Synthesis and cavity size effect of pd-containing macrocycle catalyst for efficient intramolecular hydroamination of allylurethane

Article abstract of DOI:10.1021/acs.orglett.5b00378

Palladium-containing macrocycle catalysts (PdMCs) with different ring sizes ranging from 24 to 30 members were synthesized. The intramolecular hydroamination of an allylurethane (AU) catalyzed by PdMCs proceeded efficiently to afford the corresponding oxazolidinone (OZ) in 95% isolated yield. The dependence of the hydroamination of AU to OZ on the cavity size indicated that the reaction rate was clearly controlled by both substrate uptake and product release steps.

Full text of DOI:10.1021/acs.orglett.5b00378

Letter
pubs.acs.org/OrgLett
Synthesis and Cavity Size Effect of Pd-Containing Macrocycle
Catalyst for Efficient Intramolecular Hydroamination of Allylurethane
Masahiro Ogawa,† Masaki Nagashima,† Hiromitsu Sogawa,† Shigeki Kuwata,‡ and Toshikazu Takata*,†,§
‡
†Department of Organic and Polymeric Materials and Department of Applied Chemistry, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
§Advanced Catalytic Transformation for Carbon Utilization (ACT-C), JST, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: Palladium-containing macrocycle catalysts (PdMCs) with different ring
sizes ranging from 24 to 30 members were synthesized. The intramolecular hydro-
amination of an allylurethane (AU) catalyzed by PdMCs proceeded efficiently to afford the
corresponding oxazolidinone (OZ) in 95% isolated yield. The dependence of the
hydroamination of AU to OZ on the cavity size indicated that the reaction rate was clearly
controlled by both substrate uptake and product release steps.
evelopment of supramolecular catalysts exhibiting elegant
and efficient catalytic activity is an interesting and
PdMCs with three different ring sizes were prepared via a
two-step reaction starting from the pincer-type ligand (1), as
illustrated in Scheme 1 and according to our previously
D
attractive topic for chemists.1 Among those supramolecular
catalysts, macrocycle catalysts represented by Cu-modified β-
cyclodextrins2 are characterized by facile preparation of a
flexible framework and easy cavity control through the choice of
reaction type. Because of the specific space for reactions around
the metals in these catalysts, the catalysts often exhibit size-
selective catalysis.3−5 Nolte, Rowan, and co-workers performed
detailed studies of metalloporphyrin-containing macrocycles
that effectively catalyze the epoxidation of polymeric alkenes via
a pseudorotaxane intermediate. This reaction mimics the
reaction mode of λ-exonuclease,6 which also proceeds via a
pseudorotaxane intermediate with DNA. Meanwhile, we
reported the intramolecular cyclization of allylurethanes to
oxazolidinones catalyzed by a Pd-tethering macrocycle catalyst
(PdMC)7 after our finding of a similar transformation within a
rotaxane molecule with four allylurethane moieties in the axle
component.7b Interestingly, this catalytic hydroamination of
allylurethanes with pyridyl moieties proceeds only when the
allylurethane can form a pseudorotaxane complex with PdMC,
suggesting that the reaction takes place in the catalyst cavity.7
Although such macrocycle catalysts are expected to have a
potential utility as a highly active and selective catalyst, to the
best of our knowledge the characteristic features of their
catalysis have not yet been investigated. We have studied the
functions of PdMCs and recently found a unique catalysis
specific to their macrocyclic structures that is disclosed in this
paper. This PdMC-catalyzed intramolecular hydroamination of
allylurethane depends largely upon the cavity size of PdMC,
which affected not only the initial complexation with substrate
followed by the Pd-catalyzed hydroamination but also the
elimination of the product from the cavity.
Scheme 1. Synthesis of Macrocycle Catalysts (PdMCs)
reported method for PdMC30.7a Namely, initial macrocycle
formation from pyridine bisamide ligand 1 and an oligo-
(ethylene glycol) (MC24, MC27, or MC30) was followed by
palladium insertion into the cavity of the MCs using
palladium(II) diacetate to afford the corresponding PdMCs
in good yields. The structures of PdMCs were determined
1
using H NMR, MALDI-TOF MS and X-ray crystal structure
analyses.
