Tuning ligand structure in chiral bis(phosphite) and mixed phosphite-phosphinite PCP-palladium pincer complexes

Article abstract of DOI:10.1039/c1dt10356c

A range of chiral resorcinol bis(phosphite) and phosphite-phosphinite ligands were produced and their propensity to form palladium PCP-pincer complexes examined. The ease of base-assisted C-H palladation of the ligands falls in the order bis(phosphinite)

Full text of DOI:10.1039/c1dt10356c

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Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
Published in issue 35, 2011 of Dalton Transactions
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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
ARTICLES:
Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta
Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Tuning ligand structure in chiral bis(phosphite) and mixed
phosphite–phosphinite PCP-palladium pincer complexes†
Robin B. Bedford,* Yu-Ning Chang, Mairi F. Haddow and Claire L. McMullin
Received 2nd March 2011, Accepted 21st June 2011
DOI: 10.1039/c1dt10356c
A range of chiral resorcinol bis(phosphite) and phosphite–phosphinite ligands were produced and their
propensity to form palladium PCP-pincer complexes examined. The ease of base-assisted C–H
palladation of the ligands falls in the order bis(phosphinite) > phosphite–phosphinite > bis(phosphite).
The catalytic activity of the complexes in the asymmetric allylation of benzaldehyde with allyl
tributyltin was examined and it was found that, contrary to expectations, ligands with 3,3¢-disubstituted
BINOL residues show poorer activity and stereoselectivity than unsubstituted BINOL analogues. In
addition the order of activity of the pincer complexes was established as bis(phosphite) >
phosphite–phosphinite > bis(phosphinite). Crystal structures of representative examples of a
3,3¢-disubstituted BINOL, mono- and bis(phosphite) ligands based on 2,4-di-tert-butyl resorcinol and
Pd complexes of two of the chiral complexes are presented.
ral analogues show moderate to good enantioselectivity in
the asymmetric C–H functionalisation of benzyl nitriles with
sulfonimines,12c the asymmetric condensation of sulfonimines and
isocyanoacetate,15 and the allylation of sulfonimines with allyl
stannane or tifluoroborate.12b
We previously reported the synthesis of PCP-complexes, 3,
based on a range of chiral bis(phosphites), and tested them for
activity and selectivity in the asymmetric allylation of benzalde-
hyde with allyl tributyltin.13 Of the catalysts tested only that based
on a BINOL derived bis(phosphite) – complex 3a –showed good
activity and reasonable enantiocontrol (up to 62% ee).
Introduction
Palladium bis(phosphinite) PCP-pincer complexes, 1, display a
wide range of catalytic activities.1 For instance, they have been
exploited as pre-catalysts in Heck2 and Suzuki reactions,3 the
Negishi coupling of zinc acetylides with aryl chlorides,4 allylic
alkylation,5 the allylation of aldehydes or activated imines with
allyl stannanes or borates,6,7 the a-arylation of ketones,8 the car-
boxylation of allyl stannanes,9 the condensation of sulfonimines
and isocyanoacetate10 and the C–H functionalisation of allyl
nitriles.11
By contrast, the use of palladium pincer complexes based on
bis(phosphite) ligands has been less well explored, in part due
to the greater difficulty in synthesising these complexes. The
synthetic difficulties have recently been addressed by either using
2-haloresorcinol backbones for the bis(phosphite) ligand, which
then undergo oxidative addition at a Pd(0) centre,12 by introducing
bulky groups onto the resorcinol, which facilitates base-assisted
C–H activation at a palladium(II) centre, or by using microwave
heating.13
Encouraged by these preliminary results we were keen to
see whether modifying the catalyst structure would give an
improvement in performance. We envisaged this tuning could
be brought about by controlling the catalyst architecture as
delineated in Fig. 1; stereocontrol could potentially be optimised
by varying the nature of the groups ‘R’ in 3,3¢-disubstituted
BINOL whilst varying the second donor ‘L’ should allow steric
and electronic control of the coordination sphere. We previously
found incorporating tert-butyl groups on the resorcinol backbone
significantly eases the formation of palladium pincer complexes
by preventing the formation of oligomers – which require several
The resultant palladium bis(phosphite) pincer complexes, 2,
have been used in catalytic C–P bond-formation14 while chi-
School of Chemistry, University of Bristol, Cantocks Close, Bristol, UK,
BS8 1TS. E-mail: r.bedford@bristol.ac.uk
† CCDC reference numbers 815801–815806. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10356c
9034 | Dalton Trans., 2011, 40, 9034–9041
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The Royal Society of Chemistry 2011
©

Fig. 1 Approach to ligand tuning.
days sustained heating to break – that are otherwise obtained.
This is because the tert-butyl groups force the phosphite residues
over the palladium centre and away from the resorcinol backbone,
favouring monomeric or dimeric complex formation. This steric
control also leads to greater enantioselectivity as a result of
restricted conformational freedom of the BINOL residues.13
Therefore this substitution pattern was maintained in all the
structures examined below.
Results and discussion
Chiral ligand synthesis
With the design protocol outlined in Fig. 1 in mind we decided
to synthesise bis(phosphites) with varied steric bulk in the 3,3¢
positions, accordingly we synthesised the known S-BINOL-based
chlorophosphites 4a and b (R = Me and SiMe3 respectively) and
the far bulkier, silylated analogues c and d (R = SiMePh2 and SiPh3
respectively). The chlorophosphites 4 were synthesised from the
corresponding diols 5. These were in turn prepared by the 3,3¢-
dilithiation of MOM-protected BINOL followed by quenching
with the appropriate electrophile to give the intermediates 6 from
which the MOM groups were subsequently removed (Scheme 1).
