Application of tautomerism of ferrocenyl secondary phosphine oxides in Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1021/om900435y

Several new ferrocenyl secondary phosphine oxides, (η⁵- C₅H₄-P(=O)(Ph)(H))₂Fe (7aB) and (η⁵-C₅H₄-P(=O)(R)(H))(η⁵- C₅H₅)Fe (7M, a: R = Ph; c: R =

Full text of DOI:10.1021/om900435y

6044 Organometallics 2009, 28, 6044–6053
DOI: 10.1021/om900435y
Application of Tautomerism of Ferrocenyl Secondary Phosphine Oxides
in Suzuki-Miyaura Cross-Coupling Reactions
Lin-Ying Jung, Shih-Hung Tsai, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40227,
Taiwan, Republic of China
Received May 24, 2009
Several new ferrocenyl secondary phosphine oxides, (η5-C5H4-P(dO)(Ph)(H))2Fe (7aB) and
(η5-C5H4-P(dO)(R)(H))(η5-C5H5)Fe (7M, a: R = Ph; c: R = Cy), were prepared and characterized
by spectroscopic means. Further reaction of Pd(COD)Cl2 with 2 molar equiv of (η5-C5H4-P(dO)-
(R)(H))2Fe (7bM, b: R = tBu) led to the formation of the palladium dimer [(7b0M)(7b0M-Hþ)Pd( μ-
Cl)]2, 8. In addition, the reaction of Pd(OAc)2þwith 1 molar equiv of 7bM yielded a bis-7bM-
coordinated palladium acetate, [(7b0M)(7b0M-H )Pd(OAc)], 9. The molecular structures of 7aB,
7aM, 7bM, 8, and 9 were determined by single-crystal X-ray diffraction methods. A tautomeric
equilibrium between 7aB and its isomeric form (η5-C5H4-P(OH)(Ph))2Fe (7a0B) indeed took place.
This was also true for the case of 7aM and its isomeric form (η5-C5H4-P(OH)(Ph))(η5-C5H5)Fe
(7a0M). Palladium-catalyzed Suzuki-Miyaura reactions employing 7aB and 7bM as ligands gave
satisfactory results.
1. Introduction
form (RR0POH, 1b) is well known (Scheme 1).4 It is the
phosphinous acid form 1b that exhibits good coordinating
capability toward low-valent transition metals. In the pre-
sence of base or transition metals, the balance might
shift from 1a to 1b, then to 1cM. Satisfactory catalytic
performances for palladium-catalyzed cross-coupling reac-
tions employing 1b-like phosphine ligands have been re-
ported.5
There are few published studies of the tautomerism of a
metal-containing phosphine oxide and the corresponding
phosphinous acid.6 Since the reactivity of the cyclopentadi-
nyl ring of the ferrocene has been well studied, modification
on the rings should be readily achievable. In this work, we
report the preparation of several iron-containing secondary
Over the last few decades, the realm of organic synthesis
has experienced prolific and beneficial development of tran-
sition metal-catalyzed cross-coupling reactions.1 Among
these fascinating synthetic methods, the Suzuki-Miyaura
reaction is probably the most commonly employed techni-
que in the formation of carbon-carbon bonds. This reaction
is characterized by a coupling process between an aryl halide
(or triflate) and organoborate, catalyzed by a ligand-assisted
palladium complex in basic medium.2 The success of the
Suzuki-Miyaura coupling can be ascribed to several benefi-
cial factors: (1) low toxicity of organoborates and their
byproduct; (2) commercial availability of diverse forms
of boronic acids and esters; (3) tolerable stability toward
air and water.3
(4) (a) Dixon, K. R.; Rattray, A. D. Can. J. Chem. 1971, 49, 3997–
4004. (b) Silver, B.; Luz, Z. J. Am. Chem. Soc. 1962, 84, 1091–1905. (c)
Luz, Z.; Silver, B. J. Am. Chem. Soc. 1962, 84, 1095–1098. (d) Doak, G. O.;
Freedman, L. D. Chem. Rev. 1961, 61, 31–44. (e) Luz, Z.; Silver, B. J. Am.
Chem. Soc. 1961, 83, 4518–4521.
A tautomeric equilibrium between a secondary phosphine
oxide (RR0HP(dO), 1a) and its less stable phosphinous acid
*To whom correspondence should be addressed. Fax: 886-4-
22862547. E-mail: fehong@dragon.nchu.edu.tw.
(5) (a) Yang, D. X.; Colletti, S. L.; Wu, K.; Song, M.; Li, G. Y.; Shen,
H. C. Org. Lett. 2009, 11, 381–384. (b) Pryjomska, I.; Bechowski, H. B.;
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174–238. (b) Corbet, J.-P.; Mignani, G. Chem. Rev. 2006, 106, 2651–2710.
Ciunik, Z.; Trzeciak, A. M.; Ziozkowski, J. J. Dalton Trans. 2006, 213–220.
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117, 7382–7386. Angew. Chem., Int. Ed. 2005, 44, 7216-7219. (d)
Ackermann, L.; Born, R. Angew. Chem. 2005, 117, 2497–2500. Angew.
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Hong, F.-E. Chem.;Eur. J. 2007, 13, 1583–1593. (d) Ackermann, L.
Synlett 2007, 507–526. (e) Ackermann, L. Synthesis 2006, 1557–1574.
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Published on Web 09/23/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 20, 2009 6045
Scheme 1. Tautomeric Equilibrium between a Secondary Phosphine Oxide 1a and Phosphinous Acid 1b as Well As in the Presence of Base
and Transition Metal
Scheme 2. Preparations of Ferrocenyl Phosphine Oxides and Their Tautomeric Phosphinous Acidsa
a Throughout the article the abbreviations of the species involved in the reaction are as follows: M for monodentate; B for bidentate; N for
diethylamino; C for chloride; a for R = Ph; b for R = tBu; c for R = Cy; 0 for phosphinous acid form.
Scheme 3
phosphine oxides and the exploration of their catalytic
performance in the Suzuki-Miyaura reaction.
procedures modified from the literature (Scheme 2).7 First,
ferrocene was deprotonated by equimolar amounts of nBuLi;
this was followed by adding equal moles of PPhCl(NEt2), 4a.
Compounds 6aNM and 6aNB were yielded in good quantity.
