Metalated Amino Acids and Peptides

Chapter 4

Metalated Amino Acids and Peptides: Programmable Metal Array Fabrication and Application to Supramolecular Catalyst Hikaru Takaya1, Katsuhiro Isozaki1, Ryota Yoshida1,2, Takafumi Shanoh1,2, Nobuhiro Yasuda3 and Masaharu Nakamura1 1

Institute for Chemical Research, Kyoto University, Kyoto, Japan, 2Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan, 3 Japan Synchrotron Radiation Research Institute (JASRI), Kouto, Japan

4.1 INTRODUCTION Metalated amino acids and peptides are one of the most widely used and developed bioorganometallic compounds.1 There has been numerous research in bioanalytical and biomedical applications, where various biologically active, radioactive metal complexes are conjugated into amino acids and peptides to provide biomarkers, contrast agents, and drugs.1b 3 However, there are non-biological applications of metalated amino acids and peptides to functional materials, such as supramolecular architectures and catalysts, based on their inherent properties of self-assembly and molecular recognition. We have successfully formulated programmable fabrications of sequence-controlled multi-metalated peptides and their supramolecular architectures using α-side chain-metal-conjugated amino acids and peptides. Synthesis and the precise structure determination of these novel bioorganometallic molecules and the application to supramolecular catalyst are also described in this chapter.

Advances in Bioorganometallic Chemistry. DOI: https://doi.org/10.1016/B978-0-12-814197-7.00004-2 © 2019 Elsevier Inc. All rights reserved.

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

4.2 CLASSIFICATION OF METALATED AMINO ACIDS AND PEPTIDES Reported metalated amino acids and peptides are roughly classified into three types as shown in Fig. 4.1.1,3 7 The N- and C-terminus types of amino acids, where the metals or metal complexes conjugated to the N-/C-terminus of amino acid/peptide, are the most commonly reported forms especially in bioanalytical and biomedical research. The advantage of these two types is that the metal moiety can be readily incorporated by the simple condensation reaction of a metal complex or ligand for the coordination of metals at a late stage. This merit was fully demonstrated in the synthesis of SPECT and MRI contrasting agents using short life time and labile metals, such as Tc and Gd, and protein-binding peptides, where a minimized step synthesis is required to avoid leaching metal during the synthesis.2 The third type of side-chainconjugated metalated amino acids and peptides are generally synthesized through a cumbersome multi-step process to furnish the metal conjugationsite involving protection/deprotection processes of the N-/C-terminus. However, the N-/C-terminus can be fully available for further conjugation of various amino acids, peptides, and proteins. Such diversely-oriented molecular design is suitable for the programmable construction of highly-ordered hierarchical structure and installation of various functional groups. For this reason, we choose the side chain-metalated type as the platform of our research aiming to produce metalated-amino acids/peptides-based highlyfunctional materials. 1. N-terminus type

2. C-terminus type

R1 N H

M

R1 2

M

3. Side chain type

3

R

R

O

N H

M O

3

R

N H

R2 O

n

M = metal complex R = functional groups or peptide residues

FIGURE 4.1 Classification of metalated amino acids.

4.3 SYNTHESIS OF α-SIDE CHAIN-METALATED AMINO ACIDS AND PEPTIDES Synthesis of side chain-metalated amino-acids and peptides is commonly carried out by following two routes, as shown in Fig. 4.2. The first route is the post-complexation or post-metalation route: Metalation is performed by the reaction of metal salt with amino acid/peptide-bearing coordination site conjugated into the α-side chain. The other is the metal complex conjugation route where the metal complex is conjugated by appropriate coupling chemistry, such as through a cross-coupling reaction and condensation reaction.

Metalated Amino Acids and Peptides Chapter | 4 1. Post complexation

2. Metal complex conjugation M

R

2

R

N H

metal salt

O

1

R

2

R

N H

M

X

M 1

77

O

= coordination site

R1

N H

O

M

R2 metalYcomplex R1 N H

R2 O

X, Y = functional groups for conjugation reaction

Rn = functional groups or peptide residue

FIGURE 4.2 Synthesis of side chain-metalated amino acids and peptides. R

Py

Py Py M N

N

N R

M

R

MLn

N

N

N H

O

N

N

Ln N

N

M N

N

Y R1

M Py

O

M = Mg, Zn: N. Solladie M = Zn: T. Yamamura

R1

N H

Y

Y

Y R2

R2 O

M = Zn: B. Imperiali M = Ru: B. M. Peek

R1

N H

R2 O

M = Zn, Ni: I. Hamachi

R1

N H

R2 O

M = Ru, Pt, Rh: K. Tashiro, O. M. Yaghi

FIGURE 4.3 Selected examples of metalated amino acids and peptides synthesized by post complexation route.