1
The chlorophosphites 4 were characterised by H and 31P NMR
spectroscopy; the latter showed characteristic singlets in the range
Scheme 1 Conditions: BuLi, THF, 0 ◦C–r.t., 1 h, then RX 0 ◦C–r.t.,
18 h. (ii) Amberlyst 15, THF/MeOH, reflux 48–72 h. (iii) PCl3, Et3N,
THF, r.t. 30 min. (iv) 2,4-di-tert-butylresorcinol, Et3N, THF, r.t., 1 h. (v)
2,4-di-tert-butylresorcinol, Et3N, toluene (or 1,2-dichloroethane for 8c),
r.t., 16–72 h (60 ◦C, 16 h for 8c). (vi) ClPPh2, Et3N, toluene, r.t., 18 h.
d 174.4 – 182.2 ppm.
The reaction of the chlorophosphite 4a with 2,4-di-tert-
butylresorcinol yielded the desired resorcinol bis(phosphite) lig-
and 7 in 88% yield. It was characterised both spectroscopically –
the 31P NMR spectrum showed a singlet at d 142.4 ppm – and by
single crystal X-ray diffraction (Fig. 2). From this it can be seen
that the tert-butyl groups repel the phosphite groups, forcing them
into a conformation primed for bidentate coordination to a metal
centre.
In stark contrast to the facile synthesis of ligand 7 we were
unable to generate the more sterically encumbered analogues
by equivalent reactions of 4b–d with 2,4-di-tert-butylresorcinol.
Instead the mono-phosphite based ligands 8a–c were produced.
Replacing triethylamine with BuLi in the reaction of 4d again
proved ineffective in the synthesis of the bis(phosphite). Presum-
ably the desired bis(phosphite) ligands are too crowded to be
synthetically viable, at least using the methodologies reported here.
Indeed an idea of the degree of hindrance in these systems can
be gleaned by examining the single crystal X-ray structure of the
MOM-protected diol 6d (Fig. 3), which clearly shows the crowding
about the oxygen functions.
Fig. 2 The X-ray structure of ligand S-7. Thermal ellipsoids set at 30%
probability.
1
similar to that of the bis(phosphite) ligand 7, while H NMR
The 31P spectroscopic data for the monophosphinite ligands
8a–c (singlets at d 139.3, 146.3 and 148.8 ppm, respectively) are
spectroscopy showed peaks corresponding to the phenolic protons
at d 3.65, 3.98 and 3.67 ppm, respectively. The presence of only one
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Dalton Trans., 2011, 40, 9034–9041 | 9035
©

Fig. 3 X-ray structure of S-6d. Thermal ellipsoids set at 30% probability.
phosphite residue per resorcinol was confirmed by X-ray structural
analyses of 8a and b, the results of which are shown in Fig. 4 and
5, respectively.
Fig. 5 X-ray structure of ligand S-8b. Thermal ellipsoids set at 30%
probability.
Fig. 4 X-ray structure of ligand S-8a. Thermal ellipsoids set at 30%
probability.
The monophosphites 8 represent interesting ligands in their
own right and the following paper describes their coordination to
palladium and the catalytic activity of the resultant palladacycles.
However, for the purposes of producing PCP-pincer complexes,
we next examined their use as synthons for mixed phosphite–
phosphinite ligands (Scheme 1). Gratifyingly 8a–c reacted with
chlorodiphenylphosphine in the presence of triethylamine to
generate the novel phosphite–phosphinite pincer ligands 9a–
c, respectively. The 31P NMR spectroscopy of ligands 9 was
again informative with distinct environments observed for the
phosphite (d 147.6, 149.1 and 154.2 ppm for 9a–c, respectively) and
phosphinite (d 109.5, 108.2 and 106.7 ppm, respectively) residues.
Scheme 2 Conditions: (i) Ligand 7, Et3N, 1,2-dichlorethane, 80 ◦C,
2 h. (ii) Ligand 9, Et3N, 1,2-dichlorethane, r.t., 18–24 h. (iii) Et3N,
1,2-dichlorethane, r.t., 1 h.
◦
80 C. By contrast, BINOL-based bis(phosphite) pincer ligands
Complex synthesis
with unsubstituted resorcinol backbones either require heating
for 6 days or the use of microwave heating in order to break the
insoluble oligomers formed initially.13
The X-ray structure of complex 3b is given in Fig. 6. The metal
complex has crystallographic twofold symmetry with Pd1, C1, Cl1
and C4 on a twofold axis and clearly shows that two C2-related
methyl groups in the 3,3¢-positions of the BINOL residues project
With pincer ligands 7 and 9a–c in hand, we next explored their
reactions with palladium. As anticipated, the incorporation of
tert-butyl groups into the resorcinol backbone of the ligands
and the use of triethylamine as base did indeed facilitate C–H
activation (Scheme 2) and the bis(phosphite) complex 3b could be
obtained in good yield within two hours in 1,2-dichlorethane at
9036 | Dalton Trans., 2011, 40, 9034–9041
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©

Fig. 6 X-ray structure of complex 3b. Diethyl ether solvate omitted
for clarity. Thermal ellipsoids set at 30% probability. Operator for
symmetry-generated atoms with ‘A’: x, -y, 1-z.
Fig. 7 X-ray structure of complex 10c. Phenyl residues on Si atoms and
diethyl ether solvate omitted for clarity. Thermal ellipsoids set at 30%
probability.
into the space above and below the palladium; the region where
substrates are likely to approach the metal centre, effectively giving
a better defined chiral ‘pocket’ than in the comparable complex 3a
with unsubstituted BINOL residues.13
represented a good test-bed reaction to assess the impact of
structural variation on catalyst performance and stereoselectivity.
The data from this study are summarised in Table 1.
Interestingly the synthesis of the mixed phosphite–phosphinite
palladium PCP-pincer complexes 10a–c could be conducted at
room temperature giving good yields within one day. This sug-
gested that the introduction of a phosphinite ligand eases the C–H
activation significantly with respect to bis(phosphite) ligands. In
order to confirm whether phosphinite residues support more facile
C–H functionalisation we synthesised the bis(phosphinite) ligand
11 from di-tert-butylresorcinol and chlorodiphenylphosphine and
found that it underwent rapid reaction with [PdCl2(NCMe)2] to
give complete conversion to complex 12 within 1 h at room
temperature in the presence of triethylamine.
