(π-Allyl)Pd complexes containing N-heterocyclic carbene and pseudohalogen ligands - Synthesis, reactivity toward organic isothiocyanates and isocyanides, and their catalytic activity in suzuki-miyaura cross-couplings

Article abstract of DOI:10.1002/ejic.201300608

Dinuclear (π-allyl)palladium chlorides, [(π-allyl)Pd(μ-Cl)] ₂, were cleaved by N-heterocyclic carbenes (NHCs) to give mononuclear (π-allyl)palladium-NHC chlorides, [(π-allyl)Pd(Cl)(NHC)] (1-6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPR), 1,3-bis(2,6- diisopropylphenyl)-4,5-dihydroimidazol-2-ylidine (SIPR), 1,3-bis(2,4,6- trimethylphenyl)imidazol-2-ylidene (IMes)]. Complexes 1-6 were subsequently treated with aqueous NaN₃, KSCN, KOCN, and CF₃COOAg to produce the corresponding mononuclear (π-allyl)palladium-NHC pseudohalogen complexes, [(π-allyl)Pd(X)(NHC)] (X = N₃, NCS, SCN, NCO, CF ₃COO) (7-30). These products could also be obtained by treating dinuclear pseudohalogen-bridged Pd complexes, [(π-allyl)Pd(μ-X)] ₂, which were prepared by replacing the μ-Cl ligand in [(π-allyl)Pd(μ-Cl)]₂, with aqueous NaN₃, KSCN, KOCN, or CF₃COOAg, followed by cleavage with the NHCs. Reactions of [(π-allyl)Pd(N₃)(NHC)] with organic isothiocyanates (R-NCS) or CH₃O(CO)C≡CO(CO)CH₃ resulted in selective 1,3-dipolar cycloaddition into the Pd-azido bond to give heterocyclic compounds. By contrast, analogous reactions of [(η³-allyl)Pd(N ₃)(IPr)] with an organic isocyanide (R-NC: R = tert-butyl, benzyl) gave the adduct [(η³-allyl)Pd(N₃)(IPr)]·(R-NC) as the only product or a mixture of the adduct and a dipolar cycloaddition product, [(η³-allyl)Pd{CN₄(R)}(IPr)], depending on the isocyanides used. Finally, a series of (π-allyl)Pd-NHC pseudohalogen complexes, [(π-allyl)Pd(X)(NHC)], exhibited high catalytic activity in Suzuki-Miyaura cross-coupling reactions of aryl chlorides with arylboronic acids. A series of (π-allyl)palladium complexes containing N-heterocyclic carbene and pseudohalogen ligands are prepared and their reactivity toward organic isothiocyanates and isocyanides is studied. Suzuki-Miyaura cross-coupling reactions of aryl chlorides with aryl boronic acid catalyzed by these (π-allyl)palladium complexes are reported. Copyright

Full text of DOI:10.1002/ejic.201300608

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DOI:10.1002/ejic.201300608
(π-Allyl)Pd Complexes Containing N-Heterocyclic
Carbene and Pseudohalogen Ligands – Synthesis,
Reactivity toward Organic Isothiocyanates and
Isocyanides, and Their Catalytic Activity in
Suzuki–Miyaura Cross-Couplings
Hyun-Kyung Kim,[a] Jung-Hyun Lee,[a] Yong-Joo Kim,*[a]
Zhen Nu Zheng,[b] and Soon W. Lee[b]
Keywords: Allyl ligands / Carbenes / Cross-coupling / Palladium / Pseudohalogens
Dinuclear (π-allyl)palladium chlorides, [(π-allyl)Pd(μ-Cl)]2,
were cleaved by N-heterocyclic carbenes (NHCs) to give mo-
nonuclear (π-allyl)palladium–NHC chlorides, [(π-allyl)Pd-
(Cl)(NHC)] (1–6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imid-
azol-2-ylidene (IPR), 1,3-bis(2,6-diisopropylphenyl)-4,5-di-
hydroimidazol-2-ylidine (SIPR), 1,3-bis(2,4,6-trimethylphen-
yl)imidazol-2-ylidene (IMes)]. Complexes 1–6 were sub-
sequently treated with aqueous NaN3, KSCN, KOCN, and
CF3COOAg to produce the corresponding mononuclear (π-
allyl)palladium–NHC pseudohalogen complexes, [(π-allyl)-
Pd(X)(NHC)] (X = N3, NCS, SCN, NCO, CF3COO) (7–30).
These products could also be obtained by treating dinuclear
pseudohalogen-bridged Pd complexes, [(π-allyl)Pd(μ-X)]2,
which were prepared by replacing the μ-Cl ligand in [(π-
allyl)Pd(μ-Cl)]2, with aqueous NaN3, KSCN, KOCN, or
CF3COOAg, followed by cleavage with the NHCs. Reactions
of [(π-allyl)Pd(N3)(NHC)] with organic isothiocyanates (R–
NCS) or CH3O(CO)CϵCO(CO)CH3 resulted in selective 1,3-
dipolar cycloaddition into the Pd–azido bond to give hetero-
cyclic compounds. By contrast, analogous reactions of [(η3-
allyl)Pd(N3)(IPr)] with an organic isocyanide (R–NC: R = tert-
butyl, benzyl) gave the adduct [(η3-allyl)Pd(N3)(IPr)]·(R–NC)
as the only product or a mixture of the adduct and a dipolar
cycloaddition product, [(η3-allyl)Pd{CN4(R)}(IPr)], depending
on the isocyanides used. Finally, a series of (π-allyl)Pd–NHC
pseudohalogen complexes, [(π-allyl)Pd(X)(NHC)], exhibited
high catalytic activity in Suzuki–Miyaura cross-coupling re-
actions of aryl chlorides with arylboronic acids.
compounds, specifically aryl bromides and chlorides.[8]
Their catalytic activity was dependent on the supporting
ligand, a tertiary phosphane or an NHC ligand. For exam-
ple, compounds with an NHC ligand showed higher cata-
lytic activity for aryl chlorides than those with a combina-
tion of C,N-donor and tertiary phosphane ligands. On the
basis of these results, we set out to find more effective Pd
catalysts for the Suzuki–Miyaura coupling. For this pur-
pose, we tried to combine ligands that were more flexible
than the C,N-donor ligands with other supporting ligands
such as pseudohalogens and NHC ligands.
Introduction
In transition-metal-mediated organic syntheses, Pd-cata-
lyzed C–C cross-coupling reactions are widely utilized.[1–4]
In particular, palladium compounds containing labile li-
gands such as C,N-donor ligands have emerged as efficient
precatalysts for Suzuki–Miyaura cross-coupling reac-
tions.[5,6] Furthermore, to enhance catalytic efficiency in
these catalytic reactions, several studies on cyclopalladated
compounds containing N-heterocyclic carbenes (NHCs),
one of the many sterically and/or electronically beneficial
ligands, were reported.[7]
In this context, an allyl ligand as a flexible group was
first introduced to the Pd center, because the η3-allyl ↔ η1-
allyl isomerization may facilitate C–C bond formation or
reductive elimination during catalytic reactions. This isom-
erization is known for (allyl)Pd–NHC complexes, and it is
regarded as important dynamic behavior that can be con-
trolled by supporting ligands.[9–11] In addition, we intro-
duced an NHC ligand and a pseudohalogen ligand as sup-
porting ligands. Pseudohalogen ligands are known to be
better leaving groups than halogen ligands in catalytic reac-
tions. In summary, in this study, a set of the three aforemen-
tioned ligands (i.e., NHC, pseudohalogen, and allyl) were
Recently, we reported that cyclopalladated azides con-
taining C,N-donor ligands exhibit excellent catalytic ac-
tivity in the Suzuki–Miyaura cross-coupling of haloaryl
[a] Department of Chemistry, Kangnung-Wonju National
University,
Gangneung 210-702, Korea
E-mail: yjkim@ kangnung.ac.kr
Homepage: https://chemistry.gwnu.ac.kr/
[b] Department of Chemistry, Sungkyunkwan University, Natural
Science Campus,
Suwon 440-746, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300608.
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introduced to the Pd coordination sphere to induce unique
reactivity and high catalytic activity.
