Imidazolio-substituted secondary phosphine oxides as potential carbene reagents

Article abstract of DOI:10.1016/j.poly.2015.08.041

New pre-ligands of 1-aryl-1H-imidazol-2-yl substituted secondary phosphine oxides (HRP(=O)-Im:SPO-Im) were prepared and characterized by spectroscopic means. The SPO-Im pre-ligands are able to tautomerize to their corresponding genuine ligands, the 1-aryl-1H-imidazol-2-yl substituted phosphinous acids ((HO)RP-Im:PA-Im) while the condition permitted. Further reactions of selected SPO-Im pre-ligands toward Pd(COD)Cl₂ or PdBr yielded bis-(PA-Im)-coordinated cis-palladium dichloride or dibromide complexes, respectively. These cis-palladium dihalide complexes are all having an intramolecular hydrogen bond between two coordinated PA-Im ligands (R(Im(H)⁺)P-O^-?HO-PRIm) by transferring a proton from one of the two P-OH groups to its free nitrogen site of imidazolyl substituents; and, therefore, imposes the formation of zwitterionic cis-form palladium complexes. Three crystal structures of these square-planar zwitterionic palladium complexes were determined by the X-ray diffraction methods. Additionally, two novel NHC carbene coordinated palladium complexes were obtained from the crystal-growing process of the above-mentioned zwitterionic cis-palladium dihalide complexes. The crystal structures of two NHC carbene coordinated palladium complexes were also successfully determined. Density-functional theory (DFT) calculations were carried out to substantiate the feasibility of the proposed reaction pathways for the formation of NHC carbene coordinated palladium complexes. Finally, applying selected SPO-Im pre-ligands to Heck reactions with pre-optimized reaction conditions exhibited satisfactory performances.

Full text of DOI:10.1016/j.poly.2015.08.041

Accepted Manuscript
Imidazolio-substituted Secondary Phosphine Oxides as Potential Carbene Re-
agents
Yu-Chang Chang, Chih-Hung Chang, Lyu-Wei Wang, Ya-Han Liang, Dan-Fu
Hu, Chia-Ming Weng, Kuo-Chung Mao, Fung-E Hong
PII:
S0277-5387(15)00487-8
DOI:
http://dx.doi.org/10.1016/j.poly.2015.08.041
POLY 11509
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
20 June 2015
28 August 2015
Please cite this article as: Y-C. Chang, C-H. Chang, L-W. Wang, Y-H. Liang, D-F. Hu, C-M. Weng, K-C. Mao, F-
E. Hong, Imidazolio-substituted Secondary Phosphine Oxides as Potential Carbene Reagents, Polyhedron (2015),
doi: http://dx.doi.org/10.1016/j.poly.2015.08.041
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For table of contents use only
Imidazolio-substituted Secondary Phosphine Oxides as Potential Carbene
Reagents
Yu-Chang Chang, Chih-Hung Chang, Lyu-Wei Wang, Ya-Han Liang, Dan-Fu Hu,
Chia-Ming Weng, Kuo-Chung Mao, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road,
Taichung 40227, Taiwan
E-mail: fehong@dragon.nchu.edu.tw
The reaction of SPO-Im 1h with Pd(COD)Cl yielded a bis-1h’ (PA-Im)
2
’
coordinated cis-palladium dichloride complex, 2Ah . It subsequently converted to a
carbene coordinated palladium complex, 3Ah, in the presence of moisture, and
eventually to 4Ah. The P-C(imidazole) bond of 1h’ was dissected and the dissociated
species acted as a NHC carbene ligand towards palladium metal.
1

Imidazolio-substituted Secondary Phosphine Oxides as Potential Carbene
Reagents
Yu-Chang Chang, Chih-Hung Chang, Lyu-Wei Wang, Ya-Han Liang, Dan-Fu Hu,
Chia-Ming Weng, Kuo-Chung Mao, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road,
Taichung 40227, Taiwan
E-mail: fehong@dragon.nchu.edu.tw
Abstract: New pre-ligands of 1-aryl-1H-imidazol-2-yl substituted secondary
phosphine oxides (HRP(=O)-Im: SPO-Im) were prepared and characterized by
spectroscopic means. The SPO-Im pre-ligands are able to tautomerize to their
corresponding genuine ligands, the 1-aryl-1H-imidazol-2-yl substituted phosphinous
acids ((HO)RP-Im: PA-Im) while the condition permitted. Further reactions of
selected SPO-Im pre-ligands toward Pd(COD)Cl or PdBr yielded bis-(PA-Im)-
2
coordinated cis-palladium dichloride or dibromide complexes, respectively. These
cis-palladium dihalide complexes are all having an intramolecular hydrogen bond
+
-…
between two coordinated PA-Im ligands (R(Im(H) )P-O HO-PRIm) by transfering a
proton from one of the two P-OH groups to its free nitrogen site of imidazolyl
substituents; and, therefore, imposes the formation of zwitterionic cis-form palladium
complexes. Three crystal structures of these square-planar zwitterionic palladium
complexes were determined by the X-ray diffraction methods. Additionally, two
novel NHC carbene coordinated palladium complexes were obtained from the crystal-
growing process of the above-mentioned zwitterionic cis-palladium dihalide
complexes. The crystal structures of two NHC carbene coordinated palladium
complexes were also successfully determined. Density-functional theory (DFT)
calculations were carried out to substantiate the feasibility of the proposed reaction
pathways for the formation of NHC carbene coordinated palladium complexes.
Finally, applying selected SPO-Im pre-ligands to Heck reactions with pre-optimized
reaction conditions exhibited satisfactory performances.
Keywords: Nitrogen Heterocyclic Carbene (NHC) • Secondary Phosphine Oxide
(
SPO) • Palladium • Heck reaction • Density Functional Theory (DFT)
2