In view of the better defined chiral ‘pocket’ in complex
3b compared with complex 3a,13 which contains unsubstituted
BINOL residues, it was disappointing that complex 3b showed not
only poorer activity (3a gave 82% yield under identical conditions),
as might be expected due to greater steric hindrance in the coor-
dination sphere, but also significantly poorer enantioselectivity
(entry 1, Table 1). Running the reaction for longer (entry 2, Table
1) gave improved yield but essentially led to a racemic product.
Similarly, better activity was seen at room temperature, but at the
expense of complete loss of stereoselectivity (entry 3, Table 1).
The mixed phosphite–phosphinite complexes 10a–c all showed a
sharp drop in catalyst activity (entries 4, 5 and 7, Table 1) compared
with the bis(phosphite) complex 3b, implying that increasing the
electron-density at the palladium centre is deleterious to the
reaction. Indeed when the more electron-rich bis(phosphinite)
complex 12 was examined (entry 9, Table 1) then almost no
The 31P NMR spectrum of the bis(phosphite) complex 3b and
the bis(phosphinite) complex 12 showed singlets with surprisingly
similar chemical shifts at d 145.2 and 144.8 ppm respectively,
despite the very different substitution patterns on the phosphorus
residues. The mixed phosphite–phosphinite complexes 10a–c
2
showed AB systems in their 31P NMR spectra with trans JPP
Table 1 Pincer-catalysed allylation of benzaldehydea
coupling constants of 670–681 Hz, somewhat larger than the
trans coupling seen in phosphine adducts of orthopalladated
triarylphosphites.16 A consequence of the surprising closeness of
the 31P NMR signals of the phosphite and phosphinite residues
was that the ‘AB’ patterns displayed a significant ‘roofing’ effect,
such that the ratio of ‘outside’ to ‘inside’ lines recorded varied
between approximately 1 : 18 for complex 11a and 1 : 125 for 11c.
The structure of complex 10c was confirmed by single crystal
X-ray analysis and the molecular structure is shown in Fig. 7.
Pincer
complex
Spec.
entry
Temp.
Time, h
yield,%b
e.e.,%c
1
2
3
4
5
6
7
8
9
3b
0 ◦
0 ◦
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
C
C
18
48
18
18
72
72
120
24
18
22
57
63
<1
25
34d
4
35
1
2
n.d.
18
6
n.d.
1
Catalysis
10a
10b
It was previously shown that palladium PCP-pincer complexes
based on both bis(phosphinite) and bis(diamidophosphinite)
ligands complexes show no activity in the arylation of enones with
arylboronic acids,17 therefore it was not surprising that complex
3b showed no activity in the reaction of phenylboronic acid with
cyclohexenone.18 By contrast, complex 3a has been shown to give
good activity and modest enantioselectivity (up to 62% ee) in
the addition of allyl tributyltin to benzaldehyde,13 this therefore
10c
12
50d
<1
—
a Conditions: 5 mol% Pd catalyst loading, CH2Cl2. b Determined by
1H NMR spectroscopy (1,3,5-trimethoxybenzene internal standard).
c Determined by chiral HPLC, R-isomer obtained in all cases. d Under
air.
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Dalton Trans., 2011, 40, 9034–9041 | 9037
©

activity was observed. Interestingly, slightly better activity was
observed when complexes 10b and c were used under air than
under nitrogen (compare entries 6 and 8 with 5 and 7, Table
1), however the eroded enatioselectivities observed under aerobic
conditions probably indicates that the catalysts underwent partial
decomposition to more active but less selective species.
flash column chromatography (silica gel, ethyl acetate/hexane
eluent). 6c: yield: 1.42 g (69.5%). Anal. Calcd for C50H46O4Si2:
C, 78.29; H, 6.04. Found: C, 78.04; H, 6.26. 1H NMR (400 MHz,
CDCl3) d 1.00 (s, 6, SiPh2Me), 2.47 (s, 6, OCH3), 3.97 (d, 2, J = 4.8
Hz, OCH2O), 4.03 (d, 2, J = 5.0 Hz, OCH2O), 7.28–7.43 (m, 18,
ArH), 7.59–7.61 (m,4, ArH), 7.65–7.67 (m, 4, ArH), 7.72–7.74 (m,
2, ArH), 7.87 (s, 2, ArH). 6d: Yield: 1.80 g (75.5%). Anal. Calcd
for C60H50O4Si2: C, 80.86; H, 5.65%. Found: C, 80.65; H, 6.14%.
1H NMR (400 MHz, CDCl3) d 2.27 (s, 6, OCH3), 3.79 (d, 2, J =
5.1 Hz, OCH2O), 3.88 (d, 2, J = 5.1 Hz, OCH2O), 7.33–7.48 (m,
24, ArH), 7.64–7.73 (m, 14, ArH), 7.90 (s, 2, ArH).
Conclusions
In summary, attempts to produce chiral 2,4-di-tert-butyl resorcinol
bis(phosphite) pincer ligands based on 3,3¢-disubstituted-BINOL
groups were hampered by unfavourable steric interactions, with
only the pincer based on 3,3¢-dimethyl BINOL proving accessible.
However, bulkier monophosphite residues could be introduced
and subsequent reactions with chlorodiphenylphosphine yielded
chiral mixed phosphite–phosphinite pincer ligands. The synthesis
of palladium PCP-complexes of all of these ligands by base-
assisted C–H activation was comparatively straightforward with
the ease of formation following the order bis(phosphinite) >
phosphite–phosphinite > bis(phosphite). A brief investigation
into the use of these complexes as catalysts in the allylation of
benzaldehyde with allyl tributyltin showed that in all cases the
yields and enantioselectivities obtained were lower than with an
unsubstituted BINOL-based analogue.
