Cyclopalladation of the prochiral (di-tert-butyl)(diphenylmethyl)phosphine: Kinetic lability of the corresponding (+)-phosphapalladacyclic Pd-C bond and the reluctance of the phosphine to bind in a monodentate fashion

Article abstract of DOI:10.1021/ic0702320

The ortho palladation of prochiral (di-tert-butyl)(diphenylmethyl)phosphine proceeded readily to give rise to the dimeric complex, di-μ- chlorobis{[(phenyl)(di-tert-butylphosphino)methyl]phenyl-C², P}dipalladium(II). The (S,S)-(+)-dimer was subsequently obtained by optical resolution with sodium (S)-prolinate. The absolute configuration of the optically resolved (+)-dimer was concluded from the X-ray diffraction studies of the derivatized O,O-acetylacetonate complex. The availability of the (+)-dimer is crucial to the study of the properties of the Pd-C bond. The phosphapalladacycle Pd-C bond exhibited a remarkable thermodynamic stability. It could not be permanently ruptured to give rise to the η¹-P monodentate even in a refluxing acetone solution containing concentrated hydrochloric acid. Instead, the phosphine was noted to fluctuate between the ring closed and opened states via the reversible Pd-C bond cleavage/formation under this condition. Inevitably, this resulted in the racemization of the five-membered organopalladium ring structure. In contrast, such bond cleavage was not observed at room temperature in the absence of HCl. In fact, the phosphine was observed to readily ortho palladate even under conditions not favorable to cyclopalladation. Indeed, the difficulty of isolating the phosphine as a simple η¹-P monodentate coordination complex was further noted by its lack of reactivity toward the N,N-dimethyl-1-(1′naphthyl) ethylaminate palladacycle μ-chloro dimer. Only by enhancing the Lewis acidity of the palladacycle in the form of the positively charged bis(acetonitrile) complex could the phosphine be encouraged to participate in monodentate η¹-P bonding. Even then, this form of coordination was weak and was only observed by NMR spectroscopy.

Full text of DOI:10.1021/ic0702320

Inorg. Chem. 2007, 46, 5100−5109
Cyclopalladation of the Prochiral
(Di-tert-butyl)(diphenylmethyl)phosphine: Kinetic Lability of the
Corresponding ( )-Phosphapalladacyclic Pd C Bond and the
+
−
Reluctance of the Phosphine to Bind in a Monodentate Fashion
Joseph Kok-Peng Ng,† Shuli Chen,‡ Yongxin Li,‡ Geok-Kheng Tan,† Lip-Lin Koh,† and
Pak-Hing Leung*,‡
Department of Chemistry, National UniVersity of Singapore, Kent Ridge, Singapore 119260,
and DiVision of Chemistry and Biological Chemistry, Nanyang Technological UniVersity,
Singapore 637616
Received February 7, 2007
The ortho palladation of prochiral (di-tert-butyl)(diphenylmethyl)phosphine proceeded readily to give rise to the
dimeric complex, di- -chlorobis dipalladium(II). The
[(phenyl)(di-tert-butylphosphino)methyl]phenyl-C2,
(S,S)-( )-dimer was subsequently obtained by optical resolution with sodium (S)-prolinate. The absolute configuration
of the optically resolved )-dimer was concluded from the X-ray diffraction studies of the derivatized
O,O-acetylacetonate complex. The availability of the ( )-dimer is crucial to the study of the properties of the Pd
bond. The phosphapalladacycle Pd
µ
{
P}
+
(
+
+
−C
−
C bond exhibited a remarkable thermodynamic stability. It could not be
permanently ruptured to give rise to the η1−P monodentate even in a refluxing acetone solution containing
concentrated hydrochloric acid. Instead, the phosphine was noted to fluctuate between the ring closed and
opened states via the reversible Pd−C bond cleavage/formation under this condition. Inevitably, this resulted in the
racemization of the five-membered organopalladium ring structure. In contrast, such bond cleavage was not observed
at room temperature in the absence of HCl. In fact, the phosphine was observed to readily ortho palladate even
under conditions not favorable to cyclopalladation. Indeed, the difficulty of isolating the phosphine as a simple
η1−P monodentate coordination complex was further noted by its lack of reactivity toward the N,N-dimethyl-1-
(1′naphthyl)ethylaminate palladacycle µ-chloro dimer. Only by enhancing the Lewis acidity of the palladacycle in
the form of the positively charged bis(acetonitrile) complex could the phosphine be encouraged to participate in
monodentate η1−P bonding. Even then, this form of coordination was weak and was only observed by NMR
spectroscopy.
Introduction
stoichiometric agents for the syntheses of organic com-
pounds.5 These agents have also gained attention for their
photoluminescent behavior,6 biological properties7, and as
liquid crystals.8
Ever since the unprecedented ortho metalation of azoben-
zene was reported by Cope et al.,1 the chemistry of
cyclopalladated reagents (chiral and otherwise) has developed
into a very rich one. Such complexes have generated
interests for their many roles as catalysts,2 agents for
enantiomeric excess determinations,3 resolving agents4, and
We have previously contributed to this field of interest
by utilizing the chiral N,N-dimethyl-1-(1′-naphthyl)ethy-
laminate palladacycle for the syntheses of a number of
optically active phosphorus-containing ligands.9 It is well
known that the chiral-inducing ability of this palladacycle
derives from the transmission of vital stereochemical infor-
mation originating from the conformationally robust five-
membered palladacycle9d,i,10 by (i) the dimethylamino group
of the neighboring coordination site on the Pd atom11 and
* To whom correspondence should be addressed. E-mail: pakhing@
ntu.edu.sg.
† National University of Singapore.
‡ Nanyang Technological University, Singapore.
(1) Cope, A. C.; Siekman, R. W. J. Am. Chem. Soc. 1965, 87, 3272-
3273.