1. Introduction
Various types and functions of organophosphines have probably been the most
employed ligands in transition-metal complexes catalyzed reactions, particularly in
[1]
cross-coupling reactions.
Nevertheless, these tri-substituted organophosphine
ligands often create practical problems of being sensitive towards air and moisture
[
2]
and eventually loss their coordinating capacities.
Thereby, other air-stable
[
3]
[4]
[5]
[6]
heterocyclic ligands such as pyridines, quinolines, imidazoles, tetrazoles,
[7]
[8]
oxazolines, Schiff bases and hydrazones, were introduced as alternative solutions.
Particularly, N-heterocyclic carbenes (NHCs) have merged recently as one of the
most prestigious competitors among non-phosphine ligands. Moreover, methods for
and the hybrid of these two types of
ligands have been reported and their catalytic performances were examined.
Recently, a fascinating category of phosphine ligands having the general form of
[9]
[10]
the preparation of phosphines, NHCs
[11]
2 2
PR -Im (Im: imidazolyl) has been developed (Scheme 1). For PR -Im, protonation or
alkylation on lone pair of nitrogen atom of imidazolyl ring can give rise to cationic
+
and dative-bond featured phosphenium-NHC species (PR ←NHC, NHC: N-
2
[12]
[13]
heterocyclic carbene) capable of acting as potential carbene-transfer reagent. For
instance; an NCH-coordinated gold complex was obtained from the reaction of
[14]
AuCl←(PR
Note that the PR
2
-Im) complex with methanol (proton and weak nucleophile source).
(OMe) byproduct is still a potential ligand for metal coordination.
2
Thus, protonated or alkylated phosphenium-NHC can be regarded as a two-in-one
compound ligating system.
R"
R"
N
N
N
N
+
+
H or CH OTf
3
C
PR2
C
PR2
C
PR2
N
N
R'
R'
R'
-R" = -H or -CH3
+
Scheme 1. Illustration of chemical transformation for PR
2
-Im to PR
2
←NHC adduct.
In ligand-assisted transition-metal catalyzed reactions, the application of
secondary phosphine oxides as pre-ligands (SPOs, RR'P(O)H) has attracted much
[15]
attention due both to their thermal- and air-stability. As demonstrated repeatedly,
the coordinating capacity of SPO towards transition metal is acquired through
tautomerization, in which the P(V) in RR'P(O)H is converted to P(III) in phosphinous
[
16]
acid (PA, RR'POH) in solution. The PA tautomer is having a lone pair of electrons
and functioning as authentic phosphine ligand in coordination chemistry.
Previously, we had demonstrated that SPO-imidazolios 1a and 1b are active SPO
[17]
pre-ligands for Suzuki-Miyaura cross-coupling reactions (Scheme 2). These SPO-
based pre-ligands 1a and 1b are symbolized as SPO-Im, which is analogous to the
[17]
neutral PR
treating 1a with Pd(COD)Cl
with PdBr (Scheme 2). Their crystal structures were determined by X-ray diffraction
methods. More interestingly, two unconventional dinuclear Pd(I)-Pd(I) complexes,
2
-Im.
Herein, we show that palladium complex 2Aa can be obtained by
2
; while 2Bb was synthesized from the reaction of 1b
2
1
Aa and 1Bb, were also obtained from these two reactions of 1a and 1b,
[
17]
respectively.
In the literature, only few palladium complexes Pd(I)-Pd(I) were
3

[18]
reported.
Therefore, as shown in Scheme 2 and referenced from literature, the
resulted palladium complexes in our studies were classified into two categories: (i)
[17,18]
dinuclear Pd(I)-Pd(I) complexes 1Aa and 1Bb;
complexes 2Aa and 2Bb.
and (ii) cis-form Pd(II)
[19]
X
OH
tBu
P
Pd
N
Ar
N
N
Ar
N
Pd
P
tBu
OH
O
X
N
+
Pd(COD)Cl2
1Aa: X = Cl; 1Bb: X = Br
P
H
or + PdBr2
N
R1
X
X
R2
Pd
N
N
1
1
a: R=H
b: R=2-OMe
P
P
N
N
tBu Ar
tBu
Ar
HO
H
O
2Aa: X = Cl; 2Bb: X = Br
Scheme 2. Reactions of pre-ligand SPO-Im 1a with Pd(COD)Cl
2
and 1b with
The abbreviations of the species are as follows: Aa indicates products
obtained from the reaction of 1a + Pd(COD)Cl , while Bb denotes products from 1b
[17]
2
PdBr .
2
+
PdBr
2
.
On the other hand, palladium-catalyzed coupling of olefins with aryl and vinyl
[
20]
halides, known as Heck reaction,
is one of the most important methods to the
[20j,21]
formation of substituted double bond in organic synthesis.
Heck reaction has
been proven to be a rather versatile method in the syntheses of important building
blocks in pharmaceuticals and bioactive compounds, natural products, monomers,
[1i,5a,20f,20j,21b,22]
herbicides and high performance materials.
TM
H
N
O
TM
N
N
N
N
N
+
TM
P
P
H
P
P
N
N
R
1
R
1
OH
proton-
transfer
R
1
2
R
1
Ar
O
Ar
D
O
H
D
Ar
Ar
D
D
SPO-Im
2A or 2B-like
2A' or 2B'-like zwitterionic complex
H
H
N
N
N
N
Nu:-
OH
OH
+
TM
P
+
P
TM
+
P
R1
Nu
R1
Nu
N
N
R OH
1
D
Ar
Ar
Ar
D
D
3
PA-Im
4
Scheme 3. Potential mode-of-coordination for two-in-one SPO-Im ligating system in
the presence of nucleophile. Lone-pair electrons of oxygen atom are not shown for
clarity. Ar is a molecular linker and D is a metal-coordinating site, if any.
4

Herein, we wish to report the preparations of a number of new pre-ligands of
SPO-Im 1c-1l and the related palladium complexes (zwitterionic complex 2' and
neutral complexes 3 and 4 shown in Scheme 3). In comparison with the stable
carbene-transfer analogue of NHC-phosphenium oxide or sulfide with non-
[23]
+
2
coordinating pentavalent phosphorus, [P(=Y)R -NHC] (Y = O or S), SPO-Im 1s
are potential pre-ligands for metal-coordination (complex 2’ in Scheme 3) and
catalysis. It is closely related to the SPO-to-PA (RR'P(O)H to RR'POH)
tautomerization that creating a trivalent phosphorus atom in phosphine-like PA ligand.
Upon nucleophilic attack on P(III) atom of zwitterionic PA ligand in complex 2’, the
NHC and phosphite-coordinated complex 3 could be obtained. Thus, we are
demonstrating here that SPO-Im 1 could act as two-in-one pre-ligands (PA and NHC).
If the substituent on nitrogen atom of imidazolyl ring is equipped with an extra
coordinating site, complex 3 could dissociate a phosphite ligand to yield complex 4
(
Scheme 3). In addition, DFT methods were employed to probe the probable reaction
pathways for the formation of unexpected carbene-coordinated palladium complex
3Aa) from the reaction of SPO-Im 1a with Pd(COD)Cl . Selected SPOs as efficient
(
2
mono- or bi-dentate pre-ligands in palladium-catalyzed Heck reactions were also
investigated.
II. Results and Discussion
Preparation of SPO-Im 1c-1l and the unexpected observations from the reactions
with palladium salts
Several new SPO-Im 1c-1l were prepared according to the procedures modified from
[17]
our previous work. Firstly, 1-aryl-1H-imidazole was prepared by Cu(I)-catalyzed
coupling reaction between imidazole and the matching substituted bromobenzene.
t
The first resultant intermediate was then treated with BuLi and P( Bu)Cl
2
. Finally, the
second intermediate containing one P-Cl bond was followed by hydrolysis work-up in
acidic media. Notably, cis-form of bis-PA coordinated palladium complexes 2Af’,
2
Ag’, 2Ah’ and 4Ah can be successfully obtained from the reactions of
corresponding pre-ligands of SPO-Im 1f, 1g, and 1h with Pd(COD)Cl (Scheme 4).
Another palladium complexes 3Bb that bearing NHC and phosphite ligands was
yielded from the reaction of SPO-Im 1b with PdBr (Scheme 4). These palladium
complexes were all synthesized with half molar equivalent of PdX (X = Cl or Br) at
5 °C for two hours. It is believed that the formations of 2Ax’ and 2Bx’ were from
2
2
2
2
[17]
the tautomerization of their corresponding tautomers 2Ax and 2Bx, respectively.
5

