Synthesis, structure, and function of pcp pincer transition-metal-complex- bound norvaline derivatives

Article abstract of DOI:10.1055/s-0033-1339473

A PCP pincer palladium-complex-bound norvaline, Boc-l-[Pd]Nva-OMe, was synthesized and fully characterized by NMR, FT-ICR-MS, and X-ray crystallography. Selective N- and C-terminus transformations of Boc-l-[Pd]Nva-OMe were performed by conventional deprotection-condensation procedures to afford lipophilic palladium-bound norvaline derivatives without metal detachment. The N-/C-bisfunctionalized palladium-bound norvaline showed self-assembly properties, as evidenced by supramolecular gel formation. The catalytic activity of the supramolecular gel was assessed in the 1,4-conjugate addition of phenylboronic acid.

Full text of DOI:10.1055/s-0033-1339473

1910▌
LETTER
lSetyternthesis, Structure, and Function of PCP Pincer Transition-Metal-Complex-
Bound Norvaline Derivatives
PCP Pincer Transition-Metal-Complex-Bound Norvaline Derivatives
Hikaru Takaya,*a,b,c Takashi Iwaya,a Kazuki Ogata,a,b Katsuhiro Isozaki,a,b,d Tomoya Yokoi,a,b Ryota Yoshida,a,b
Nobuhiro Yasuda,e Hirofumi Seike,a Toshio Takenaka,a Masaharu Nakamura*a,b
a
International Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Fax +81(774)383186; E-mail: takaya@scl.kyoto-u.ac.jp; E-mail: masaharu@scl.kyoto-u.ac.jp
b
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyoku, Kyoto 615-8510, Japan
c
PRESTO, Japan Science and Technology Agency
d
CREST, Japan Science and Technology Agency
e
JASRI, SPring-8, 1-1 Kouto, Sayo, Hyogo 679-5198, Japan
Received: 22.05.2013; Accepted after revision: 27.06.2013
amino acid-based fabrication of supramolecular function-
Abstract: A PCP pincer palladium-complex-bound norvaline,
al materials, and led us to further exploit pincer transition-
Boc-L-[Pd]Nva-OMe, was synthesized and fully characterized by
metal-complex-bound amino acids. Herein, we report the
synthesis, characterization, and self-assembly properties
NMR, FT-ICR-MS, and X-ray crystallography. Selective N- and
C-terminus transformations of Boc-L-[Pd]Nva-OMe were per-
formed by conventional deprotection–condensation procedures to of novel amino acids tethered to PCP pincer palladium
afford lipophilic palladium-bound norvaline derivatives without
metal detachment. The N-/C-bisfunctionalized palladium-bound
norvaline showed self-assembly properties, as evidenced by supra-
the 1,4-conjugate addition of arylboronic acid to cyclo-
molecular gel formation. The catalytic activity of the supramolecu-
lar gel was assessed in the 1,4-conjugate addition of phenylboronic
and platinum complexes 1–5 (Figure 1). Further, we de-
scribe the catalytic activities of supramolecular gel of 4 in
hexenone.
acid.
Ph2
P
Keywords: amino acids, cross-coupling, catalysis, palladium, self-
M = Pd
O
assembly
Cl
1: R1 = Boc, R2 = OMe
M
2: R1 = CO(CH2)10Me, R2 = OMe
3: R1 = Boc, R2 = NH(CH2)10Me
4: R1 = Me(CH2)10CO, R2 = NH(CH2)10Mea
Ph2P
O
Metalated amino acids – hybrids of organometallic and bi-
ologically important molecules – have been considered as
promising bioorganometallic materials.1 The properties
arising from the conjugation of amino acids and organo-
metallic compounds exhibit naturally inaccessible new
functions and activities without loss of the parents’ inher-
ent chemical, physical, and biological characteristics. For
instance, the combination of amino acid bioaffinity and
the photochemical, electrochemical, and radioactive prop-
erties of metal complexes has been widely investigated,
providing various metalated amino acid-based bioimag-
ing and biosensing reagents.2 However, the application of
metalated amino acids to functional materials remains a
largely untouched subject, despite there having been sev-
eral reports on metal array fabrication based on the self-
assembly of metalated peptides3 and a variety of research
for fabricating functional materials based on the self-as-
sembly of amino acids.4 We recently found that metalated
amino acids tethered to a NC-half-pincer platinum
complex5 and a NCN pincer palladium complex6 self-as-
sembled to form supramolecular fibrous aggregates with
well-regulated metal arrays; some of these exhibited en-
hanced catalytic activity as supramolecular gel catalysts.7
These findings demonstrate the potential of metalated
R1
R2
M = Pt
5: R1 = Boc, R2 = OMe
N
H
O
Figure 1 PCP pincer metal-complex-bound norvalines
We designed and synthesized the PCP pincer palladium-
complex-bound norvaline derivatives 1–4 and their plati-
num analogue 5. The robust cyclometalated PCP pincer
structure was expected to tolerate acidic or alkaline condi-
tions during various synthetic transformations without un-
desired metal leaching.8,9 Additionally, the inert alkyl
linkage between the palladium/platinum complex and the
amino acid functionalities probably contributes to the pre-
vention of undesirable metal-complex detachment, even
under severe synthetic conditions.