5100 Inorganic Chemistry, Vol. 46, No. 12, 2007
10.1021/ic0702320 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/10/2007

Cyclopalladation and Kinetic Lability
(ii) the protruding aromatic proton H′ of the site adjacent to
the Pd-C bond.10e
Encouraged by these results, we sought to develop new
variants of this type.12,13 Palladacycles containing phosphorus
donors, or especially those of other heteroatoms such as
arsenic,13a,14 oxygen15, and sulfur16, are relatively limited with
respect to their numerous nitrogen counterparts. Even among
the P-donor palladacycles,2c,d, 17 or aptly known as “phos-
phapalladacycles”, those prepared in the optically active
forms13a,18 are a rarity. Also, with the very promising roles
of phosphapalladacycles as catalysts,2c,d,h,l the development
of new variants of these unique classes of organopalladium
complexes seems necessary.
ing reaction sites on the Pd atom in all of the future
applications of the palladacycle as a reaction promoter.
Experimental Section
The delivery of new properties is expected with the
replacement of the nitrogen atom with a congener. For
instance, the phosphorus atom behaves as more than a σ
donor, as, depending on the substituents available, it can
possess various degrees of π-accepting properties as well.19
This article describes the preparation of a novel phosphapal-
ladacycle 1 in the optically active form and discusses the
kinetic lability of the Pd-C bond from optical rotation data.
The inclusion of the bulky t-butyl substituents was aimed at
providing a greater stereochemical influence to the neighbor-
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5102 Inorganic Chemistry, Vol. 46, No. 12, 2007

Cyclopalladation and Kinetic Lability
and was isolated as a white solid. 1H NMR (300 MHz, CDCl3, δ):
0.98 (d, 18H, JPH ) 10.0 Hz, t-Bu), 4.42 (d, 1H, JPH ) 4.8 Hz,
R-CH), 7.08-7.55 (m, 10H, aromatic protons). 31P{1H} NMR (121
MHz, CDCl3, δ): 47.1 (s).
chromatographed in a short silica gel column using CH2Cl2/hexanes
(in increasing polarities from 1:4 to 1:1, v/v). Purified chloro dimer
3 was eluted from the column and was crystallized as pale-yellow
blocks from acetone/ether, mp (dec) 243.9-248.9 °C; 0.107 g
(61.6% yield). Anal. Calcd for C42H56Cl2P2Pd2: C, 55.7; H, 6.2.
3
2
Di-µ-chlorobis{1-[(di-tert-butylphosphino)(phenyl)methyl]-
phenyl-C2,P}dipalladium(II) ((()-3). Method A: Prepared phos-
phine 2 (2.8 g, 8.9 mmol) and palladium(II) acetate (2.0 g, 8.9
mmol) were suspended in degassed toluene (100 mL), and the
reacting mixture was stirred at 50 °C for 6.5 h to give a yellow
solution. The excess solvents were evaporated from the mixture to
give a yellow-orange oil, which was then dissolved in acetone (15
mL). A solution of lithium chloride (0.76 g, 17.8 mmol) in acetone/
methanol (5:15, v/v) was added, and the mixture was then stirred
for 90 min, evaporated to dryness, and suspended in dichlo-
romethane. The suspension was then washed with water (50 mL),
and the aqueous layer was removed. The organic extract was
concentrated and chromatographed on a silica gel column using
dichloromethane/hexanes (1:2, v/v) from which chloro-dimer 3 was
eluted as a pale yellowish-green fraction. Crystallization from
acetone/diethylether yielded pale-yellow blocks, 3.34 g (82.7%
yield). Method B: A dichloromethane solution (5 mL) of prepared
phosphine 2 (0.241 g, 0.770 mmol) was added via cannular into a
freshly deoxygenated aqueous solution (3 mL) of Li2[PdCl4] (0.39
mmol; prepared in-situ from PdCl2 (0.068 g, 0.39 mmol) and excess
LiCl (0.032 g, 0.77 mmol) in water) followed by rapid stirring at
room temperature for 30 min. The organic layer was separated,
washed with water (2 × 10 mL), dried with MgSO4, and
(18) (a) Longmire, J. M.; Zhang, X.; Shang, M. Organometallics 1998,
17, 4374-4379. (b) Dunina, V. V.; Gorunova, O. N.; Kuz’mina, L.
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10, 3951-3961. (c) Dunina, V. V.; Gorunova, O. N.; Livantsov, M.
V.; Grishin, Y. K.; Kuz’mina, L. G.; Kataeva, N. A.; Churakov, A.
V. Tetrahedron: Asymmetry 2000, 11, 3967-3984. (d) Williams, B.
S.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. HelV. Chim. Acta
2001, 84, 3519-3530. (e) Morales-Morales, D.; Cramer, R. E.; Jensen,
C. M. J. Organomet. Chem. 2002, 654, 44-50.
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4879-4884. (b) Pacchioni, G.; Bagus, P. S. Inorg. Chem. 1992, 31,
4391-4398.
(20) Gru¨etzmacher, H. In Synthetic Methods of Organometallic and
Inorganic Chemistry; Herrmann, W. A., Ed.; Phosphorus Arsenic
Antimony and Bismuth; Thieme Medical Publishers: New York, 1996;
Vol. 3, pp 85-88.
1
Found: C, 55.8; H, 6.4. H NMR (300 MHz, CDCl3, δ): 1.33-
2
1.43 (br d, 18H, t-Bu), 4.51 (d, 1H, JPH ) 11.2 Hz, t-Bu2PCH),
6.70-7.95 (br m, 9H, aromatic protons). 31P{1H} NMR (121 MHz,
CDCl3, δ): 111.5, 111.8, 111.9 (br s), (223 K, δ): 110.0 (s), 110.2
(s), 110.3 (s), 110.5 (s).