The post-complexation route has the merit of late-stage metalation and one can readily introduce a variety of metals into the desired amino acids and peptides through simple and easy synthetic operation. Therefore, most sidechain metalated amino acids/peptides have been synthesized by this route and widely applied to bioanalytical chemistry.1a,2,4 N-Heterocyclic scaffolds such as pyridines8 and porphyrins9 are commonly used for the coordination site (Fig. 4.3) while their rich coordination chemistry provides significant contributions to expand synthetic diversity of metalated amino acids/peptides. In comparison with the post-complexation route, the second metal complex conjugation route is generally troublesome and time-consuming because of the necessity to carefully consider choosing the molecular design of metal complexes to avoid undesirable side reactions, such as metal- and ligandleaching during the coupling reaction. However, if one can find substantially robust metal complexes under coupling conditions, the merit of higher metal unit convertibility enables us to establish a programmable synthesis of sequence-controlled metal arrays, as described in the next section.

4.4 SYNTHESIS OF α-SIDE CHAIN-METALATED PEPTIDES Synthesis routes of α-side chain-metalated peptides are summarized in Fig. 4.4. The post-metalation route is the rewriting of the routes depicted in

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

1. Post metallation (metal complex conjugation or post complexation)

R1

X

O

H N

or 2

N H X

R

2. Conjugation of metalated amino acids M2

M

Y metal complex or metal salt

R1

Rn = functional groups or peptide residue

M1

O

H N

N H

O

X, Y = functional groups for metallation

M2

M

M

M

R1

R2 R1 O

N H

R2 O

N H

R2 O

R1

O

H N

condensation

N H M1

R2 O

Rn = functional groups or peptide residue

FIGURE 4.4 Synthesis of α-side chain-metalated peptides.

Fig. 4.2, where the metalation of peptides is simply performed in one-step by conjugation of the metal complex to the α-side chain by appropriate coupling reactions or by incorporation of metal salt into the coordination site conjugated to the α-side chains prior to metalation.4 The second route of conjugation of metalated amino acids is carried out by the condensation of metalated amino acids or peptides to afford the corresponding multi-metalated peptides. This route requires multi-step synthesis including N-/C-terminus protection/deprotection processes to obtain desired peptides while being aware of undesirable metal- or ligand-detaching; however, there is a crucial advantage that metal sequence can be absolutely defined as the condensation sequence of metalated amino acids. For this reason, we chose this route to synthesize multi-metalated peptides to construct dimensionally- or hierarchicallycontrolled metal arrays, as described in the following section.

4.5 PD- AND PT-COMPLEX-CONJUGATED GLUTAMIC ACID We designed a glutamic acid-based metalated amino acid as a platform of sequence-controlled metal array fabrication (Fig. 4.5). The significantly robust N-benzylidenalkylamine complexes of palladium and platinum

FIGURE 4.5 Pd- and Pt-conjugated glutamic acid (R1-L-[MCl(BzAA)]Glu-R2) and X-ray structures of the corresponding benzilydenealkylamine complexes (M 5 Pd and Pt, n 5 2).

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79

(Pd(BzAA)) enabled us to perform conventional solution-phase peptide synthesis involving N-/C-terminus protection/deprotection processes without metal detaching, affording the desired multi-metalated glutamic acids peptides. By using these metalated-peptides, dimensionally-controlled metallosupramolecular architecture was successfully fabricated through an ultrasound-induced, self-assembly process.6,10 12

4.6 SYNTHESIS OF PD- AND PT-COMPLEX-CONJUGATED GLUTAMIC ACIDS/PEPTIDES The palladium and platinum-conjugated glutamic acid R1-L-[MCl(BzAA)] Glu-R2 can be synthesized in three steps including syntheses of BZAA ligands from benzaldehyde and its complexes, as shown in Scheme 4.1.6,10,12 O

OH PPh 3 M

R1 NH2 HO H

O

toluene, rt

PPh 3

MCln, PPh 3

n N HO

n

BzAOHA

MeOH, reflux

M Cl

N HO

n

MCl(BzAOHA)]

N H

N

OR2 O

O R1-L-Glu-R2 EDCI, DMAP, CH2Cl2 , rt

R1

N H

O

Cl n

OR2

O R1-L-[MCl(BzAA)]Glu-R2

SCHEME 4.1 Synthesis of Pd- and Pt-conjugated glutamic acid (R1-L-[MCl(BzAA)]Glu-R2).