The 31P NMR spectra of 6aNM and 6aNB display signals at
55.0 and 54.2 ppm, respectively. The conversion of 6aNB (or
6aNM) to its corresponding secondary phosphine oxide
(η5-C5H4-P(dO)(Ph)(H))2Fe (7aB) (or (η5-C5H5)(η5-C5H4-
P(dO)(Ph)(H))Fe, 7aM) was achieved in acidic media accord-
ing to the ingenious preparative procedure described by Denis.7
The corresponding 6bNB (or 7bB) could not be made by the
same procedure probably due to severe steric hindrance bet-
ween the two bulky substituents.
2. Results and Discussion
2.1. Preparations of (η5-C5H5)(η5-C5H4-P(NEt2)(Ph))Fe,
6aNM, and (η5-C5H4-P(NEt2)(Ph))2Fe, 6aNB. A mono-
phosphino-subtituted ferrocene, (η5-C5H5)(η5-C5H4-
P(NEt2)(Ph))Fe, 6aNM, and diphosphino-subtituted
ferrocene, (η5-C5H4-P(NEt2)(Ph))2Fe, 6aNB, were prepared by
(7) (a) Baba, G.; Pilard, J.-F.; Tantaoul, K.; Gaumont, A.-C.; Denis,
J.-M. Tetrahedron Lett. 1995, 36, 4421–4424. (b) Pilard, J.-F.; Baba, G.;
Gaumont, A.-C.; Denis, J.-M. Synlett 1995, 1168–1170.

6046 Organometallics, Vol. 28, No. 20, 2009
Jung et al.
Compound 7aB was characterized by spectroscopic meth-
ods. There are eight discrete signals being observed for the
cyclopentadienyl protons in the 1H NMR. This implies that
these two cyclopentadienyl rings are not in an equivalent
environment. Presumably, an intramolecular hydrogen
bonding takes place between the two phosphino substituents
that causes discrete chemical shifts for the hydrogen atoms
(Scheme 3). The existence of P-H bonds in 7aB is also
evidenced by two sets of large coupling constants (JP-H
=
487.3 and 489.3 Hz) between them.
Fortunately, crystals of 7aB were grown from the CH2Cl2
solution, and the structure was determined by single-crystal
X-ray diffraction methods. The ORTEP diagram of 7aB
depicted in Figure 1 shows the presence of a ferrocenyl
secondary phosphine oxide. The presence of a PdO bond
˚
is also evidenced by its short bond length, 1.504(5) A. The
two phosphino groups are situated on the opposite ends
of the molecule (Figure 1). Interestingly, the crystal structure
does not confirm the required geometry to support the
existence of intramolecular hydrogen bonding. The 31P
NMR spectrum of 7aB displays only one signal at
19.2 ppm, which reflects the fact that 31P NMR is not as
sensitive as 1H NMR to a small environmental variation.
In principle, a tautomeric equilibrium between 7aB and
the less stable secondary phosphinous acid 7a0B could take
place in solution. Nevertheless, the 7aB form must be
strongly favored in the tautomeric equilibrium without the
presence of base or metal, judging from the pattern of the 31P
NMR spectrum with a large JP-H coupling constant. Theo-
retically, 7a0B could act as a bidentate ligand toward a
palladium complex. Therefore, the coordination of 7a0B to
palladium in basic media could yield a catalyst precursor that
is ready for the forthcoming coupling reactions (Scheme 2).
The 31P NMR spectra of 6aNM and 7aM display signals
at 55.0 and 19.8 ppm, respectively. As expected, the chemical
shift of 7aM is close to that of 7aB. The crystal structure of
7aM was determined by X-ray diffraction methods. The
ORTEP drawing of 7aM shows the presence of a ferrocenyl
secondary phosphine oxide (Figure 2). A tautomeric equi-
librium between 7aM and its less stable secondary phosphi-
nous acid 7a0M in solution is evidenced by the formation of
palladium complexes by using 7a0M as the coordinating
ligand. This will be demonstrated later. Therefore, 7a0M is
able to act as a monodentate ligand in palladium-catalyzed
cross-coupling reactions.
Figure 1. ORTEP drawing of 7aB. Hydrogen atoms are
omitted for clarity.
Figure 2. ORTEP drawing of 7aM. Hydrogen atoms are
omitted for clarity.
7.06 (JP-H = 458.5 Hz) for 7aM, 7bM, and 7cM, respec-
tively.
2.2. Preparations of (η5-C5H4-P(Cl)(tBu))(η5-C5H5)Fe,
6bCM, and (η5-C5H4-P(Cl)(Cy))(η5-C5H5)Fe, 6cCM. An
alternative preparation of monophosphino-subtituted ferro-
cenes (η5-C5H4-P(dO)(R)(H))(η5-C5H5)Fe (7M, a: R = Ph;
b: R = tBu; c: R = Cy) can be achieved by taking route 2
(Scheme 2). First, ferrocene was deprotonated by equimolar
amounts of BuLi, followed by adding equal moles of PRCl2
2. The formation of 6aCM (or 6bCM and 6cCM) was
completed within 24 h. The conversion of 6aCM (or 6bCM
and 6cCM) to its corresponding 7aM (or 7bM and 7cM) is
readily done by adding water. Compounds 7aM, 7bM, and
7cM were yielded in good quantity. The 31P NMR spectra of
7aM, 7bM, and 7cM display signals at 19.8, 47.5, and 37.3
ppm, respectively. The 1H NMR spectra of 7aM, 7bM, and
7cM show matching signals for cyclopentadienyl rings at
4.33, 4.36, and 4.36 ppm. The chemical shifts of the phos-
phorus attached protons (P-H) vary with the surround-
ing environments and have large coupling constants. They
are 8.03 (JP-H = 483.3 Hz), 6.88 (JP-H = 454.5 Hz), and
A tautomeric equilibrium between 7bM and its less stable
secondary phosphinous acid 7b0M in solution is evidenced by
the formation of palladium complexes via 7b0M as the
coordinating ligand. Compound 7bM has been reported;
however, no crystal structure was available.8,6a Fortunately,
we were able to obtain crystals of 7bM, and its structure
was determined by X-ray diffraction methods. Again, the
ORTEP drawing of 7bM reveals the presence of a ferrocenyl
secondary phosphine oxide (Figure 3). The PdO bond is
˚
evidenced by its short bond length, 1.484(2) A. A large
coupling constant (JP-H = 454.5 Hz) substantiates the
existence of a P-H bond in 7bM.