[(S)-Prolinato-N,O] {1-[(di-tert-butylphosphino)(phenyl)-
methyl]phenyl-C2,P}palladium(II) (4). A methanol solution (2
mL) of potassium (S)-prolinate (0.12 g, 0.78 mmol) was added to
chloro-dimer (()-3 (0.23 g, 0.25 mmol) dissolved in dichlo-
romethane (2 mL), and the mixture was stirred vigorously at room
temperature for 30 min. The resulting white suspension was
evaporated to dryness, washed with water (10 mL), and was
extracted with CH2Cl2 (2 × 10 mL). The combined organic extracts
were dried with MgSO4 to give a colorless solution, which was
evaporated to dryness to yield 4 as a white solid, mp (dec) 262-
264 °C, [R]D +105°, [R]578 +110°, [R]546 +128°, [R]436 +250°,
[R]365 +541° (c, 1.0, CH2Cl2); 0.25 g (92.6% yield). Anal. Calcd
for C39H43ClP2Pd: C, 58.7; H, 6.8; N, 2.6. Found: C, 58.9; H,
6.8; N, 2.4. 1H NMR (500 MHz, CDCl3), two sets of signals
corresponding to the (R,S) and (S,S) diastereomers. Assignment of
signals to either diastereomer was made by comparison with the
1H NMR spectrum of the isolated (S,S)-4 diastereomer (below), δ,
1.24 (d, 9H, 3JPH ) 13.8 Hz, t-Bu of (R,S)-diastereomer), 1.25 (d,
9H, 3JPH ) 14.2 Hz, t-Bu of (S,S)-diastereomer), 1.26 (d, 9H, 3JPH
3
) 12.9 Hz, t-Bu of (S,S)-diastereomer), 1.36 (d, 9H, JPH ) 13.2
Hz, t-Bu of (R,S)-diastereomer), 1.75-4.15, (S)-prolinate signals:
δ, 1.75-1.87 (m, 2H, γ-H of both diastereomers), 2.02-2.21 (m,
3H, â- and γ-H of (R,S)-diastereomer, γ-H of (S,S)-diastereomer),
2.29-2.42 (m, 2H, both â-H of (S,S)-diastereomer), 2.54 (m, 1H,
â-H of (R,S)-diastereomer), 3.33-3.50 (m, 5H, all δ-H of both
diastereomers and N-H of (S,S)-diastereomer), 3.60 (m, 1H, N-H
of (R,S)-diastereomer), 4.07-4.14 (m, 2H, R-H of both diastere-
2
omers), 4.54 (d, 1H, JPH ) 11.3 Hz, palladacycle R-CH of (S,S)-
diastereomer), 4.64 (d, 1H, 2JPH ) 11.6 Hz, palladacycle R-CH of
(R,S)-diastereomer), 6.73-7.57, aromatic signals: δ, 6.75 (m, 1H,
H5 of (R,S)-diastereomer), 6.80 (m, 1H, H5 of (S,S)-diastereomer),
6.91-6.96 (m, 4H, H3 and H4 of both diastereomers), 7.07-7.12
(m, 2H, H2 of both diastereomers), 7.18-7.22 (m, 4H, Ph ring m-
and p-H of both diastereomers), 7.27-7.31 (m, 2H, Ph ring m-H
of both diastereomers), 7.34-7.36 (br d, 2H, Ph ring o-H of both
diastereomers), 7.50 (m, 1H, Ph ring o-H of (R,S)-diastereomer),
7.56 (m, 1H, Ph ring o-Ph of (S,S)-diastereomer). 31P{1H} NMR
(202 MHz, CDCl3) four sets of signals in 99.0, 98.0, 1.0, 2.0;
relative intensities, δ: 108.2 (s), 109.8 (s), 110.1 (s), 111.2 (s).
(21) (a) Shaw, B. L. J. Organomet. Chem. 1980, 200, 307-318. (b) Cheney,
A. J.; Mann, B. E.; Shaw, B. L.; Slade, R. M. J. Chem. Soc. D 1970,
1176-1177.
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Inorg. Chim. Acta 1978, 31, L441-L442. (b) Cheney, A. J.; Shaw B.
L. J. Chem. Soc., Dalton Trans. 1972, 860-865.
(23) (a) Dehand, J.; Mutet, C.; Pfeffer, M. J. Organomet. Chem. 1981,
209, 255-270. (b) Hiraki, K.; Fuchita, Y.; Takakura, S. J. Organomet.
Chem. 1981, 210, 273-280. (c) Hiraki, K.; Fuchita, Y.; Maruta, T.
Inorg. Chim. Acta 1980, 45, L205-L206. (d) Mutet, C.; Pfeffer, M.
J. Organomet. Chem. 1979, 171, C34-C36.
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18, 2803-2808.
Optical Resolution of Di-µ-chlorobis{1-[(di-tert-butylphos-
phino)(phenyl)methyl]phenyl-C2, P}dipalladium(II). Isolation of
(+)-[(S)-Prolinato-N,O]{(S)-1-[(di-tert-butylphosphino)(phenyl)-
methyl]phenyl-C2,P}palladium(II) ((S,S)-4). A dichloromethane
solution (10 mL) of the 1:1 diastereomeric mixture of (R,S)- and
(S,S)-prolinate 4 (1.18 g, 2.2 mmol) was diluted with toluene of
the same volume, and the colorless solution was concentrated in
vacuo. The resulting solution was mainly enriched in toluene as
the solvent was left to stand at room temperature, from which a
white solid that was 71% de enriched in favor of the (S,S) isomer
was obtained. The obtained white solid was recrystallized
twice from the above solvent system to yield the (S,S)-4 isomer as
a white solid with a final optical purity of 98.1% de, mp (dec)
287-288 °C; [R]D +287°, [R]578 +307°, [R]546 +353°, [R]436
(25) Garrou, P. E. Chem. ReV. 1981, 81, 229-266.
(26) Dunina, V. V.; Razmyslova, E. D.; Kuz’mina, L. G.; Churakov, A.
V.; Rubina, M. Y.; Grishin, Y. K. Tetrahedron: Asymmetry 1999,
10, 3147-3155.
