Changing the palladium coordination to phosphinoimidazolines with a remote triazole substituent

Article abstract of DOI:10.1002/adsc.201100684

Phosphinoimidazoline (PHIM) ligands bearing a triazolylmethyl substituent at the sp³ nitrogen atom in the imidazoline ring lead to highly improved enantioselectivity (up to 99% ee) in allylic substitution reactions with respect to analogous ligands with substituents lacking the triazole unit. NMR and theoretical studies support a shift in the coordination mode of the PHIM ligand to palladium, triggered by a very favourable interaction with the triazole unit. Copyright

Full text of DOI:10.1002/adsc.201100684

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DOI: 10.1002/adsc.201100684
Changing the Palladium Coordination to Phosphinoimidazolines
with a Remote Triazole Substituent
Verꢀnica de la Fuente,a Rocꢁo Marcos,b Xacobe C. Cambeiro,b Sergio Castillꢀn,c,*
Carmen Claver,a,* and Miquel A. Pericꢂsb,d,*
a
Departament de Quꢁmica Fꢁsica i Quꢁmica Inorgꢂnica, Universitat Rovira i Virgili; C/Marcel·lꢁ Domingo s/n, 43007
Tarragona, Spain
Fax : (+34)-977-559-563; phone: (+34)-977-559-574; e-mail: carmen.claver@urv.cat
Institute of Chemical Research of Catalonia (ICIQ); Av. Paꢃsos Catalans 16; 43007 Tarragona, Spain
b
Fax : (+34)-977-920-222; phone: (+34)-977-920-200; e-mail: mapericas@iciq.es
Departament de Quꢁmica Analꢁtica i Quꢁmica Orgꢂnica, Universitat Rovira i Virgili; C/Marcel·lꢁ Domingo s/n, 43007
c
Tarragona, Spain
E-mail: sergio.castillon@urv.cat
Departament de Quꢁmica Orgꢂnica, Universitat de Barcelona; 08028 Barcelona, Spain
d
Received: July 17, 2011; Revised: August 29, 2011; Published online: December 6, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100684.
Abstract: Phosphinoimidazoline (PHIM) ligands
bearing a triazolylmethyl substituent at the sp3 ni-
trogen atom in the imidazoline ring lead to highly
improved enantioselectivity (up to 99% ee) in allyl-
ic substitution reactions with respect to analogous
ligands with substituents lacking the triazole unit.
NMR and theoretical studies support a shift in the
coordination mode of the PHIM ligand to palladi-
um, triggered by a very favourable interaction with
the triazole unit.
PHIM ligands have shown to be very efficient in
the Ir-catalyzed enantioselective hydrogenation of
prochiral olefins[2] and imines,[3] as well as in the Pd-
catalyzed asymmetric Heck reaction.[4] However, they
have never been used in Pd-catalyzed asymmetric al-
lylic substitution reactions, a domain where PHOX li-
gands are one of the most important players.[1d,5] We
wish to report in the present paper the structural opti-
mization of modular PHIM ligands using the Pd-cata-
lyzed asymmetric allylic substitution reaction as a
benchmark. A first generation of PHIM ligands re-
sults from the optimization of the substituents at C-4
and C-5 on the imidazoline moiety (R1), the substitu-
ent at the non-coordinating nitrogen atom of the imi-
dazoline ring (R2), and the substituents of the phos-
Keywords: allylic substitution; asymmetric catalysis;
palladium; phosphinoimidazoline ligands; triazole
ligands
Phosphinooxazolines (PHOX)[1] are versatile C1-sym-
metric chiral ligands (Figure 1, left) with ample appli-
cations in a variety of metal-catalyzed reactions. The
analogous phosphinoimidazoline (PHIM) ligands
(Figure 1, right) could represent an even more con-
venient alternative: the topology of the imidazoline
ligand allows the use of readily available C2-symmet-
ric diamine fragments, while the second nitrogen
atom represents an additional source of molecular di-
versity (R2), allowing the programmed modification
of the electronic properties of the coordinating nitro-
gen atom. The additional nitrogen atom could also
serve for the heterogenization of the phosphinoimida-
zoline ligand onto insoluble organic resins, a field
which still remains completely unexplored.