The unreported parent PCP pincer complexes, [4-bromo-
2,6-bis(diphenylphosphinoxy-κP,κP)phenyl-κC]chloro-
palladium(II) (6) and [4-bromo-2,6-bis(diphenylphosphi-
noxy-κP,κP)phenyl-κC]chloroplatinum(II) (7), were
prepared in one pot, similarly to literature methods
(Scheme 1).10 The reaction of 1-bromo-3,5-dihydroxy-
benzene with excess chlorodiphenylphosphine (2.4 equiv)
and triethylamine (3.0 equiv) was carried out in refluxing
toluene for eight hours. Metalation of the resulting phos-
phinite intermediate was performed by the addition of
PdCl2 or PtCl2·SMe2 with additional refluxing for 18
SYNLETT 21709013, 2415, 1910–1914
Advanced online publication: 13.08.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339473; Art ID: ST-2013-U0474-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
PCP Pincer Transition-Metal-Complex-Bound Norvaline Derivatives
1911
Cl
Ph2P
M
PPh2
O
Ph2P
O
PPh2
O
HO
OH
(i)
O
(ii)
Br
Br
Br
6: M = Pd, 48%
7: M = Pt, 30%
Scheme 1 Synthesis of PCP pincer palladium and platinum complex-
es. Reagents and conditions: (i) Ph2PCl, Et3N, toluene, reflux, 8 h; (ii)
MCl2 (M = Pd or Pt), toluene, reflux, 18 h.
Figure 2 X-ray crystal structure of PCP pincer palladium complex
6. Selected bond lengths (Å): Pd1–Cl1 = 2.3508(10), Pd1–C1 =
1.980(4), Pd1–P1 = 2.2597(12), Pd1–P2 = 2.2756(12); selected bond
angles (°): P1–Pd1–P2 = 160.64(4), Cl1–Pd1–C1 = 177.08(13), Cl1–
Pd1–P1 = 96.88(4), Cl1–Pd1–P2 = 102.47(4), C1–Pd1–P1 =
80.36(12), C1–Pd1–P2 = 80.28(12); selected dihedral angles (°):
C11–Pd1–P1–O1 = –177.72(4), Cl1–Pd1–P2–O2 = –179.19(4), C1–
Pd1–P1–O1 = 1.27(9), C1–Pd1–P2–O2 = 1.83(9).
hours to afford the corresponding PCP pincer complexes
6 and 7, respectively. The PCP coordination geometry of
complex 6 was unequivocally determined by single-crys-
tal X-ray structure analysis as shown in Figure 2.11 Conju-
gation of the PCP pincer complexes to the norvaline
moiety was achieved by sequential hydroboration–
Suzuki–Miyaura coupling starting from allylglycine – a
route developed by Taylor12 and van Koten,13 and recently
reconfirmed in effectiveness by us (Scheme 2).6,7 The syn-
thesis began with Boc-L-allylgly-OMe, which was quan-
titatively prepared from commercially available Boc-L-
allylgly-OH·HNCy2 by one-pot amine neutralization with
HCl and trimethylsilyldiazomethane esterification. Cross-
coupling of the 9-BBN adduct of Boc-L-allylgly-OMe
with the PCP pincer complexes afforded mixtures of si-
multaneously formed halide-exchanged byproducts of M–
Br and unreacted M–Cl complexes. Treatment with ex-
cess KCl reconverted the M–Br species into M–Cl species
and afforded the desired PCP pincer palladium- and plat-
inum-bound norvalines 114 and 5, respectively. No metal
leaching was observed in the reaction mixture at any step
of the process.15 Preservation of the enantiomeric purity of
the norvaline subunit during the coupling reaction in the
presence of excess K3PO4 base was confirmed by chiral
HPLC analysis, as in our previous study.6 The absolute
configuration of 1 was unequivocally determined by sin-
gle-crystal X-ray crystallography,16 in which the observed
bond lengths and angles around the palladium complex
subunit were essentially unchanged from those of the par-
ent complex 6 (Figure 3).