(27) Chooi, S. Y. M.; Siah, S. Y.; Leung, P. H.; Mok, K. F. Inorg. Chem.
1993, 32, 4812-4818.
(28) McFarlane, W.; Swarbrick, J. D.; Bookham, J. L. J. Chem. Soc., Dalton
Trans. 1998, 3233-3238.
(29) Leung, P. H.; Martin, J. W. L.; Wild, S. B. Inorg. Chem. 1986, 25,
3396-3400.
(30) (a) Pearson, R. G. Inorg. Chem. 1973, 12, 712-713. (b) Jørgensen,
C. K. Inorg. Chem. 1964, 3, 1201-1202.
(31) (a) Vicente, J.; Arcas, A.; Bautista, D.; Jones, P. G. Organometallics
1997, 16, 2127-2138. (b) Vicente, J.; Abad, J. A.; Frankland, A. D.;
de Arellano, M. C. R. Chem.sEur. J. 1999, 5, 3066-3075.
(32) Chooi, S. Y. M.; Leung, P.-H.; Lim, C. C.; Mok, K. F.; Quek, G. H.;
Sim, K. Y.; Tan, M. K. Tetrahedron: Asymmetry 1992, 3, 529-532.
Inorganic Chemistry, Vol. 46, No. 12, 2007 5103

Ng et al.
Table 1. Selected Bond Lengths (angstroms) and Angles (degrees) of
(S)-5
[R]436 +645°, [R]365 +1337° (c 0.5, CH2Cl2); 0.059 g (54.4%). Anal.
Calcd for C26H35O2PPd: C, 60.4; H, 6.8. Found: C, 60.5; H, 7.0.
3
1H NMR (500 MHz, CDCl3, δ): 1.27 (d, 9H, JPH ) 13.8 Hz,
Pd(1)-C(1)
1.997(4)
2.106(3)
1.880(4)
1.523(5)
1.881(4)
178.6(2)
87.7(1)
173.46(8)
122.9(3)
102.9(3)
Pd(1)-P(1)
2.2172(11)
2.084(3)
1.403(5)
1.521(5)
1.883(4)
92.0(1)
equatorial t-Bu), 1.33 (d, 9H, 3JPH ) 13.0 Hz, axial t-Bu), 1.90 (s,
Pd(1)-O(1)
Pd(1)-O(2)
2
P(1)-C(7)
C(1)-C(6)
3H, acac-Me), 2.14 (s, 3H, acac-Me), 4.54 (d, 1H, JPH ) 11.5
C(6)-C(7)
C(7)-C(8)
3
Hz, R-CH), 5.37 (s, 1H, acac-CH), 6.71 (dd, 1H, JHH ) 7.6 Hz,
P(1)-C(14)
P(1)-C(18)
3
3
4JHH ) 1.57 Hz, H5), 6.88 (dddd, 1H, JHH ) 7.6 Hz, JHH ) 7.3
Hz, 4JHH ) 1.3 Hz, 5JPH ) 2.1 Hz, H4), 6.98 (ddd, 1H, 3JHH ) 7.8
Hz, 3JHH ) 7.3 Hz, 4JHH ) 1.6 Hz, H3), 7.15-7.18 (m, 2H, m-Ph,
C(1)-Pd(1)-O(1)
O(2)-Pd(1)-O(1)
O(2)-Pd(1)-P(1)
Pd(1)-C(1)-C(6)
C(6)-C(7)-P(1)
C(1)-Pd(1)-O(2)
C(1)-Pd(1)-P(1)
O(1)-Pd(1)-P(1)
C(1)-C(6)-C(7)
Pd(1)-P(1)-C(7)
81.6(1)
98.84(9)
119.3(3)
104.8(1)
3
R-p-Ph), 7.24 (m, 1H, R-m-Ph), 7.33 (br d, 1H, JHH ) 8.4 Hz,
R-o-Ph), 7.69 (m, 1H, R-o-Ph), 7.91 (ddd, 1H, 3JHH ) 7.8 Hz, 4JHH
4
) 1.3 Hz, JPH ) 3.9 Hz, H2). 31P{1H} NMR (202 MHz, CDCl3,
+650°, [R]365 +1200° (c, 0.3, CH2Cl2); 0.281 g (47.8%). Anal.
Calcd for C39H43ClP2Pd: C, 58.7; H, 6.8; N, 2.6. Found: C, 58.8;
δ): 107.5 (s).
1
3
H, 6.9; N, 2.6. H NMR (500 MHz, CDCl3, δ): 1.25 (d, 9H, JPH
) 14.0 Hz, t-Bu), 1.26 (d, 9H, 3JPH ) 13.2 Hz, t-Bu), 1.83 (m, 1H,
γ-H′), 2.13 (m, 1H, γ-H′′), 2.29-2.43 (m, 2H, both â-H), 3.40-
3.51 (m, 3H, N-H and both δ-H), 4.09 (m, 1H, prolinate R-H),
2
4.54 (d, 1H, JPH ) 11.4 Hz, palladacycle R-CH), 6.80 (m, 1H,
H5), 6.91-6.96 (m, 2H, H4, H3), 7.09 (m, 1H, H2), 7.18-7.21 (m,
2H, Ph ring m- and p-H), 7.27 (m, 1H, Ph ring m-H), 7.35 (br d,
1H, JHH ) 7.4 Hz, Ph ring o-H), 7.56 (m, 1H, Ph ring o-H). 31P-
{1H} NMR (202 MHz, CDCl3): three sets of signals in 100, 0.97,
1.93; relative intensities, δ: 108.2 (s, Z-(S,S)-isomer), 109.8 (s,
(R,S)-isomer), 110.1 (s, E-(S,S)-isomer).