Nolan and co-workers reported that several (π-allyl)Pd–
NHC chloride complexes, [(π-allyl)Pd(Cl)(NHC)] [NHC =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
(IPR),
1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylid-
ine (SIPR), 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylid-
ene (IMes), etc.], exhibit highly efficient catalytic activity
in the Suzuki–Miyaura coupling of aryl chlorides and aryl
boronic acid.[12] The spectroscopic and dynamic behavior of
(π-allyl)Pd complexes possessing halogen or pseudohalogen
ligands, [(η3-allyl)Pd(X)(IPr)] (X = Cl, Br, N3, SCN, etc.),
were previously reported.[9,10] Several ionic complexes, [(η3-
allyl)Pd(NHC)](X) (X = OTf, BF4, PF6, etc.), and neutral
complexes, [(η3-allyl)Pd(X)(NHC)] (X = I, Cl, Me), were
also examined by spectroscopy.[13] However, the chemical
reactivity (e.g., dipolar cycloaddition with organic unsatu-
rated molecules) of the (π-allyl)palladium complexes con-
taining various pseudohalogen ligands has not been re-
ported. Furthermore, comparative catalytic evaluation of
the pseudohalogen complexes and their halogen counter-
parts has yet to be explored. In this work, we prepared a
series of neutral (π-allyl)Pd–NHC pseudohalogen com-
plexes and examined their reactivity as well as their catalytic
activity in Suzuki–Miyaura cross-coupling reactions.
Scheme 1.
Results and Discussion
Scheme 2.
Preparation of (π-Allyl)Pd–NHC Pseudohalogen Complexes
Dinuclear (π-allyl)palladium chlorides [(π-allyl)Pd(μ- the dinuclear pseudohalogen-bridged Pd complexes [(π-
Cl)]2 were cleaved by the NHCs (2 equiv.) to give neutral allyl)Pd(μ-X)]2 (X = N3, NCS), which were then cleaved
mononuclear (π-allyl)palladium–NHC chlorides 1–6, [(π- with the NHCs (2 equiv.) to yield the desired complexes. In
allyl)Pd(Cl)(NHC)] (NHC = IPr, SIPr, IMes). Subsequent addition, for the preparation of complexes 25–30, [(π-allyl)-
transmetalation of complexes 1–6 by using an aqueous Pd(Cl)(NHC)] was treated with CF3COOAg instead of
solution of NaN3, KSCN, KOCN, or CF3COOAg afforded CF3COONa, which is commonly used to introduce the
the corresponding (π-allyl)palladium–NHC pseudohalogen CF3COO group to the transition-metal system. One pos-
complexes 7–30, [(π-allyl)Pd(X)(NHC)] (X = N3, NCS, sible explanation for the formation of the unidentifiable
SCN, NCO, CF3COO) (Schemes 1).
products by the nucleophilic attack of strong nucleophiles
Complexes 7–30 could also be obtained by treating di- (NaN3, KSCN, and CF3COONa) at the π-allyl ligand may
nuclear pseudohalogen-bridged Pd complexes, [(π-allyl)- be the partial dissociation of the π-allyl ligand as well as
Pd(μ-X)]2, which were prepared from [(π-allyl)Pd(μ-Cl)]2, the formation of the desired product.
with an aqueous solution of NaN3, KSCN, KOCN, or
CF3COOAg followed by cleavage with the NHCs (2 equiv., ported that dinuclear N3-, NCS-, CH3COO-, and
see Scheme 2). CF3COO-bridged complexes, [(π-allyl)Pd(μ-X)]2, are
In earlier work, Busetto,[14] Shaw,[15] and Wilke[16] re-
However, if [(η3-crotyl)Pd(Cl)(IPr)] and [(η3-cinnamyl)- formed by the metathesis of a dinuclear (π-allyl)Pd chloride
Pd(Cl)(SIPr)] were treated with an equimolar amount of with nBu4N3, KSCN, CH3COOAg, and CF3COOAg,
NaN3 or KSCN to prepare complexes 12, 17, and 18 (path- respectively. In addition, mononuclear and dinuclear com-
way outlined in Scheme 1), impure products containing an plexes, [(π-allyl)Pd(solvent)(X)] (X = BF4, SbF6), [(π-allyl)-
unidentifiable contaminant were obtained. For this reason, Pd(OTf)(NHC)] (Tf = trifluoromethanesulfonyl), and [(π-
the pathway outlined in Scheme 2 was used to synthesize allyl)Pd(μ-XЈ)]2 (XЈ = BF4, SbF6, OTf) were also re-
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ported.[13,17] Moreover, allyl trifluoroacetate was shown to
oxidatively add to [Pd(dba)2] (dba = dibenzylideneacetone) plexes 19–24 (Table 1) display a characteristic strong band
to give a trifluoroacetate-bridged (π-allyl)Pd complex.[18] at 2158–2186 cm–1 that can be assigned to ν(NCO), and this
(π-Allyl)Pd isocyanato [(π-allyl)Pd(NCO)(NHC)] com-
The Pd–NHC pseudohalogen complexes were isolated in is consistent with that found in other palladium isocyanato
moderate-to-good yields. A few (π-allyl)Pd–NHC chlorides complexes.[22–24] Table 1 shows the strong IR bands charac-
were previously prepared from NHC·HCl and dinuclear (π- teristic of νasym(CO2) and νsym(CO2) at approximately 1680
allyl)Pd chloride dimers,[12c,19] and several (π-allyl)Pd–NHC and 1400 cm–1 for (π-allyl)palladium trifluoroacetates 25–
chlorides are commercially available. Although many cleav- 30, and this is indicative of an η1-trifluoroacetato ligand.
age reactions of Cl-bridged (π-allyl)palladium complexes by The difference in these stretching frequencies (Δν ≈
NHCs are currently known,[10,12,13] examples involving di- 280 cm–1) is typical of metal–(η1-trifluoroacetato) com-
1
nuclear azido (or other pseudohalogen)-bridged (π-allyl)- plexes.[25,26] The H NMR and 13C NMR assignments for
palladium complexes have not been reported. The form- the (π-allyl)Pd–NHC pseudohalogen complexes were made
ation of (π-allyl)Pd–NHC azides 7–12 was followed by IR on the basis of those reported in the literature.[9,10,12,13] Par-
spectroscopy by monitoring the characteristic N3 stretching ticularly, the 13C NMR peaks at δ = 52.5–55.0 ppm, which
band at 2022–2031 cm–1 (see Table 1). Interestingly, treat- are due to the terminal π-allyl carbon atom trans to the X
ment of [(π-allyl)Pd(Cl)(NHC)] complexes 1–6 with KSCN ligand (S- or N-coordinated NCS), for complexes 13–18 are
afforded S- and N-coordinated (NCS) palladium complexes downfield shifted relative to the same peaks at δ = 40.3–
13–19, depending on the starting Pd compounds. The IR 47.4 ppm for complexes 7–12 and 19–30, and this is a reflec-
spectra of the complexes clearly display characteristic bands tion of some electronic influence of the trans thiocyanato
that can be attributed to S- and N-coordination of the NCS group in these complexes.
ligand to the Pd center. S-Coordinated complexes 13–16,
[(π-allyl)Pd(SCN)(NHC)], exhibit a sharp and strong
stretching band above 2100 cm–1, whereas N-coordinated
complexes 17 and 18, [(π-allyl)Pd(NCS)(NHC)], exhibit a
strong but broad band below 2100 cm–1. Similar IR spec-
troscopic assignments were made by Maitlis[20] and Nor-
bury.[21] The bent Pd–SCN (in complexes 13–16) and linear
Pd–NCS (in complexes 17 and 18) bonding modes can
probably be explained in terms of steric congestion around
the Pd metal. Complexes 17 and 18 contain the π-allyl li-
gand with a substituent (CH3 or Ph) at the terminal carbon
atom that would require a large space around the Pd metal,
whereas complexes 13–16 do not. Consequently, the Pd
metal in complexes 17 and 18 is exposed to greater steric
congestion, and the linear Pd–NCS (N-coordination) frag-
ment would be formed favorably. X-ray crystallographic
analysis fully supports the formation of S- and N-coordi-
nated NCS complexes (see Figure 3 for 14 and Figure 4 for
17).
Structures of the Compounds
Single crystals suitable for X-ray crystallographic analy-
sis were grown from solutions of THF/hexane at –35 °C.
The crystal data, intensity collection, and refinement details
are given in the Supporting information (Table S1). The
molecular structures of complexes 7, 8, 14, 17, 19, 22, 25,
and 31 are given in Figures 1, 2, 3, 4, 5, 6, 7, and 8, respec-
tively, in which each Pd metal is coordinated to one NHC
ligand, one pseudohalogen ligand, and one η3-allyl group.
The coordination sphere of all the complexes can be best
described as a distorted square plane if the coordination of
the central allyl carbon atom to the Pd metal is ignored. In
Table 2, selected bond lengths for all complexes are listed.