X
X
X
X
NH
Pd
N
N
Pd
N
O
N
N
+
Pd(COD)Cl
2
N
P
P
N
P
N
P
P
N
H
or PdBr
2
tBu
tBu
tBu HO
tBu
Ar
tBu
O
O
Ar
O
Ar
Ar
H
H
Ar
1
1
7
2
Ax: 2Aa (X = Cl)
2Ax' (X = Cl): 2Af', 2Ag' and 2Ah'
2Bx' (X = Br): -
17
2
Bx: 2Bb (X = Br)
N
Ar:
l: 2-pyridyl
g: R=2,4,6-Me
R
X
X
X
X
a: R=H
Pd
Pd
b: R=2-OMe
c: R=3-OMe
d: R=4-OMe
e: R=2,4,6-OMe
f: R=2-Me
h: R=2-NMe
2
Me
Me
HN
HN
i: R=3-NMe
j: R=4-NMe
k: R=2,6- Pr
2
N
P
OH
tBu
N
N
2
OH
i
Ar
4: 4Ah
3: 3Bb
Scheme 4. Reactions of pre-ligands SPO-Im 1s with Pd(COD)Cl
2
2
or PdBr . The
abbreviations for the species are as follows: A is for X = Cl; B is for X = Br;
lowercases a-l are for substituents on either phenyl or pyridyl ring. SPO-Im 1c-1k
were newly synthesized in this work, while SPO-Im 1a-1b and palladium complexes
2
Aa’ and 2Bb’ were published in ref. [17]. SPO-Im 1b, 1f, 1g, and 1h (in blue) were
employed to synthesize palladium complexes, respectively. Names of complexes
indicate the obtained species from their respective reactions.
The structures of three cis-form complexes, 2Af’, 2Ag’ and 2Ah’, were
determined by X-ray diffraction methods. As shown in Figure 1, a hydrogen bonding
…
between O(1)-H(1) O(2) is present in each case. Compared with the bonding modes
of conventional tri-substituted phosphines toward metals, this manner is quite unique
[
5]
for these cis-form of bis-phosphine ligands coordinated palladium complexes.
+
Interestingly, the released H dissociated from one of the SPOs is relocated to the
nitrogen atom of imidazole, forming zwitterionic type complexes 2Af’, 2Ag’ and
[
24]
2
Ah’. Thereby, it is believed that the conversion of 2Ax to its more stable tautomer
[25]
2
Ax’ shall take place in solution.
6

Figure 1. ORTEP drawings of 2Af’ (top), 2Ag’ (middle) and 2Ah’ (bottom). Most
hydrogen atoms are omitted for clarity.
7

For comparison, the selected structural data of several related compounds are
listed in Table 1. Except for 2Ah’, the distances between the protonated N and the
closest X (X = Cl, Br) range from 3.047Å to 3.197 Å, those are shorter than the sums
of their van der Waals radius. An intra-molecular hydrogen bond is present in either
2
Af’ or 2Ag’ where the protonated imidazole ring titles away from chloride.
Generally speaking, trans-form of bis-phosphine coordinated palladium complexes
are presumably more stable than that of cis-form complexes in terms of less steric
hindrance between two bulky ligands. Nevertheless, the fact that the cis-
conformations are dominated here is most likely due to the existence of an intra-
molecular hydrogen bond between two ligands. Moreover, the environments of the
two coordinated phosphine ligands are not the same. The differences in the P-C(im)
bond lengths range from 0.019 Å to 0.052 Å. The reason could be that the bonding
between C(im) and P(2) can be regarded as a carbon-to-phosphorus dative bond after
[26]
the imidazolophosphine is converted to a NHC-phosphenium in 2Ax’.
Table 1. . Selected bond lengths and distances for 2Af’, 2Ag’ and 2Ah’.
…
N Cl
…
Cl H
…
N-H Cl
N-H
P(1)-C(im)
1.806(3)
1.813(3)
1.828(7)
P(2)-C(im)
1.858(3)
1.858(2)
1.847(6)
2
2
2
Af’
3.164
3.083
3.710
0.77(4)
0.85(3)
0.8800
2.476
2.334
3.871
148.76
148.77
73.00
Ag’
Ah’
Figure 2. ORTEP drawing and the crystal packing of 3Bb.
Unexpectedly, a unique palladium complex, 3Bb was obtained from the reaction
of 1b with PdBr . This square planar palladium complex is consisted of two
2
coordinated ligands, di-tert-butylphosphonous acid and a 3H-NHC carbene, as well as
two bromides all in cis-positions. We believe that the formation of 3Bb was from a
[
17]
series of chemical conversions of 2Bb. Firstly, the tautomerization of 2Bb to 2Bb’
via intra-molecular proton transfer. Subsequently, the nucleophilic attack of H O on
2
phosphorus atom followed by intra-molecular deprotonation reaction of the reacted
8

-
2
H O by P-O moiety. Next, the dissociation of the spectating 1Bb, and eventually, the
cleavage of the P-C(im) bond from the coordinated 1Bb. To our knowledge, the case
of 3Bb is among the few examples of palladium complexes having both coordinating
phosphine and carbene ligands directly from cleavage of the P-C(im) bond of an
[27]
imidazole substituted SPO. As shown, there are inter-molecular hydrogen bonding
between the protonated nitrogen atoms and the bromides from two neighboring
molecules (N…Br = 3.346 Å) in solid state (Figure 2).
An interesting observation was the formation of carbene-amine-coordinated 4Ah,
which was obtained from the crystal-growing process of the resulted solution from the
reaction of 1h with Pd(COD)Cl
ray diffraction methods (Figure 3). It is obvious that the release of the P(OH)
2
. The identification of 4Ah was validated by the X-
t
2
( Bu)
ligand from 3Ah, the counterpart of 3Bb, creates a vacuum site for the potential
coordination by nitrogen atom from N,N-dimethylaniline moiety in 3Ah. As revealed
in Figure 3, the palladium metal center is coordinated by both carbene and amine
2
moieties in 4Ah. Alternatively, 4Ah can be regarded as a PdCl skeleton being
2
chelated by a κ -C,N-bidentate ligand. Judging from the composition of 4Ah, only
one molar equivalent of 1h is required to form the complex, although the reaction was
started with two molar equivalents of 1h.
Figure 3. ORTEP drawing of 4Ah.
9