Attempted Isolation of (Acetonitrile){(R)-1-[1′-(N,N-dimethyl-
amino)ethyl]naphthyl-C2,N}[(di-tert-butyl)(diphenylmethyl)-
phosphine]palladium(II) Hexafluorophosphate(V) ((R)-12). A
dichloromethane solution (5 mL) of 2 (0.12 g, 0.38 mmol) was
transferred via cannular under argon to crystalline (R)-11 (0.20 g,
0.38 mmol) with stirring, which led to the immediate formation of
a golden-yellow solution. Attempts made to isolate the product by
crystallization have been unsuccessful. 1H NMR (500 MHz, CDCl3,
3
3
δ): 1.41 (d, 9H, JPH ) 14.4 Hz, t-Bu), 1.60 (d, 9H, JPH ) 14.4
3
4
Hz, t-Bu), 1.82 (d, 3H, JHH ) 6.6 Hz, R-CMe), 1.85 (d, 3H, JPH
) 3.8 Hz, equatorial NMe), 2.11 (br s, NCMe), 2.42 (d, 3H, JPH
4
(+)-(S,S)-Di-µ-chlorobis{1-[(di-tert-butylphosphino)(phenyl)-
methyl]phenyl-C2, P}dipalladium(II) ((S,S)-3). HCl (10 mL, 1
M) was added to a CH2Cl2 solution (12 mL) of resolved (S,S)-4
prolinate (0.23 g, 0.43 mmol), and the two-phase mixture was
vigorously stirred for 5 min at room temperature. The aqueous layer
was separated, and the procedure was repeated once using the same
amount of HCl. The organic layer was removed, and it was washed
with water (2 × 10 mL), dried with MgSO4, and evaporated to
dryness in vacuo to afford the optically active dimer (S,S)-3 as a
pale-yellow amorphous powder, mp (dec) 274-275 °C, [R]D +342°,
[R]578 +358°, [R]546 +412°, [R]436 +723° (c, 0.5, CH2Cl2); 0.193
g (99.0%). Anal. Calcd for C42H56Cl2P2Pd2: C, 55.7; H 6.2.
3
4
) 1.8 Hz, axial NMe), 4.07 (dq, 1H, JHH ) 6.6 Hz, JPH ) 5.0
Hz, R-CH), 5.27 (d, 1H, 2JPH ) 11.4 Hz, phosphine R-CH), 7.23-
7.26 (m, 3H, aromatic protons), 7.29-7.32 (m, 2H, aromatic
protons), 7.37 (d, 1H, JHH ) 8.8 Hz, H3), 7.44-7.46 (m, 2H,
3
3
aromatic protons), 7.58 (dd, 1H, JHH ) 8.8 Hz, JPH ) 5.6 Hz ,
H2), 7.60 (m, 1H, H8), 7.71 (m, 1H, aromatic proton), 7.76-7.80
3
(m, 3H, aromatic protons), 8.21 (d, 2H, JHH ) 7.9 Hz, R-o-Ph).
31P{1H} NMR (202 MHz, CDCl3, δ): 54.6 (s, phosphine P), -143.9
(septet, JPF ) 711 Hz, PF6-).
1
1
Found: C, 55.2; H, 6.3. H NMR (500 MHz, CDCl3, δ): 1.28-
1.42 (m, 18H, t-Bu), 4.51 (d, 1H, 2JPH ) 11.4 Hz, R-CH), 6.70 (m,
1H, aromatic), 6.85-6.94 (br m, 2H, aromatic protons), 7.04-7.33
(br m, 4H, aromatic protons), 7.81-7.97 (br m, 2H, aromatic
protons). 31P{1H} NMR (202 MHz, CDCl3) two sets of closely
spaced signals at δ: 111.8 (s), 112.2 (s), (223 K) δ: 110.5 (s) and
110.0 (s).
(Acetylacetonato-O,O′){(S)-1-[(di-tert-butylphosphino)(phenyl)-
methyl]phenyl-C2,P}palladium(II) ((S)-5). Na(acac)·H2O (0.029
g, 0.21 mmol) was added to an acetone solution (2 mL) of the
resolved dimer (S,S)-3 (0.095 g, 0.11 mmol), and the mixture was
stirred vigorously for 2 h at room temperature. The resulting white
suspension was filtered through a plug of celite to give a clear and
colorless solution, which was concentrated. The optically active
product was isolated as colorless blocks upon slow crystallization,
mp (dec) 245-248 °C; [R]D +267°, [R]578 +282°, [R]546 +327°,
Crystal Structure Determination of (S)-5. Crystal data for the
complex and a summary of the crystallographic analyses are given
in Table 2. Diffraction data were collected on a Siemens SMART
CCD diffractometer with Mo KR radiation (graphite monochro-
mator) using ω-scans. SADABS absorption corrections were
applied, and refinements by full-matrix least-squares were based
5104 Inorganic Chemistry, Vol. 46, No. 12, 2007

Cyclopalladation and Kinetic Lability
Table 2. Crystallographic Data for Complexes of (S)-5
formula
mol wt
No.
cryst syst
a (Å)
b (Å)
c (Å)
V (Å3)
Z
C26H35O2PPd
516.91
P2(1)2(1)2(1)
orthorhombic
9.9659(15)
15.391(2)
16.128(2)
2473.9(6)
4
T (K)
λ (Å)
223(2)
0.71073
0.834
0.0434
0.0801
µ (mm-1
)
R1 (obs. data)a
wR2 (obs. data)b
flack parameter
-0.03(3)
on SHELXL 93.3333 All of the non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were introduced at a fixed distance
from carbon and nitrogen atoms and were assigned fixed thermal
parameters.
Figure 1. Molecular structure of (S)-5.
supported by a coordination shift (∆δ) value of about +65,25
when comparing the 31P{1H} NMR chemical shifts of the
free phosphine (δ 47.1) and the dimer.
Results and Discussion
Synthesis of the Optically Active Phosphapalladacycle.