Figure 1. Modular phosphinooxazoline (PHOX) and phos-
phinoimidazoline (PHIM) ligands.
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Figure 2. First generation PHIM ligands 1–7.
phine moiety (R3). A second generation of PHIM li-
gands, resulting from the introduction of triazolyl-
methyl moieties as nitrogen substituents (R2), has un-
expectedly led to catalytic systems with highly im-
proved enantioselectivities for a variety of substrates
and nucleophiles. A combined theoretical and NMR
study provides a satisfactory explanation for this be-
haviour.
A small modular library of phosphinoimidazoline
ligands 1–7, including variations of the employed C2-
Scheme 1. Synthesis of second-generation PHIM ligands 11
symmetrical 1,2-diamine, the diorganylphosphino and 12.
moiety, and the substituent at the sp3-type nitrogen
atom in the imidazoline was initially prepared by
Table 1. Alkylation and amination of 1,3-diphenylallyl ace-
tate (8) with Pd/PHIM catalysts.
standard methodology[3,6] (Figure 2, see Supporting
Information for details).
With these ligands in hand, the alkylation of (E)-
1,3-diphenylallyl acetate (8) with dimethyl malonate
and the amination of the same substrate with benzyl-
(h3-C3H5)Cl]2 were taken
ACHTUNGTRENNUNGamine in the presence of [PdHCATUNGTRENNUGN
as model reactions (Table 1). For the alkylation reac-
tion, optimal reaction conditions involved working in
dichloromethane, with microwave irradiation at 658C,
in the presence of KOAc and BSA (Table 1, en-
tries 1–7, Conditions A). Under these conditions, all
the ligands tested resulted in good conversion in a
reasonable reaction time. In terms of enantioselectivi-
ty, N-benzylated ligands 3 and 4 (entries 3 and 4),
gave the best results (85 and 87% ee, respectively),
with little influence of the 1,2-diphenylethylene or cy-
clohexane-1,2-diyldiamine backbones being used. On
the other hand, comparison of ligands 1 and 7 (en-
tries 1 and 7) shows that the diphenylphosphino
moiety provides better results than the dicyclohexyl-
phosphino one. The corresponding amination reaction
with benzylamine was next studied with the most syn-
thetically interesting N-substituted ligands 3 and 4
(Table 1, entries 8 and 9, Conditions B). While satis-
factory conversions were registered under thermal
conditions, enantioselectivity was only ca. 80%.
Entry Ligand Conditions[a] Conversion [%][b] ee [%][c]
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
3
4
A
A
A
A
A
A
A
B
100
91
71
88
99
100
74
95
83
53
85
87
81
73
78
79
82
B
100
[a]
All reactions run with 0.02 mmol [PdACTHNUTRGNEUNG
(h3-C3H5)Cl]2,
0.044 mmol ligand, 1 mmol substrate, 3 mmol dimethyl
malonate (Conditions A) or 3 mmol benzylamine (Con-
ditions B), 3 mmol BSA and 0.04 mmol KOAc in 2 mL
CH2Cl2.
By NMR.
By HPLC on a chiral stationary phase.