Figure 4. Depicted solid-state packing motif of 4Ah’.
DFT studies of the reaction pathways for the formation of 3Aa using model pre-
ligand SPO-Im 1
Detailed analysis of the crystal structure of 4Ah indicates that non-covalent
interactions play essential roles in the solid-state packing motif, including π-π,
hydrogen-bond and long-distant ligand-to-metal interactions (Figure 4). It is revealed
that, along the unit cell a-axis, parallel-displaced π-π interaction between two inter-
molecularly adjacent phenyl rings have center-to-center distances of 3.764 and 3.677
[28]
Å, which are in the range of normal aromatic π-π stacking.
Along the c-axis, a
molecular dimer mutually forms two inter-molecular hydrogen bonds via the Cl(2)
and hydrogen of N(2)’-H’ at distances of 2.229 and 2.220 Å (angles of N(2)’-
…
…
o
H’ Cl(2)/N(2)-H Cl(2)’ are 155.3 and 164.4 ), respectively. In the dimer,
furthermore, there are two short contacts, 3.235 and 3.332 Å, between intermolecular
…
…
Cl(1) Pd’ and Cl(1)’ Pd atoms, respectively.
The density functional theory (DFT) methods were employed to evaluate the
plausible mechanism for the P-C(im) bond cleavage of palladium-coordinated SPO-
Im that leading to the formation of complex 3Ax (Scheme 4 and 5). To make
computations more efficient and feasible, a simplified model for pre-ligand SPO-Im 1
(
R = CH ) was used in our DFT study. There are two reaction pathways, Route A &
3
B, were proposed and evaluated. The major difference between these two pathways is
the location of the proton of P-OH either on oxygen (model 2A) or nitrogen atom
(
model 2A’). The conformer 2A’ is energetically more stable than that of 2A by 10.2
kcal/mol. This is consistent with the observation of solid-state structures of 2Af’, 2Ag’
and 2Ah’.
1
0

H
Cl
H
Cl
Cl
Cl
P
O
Cl
Cl
Pd
P
H
Pd
O
Route A
N
N
-
L
Pd
N
H
P
N
N
+H
2
O
P
OH
Cl
Cl
R
R
N
Ph
N
R
Ph HO H O
Ph
R
OH
2A_1 (29.5)
N
Pd
Ph
A_TS (48.9)
Pd(COD)Cl
2
H
2
A
(10.6)
N
P
OH
OH
2
+
N
-
L
+H O
R
N
2
Ph
3A'_1 (40.4)
P
N
Cl
Cl
Cl
Cl
P
Cl
Cl
R
OH
Ph
Pd
H
H
Pd
H
Pd
- L
+H
N
N
N
H
H
N
P
O
P
N
2
O
P
H
Route B
N
Ph
N
Ph
H
R
R
R
N
R
Ph
Ph
O
O
O
O
H O
N
O
P
2A' (0.0)
2A'_1 (28.5)
2A'_TS (64.5)
N
H
R
Ph
1: -R = -CH
3
Cl
Cl
Cl
Cl
Cl
Cl
Pd
Pd
Ph
Pd
H
N
HN
HN
P
OH
OH
P
OH
OH
P
OH
OH
N
N
N
R
R
R
Ph
Ph
3A
(9.0)
3A_TS (39.9)
3A'_2 (21.3)
Scheme 5. Proposed reaction pathways for the formation of 3A. Calculated free
energies related to 2A’ are provided in parentheses in kcal/mol.
Subsequently, 2A_1 and 2A’_1 represent the tri-coordinated intermediates after
the dissociation of one neutral PA-Im 1 ligand and picking up one H O molecule from
2
surrounding. The source of water might come from the moisture that sneaked into the
solution during the crystal-growing process. The incoming water shows hydrogen-
bond interaction with nitrogen atom in complex 2A_1 or with oxygen atom in 2A’_1.
These two intermediates are energetically similar with each other. In 2A_TS, one of
the O-H bonds from water is breaking and the P-OH bond is forming simultaneously.
‡
Instead, the less energy-demanding process of 2A_1 to 3A’_1 via 2A_TS (∆G = 38.1
‡
kcal/mol) than 2A’_1 to 3A’_1 via 2A’_TS (∆G = 37.4 kcal/mol) was elucidated by
DFT calculations. The formation of a four-membered ring in 2A’_TS is believed to
be less favorable than forming a five-membered ring in 2A_TS.
Thereafter, both Route A and B lead to the same conformation of a zwitterionic
type intermediate 3A’_1. The formation of P-C(im) bond that followed by the
breaking of a P-Pd bond lead to 3A’_2. This is an exothermic process (∆G = -21.7
kcal/mol). Eventually, the conversion of 3A’_2 to the final product 3Aa through the
cleavage of a P-C(im) bond and formation of a P-Pd bond in 3Aa_TS is achieved
with a moderate activation energy of 18.4 kcal/mol. Consequently, the formation of
4
2
Ah from 3Ah could be anticipated by the releasing of a PR(OH) ligand from 3Ah
and followed by the coordination of amino group to the unsaturated palladium center.
Palladium-catalyzed Heck reactions using selected 1c~1l as ligands
As known, only small fraction of secondary phosphine oxide (SPO, RR’P(=O)H)
might be converted to its less stable tautomeric form, phosphinous acid (PA,
RR’POH), in solution unless strong electron-withdrawing substituent(s) is/are
employed. Nevertheless, the formation of PA-coordinated metal complex is resulted
in the presence of palladium salt even started with SPO. Recently, we reported the
[
17]
preparation of ligand (1b) and its application in cross-coupling reactions.
In this
1
1

