination. Further work will be
disclosed on their application in allylic alkylation and
on our NMR studies of substituted allyl complexes of
these ligands [29].
3.1. Preparation of [{1-[4-(S)-iso-propyl-2-
oxazolin-2-yl]-2-(S)-(diphenylphosphino)ferrocene}-
[y-allyl] palladium]tetrafluoroborate, (9)
Di-m-chloro-bis(p-allyl)dipalladium (0.013 g, 0.032
mmol), 1-[4-(S)-iso-propyl-2-oxazolin-2-yl]-2-(S)-(di-
phenylphosphino)ferrocene (0.036 g, 0.064 mmol), and
Scheme 3.

114
Y.M. Malone, P.J. Guiry / Journal of Organometallic Chemistry 603 (2000) 110–115
sodium tetrafluoroborate, (0.022 g, 0.2 mmol), were
placed in a Schlenk under nitrogen. Dried and degassed
chloroform was added via syringe to give an orange
suspension which was stirred at ambient temperature
for 24 h. The solid was removed by filtration and the
solvent was removed in vacuo to give diastereomeri-
cally pure [{1-[4-(S)-iso-propyl-2-oxazolin-2-yl]-2-(S)-
(diphenylphosphino)ferrocene} - [p - allyl]palladium] te-
trafluoroborate (0.42 g, 91%), as a brown solid, m.p.
118–120°C; [h]2D3−35 (c 0.33, chloroform); (Found: C,
51.86; H, 4.46; N,1.81. C31H33NOPFePdBF4 requires C,
52.03; H, 4.62; N, 1.96%); wmax (KBr) 1620 (CꢀN), 1054
(BꢁF) cm−1; lH (270 MHz, CDCl3) 1.01 (3H, br s,
(CH3)), 1.09 (3H, br s, (CH3)), 1.7 (1H, br s, allyl), 2.4
(1H, s, CH(CH3)2), 3.01 (1H, d, J 11.7, ꢁOCH2), 3.6
(1H, s, allyl), 3.83 (5H, s, ꢁC5H5), 4.08 (1H, d, J 6.45,
ꢁOCH2), 4.33 (br s, allyl), 4.51 (br s, allyl), 4.65 (1H, br
s, ꢁFc), 4.91 (1H, br s, ꢁFc), 5.18 (1H, br s, ꢁFc), 5.4
(m, CHN), 5.9 (br s, allyl), 7.08 (2H, br m, Ph), 7.35
(3H, br s, Ph), 7.68 (3H, br s, Ph), 7.9 (2H, br s, Ph); lC
(67.5 MHz, CDCl3) 15.07 (CH3), 19.01(CH3), 30.69
(CH(CH3)2), 68.8 (ꢁOCH2), 71.96 (C5H5), 72.00 (J 6.4,
ipso-Fc), 74.26 (J 7.5), 75.56, 75.88, 76.32, 77.24,
128.85, 128.9, 129.44, 129.61, 130.34, 130.95 (J 11.8),
132.65, 132.68, 135.04 (J 15.1) and 171.2 (CꢀN); lP
(109.3 MHz, CDCl3) 17.5, 18.5; m/z (ESI/pos in
CH3OH) 628 MꢁBF4.
7.42–7.32 (4H, m), 7.74–7.65 (3H, br m), 8.3–7.86
(2H, v br s, 2H); lC (67.5 MHz, CDCl3) 26.47((CH3)3),
34.33 (ꢁC(CH3)3), 70.44 (ꢁOCH2), 71.99 (C5H5), 74.47,
76.63, 128.81, 128.97, 129.35, 129.52, 130.31, 131.1,
132.89, 135.48; lP (109.3 MHz, CDCl3) 17.1, 19.0; m/z
(ESI/pos in CH3OH) cation 642 MꢁBF4.