Preparation of 5c and 5d. A mixture of 6c or d (1.11 and
1.88 mmol respectively) and Amberlyst 15 resin (1.1 g per mmol
substrate) in THF : MeOH (1 : 1, 30 ml) was stirred and heated
at reflux temperature for 48 h, then cooled to room tempera-
ture. The resin was removed by filtration and the filtrate was
concentrated by rotary evaporation. The products were purified
by column chromatography (silica). 5c: Purification by column
chromatography (EtOAc : hexane 1 : 5 eluent) afforded an off-
white solid. Yield: 0.616 g (81.5%). 1H NMR (400 MHz, CDCl3):
d 0.98 (s, 6, SiMePh2), 5.23 (s, 2, OH), 7.16–7.20 (m, 2, ArH),
7.29–7.46 (m, 16, ArH), 7.58–7.62 (m, 8, ArH), 7.73–7.77 (m, 2,
ArH), 7.87 (s, 2,ArH). 5d: Purification by column chromatography
(EtOAc : hexane = 1 : 5) to afforded a white solid. Yield: 1.37 g
1
(90.5%). H NMR (400 MHz, CDCl3): d 5.35 (s, 2, OH), 7.28–
Experimental
7.53 (m, 24, ArH), 7.65–7.72 (m, 12, ArH), 7.73–7.76 (m, 2, ArH),
7.96 (s, 2, ArH).
General notes
Preparation of 4c and d. A mixture of 5c or d (1.03 and
0.55 mmol respectively) and phosphorus trichloride (1 equivalent)
in THF (6 ml) was treated with NEt3 (2.2 equiv.) and stirred at
room temperature for 30 min. The resultant mixture was filtered
through Celite and the volatiles removed under reduced pressure
to give the title compound as a white solid which was used without
further purification. 4c: 0.634 g (83%). 1H NMR (300 MHz,
CDCl3): d 1.05 (s, 6, SiMePh2), d 1.06 (s, 6, SiMePh2), 7.23–7.60
(m, 20, ArH), 7.63–7.71(m, 6, ArH), 7.80–7.89 (m, 2, ArH), 7.99
All reactions were carried out under a nitrogen atmosphere in
either a glove box or using standard Schlenk techniques. Dry N2-
saturated solvents were collected from a Grubbs solvent system.19
All other chemicals were used as received unless otherwise stated.
All NMR spectra were recorded using a Jeol GX270, Jeol GX400,
Jeol ECP300 and Jeol Lambda 300 spectrometers. HPLC analysis
was carried out using a Varian Prostar 210 solvent delivery
module equipped with a Varian 320 UV/Vis detector fitted with a
Chiralcel OD column (0.46 ¥ 25 cm) and using STAR workstation
software (v. 6.41). 2,2¢-Bis(methoxymethoxy)-1,1¢-binaphthyl, 6a
and b,20 5a and b,21,22 and 4a and b23,24 were prepared by literature
methods. ESI and EI mass spectra were recorded on VG Analytical
Quattro and VG Analytical Autospec spectrometers respectively.
Microanalyses were carried out by the Microanalytical Laboratory
of the School of Chemistry at the University of Bristol.
1
(s, 1, ArH), 8.06(s, 1, ArH). 31P{ H} NMR (121.5 MHz, CDCl3): d
178.2 (s). 4d: Yield: 0.435 mg (91%). 1H NMR (300 MHz, CDCl3):
d 7.20–7.45 (m, 23, ArH), 7.56–7.66 (m, 13, ArH), 7.76–7.85 (m,
1
2, ArH), 8.01 (s, 1, ArH), 8.11 (s, 1, ArH). 31P{ H} NMR (121.5
MHz, CDCl3): d 182.2 (s).
Preparation of ligand 7. A solution of 2,4-di-tert-butyl resorci-
nol (0.10 g, 0.45 mmol) and 4a (0.341 g, 0.9 mmol) in THF (20 ml)
was treated with NEt3 (0.88 ml, 6.3 mmol) dropwise. The reaction
mixture was stirred at room temperature for 1 h. The mixture
was filtered through Celite, the volatiles removed under reduced
pressure and the product was purified by column chromatography
(silica, CH2Cl2 : hexane 1 : 1 eluent) to afford 7 as a white solid.
Yield: 0.36 g (88%). Crystals suitable for X-ray analysis were grown
from CH2Cl2/hexane. Anal. Calcd for C58H52O6P2: C, 76.81; H,
Preparation of 6c and d. To
a solution of (S)-2,2¢-
bis(methoxymethoxy)-1,1¢-binaphthyl (1.0 g, 2.67 mmol) in THF
(20 ml) at 0 ◦C was added n-BuLi (5.34 ml, 1.5 M, 8.02 mmol)
dropwise over 10 min. The resulting suspension was stirred at
room temperature for 1 h during which time a colour change from
yellow to brown was observed. The mixture was re-cooled to 0 ◦C
and after 15 min at this temperature a solution of the appropriate
chorosilane (8.0 mmol) in THF (10 mL) was added. The reaction
mixture was allowed to warm to room temperature and stirred
for 18 h, during which time the colour changed from brown,
through green to pale yellow. The reaction was then quenched by
the addition of aqueous saturated NH4Cl solution and the organic
component extracted into CH2Cl2. The organic layer was washed
with brine and dried over MgSO4, filtered and concentrated to
yield the crude product as viscous yellow oil which was purified
1
5.78%. Found: C, 76.59; H, 6.00%. 31P{ H} NMR (121.5 MHz,
1
CDCl3): d 142.4 (s). H NMR (300 MHz, CDCl3): 1.33 (s, 18,
C(CH3)3), 2.59 (s, 6, ArMe), 2.61 (s, 6, ArMe),7.16–7.24 (m, 4,
ArH), 7.28–7.45 (m, 10, ArH), 7.76–7.88 (m, 8, ArH). MS (ESI)
m/z: 907.35 (M+H+), 929.33 (M+Na)+.