The Pd–C1, Pd–C2meso, and Pd–C3 bond lengths [2.075(5)–
2.219(5) Å] in the π-allyl ligand are close to the values
[2.082(9)–2.284(9) Å] for (π-allyl)Pd–NHC chlorides re-
ported in the literature.[10,12b] Consistent with the relative
strength of the trans effect, the Pd–C bond trans to the
NHC is longer than that trans to the pseudohalogen.[9,10,12b]
Table 1. Characteristic IR bands for complexes 7–30.[a]
Complex
Pd–N3
Complex Pd–SCN or Pd–NCS
ν(N3) [cm–1]
ν(SCN) or ν(NCS) [cm–1]
7
8
9
10
11
12
2022
2022
2022
2030
2030
2031
13
14
15
16
17
18
2104
2104
2101
2101
2088
2098
Complex
Pd–NCO
ν(NCO)
[cm–1]
Complex Pd–OC(O)CF3
νsym(CO2)/νasym(CO2)
[cm–1]
19
20
21
22
23
24
2158
2194
2214
2214
2199
2186
25
26
27
28
29
30
1681/1403
1686/1406
1685/1401
1684/1409
1683/1404
1682/1409
[a] KBr.
Figure 1. ORTEP drawing of [(η3-2-methylallyl)Pd(N3)(IPr)] (7).
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Figure 2. ORTEP drawing of [(η3-allyl)Pd(N3)(IPr)] (8).
Figure 5. ORTEP drawing of [(η3-2-methylallyl)Pd(NCO)(IPr)]
(19).
Figure 3. ORTEP drawing of [Pd(η3-allyl)(SCN)(NHC)] (14).
Figure 6. ORTEP drawing of [(η3-allyl)Pd(NCO)(IMes)] (22).
Figure 4. ORTEP drawing of [(η3-crotyl)Pd(NCS)(IPr)] (17).
The NHC cyclic ring in all of the complexes is oriented
essentially perpendicular to the molecular plane, and the
Pd–C (NHC) bond lengths [2.034(3)–2.066(2) Å] fall in the
range of the reported values [2.028(4)–2.062(2) Å] in [(π-
Figure 7. ORTEP drawing of [(η3-2-methylallyl)Pd(OCOCF3)(IPr)]
(25).
allyl)Pd(Cl)(NHC)] complexes.[10,12b] The Pd–N3 (azido) the phosphane-containing Pd azido complexes, which indi-
bond length [2.087(3) Å for 7 and 2.078(2) Å for 8] is close cates a relatively weak trans influence of the π-allyl ligand.
to that in trans-[MePd(N3)(PMe3)2] [2.132(9) Å][27a] and The ORTEP drawings of 14 (Figure 3) and 17 (Figure 4)
that in trans-[(p-MeC6H4)Pd(N3)(PMe3)2] [2.114(3) Å][23] of clearly demonstrate the S- and N-coordination of the NCS
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tion-metal azides to afford complexes containing hetero-
cycles is well known. In particular, organic isocyanides and
isothiocyanates are known to react with transition-metal
azides to give complexes containing C-coordi-
nated[23,27,30–35] and S-coordinated[14,36–41] heterocyclic li-
gands. We previously reported that unsaturated organic
compounds react with group 10 metal azides containing
C,N-donor or phosphane ligands to give the corresponding
carbodiimide or S-coordinated tetrazole compounds.[8a]
With these results in mind, we first attempted the reactions
of [(π-allyl)Pd(N3)(NHC)] with various organic isothio-
cyanates (R–NCS: R = Ph, CH2Ph, C6H4NCS; see
Scheme 3). Consistent with our expectation, the S-coordi-
nated heterocycles were isolated, which were formed by the
dipolar cycloaddition of the isothiocyanate to the Pd–azido
bond. By this reaction, mono- and dinuclear tetrazole–
thiolato (π-allyl)Pd–NHC complexes were readily formed in
moderate-to-good yields at room temperature.
Figure 8. ORTEP drawing of [(η3-2-methylallyl)Pd{CN4(Ph)}(IPr)]
(31).
Table 2. Selected bond lengths for complexes 7, 8, 14, 17, 19, 22, 25, and 31.
Complex
Pd–C1 [Å]
Pd–C2 [Å]
Pd–C3 [Å]
Pd–C (NHC) [Å]
Pd–S [Å]
Pd–N [Å]
Pd–O [Å]
7
2.097(4)
2.103(3)
2.136(2)
2.075(5)
2.091(3)
2.091(2)
2.080(2)
2.114(3)
2.151(4)
2.110(3)
2.144(3)
2.116(4)
2.150(3)
2.118(2)
2.132(2)
2.161(3)
2.143(4)
2.168(3)
2.170(2)
2.219(5)
2.153(3)
2.202(2)
2.215(2)
2.164(3)
2.046(3)
2.041(2)
2.066(2)
2.034(3)
2.039(2)
2.038(2)
2.041(2)
2.047(2)
–
–
2.087(3)
2.078(2)
–
2.073(3)
2.071(3)
2.067(2)
–
–
–
–
–
–
–
8
14
17
19
22
25
31
2.364(6)
–
–
–
–
2.142(1)
–
2.335(7)
–
ligand, respectively. The Pd1–S1 bond length [2.364(6) Å] in
complex 14 is almost the same as that in [Pd(SCN)2(dppm)]
[2.364(2) Å, dppm = 1,1-bis(diphenylphosphanyl)methane]
and that in [Pd(NCS)(SCN)(dppe)] [2.364(4) Å, dppe = 1,2-
bis(diphenylphosphanyl)ethane]. The Pd1–N3 (NCS) bond
length [2.073(3) Å] in complex 17 is consistent with that
found in [Pd(NCS)(SCN)(dppe)] [2.062(10) Å] and that in
[Pd(Ph2P(CH2)3N(CH3)2)(SCN)(NCS)] [2.063(7) Å].[28] The
Pd–N [2.067(2) and 2.071(3) Å] bond length in (π-allyl)Pd
isocyanates [(η3-2-methylallyl)Pd(NCO)(IPr)] (19) and [(η3-
allyl)Pd(NCO)(IMes)] (22) is close to that in 17, but shorter
than that [2.119(5) Å] in trans-[Pd(CH3CO)(NCO)-
(PMe3)2].[23] The N–C bond length [1.128(4) for 19 and
1.114(3) Å for 22] in the Pd–NCO fragment is shorter than
the C–O bond length [1.204(4) Å for 19 and 1.198(3) Å for
22], and this phenomenon is common in other metal iso-
cyanato complexes.[29] The molecular structure of complex
25 is given in Figure 5, which shows a distorted square
plane with an NHC, an η3-2-methylallyl group, and an η1-
OCOCF3 ligand.
Scheme 3.
In contrast, the room-temperature reaction of complex
7 with dimethyl acetylenedicarboxylate (DMAD, 1 equiv.)
proceeded to produce quantitatively the N-coordinated tri-
azolato Pd [(η3-2-methylallyl)Pd{(N3C2)(COOMe)2}] com-
plex (see Scheme 3). The formation of the N1-bound isomer
could not be confirmed by spectroscopy. The IR spectra of
complexes 31–35 do not display the ν(N3) band of the start-
ing compounds, which supports the dipolar cycloaddition
Reactivity toward Organic Isothiocyanates (R–NCS),
Isocyanides (R–NC), and Alkynes
Dipolar cycloaddition of unsaturated organic molecules of the organic substrates to the Pd–N3 bond. Moreover, the
such as organic isothiocyanates (R–NCS), isocyanides (R– molecular structure of 31 in Figure 8, determined by X-ray
NC), organonitriles (R–CN), and alkynes with late-transi- diffraction, further confirms this reactivity.
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Reactions of [(π-allyl)Pd(N3)(NHC)] with equimolar Catalytic Application to Suzuki–Miyaura Cross-Coupling
amounts of organic isocyanides (tert-butyl or benzyl isocy- Reactions
anide) gave products that were quite different from those
The Suzuki–Miyaura cross-coupling of aryl halides with
obtained from reactions with isothiocyanates. Upon reac-
arylboronic acids is a well-known tool to form various C–
tion with tert-butyl isocyanide, the [(η3-allyl)Pd(N3)(IPr)]·
C coupled compounds.[43] Many palladacycles containing
(CN–tBu) (36) adduct was obtained as the sole product
a supporting NHC ligand are widely utilized for Suzuki–
Miyaura C–C coupling reactions.[43c,43d] Recently, we also
reported that palladacycles containing C,N-donor, NHC,
and azido ligands act as highly efficient catalysts in C–C
cross-coupling reactions.[8b] Our study also showed that the
combination of a functional moiety and suitable supporting
ligands around the metal center enhances the catalytic ac-
tivity. In addition, studies on the effects of the pseudo-
halogen supporting ligands of the Pd–NHC complexes on
the cross-coupling catalytic reactions are relatively rare.
