Extending the range of neutral N-donor ligands available for metal catalysts: N-[1-Alkylpyridin-4(1 H)-ylidene]amides in palladium-catalyzed cross-coupling reactions

Article abstract of DOI:10.1021/ic201527s

N-[1-Alkylpyridin-4(1H)-ylidene]amides (PYAs) are a new class of easily prepared, neutral N-donor ligands that share some features in common with N-heterocyclic carbenes. They are strongly electron-donating toward metal centers, and a palladium(II) complex of one of these ligands has been shown to successfully catalyze both the Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions.

Full text of DOI:10.1021/ic201527s

COMMUNICATION
pubs.acs.org/IC
Extending the Range of Neutral N-Donor Ligands Available for
Metal Catalysts: N-[1-Alkylpyridin-4(1H)-ylidene]amides in
Palladium-Catalyzed Cross-Coupling Reactions
Peter D. W. Boyd, L. James Wright,* and M. Naveed Zafar
School of Chemical Sciences, University of Auckland, Auckland, New Zealand
S Supporting Information
b
Scheme 1. Similarities between Valence-Bond Representa-
tions of NHCs, Remote NHCs, PYEs, and PYAs
ABSTRACT: N-[1-Alkylpyridin-4(1H)-ylidene]amides
(PYAs) are a new class of easily prepared, neutral N-donor
ligands that share some features in common with N-hetero-
cyclic carbenes. They are strongly electron-donating toward
metal centers, and a palladium(II) complex of one of these
ligands has been shown to successfully catalyze both the Heckꢀ
Mizoroki and SuzukiꢀMiyaura cross-coupling reactions.
he developmentof effective homogeneousmetal catalysts has
T
long been recognized as an important scientific endeavor.1
A growing appreciation for the urgent need to develop sustainable
chemical practices is currently providing significant impetus to
studies in this area.2,3 While the ligand compliment is a funda-
mental part of any homogeneous metal catalyst, it is noteworthy
that relatively few classes of neutral donor ligands have been
commonly utilized in successful homogeneous catalysts.4 Among
these are the phosphanes,5 amines,6 nitrogen-containing hetero-
cycles such as pyridines and oxazolines,7,8 imines,9ꢀ11 and, more
recently, N-heterocyclic carbenes (NHCs).12ꢀ14 Thediscoveryof
new ligand classes that can be widely utilized in catalyst develop-
ment is therefore an important goal.
knowledge, these have not been used directly as ligands to form
coordination compounds. A small number of complexes that
have PYA-like ligands and were prepared in alternative ways have
been described in the literature.24
The PYA ligands reported in this study are simple to prepare,
as indicated in Scheme 2. Details of the syntheses and characteri-
zation data for all new compounds are available in the Supporting
Information. The precursor pyridinium carboxamides, [HL1ꢀ3]X,
are easily deprotonated through treatment with bases such as
sodium carbonate (aqueous) or sodium hydride to give L1ꢀ3
in excellent yield. In the IR spectrum, the bands in the amide
I region of [HL1ꢀ3]X all decrease by ca. 40ꢀ60 cmꢀ1 upon
deprotonation.
It has recently emerged that there is a set of ligands that share
the common features of neutral overall charge and one valence-
bond representation in which the donor atom is negatively
charged but is in π conjugation with a positively charged
“iminium-like” group. Examples of classes of ligands with these
features are NHCs,12 “remote” NHCs,14,15 and the recently
reported N-[1-alkylpyridin-4(1H)-ylidene]amines (PYEs).16ꢀ19
Valence-bond representations of each of these classes of ligands
that emphasize the relationships between them are illustrated
in Scheme 1. In this Communication, we now extend this set
of ligands by reporting the N-[1-alkylpyridin-4(1H)-ylidene]-
amides (PYAs; see Scheme 1) as a new class of neutral N-donor
ligands. The valence structures A and B illustrate the relationship
between PYAs and the other classes of ligands depicted in
Scheme 1. The use of palladium PYA complexes to successfully
catalyze cross-coupling reactions is also reported.
Amidates, which are obtained via deprotonation of the nitro-
gen atom of carboxamides, are well-known anionic ligands.20
Coordination usually occurs through the nitrogen atom, and
in this form, they are characteristically strong σ donors.21,22
Deprotonation of the corresponding N-pyridinium-substituted
carboxamides gives neutral PYAs directly. Although some com-
pounds of this type have been reported previously,23 to our
Single-crystal X-ray structure determinations of [HL2]Cl
and L2 have been obtained (see the Supporting Information).
The atom numbering schemes used for these compounds are
identical with that used for the L2 ligand in the structure of
[PdCl2(L2)] (2), depicted in Figure 2. Upon deprotonation of
the pyridinium carboxamide [HL2]+ to form L2, significant
decreases are observed in the distances C6ꢀN2 [1.382(2)ꢀ
1.3562(14) Å] and N2ꢀC7 [1.380(2)ꢀ1.3414(14) Å], while
accompanying increases occur in the distances C7ꢀC8 [1.405(2)ꢀ
1.4280(15) Å] and C6ꢀO1 [1.216(2)ꢀ1.2357(13) Å]. These
data suggest that both valence-bond structures A and C in
Scheme 1 should be considered in addition to B in a simple
description of the delocalized bonding for L2.
Received: July 19, 2011
Published: September 30, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic201527s Inorg. Chem. 2011, 50, 10522–10524
|