Preparation of 8a. A solution of 2,4-di-tert-butyl resorcinol
(0.204 g, 0.92 mmol) and 4b (0.453 g, 0.92 mmol) in toluene (20 ml)
9038 | Dalton Trans., 2011, 40, 9034–9041
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The Royal Society of Chemistry 2011
©

at room temperature was treated with NEt3 (0.9 ml, 6.4 mmol)
dropwise and the resultant mixture stirred for 3 days. The mixture
was filtered through Celite, the volatiles removed under reduced
pressure and the product purified by column chromatography
(silica, CH2Cl2 : hexane 2 : 3 eluent) to afford 8a as a white solid.
Yield: 0.52 g (83%). Crystals suitable for X-ray analysis were grown
from diethyl ether. Anal. Calcd for C40H49O4PSi2: C, 70.55; H,
Preparation of ligand 9b. As for 9a with 8b (0.280 g, 0.30 mmol)
and PPh2Cl (0.066 g, 0.30 mmol) to give 9b as a white solid (0.294 g,
1
80%), which was used without further purification. 31P{ H} NMR
1
(121.5 MHz, CDCl3): d 108.2 (s), 149.1 (br, s). H NMR (300
MHz, CDCl3): 0.69 (s, 3, SiPh2Me), 085 (s, 9, C(CH3)3), 087 (s, 3,
SiPh2Me), 1.37 (s, 9, C(CH3)3), 6.72–6.74 (m, 1, ArH), 7.09–7.26
(m, 10, ArH), 7.28–7.83 (m, 30, ArH), 7.96 (s, 1, ArH).
1
7.25%. Found: C, 70.93; H, 7.36%. 31P{ H} NMR (161.8 MHz,
1
CDCl3): d 139.3 (s). H NMR (400 MHz, CDCl3): 0.33 (s, 9,
Preparation of ligand 9c. As for 9a with 8c (0.185 g, 0.18 mmol)
and PPh2Cl (0.039 g, 0.18 mmol) to give 9c as a white solid
(0.178 g, 81.6%), which was used without further purification.
SiMe3), 0.36 (s, 9, SiMe3), 1.13 (s, 9, C(CH3)3), 1.29 (s, 9, C(CH3)3),
3.65 (s, 1, OH), 5.74 (d, 1, ArH), 7.08 (s, 1, ArH), 7.16–7.28 (m,
4, ArH), 7.38–7.47 (m, 2, ArH), 7.90–7.95 (m, 2, ArH), 8.06 (s,
1, ArH), 8.08 (s, 1, ArH). MS (ESI) m/z: 681.27 (M+H)+, 703.25
(M+Na)+.
1
31P{ H} NMR (121.5 MHz, CDCl3): d 106.7 (s), 154.2 (br, s). 1H
NMR (300 MHz, CDCl3): 076 (s, 9, C(CH3)3), 1.42 (s, 9, C(CH3)3),
6.28–6.30 (m, 1, ArH), 7.10 (s, 1, ArH), 7.20–7.49 (m, 30, ArH),
7.62–7.70 (m, 16, ArH), 7.82–7.7 (m, 2, ArH), 8.09 (s, 1, ArH),
8.15 (s, 1, ArH).
Preparation of 8b. A solution of 2,4-di-tert-butylresorcinol
(0.327 g, 1.47 mmol) and 4c (1.092 g, 1.47 mmol) in toluene
(10 ml) at room temperature was treated with NEt3 (1.4 ml,
10.3 mmol) dropwise and the resultant mixture was stirred for
18 h. The mixture was filtered through Celite, the volatiles removed
under reduced pressure and the product purified by column
chromatography (silica, CH2Cl2 : hexane 1 : 3 eluent) affording 8b
as a white solid. Yield: 1.06 g (78%). Crystals suitable for X-
ray analysis were grown by slow evaporation of an Et2O/hexane
solution. Anal. Calcd for C60H57O4PSi2: C, 77.55; H, 6.18%.
Preparation of complex 3b. A mixture of ligand 7 (0.252 g,
0.278 mmol) and [PdCl2(NCMe)2] (0.072 g, 0.278 mmol) in
1,2-dichloroethane (10) ml was treated with NEt3 (0.04 ml,
0.278 mmol) and then heated at 80 ◦C for 2 h. The resulting
mixture was filtered through Celite to remove palladium black,
the volatiles removed under reduced pressure and the product
purified by column chromatography (silica, CH2Cl2 : hexane 2 : 1
eluent) to afford 3b as a white solid. Yield: 0.24 g (82.5%). Crystals
suitable for X-ray analysis were grown from CH2Cl2/diethyl ether.
Anal. Calcd for C58H51ClO6P2Pd: C, 66.48; H, 4.91%. Found: C,
1
Found: C, 77.81; H, 6.69%. 31P{ H} NMR (121.5 MHz, CDCl3):
1
d 146.3 (s). H NMR (300 MHz, CDCl3): 0.78 (s, 3, SiMePh2),
1
0.88 (s, 3, SiMePh2), 0.91 (s, 9, C(CH3)3), 1.38 (s, 9, C(CH3)3), 3.98
(s, 1, OH), 5.55 (d, 1, ArH), 7.06 (s, 1, ArH), 7.19–7.51 (m, 26,
ArH), 7.74–7.82 (m, 3, ArH), 7.93 (s, 1, ArH). MS (ESI) m/z:
929.36 (M+H)+.
66.29; H, 5.28%. 31P{ H} NMR (121.5 MHz, CDCl3): d 145.2 (s).