work we wish to extend the scope of these SPO-Im 1c-1l as pre-ligands in transition-
metal catalyzed Heck cross-coupling reactions (Scheme 6).
Scheme 6. Heck reactions were catalyzed by in situ combination of palladium salts
with selected 1 as ligands.
To optimize the reaction conditions, a series of experiments under various
reaction conditions in terms of base, solvent and reaction temperature for a model
coupling reaction of styrenes and bromobenzenes were pursued. As known, Heck
reaction normally requires more severe reaction condition than that of Suzuki reaction.
[2b,29]
Thereby, Heck reactions of substituted bromobenzene with styrene were carried
out at elevated temperature using a high boiling point solvent DMF as the acting
medium as shown in Scheme 6. For efficiency comparison with the reported work of
[30]
Hayashi.
carried out as follows. A suitable Schlenk tube was charged with 1.0 mmol of styrene,
.1 mmol of bromobenzene, 2.0 mmol of base. Then, the combination of 2.0 mol % of
similar procedures for the catalytic reactions under investigation were
1
palladium salt and 4.0 mol % of ligand as catalyst precursor was added. The mixture
was stirred at a preset reaction temperature and time depending on the reaction
executed. It was then followed by an acidic work-up. Due to steric hindrance, the
formation of 1,2-disubstituted ethylene in trans-form (5) is predominated as the major
product than that of cis-form. Meanwhile, 1,1’-disubstituted ethylene (6) is expected
as a minor side product. After screening all the influential factors, it was concluded
that the optimal reaction condition comes by using 1e/Pd(Oac)
2
(2 mol%) as the
catalyst precursor with the combination of K CO (2.0 mmol) in DMF (1.0 mL) and
2
3
o
[31]
reacting at 90 C for 12 h.
The scope of this Heck coupling reaction using various aryl bromides was
examined by employing the optimized condition from the previous studies. The
results are presented in Table 2. As shown, compatible efficiencies were obtained for
either employing electron-withdrawing or -donating group substituted arylbromides
(
Entries 1-6). Besides, small amount of side products either from self-coupling of
arylbromide or 1,1’-disubstituted ethylene were observed, thus also lessens the
conversion rate. Moderate performance was observed for the case having -OMe on
ortho-position of arylbromide probably due to steric hindrance (Entry 7). No
conversion was observed for either with severe hindrance case such as 2-bromo-1,3,5-
trimethoxybenzene (Entry 8) or heterocyclic case such as 2-bromopyridine (Entry 9).
1
2

[a]
Table 2. Heck reactions employed various substrates
[b, c]
Entry
Substate
Product
Yield(%)
1
98(99:1)
93(100:0)
98(97:3)
94(100:0)
92(100:0)
90(97:3)
67(97:3)
2
3
4
5
6
7
8
9
0
0
[
a] Conditions: 1.1 mmol arylbromide, 1.0 mmol styrene, 2.0 mmol K
2
CO
3
, 1 mL
o
DMF (dried), 0.02 mmol Pd(OAc)
yield; [c] In parentheses, the ratio of 5 with 6 was determined by H NMR.
2
, 0.04 mmol 1e, 90 C, 12 h; [b] Total isolated
1
1
3

Conclusions
Several new air- and moisture-stable imidazolio-secondary phosphine oxides
SPO-Im, 1c-1l) were prepared and applied in palladium-catalyzed Heck cross-
(
coupling reactions. Excellent efficiencies were shown for various styrene derivatives
towards aryl bromides. Several functional groups and heteroaryl were tolerated in this
reaction. The formation of unexpected carbene coordinated palladium complexes,
3
Bb and 4Ah, from the reactions of corresponding SPO-Im 1b and 1h with palladium
salts is noteworthy and the reaction pathways for the formations of NHC-phosphite
coordinated pallasium complex 3Bb has been proposed based on the characterized
structures of 2Af’, 2Ag’ and 2Ah’. By using model pre-ligand SPO-Im 1, the reaction
mechanism for the formation of 3Bb analogue was also investigated with DFT
calculations.
Experimental Section
General
All reactions were carried out under a nitrogen atmosphere using standard Schlenk
techniques or in a nitrogen-flushed glove box. 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
chromatography. GC-MS analysis was performed on an Agilent 5890 gas
chromatograph (Restek Rtx-5MS fused silica capillary column: 30m, 0.25mm,
®
1
0
.5µm) with an Agilent 5972 mass selective detector. Routine H NMR spectra were
recorded on a Varian-400 spectrometer at 399.756 MHz. The chemical shifts are
3
1
13
reported in ppm relative to internal standards TMS (δ = 0.0 ppm). P and C NMR
spectra were recorded at 161.835 and 100.529 MHz, respectively. The chemical shifts
for the former and the latter are reported in ppm relative to internal standards H
3
PO
4
(
δ = 0.0 ppm) and CHCl (δ = 77.0 ppm) or CH Cl (δ = 53.0 ppm), respectively.
3
2
2
Mass spectra were recorded on JOEL JMS-SX/SX 102A GC/MS/MS spectrometer.
Electrospray ionization-high resolution mass spectra (ESI-HRMS) were recorded on a
Finnigan/Thermo Quest Mat 95 XL mass spectrometer.
Synthesis and characterization of substituted 2-(tert-butylhydrophosphoryl)-1-
phenyl-1H-imidazole derivatives (1c-1l)
A N
2
flashed 100 ml round-bottomed flask was placed 25 mL THF with 5.0 mmol of
1
-(3-methoxyphenyl)-1H-imidazole (0.869 g), which was prepared by procedures
[17]
reported elsewhere. The solution had been cooled to -78 °C before 2.2 mmol of n-
BuLi (0.88 mL, 2.5 M in hexane) was added. The suspension was slowly warmed up
to 0 °C and stirred for another 30 minutes. Subsequently, the solution was slowly
transferred to another flask which contained 5.5 mmol of tert-butyldichlorophosphine
(
0.869 g) in 25 mL THF at 0 °C. The reaction mixture was then heated up gradually
from 0 °C to 50 °C, and stirred for another 2 hours. After completion, the solution
was diluted with 50 mL ethyl acetate and the organic layer was collected. It was again
extracted twice with ethyl acetate (2 x 5 mL). Then, the combined organic layer was
dried with MgSO , filtered and concentrated. The residue was purified by flash
4
1
4