The association constant (Ka) between PdMC and a
pyridine-containing bis(allylurethane) (AU) was evaluated by
1H NMR spectra in d6-DMSO at various temperatures (from 40
to 120 °C), as shown in Table 1.8 The formation of a 1:1
inclusion complex (AU ⊂ PdMC) was readily confirmed by the
Received: February 6, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00378
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Table 1. Complexation between PdMCs and AU
ab
,
Ka (M−1
)
40 °C
60 °C
80 °C
100 °C
120 °C
PdMC24
PdMC27
0
0
0
0
0
22
59
150 59
110 40
460 60
65 15
190 10
47
6
6
6
PdMC30
880 210
120 10
a
b
1
Estimated by H NMR spectrum. [PdMCs] = [AU] = 0.04 M.
upfield shift of the methylene proton peaks of AU (Figure S1).
This result strongly suggested the formation of a pseudorotax-
ane complex on the basis of the similarity of NMR spectra to
those obtained in our previous study.7b The thermodynamic
parameters for pseudorotaxane formation were also determined
by van’t Hoff plot in d6-DMSO (Figure S2 and Table S1).
Although no complex was formed with PdMC24, complexes
were obtained with PdMC27 and PdMC30, which had larger
cavity sizes. As can be seen in Table 1, AU ⊂ PdMC30
exhibited a larger Ka than AU ⊂ PdMC27 at all temperatures,
indicating that AU ⊂ PdMC30 is more stable than AU ⊂
PdMC27. Clear MALDI-TOF-MS peaks corresponding to AU
⊂ PdMC27 (m/z 1047.77 [M + Na]+, m/z 1047.25 calcd for
C52H50N6O10Pd) and AU ⊂ PdMC30 (m/z 1091.63 [M +
Na]+, m/z 1091.28 calcd for C54H54N6OO11Pd) were observed.
The cavity sizes of PdMCs have been estimated by X-ray
analysis of the acetonitrile complexes or their derivative (Table
2). The structural data revealed that the distances between the
Pd atoms and oxyethylene portions of PdMC24, PdMC27, and
PdMC30 were ca. 8.2, 9.8, and 9.9 Å,7b respectively. The cavity-
sizes clearly increased as the length of the oxyethylene spacer
increased, as expected. Furthermore, two face-to-face benzene
rings in PdMC24 were twisted to shrink its cavity width (5.8
Å), which is likely due to the shorter spacer length, which was
not observed in PdMC27 (7.0 Å) or PdMC30 (7.1 Å). This
constrained structure should control the formation of the
pseudorotaxane-type complex with AU.
Table 2. Cavity Sizes for PdMCs Estimated by X-ray Analysis
a
Calculated from the crystal structures of the acetonitrile complexes.
Structural parameters have been estimated from the crystal structure
b
of a cyclic tetrameric derivative without acetonitrile ligands. Averaged
values in the four crystallographically independent units are shown.
See Supporting Information for details.
Next, a solution of AU, triethylamine (200 mol %), and
PdMC27 (20 mol %) in d6-DMSO was heated at 80 °C for 95
h to give the corresponding hydroamination product (OZ) in
95% isolated yield with a diastereomer ratio of ca. 50:50. With
PdMC30, the yield of OZ was 72%, and no OZ was obtained
with PdMC24. The catalytic activity of PdMC was investigated
by monitoring the 1H NMR spectral changes during the
reaction of AU in the presence of PdMC (20 mol %) and
The time−conversion curves clearly indicate two different
reaction stages (early and late). Considering all of the above-
described results, a plausible reaction mechanism for the
PdMC-catalyzed hydroamination of AU was proposed and is
presented in Scheme 2.
The initial equilibrium formation of a pseudorotaxane
complex between PdMC and AU is followed by the Pd-
catalyzed hydroamination of one allylurethane moiety of AU.
Successive hydroamination of the remaining allylurethane
moiety proceeds with retention of the pseudorotaxane
structure. Final PdMC elimination from the product−PdMC
complex (OZ ⊂ PdMC) concludes a successful reaction cycle.