1H NMR (300 MHz, CDCl3): 1.29 (s, 18, C(CH3)3), 2.57 (s, 6,
ArMe), 2.79 (s, 6, ArMe), 7.17–7.30 (m, 7, ArH), 7.37–7.43 (m,
4, ArH), 7.48–7.52 (m, 2, ArH), 7.81–7.83 (m, 4, ArH), 7.88 (s, 2,
ArH), 7.91–7.94 (m, 2, ArH). MS (ESI) m/z: 1012.41 (M-Cl)+.
Preparation of 8c. A solution of 2,4-di-tert-butyl resorcinol
(0.239 g, 1.08 mmol) and 4d (0.932 g, 1.08 mmol) in 1,2-
dichloroethane (20 ml) at room temperature was treated with
NEt3 (1.0 ml, 7.5 mmol) dropwise. The reaction mixture was
heated at 60 ◦C for 16 h. After cooling, the reaction mixture was
filtered through Celite, the volatiles were removed under reduced
pressure and the product purified by column chromatography
(silica, CH2Cl2 : hexane 1 : 1 eluent) to afford 8c as a white solid.
Yield: 0.798 g (70.5%). Anal. Calcd for C70H61O4PSi2: C, 79.81; H,
Preparation of ligand 11. A mixture of 2,4-di-tert-butyl re-
sorcinol (0.56 g, 2.50 mmol), chlorodiphenylphosphine (0.96 ml,
5.24 mmol) and NEt3 (1.4 ml, 9.98 mmol) in toluene (10 ml)
was heated at reflux temperature for 18 h. The reaction mixture
was allowed to cool, filtered through celite and the volatiles were
removed under reduced pressure to give 11 as a white solid (1.3 g,
1
88%), which was used without further purification. 31P{ H} NMR
1
(121.5 MHz, CDCl3): d 109.4 (s). H NMR (300 MHz, CDCl3):
1
5.84%. Found: C, 79.59; H, 5.96%. 31P{ H} NMR (161.8 MHz,
1.30 (s, 9, C(CH3)3), 7.24–7.32 (m, 14, ArH), 7.47–7.53 (m, 8,
ArH).
1
CDCl3): d 148.8 (s). H NMR (300 MHz, CDCl3): 0.74 (s, 9,
C(CH3)3), 1.38 (s, 9, C(CH3)3), 3.67 (s, 1, OH), 4.66 (d, 1, ArH),
6.94 (s, 1, ArH), 7.20–7.55 (m, 36, ArH), 7.79–7.86 (m, 2, ArH),
8.00 (s, 1, ArH), 8.06 (s, 1, ArH). MS (ESI) m/z: 1053.39 (M+H)+,
1075.37 (M+Na)+.
Preparation of complex 10a. A solution of ligand 9a (0.102 g,
0.117 mmol) and [PdCl2(NCMe)2] (0.031 g, 0.117 mmol) in 1,2-
dichloroethane (5 ml) was treated dropwise with NEt3 (0.03 ml,
0.167 mmol) at room temperature and then stirred for 18 h.
The resultant mixture was filtered through Celite, the volatiles
removed under reduced pressure and the product purified by
column chromatography (silica, CH2Cl2 : hexane 10 : 1 eluent) to
afford 10a as a yellow solid. Yield: 0.079 g (67%). Anal. Calcd
for C52H57ClO4P2PdSi2: C, 62.08; H, 5.71%. Found: C, 62.34; H,
Preparation of ligand 9a. A solution of 8a (0.100 g, 0.15 mmol)
and PPh2Cl (0.036 g, 0.15 mmol) in toluene (5 ml) was treated
dropwise with NEt3 (0.06 mL, 0.45 mmol) at room temperature
and the resultant mixture stirred at r.t. for 18 h. The resultant
mixture was filtered through Celite and the volatiles removed
under reduced pressure to give 9a as a white solid (0.102 g, 80%),
1
5.91%. 31P{ H} NMR (121.5 MHz, CDCl3): d 144.66 (d, J =
1
which was used without further purification. 31P{ H} NMR (121.5
679.2 Hz), 151.01 (d, J = 681.0 Hz). 1H NMR (300 MHz, CDCl3):
0.31 (s, 9, SiMe3), 0.44 (s, 9, SiMe3), 1.13 (s, 9, C(CH3)3), 1.42
(s, 9, C(CH3)3), 7.09–7.12 (m, 2, ArH), 7.18–7.30 (m, 3, ArH),
7.41–7.57 (m, 8, ArH), 7.90–8.12 (m, 8, ArH). HRMS (ESI) m/z:
1029.1912 (M+Na)+, 971.2336 (M-Cl)+.
1
MHz, CDCl3): d 109.5 (s), 147.6 (br, s). H NMR (300 MHz,
CDCl3): 0.30 (s, 9, SiMe3), 0.33 (s, 9, SiMe3), 0.99 (s, 9, C(CH3)3),
1.33 (s, 9, C(CH3)3), 7.11–7.27 (m, 6, ArH), 7.38–7.43 (m, 8, ArH),
7.58–7.63 (m, 4, ArH), 7.89–8.10 (m, 4, ArH).
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9034–9041 | 9039
©

1
1
H, 5.58%. 31P{ H} NMR (121.5 MHz, CDCl3): d 144.8 (s). H
NMR (300 MHz, CDCl3): 1.41 (s, 18, C(CH3)3), 7.12 (s, 1, ArH),
7.46–7.50 (m, 12, ArH), 7.97–8.04 (m, 8, ArH). HRMS (ESI) m/z:
753.1068 (M+Na)+, 695.1483 (M-Cl)+.