Ph2
P
O
Cl
M
Ph2P
O
B
(i)
Boc
(ii), (iii)
OMe
Boc
OMe
N
Boc
OMe
H
N
O
H
N
O
H
O
Boc-Allylgly-OMe
1: M = Pd, 52%
5: M = Pt, 10%
Scheme 2 Synthesis of palladium- and platinum-complex-bound
norvalines. Reagents and conditions: (i) 9-BBN, THF, 0 °C, 5 min,
then r.t., 3 h; (ii) 6 or 7, Pd(OAc)2 (5 mol%), SPhos (10 mol%),
K3PO4, THF–H2O, r.t., 12 h; (iii) excess KCl, THF–H2O, r.t., 6 h.
Reactivity at the N-terminus was investigated on the pal-
ladium-bound norvaline 1, as shown in Scheme 3. The
Boc deprotection of 1 under acidic conditions using ex-
cess HCl afforded the palladium-bound norvaline-free
amine 9. Subsequent reaction with dodecanoic acid medi-
ated by the condensing reagent DMTMM·PF617 gave the
corresponding N-alkanoyl palladium-complex-bound
norvaline C12-L-[Pd]Nva-OMe (2) in 89% yield without a
metal-leaching byproduct. Conventional trifluoroacetic
acid conditions were also applicable for the Boc deprotec-
tion of 1, giving 9 in 68% yield, as revealed by NMR.
Figure 3 X-ray crystal structure of palladium-complex-bound nor-
valine 1. Selected bond lengths (Å): Pd1–Cl1 = 2.376(1), Pd1–C1 =
1.991(4), Pd1–P1 = 2.276(1), Pd1–P2 = 2.266(1); selected bond an-
gles (°): P1–Pd1–P2 = 159.49(5), Cl1–Pd1–C1 = 178.6(1), Cl1–Pd1–
P1 = 99.63(5), Cl1–Pd1–P2 = 100.80(5), C1–Pd1–P1 = 79.3(1), C1–
Pd1–P2 = 80.3(1); selected dihedral angles (°): C11–Pd1–P1–O1 =
–174.02(9), Cl1–Pd1–P2–O2 = –177.4(1), C1–Pd1–P1–O1 = 5.0(2),
C1–Pd1–P2–O2 = 3.5(2).
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1910–1914

1912
H. Takaya et al.
LETTER
minutes, the TBME solution gradually thickened to form
an opaque gel (Figure 4, b). Further, upon heating, the gel
of 4 melted to afford the solution once again, and this sol–
gel transition was completely reversible through heating
and cooling cycles. Such a thermoreversible phase transi-
tion strongly suggested that the observed gelation was in-
duced by a self-assembly process among the gelator
molecules through noncovalent interactions such as hy-
drogen bonding and π–π interactions.5,6 Scanning electron
microscopy (SEM) analyses of dried gel particles (xero-
gels) of 4 showed an entangled network of beltlike micro-
fibrils, which is typical of supramolecular gels prepared
from lipophilic amino acids and peptides (Figure 5).3
Ph2
Ph2
P
P
O
O
Cl
Cl
Pd
O
Pd
O
Ph2P
Ph2P
(i)
O
(ii)
1
OMe
OMe
H2N
Me(CH2)10
N
H
O
O
9
2 89%
Scheme 3 N-Terminal functionalization of 1. Reagents and condi-
tions: (i) excess 4.0 M HCl, dioxane, r.t., 2 h; (ii) Me(CH2)10CO2H,
DMTMM·PF6, Et3N, DMF, r.t., 6 h.
The C-terminal convertibility was demonstrated by LiI-
promoted C-deprotection,18 followed by DMTMM·PF6-
mediated condensation with primary amines. Standard ba-
sic C-hydrolysis using LiOH or TMSOK failed and result-
ed in undesirable Boc deprotection and decomplexation of
the PCP pincer palladium complex. As depicted in
Scheme 4, condensation of the palladium-bound norva-
line free acid 10 with undecylamine proceeded smoothly,
and sequential halide exchange with AgBF4/NaCl trans-
formed the tentatively formed Pd–I byproduct to the Pd–
Cl derivative, giving the desired C-alkylamido palladium-
bound norvaline Boc-L-[Pd]Nva-C11 (3) in 61% yield. In
addition, C-terminal amidation of 2 was also performed
via the metalated free acid 11 to give the double-tailed
palladium-bound norvaline C12-L-[Pd]Nva-C11 (4) in 55%
yield.