As illustrated in Scheme 1, the synthesis of the phosphapal-
ladacycle was readily achieved by the direct treatment of
the prochiral phosphine (2)20 and palladium(II) acetate in
1:1 stoichiometry followed by a chloride ion metathesis
reaction using lithium chloride to arrive at the chloro-bridged
dimer 3 as an air-stable yellow solid in 82.7% yield. The
ease of cyclopalladation can be attributed to the steric
stimulation presented by bulky substituents available on the
donor,21which in the case of 2, corresponds to the t-butyl
groups.Thisisingeneralagreementwithpreviousobservations,17k, 22
in which enhancement to cyclopalladation was accredited
to the presence of such bulky groups. In fact, the fluency of
cyclopalladating 2 was manifested by employing the even
weaker cyclopalladating agent,17l, 23 [PdCl4]2- in a 2:1 (ligand:
Pd(II)) stoichiometric ratio in the absence of an external
Brønsted base, albeit a lower yield of 61.6%. Despite the
use of such a reaction stoichiometry to encourage the
formation of the bis(phosphine) complex of the type [PdCl2L2]
(L ) 2), such a product was not observed at the end of the
reaction. Notably, the reaction was completed within
5 min at room temperature. As demonstrated in preceding
examples,14b,17c,22,24 the (ligand:Pd(II)) stoichiometry becomes
rather inconsequential in such systems when bulky substit-
uents are present.
Whereas dimeric 3 was represented as a poorly resolved
resonance at ca. δ 112 from the 31P{1H} NMR spectrum in
(CDCl3), signal resolution could be improved upon cooling
the NMR sample to 223 K, such that the appearance of four
singlet peaks at δ 110.0, 110.2, 110.3 and 110.5 became
apparent. These four peaks must reflect the existence of the
dimer as a mixture of six possible isomers in solution, namely
the chiral syn- and anti-(R,R)/(S,S) as well as the meso syn-
and anti-(R,S) isomers, which is therefore evident of the
chiral nature of the palladacycle. Moreover, the formation
of the palladacycle as a five-membered ring structure was
A very high regioselectivity was noted in the reaction of
the dimer with the chelating ligand, (S)-prolinate. Two pairs
of geometric isomers, 4, were observed. These were pre-
sented as four singlet peaks from the 31P{1H} NMR spectrum
(CDCl3) of the crude product at δ 108.2, 109.8, 110.1, and
111.2 in a relative ratio of 99:98:1:2. The isolation of the
(S,S) isomer with 98.1% de (from 31P{1H} NMR spectro-
scopic data) in 47.8% yield with [R]D +287° (CH2Cl2) from
the mixture was achieved via an optical resolution procedure
of repeated fractional crystallization from the mother liquor
that was enriched in toluene. A standard two-phase treatment
of the isolated prolinate derivative (S,S)-4 with dilute HCl
(1 M) readily led to its conversion to the optically resolved
dimer (S,S)-3 as an amorphous yellow-green powder in the
optically active form with [R]D +342° (CH2Cl2). In CDCl3,
the optically resolved (S,S)-3 was displayed as two suf-
ficiently resolved singlet peaks from 31P{1H} NMR spectra
at δ 111.8 and 112.2 at room temperature and δ 110.5 and
110.0 at 223 K, which support the existence of the dimer as
a mixture of anti and syn regioisomers. Notably, the absence
of the extra NMR signals that corresponded to the meso syn-
and anti-(R,S)/(S,R) isomers (present in the unresolved parent
dimer) must also point to the optically active form of the
phosphapalladacycle complex. This method of optical resolu-
tion, that is, by the use of chiral amino acidates, has been
previously applied to the isolation of optically active
palladacycles.3c,12c,18b,c,26 No loss in optical purity was
detected from this transformation. This was verified from
the 31P{1H} NMR data of the prolinate derivative (S,S)-4
obtained from the rederivatization of the resolved dimer
(S,S)-3 by treatment with potassium (S)-prolinate in excess.
Whereas the isolated prolinate derivative (S,S)-4 could not
be obtained as single crystals of a satisfactory quality to allow
a determination of the phosphapalladacycle absolute con-
figuration by X-ray crystallography, this problem was readily
circumvented by the use of the â-diketonate derivative (S)-5
instead. This was obtained as white crystals with [R]D
(33) Sheldrick, G. M. SHELXL 93, Program for Crystal Structure;
University of Gottingen: Gottingen, Germany, 1993.
Inorganic Chemistry, Vol. 46, No. 12, 2007 5105

Ng et al.
Scheme 1
+267° (CH2Cl2) by the direct treatment of the optically
resolved (S,S)-3 with sodium acetylacetonate. The molecular
structure and numbering scheme of (S)-5 are presented in
Figure 1, and selected bond lengths and bond angles are
given in Table 1. Importantly, the (S) absolute configuration
at the stereogenic R-C center of the optically resolved
phosphapalladacycle was confirmed by the Flack parameter
[-0.03(3)]. Notably, the five-membered palladacycle ring
was noted to adopt a puckered conformation, and the
nonplanarity of the ring was given by the mean intrachelate
torsion angle of 18.3°. This value is comparable to those of
the ortho-palladated 1-(1′-naphthyl)ethyldiphenylphosphine
(16.7-21.0°) and 1-(1′-naphthyl)ethyldiphenylarsine (16.0-
20.3°) systems13a but is weak with respect to that obtained
from the phosphapalladacycle chloro dimer of 1-[(2′, 5′-
dimethyl)-1-phenyl]ethyldiphenylphosphine (31.4°).13b The
R-C phenyl substituent takes up the axial disposition. This
was inferred from the angle of 11.0° between the C(8)-
C(7) bond to the normal of the mean coordination plane
surrounding the central Pd atom.