2.3. Reactions of 7bM with Pd(COD)Cl2 and Pd(OAc)2.
The reaction of Pd(COD)Cl2 with 2 equiv of (η5-C5H4-P-
(dO)(tBu)(H))2Fe (7bM) led to the formation of a palladium
(8) Intramolecular hydrogen bonding of related cases: (a) Wolf,
C.; Lerebours, R. Org. Lett. 2004, 6, 1147–1150. (b) Cobley, C. J.;
van den Heuvel, M.; Abbadi, A.; de Vries, J. G. Tetrahedron Lett. 2000,
41, 2467–2470. (c) Ref 6a
.

Article
Organometallics, Vol. 28, No. 20, 2009 6047
dimer, [(7b0M)(7b0M-Hþ)Pd( μ-Cl)]2 (8), accompanied with
the release of 2 molar equiv of HCl (Scheme 4). Note that
the ligand used here, 7bM, is a mixture of racemic forms.
Compound 8 was characterized by spectroscopic methods as
well as single-crystal X-ray diffraction. In 1H NMR, singlets
by spectroscopic methods as well as single-crystal X-ray
diffraction. Unexpectedly, the crystal structure of
[(7b0M)(7b0M-Hþ)Pd(OAc)] (9) reveals that it is a bis-7bM-
coordinated palladium acetate. It is believed that an original
ligand/palladium = 1:1 complex was converted to the more
stable complex 9 during the crystallization process. The
palladium metal is in fact coordinated by two phosphinous
acid ligands (Figure 5). The formation of the product was
accompanied with the release of 1 molar equiv of HOAc. One
was a ferrocenyl secondary phosphine oxide; the other a
deprotonated ferrocenyl secondary phosphine oxide coordi-
nated with palladium. One out of two acetates remained to
chelate to palladium metal via η2-means. The bond angle of
O(3)-Pd-O(4) is 59.67(12)°. Two Pd-O bond lengths,
t
at 1.10 and 4.76 ppm were shown for the Bu group and
cyclopentadienyl ring, respectively. A set of multiplets
around 4.40-4.65 ppm was assigned for the substituted
cyclopentadienyl ring. The 31P NMR spectrum also exhibited
a singlet at 108.1 ppm. The crystal structure of 8 reveals
that each palladium center is coordinated by two phosphi-
nous acid ligands, though one of the ligand is deprotonated
(Figure 4). There is an encapsulated diethyl ether molecule in
each of the asymmetric units. A hydrogen bond between the
oxygen atom (PdO) of the deprotonated phosphinous acid
and the hydrogen atom (P-OH) of the phosphinous acid
is observed.8,6a Two palladium metals are bridged by two
chlorides. The four atoms Pd(1), Cl(1), Pd(2), Cl(2) are
almost coplanar and in a diamond-shaped geometry. Two
ferrocenyl moieties are above the plane; the other two are
below the plane. The oxidation state of the palladium metal
remains þ2. The formation of 8 demonstrates that tauto-
merism indeed exists between ferrocenyl secondary phos-
phine oxide 7bM and its phosphinous acid form 7b0M. The
latter form is responsible for the coordination toward the
palladium metal.
Another reaction of 1 molar equiv of 7bM with Pd(OAc)2
was carried out in THF. The new product was characterized
Figure 4. ORTEP drawing of 8. Hydrogen atoms are omit-
˚
ted for clarity. Selected bond lengths (A) and angles (deg):
Pd(1)-P(1) 2.248(3); Pd(1)-P(2) 2.252(2); P(1)-O(1) 1.557(7);
P(1)-Pd(1)-P(2) 92.57(9); Cl(1)-Pd(1)-Cl(2) 82.66(9); Pd-
(2)-Cl(1)-Pd(1) 97.23(9); O(1)-P(1)-C(1) 106.1(5); O(1)-
P(1)-C(11) 106.0(5); C(1)-P(1)-C(11) 112.8(6); P(1)-O(1)-
H(1A) 109.5.
Figure 3. ORTEP drawing of 7bM. Hydrogen atoms are
omitted for clarity.
a
Scheme 4. Preparations of Palladium Complexes from the Reactions 7bM with Pd(COD)Cl2 and Pd(OAc)2
a Racemic mixtures of 7bM were employed.

6048 Organometallics, Vol. 28, No. 20, 2009
Jung et al.
˚
Figure 5. ORTEP drawing of 9. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (deg): Pd-P(1)
2.2265(11); Pd-P(2) 2.2450(11); P(1)-O(1) 1.546(3); P(1)-Pd-P(2) 91.51(4); P(1)-O(1)-H(1A) 109.5; C(30)-C(29)-Pd 178.5(3);
O(3)-Pd-O(4) 59.67(12).
Scheme 5. Secondary Phosphine Oxide Assisted Palladium-
Catalyzed Suzuki-Miyaura Reactions
˚
2.181(3) and 2.194(3) A, from acetate to palladium are
almost the same. Again, intramolecular hydrogen bondings
between deprotonated and non-deprotonated ferrocenyl
secondary phosphine oxides were observed.8,6a The bond
˚
length of PdO is 1.546(3) A, indicating the existence of a
double bond. The two bonds between P and Pd can be
regarded as one dative bond plus a covalent bond. The six
atoms Pd, P(1), P(2), O(3), O(4), and C(29) are almost
coplanar. Two ferrocenyl moieties are on the same side of
Table 1. Suzuki-Miyaura Coupling Reactions Employed Various
Palladium Sourcesa
entry
palladium source
NMR conv (%)b
t
the plane; two Bu groups are on the other side. The bond
angle of P(1)-Pd-P(2) is 91.51(4)°. The charge of the
1
2
3
4
5
6
Pd(OAc)2
Pd2(dba)3
PdCl2
PdCl2(CH3CN)2
[(η3-C3H5)PdCl]2
Pd(COD)Cl2
94
83
88
71
46
40
1
palladium metal remains þ2. Judging from the H and 31P
NMR spectra of 9, the two ligands are in slightly different
environment. Although secondary phosphine oxide chelated
PdCl2 complexes are known,8a to the best of our knowledge
this is the first example of a two secondary phosphine oxide
ligand coordinated Pd(II)(OAc) complex.9
a Conditions: 1.0 mmol of 4-bromobenzaldehyde, 1.5 mmol of phe-
nylboronic acid, 2.0 mmol of K2CO3, 1 mL of THF, and 1 mol % Pd/
7aB, Pd/7aB = 1:1, 60 °C, 15 h. b Determined by 1H NMR.