Having in mind the ultimate goal of supporting the
PHIM ligands onto polymer supports for easy recov-
ery and recycling,[7,8] we decided to study the catalytic
behaviour of PHIM derivatives bearing triazolylmeth-
yl substituents on the imidazoline moiety. Thus, li-
[b]
[c]
gands 11 and 12 were prepared through CuAAC reac- reaction with benzyl bromide in the presence of
tions[9] from N-propargylic derivatives 13 and 14 sodium azide, l-ascorbic acid and catalytic amounts of
(Scheme 1). The triazole moieties were installed by CuSO4, and the phosphino units were introduced in
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Changing the Palladium Coordination to Phosphinoimidazolines with a Remote Triazole Substituent
Table 2. Pd-catalyzed allylic alkylation with second genera-
tion PHIM ligands 11 and 12.[a]
Table 3. Pd-catalyzed allylic amination with second genera-
tion PHIM ligands 11 and 12.[a]
Entry Substrate Ligand Conversion [%][b] ee [%][c]
1
2
3
4
8
8
17
17
11
12
11
12
94
85
63
32
96
86
99
75
[a]
All reactions run with 0.02 mmol [PdACTHNUTRGNEUNG
(h3-C3H5)Cl]2,
Entry Ligand Nucleophile Conversion [%][b] ee [%][c]
0.044 mmol ligand, 1 mmol substrate, 3 mmol dimethyl
malonate, 3 mmol BSA and 0.04 mmol KOAc in 2 mL
CH2Cl2.
By NMR.
By HPLC on a chiral stationary phase.
1
2
3
4
11
11
11
11
11
11
12
12
12
19
20
21
22
23
24
19
22
24
99
99
99
99
99
47
99
49
0
98
96
96
99
97
99
84
67
–
[b]
[c]
5
6[d]
7
the last step of the sequence to prevent oxidative de-
terioration of the PHIM ligands.
8
9[d]
The performance of ligands 11 and 12 was first ex-
amined in the alkylation of substrates 8 and 17 with
dimethyl malonate under the previously optimized re-
action conditions. The results (Table 2) showed the
1,2-diphenylethylenediamine-derived ligand 11 to be
clearly superior to the 1,2-cyclohexanediamine-de-
rived one 12, both in terms of catalytic activity and
enantioselectivity. In addition, and quite notably,
these N-triazolylmethyl PHIM ligands turned out to
[a]
All reactions run with 0.02 mmol [PdACTHNUTRGNEUNG
(h3-C3H5)Cl]2,
0.044 mmol ligand, 1 mmol substrate, 3 mmol nucleo-
phile, and 3 mmol BSA in 2 mL CH2Cl2.
By NMR.
By HPLC on a chiral stationary phase.
48 h.
[b]
[c]
[d]
induce notably higher enantioselectivities than their 22, 23), complete conversion with virtually complete
N-alkyl or unsubstituted counterparts (see Table 1, enantioselectivity was recorded. In fact, the enantio-
above).
selectivities achieved with ligand 11 are among the
Ligands 11 and 12 were also tested in the palladi- highest reported for this type of reaction.[10]
um-catalyzed allylic amination of 8 using amines 19–
The very positive effect on enantioselectivity of a
24 (Table 3). The results confirmed that the structure triazolylmethyl substituent at the sp3 nitrogen atom of
of 11 incorporates the optimal chiral fragment for the imidazoline ring in PHIM ligands is hardly under-
high enantioselectivity in the amination reaction. standable by analysis of the generally accepted transi-
Very remarkably, when this ligand was used for the tion states for the Pd-catalyzed substitution reactions,
amination with synthetic equivalents of ammonia (19, since the aromatic part of the substituent on the imi-
Figure 3. Alternative coordination modes in [Pd (h3-C3H5) (PHIM)]+ cationic complexes. The nature of the coordinating N
atom is shown in brackets.
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Figure 4. DFT-optimized structures and energies of the [Pd(h3-diphenylallyl)(11)]+ cationic complex. Non-relevant hydrogen
atoms have been omitted for clarity. Exoskeletal substituents are represented as a wireframe; green: Pd; orange: P; blue: N;
grey: C; white: H.