The simple and practical solution-phase reactions allowed large-scale preparation of 10 g per batch using a 3 L flask. The resulting metalated glutamic acids show sufficient stability under conventional conditions of peptide synthesis for acidic N-/C-deprotection with elevated temperature (ca. 100  C). Any decompositions including metal leaching could not be clearly observed under these conditions by 1H NMR monitoring. We examined the condensation of metalated-amino acids by the reaction of in situ prepared C-terminus free Pd-conjugated glutamic acids Fmoc-L[PdCl(BzAA)]Glu-OAllyl 113 and N-terminus free Pd-conjugated glutamic acid H-L-[PdCl(BzAA)]Glu-NHBu 2 using EDCI as a condensation reagent. As a result, the corresponding Pd-conjugated glutamic acid dipeptides FmocL-{[PdCl(BzAA)]Glu}2-NHBu 3 6 were formed in 50% 62% yields as shown in Scheme 4.2.6,10 Although the product yields stayed moderate due to the steric hindrance of bulky Pd-complex units, no metal leaching byproducts were observed in the reaction mixture. The resulting selective formation of dipeptides 3 6 with recovery of unreacted C/N-terminus free Pd-glutamic acids indicates that this motif and Pd moiety is stable and welltolerated under the peptide condensation conditions.

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

PPh 3 Pd PPh 3 Cl

N O

Fmoc

Pd(PPh 3)4 (10 mol%) dimedone (1.2 eq) EDCI (2.0 eq.)

n

O

OAllyl

N H

O

H2N 2

CH2Cl2 rt, 30 min

O 1

Pd

n

O

H N

PPh3

Cl

N

Pd

Cl

N n

O

O Bu

O

Fmoc

rt, 12 h

O

H N

N H

Bu

N H

O O

O

n N

3: 4: 5: 6:

n= n= n= n=

2 (50%) 3 (52%) 4 (47%) 5 (62%)

Cl Pd PPh3

SCHEME 4.2 Synthesis of Pd-conjugated glutamic acid dipeptides Fmoc-{L-[PdCl(BzAA)] Glu}2-NH-Bu.

Programmable fabrication of the heterometallic array was successfully accomplished through peptide synthesis using Pd- and Pt-conjugated glutamic acids as metal units.6 We can synthesize two possible sequence isomers of Pd-Pt and Pt-Pd heterometallic dipeptides 9 and 10 selectively, as well as Pd-Pd and Pt-Pt homometallic dipeptides 3 and 11, by the condensation of Fmoc-L-[MCl(BzAA)]Glu-H (M 5 Pd or Pt) prepared from Fmoc-L-[MCl (BzAA)]Glu-OAllyl (1: M 5 Pd, 7: M 5 Pt) and H-L-[MCl(BzAA)]GluNHBu (2: M 5 Pd or 8: M 5 Pt) as shown in Scheme 4.3. Notably, this is the first example of comprehensive metal sequence fabrication using metalated amino acid as a metal unit. Construction of metallo-supramolecular frameworks based on the ultrasound-induced self-assembly of metalated dipeptides 3 and 9 11 will be discussed later. PPh 3

PPh 3

M2 Cl

N PPh 3

O

M1 Cl

N O

Fmoc

N H

O

2

OAllyl

O 1: M = Pd or 7: M1 = Pt 1

Pd(PPh 3)4 (10 mol%) dimedone (1.2 eq) EDCI (2.0 eq.) CH2Cl2 rt, 30 min

H2 N

O

H N

M1

2

Bu

O 2: M2 = Pd or 8: M2 = Pt

O

Fmoc

Cl

N

N H

2

O

O

H N

N H

O O

rt, 12 h 9: M1 = Pd, M2 = Pt (55%) 10: M1 = Pt, M2 = Pd (54%) 11: M1 = Pt, M2 = Pt (44%)

Bu

O

2 N

Cl M2 PPh 3

cf. 3: M1 = Pd, M2 = Pd (50%)

SCHEME 4.3 Synthesis of controlled metal arrays using Pd- and Pt-conjugated glutamic acids.