2.4. Application of 7aB in Palladium-Catalyzed Suzu-
ki-Miyaura Reactions. As demonstrated, the best perfor-
mance of a palladium-catalyzed carbon-carbon cross-
coupling reaction can be achieved by optimizing the major
factors involved.10 Palladium-catalyzed Suzuki-Miyaura
reactions of bromobenzene with 4-bromobenzaldehyde were
carried out by employing 7aB as ligand precursor
(Scheme 5).
To begin with, the efficiencies of several commonly used
palladium sources were screened. Suzuki-Miyaura reac-
tions of 4-bromotoluene and phenylboronic acid employing
7aB with various Pd sources in a K2CO3/THF system were
carried out at 60 °C for 15 h (Table 1). As shown, the best
yield was obtained while using Pd(OAc)2 as the palladium
source (entry 1). This is a consistently common obser-
vation.11
The ratio of ligand to metal is also a critical factor in the
ligand-assisted palladium complex-catalyzed cross-coupling
reactions. Conventionally, the ratio of 7aB/Pd(OAc)2 should
be 1:1 according to the presumed composition of the active
catalyst precursor. The impact of the 7aB/Pd(OAc)2 ratio on
the catalytic efficiency was evaluated here. In order to be able
to differentiate the performances of the catalytic reactions, a
less efficient 4-bromotoluene rather than 4-bromobenzalde-
hyde was employed. Unexpectedly, better catalytic efficien-
cies are shown for the ratio of 7aB/Pd(OAc)2 greater than 1.
(9) Amoroso, D.; Bell, A.; Protasiewicz, J. D. Organometallics 2005,
24, 4099–4102.
(10) (a) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.;
Clapp, A. R.; Uyeda, H. T.; Deschamps, J. R.; Lassman, M. E.;
Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 6744–6751. (b) Braga,
A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem. Soc.
2005, 127, 9298–9307.
(11) (a) Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314–321.
ꢀ
(b) Amatore, C.; Carre, E.; Jutand, A.; M'Barki, M. A.; Meyer, G. Organo-
metallics 1995, 14, 5605–5614.

Article
Organometallics, Vol. 28, No. 20, 2009 6049
Table 2. Suzuki-Miyaura Coupling Reactions Employing
Table 4. Suzuki-Miyaura Coupling Reactions Employing
Various Pd(OAc)2/7aBa
[Pd]/7aB
Various Solvent Ratios and Basesa
entry
NMR conv (%)b
entry
solvent
base
time (h) NMR conv (%)b
1
2
3
4
5
6
7
8
1/0
1/0.5
1/1
1/2
1/3
1/1
1/2
1/3
11
49
55
62
1
2
3
4
5
6
7
8
H2O
H2O
THF
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOH
2
4
4
4
4
4
4
4
2
2
2
2
2
2
1
1
1
91
>99
91
89
78
35
11
1
70
0
14
21
53
69
CH3OH
68
1,4-dioxane
toluene
DMF
58c
64c
73c
CH3CN
9
tBuOH/H2O (0:1)
tBuOH/H2O (0:1)
tBuOH/H2O (1:0)
tBuOH/H2O (1:0)
a Conditions: 1.0 mmol of 4-bromotoluene, 1.5 mmol of phenylboro-
nic acid, 2.0 mmol of K2CO3, 1 mL of THF, and 1 mol % Pd(OAc)2, 19
h, 60 °C. b Determined by 1H NMR, average of two runs. c 24 h.
10
11
12
13
14
15
16
17
NaOtBu
NaOH
tBuOH/H2O (0.5:0.5) NaOtBu
tBuOH/H2O (0.5:0.5) NaOH
Table 3. Suzuki-Miyaura Coupling Reactions Employing
H2O (1 mL)
H2O (2 mL)
H2O (3 mL)
NaOtBu
NaOtBu
NaOtBu
52 (70)c
39 (56)c
26 (52)c
Various Basesa
entry
base
time (h)
NMR conv (%)b
a Conditions for entries 1-8: 1.0 mmol of 4-bromotoluene, 1.5 mmol
of phenylboronic acid, 2.0 mmol of base, 1 mL of solvent, and 1 mol %
Pd(OAc)2/7aB = 1:1, 50 °C; 40 °C for entries 9-14. b Determined by 1H
NMR. c 2 h.
1
2
3
4
5
6
7
8
NaOtBu
NaOtBu
K3PO4
KOH
4
12
19
19
19
19
19
19
19
19
19
90
>99
88
88
80
78
66
52
29
CsF
KOtBu
NaOH
K2CO3
KF
either by adding it directly or produced indirectly from the
reaction with water. In addition, the concentration of the
reactant, by changing the amount of solvent, indeed affects
the rate of the reaction (entries 15-17). A more concentrated
solution leads to the better performance.
9
10
11
Cs2CO3
CsOH
27
14
It was also a common observation that in Suzuki-
Miyaura coupling reactions a better conversion is achieved
with aryl halides bearing an electron-withdrawing than with
an electron-donating substituent.13 In fact, this is mostly
correct for the reaction with the oxidative addition process as
the rate-determining step (rds).14 As shown in Table 5, less
effective performances were observed for substances with
electron-donating groups than the unsubstituted case
(entries 2-5 vs 1). Nevertheless, the steric effect plays a more
critical role than the electronic effect here (entries 6, 7).
Unfortunately, this system does not work well for the cases
of aryl chlorides (entries 8-10).