In this context, we evaluated the catalytic activity of [(π-
allyl)Pd(X)(NHC)] (X = halide, pseudohalide). To find the
optimum conditions for the Suzuki–Miyaura cross-coupling
reactions, we examined the catalytic activity of [(η3-2-meth-
ylallyl)Pd(N3)(IPr)] (7, 1 mol-%) for the reaction of p-
chloroacetophenone, phenylboronic acid, and a base in a
molar ratio of 1:1.2:2 in several solvents. Reaction condi-
tions and product yields are listed in Table 3, which shows
that the conditions outlined in entry 5 resulted in high ac-
tivity for the coupling of p-chloroacetophenone with phen-
ylboronic acid.
(Scheme 4).
A
similar reaction of [(η3-2-methylallyl)-
Pd(N3)(IPr)] with benzyl isocyanide gave a 1:1 mixture of
[(η3-2-methylallyl)Pd(N3)(IPr)]·(CN–CH2Ph) and [(η3-2-
methylallyl)Pd{CN4(CH2Ph)}(IPr)].
Scheme 4.
The IR spectrum of 36 displays strong absorption bands
at 2081 and 2042 cm–1 that are assignable to ν(CϵN) and
ν(N3), respectively, whereas the IR spectrum of the starting
isocyanide complex shows the ν(CϵN) band at 2136 cm–1.
The shift of the ν(CϵN) band to lower frequency may arise
from the formation of the adduct (i.e., complex 36). The
1H NMR and 13C NMR spectra of complex 36 do not indi-
cate that isocyanide insertion to the Pd–C or Pd–N bond
took place. If less-hindered benzyl isocyanide was em-
ployed, the IR spectrum of the product displays absorption
bands at 2215 [ν(CϵN)] and 2024 [ν(N3)] cm–1 [Scheme 4,
Equation (2)]. In this case, the ν(CϵN) band is shifted to a
high frequency [ν(CϵN) at 2152 cm–1 for free benzyl iso-
cyanide]. The reason for the opposite shifts of the ν(CϵN)
band for the tBu–NC and PhCH2–NC cases is not clear
because of the limited data. In addition, the 1H NMR spec-
trum of the first isolated product shows two distinct CH3
signals with a 1:1 integration ratio, which is probably the
result of a mixture of [(η3-2-methylallyl)Pd(N3)(IPr)]·(CN–
R) and [(η3-2-methylallyl)Pd{CN4(R)(IPr)}] (R = benzyl).
The signal at δ = 168.7 ppm in the 13C NMR spectrum is
Table 3. Optimization of the Suzuki–Miyaura cross-coupling of 4-
chloroacetophenone with phenylboronic acid catalyzed by [(η3-2-
methylallyl)Pd(N3)(IPr)] (7).
Entry Solvent
Base
T [°C] t [h]
Isolated yield [%]
1
2
3
4
5
6
EtOH
K2CO3 80
K2CO3 80
K2CO3 80
Cs2CO3 80
Cs2CO3 80
Cs2CO3 60
Cs2CO3 r.t.
Cs2CO3 80
Cs2CO3 80
Cs2CO3 80
KOtBu 80
Cs2CO3 80
KOtBu r.t.
14
3
2
trace
83
99
84
99
80
trace
50
49
76
toluene
MeOH
EtOH
0.5
0.5
1
14
0.5
0.5
0.5
0.5
0.5
15
MeOH
MeOH
MeOH
MeOH
dioxane
iPrOH
iPrOH
iPrOH
iPrOH
7
8[a]
9
10
11[b]
93
91
51
assigned to the carbon atom of the tetrazolato ring (CN4) 12[b]
13[b]
formed by cycloaddition of the isocyanide to the Pd–N3
bond, which is characteristic of Pd compounds having a C-
coordinated tetrazolato ring.[30–35] The rather distinct reac-
tivity between the two isocyanides in Scheme 4 may be ex-
[a] Under an atmosphere of O2. [b] Technical-grade iPrOH.
The optimized conditions were applied to other C–C
plained on the basis of steric factors. The bulkier tert-butyl cross-coupling reactions of various activated and unacti-
isocyanide seems to be reluctant to undergo cycloaddition vated aryl chlorides with arylboronic acids in the presence
to the Pd–N3 bond. On the contrary, the steric problem is of Pd catalyst 7 (1 mol-%) at 80 °C in MeOH with Cs2CO3
somewhat relieved for less-bulky benzyl isocyanide, and so as the base (Table 4). As expected, the coupling reactions
a mixture of an adduct and a cycloaddition product is involving various organoboronic acids (Table 4, entries 9–
formed. In addition, related adducts were previously pre- 12) gave moderate-to-good yields. However, the relatively
pared from organic isocyanides and NiII or PdII isothio- low yield for entry 7 is probably due to the steric hindrance
cyanato complexes.[40a,42]
or deactivation of the substituent of the organic halide dur-
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ing oxidative addition within the catalytic cycle. Biphenyl complexes gave high yields (see Table 5 and Figure 9), ex-
was also observed as a major byproduct for the reaction of cept thiocyanato (SCN) and isothiocyanato (NCS) com-
entry 7.
plexes 13–18. For this unusually low catalytic efficiency, we
speculate that the dissociated SCN– or NCS– ion coordi-
Table 4. Suzuki–Miyaura cross-coupling reactions of aryl chlorides nates to the Pd center to prevent the formation of a zerova-
with arylboronic acids catalyzed by complex 7.[a]
lent Pd0–(NHC) intermediate, which is necessary for the
subsequent oxidative addition of the organic halide to yield
the final coupling products. Whereas the catalytic activity
of Pd–NHC halogen complexes has been widely explored,
the activity of the pseudohalogen analogues has not been
investigated to date. The catalytic results in Figure 9 clearly
demonstrate that the (π-allyl)Pd–NHC pseudohalogen (N3,
NCO, CF3COO) and halogen (Cl) complexes exhibit high
catalytic efficiency in Suzuki–Miyaura cross-coupling reac-
tions.
Table 5. Suzuki–Miyaura cross-coupling yields in the reaction of 4-
chloroacetophenone with phenylboronic acid catalyzed by com-
plexes 1–30.[a]
Pd cat.
Isolated yield [%] Pd cat.
Isolated yield [%]
1
98
99
99
96
93
96
99
94
90
99
97
82
7
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
1
3
2
3
13
99
98
99
98
97
99
98
99
88
90
98
77
4
5
6
7
8
9
10
11
12
13
14
15
10
6
[a] All reactions were performed at 80 °C in MeOH with Cs2CO3
for 30 min.
[a] p-Tolylboronic acid was used. [b] 4-Acetylphenylboronic acid
was used. [c] Naphthalene-1-boronic acid was used. [d] 2-Methoxy-
phenylboronic acid was used.
Figure 9. Comparison of [(π-allyl)PdX(NHC)]-catalyzed cross-cou-
pling yields for the reaction of 4-chloroacetophenone with phen-
ylboronic acid (vertical axis: % yield).
To compare the catalytic activity of the (π-allyl)palla-
dium–NHC halogen and pseudohalogen complexes, [(π-
allyl)Pd(X)(NHC)] (X = N3, Cl, NCS, SCN, NCO,
CF3COO), the cross-coupling reactions of p-chloroaceto-
Conclusions
In summary, we prepared a series of (π-allyl)Pd–NHC
phenone with arylboronic acids were investigated. Most pseudohalogen (N3, SCN, NCS, NCO, CF3COO) com-
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plexes. In particular, treatment of [(η3-crotyl)Pd(Cl)(IPr)]
and [(η3-cinnamyl)Pd(Cl)(SIPr)] with an aqueous solution
of NaN3 or KSCN led to incomplete substitution. Also,
reactions of (π-allyl)Pd chlorides with CF3COONa resulted
in partial replacement of Cl by CF3CO2. These results
strongly indicate the liability of the π-allyl moiety toward
such nucleophiles, which is closely related to the potential
catalytic activity of these complexes in Suzuki–Miyaura
cross-coupling reactions. However, we observed selective di-
polar cycloaddition of organic isothiocyanates (R–NCS)
into the Pd–azido bond of (π-allyl)Pd–NHC azide com-
plexes to give heterocyclic tetrazole–thiolato compounds. In
contrast, reactions of (π-allyl)Pd–NHC azides with organic
isocyanides (R–NC) did not give cycloaddition products
but a [(π-allyl)Pd(N3)(IPr)]·(R–NC) adduct or a mixture of
the adduct and a dipolar cycloaddition compound, de-
pending on the nature of the isocyanide. These results show
the different behavior of electrophiles and nucleophiles to-
ward our (π-allyl)Pd–NHC pseudohalogen complexes. Fi-
nally, for the Suzuki–Miyaura coupling of aryl chlorides
with arylboronic acids, the (π-allyl)Pd–NHC pseudohalo-
gen (N3, NCO, CF3CO2) complexes exhibited higher cata-
lytic efficiency relative to that exhibited by the thiocyanato
(S- or N-coordinated) analogues.