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81

The advantage of the molecular design of our Pd- and Pt-glutamic acids was demonstrated in the elongation of the peptide chain to synthesize triand tetra-metalated peptides. Generally, solubility of metalated peptides in organic solvent drastically decreases with an increasing number of amino acid residues. However, our Pd- and Pt-conjugated amino acids and peptides show excellent solubility in various organic solvents such as benzene, chlorobenzene, toluene, CH2Cl2, CHCl3, EtOAc, THF, and Et2O by the effect of the highly soluble PPh3 ligand. Therefore, solution-phase synthesis of tripeptide Fmoc-L-{[MCl(BzAA)]Glu}3-NHBu (12: M 5 Pd, 13: M 5 Pt) and tetrapeptide Fmoc-L-{[MCl(BzAA)]Glu}4-NHBu (14: M 5 Pd, 15: M 5 Pt) can be performed by the same procedure of dipeptide synthesis with similar product yields (Fig. 4.6).10,11

PPh 3

PPh 3 N O

Fmoc

N H

O

N

Cl 2

O

O O

H N

H N

N H

O O

O

N O

Fmoc

N H

O

O

PPh 3

O

O

H N O

O N H

O

O

O

2 Cl

N 14: M = Pd (51% from 12) 15: M = Pt (41% from 13)

Cl 2

H N

N H

Cl M

N

Cl 2

O

2 N

12: M = Pd (51% from 3) 13: M = Pt (41% from 11 )

Cl

O

M

M

2

Bu

PPh 3

PPh 3

M

M

Bu

O

2 N

Cl M

M PPh 3

PPh 3

FIGURE 4.6 Tri- and tetrametalated-peptides derived from Pd- and Pt-conjugated glutamic acids.

4.7 ULTRASOUND-INDUCED SELF-ASSEMBLY OF PD- AND PT-COMPLEX-CONJUGATED GLUTAMIC PEPTIDES Peptide self-assembly has been recognized as a key platform to construct precisely structure-controlled materials gained intense interest from interdisciplinary research areas.14 In the self-assembling process of peptides, programmed formation of amide-amide hydrogen bonds (C 5 O


H N) at the desired point, direction, and timing is an essential factor to obtain dimensionally- and hierarchically-controlled architectures. During the course of research on metalated amino acids/peptide-based supramolecular gel formation, we found an interesting ultrasound-induced hydrogen-bond switching mechanism as shown in Scheme 4.4.6,10 In the metalated glutamic acids/peptides dissolved into aprotic organic solvents, a self-locked structure 16 is predominantly formed by the intramolecular hydrogen bonding of

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

Cl

hydrogen bond M

Cl

M

Cl intramolecular hydrogen bonding H

ultrasound

N

H

self-assebly

N O 16

self-locked structure

M

17

O

N H

O

O

H

intermolecular hydrogen bonding

N

unlocked structure 18 M

Cl

inter-locked structure

SCHEME 4.4 Ultrasound-induced switching of intra- and intermolecular hydrogen bonds of metalated amino acids/peptides.

M Cl


H N between the chloride ligand and amide N-H.15 Irradiation of ultrasound to the solution can break the intramolecular hydrogen bond to give hydrogen a bond-free unlocked structure 17 as a short-lived unstable intermediate, followed by self-assembly of amino acids/peptides which proceeds along with the formation of an inter-locked structure 18 through intermolecular amide-amide hydrogen bonds to give the corresponding supramolecular assembly possessing a β-sheet-type structural topology.16,17 Representative results of the ultrasound-induced self-assembly of Pd- and Pt-conjugated glutamic dipeptide are illustrated in Fig. 4.7. Upon 60 s irradiation of ultrasound (0.45 W/cm2, 40 kHz) to EtOAc solutions of dipeptides 3 and 9 11, the peptides undergo self-assembly with the spontaneous formation of opaque gels. The observed reversible sol-gel transformation under a sonication-heating cycle exhibits that the gelation is driven by non-covalent interaction of hydrogen-bonds. This gelation specifically occurs under sonication and usual heat cool procedures gives an amorphous precipitate without the formation of gel.18 SEM images of xerogel of 3 show an entangled fiber structure being typical of supramolecular peptide assemblies and the lamellar pattern with 3 nm pitch found on the surface of belt-like fibers coincides with the formation of β-sheet type aggregate. Interestingly, the minimum gelation concentration increased with the increase of Pt-conjugated glutamic acids, such as Pd-Pd , Pd-Pt  Pt-Pd , Pt-Pt. This phenomenon can be explained by the hydrogen bond strength of M Cl


H N where the hydrogen bond of Pd Cl


H N is more stable than Pt Cl


H N because of the larger electric dipole moment of Pd-Cl compared with PtCl6,15 and higher stabilization of inter-β-sheet interaction would contribute to reducing the minimum gelation concentrations of Pd-conjugated dipeptides. To determine the precise self-assembled structures of Pd- and Ptconjugated glutamic acid dipeptides, we carried out X-ray scattering analysis (WAX and SAXS) and IR analysis of the obtained gels. Structural