Figure 4 Thermoreversible gelation of 4 in TBME (3.010–2 M): a)
TBME solution, and b) gel state.
Ph2
Ph2
P
O
P
O
Cl
Cl
Pd
O
Pd
O
Ph2P
Ph2P
1
or
2
(i)
(ii), (iii)
H
R1
OH
R1
N
Figure 5 SEM image of TBME gel of 4 (× 11000).
N
N
(CH2)10Me
H
H
O
O
10: R1 = Boc
11: R1 = CO(CH2)10Me
3: R1 = Boc, 61%
4: R1 = CO(CH2)10Me, 55%
We examined the catalytic activity of the supramolecular
gel of 4 because palladium-complex-based supramolecu-
lar materials have recently been widely recognized as ef-
ficient supramolecular catalysts.7,19 The xerogel particles
of 4 prepared from the TBME gel showed catalytic activ-
ity for the 1,4-conjugate addition of phenylboronic acid to
cyclohexenone under heterogeneous conditions (Scheme
5),20 affording the corresponding adduct in 12% yield, de-
spite that the parent complex, [2,6-bis(diphenylphosphi-
noxy-κP,κP)phenyl-κC]chloropalladium(II) and catalysts
1–4 gave less than 3% yield of products under homoge-
neous nongelated conditions in CHCl3 and THF. It is note-
worthy that these types of PCP pincer palladium
complexes have been reported as ineffective catalysts in
the 1,4-conjugate addition of arylboron derivatives to
enones in anhydrous toluene.21
Scheme 4 C-Terminal functionalization of 1 and 2. Reagents and
conditions: (i) LiI, pyridine, reflux, 3 h; (ii) Me(CH2)10NH2,
DMTMM·PF6, Et3N, DMF, r.t., 6 h; (iii) AgBF4, MeCN, r.t., 1 h, then
NaCl, 1 h.
We have reported that NCN-pincer palladium-complex-
bound C-alkylamido norvalines showed remarkable gela-
tion properties in organic solvents, producing supramo-
lecular gels.6 Here, also, the PCP pincer palladium-bound
norvaline 4 was found to be an efficient gelator in ethereal
and aromatic solvents. Typically, a mixture of 4 in tert-
butyl methyl ether (TBME) was heated until complete dis-
solution to afford a clear solution (Figure 4, a). Upon cool-
ing to room temperature and allowing to stand for a few
Synlett 2013, 24, 1910–1914
© Georg Thieme Verlag Stuttgart · New York

LETTER
PCP Pincer Transition-Metal-Complex-Bound Norvaline Derivatives
1913
(c) Fujimura, F.; Kimura, S. Org. Lett. 2007, 9, 793.
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4 gel (5 mol% Pd)
K3PO4 (1.0 equiv)
OH
OH
B
O
+
toluene–H2O (1 :1)
60 °C, 6 h
O
10.1039/C3DT51696B.
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Scheme 5
In conclusion, we successfully synthesized chemically
and physically robust novel PCP pincer palladium-bound
norvaline derivatives. Self-assembly of the N-/C-double
alkylated palladium-bound norvalines enabled us to pre-
pare palladium-immobilized metallogels. We also found
that a supramolecular gel of the palladium-bound norva-
line showed catalytic activity in the 1,4-conjugate addi-
tion of phenylboronic acid. The palladium-immobilized
metallogel acted as a heterogeneous catalyst for this reac-
tion in the aqueous phase. Improvement of the catalytic
efficiency of the PCP pincer palladium-bound norvaline
gels and application of the supramolecular-gel-catalyzed
reaction to asymmetric C–C bond formation are currently
ongoing in our laboratory and will be reported in due
course.
(5) Isozaki, K.; Ogata, K.; Haga, Y.; Sasano, D.; Ogawa, T.;
Kurata, H.; Nakamura, M.; Naota, T.; Takaya, H. Chem.
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Yokoi, T.; Isozaki, K.; Yoshida, R.; Takenaka, T.; Seike, H.;
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M. Chem. Eur. J. 2013, 19, DOI: 10.1002/chem.201301513.
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Hao, X.-Q.; Gong, J.-F.; Song, M.-P. Dalton Trans. 2011,
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Curr. Org. Synth. 2009, 6, 169. (c) van der Boom, M. E.;
Milstein, D. Chem. Rev. 2003, 103, 1759.