2.5. Application of 7bM in Palladium-Catalyzed Suzuki-
Miyaura Reactions. In principle, 7bM should act as a mono-
dentate ligand. Its reactivity is expected to be different from
that of the disubstituted 7aB. Suzuki-Miyaura reactions
were carried out with 7bM employed as ligand. Several
factors that might be crucial to the performance were
examined. First, the efficiencies of several commonly used
palladium sources were screened. Here, Suzuki-Miyaura
reactions of 4-bromoaldehyde and phenylboronic acid em-
ploying 7bM with various Pd sources in a K2CO3/THF
system were carried out at 60 °C for 7 h (Table 6). In order
to differentiate the reactivities of various palladium sources,
a less efficient base, K2CO3, was used. As shown, the best
yield was obtained when using Pd(OAc)2 as the palladium
source (entry 1).
a Conditions: 1.0 mmol of 4-bromotoluene, 1.5 mmol of phenylboro-
nic acid, 2.0 mmol of base, 1 mL of THF, and 1 mol % Pd(OAc)2, Pd:7aB
= 1:1, 50 °C. b Determined by 1H NMR.
The optimum ratio might even be as high as 3:1 (Table 2,
entries 1-5). In order to increase the amount of 7a0B, the
effective phosphine form in the coordination toward palla-
dium center, a large excess of 7aB is needed. This can be
explained by the imbalanced tautomeric equilibrium that
favors 7aB over 7a0B. Otherwise, long reaction hours did not
improve the yield (entries 6-9).
As demonstrated, the type of base employed in the Suzu-
ki-Miyaura reaction is crucial to the catalytic efficiency.12 It
is also the most unpredictable element among all the factors
that affect performance. The influence of the bases used in
these secondary phosphine oxide-catalyzed coupling reac-
tions was examined (Table 3). As shown, the best result came
with NaOtBu used as the active base.
The solubility of substances and catalyst in solvent is
greatly affected by the polarity. A careful selection of
appropriate solvent is critical to the performance of the
catalytic reaction. Interestingly, the best result was observed
in aqueous solution using NatOBu as base (Table 4, entry 2).
This might be partly attributed to the formation of highly
polar deprotonated 7a0B-chelated palladium complex in
base media, which is best dissolved in water. Since the
organic reactants are not particularly soluble in aqueous
solution, the tBuOH, yielded from the addition of NatOBu in
water, is responsible for their dissolubility in solution. A
careful examination by varying the ratio of tBuOH/H2O in
either NaOtBu or NaOH as base (entries 9-14) found that
Since various bonding modes are possible for different
types of ligands, the ratio of ligand to palladium is also crucial
in Suzuki-Miyaura coupling reactions. Suzuki-Miyaura
t
the yields are rather poor without the presence of BuOH
(13) Kataoka, N.; Shelby, Q.; Stambuli, J. P.; Hartwig, J. F. J. Org.
Chem. 2002, 67, 5553–5566.
(14) (a) Singh, R.; Viciu, M. S.; Kramareva, N.; Navarro, O.; Nolan,
S. P. Org. Lett. 2005, 7, 1829–1832. (b) Limmert, M. E.; Roy, A. H.;
Hartwig, J. F. J. Org. Chem. 2005, 70, 9364–9370. (c) Xiong, Z.; Wang, N.;
Dai, M.; Li, A.; Chen, J.; Yang, Z. Org. Lett. 2004, 6, 95–98. (d) Dai, C.; Fu,
G. C. J. Am. Chem. Soc. 2001, 123, 2719–2724.
(12) (a) Dubbaka, S. R.; Vogel, P. Org. Lett. 2004, 6, 95–98. (b) Grasa,
G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077–1080. (c) Miyaura,
N.; Yamada, K.; Suginome, H.; Suzuki, A. J. Am. Chem. Soc. 1985, 107,
972–980.

6050 Organometallics, Vol. 28, No. 20, 2009
Jung et al.
Table 5. Suzuki-Miyaura Coupling Reactions Employing
Table 7. Suzuki-Miyaura Coupling Reactions Employing
Various Pd(OAc)2/7bM a
Various Aryl Halidesa
entry
[Pd]/7bM
NMR conv (%)b
1
2
3
4
5
6
7
8
1:0
1:1
1:2
1:3
1:0
1:1
1:2
1:3
9
36
27
29
15c
84c
57c
68c
a Conditions: 1.0 mmol of 4-bromotoluene, 1.5 mmol of phenylboro-
nic acid, 2.0 mmol of K2CO3, 1 mL of THF, and 1 mol % Pd(OAc)2, 4 h,
60 °C. b Determined by 1H NMR, average of two runs. c 24 h.
Table 8. Suzuki-Miyaura Coupling Reactions Employing
Various Basesa
entry
base
time (h)
NMR conv (%)b
1
2
3
4
5
6
7
8
NaOtBu
K2CO3
NaOH
K3PO4
NaOtBu
K2CO3
NaOH
K3PO4
NaOtBu
K3PO4
NaOtBu
K3PO4
1
1
1
1
2
2
2
2
2
2
4
4
67
7
19
72
>99
10
28
90
68
9
10
11
12
19
70
27
a Conditions: 1.0 mmol of 4-bromotoluene (entries 1-8) or 4-bro-
moanisole (entries 9-12), 1.5 mmol of phenylboronic acid, 2.0 mmol of
base, 1 mL of THF, and 1 mol % Pd(OAc)2, [Pd]:7bM = 1:1, 60 °C.
b Determined by 1H NMR.
Table 9. Suzuki-Miyaura Coupling Reactions Employing
Various Solventa
entry
solvent
THF
toluene
H2O
CH3OH
1,4-dioxane
DMF
THF
toluene
H2O
time (h)
NMR conv (%)b
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
2
2
2
2
2
67
61
NR
77
40
22
94
63
NR
80
62
34
a Conditions: 1.0 mmol of aryl bromide, 1.5 of mmol phenylboronic
acid, NaOtBu 2.0 mmol, H2O 1 mL, and 1 mol % Pd(OAc)2, Pd:L = 1:1,
60 °C, 2 h. b Isolated yield, average of two runs. c NMR conversion.
Table 6. Suzuki-Miyaura Coupling Reactions Employed Various
Palladium Sourcesa
entry
palladium source
NMR conv (%)b
9
10
11
12
CH3OH
1,4-dioxane
DMF
1
2
3
4
5
6
Pd(OAc)2
Pd2(dba)3
PdCl2
PdCl2(CH3CN)2
[(η3-C3H5)PdCl]2
Pd(COD)Cl2
77
54
57
69
10
38
a Conditions: 1.0 mmol of 4-bromotoluene, 1.5 mmol of phenylboro-
nic acid, 2.0 mmol of base, 1 mL of solvent, 1 mol % Pd(OAc)2, [Pd]:7bM
= 1:1, 60 °C. b Determined by 1H NMR.
a Conditions: 1.0 mmol of 4-bromobenzaldehyde, 1.5 mmol of phe-
nylboronic acid, 2.0 mmol of K2CO3, 1 mL of THF, and 1 mol %
palladium salt, [Pd]/7bM = 1:1, 60 °C, 7 h. b Determined by 1H NMR.
the reaction was run in THF with 4-bromotoluene (entries
1-8). Again, a less efficient 4-bromoanisole, rather than 4-
bromotoluene, was used to differentiate the reactivities of
these two bases (entries 9-12). It was found that NaOtBu is
more effective than K3PO4 in this case.