3.3. General procedure for allylic amination
Ethyl-(2E)-1,3-diphenyl-2-propenyl carbonate (0.078
g, 0.276 mmol) in dry and degassed acetonitrile (0.5 ml)
was added to a solution of [{1-[4-(S)-trisubstituted-2-
oxazolin-2-yl]-2-(S)-(diphenylphosphino) ferrocene}-[p-
allyl]palladium]tetrafluoroborate (2 mol%) in dry and
degassed acetonitrile (0.5 ml). To this was added benzy-
lamine (0.037 ml, 0.331 mmol) and the reaction was
stirred at the relevant temperature under a nitrogen
atmosphere. Reaction progress was monitored by TLC
(hexane:ethyl acetate; 3:1 as the eluent). The reaction
was purified directly using silica-gel plates (eluent=
hexane–ethyl acetate, 3:1) to afford (R)-N-benzyl-(1,3-
diphenyl-1,2-propenyl)amine (12) as
a
clear oil,
Rf=0.55 (hexane:ethyl acetate; 3:1); wmax (Nujol); lH
(270 MHz, CDCl3) 1.77 (1H, br s, NH), 3.77 (2H, AB,
J 13.4, CH2Ph), 4.38 (1H, d, J1,2 7.3, H1), 6.3 (1H, dd,
J1,2 7.3 J2,3 15.7, H2),. 6.57 (1H, d, J2,3 15.8, H3) and
7.44–7.16 (15H, m, ꢁPh); m/z 299 (M+, 9%), 208 (40),
115 (28), 91 (100) and 77 (30). Enantiomeric excesses
were determined by HPLC using a Daicel Chiralcel OJ
column and eluting with a hexane–i-PrOH mixture
(87:13 v/v, 0.5 ml min−1; T=25°C, retention times:
(S), 18.3 min; (R), 20.9 min).
3.2. [{1-[4-(S)-tert-butyl-2-oxazolin-2-yl]-2-(S)-
(diphenylphosphino)ferrocene}-[y-allyl]palladium]
tetrafluoroborate, (10)
Di-m-chloro-bis(p-allyl)dipalladium (0.008 g, 0.02
mmol), 1-[4-(S)-tert-butyl-2-oxazolin-2-yl]-2-(S)-(diphe-
nylphosphino)ferrocene (0.02 g, 0.04 mmol), and
sodium tetrafluoroborate, (0.013 g, 0.121 mmol), were
placed in a Schlenk under nitrogen. Dried and degassed
chloroform was added via syringe to give an orange
suspension which was stirred at ambient temperature
for 24 h. The solid was removed by filtration and the
solvent was removed in vacuo to give [{1-[4-(S)-tert-
Acknowledgements
We thank University College Dublin for a startup
grant, Zeneca UK for a New Researchers grant to PJG
and Dr John Blacker of Avecia Ltd for his interest in
this work. We thank Enterprise Ireland for a Research
Scholarship (BR/941024) to support YMM. The award
of the 1996 BOC Gases Postgraduate Bursary to YMM
and the loan of Pd salts from Johnson Matthey are
gratefully acknowledged. Many thanks to Mr Patrick
Lacey for a critical reading of this manuscript.
butyl-2-oxazolin-2-yl]-2-(S)-(diphenylphosphino)
fer-
rocene}-[p-allyl]palladium] tetrafluoroborate (0.02 g,
90%), as a brown solid, m.p. 150–152°C; (Found: C,
52.41; H, 4.53; N, 1.89. C32H35NOPFePdBF4 requires
C, 52.67; H, 4.79; N, 1.92%); wmax (KBr) 1616 (CꢀN),
1054 (BꢁF) cm−1; lH (270 MHz, CDCl3) major
diastereomer 1.13 (9H, s, (CH3)3), 1.7 (1H, br s, allyl),
3.03 (1H, br s, allyl), 3.28 (1H, br s, allyl), 3.79 (5H, s,
ꢁC5H5), 4.03 (1H, br s, allyl), [4.36 (1H, dd, J 4.2 9.04),
4.59 (1H, m), 4.62 (1H, br s) ꢁOCH2, CHN], 4.57 (1H,
s, ꢁFc), 4.88 (1H, s, ꢁFc), 5.19 (1H, s, ꢁFc), 5.68 (1H,
br s, allyl), minor diastereomer 1.33 (9H, s, (CH3)3), 2.88
(1H, br s, allyl), 3.85 (5H, s, ꢁC5H5), 4.22 (1H, d, J 8.9,
allyl), [4.39 (1H, s), 4.8 (1H, t) ꢁOCH2, CHN], major
and minor diastereomers 7.12 (app t, J 10.15 7.33),
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