Inorganic Chemistry
COMMUNICATION
Scheme 2. Syntheses of Alkylpyridinium Carboxamides
[HL1-3]X, PYA Ligands L1ꢀ3, and Methylpyridinium Amine
[HL4]X
Figure 2. X-ray structure of 2.
Scheme 3. Syntheses of Complexes 1ꢀ4
Figure 1. Frontier NBO orbitals: (I) lone pair on N2 and (II) the π
bond between N2 and C7 in L3.
conveniently, a mixture of H[L2]Cl and 1,8-diazabicyclo[5.4.0]-
undec-7-ene (DBU; Scheme 3). The closely related complex
[PdCl2(L3)] (3) can be obtained through parallel reactions with
L3 or H[L3][O3SCF3]. In both 2 and 3, the ring protons of the
alkylpyridinium groups are observed in the 1H NMR spectra at
positions intermediate between those found for the correspond-
ing protonated (H[L2,3]X) and free ligands ([L2,3]).
A single-crystal X-ray structure determination of 2 has been
obtained (see the Supporting Information), and the molecular
structure is shown in Figure 2. This confirms that coordination
to palladium occurs through the two nitrogen donors of L2.
The distances C6ꢀN2 [1.363(2) Å], N2ꢀC7 [1.397(2) Å],
C7ꢀC8 [1.404(2) Å], and C6ꢀO1 [1.230(2) Å] in 2 are either
between the corresponding distances observed in [HL2]Cl
and L2 or close to those recorded for [HL2]Cl. The PdꢀCl2
distance [2.2917(4) Å] is normal for chlorine trans to pyridine.
In contrast, the PdꢀCl1 [2.3115(4) Å] distance is longer,
indicating that the PYA ligand has a stronger trans influence
than pyridine. The PdꢀN2 distance of 2.0392(14) Å is similar to
that reported for a corresponding PdꢀN distance in a related
PYE-containing complex.18
Density functional theory calculations (B3LYP/def2-TZVP)
carried out on L3 and [HL3]+ support the valence-bond repre-
sentation A in Scheme 1 as the predominant Lewis structure for
L3. A natural bond orbital (NBO) analysis of L3 indicates the σ
and π character of the double bonds represented in A (see the
Supporting Information). The lone pair on N2 and the π bond
between N2 and C7 obtained by this analysis are illustrated in
Figure 1.
The ligands L1ꢀ3 (Scheme 2) are all air-stable crystalline
solids. They remain essentially unchanged upon heating under
reflux in methanol for 30 min or standing at 20 °C for 48 h in
dimethyl sulfoxide containing 10% water.
Estimates of the comparative donor properties of a range
of neutral ligands L (including NHCs) have been obtained
previously by measuring νav(CO) for the complexes [cis-RhCl-
(CO)2L].25 The treatment of [{Rh(μ-Cl)(COD)}2] with L1
gives [Rh(Cl)(COD)L1], and subsequent carbonylation pro-
duces [cis-RhCl(CO)2L1] (1). A preliminary crystal structure
determination of [Rh(Cl)(COD)L1] confirms that L1 bonds as a
monodentate ligand through nitrogen in this complex, and it is
reasonable to assume this is also the case in 1. In an alterna-
tive synthesis, 1 can be obtained directly by the treatment of
[{Rh(μ-Cl)(CO)2}2] with L1 (Scheme 3). The two ν(CO) bands
for the cis-carbonyl ligands in 1 are observed in the IR spectrum at
2069 and 1987 cmꢀ1 (av 2028 cmꢀ1). These values are slightly
lower than those reported for [cis-RhCl(CO)2L], where L is either
a saturated NHC (2081, 1996; av 2038 cmꢀ1),25 or a PYE ligand
(2077, 1998; av 2038 cmꢀ1).17 On the basis of these data, the
donor strength ofthe PYAligand L1 in1 is atleast asgreat asthat of
typical NHC or PYE ligands, even though there are differences in
the electronic structures of these three ligand classes.
To gauge the potential utility of the PYA ligands L2 and L3 as
supporting ligands for the important palladium-catalyzed Heckꢀ
Mizoroki and SuzukiꢀMiyaura cross-coupling reactions (NHCs
have been extensively studied as supporting ligands for these
reactions26,27), preliminary experiments with 2 and 3 were carried
out. For comparative purposes, the PYE-containing palladium
complex [PdCl2(L4)] (4; see Scheme 3) was prepared from
[HL4]O3SCF3 (Scheme 2) and also studied in these reactions.
The PYE ligand L4 was designed so that it incorporates a chelating
arm that is similar to the chelating pyridyl groups of L2,3.
The results obtained for the HeckꢀMizoroki reactions (not
optimized) between phenyl bromide and styrene with 2, 3, or 4
added as precatalysts (1 mol %) are summarized in Table 1.
Nearly complete conversion to stilbenes (>93% trans-stilbene)
The palladium complex 2 is formed in ca. 70% isolated yield
through the treatment of [PdCl2(COD)] with either L2 or, more
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dx.doi.org/10.1021/ic201527s |Inorg. Chem. 2011, 50, 10522–10524