Preparation of [(η3-2-Methylallyl)Pd(μ-N3)]2, [(η3-Allyl)Pd(μ-N3)]2,
[(η3-Crotyl)Pd(μ-N3)]2, and [(η3-Cinnamyl)Pd(μ-N3)]2
[(η3-2-Methylallyl)Pd(μ-N3)]2 was prepared by a modified literature
method.[8] To a yellowish suspension of [(η3-2-methylallyl)Pd(μ-
Cl)]2 (1.07 g, 2.73 mmol) in CH2Cl2 (10 mL) was added a solution
of NaN3 (0.532 g, 8.19 mmol) dissolved in N2-bubbled H2O (1 mL)
by cannula. After stirring for 3 h at room temperature, the reaction
mixture was completely evaporated to yield a crude solid, which
was extracted with CH2Cl2 (20 mL). The collected solution was
evaporated to give a pale yellow solid, which was washed with n-
hexane. The product was dried under vacuum to give [(η3-2-methyl-
allyl)Pd(μ-N3)]2 as a pale yellow product (1.10 g, 98%).
[(η3-Allyl)Pd(μ-N3)]2 (98%), [(η3-crotyl)Pd(μ-N3)]2 (97%), and
[(η3-cinnamyl)Pd(μ-N3)]2 (98%) were prepared similarly and were
used as starting materials in subsequent reactions (Scheme 2) with-
out further crystallization because of their poor solubility in com-
mon organic solvents.
Cleavage Reactions of [(η3-2-Methylallyl)Pd(μ-N3)]2, [(η3-Allyl)-
Pd(μ-N3)]2, [(η3-Crotyl)Pd(μ-N3)]2, and [(η3-Cinnamyl)Pd(μ-N3)]2
with NHCs (2 equiv.)
Experimental Section
According to Scheme 2: A solution of IPr (0.631 g, 1.62 mmol) in
THF (4 mL) was added to a Schlenk flask containing [(η3-2-meth-
ylallyl)Pd(μ-N3)]2 (0.332 g, 0.81 mmol) by cannula, and THF
(2 mL) was added. The initial pale yellow suspension turned into
a homogeneous pale orange solution. After stirring for 2 h at room
temperature, the solvent was removed under vacuum. The resulting
residue was washed with n-hexane (3ϫ) to yield a white solid.
Recrystallization from THF/n-hexane gave white crystals of [(η3-2-
methylallyl)Pd(N3)(IPr)] (7, 0.769 g, 80%). C31H43N5Pd (592.13):
calcd. C 62.88, H 7.31, N 11.83; found C 62.95, H 7.58, N, 11.87.
General Methods, Materials, and Measurements: All manipulations
of air-sensitive compounds were performed under an atmosphere
of N2 or Ar by standard Schlenk-line techniques. Solvents were
distilled from Na–benzophenone. The analytical laboratories at
Kangnung-Wonju National University performed the elemental
analyses with a CE instruments EA1110. IR spectra were recorded
with a Perkin–Elmer BX spectrophotometer. NMR (1H, 13C{1H},
and 31P{1H}) spectra were obtained with JEOL Lamda 300 MHz
and ECA 600 MHz spectrometers. Chemical shifts were referenced
to internal Me4Si. Mass (FAB) spectra were obtained at the Korea
Basic Science Institute (Seoul). The [(η3-2-methylallyl)Pd(μ-Cl)]2,
[(η3-allyl)Pd(μ-Cl)]2, [(η3-crotyl)Pd(μ-Cl)]2, and [(η3-cinnamyl)-
Pd(μ-Cl)]2 derivatives were prepared by literature methods[44] or
modified methods or purchased from Aldrich Co. or Strem Co.
Cleavage reactions of [(η3-allyl)Pd(μ-N3)]2, [(η3-crotyl)]Pd(μ-N3)]2,
and [(η3-cinnamyl)Pd(μ-N3)]2 with the NHCs (IPr, SIPr, IMes;
2 equiv.) were analogously performed to give corresponding mono-
nuclear (π–allyl)palladium–NHC azides 8–12.
According to Scheme 1: To a solution of [(η3-allyl)Pd(Cl)(IMes)] (4,
0.984 g, 2.02 mmol) in THF (6 mL) was added a solution of NaN3
(0.144 g, 2.22 mmol) in N2-bubbled H2O (1 mL) by cannula. The
initial yellow suspension turned into an orange solution. After stir-
ring for 4 h at room temperature, the solvent was completely re-
moved under vacuum, and the remaining solid was extracted with
CH2Cl2 and washed with diethyl ether (2 mLϫ3). Recrystallization
from THF/hexane gave pale yellow crystals of [(η3-allyl)-
Pd(N3)(IMes)] (10, 0.928 g, 93%).
Cleavage Reactions of [(η3-2-Methylallyl)Pd(μ-Cl)]2, [(η3-Allyl)-
Pd(μ-Cl)]2, [(η3-Crotyl)Pd(μ-Cl)]2, and [(η3-Cinnamyl)Pd(μ-Cl)]2
with NHCs (2 equiv.)
A solution of IPr (0.648 g, 1.67 mmol) in THF (4 mL) was added
to a Schlenk flask containing [(η3-2-methylallyl)Pd(μ-Cl)]2 (0.323 g,
0.83 mmol) by cannula, and then THF (2 mL) was added. The ini-
tial pale yellow suspension turned into a homogeneous pale yellow
solution. After stirring for 2 h at room temperature, the solvent was
removed under vacuum. The resulting residue was washed with n-
hexane (3 times) to yield a white solid. Recrystallization from THF/
n-hexane gave white crystals of [(η3-2-methylallyl)Pd(Cl)(IPr)] (1,
0.960 g, 98%).
Similar reactions of [(η3-allyl)Pd(μ-Cl)]2, [(η3-crotyl)Pd(μ-Cl)]2,
and [(η3-cinnamyl)Pd(μ-Cl)]2 with the NHCs (IPr, SIPr, IMes,
2 equiv.) gave corresponding mononuclear (π-allyl)palladium–
NHC chlorides 2–6. Complexes 2, 4, and 6 are commercially avail-
able. The spectroscopic data for the (π-allyl)Pd–NHC halogen and
pseudohalogen complexes are available in the Supporting Infor-
mation.
The [(η3-2-methylallyl)Pd(Cl)(IPr)], [(η3-2-methylallyl)Pd(Cl)-
(SIPr)], [(η3-allyl)Pd(Cl)(IPr)], [(η3-crotyl)Pd(Cl)(IPr)], and [(η3-
cinnamyl)Pd(Cl)(SIPr)] complexes were treated further with NaN3
to prepare corresponding azido complexes 7–9, 11, and 12. Spec-
troscopic data of the azido complexes were in agreement with those
obtained by following the path outlined in Scheme 2.
[(η3-Allyl)Pd(N3)(IPr)] (8): Yield: 97%. C30H41N2Pd (536.08):
calcd. C 62.33, H 7.15, N 12.11; found C 62.22, H 7.68, N 11.94.
Complexes 8, 14, and 20 were also reported by Pregosin and
Albinati co-workers.[9]
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[(η3-2-Methylallyl)Pd(N3)(SIPr)] (9): Yield: 96%. C31H45N5Pd Preparation of [(η3-2-Methylallyl)Pd(NCO)(NHC)] (NHC = IPr,
(594.14): calcd. C 62.67, H 7.63, N 11.79; found C 63.08, H 7.99,
N 11.67.
SIPr), [(η3-Allyl)Pd(NCO)(IPr)], [(η3-Allyl)Pd(NCO)(IMes)], [(η3-
Crotyl)Pd(NCO)(IPr)], and [(η3-Cinnamyl)Pd(NCO)(SIPr)] (19–
24)
[(η3-Allyl)Pd(N3)(IMes)] (10): Yield: 93%. C24H29N5Pd (493.94):
According to Scheme 1: To
a
solution of [(η3-2-methylallyl)
calcd. C 58.36, H 5.92, N 14.18; found C 58.72, H 6.37, N 13.93.
[(η3-Crotyl)Pd(N3)(IPr)] (11): Yield: 83%. C31H43N5Pd (592.13):
calcd. C 62.88, H 7.32, N 11.83; found C 63.07, H 7.75, N 11.70.