dazoline N-1 position is both electronically isolated palladium, from the usual N-3 nitrogen atom of the
and spatially remote from the molecular region where imidazoline to the N-1 one (type B) or even to the tri-
the bond-making process takes place (Figure 3, type azole group (type C).[11,12]
A). This led us to speculate with the possibility of the
To test this hypothesis, we decided to study by the-
triazolylmethyl substituent triggering a change in the oretical means (DFT) the h3-(1,3-diphenylallyl)palla-
coordination mode of the phosphinoimidazoline with dium intermediates arising from the three proposed
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Changing the Palladium Coordination to Phosphinoimidazolines with a Remote Triazole Substituent
coordination modes of the PHIM ligand to palladium. ly strong stabilization of the type C structures with re-
For each coordination mode, exo and endo isomers of spect to the normal type A ones (>11 kcalmolÀ1) as
the syn-syn type were considered. The calculations well as with respect to the type B ones (>6 kcal
were performed with the Minnesota 06 (M06) func- molÀ1). This result clearly indicates the triazole to be
tional,[13] as implemented in Gaussian 09.[14] The a good ligand for palladium, thus allowing it to be sig-
triple-z Stuttgart–Dresden (SDD) basis set and effec- nificantly competitive even in this situation where the
tive core potentials[15] were used for palladium and imidazoline coordination leads to a very stable six-
the split-valence double-z 6-31G basis set with addi- membered chelate ring.
tional d polarization functions[16] for all the other
To check this conclusion against experimental data,
atoms. After an exhaustive conformational analysis, we decided to study in detail the cationic Pd(II) com-
absolute energy minima for the endo and exo isomers plex [Pd(h3-diphenylallyl)(11)]PF6, which could be
of the three coordination modes were located and easily obtained upon treatment of 11 with the dimeric
characterized. To simulate the employed experimental precursor [Pd(h3-diphenylallyl)Cl]2 and NH4PF6. Un-
conditions, single point energy calculations were per- fortunately, no crystals suitable for X-ray analysis
formed on the optimized structures with the scrf sol- could be obtained; however, a comprehensive NMR
vent model[17] for dichloromethane. The results, sum- study (see Supporting Information) provided strong
marized in Figure 4, surprisingly revealed an extreme- support to the conclusions of the theoretical study.
Figure 5. NOESY spectrum of the [Pd(h3-diphenylallyl)(11)]PF6 complex in CD2Cl2. Peaks in yellow correspond to positive
NOEs and peaks in blue to negative ones, likely indicating an exchange process to be taking place (endo-exo interchange).
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Table 4. NOE-relevant H-H distances in the calculated mini-
mal energy conformers of the [Pd(h3-1,3-diphenylallyl)(11)]+
cationic complex and its comparison with the observed
NOEꢇs in the NMR study of the same complex.[a]
Particularly remarkable are the results of NOESY ex-
periments (Figure 5), which showed significant differ-
ences with complexes of PHIM ligands lacking the tri-
azolylmethyl moiety. Namely, (i) in contrast with what
observed for alkyl-substituted PHIM ligands (5, see
Supporting Information), an NOE interaction be-
tween one of the methylene hydrogens (Hb’) in the
triazolylmethyl substituent and the allyl terminal hy-
drogen in a position trans to phosphorus (Ht) was ob-
served for [Pd(h3-diphenylallyl)(11)]PF6, and (ii) also
in contrast with what observed for alkyl-substituted
PHIM ligands, no NOE between the imidazoline C-4
or C-5 hydrogens (Hd and He) and the allyl hydrogens
(Ht and Hc) was observed for [Pd(h3-diphenylal-
lyl)(11)]PF6.
H
H
NOE
Distance [ꢅ]
observed? Aexo Aendo Bexo Bendo Cexo Cendo
Ha Hb YES
Ha Hb’ YES
Hb’ Ht YES
Ht Hd NO
Ht He NO
Hc Hd NO
Hc He NO
2.36 2.43 3.67 3.53 3.15 3.17
3.77 3.82 4.05 3.59 4.66 4.68
7.23 7.59 4.55 5.59 3.61 4.89
2.97 3.77 6.28 4.81 5.03 7.21
5.52 6.38 3.51 2.88 5.24 7.05
4.78 4.83 5.70 6.96 7.07 5.36
7.45 6.73 3.79 4.37 7.74 5.80
[a]
Highlighted in boldface italics are the distances consid-
ered not to be consistent with the experimental data. For
this simplification we have assumed a cut between ob-
servable or not observable NOE at 5.00 ꢅ, which is a
somewhat extreme, although valid distance.[18]
If we consider type A structures resulting from the
theoretical study, the very long distances between Hb’
and Ht (7.23 ꢅ in A-exo and 7.59 ꢅ in A-endo) would
prevent the observation of NOE interactions between
these protons. In addition, the rather short distances
between Hd and Ht (2.97 ꢅ in A-exo and 3.77 ꢅ in A- by the imidazoline moiety. This new coordination be-
endo) should result in the observation of NOE inter- haviour offers potential for the design of P,N ligands
actions between these protons. According to these ob- with increased catalytic efficiency and enantioselectiv-
servations, type A structures involving the normal co- ity in different reaction types. Work along these lines
ordination of the imidazoline to palladium must be is now in progress in our laboratories.