Kinetic Lability of the Phosphapalladacycle Pd-C
Bond. In many publications devoted to the chemistry of
palladacycles, much has been investigated about the stability
and therefore the Pd-C bond reactivity. In particular, this
bond was known to be reactive toward various nucleophiles.5
In fact, one of the other main interests of cyclopalladated
complexes was based on the exploitation of the reactivity
of this bond as a means of functionalizing the metalated
chelate so that a plethora of organic molecules could be
derived thereafter. For the palladacycles of the 1-(1′-napthyl)-
ethyldiphenylphosphine/arsine13a and later, 1-(2′,5′-
dimethylphenyl)ethyldiphenylphosphine13b that we have de-
veloped, the Pd-C bonds of such systems were found to be
unstable in the presence of concentrated HCl, so that the
addition of one drop of the latter to NMR samples of the
CDCl3 solutions of their chloro-bridged dimers at room
temperature was sufficient to bring about the immediate
rupture of these Pd-C bonds. In all the three cases, the
palladacycles could not survive and were dechelated instan-
taneously to the η1-P (or η1-As) monodentate dimers via
Pd-C bond cleavage and protonation.
In contrasting fashion, 31P{1H} NMR spectroscopy re-
vealed no chemical transformation when the same test
was applied to the current phosphapalladacycle dimer, (()-
3. To determine if the phosphapalladacycle was able to
withstand an even harsher reaction condition, the same
complex was subsequently treated with excess concentrated
HCl in refluxing acetone for 2 h. Quite remarkably, a similar
31P{1H} NMR spectral picture of the original dimer was
observed, suggesting that no reaction had taken place and
that the Pd-C bond had remained intact in spite of such
harsh conditions. However, it has to be emphasized that
the above treatment was applied to 3 in the racemic form
because repeating the above experiment using the
optically active (S,S)-3 led unexpectedly to the racemization
of the phosphapalladacycle. This important fact was estab-
lished from two experimental observations. First, dimer 3
was no longer optically active, with [R] ) 0 determined at
various wavelengths. Additionally, treatment of the product
with excess potassium (S)-prolinate revealed the presence
of two sets of diastereomers in equal intensities from
the 31P{1H} NMR spectrum of the obtained prolinate
derivatives 4, that is, an identical spectral pattern that
was also obtained from (()-3. Racemization must have
proceeded via a Pd-C bond cleavage because this generates
symmetry on the η1-P coordinated phosphine ligand.
It then follows that either one of the available two
enantiotopic phenyl rings could ortho palladate thereafter
(Scheme 2).
The above results seemed to suggest that the phosphapal-
ladacycle was undergoing reversible ring opening/closing via
the repeated Pd-C bond cleavage/re-formation under the
above reflux condition. Such an activity could not have been
detected from the racemic dimer alone because no change
in optical activity could be anticipated before and after the
experiment. In other words, in such acidic medium, the
5106 Inorganic Chemistry, Vol. 46, No. 12, 2007

Cyclopalladation and Kinetic Lability
Scheme 2
Scheme 3
Scheme 4
Scheme 5
phosphapalladacycle is unstable kinetically, but is itself the
thermodynamically stable product. The strong tendency of
the phosphine to undergo ring closure from the η1-P
monodentate state, after hydrolysis by HCl, is therefore
obvious here. This inclination is most probably stimulated
by the steric effects of the bulky t-butyl substituents. This
tendency was so strong that the presence of concentrated
HCl was only sufficient to labilize the Pd-C bond and not
bring about its permanent dissection.
It is important to understand that the racemization
process could not have proceeded via the alternate P f Pd
dative bond cleavage as illustrated in Scheme 3. This process
is immediately ruled out because ring opening of the
phosphapalladacycle by acid protonation of the P donor
would not have led to a symmetrization of the phosphine
ligand but would preserve the stereogenic integrity of the
R-C atom and hence the overall chirality of the Pd-C-bonded
phosphonium ligand in the intermediate (S,S)-7, by the
continued presence of four different substituents on the R-C
atom.
The phosphine has therefore displayed a very strong
tendency to remain in the ortho-palladated state and that this
mode of coordination is much preferred over the simple η1-P
monodentate. In fact, the strong inclination of the phosphine
toward the ortho-palladated state over the latter coordination
mode was earlier exemplified from its reaction with the agent
Li2[PdCl4] in 2:1 (ligand/metal) stoichiometry in the absence
of any external Brønsted base. Notably, neither the formation
Inorganic Chemistry, Vol. 46, No. 12, 2007 5107

Ng et al.
of the coordinated complex, dichlorobisphosphinepalladium-
(II), nor the adduct of the type 8 was isolated, in which both
Figure 2. Spectroscopic evidence for phosphine coordination in (R)-12
by comparison of (a) 1H and (b) 1H{31P} spectra for the signals of the
(R)-1-(1′-naphthyl)ethylaminate palladacycle R-CH proton.
complexes contain at least one simple P-coordinated mono-
dentate unit of the phosphine ligand. Their formations may
not be ruled out given the initial 2:1 (ligand/metal) stoichi-
ometry. Clearly, the strong tendency of the phosphine to
achieve an ortho-palladated structure and its difficulty to
remain in a simple η1-P monodentate coordination bonding
as a consequence of the stimulating effect of the bulky t-butyl
groups is again manifested here. Therefore, the η1-P
monodentate type of coordination alone does not impart
improved stability to the free, uncoordinated ligand. As such,
the ortho-palladated mode of coordination can be seen as
the more-stable mode of coordination over the former η1-P
monodentate type, so that it converts spontaneously to the
cyclopalladated mode whenever pathways leading to the
latter are available. This behavior is further supported by
the observations in the following.
Figure 3. Spectroscopic evidence for phosphine coordination in (R)-12:
by comparison of (a) 1H and (b) 1H{31P} spectra for the signals of the
(R)-1-(1′-naphthyl)ethylaminate palladacycle axial and equatorial NMe
protons.
It is well-known that µ-chloro dimers of cyclopalladated
amines such as the chiral N,N-dimethylamino-1-(1′-naphth-
yl)ethylaminate system react rather easily with a wide range
of monodentate phosphines and arsines via the cleavage of
the chloro bridges to form trans-(N,E) mononuclear adducts
(where E ) P/As). These contain the stable Pd-Cl bond,
both thermodynamically and kinetically.27 The stability of
this bond was exemplified from the possibility to isolate the
optically resolved forms of the mononuclear compounds of
1-(1′-napthyl)ethyldiphenylphosphine/arsine13a and 1-(2′,5′-
dimethylphenyl)ethyldiphenylphosphine13b in the η1-E co-
ordination modes despite their abilities to ortho palladate.