[(η3-Cinnamyl)Pd(N3)(SIPr)] (12): Yield: 64%. C36H47N5Pd
(656.21): calcd. C 65.89, H 7.22, N 10.67; found C 65.80, H 7.59,
N 10.22.
Pd(Cl)(IPr)] (1, 0.156 g, 0.26 mmol) in CH2Cl2 (4 mL) was added
a solution of KOCN (0.026 g, 0.32 mmol) in N2-bubbled H2O
(1 mL) by cannula. After stirring for 3 h at room temperature, the
reaction mixture was completely evaporated to yield a crude solid,
which was extracted with CH2Cl2 (20 mL). The collected solution
was evaporated to give a crude solid, which was washed with n-
hexane. Recrystallization from THF/hexane gave white crystals of
Preparation of [(η3-2-Methylallyl)Pd(X)(NHC)] (NHC
= IPr,
SIPr)], [(η3-Allyl)Pd(X)(IPr)], [(η3-Allyl){Pd(X)(IMes)}], [(η3-
Crotyl)Pd(X)(IPr)], and [(η3-Cinnamyl)Pd(X)(SIPr)] (X = SCN,
NCS) (13–18)
[(η3-2-methylallyl)Pd(NCO)(IPr)]
(19,
0.152 g,
96%).
C32H43N3OPd (592.12): calcd. C 64.90, H 7.32, N 7.09; found C
64.55, H 7.33, N 7.25.
According to Scheme 1: To a solution of [(η3-2-methylallyl)-
Pd(Cl)(IPr)] (1, 0.102 g, 0.17 mmol) in THF (2 mL) was added a
solution of KSCN (0.019 g, 0.19 mmol) dissolved in N2-bubbled
H2O (1 mL) by cannula. After stirring for 3 h at room temperature,
the solvent was completely removed under vacuum, and the re-
maining solid was extracted with CH2Cl2 and washed with a n-
hexane (2 mLϫ3). Recrystallization from THF/hexane gave white
crystals of [(η3-2-methylallyl)Pd(SCN)(IPr)] (13, 0.102 g, 97%).
C32H43N3SPd (608.19): calcd. C 63.19, H 7.13, N 6.91; found C
63.17, H 7.24, N, 6.75.
Analogous reactions of [(η3-2-methylallyl)Pd(Cl)(SIPr)], [(η3-
allyl){Pd(Cl)(IPr)}], [(η3-allyl)Pd(Cl)(IMes)], [(η3-crotyl)Pd(Cl)-
(IPr)], and [(η3-cinnamyl)Pd(Cl)(SIPr)] by using KSCN (1.1 equiv.)
gave corresponding thiocyanato (SCN) or isothiocyanato (NCS)
palladium–NHC complexes 14–18.
According to Scheme 2: To a solution of [(η3-2-methylallyl)Pd(μ-
Cl)]2 (0.301 g, 0.76 mmol) in CH2Cl2 (3 mL) was added a solution
of KSCN (0.156 g, 1.60 mmol) in N2-bubbled H2O (1 mL) by can-
nula. After stirring for 3 h at room temperature, the reaction mix-
ture was completely evaporated to yield a crude solid, which was
extracted with CH2Cl2 (17 mL). The collected solution was evapo-
rated to give a crude solid, which was washed with n-hexane.
Recrystallization from THF/hexane (1:4) gave pale yellow crystals
of [(η3-2-methylallyl)Pd(μ-SCN)]2 (0.280 g, 84%). A solution of IPr
(0.268 g, 0.69 mmol) in CH2Cl2 (2 mL) was added to a solution
of [(η3-2-methylallyl)Pd(μ-SCN)]2 (0.151 g, 0.34 mmol) in CH2Cl2
(3 mL) by cannula. The initial pale yellow suspension turned into
a yellow solution. After stirring for 3 h at room temperature, the
solvent was removed under vacuum. The resulting residue was
washed with n-hexane (3ϫ) to yield a gray solid. Recrystallization
from THF/n-hexane (1:6) gave white crystals of [(η3-2-methylallyl)-
Pd(SCN)(IPr)] (13, 0.372 g, 89%).
Analogous reactions of [(η3-2-methylallyl)Pd(Cl)(SIPr)], [(η3-allyl)-
Pd(Cl)(IPr)], [(η3-allyl)Pd(Cl)(IMes)], [(η3-crotyl)Pd(Cl)(IPr)], and
[(η3-cinnamyl)Pd(Cl)(SIPr)] by using KOCN (1.1 equiv.) gave cor-
responding isocyanato palladium–NHC complexes 20–24.
According to Scheme 2: To a solution of [(η3-2-methylallyl)Pd(μ-
Cl)]2 (0.302 g, 0.77 mmol) in CH2Cl2 (3 mL) was added a solution
of KOCN (0.131 g, 1.61 mmol) in N2-bubbled H2O (1 mL) by can-
nula. After stirring for 3 h at room temperature, the reaction mix-
ture was completely evaporated to yield a crude solid, which was
extracted with CH2Cl2 (6 mL). The collected solution was evapo-
rated to give a crude solid, which was washed with n-hexane.
Recrystallization from THF/hexane gave pale yellow crystals of
[(η3-2-methylallyl)Pd(μ-NCO)]2 (0.282 g, 97%). An solution of IPr
(0.286 g, 0.74 mmol) in CH2Cl2 (4 mL) was added to a solution
of [(η3-2-methylallyl)Pd(μ-NCO)]2 (0.150 g, 0.37 mmol) in CH2Cl2
(2 mL) by cannula. The initial pale yellow suspension turned into
a pale orange solution. After stirring for 3 h at room temperature,
the solvent was removed under vacuum. The resulting residue was
washed with n-hexane (3 times) to yield a white solid. Recrystalli-
zation from THF/n-hexane gave white crystals of [(η3-2-methyl-
allyl)Pd(NCO)(IPr)] (19, 0.400 g, 92%).
[(η3-Allyl)Pd(NCO)(IPr)] (20): Yield: 82%. C31H41N3OPd
(578.10): calcd. C 64.40, H 7.15, N 7.26; found C 64.28, H 7.25, N
7.25.
[(η3-2-Methylallyl)Pd(NCO)(SIPr)]
(21):
Yield:
97%.
C32H45N3OPd (594.14): calcd. C 64.68, H 7.63, N 7.07; found C
64.47, H 7.75, N 7.10.
[(η3-Allyl)Pd(NCO)(IMes)] (22): Yield: 97%. C25H29N3OPd
(493.94): calcd. C 60.79, H 5.92, N 8.51; found C 60.61, H 6.03, N
8.43.
[(η3-Crotyl)Pd(NCO)(IPr)] (23): Yield: 93%. C32H43N3OPd
(592.12): calcd. C 64.91, H 7.32, N 7.09; found C 65.09, H 7.49, N
7.13.
[(η3-Allyl)Pd(SCN)(IPr)] (14): Yield: 95%. C31H41N3PdS (594.16):
calcd. C 62.66, H 6.95, N 7.07; found C 62.59, H 7.03, N 6.82.
[(η3-2-Methylallyl)Pd(SCN)(SIPr)] (15): Yield: 98%. C32H45N3PdS
(610.21): calcd. C 62.98, H 7.43, N 6.88; found C 62.96, H 7.60, N
6.49.
[(η3-Cinnamyl)Pd(NCO)(SIPr)] (24): Yield: 89%. C37H47N3OPd
(656.21): calcd. C 67.72, H 7.22, N 6.40; found C 67.40, H 7.33, N
6.43.
[(η3-Allyl)Pd(SCN)(IMes)] (16): Yield: 98%. C25H29N3PdS
(510.00): calcd. C 58.88, H 5.73, N 8.24; found C 58.99, H 5.81, N
7.88.
[(η3-Crotyl)Pd(NCS)(IPr)] (17): Yield: 95%. C32H43N3PdS
(608.19): calcd. C 63.19, H 7.13, N 6.91; found C 63.23, H 7.32, N
6.35.
[(η3-Cinnamyl)Pd(NCS)(SIPr)] (18): Yield: 63%. C36H47N5Pd
(656.21): calcd. C 65.89, H 7.22, N 10.67; found C 65.80, H 7.59,
N 10.22.