Preparation of complex 10b. As for 10a with ligand 9b (0.100 g,
0.09 mmol) and [PdCl2(NCMe)2] (0.023 g, 0.09 mmol) to afford
10b as a yellow solid. Yield: 0.086 g (76%). Anal. Calcd for
C72H65ClO4P2PdSi2: C, 68.95; H, 5.22%. Found: C, 69.41; H,
1
5.70%. 31P{ H} NMR (121.5 MHz, CDCl3): d 142.68 (d, J = 676.5
General method for the allylation of benzaldehyde. To a
dried reaction tube, charged with a solution of the appropriate
palladacycle (0.025 mmol Pd) in CH2Cl2 (0.5 ml), were added
benzaldehyde (0.050 ml, 0.053 g, 0.5 mmol) and allyltributyl
tin (0.200 ml, 0.212 g, 0.64 mmol). Additional CH2Cl2 (0.5 ml)
was added to ensure all reagents were washed into the reaction
mixture and the resulting solution was stirred at the appropriate
temperature for 18–120 h (see Table 1 for details). The reaction
mixture was quenched with water (10 ml), the product extracted
with CH2Cl2 (3 ¥ 10 ml), the combined organic phase dried
(MgSO4) and the solvent removed under reduced pressure. 1,3,5-
Trimethoxybenzene (internal standard) was added and the spec-
troscopic yield was determined by 1H NMR spectroscopy (CDCl3,
300 MHz). The product was purified by column chromatography
(silica, EtOAc : hexane 1 : 6 eluent) prior to determining optical
purity by chiral HPLC.
Hz), 146.99 (d, J = 677.1 Hz). 1H NMR (300 MHz, CDCl3): 0.82
(s, 3, SiPh2Me), 1.05 (s, 9, C(CH3)3), 1.07 (s, 3, SiPh2Me), 1.53
(s, 9, C(CH3)3), 6.95–7.34 (m, 17, ArH), 7.39–7.54 (m, 14, ArH),
7.60–7.62 (m, 2, ArH), 7.81–7.90 (m, 2, ArH), 8.02 (s, 1, ArH),
8.07–8.14 (m, 5, ArH). HRMS (ESI) m/z: 1277.2550 (M+Na)+,
1219.2955 (M-Cl)+.
Preparation of complex 10c. As for 10a with ligand 9c (0.206 g,
0.167 mmol) and [PdCl2(NCMe)2] (0.044 g, 0.167 mmol) for 1
day. The product was purified by column chromatography (silica,
CH2Cl2 : hexane 2 : 1) to afford 10c as a yellow solid. Yield: 0.117
g (51%). Crystals suitable for X-ray analysis were grown by
slow evaporation of a THF/hexane solution. Anal. Calcd for
C82H69ClO4P2PdSi2: C, 71.45; H, 5.05%. Found: C, 71.53; H,
1
5.33%. 31P{ H} NMR (121.5 MHz, CDCl3): d 139.53 (d, J =
670.1 Hz), 143.76 (d, J = 670.8 Hz). 1H NMR (300 MHz, CDCl3):
1.15 (s, 9, C(CH3)3), 1.62 (s, 9, C(CH3)3), 6.89–7.07 (m, 19, ArH),
7.21–7.24 (m, 1, ArH), 7.31–7.54 (m, 23, ArH), 7.89–7.94 (m, 2,
ArH), 8.22–8.30 (m, 6, ArH). MS (ESI) m/z: 1341.32 (M-Cl)+,
1399.29 (M+Na)+.
X-ray crystallography. X-ray diffraction experiments on all
samples were carried out at 100 K on a Bruker APEX II
diffractometer using Mo-Ka radiation (l = 0.71073 A), with the
exception of 8b, where the data collection was carried out at 100 K
˚
Preparation of complex 12. A solution of ligand 11 (0.147 g,
0.25 mmol) and [PdCl2(NCMe)2] (0.65 g, 0.25 mmol) in 1,2-
dichloroethane (10 ml) was treated dropwise with NEt3 (0.1 ml,
0.76 mmol) at room temperature and the resultant mixture was
stirred for 1 h. The mixture was filtered through Celite, the volatiles
removed under reduced pressure and the resultant crude product
purified by column chromatography (silica, CH2Cl2 : hexane 7 : 1
eluent) to afford 12 as a yellow solid. Yield: 0.111 g (61%). Anal.
Calcd for C39H42ClO2P2Pd: C, 62.39; H, 5.37%. Found: C, 62.60.;
on a Bruker Microstar rotating anode diffractometer using Cu-Ka
radiation (l = 1.54178 A). Data collections were performed using
˚
a CCD area detector from a single crystal mounted on a glass fibre.
Intensities were integrated25 from several series of exposures mea-
suring 0.5◦ in w or f. Absorption corrections were based on equiv-
alent reflections using SADABS or TWINABS.26 The structures
2
were solved using SHELXS and refined against all Fo data with
hydrogen atoms riding in calculated positions using SHELXL.27
Crystal structure and refinement data are given in Table 2.