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83

FIGURE 4.7 Ultrasound-induced supramolecular gel formation of Pd- and Pt-bound glutamic acid dipeptides 3 and the minimum gelation concentration of dipeptides 3 and 9 11 in EtOAc with a SEM image of supramolecular xerogel of 3.

characterization revealed that the structure of dipeptide supramolecular assembly is influenced by the solvent used for the sonication gelation. The supramolecular gels of Pd-Pt dipeptide 9 and Pt-Pd dipeptide 10 prepared from a chlorobenzene solution consist of parallel β-sheet assemblies with the formation of intermolecular π π stacking of N-terminus fluorenyl moieties, as illustrated in Fig. 4.8. In the parallel β-sheet aggregates, the Pd- and Pt complex moieties are aligned into the same side and the same orientations along the N/C-direction of the peptide backbone to afford a dimensionally controlled heterometal array through the supramolecular gel formation, as we expected for this molecular design (Fig. 4.9).

FIGURE 4.8 Parallel β-sheet type self-assembled structures of Pd- and Pt-conjugated glutamic acid dipeptides.

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

FIGURE 4.9 Controlled heterometal assembling in parallel β-sheet type self-assemblies.

To gain insight into ultrasound-induced supramolecular gelation, we conducted a measure of the kinetics of gelation to determine the rate constant under varied sonication times and frequencies. A kinetic experiment using 7.0 mM EtOAc solution of Pd-Pd dipeptide 3 revealed that the gelation rate of dipeptide obeys a first-order dependence on the sonication time of over 80 s irradiation, but an induction period was observed for short-time irradiation of less than 60 s without the formation of gel. This result suggested that the ultrasound-induced gelation process involves a sonication-induced initiation step followed by a spontaneous propagation process. The gelation rate also depends on the frequency of ultrasound: The rate exponentially increases with decreasing frequencies and the curve profile showed agreement with the relationship of the frequency and energy of micro-cavitation which was formed by ultrasound irradiation.19 These results strongly indicate that microcavitation is a key factor to promote the self-assembly of metalated peptides. Considering the results, we proposed a mechanism of ultrasound-induced gelation as depicted in Fig. 4.10. The intramolecular hydrogen bond of selflocked metalated dipeptides is cleaved under extremely high temperature and

FIGURE 4.10 Proposed mechanism of ultrasound-induced gelation of Pd- and Pt-bound glutamic acid dipeptides.

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85

pressure conditions arisen from ultrasound micro-cavitation to give the corresponding unlocked metastable intermediate. Supersaturation at the interfacial surface of the cavitation bubble induces an intermolecular association of the unlocked intermediate to give metastable, tiny oligomers. Notably, such extremely high energy and high concentration conditions cannot be provided by thermal processes. After collapsing the micro-cavitation, the resulting well-grown insoluble oligomers act as nuclei for the spontaneous propagation of self-assembling peptides to provide β-sheet supramolecular aggregates. Such supramolecular polymerization have gained an upsurge of interest as a living supramolecular polymerization and variety of monomers bearing both π-stacking and hydrogen bonding parts as key factors have been developed.20

4.8 SYNTHESIS OF NCN-PINCER PD- AND RU-COMPLEXCONJUGATED NORVALINES In this decade, transition-metal complexes on a pincer scaffold have emerged with an upsurge of interest as a privileged class of organometallic compounds, owing to their extremely robust nature, high catalytic activity, excellent photo-/electric properties, and their extensive versatility in interdisciplinary research areas.21 We consider that these properties of pincer complexes will open up new horizons for bioorganometallic chemistry especially for the platform of advanced catalysts. In contrast to supramolecular gel preparation, catalytic applications of metalated amino acids and peptides are required to be more stable and robust under more severe and varied conditions in the presence of highly reactive reagents, bases, oxidizing agents, and so on. We have developed a variety of pincer complex-conjugated norvalines in which stable and catalytically active NCN-pincer palladium- and ONO-pincer ruthenium-complexes are covalently conjugated into α-side chain of norvaline derivatives, as depicted in Fig. 4.11.7,22,23 The molecular design strategy based on pincer scaffold conjugated through robust carbon carbon bond linkage enabled us to perform various catalytic reactions, such as nucleophilic cycloaddition, cross-coupling, and oxidation reactions.

FIGURE 4.11 NCN-pincer and ONO-pincer complex-conjugated norvalines.