chromatography by changing the polarity of mixed solvent. It was started from ethyl
acetate/n-hexane=1:3 then to pure ethyl acetate and finally to ethyl
acetate/methanol=20:1. The isolated white solid 1c (0.598 g, 2.15 mmol) was
obtained in 43% yield.
Similar procedures as the preparation of 1c were applied to the syntheses of
substituted 2-(tert-butylhydrophosphoryl)-1H-imidazole derivatives (1d-1l) starting
from each corresponding substituted imidazoles. The yields of purified 1d, 1e, 1f, 1g,
1
h, 1i, 1j, 1k, and 1l are 43 %, 72 %, 50 %, 41 %, 51 %, 49 %, 23 %, 5-10%, 18 %,
respectively.
1
‡
Spectroscopic data for 1c H NMR (CDCl
3
, δ/ppm): 7.32 (d, J = 8.0 Hz, 1H, Ar),
7
.27 (1H, Imidazole), 7.24 (1H, Imidazole), 7.20 (1H, Ar), 7.17 (t, J = 24.0 Hz, 1H,
Ar), 7.10 (d, JP-H = 484 Hz, 1H, P-H), 7.06 (d, J = 8.0 Hz, 1H, Ar), 6.93 (d, J = 8.0 Hz,
1
13
H, Ar), 3.87 (s, 3H, OCH
3 3 3 3
), 1.12 (d, J = 16.0 Hz, 9H, C(CH ) ); C NMR (CDCl ,
δ/ppm): 139.5 (d, JP-H = 125.8 Hz, 1C, Imidazole), 137.5, 130.4 (2C, Imidazole) ,
1
58.8, 130.3, 124.6, 117.1, 114.9, 111.1 (6C, Ar), 55.3 (1C, OMe), 32.4 (d, JP-C
=
t
t
31
70.4 Hz, 1C, Bu), 23.4 (3C, Bu); P NMR (CDCl
3
, δ/ppm): 32.2 ppm (d, JP-H = 482
Hz).
1
‡
7
6
Spectroscopic data for 1d H NMR (CDCl
3
, δ/ppm): 7.40 (d, J = 8.0 Hz, 2H, Ar),
.23 (s, 1H, Imidazole), 7.17 (s, 1H, Imidazole), 6.99 (d, JP-H = 480 Hz, 1H, P-H),
t
13
.91 (d, J = 6.0 Hz, 2H, Ar), 3.77 (s, 3H, OCH
, δ/ppm): 139.8 (d, JP-C = 130.7 Hz, 1C, Imidazole), 130.4, 130.3 (2C,
Imidazole), 159.7, 127.1, 125.2, 125.2, 122.9, 114.6, 114.2 (6C, Ar), 55.3 (1C, OMe),
3
), 1.02 (d, J = 16.0 Hz, 9H, Bu); C
NMR (CDCl
3
t
t
31
3
2.4 (d, JP-C = 70.0 Hz, 1C, Bu), 23.4 (3C, Bu); P NMR (CDCl
3
, δ/ppm): 32.1 ppm
(
d, JP-H = 480 Hz).
1
‡
3
Spectroscopic data for 1e H NMR (CDCl
3
, δ/ppm): 7.37 (1H, Imidazole), 7.02 (1H,
Imidazole), 6.88 (d, JP-H = 476 Hz, 1H, P-H), 6.17 (s, 2H, Ar), 3.85 (s, 3H, OCH
3
),
), 1.15 (d, J = 16.0 Hz, 9H, Bu); C NMR
, δ/ppm): 139.8 (d, JP-C = 130.7 Hz, 1C, Imidazole), 130.1, 125.8 (2C,
Imidazole), 161.8, 156.5, 156.1 107.3, 90.4 (6C, Ar), 55.8, 55.3, (3C, OMe), 32.2 (d,
t
13
.76 (s, 3H, OCH
3
), 3.75 (s, 3H, OCH
3
(
CDCl
3
t
t
31
J
3
P-C = 80.4 Hz, 1C, Bu), 23.1 (3C, Bu); P NMR (CDCl , δ/ppm): 28.0 ppm (d, JP-H
=
476 Hz).
Spectroscopic data for 1f H NMR (CDCl
1
‡
3
, δ/ppm): 7.41-7.31 (m, 5H, Ar and
Imidazole), 7.16 (s, 1H, Imidazole), 6.89 (d, 1H, P-H, JP-H = 474 Hz), 2.14 (s, 3H,
t
13
Me), 1.19 (d, J = 18.0 Hz, 9H, Bu); C NMR (CDCl
3
, δ/ppm): 140.0 (d, JP-C = 126.0
Hz, 1C, Imidazole), 135.76, 135.28 (2C, Imidazole), 131.03, 130.81, 129.86, 127.88,
t
t
1
26.60, 124.96 (6C, Ar), 32.38 (d, JP-C = 73.0 Hz, 1C, Bu), 23.50 (3C, Bu), 17.30
3
1
(
1C, Me); P NMR (CDCl
3
, δ/ppm): 28.8 (d, JP-H = 472 Hz).
1
‡
Spectroscopic data for 1g H NMR (CDCl
3
, δ/ppm): 7.43 (1H, Imidazole), 7.04 (1H,
Imidazole), 6.99 (1H, Ar), 6.96 (1H, Ar), 6.65 (d, 1H, P-H, JP-H = 472 Hz), 2.34 (3H,
t
13
p-Me), 2.07 (3H, o-Me), 1.95 (3H, o-Me), 1.30 (d, J = 18.0 Hz, 9H, Bu); C NMR
CDCl , δ/ppm): 140.73 (d, JP-C = 127.0 Hz, 1C, Imidazole), 139.62, 136.20 (2C,
Imidazole), 134.84, 132.49, 131.30 , 129.45, 128.91, 123.96 (6C, Ar), 32.27 (d, JP-C
(
3
=
t
t
31
7
2
4.4 Hz, 1C, Bu), 23.62 (3C, Bu), 21.07, 17.52 (3C, Me); P NMR (CDCl
4.8 (d, JP-H = 472 Hz).
3
, δ/ppm):
1
5