Table 2 Crystallographic data
Compound
3b·C4H10O
6d
7
8a
8b
10c·1.5C4H8O
Colour, habit
Size/mm
Empirical Formula
M
colourless, plate colourless, block colourless, block colourless, cut
0.93 ¥ 0.61 ¥ 0.15 0.35 ¥ 0.20 ¥ 0.12 0.49 ¥ 0.35 ¥ 0.18 0.51 ¥ 0.17 ¥ 0.14 0.17 ¥ 0.06 ¥ 0.06 0.50 ¥ 0.27 ¥ 0.13
colourless, cut
colourless, block
C62H61ClO7P2Pd C60H50O4Si2
C58H52O6P2
906.94
C40H49O4PSi2
680.94
C66H57O4PSi2
1001.27
Orthorhombic
P 212121
9.9513(10)
19.713(2)
28.496(3)
90.00
90.00
90.00
5590.1(10)
4
1.217
C96H99ClO6P2PdSi2
1608.72
Monoclinic
P21
13.5371(3)
23.2484(5)
14.3898(3)
90.00
114.0080(10)
90.00
4136.91(15)
2
0.379
100
1.55, 33.32
1121.90
Orthorhombic
C 2221
891.18
Crystal system
Space group
Orthorhombic
P 212121
9.9481(4)
17.2684(6)
31.2989(12)
90.00
Orthorhombic
P 212121
9.8619(2)
18.1864(3)
26.4065(5)
90.00
Orthorhombic
P 212121
9.9085(4)
11.2581(4)
34.2755(13)
90.00
˚
a/A
15.1839(5)
15.9875(6)
22.2064(7)
90.00
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90.00
90.00
90.00
90.00
90.00
90.00
90.00
90.00
3
˚
V/A
5390.7(3)
4
0.507
5376.8(4)
4
0.109
4736.07(15)
4
0.145
3823.5(3)
4
0.173
Z
m/mm-1
T/K
100
1.83, 27.61
100
1.30, 27.56
100
2.57, 27.52
100
1.19, 28.29
100
2.73, 65.18
qmin,max
Completeness
0.999 to q = 27.61◦ 0.998 to q = 27.56◦ 0.997 to q = 27.52◦ 1.000 to q = 28.29◦ 0.995 to q = 65.18◦ 0.996 to q = 33.32◦
Reflections: total/independent
Rint
46257/6267
0.0476
12378/12378
n/a
34556/10860
0.0504
40852/9480
0.0407
57780/57780
n/a
0.0536, 0.1437
0.742, -0.298
1.190
147604/31588
0.0374
0.0375, 0.1033
1.531, -0.878
1.291
Final R1 and wR2
0.0250, 0.0601
0.835, -0.508
1.382
0.0561, 0.1535
0.817, -0.398
1.101
0.23(11)
220
0.0461, 0.1193
0.291, -0.355
1.272
0.0329, 0.0762
0.327, -0.243
1.183
-3
˚
Largest peak, hole/eA
rc/g cm-3
Flack parameter
-0.008(15)
-0.06(7)
0.07(6)
0.001(10)
-0.013(9)
Squeeze contribution per unit cell. n/a
9040 | Dalton Trans., 2011, 40, 9034–9041
This journal is
The Royal Society of Chemistry 2011
©

In 3b, the oxygen atom in the disordered diethyl ether molecule
is sitting on a two-fold axis.
6 (a) N. Solin, J. Kjellgren and K. J. Szabo´, Angew. Chem., Int. Ed., 2003,
42, 3656; (b) N. Solin, J. Kjellgren and K. J. Szabo´, J. Am. Chem. Soc.,
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1829.
7 O. A. Wallner and K. J. Szabo´, Chem.–Eur. J., 2006, 12, 6976.
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9 R. Johansson and O. F. Wendt, Dalton Trans., 2007, 488.
10 J. Aydin, K. S. Kumar, L. Eriksson and K. J. Szabo´, Adv. Synth. Catal.,
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11 J. Aydin and K. J. Szabo, Org. Lett., 2008, 10, 2881.
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15 J. Aydin, A. Ryde´n and K. J. Szabo´, Tetrahedron: Asymmetry, 2008, 19,
1867.
16 These are typically in the range of ~500–600 Hz, see: (a) A. Albinati,
S. Affolter and P. S. Pregosin, Organometallics, 1990, 9, 379; (b) R.
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2003, 9, 3216.
The difference map of 6d contained residual electron density
peaks which could not be modelled satisfactorily. In this case,
the data were modelled with the PLATON/SQUEEZE algo-
rithm, which estimated 110 electrons in each void. This equates
to approximately four extra molecules of dichloromethane or
ether per unit cell, although the nature of the solvent is not
known.
Crystals of 8b were a non-merohedral twin with three domains.
These were indexed independently and integrated simultaneously
using the APEX II software. A smooth refinement was hindered
by the presence of disordered solvent (either diethyl ether or
hexane), which could not be modelled satisfactorily. The region
containing solvent has been modelled by using six fully occupied
carbon carbons with free anisotropic displacement parameters.
No attempt has been made to impose a chemically sensible model
on the region since the use of an unreasonable number of restraints
and constraints would be inevitable and their use would not
necessarily improve the part of the structure that is of most
interest.
17 R. B. Bedford, M. Betham, J. P. H. Charmant, M. F. Haddow, A. G.
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18 PhB(OH)2 (1.0 mmol), 2-cyclohexenone (0.5 mmol), K3PO4 (0.106 g,
0.5 mmol) and catalyst 3b (5 mol% Pd) in toluene (2 ml) stirred at r.t.
for 24 h.
19 A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J.
Timmers, Organometallics, 1996, 15, 1518.
Acknowledgements
We thank the CCDC for a studentship (CLM) and Johnson
Matthey for the loan of palladium salts.
20 P. J. Cox, W. Wang and V. Snieckus, Tetrahedron Lett., 1992, 33, 2253.
21 R. C. Helgeson, J. M. Timko, P. Moreau, S. C. Peacock, J. M. Mayer
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22 K. Maruoka, T. Ito, Y. Araki, T. Shirasaka and H. Yamamoto, Bull.
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23 A. H. M. de Vries, A. Meetsma and B. L. Feringa, Angew. Chem., Int.
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Notes and references
1 For a recent review see: (a) N. Selander and K. J. Szabo´, Chem.
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2 (a) D. Morales-Morales, R. Redo´n, C. Yung and C. M. Jensen, Chem.
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3 R. B. Bedford, S. M. Draper, P. N. Scully and S. L. Welch, New J. Chem.,
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24 G. J. H. Buisman, L. A. van der Veen, A. Klootwijk, W. G. J. de Lange,
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25 Bruker-AXS SAINT, Madison, Wisconsin.
4 M. R. Eberhard, Z. Wang and C. M. Jensen, Chem. Commun., 2002,
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26 G. M. Sheldrick, SADABS V2008/1 or TWINABS V2008/2, University
of Go¨ttingen, Germany.
5 Z. Wang, M. R. Eberhard, C. M. Jensen, S. Matsukawa and Y.
Yamamoto, J. Organomet. Chem., 2003, 681, 189.
27 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
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This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9034–9041 | 9041
©