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

NCN-pincer palladium complex-conjugated norvalines can be synthesized by using Suzuki-Miyaura cross-coupling reactions originally developed by Taylor24 and von Koten25 for functionalization of amino acids side chains as shown in Scheme 4.5.7,26 The cross-coupling reaction was performed by the reaction of 9-BBN adduct L/D-19 in situ prepared from commercially available protected allylglycines Boc-L/D -Allygly-OMe with NCN-pincer palladium complex PdCl(dpb-Br). After treatment of the resulting reaction mixture with KCl for conversion of Pd-Br to Pd-Cl of the conjugated pincer complex gave the desired Pd-conjugated norvalines L/D-20 through a one-pot, three-step process.

N Cl 1) PdCl(dpb-Br) (1.0 eq) Pd(OAc)2 / S-Phos (3 mol%) K3PO4 (2.4 eq) THF-H2O-DMF, rt, 18 h

B 9-BBN (2.2 eq) Boc

OMe

N ∗ H

O Boc-L/D-Allylgly-OMe

THF, 0 oC, 5 min then rt, 1 h

Boc

Pd

OMe

N ∗ H

N

2) excess KCl, rt, 2 h

Boc

O L/D-19

N ∗ H

OMe

O L-20 (72%) D-20 (54%)

SCHEME 4.5 Synthesis of NCN-pincer Pd-complex-bound norvalines.

Large-scale synthesis of L/D-20 in a 5 g per batch using a 3 L flask can be performed without loss of chiro-optical purity which was unequivocally confirmed by chiral HPLC and single X-ray structure analysis. Notably, only a few examples have been reported on the determination of the absolute configuration of metalated amino acids, including our works,7,23,24,26,27 due to its poor crystallinity caused by inherent fluxionality of the molecules. As we expected, the NCN-pincer Pd-conjugated norvaline 20 showed sufficient stability under acidic and basic conditions for C- and N-terminus conversions (Schemes 4.6 and 4.7). Deprotections of C-OMe and N-Boc were efficiently carried out by treatment of 20 with LiOH or HCl to provide the corresponding C/N-free Pd-conjugated norvalines 21 and 23 almost quantitatively without leaching palladium. The resulting C/N-free intermediates 21 and 23 underwent condensation with various amines and carboxylic acids

N

N

N

Cl

Cl

Cl

Pd

Pd LiOH H2O (2.0 eq)

N

Boc

N H

OMe O L-20

THF-H2 O, rt, 1 h

DMT-MM Cl (2.0 eq) NEt3 (1.1 eq) CH2Cl2 , rt, 18 h Boc

Pd

H2N C11 H23 (1.1 eq)

N

N H

OH O

L-21

SCHEME 4.6 C-terminus conversion of Pd-conjugated norvaline.

N

Boc

N H

O L-22: 88%

H N

C11 H23

87

Metalated Amino Acids and Peptides Chapter | 4

N

N

Cl

N

Cl

Cl

Pd

Pd 4N HCl (150 eq)

N

Boc

DMT-MM BF 4 (2.0 eq) NEt3 (1.1 eq)

N O

DMF, rt, 18 h

dioxane, OMe rt, 5 h

N ∗ H

Pd

R CO2H (1.1 eq)

N

OMe

H2 N ∗

O L/D-20

N ∗ H

R

O

OMe

O L-24: R = C11 H23 (88%) D-25: R = C 11 H 23 (88%) L-26: R = CH3 (75%) L-27: R = 2,4-NO2 -C6H3 (44%)

L/D-23

SCHEME 4.7 N-terminus conversion of Pd-conjugated norvaline.

using a DMT-MM condensing agent28 to provide C/N-functionalized metalated norvalines 22 and 25 28 in good yields without the formation of metal leaching by-products and loss of optical purity. Notably, these C/N-conversions can be performed in a one-batch, one-pot process without isolation and purification of 21 and 23. The DMT-MM-mediated condensation proved highly effective for the coupling of extremely sterically bulky metalated norvalines to afford the corresponding metalated norvaline peptides, as shown in Scheme 4.8. The coupling of N/C-free Pd- or Pt-conjugated norvalines, which can be used as synthesized without purification after the deprotection process, proceeded efficiently by DMT-MM-mediated condensation to provide the corresponding Pd-/Pt-conjugated norvaline dipeptides 31 34. No metal leaching or scrambling was observed with recovery of starting N-/C-free metalated norvalines. Up to tetrato hexa-metalated peptides were successfully synthesized both for NCN-pincer

N

N Cl

Cl

1

M

M1

N Cl

N

N

LiOH H2O (2.0 eq)

M1

N Boc

N H

OMe THF-H2O, rt, 1 h

O M = Pd: 20, Pt: 28 1

Boc

O

M = Pd: 21, Pt: 29

M

4N HCl (150 eq)

N

N H

O

Boc

H N

N H

O OMe

O

N M2

M2

Cl N

N

dioxane, OMe rt, 5 h

M2 = Pd: 20, Pt: 28

DMT-MM Cl (2.0 eq) NEt(i-Pr)2 (11.7 eq) THF-i-PrOH, 80 °C, 9-32 h

N Cl

2

Boc

OH

1

N Cl

N H

H2 N

OMe

O M2 = Pd: 23, Pt: 30

31: M1 = Pd, M2 = Pd (57%) 32: M1 = Pd, M2 = Pt (56%) 33: M1 = Pt, M2 = Pd (68%) 34: M1 = Pt, M2 = Pt (73%)

SCHEME 4.8 Controlled metal array fabrication based on Pd- and Pt-conjugated norvalines.