1
‡
1
5
1
1
7
Spectroscopic data for 1h H NMR (CDCl
3
, δ/ppm): 7.40~7.35 (m, 2H, Ar), 7.29 (s,
H, Imidazole), 7.24 (1H, Imidazole), 7.09~7.02 (m, 2H, Ar), 6.74 (d, 1H, P-H, JP-H
=
t
13
32 Hz), 2.48 (6H, Nme
2
), 1.20 (d, J = 20.0 Hz, 9H, Bu); C NMR (CDCl
3
, δ/ppm):
40.1 (d, JP-C = 120.6 Hz, 1C, Imidazole), 147.97, 130.9 (2C, Imidazole), 129.8,
28.6, 127.5, 126.37, 123.5, 121.6, 118.9 (6C, Ar), 42.0 (2C, Nme ), 32.2 (d, JP-C
, δ/ppm): 25.5 ppm (d, JP-H = 532
2
=
t
t
31
0.4 Hz, 1C, Bu), 23.3 (3C, Bu); P NMR (CDCl
3
Hz).
1
‡
1
4
Spectroscopic data for 1i H NMR (CDCl
3
, δ/ppm): 7.33 (s, 1H, Imidazole), 7.31 (s,
H, Imidazole), 7.28 (d, J = 4.0 Hz, 1H, Ar), 7.00 (s, 1H, Ar), 6.95 (d, 1H, P-H, JP-H
=
t
95 Hz), 6.76 (t, J = 8.0 Hz, 1H, Ar), 3.02 (6H, Nme
2
), 1.13 (d, J = 20.0 Hz, 9H, Bu);
, δ/ppm): 140.2 (d, JP-C = 130.7 Hz, 1C, Imidazole), 137.8, 130.5
2C, Imidazole), 151.1, 130.4, 129.9, 124.8, 112.4, 112.3, 109.9 (6C, Ar), 40.3 (2C,
1
3
C NMR (CDCl
3
(
t
t
31
2 3
Nme ), 32.7 (d, JP-C = 70.0 Hz, 1C, Bu), 23.8 (3C, Bu); P NMR (CDCl , δ/ppm):
31.2 ppm (d, JP-H = 495 Hz).
1
‡
3
Spectroscopic data for 1j H NMR (CDCl , δ/ppm): 7.33 (d, 1H, P-H, J = 8.0 Hz,
Ar), 7.27 (1H, Imidazole), 7.18 (1H, Imidazole), 7.00 (d, 1H, P-H, JP-H = 476 Hz),
t
6
.70 (d, 1H, P-H, J = 8.0 Hz, Ar), 2.97 (6H, Nme
2
), 1.08 (d, J = 16.0 Hz, 9H, Bu);
, δ/ppm): 140.0 (d, JP-C = 130.0 Hz, 1C, Imidazole), 130.3, 130.2
2C, Imidazole), 150.5, 126.7, 125.3, 111.8 (6C, Ar), 40.2 (2C, Nme ), 32.4 (d, JP-C
, δ/ppm): 31.1 ppm (d, JP-H = 476
1
3
C NMR (CDCl
3
(
2
=
t
t
31
6.0 Hz, 1C, Bu), 23.6 (3C, Bu); P NMR (CDCl
3
Hz).
1
‡
7
(
2
Spectroscopic data for 1k H NMR (CDCl
.42 (1H, Imidazole), 7.31 (d, J = 8.0 Hz, 1H, Ar), 7.25 (d, J = 8.0 Hz, 1H, Ar), 7.07
),
), 1.37, 1.32 (d, J = 18.0 Hz, 9H, Bu), 1.30 (d, J = 8.0 Hz,
), 1.20 (d, J = 8.0 Hz, 3H, CH(CH ), 1.09 (d, J = 8.0 Hz, 3H,
); C NMR (CDCl , δ/ppm): 141.33 (d,
P-C = 127.0 Hz, 1C, Imidazole), 147.25, 145.59 (2C, Imidazole), 131.68, 130.73,
3
, δ/ppm): 7.48 (t, J = 8.0 Hz, 1H, Ar),
s, 1H, Imidazole), 6.60 (d, 1H, P-H, JP-H = 464 Hz), 2.50-2.42 (m ,1H, CHMe
2
t
.23-2.15 (m ,1H, CHMe
H, CH(CH
), 1.06 (d, J = 8.0 Hz, 3H, CH(CH
2
3
3
)
2
)
3 2
1
3
CH(CH
3
)
2
3
)
2
3
J
1
2
2
t
30.59, 125.48, 124.37, 123.58 (6C, Ar), 31.69 (d, JP-C = 75.0 Hz, 1C, Bu), 28.46,
8.24, 26.06, 25.98, 3.74 (6C, Pr), 22.72, 22.43 (3C, Bu); P NMR (CDCl
2.4 (d, JP-H = 464 Hz).
i
t
31
3
, δ/ppm):
1
‡
7
(
Spectroscopic data for 1l H NMR (CDCl
3
, δ/ppm): 8.57 (d, J = 8.0 Hz, 1H, Ar),
.91~7.89 (m, 2H), 7.66 (1H, Imidazole), 7.41 (d, 1H, P-H, JP-H = 496 Hz), 7.39~7.35
m, 2H), 1.20 (d, J = 20.0 Hz, 9H, Bu); C NMR (CDCl
t
13
3
, δ/ppm): 139.1 (d, JP-C
=
1
20.0 Hz, 1C, Imidazole), 149.2, 148.9 (2C, Imidazole), 130.8, 130.7, 123.5, 122.9,
t
t
31
117.9 (6C, Ar), 33.0 (d, JP-C = 0.0 Hz, 1C, Bu), 23.88 (3C, Bu); P NMR (CDCl ,
3
δ/ppm): 34.5 ppm (d, JP-H = 496 Hz).
Reactions of substituted 2-(tert-butylhydrophosphoryl)-1-phenyl-1H-imidazole
derivatives with palladium halides
Under nitrogen, a 20 mL Schlenk tube with stir bar was charged with 1 mL of
THF, 0.02 mmol of Pd[COD]Cl
2
and
2
molar equivalents of (t-
o
butylhydrophosphoryl)-1-o-tolyl-1H-imidazole (1f). The solution was stirred at 60 C
o
(
or 25 C for 1h) for 2 hours. The solution was cooled down to room temperature and
solvent was completely removed under reduced pressure. The residue was completely
dissolved in 1 mL of CH Cl ; then, 1 mL of n-hexane was added gradually. Crystals
2
2
1
6