The effects of solvents used in the coupling reaction were
evaluated also (Table 9). The best yield was observed using
THF as the solvent (entry 7). Although their polarities are
close, 9.8 for 1,4-dioxane and 9.1 for THF, the catalytic
performances are quite different. Water is not an adequate
solvent due to poor solubility and hydrolysis of the ligand
(entries 3 and 9).
coupling reactions were carried out in situ by employing
various ratios of 7bM/Pd(OAc)2. A less efficient 4-bromoto-
luene was used than 4-bromobenzaldehyde to discriminate the
performances of various ratios of ligand to palladium salt
system (Table 7). Unexpectedly, the best yield was obtained
with 7bM/Pd(OAc)2 = 1:1 (entries 2 and 6). The catalytic
performances are consistently better for systems fixed in this
ratio even with different reaction times.
Subsequently, the impact of the base used in the reac-
tion was examined (Table 8). Among all the bases used,
NaOtBu and K3PO4 were found to be more effective when
The following table shows the results from the 7bM-
assisted Suzuki-Miyaura cross-coupling reactions with

Article
Organometallics, Vol. 28, No. 20, 2009 6051
Table 10. Suzuki-Miyaura Coupling Reactions Employing
The crystal structures of both 8 and 9 reveal that the ratio
of the coordinated ligand to the palladium metal is 2:1.
Nevertheless, the before-mentioned work shows that the
catalytic performance is better with an equal ratio of ligand
and palladium. However, the composition of a crystallized
compound may not necessarily represent the true formula of
the catalytically active species. In an 31P NMR experiment,
the 7bM-coordinated catalytically active species was mon-
itored while adding 7bM to Pd(OAc)2 in the ratio of 1:1.
Initially, a signal was observed in the 31P NMR at 118 ppm;
this slowly changed to 108 ppm in 60 °C within 1 h.
Precipitation of gray metal particles, presumably aggregated
palladium black, was observed along with the change of the
NMR signal. Latterly, the composition of the palladium
complex in solution was identified as 9 by 31P NMR mea-
surement since it showed identical chemical shift to that of 9
from a purified crystalline form. The unstable catalytic
precursor 90, which is responsible for the chemical shift at
118 ppm, was proposed as the original product (Scheme 4).
Presumably, it converted to 9 by the process of dispropor-
tionation and elimination of part of the palladium moiety. It
is assumed that all these potentially catalytic precursors have
the divalent palladium metal in the center of a tetracoordi-
nated, square-planar environment. These Pd(II) species are
all be reduced to Pd(0) before the catalytic cycle starts.
Summary. We have developed a general route for the
preparation of ferrocenyl secondary phosphine oxides. They
have been employed as phosphine ligands through tautomeric
equilibrium in basic media and proven themselves to be fairly
efficient in the palladium-catalyzed Suzuki-Miyaura reac-
tion. Since the secondary phosphine oxides are not prone to
oxidation, employment of secondary phosphine oxides is an
applicable choice for the purpose of long-term storage.
Various Aryl Halidesa
3. Experimental Section
3.1. General. All operations were performed in a nitrogen-
flushed glovebox or in a vacuum system. Freshly distilled
solvents were used. All processes of separations of the products
were performed by centrifugal thin layer chromatography
(CTLC, Chromatotron, Harrison model 8924) or column chro-
matography. GC analyses were performed on a HP-5890 FID
GC with a QUADREX 007-CW fused silica 30 m column, and
data were recorded on a HP ChemStation. Most of the 1H NMR
spectra were recorded on a 300 MHz Varian VXR-300S spectro-
meter. The chemical shifts are reported in ppm relative to
internal standards CHCl3 (δ = 7.26), CH2Cl2 (δ = 5.30), or
CH3C(dO)CH3 (δ = 2.09). 31P and 13C NMR spectra were
recorded at 121.44 and 75.46 MHz, respectively. The chemical
shifts for the former and the latter are reported in ppm relative to
internal standards H3PO4 (δ = 0.0) and CHCl3 (δ = 77) or
CH2Cl2 (δ = 53), respectively. In addition, some routine 1H
NMR spectra were recorded on either a Gemini-200 spectro-
meter at 200.00 MHz or a Varian-400 spectrometer at 400.00
MHz. Mass spectra were recorded on a JEOL JMS-SX/SX
102A GC/MS/MS spectrometer. Elemental analyses were re-
corded on a Heraeus CHN-O-S-Rapid.
a Conditions: 1.0 mmol of aryl bromide, 1.5 mmol of phenylboronic
acid, NaOtBu 2.0 mmol, THF 1 mL, and 1 mol % Pd(OAc)2, [Pd]:7bM =
1:1, 60 °C, 1 h. b Isolated yield; average of two runs. c NMR conversion.
substituted aryl bromides (Table 10). It was less effective for
the substrate with an electron-donating group. As expected,
the catalytic efficiency was lower when the substrate bearing
substituent had severe steric hindrance (entry 7). Never-
theless, an unexpected good performance was observed for
2-bromotoluene (entry 3). Less effective performances for
the cases of aryl chlorides were observed (entries 8-12).
Good to excellent yields were reported by Wolf for the
POPd- or POPd1-catalyzed Suzuki-Miyaura cross-
coupling of 4-chloroquinoline derivatives and boronic acid.
Here, POPd and POPd1 stand for trans-[(tBu)2P(OH)]2-
PdCl2 and [[(tBu)2P(OH)(tBu)2PO)]PdCl]2, respectively.