Preparation of [(η3-2-Methylallyl)Pd(OCOCF3)(NHC)](NHC
=
IPr, SIPr), [(η3-Allyl)Pd(OCOCF3)(IPr)], [(η3-Allyl)Pd(OCOCF3)-
(IMes)], [(η3-Crotyl Pd(OCOCF3)(IPr)], and [(η3-Cinnamyl)Pd-
(OCOCF3)(SIPr)] (25–30)
According to Scheme 1: To a solution of [(η3-2-methylallyl)-
Pd(Cl)(IPr)] (19, 0.154 g, 0.26 mmol) in CH2Cl2 (3 mL) was added
a solution of AgOCOCF3 (0.058 g, 0.26 mmol) in N2-bubbled H2O
(2 mL) by cannula. After stirring for 1 h at room temperature, the
reaction mixture was filtered to remove the precipitated salts, and
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the filtrate was completely evaporated and then washed with n-
hexane (2 mLϫ3) to yield the crude product (0.169 g, 97%).
Recrystallization from THF/hexane gave white crystals of [(η3-2-
methylallyl)Pd(OCOCF3)(IPr)] (25). C33H43N2O2F3Pd (663.12):
calcd. C 59.77, H 6.54, N 4.22; found C 60.14, H 4.07, N, 6.38.
2 H, Ar-H) ppm. 13C{1H} NMR (75 MHz, CDCl3, 23 °C): δ =
22.2, 22.4, 22.8, 25.7, 26.5, 28.5 (s, iPr CH), 28.6 (s, iPr CH), 55.5
(C1), 69.6 (C3), 123.8 (C2meso), 123.9, 124.3, 127.6, 128.4, 129.7,
130.4, 135.9, 136.3, 145.5, 145.9, 163.1 (CN4), 186.2 (NCN) ppm.
Analogous reactions with benzyl isothiocyanate and p-phenylene
Analogous reactions of [(η3-2-methylallyl)Pd(Cl)(SIPr)], [(η3-allyl)-
Pd(Cl)(IPr)], [(η3-allyl)Pd(Cl)(IMes)], [(η3-crotyl)Pd(Cl)(IPr)], and
[(η3-cinnamyl)Pd(Cl)(SIPr)] with AgOCOCF3 (1 equiv.) gave corre-
sponding palladium–NHC trifluoroacetates 26–30.
diisothiocyanate were performed.
[(η3-Allyl)Pd{CN4(CH2Ph)}(IPr)] (32): Yield: 91%. C38H48N6PdS
(727.31): calcd. C 62.75, H 6.65, N 11.55; found C 62.59, H 6.80,
N 11.18. 1H NMR (300 MHz, CDCl3, 23 °C): δ = 1.08 (d, J =
7.0 Hz, 6 H, iPr CH3), 1.17 (d, J = 6.6 Hz, 6 H, iPr CH3), 1.32 (d,
J = 7.0 Hz, 6 H, iPr CH3), 1.38 (d, J = 7.0 Hz, 6 H, iPr CH3), 1.68
(d, J = 12 Hz, 1 H, CH), 2.46 (d, J = 13 Hz, 1 H, CH), 2.81 (sept.,
J = 7.0 Hz, 2 H, iPr CH), 2.96 (d, J = 7.0 Hz, 1 H, CH), 3.03
(sept., J = 6.6 Hz, 2 H, iPr CH), 3.23 (dd, J = 2.2, 7.3 Hz, 1 H,
CH), 4.52 (m, 1 H, CH), 5.18 (s, 2 H, CH2), 7.05–7.08 (m, 2 H,
Ar-H), 7.18–7.19 (m, 4 H, Ar-H), 7.21 (s, 2 H, CH=), 7.26–7.29
(m, 4 H, Ar-H), 7.39–7.44 (m, 3 H, Ar-H) ppm. 13C{1H} NMR
(75 MHz, CDCl3, 23 °C): δ = 22.8, 22.9, 25.9, 26.6, 28.5 (s, iPr
CH), 28.6 (s, iPr CH), 49.6 (C1), 55.8 (CH2), 70.3 (C3), 115.2
(C2meso), 123.8, 123.9, 127.5, 128.2, 128.3,130.0, 135.4, 135.7, 145.7,
146.0, 163.9 (CN4), 185.3 (NCN) ppm.
According to Scheme 2: To a solution of [(η3-2-methylallyl)Pd(μ-
Cl)]2 (0.320 g, 0.81 mmol) in CH2Cl2 (3 mL) was added a solution
of AgOCOCF3 (0.376 g, 1.70 mmol) in N2-bubbled H2O (2 mL) by
cannula. After stirring for 3 h at room temperature, the reaction
mixture was completely evaporated to yield a crude solid, which
was extracted with CH2Cl2 (20 mL). The collected solution was
evaporated to give a crude solid, which was washed with n-hexane.
Recrystallization from THF/hexane (3:30) gave pale yellow crystals
of [(η3-2-methylallyl)Pd(μ-OCOCF3)]2 (0.361 g, 81%). A solution
of IPr (0.216 g, 0.55 mmol) in CH2Cl2 (2 mL) was added to a solu-
tion of [(η3-2-methylallyl)Pd(μ-OCOCF3)]2 (0.152 g, 0.28 mmol) in
CH2Cl2 (2 mL) by cannula. The initial yellow solution turned
green. After stirring for 3 h at room temperature, the solvent was
removed under vacuum. The resulting residue was washed with n-
hexane (3 times) to yield a gray solid. Recrystallization from THF/
n-hexane (7:40) gave white crystals of [(η3-2-methylallyl)Pd(OCOC-
F3)(IPr)] (25, 0.318 g, 86%).
[(η3-2-Methylallyl)Pd{SCN4-}(SIPr)]2(μ-C6H4) (33): Yield: 97%.
C70H94N12Pd2S2 (1380.55): calcd. C 60.90, H 6.86, N 12.17; found
1
C 60.55, H 6.93, N 12.08. H NMR (300 MHz, CDCl3, 23 °C): δ
= 1.09 (s, 6 H, CH3), 1.18 (d, J = 7.0 Hz, 12 H, iPr CH3), 1.25 (d,
J = 6.6 Hz, 12 H, iPr CH3), 1.26 (d, J = 6.6 Hz, 12 H, iPr CH3),
1.38 (d, J = 7.0 Hz, 12 H, iPr CH3), 1.89 (s, 2 H, CH), 2.54 (d, J
= 2.2 Hz, 2 H, CH), 2.94 (d, J = 2.9 Hz, 2 H, CH), 3.30 (sept., J
= 7.0 Hz, 8 H, iPr CH), 3.61 (m, 2 H, CH), 4.00 (m, 8 H, CH2),
7.19–7.22 (m, 8 H, Ar-H), 7.33–7.38 (m, 4 H, Ar-H), 7.76 (s, 4 H,
Ar-H) ppm. 13C{1H} NMR (75 MHz, CDCl3, 23 °C): δ = 22.2
(CH3), 23.5, 23.7, 26.7, 26.8, 28.5 (s, iPr CH), 28.6 (s, iPr CH), 53.9
(CH2), 55.8 (C1), 70.0 (C3), 123.9 (C2meso), 124.3, 124.6, 129.0,
130.9, 135.4, 136.2, 146.5, 147.2, 163.6 (CN4), 213.1 (NCN) ppm.
[(η3-Allyl)Pd(OCOCF3)(IPr)] (26): Yield: 97%. C32H41F3N2O2Pd
(649.10): calcd. C 59.21, H 6.37, N 4.32; found C 59.46, H 6.62, N
4.16.
[(η3-2-Methylallyl)Pd(OCOCF3)(SIPr)]
(27):
Yield:
96%.
C33H45F3N2O2Pd (665.14): calcd. C 59.59, H 6.81, N 4.21; found
C 59.89, H 7.18, N 4.60.
[(η3-Allyl)Pd(OCOCF3)(IMes)] (28): Yield: 96%. C26H29F3N2O2Pd
(564.94): calcd. C 55.27, H 5.17, N 4.95; found C 55.57, H 5.52, N
5.06.
[(η3-Allyl)Pd{SCN4-}(IPr)]2(μ-C6H4)
(34):
Yield:
98%.
C68H86N12Pd2S2 (1348.46): calcd. C 60.57, H 6.43, N 12.46; found
1
C 60.86, H 6.43, N 12.27. H NMR (300 MHz, CDCl3, 23 °C): δ
[(η3-Crotyl)Pd(OCOCF3)(IPr)] (29): Yield: 96%. C33H43F3N2O2Pd
(663.12): calcd. C 59.77, H 6.53, N 4.22; found C 59.45, H 6.77, N
4.57.