of 2Af’ with suitable size for x-ray diffraction were obtained after placing the solution
o
at 4 C for several days. Similar procedures were taken for preparing 2Ag’ and 2Ah’
from corresponding 1g and 1h, respectively. Crystals of 3Bb were obtained from the
side reaction of 1b with PdBr . Crystals of 4Ah were obtained presumably from the
2
conversion of 2Ah’. Nevertheless, almost all of 2Ah’ was converted to 4Ah after
purification process by column chromatography. Therefore, it is not enough sample of
2
Ah’ for characterization by spectroscopic methods.
1
‡
2
(
Spectroscopic data for 2Af’ H NMR (CDCl
3
, δ/ppm): 13.86 (b, 1H, N-H), 7.72 (s,
), 1.36
H), 7.53 (d, J =7.6 Hz, 2H), 7.18~7.11 (m, 6H), 7.04 (s, 2H), 2.1 6(s, 6H, CH
3
1
3
6
d, J =12.4 Hz, 18H, t-Bu); C NMR (DMSO-d , δ/ppm): 142.5 (d, J = 48.0 Hz),
1
35.9, 130.9, 130.0, 129.4, 128.4, 127.4, 126.8, 126.0, 27.8, 17.9 (d, J = 95.8 Hz),
31
17.7; P NMR (CDCl
3
, δ/ppm): 92.7.
1
‡
2
1
1
2
Spectroscopic data for 2Ag’ H NMR (CDCl
3
, δ/ppm): 8.15 (b, 1H, N-H), 6.94(s,
H), 6.87(s, 2H), 6.83(s, 2H), 6.78(s, 2H), 2.21(s, 6H) , 2.09(s, 6H), 1.92(s, 6H),
.38(d, J = 16.8 Hz, 18H, t-Bu); C NMR (CDCl
36.3, 135.2, 133.0, 129.5, 129.0, 128.8, 126.8, 124.7, 41.6, 41.3, 27.8, 27.4, 21.1,
1.0, 19.4, 18.0; P NMR (CDCl
13
3
, δ/ppm): 143.8, 143.3, 139.1,
31
3
, δ/ppm): 97.8.
1
‡
=
=
Spectroscopic data for 3Bb H NMR (CDCl , δ/ppm): 11.72 (s, 1H, N-H), 8.72 (d, J
3
7.6 Hz, 1H, Ar), 7.49 (t, J = 11.2 Hz, 1H, Ar), 7.38 (t, J = 2.0 Hz, 1H, Ar), 7.31 (t, J
2.0 Hz, 1H, Ar), 7.25 (t, J = 7.2 Hz, 1H, Ar), 7.05 (d, J = 8.4 Hz, 1H, Ar), 3.84 (s,
H, OMe), 0.77 (d, J = 8.0 Hz, 9H, Bu); C NMR (CDCl
t
13
3
3
, δ/ppm): 154.4 (NHC),
1
52.4, 130.6, 128.8, 127.4, 124.7, 121.7, 120.0, 111.7 (Aromatic), 55.7 (OMe), 37.4(d,
t
t
31
3 3
J = 56, Bu, ipso), 23.6( Bu, CH ); P NMR (CDCl , δ/ppm): 127.8.
1
‡
(
Spectroscopic data for 4Ah H NMR (CDCl
s, 1H, N-CH=C), 7.33 (t, J = 8.0 Hz, 1H, Ar), 7.15 (d, J = 12.0 Hz, 1H, Ar), 7.08 (d,
J = 8.0 Hz, 1H, Ar), 7.01 (t, J = 6.0 Hz, 1H, Ar), 6.30 (s, 1H, C-NH-C), 2.50 (s, 6H,
3
, δ/ppm): 8.39 (s, 1H, N-CH=C), 7.56
13
N-(CH
3
)
2
). C NMR (CDCl
3
, δ/ppm): 42.37 (s, N-(CH
3
2
) ), 116.70, 119.35, 119.22,
1
16.70, 126.26, 129.72 (s, Ar), 139.31, 129.88 (s N-CH=CH), 147.58 (s, N-C-N); MS
+
(
EI, m/z): 187.1 (M-PdCl ) .
2
General procedure for the Heck reactions
Schlenk tube was charged with Pd(Oac) (0.03 mmol, 0.0067 g), ligand (0.06
2
mmol) and salt (3 mmol). This tube was subjected to vacuum for 10 minutes and then
toluene (1 mL) was added. To this mixture styrene (1.2 mmol, 0.137 mL) and aryl
bromide (1.0 mmol) were added slowly with the help of syringe. The resulting
o
suspension was stirred at 90 C for 12 hours. After completion of reaction, the
solution was diluted with EtOAc and filtered through a pad of Celite. The organic
layer was then extracted with ethylacetate (10 x 3 mL) and water. The organic layer
was collected separately and dried over anhydrous Na
2
SO and concentrated under
4
reduced pressure to obtain crude residue. The product was purified by flash
chromatography (5% EtOAc/hexane), which yielded mostly the desired trans-stilbene.
X-ray crystallographic studies
Suitable crystals of 2Af’, 2Ag’, 2Ah’, 3Bb and 4Ah were sealed in thin-walled
glass capillaries under nitrogen atmosphere and mounted on a Bruker AXS SMART
1
7

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 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 a
[32]
SHELXTL package. 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
[
33]
for H atoms.
Computational methods
All calculations were done with Gaussian 09.
[
34]
[
Gas-phase geometry
For palladium, the
35]
optimizations were done with the M06 density functional.
LANL2DZ(f) effective core potential in conjunction with the corresponding basis set
of double zeta quality augmented it with a set of f functions was employed; while for
the other atoms, the 6-31G(d,p) was used (denoted as 631LAN(f)). Frequency
calculations were performed so as to verify whether the optimized geometries were
local minima (no imaginary frequency) or transition states (one imaginary frequency)
at the same level of theory. Considering the solvation effect of THF with PCM
[36]
solvation model,
the subsequent single-point calculations were performed at the
M06/631+LAN(f) level of theory in which the 6-31+G(d,p) basis set was employed
for non-metal atoms instead of 6-31G(d,p). Free energy for each species in THF
solution was estimated with the sum of PCM energy (G(631+LAN(f))) and the zero-point
vibrational energy plus the gas-phase thermal correction to enthalpy (Gcorr = Hcorr
–
TStot) obtained at the M06/631LAN(f) level. Accordingly, relative free energies were
calculated to construct the potential energy surface of the formation of the NHC-
coordinated complex 3A from the reaction of SPO-Im and Pd(COD)Cl .
2
Appendix A. Supplementary data
CCDC 901014, 901015, 901016, 901017, and 901018 contain the supplementary
crystallographic data for 2Af’, 2Ag’, 2Ah’, 3Bb and 4Ah, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
We thank the Ministry of Science and Technology of the ROC (Grant NSC 101-2113-
M-005-011-MY3) for financial support. We are also grateful to the National Center
for High-performance Computing (NCHC) for providing computing time and
facilities.
References:
[
1]
(a) A. Suzuki, Angew. Chem. Int. Ed. 50 (2011) 6723-6737; (b) E. Negishi,
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2
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2
2

Imidazolio-substituted Secondary Phosphine Oxides as Carbene Precursor
Yu-Chang Chang, Chih-Hung Chang, Lyu-Wei Wang, Ya-Han Liang, Dan-Fu Hu,
Chia-Ming Weng, Kuo-Chung Mao, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road,
Taichung 40227, Taiwan
E-mail: fehong@dragon.nchu.edu.tw
Graphical abstract:
Cl
Cl
P
Cl
Cl
Cl
Cl
H
N
tBu
Pd
Pd
N
N
N
Pd
Ar
+
Pd(COD)Cl
2
+ H
2
O
Me
Me
C
P
C
C N
H
P
HN
C
N
HN
C
N
P
OH
O
Ar tBu
t
Bu
N
Ar
NMe
2
O
N
tBu
O
H
Ar
PA-Im 1h'
Ar:
OH
SPO-Im 1h
2Ah'
3Ah (proposed)
4Ah
2
3

Imidazolio-substituted Secondary Phosphine Oxides as Carbene Precursor
Yu-Chang Chang, Chih-Hung Chang, Lyu-Wei Wang, Ya-Han Liang, Dan-Fu Hu,
Chia-Ming Weng, Kuo-Chung Mao, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road,
Taichung 40227, Taiwan
E-mail: fehong@dragon.nchu.edu.tw
Synopsis:
The reaction of SPO-Im 1h with Pd(COD)Cl yielded a bis-1h’ (PA-Im)
2
’
coordinated cis-palladium dichloride complex, 2Ah . It subsequently converted to a
carbene coordinated palladium complex, 3Ah, in the presence of moisture, and
eventually to 4Ah. The P-C(imidazole) bond of 1h’ was dissected and the dissociated
species acted as a NHC carbene ligand towards palladium metal.
2
4