The reactions were carried out in 1,4-dioxane at 100 °C for
20 h.6g Recent work of Li showed similar efficiency in
Suzuki-Miyaura cross-coupling of aryl halides and 6-meth-
oxypyridyl-2-boronic ester using PXPd, trans-[(tBu)2PCl]2-
PdCl2, as the catalyst precursor.5a
3.2. Synthesis and Characterization of 7aB. A 100 mL round-
bottomed flask charged with a magnetic stir bar was placed with
2.00 mmol of ferrocene (0.372 g) and 12 mL of hexanes. The
solution was kept stirring at 0 °C while 2.2 molar equiv of
n-BuLi and TMEDA were slowly added. After 0.5 h the solution
was allowed to warm to 25 °C and stirred for another 8 h. A dark
red solid, 1,10-dilithioferrocene, was obtained after the removal
of solvent under reduced pressure.
In a drybox, dichlorophenylphosphine (2.00 mmol, 0.358 g)
and bis(diethylamino)phenylphosphine (2.00 mmol, 0.505 g)

6052 Organometallics, Vol. 28, No. 20, 2009
Jung et al.
were dissolved in 2.5 mL of THF separately in 50 mL flasks.
Then, these two solutions were mixed and stirred at 25 °C for
1.5 h. A colorless oily product, chloro(diethylamino)phe-
nylphosphine, resulted.
3.4. Synthesis and Characterization of 8. In a N2-flashed
100 mL round-bottomed flask, 0.290 g (0.100 mmol) of 7bM
and 0.285 g (0.10 mmol) of Pd(COD)Cl2 were placed with 8 mL
of CH2Cl2. The solution was kept stirring at 60 °C for 1 h before
the solvent was removed under reduced pressure, and the residue
was separated by CTLC. A red-colored band was eluted using
a mixed solvent mobile phase (EA/CH2Cl2 = 10:1) and was
identified as [(7b0M)(7b0M-Hþ)Pd( μ-Cl)]2, 8. The yield of 8 is
86% (0.064 g, 0.043 mmol).
At 0 °C, chloro(diethylamino)phenylphosphine was added to
the solution of 1,10-dilithioferrocene, in 10 mL of THF. The
solution was allowed to warm to 25 °C and stirred for another
5 h. Subsequently, the solvent was removed under reduced
pressure and the residue was separated by CTLC. A dark yellow
colored band was eluted using a mixed solvent mobile phase
(MeOH/CH2Cl2 = 1:6) and was identified as (η5-C5H4-P(dO)-
(Ph)(H))2Fe, 7aB. The yield of 7aB is 68.4% (0.594 g, 1.37
mmol).
Selected Spectroscopic Data for 8. 1H NMR (CDCl3, δ/ppm):
1.10 (d, JP-H = 14.8 Hz, 36 H), 4.76 (s, 20 H), 4.40-4.65 (m,
16 H). 13C NMR (CDCl3, δ/ppm): 72.8 (s, 20 C, Cp), 70.6, 70.1,
69.1 (t, 20 C, Cp), 41.2, 41.0, 40.7 (t, 12 C, -C(CH3)3), 27.1 (s, 4
C, -C(CH3)3). 31P NMR (CDCl3, δ/ppm): 108.1. MS (FAB):
m/z=1442 (M - tBu - OH)þ. Anal. Calcd: C, 46.64; H, 5.17.
Found: C, 45.86; H, 5.24.
1
Selected Spectroscopic Data for 7aB. H NMR (CDCl3, δ/
ppm): 4.49-4.83 (m, 8 H, (η5-C5H4)2), 7.47-7.73 (m, 10 H, Ph),
7.92, 8.02 (d, 2 H, P-H, JP-H = 487.3, 489.3 Hz). 13C NMR
(CDCl3, δ/ppm): 72.5-73.3 (m, 10 C, (η5-C5H4)2), 128.6-132.1
(m, 12 C, Ph). 31P NMR (CDCl3, δ/ppm): 19.2 (d, 2 P, P-H,
3.5. Synthesis and Characterization of 9. In a N2-flashed
100 mL round-bottomed flask, 0.290 g (0.10 mmol) of 7bM
and 0.224 g (0.10 mmol) of Pd(OAc)2 were placed with 8 mL of
THF. The solution was kept stirring at 60 °C for 1 h before the
solvent was removed under reduced pressure, and the residue
was separated by CTLC. A red-colored band was eluted using
a mixed solvent mobile phase (EA/CH2Cl2 = 20:1) and was
identified as [(7b0M)(7b0M-Hþ)Pd(OAc)], 9.
J
P-H = 489.4 Hz). MS (FAB): m/z = 432 (Mþ). Anal. Calcd: C,
60.86; H, 4.64. Found: C, 59.50; H, 4.44.
3.3. Synthesis and Characterization of 7aM, 7bM, and 7cM. A
100 mL round-bottomed flask charged with a magnetic stir bar
was placed with 4.00 mmol of ferrocene (0.372 g) and 10 mL of
THF. The solution was kept stirring at 0 °C while 4.4 equiv of
1.9 mL of t-BuLi (2.3 M in penteane, 4.4 mmol) was slowly
added. After 1.0 h the solution was allowed to warm to 25 °C and
stirred for another 3 h. A dark red solid, 1,10-dilithioferrocene,
was obtained after the removal of solvent under reduced
pressure.
Selected Spectroscopic Data for 9. 1H NMR (CDCl3, δ/ppm):
1.28 (d, JP-H = 16.8 Hz, 9 H), 1.18 (d, JP-H=17.2 Hz, 9 H), 2.05
(s, 3 H), 4.16-4.58 (m, 18 H). 13C NMR (CDCl3, δ/ppm):
72.4 (s, 10 C, Cp), 70.6, 69.9, 67.8 (t, 10 C, Cp), 39.9, 39.3, 38.7 (t,
6
C, -C(CH3)3), 26.9-24.1 (m, 2
C, -C(CH3)3). 31P
First, 2.00 mol of tert-butyldichlorophosphine was dissolved
in 2.5 mL of THF. Then it was added to the solution of 1,10-
dilithioferrocene at 25 °C. The solution was allowed to warm to
room temperature and stirred for another 12 h at 60 °C.