Given the restriction (to cyclopalladate) imposed by the Pd-
Cl bond, it was therefore of little surprise when no reaction
was observed (from 31P NMR spectroscopy) when phosphine
2 was treated with the chloro dimer of (R,R)-N,N-dimethy-
lamino-1-(1′-naphthyl)ethylaminate in the usual 2:1 stoichi-
ometry. Here, the transformation of the phosphine from the
monodentate mode to the cyclopalladated mode becomes
remote because of the presence of the Pd-Cl bond. Because
the monodentate η1-P coordination presents little incentive
over the free ligand in the uncoordinated state, the phosphine
remains uncoordinated.
protons and the R-C methine proton of the N,N-dimethy-
lamino-1-(1′-naphthyl)ethylaminate palladacycle in the H
1
NMR spectrum of the crude compound. These were deter-
mined to be 5.0, 1.8, and 3.8 Hz, respectively. Moreover,
the existence of 4JPH coupling as the only observed 1H-31P
spin-spin coupling for the palladacycle R-C methine proton
is also indicative of the usual trans-(P, N) geometry of the
complex. This conclusion was made on the basis of a
previous report of similar complexes of C2-symmetric chiral
diphosphine ligands chelated to the N,N-dimethylamino-1-
(1′-phenyl)ethylamine palladacycle. It revealed that the
palladacycle R-C methine proton had spin-coupled to only
one of the two 31P nuclei and, specifically, the 31P nucleus
cis to the adjacent Pd-C bond.28 This trans-(P, N) arrange-
ment was dictated by the electronic directing effects of the
hard/soft natures of the N and C- donors of the aminate
palladacycle10c,11,29 and is attributed to the result of “anti-
symbiosis”.30 Support for the direct phosphine attachment
was adequately provided by a comparison of the 1H and 1H-
{31P} NMR spectra for the resonance of these three sets of
protons (Figures 2 and 3). The 31P{1H} NMR spectroscopic
chemical shift of the observed complex, at δ 54.6 (CDCl3)
only indicates a monodentate η1-P coordination mode of
the phosphine. This is because the rather small value of the
coordination shift, ∆δ, at 7.5 ppm with respect to the
chemical shift of the free ligand, does not justify the
formation of a five-membered ring that is satisfied by the
ortho palladated structure.25 Even then, the monodentate
η1-P coordination was only detected in solution spectro-
However, phosphine 2 could be coerced to bind in the
η1-P coordination mode when an extra stimulus was
provided by the improved Lewis acidity of the Pd(II) center
in the form of positively charged complex (R)-11. This was
observed from the presence of the 1H-31P spin-spin
couplings for the axial and equatorially disposed NMe
5108 Inorganic Chemistry, Vol. 46, No. 12, 2007

Cyclopalladation and Kinetic Lability
scopically, as attempts made to isolate the complex by slow
crystallization have led to the isolation of the starting material
(R)-11 in almost quantitative yield. The gradual de-coordina-
tion of the phosphine ligand is therefore evident, which once
again points to the low thermodynamic stability of complex
(R)-12 and therefore, the continued difficulty of the phos-
phine to be engaged in a simple η1-P monodentate coor-
dination. Ring closure following the monodentate coordina-
tion in this case was probably prohibited by the unfavorable
trans-(aryl, aryl) arrangement known as the phenomenon of
“transphobia”,31 notwithstanding the possible coordination
vacancy that could be generated by the withdrawal of the
rather weakly coordinated acetonitrile ligand from the Pd-
(II) center.
phosphine from the cyclopalladated state to the monodentate
η1-P state was not observed. The ortho-palladated state,
rather than the monodentate η1-P state, was the more-
favored choice of coordination, so that whenever the forma-
tion of the former was rendered impossible, the phosphine
would preferably remain in the free, uncoordinated state. This
was supported by its lack of reactivity toward the µ-chloro
dimer of the N,N-dimethyl-1-(1′naphthyl)ethylaminate pal-
ladacycle. The tendency of the phosphine to cyclopalladate
is attributed to the stimulating steric effect presented by the
bulky t-butyl substituents at the P donor.
It is noteworthy that the kinetic lability of the Pd-C bond
in the phosphapalladacycle could not have been discovered
in the absence of the resolved form of the complex. At the
time of writing, investigations related to the asymmetric ortho
palladation of the phosphine and the synthetic applications
of the enantiomeric form of the newly prepared phosphapal-
ladacycle are in progress.
Conclusions
Direct ortho palladation of the title phosphine was readily
accomplished by the use of palladium(II) acetate or even
the less effective metallating agent Li2[PdCl4],17l,23 despite
employing 1:2 (metal/ligand) stoichiometry. The resulting
phosphapalladacycle was then obtained in the optically active
form by optical resolution involving the separation of the
(S)-prolinate diastereomeric derivatives. Racemization of the
optically active phosphapalladacycle was noted in refluxing
acetone/HCl. This was the result of the lability of the Pd-C
bond, which was undergoing reversible cleavage under this
condition. Notably, in contrast to previous experiences
involving other similar palladacycles, a permanent rupture
of the Pd-C bond and therefore the transformation of the
Acknowledgment. We thank the National University of
Singapore and Nanyang Technological University for the
awards of the research scholarships to J.K.P.N. and S.C.,
respectively.
Supporting Information Available: Crystal data, data collec-
tion, solution and refinement, final positional parameters, bond
distances and angles, thermal parameters of non-hydrogen atoms,
and calculated hydrogen parameters for (S)-5. This material is
available free of charge via the Internet at http://pubs.acs.org
IC0702320
Inorganic Chemistry, Vol. 46, No. 12, 2007 5109