= 1.07 (d, J = 7.0 Hz, 12 H, iPr CH3), 1.14 (d, J = 7.0 Hz, 12 H,
iPr CH3), 1.26 (d, J = 6.6 Hz, 24 H, iPr CH3), 1.76 (d, J = 12 Hz,
2 H, CH), 2.77 (sept., J = 7.0 Hz, 4 H, iPr CH), 2.79 (d, J = 14 Hz,
2 H, CH), 2.99 (sept., J = 7.0 Hz, 4 H, iPr CH), 3.27 (dd, J = 2.2,
7.3 Hz, 2 H, CH), 3.74 (d, J = 7.3 Hz, 2 H, CH), 4.76 (m, 2 H,
CH), 7.18 (s, 4 H, CH=), 7.22–7.26 (m, 8 H, Ar-H), 7.39–7.45 (m,
4 H, Ar-H), 7.72 (s, 4 H, Ar-H) ppm. 13C{1H} NMR (75 MHz,
CDCl3, 23 °C): δ = 22.6, 22.7, 25.8, 26.5, 28.6 (s, iPr CH), 28.7 (s,
iPr CH), 55.9 (C1), 70.3 (C3), 115.5 (C2meso), 123.9, 124.0, 124.2,
124.4, 130.0, 135.3, 135.5, 145.6, 145.9, 163.4 (CN4), 184.9 (NCN)
ppm.
[(η3-Cinnamyl)Pd(OCOCF3)(SIPr)]
(30):
Yield:
86%.
C38H47F3N2O2Pd (727.21): calcd. C 62.76, H 6.51, N 3.85; found
C 62.42, H 6.91, N 4.04.
Reactions of [(η3-2-Methylallyl)Pd(N3)(IPr)], [(η3-2-Methylallyl)-
Pd(N3)(SIPr)], and [(η3-Allyl)Pd(N3)(IPr)] with Organic Isothio-
cyanates (R–NCS: R = Ph, CH2Ph, SCN-p-C6H4) and DMAD: To
a
solution of [(η3-2-methylallyl)Pd(N3)(IPr)] (7, 0.244 g,
0.41 mmol) in THF (4 mL) was added phenyl isothiocyanate
(0.086 mL, 0.45 mmol). After stirring for 3 h, the reaction mixture [(η3-2-Methylallyl)Pd{(N3C2)(COOMe)2}] (35): To a Schlenk flask
was evaporated completely under vacuum, and then the resulting
residue was solidified with n-hexane. The solid was filtered and
washed with hexane (5 mLϫ2). Recrystallization from diethyl
ether/n-hexane gave yellow crystals of [(η3-2-methylallyl)Pd-
{CN4(Ph)}(IPr)] (31, 0.170 g, 53%). C38H48N6PdS (727.31): calcd.
containing [(η3-2-methylallyl)Pd(N3)(IPr)] (7, 0.209 g, 0.35 mmol)
was added THF (5 mL) and then DMAD (48 μL, 0.39 mmol). The
orange solution immediately turned yellow. After stirring for 4 h at
room temperature, the reaction mixture was fully evaporated under
vacuum, and then the resulting solid was washed with hexane.
C 62.75, H 6.65, N 11.55; found C 62.98, H 6.85, N 11.61. 1H Recrystallization from diethyl ether gave pale yellow crystals of 35
NMR (300 MHz, CDCl3, 23 °C): δ = 1.02 (d, J = 7.0 Hz, 6 H, iPr
(0.235 g, 91%). IR (KBr): ν = 1731 (CO) cm–1. C H N O Pd
CH3), 1.05 (d, J = 7.0 Hz, 6 H, iPr CH3), 1.19 (s, 3 H, CH3), 1.22 (734.24): calcd. C 60.52, H 6.73, N 9.53; found C 60.42, H 6.96, N
˜
37 49
5
4
1
(d, J = 7.0 Hz, 6 H, iPr CH3), 1.29 (d, J = 7.0 Hz, 6 H, iPr CH3),
9.38. H NMR (300 MHz, CDCl3, 23 °C): δ = 0.97 (d, J = 6.9 Hz,
1.86 (s, 1 H, CH), 2.60 (s, 1 H, CH), 2.79 (sept., J = 6.6 Hz, 2 H, 6 H, iPr CH3), 1.05 (d, J = 6.9 Hz, 12 H, iPr CH3), 1.12 (d, J =
iPr CH), 3.00 (d, J = 3.3 Hz, 1 H, CH), 3.05 (sept., J = 6.6 Hz, 2 6.9 Hz, 6 H, iPr CH3), 1.24 (s, 3 H, CH3), 1.80 (s, 2 H, CH), 2.72
H, iPr CH), 3.75 (d, J = 2.9 Hz, 1 H, CH), 7.17 (s, 2 H, CH=), (br., 1 H, CH), 2.89 (sept., J = 6.9 Hz, 2 H, iPr CH), 3.32 (sept., J
7.22–7.26 (m, 4 H, Ar-H), 7.32–7.44 (m, 5 H, Ar-H), 7.72–7.75 (m,
= 6.9 Hz, 2 H, iPr CH), 3.82 (s, 6 H, CH3), 3.81 (overlap, 1 H,
Eur. J. Inorg. Chem. 2013, 4958–4969
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FULL PAPER
CH), 7.20–7.28 (m, 6 H, Ar-H), 7.36–7.42 (m, 2 H, Ar-H) ppm. SADABS based upon the Laue symmetry by using equivalent re-
13C{1H} NMR (75 MHz, CDCl3, 23 °C): δ = 22.3, 22.4 (CH3),
flections.[45] All calculations were performed with the SHELXTL
22.6, 25.9, 26.5, 28.1 (s, iPr CH), 51.5 (C1), 76.6 (C3), 123.9 programs.[46] All structures were solved by direct methods. All non-
(C2meso), 124.0, 124.5, 129.8, 132.2, 136.1, 145.9, 146.4, 163.1 (CO), hydrogen atoms were refined anisotropically, and all hydrogen
186.9 (NCN) ppm.
Reaction of [(η3-Allyl)Pd(N3)(IPr)] or [(η3-2-Methylallyl)Pd(N3)-
atoms were generated in ideal positions and refined in a riding
model.
CCDC-937998 (for 7), -937999 (for 8), -938000 (for 14), -938001
(for 17), -938002 (for 19), -938003 (for 22), -938004 (for 25), and
-938005 (for 31) contain the supplementary crystallographic data
for this paper. These data can be free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-re-
quest/cif.
(IPr)] with Organic Isocyanides
To a solution of [(η3-allyl)Pd(N3)(IPr)] (8, 0.187 g, 0.32 mmol) in
THF (4 mL) was added tert-butyl isocyanide (0.040 mL,
0.35 mmol). After stirring for 3 h, the reaction mixture was evapo-
rated completely under vacuum, and then the resulting residue was
solidified with n-hexane. The solid was filtered and washed with
hexane (2 mLϫ2). Recrystallization from diethyl ether at –70 °C
gave pale yellow crystals of [(η3-allyl)Pd(N3)(IPr)]·(CN-tBu) (36,
0.150 g, 70%). C35H50N6Pd (661.23): calcd. C 63.57, H 7.62, N
12.71; found C 63.32, H 8.04, N 12.36. 1H NMR (300 MHz,
CDCl3, 23 °C): δ = 1.08 (s, 9 H, CH3), 1.09 (d, J = 7.0 Hz, 6 H,
iPr CH3), 1.16 (d, J = 7.0 Hz, 6 H, iPr CH3), 1.26 (d, J = 12 Hz,
1 H, CH), 1.35 (d, J = 7.0 Hz, 6 H, iPr CH3), 1.38 (d, J = 7.0 Hz,
6 H, iPr CH3), 2.64 (d, J = 13 Hz, 1 H, CH), 2.76 (sept., J = 7.0 Hz,
2 H, iPr CH), 2.77 (overlap, 1 H, CH), 2.95 (sept., J = 6.6 Hz, 2
H, iPr CH), 3.84 (dd, J = 2.2, 7.3 Hz, 1 H, CH), 4.71 (m, 1 H,
CH), 7.11 (s, 2 H, CH=), 7.24–7.29 (m, 4 H, Ar-H), 7.40–7.45 (m,
2 H, Ar-H) ppm. 13C{1H} NMR (75 MHz, CDCl3, 23 °C): δ =
22.7, 22.8, 25.7, 26.4, 28.6 (s, iPr CH), 28.7 (s, iPr CH), 31.8 (s,
CH3), 47.5 (C1), 51.1 [s, C(CH3)3], 68.9 (C3), 114.4 (C2meso), 123.7,
123.9, 129.8, 135.8, 146.0, 146.3, 186.0 (NCN) ppm. The carbon
signal for CϵN was not observed due to its weak intensity.
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic and NMR spectroscopic data for (π-allyl)Pd–
NHC pseudohalogen complexes and organic products.
Acknowledgments
This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(Grant No. 2012R1A1B3001569).
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Received: May 9, 2013
Published Online: July 31, 2013
Eur. J. Inorg. Chem. 2013, 4958–4969
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