(9) For the stability of POCOP-type PCP pincer Pd complexes,
see the following pioneering works: (a) Miyazaki, F.;
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Acknowledgment
This work was supported by a Grant-in-Aid for Scientific Research
of no. 22550099 and ‘The Coordination Programming – Science of
Supermolecular Structure and Creation of Chemical Elements’ of
no. 24108719 from the MEXT, CREST (11103784 and 11102545)
and PRESTO (07051361) from the JST, and through the ‘Funding
Program for Next generation World-Leading Researchers’ initiated
by the CSTP. FT-ICR-MS analyses were supported by the JURC at
ICR, Kyoto University. K.O. extends special thanks to the Global
COE programs ‘International Center for Integrated Research and
Advanced Education in Materials Science’ for its financial support.
K.I. and T.T. express special thanks to the MEXT project ‘Inte-
grated Research on Chemical Synthesis’.
(11) For crystallographic data of 6, see Supporting Information.
(12) (a) Collier, P. N.; Campbell, A. D.; Patel, I.; Taylor, R. J. K.
Tetrahedron Lett. 2000, 41, 7115. (b) Collier, P. N.;
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
References and Notes
(14) To a THF solution of Boc-L-allylglycine methyl ester (1.7
mmol) 9-BBN (3.4 mmol) was added at 0 °C and stirred at
r.t. for 3 h. Pd-catalyzed cross-coupling with 6 (1.4 mmol)
was carried out in the presence of 5 mol% of Pd(OAc)2–
SPhos with K3PO4 (3.3 mmol) followed by halide exchange
of the resulting Pd–Br species by excess amount of KCl (170
mmol) to afford 5 (851 mg, 71%) after appropriate
purification; mp 97.5–98.5 °C. 1H NMR (392 MHz, CDCl3):
δ = 7.91–8.03 (m, 8 H, m-ArH), 7.42–7.54 (m, 12 H, ArH),
6.59 (s, 2 H, ArH), 5.00 (d, J = 8.2 Hz, 1 H, NH), 4.26–4.38
(m, 1 H, NHCHCO), 3.74 (s, 3 H, OCH3), 2.47–2.63 (m, 2
H, ArCH2), 1.58–1.90 (m, 4 H, CHCH2CH2CH2), 1.43 [s, 9
H, (CH3)3C]. 13C NMR (99.5 MHz, CDCl3): δ = 173.2 (1 C,
COOCH3), 164.3 (1 C, C6H2), 155.3 (1 C, OCONH), 143.6
(2 C, C6H2), 133.2 (4 C, C6H5), 131.9 (4 C, C6H5), 131.6 (8
C, C6H5), 128.9 (8 C, C6H5), 107.3 (2 C, C6H2), 53.1 (1 C,
NHCH), 52.2 (1 C, COOCH3), 35.2 (1 C, CHCH2CH2CH2),
32.2 (1 C, CHCH2CH2CH2), 28.2 [3 C, OC(CH3)3], 26.7 (1
C, CHCH2CH2CH2). 31P NMR (159 MHz, CDCl3): δ =
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Synlett 2013, 24, 1910–1914

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LETTER
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1.60. Found: C, 56.95; H, 4.98; N, 1.64. [α]D25 +8.67
(c 0.473, CHCl3).
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(15) Undesired aryl bromide reduction of 7 to [2,6-bis(diphenyl-
phosphinoxy-κP,κP)phenyl-κC]chloroplatinum(II) leads to
low yields of platinum-bound norvaline 5.
(16) X-ray diffraction measurements were obtained at SPring-8
(BL02B1: 2012B1109 and 2012B1790; BL40XU:
2011B1545, 2012A1161, 2012A1625, and 2012B1815).
Flack parameter = –0.01(3). For the detailed
crystallographic data of 1, see Supporting Information and
also, Yasuda, N.; Murayama, H.; Fukuyama, Y.; Kim, J. E.;
Kimura, S.; Toriumi, K.; Tanaka, Y.; Moritomo, Y.;
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(17) DMTMM [4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-
methylmorpholinium chloride], see: (a) Kamiński, Z. J.;
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(18) For LiI deprotection, see: (a) McMurry, J. Org. React. 1976,
24, 187. (b) Boger, D. L.; Danielle, R. S.; Christopher, W.
(20) Unfortunately, reusability of the gel catalyst 4 could not be
examined, because the extremely gummy gel made it
difficult to recover the catalyst by filtration.
(21) Bedford, R. B.; Betham, M.; Charmant, J. P. H.; Haddow, M.
F.; Orpen, A. G.; Pilarski, L. T.; Coles, S. J.; Hursthouse, M.
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Synlett 2013, 24, 1910–1914
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