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PART | I Synthesis, Structure and Reactivity of Bioorganometallic Compounds

Pd/Pt-conjugated norvalines and ONO-pincer Ru-conjugated norvalines on a gram scale without loss of optical purity.7,29

4.9 SELF-ASSEMBLY OF NCN-PINCER PD- AND ONO-PINCER RU-COMPLEX-CONJUGATED NORVALINES AND APPLICATIONS TO SUPRAMOLECULAR CATALYSTS Supramolecular self-assembly has been established as a powerful technique for the fabrication of well-defined nano-/micro-scale molecular architectures. The embedded side-chain functional groups in the resulting hierarchically structured porous supramolecular architecture can provide an efficient catalytic site as well as an enzyme-active site to promote chemo- and stereoselective organic reactions.30 We envision that self-assemblies of metalated amino acids and peptides act as a supramolecular catalyst possessing metalaccumulated nano-size pores for substrate binding and catalytic sites.6,10 Supramolecular gel of NCN-pincer Pd-conjugated norvaline L-22 was prepared through the conventional heat cool process from the toluene solution of process L-22. Microscopic observation of the resulting gel by means of cryo-TEM demonstrated the formation of a typical fibrous network structure with well-regulated nanoscale patterns, as illustrated in Fig. 4.12. The

FIGURE 4.12 Supramolecular gel of L-22 and cryo-TEM images.

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FIGURE 4.13 Supramolecular structure of L-22 in gel state.

precise supramolecular structure was successfully solved through a computer-aided molecular modeling study using all the data obtained from X-ray scattering analysis (WAX, SAXS), IR, and UV-vis spectroscopy, as shown in Fig. 4.13.7,26 The observed 3.8 nm-spaced periodic pattern on the fiber surface is wellagreed with the alternating arrangement of Pd-containing layers and alkyl chain layers. Inside the supramolecular assembly, formation antiparallel β-sheet type association through intermolecular hydrogen-bonding is clearly confirmed with accumulation of PdCl(dpb) moieties stacking up along with the hydrogen-bonding axis (Fig. 4.13). Firstly, we conducted a catalytic application of the supramolecular selfassembly of a NCN-pincer Pd-complex conjugated norvaline L-22 as a heterogeneous catalyst in aqueous reaction conditions, where hydrophobic organic substrate is readily adsorbed and condensed into supramolecular gel to accelerate the reaction. In fact, the cyclization of 4-alkynoic acid proceeded efficiently in water to afford the corresponding ene-lactone in the presence of a xerogel particle prepared from toluene gel of L-22 (Fig. 4.14).30 Notably, higher catalytic efficiency of gel state catalyst was clearly demonstrated by lower yields of the product under homogeneous condition in CH2Cl2. When substrate 4-alkynoic acid added to the water in the presence of a xerogel particle, the lemon yellow xerogel promptly swelled through the absorption of the substrate to afford an orange gel. This result adequately corroborates the proposed absorption/condensation effects of supramolecular gel catalyst. Supramolecular catalysis was also investigated in ONO-pincer Ru-conjugated norvaline-catalyzed oxidation.23,31 The inherent oxidation resistance of amino acids and peptides often observed in enzymes and proteins is suitable as a scaffold catalyst under oxidation conditions using hydrogen peroxide. We designed and synthesized Ru-conjugated norvaline L-3523,27 In this amino acid, the α-side chain is covalently bound to ONO-pincer ruthenium complex of Ru(pydc)(terpy) which shows excellent catalytic activity toward oxidation of various organic compounds.32 In the presence of very

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FIGURE 4.14 Supramolecular gel-catalyzed cycloaddition of alkynoinc acid. Pictures are before/after substrate addition to water in the presence of a xerogel particle of L-22.