Inorganic Chemistry
COMMUNICATION
Table 1. Results for the Catalytic Experiments
NBO analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
HeckꢀMizoroki reactionsa
SuzukiꢀMiyaura reactionsc
’ AUTHOR INFORMATION
catalyst
time
(h)
product
(%)b
catalyst
no.
time
(h)
product
(%)d
Corresponding Author
*E-mail: lj.wright@auckland.ac.nz.
no.
4
2
2
2
3
6
6
49
61
75
93
93
4
2
4
2
3
4
4
6
6
6
38
49
56
76
76
’ ACKNOWLEDGMENT
We gratefully acknowledge the HEC for providing a post-
graduate scholarship to M.N.Z.
12
24
24
’ REFERENCES
a Conditions: DMA solvent, 140 °C, catalyst (1 mol %), sodium acetate
(1.1 equiv), styrene (1.4 equiv) vs bromobenzene (Pd(OAc)2 blank, 6 h,
9% conversion). b trans-1,2-Stilbene, cis-1,2-stilbene, and α-1,10-stilbene.
c Conditions: DMF solvent, 100 °C, catalyst (1 mol %), Cs2CO3
(2 equiv), p-tolylboronic acid (1.5 equiv) vs bromobenzene (Pd-
(OAc)2 blank, 6 h, 38% conversion). d 4-Methylbiphenyl.
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’ ASSOCIATED CONTENT
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S
Supporting Information. Experimental procedures and
b
characterizing data, crystallographic data and material in CIF
format (CCDC 825923ꢀ825925), optimized geometries, and
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