discarded.
In type B structures, significant NOE interactions
should be expected between He and the close Hc and Experimental Section
Ht protons on the allyl moiety (the calculated distan-
ces between He and Ht are 3.51 ꢅ in B-exo and General Procedure for the Palladium-Catalyzed
2.88 ꢅ in B-endo). However, these interactions are Allylic Substitution of 8 and 17 with Different
not observed experimentally with [Pd(h3-diphenylal- Nucleophiles using the Pd/11 Catalytic System
lyl)(11)]PF6, and this is a solid argument against type
B structures.
To a Schlenk tube under argon, [PdACHTUNGTRNEUNG
(h3-C3H5)Cl]2 (1.84 mg,
5 mmol, 2%) and ligand 11 (0.011 mmol) in CH2Cl2 (2 mL)
were introduced. The resulting solution was stirred for
20 min. Then, the substrate (250 mmol), the nucleophile
(750 mmol), BSA (185 mL, 750 mmol) and 5 mmol KOAc
were successively added. The mixture was stirred under
reflux conditions until reaction completion. The reaction
mixture was then diluted with diethyl ether, filtered over
Celite, and washed with water (3ꢆ5 mL). The organic phase
was dried over anhydrous MgSO4. The residue was purified
through a short SiO2 pad eluting with (hexane/EtOAc from
Finally, a thorough analysis of the geometrical char-
acteristics of type C minima and of the results ob-
tained in the NOESY experiments with [Pd(h3-diphe-
nylallyl)(11)]PF6 revealed a perfect correlation be-
tween both sets of data (Table 4). Since type C struc-
tures are the most stable ones according to theoretical
calculations, the results of the combined NMR and
DFT study provide a rather solid support for the pro-
posed change of the coordination mode of 11 to palla-
dium, with the triazole unit instead of the imidazoline
moiety coordinatively bonded to the metal.
1
100:0 to 80:20). Conversion was determined by H NMR of
the crude product after solvent removal under vacuum.
In conclusion, PHIM ligands bearing a 4-(1,2,3-tria-
zolyl)methyl substituent at the NACHTUNGTRNEUNG
(sp3) atom in the imi-
dazoline ring, have been shown to be much more effi-
cient chiral controllers (up to 99% ee) than the corre-
sponding analogues lacking the triazole group in Pd-
catalyzed allylic substitutions (ca. 80% ee). A com-
bined NMR and DFT study has shown that these P,N
ligands most likely coordinate to palladium through
the triazole N-3 nitrogen atom as the N component,
giving rise to a highly asymmetric, unprecedented
structure with increased bite angle, and with one of
Acknowledgements
We thank MICINN (Grants No. CTQ2007-62288/BQU and
CTQ2008-00947/BQU), AGAUR (Grant No. 2009SGR623),
Consolider Ingenio 2010 (Grant No. CSD2006-0003), ICIQ
Foundation and GC (Distinction for Research Promotion
2003 CC) for financial support. V. dlF and R. M. are grateful
to GC and MICINN, for predoctoral fellowships. We are also
grateful to R. Guerrero and M. A. Rodrꢀguez (SRCT, URV)
the faces of the chelate plane very efficiently shielded and Dr. C. Godard for assistance and advice in NMR studies
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Changing the Palladium Coordination to Phosphinoimidazolines with a Remote Triazole Substituent
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