Because no monohydroaminated product was detected in the
1H NMR, the second hydroamination must be faster than the
first one. In fact, the faster reaction with PdMC30 versus
PdMC27 can be explained if the rate-limiting step is the initial
monohydroamination from AU ⊂ PdMC (see the difference in
1
triethylamine in DMSO (Figure 1). No changes in the H
NMR spectrum were observed with PdMC24, even after 57 h,
which was in contrast to PdMC27 and PdMC30; the
intramolecular hydroamination of AU to OZ with these
catalysts readily occurred with 100% conversion, as shown in
Figure 1a. These results clearly reveal that the complexation of
AU with PdMC plays a key role in the reaction. Interestingly,
the initial rate of the reaction with PdMC30 was greater than
that with PdMC27, although the reaction with PdMC30 took
longer to reach completion than the reaction with PdMC27.9
B
DOI: 10.1021/acs.orglett.5b00378
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To verify our proposed mechanism, density functional theory
(DFT) calculations were performed using Gaussian 09.10 The
geometries of AU, OZ, PdMCs, AU ⊂ PdMCs, and OZ ⊂
PdMCs were optimized without any constraints using DFT
method at B3LYP/lanl2dz levels in the presence of DMSO.11
The initial geometries of PdMCs and their derivatives were
determined or estimated using the results from the X-ray
structure analyses. Figure 2 summarizes the relative energy
Figure 2. Relative energies for various intermediate structures involved
in the PdMC-catalyzed intramolecular hydroamination of AU, for
PdMCs with different cavity sizes. The energy values for AU +
PdMC27 (−2 103 116.267 kcal/mol) and AU + PdMC30
(−2 199 643.318 kcal/mol) were calculated at the B3LYP/lanl2dz
levels and set to zero for the PdMC27 and PdMC30 derivatives,
respectively.
Figure 1. Time−conversion curves for the intramolecular hydro-
amination of AU catalyzed by PdMCs at (a) 80 and (b) 100 °C.
values for each conformation. The energy values for AU ⊂
PdMCs are lower than the totals of the individual energy values
for AU and the corresponding PdMCs. Furthermore, AU ⊂
PdMC30 is more stable than AU ⊂ PdMC27, being consistent
with the actual Ka values (Table 1). Meanwhile, the relative
energy increases dramatically for OZ ⊂ PdMCs. Notably,
because the energies are even higher than the total values for
OZ and PdMCs, PdMCs can form complexes with AU
remaining in the reaction mixture immediately after dissocia-
tion of OZ ⊂ PdMC intermediate. In addition, the difference in
the relative energies for AU ⊂ PdMC and OZ ⊂ PdMC
intermediates increases for PdMC27 (ΔE27 − ΔE30 = 1.1 kcal/
mol), as does the difference for OZ ⊂ PdMC and OZ + PdMC
(4.4 kcal/mol). The lower the concentration of AU or AU ⊂
PdMC, the larger the contribution of the dissociation of OZ ⊂
PdMC to the reaction rate. These calculated results are in good
agreement with the experimental results, including the unusual
acceleration at the latter reaction stage and the proposed
mechanism.
Scheme 2. Plausible Reaction Mechanism for the
Intramolecular Hydroamination of AU Catalyzed by PdMC
Note that if the proposed mechanism is correct then an
increase in both the reaction temperature and catalyst
concentration should affect the reaction profile, i.e., reduce
the importance of the OZ ⊂ PdMC dissociation step. This
assumption was confirmed: when the reaction was performed at
100 °C, only a slight difference in the completion time was
observed for the catalysts with different ring sizes (Figure 1b).
Namely, the difference in the energy differences ΔE30 and ΔE27
was diminished, leading to a reduction in the difference in the
reaction rates for PdMC30 and PdMC27. Meanwhile, when
the PdMC concentration was increased to 40 mol % (Figure 3),
the reaction proceeded more rapidly than that with 20 mol %
(Figure 1a), and the reaction with PdMC30 reached
completion sooner than that with PdMC27. Thus, a high
catalyst concentration appears to weaken the acceleration effect
observed with 20 mol % PdMC27 because the reaction is
complete before the effect (i.e., that PdMC27 was eliminated
Ka values in Table 1). In addition, the faster completion of the
reaction with PdMC27 can be accounted for if OZ ⊂ PdMC
dissociates more rapidly to regenerate PdMC, which can be
attributed to greater steric strain. This conclusion is supported
by previous results obtained for PdMC30, which cannot
translate over the two oxazolidinone moieties formed in the
axial positions because of the bulkiness in the rotaxane
system.7b Next, to examine the complexation between OZ
and PdMCs, the catalysts were each mixed with OZ under
various conditions. Interestingly, neither PdMC27 nor
PdMC30 formed a complex with OZ in d6-DMSO at 25−
100 °C. This result also supports the conclusion that the rate of
the reaction with PdMC27 in the latter period does not
decrease significantly, even though the concentration of
substrate AU is lower.
C
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and analytical and spectroscopic data
for all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ttakata@polymer.titech.ac.jp.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
This work was financially supported by the ACT-C program of
JST, a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan
(no. 23245031).
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