Subsequently, the solution was allowed to cool to room tem-
perature, and a small amount of water was added to quench the
reaction. The organic portion was collected from the separatory
funnel after washing with ethyl acetate several times. Then, the
solvent was removed under reduced pressure, and the residue
was separated by CTLC. A dark yellow colored band was
eluted using a mixed solvent mobile phase (EA/CH2Cl2 = 3:1)
and was identified as (η5-C5H4-P(dO)(Ph)(H))2Fe, 7bM. The
same procedures were executed for the preparations of 7aM and
7cM except the starting materials were dichlorophenylpho-
sphine and dichlorocyclohexylphosphine, respectively. The
yields of 7aM, 7bM, and 7cM were 34% (0.210 g, 0.68 mmol),
64% (0.185 g, 1.28 mmol), and 46% (0.290 g, 0.92 mmol),
respectively.
NMR (toluene-d6, δ/ppm): 104.0, 105.5. MS (FAB): m/z=685
(M - CH3COO)þ.
3.6. General Procedures for the Suzuki-Miyaura Cross-
Coupling Reactions. Suzuki-Miyaura cross-coupling reactions
were performed according to the following procedures. Reac-
tants including 7aB (8.78 mg, 0.01 mmol), phenylboronic acid
(0.183 g, 1.50 mmol), and NaOH (0.120 g, 3.00 mmol) were
placed into a 20 mL Schlenk flask. The flask was evacuated and
backfilled with nitrogen before adding toluene (1.0 mL) and
2-bromothiophene (0.11 mL, 1.00 mmol). The solution was
stirred at 40 °C for 3 h. Subsequently, an excess amount of
water was added and the product was extracted with ether (3 ꢀ
20 mL). The combined organic extracts were dried over anhy-
drous MgSO4 and concentrated under vacuum. The crude
residue was purified by flash chromatography on silica gel.
Similar procedures are applied to the case of 7aM.
3.7. X-ray Crystallographic Studies. Suitable crystals of 7aB,
7aM, 7bM, 8, and 9 were sealed in thin-walled glass capillaries
under a nitrogen atmosphere and mounted on a Bruker AXS
SMART 1000 diffractometer. Intensity data were collected in
1350 frames with increasing ω (width of 0.3° per frame). The
absorption correction was based on the symmetry equivalent
reflections using the SADABS program. The space group
determination was based on a check of the Laue symmetry
and systematic absences and was confirmed using the structure
solution. The structure was solved by direct methods using the
SHELXTL package.15 All non-H atoms were located from
successive Fourier maps, and hydrogen atoms were refined
using a riding model. Anisotropic thermal parameters were used
for all non-H atoms, and fixed isotropic parameters were used
for H atoms.16 Crystallographic data for compounds 7aB, 7aM,
7bM, 8, and 9 are available from the Supporting Information.
1
Selected Spectroscopic Data for 7aM. H NMR (CDCl3, δ/
ppm): 4.33 (s, 5 H, η5-C5H5), 4.44-4.48 (m, 4 H, η5-C5H4),
7.48-7.76 (m, 5 H, Ph), 8.03 (d, 1 H, P-H, JP-H = 483.3 Hz).
13C NMR (CDCl3, δ/ppm): 69.6 (s, 5 C, Cp), 70.4-71.9 (m, 5 C,
Cp), 128.5-132.1 (m, 6 C, Ph). 31P NMR (CDCl3, δ/ppm): 19.8
(d, 1 P, P-H, JP-H = 484.5 Hz). MS (FAB): m/z = 310 (Mþ).
Anal. Calcd: C, 61.97; H, 4.88. Found: C, 60.33; H, 5.63.
1
Selected Spectroscopic Data for 7bM. H NMR (CDCl3, δ/
ppm): 1.10 (d, JP-H = 16.8 Hz, 9 H), 4.36 (s, 5 H), 4.27-4.62 (m,
4 H), 6.88 (d, JP-H=454.5 Hz, 1 H). 13C NMR (CDCl3, δ/ppm):
23.2, 32.6 (d, JP-C = 71.1 Hz), 67.6(d, JP-C=110.1 Hz), 70.31,
71.2 (d, JP-C = 37.5 Hz), 72.2 (d, JP-C=33.1 Hz). 31P NMR
(CDCl3, δ/ppm): 47.5(s). (d, 1 P, P-H, JP-H = 454.0 Hz). MS
(FAB): m/z = 290 (Mþ). Anal. Calcd.: C, 57.96; H, 6.60. Found:
C, 56.84; H, 6.65.
1
Acknowledgment. We thank the National Science
Council of the ROC (grant NSC 95-2113-M-005-015-
MY3) for financial support.
Selected Spectroscopic Data for 7cM. H NMR (CDCl3, δ/
ppm): 1.21 (d, 4 H), 1.67 (m, 6 H), 4.36 (s, 5 H), 4.13-4.62 (m, 4
H), 7.06 (d, JP-H = 458.5 Hz, 1 H). 13C NMR (CDCl3, δ/ppm):
25.0 (d, JP-C = 9.1 Hz), 25.9 (d, JP-C = 15.2 Hz), 39.4 (d, JP-C
= 72.6 Hz), 68.3 (d, JP-C = 103.8 Hz), 69.6, 71.3 (d, JP-C
=
(15) Sheldrick, G. M. SHELXTL PLUS User’s Manual, Revision 4.1 ;
Nicolet XRD Corporation: Madison, WI, 1991.
(16) The hydrogen atoms were riding on carbons or oxygens in their
10.9 Hz), 71.6 (d, JP-C = 9.1 Hz). 31P NMR (CDCl3, δ/ppm):
37.3(d, 1 P, P-H, JP-H = 457.8 Hz). MS (FAB): m/z = 316
(Mþ). Anal. Calcd: C, 60.78; H, 6.70. Found: C, 59.72; H, 6.86.
˚
idealized positions and held fixed with C-H distances of 0.96 A.

Article
Supporting Information Available: Crystallographic data for
Organometallics, Vol. 28, No. 20, 2009 6053
Cambridge CB2 1EZ, UK (fax: þ44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.
uk). These files in CIF format for 7aB, 7aM, 7bM, 8, and 9
and spectroscopic data, 1H, 13C, 31P NMR, MS, and EA for new
compounds are available free of charge via the Internet at http://
pubs.acs.org.
the structural analysis have been deposited with the Cambridge
Crystallographic Data Center, CCDC nos. 678966, 678967,
719989, 719989, and 719991 for compound 7aB, 7aM, 7bM, 8,
and 9, respectively. Copies of this information may be obtained
free of charge from the Director, CCDC, 12 Union Road,