small amounts of L-35 (0.01 mol%), oxidation of alcohols provides the corresponding aldehydes and ketones highly efficiently using hydrogen peroxide as a terminal oxidant. As shown in Fig. 4.15, the higher catalytic activity of Ru-conjugated norvaline L-35 compared with the parent Ru(pydc)(terpy) complex was found out through the higher yield of cyclohexanone. To our best knowledge, this is the first example of enhanced catalytic activity among Ru-conjugated amino acids ever reported. Gilbertson, Robinson, and Albrecht independently developed NHC Ru complex-conjugated amino acids for catalytic olefin metathesis and transfer hydrogenation reactions; however, no enhancement of catalytic activities was found to be comparable or less than the parent ruthenium complexes.33 In situ SAXS measurement showed formations of micellar nanoparticles in both the aqueous and organic phases of the reaction mixture. We consider that these micelle act as a phase-transfer catalyst to increase the catalytic efficiency. Actually, the addition of surfactant such as sodium dodecyl sulfate to the Ru(pydc)(terpy)-catalyzed oxidation of cyclohexane is effective to improve the yield of cyclohexanone drastically. IPC analysis also supports the proposed phasetransfer ability of L-35; the content of ruthenium in L-35-catalyed oxidation mixture were almost equal, despite a small amount of ruthenium detected in organic phase of Ru(pydc)(terpy)-catalyzed oxidation mixture.

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FIGURE 4.15 ONO-pincer Ru-conjugated norvaline-catalyzed oxidation of alcohol. The pictures and images show the biphasic reaction mixtures of L-35- and Ru(pydc)(terpy)-catalyzed oxidation of cyclohexanol and the Ru contents measured by ICP, and the schematic illustrations of micellar particles found in the reaction mixture by SAXS measurement.

The role of norvaline moiety of L-35-catalyzed oxidation investigated on the catalytic oxidation of 1,3-dimethoxybenzene to 2-methoxybenzophenone as depicted in Fig. 4.16. A series of ruthenium complexes 36 and 37 bearing norvaline analogous parts was prepared to compare the catalytic activity with Ru-conjugated norvaline L-35 and the parent Ru(pydc)(terpy). Among

FIGURE 4.16 Structure-catalytic activity relationship for a series of analogous ruthenium catalyst based on Ru-conjugated norvaline L-35.

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all the catalysts examined, L-35 showed the highest catalytic activity toward the oxidation of methoxybenzene. Interestingly, the initial reaction revealed a second-order dependence on the concentration of ruthenium catalyst in L-35-catalyzed oxidation despite the first-order dependence in Ru(pydc)(terpy)-catalyzed oxidation. These results strongly support the hypothesis that the norvaline moiety is an essential part for enhanced catalysis. This unique catalytic property is induced by the formation of micellar supramolecular selfassembly acting as a phase-transfer catalyst in this biphasic oxidation system.

4.10 CONCLUSION We synthesized a series of α-side chain-conjugated metalated amino acids based on glutamic acid and norvaline in which stable and robust half-pincerand pincer-complexes of palladium, platinum, and ruthenium were covalently conjugated through appropriate coupling reactions. The robust nature of these metalated amino acids enabled us to synthesize multi-metalated oligopeptides by the condensation of metalated amino acids based on conventional peptides chemistry involving deprotection processes under acidic, basic, or high-temperature conditions. Notably, the molecular design of these metalated amino acids and peptides allowed various N-/C-functionalization without any detectable metal leaching and loss of optical purity. The scalability of the synthesis of metalated amino acids/peptides in up to 10 g makes it possible to supply adequate quantities of metalated amino acids and peptides, being important material and catalyst research for screening a huge number of reaction parameters. Because of these advantages of our metalated amino acids and peptides, we succeeded to achieve sequence-controlled metal array fabrication and dimensionally-controlled supramolecular self-assembly possessing multi-lamellar β-sheet architectures. Applications of these amino acids and peptides self-assemblies to a supramolecular catalyst were also accomplished together with a substantially large enhancement of catalytic activity compared to the parent complexes. The results clearly demonstrate that the emergent effect on metalated amino acids and peptides can be achieved by the conjugation of biomolecules and organometallic complexes. We believe that α-side chain-metalated amino acids and peptides will open up new horizons for the further application of bioorganometallic compounds in multidisciplinary areas and contribute to the design of innovative bio-inspired organometallic catalysts, such as peptide-based artificial metalloenzymes.

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FURTHER READING Xu et al., 2005 2011 2015(a) Xu G, Gilbertson SR. Org. Lett. 2005;7:4605. (b) Monney A, Venkatachalam G, Albrecht M. Dalton Trans. 2011;40:2716. (c) Gleeson EC, Wang ZJ, Jackson WR, Robinson AJ. J. Org. Chem. 2015;80:7205.