A mild and selective Pd-mediated methodology for the synthesis of highly fluorescent 2-arylated tryptophans and tryptophan-containing peptides: A catalytic role for Pd0 nanoparticles?

Article abstract of DOI:10.1039/c3cc48481e

A Pd-mediated direct C-H bond functionalisation of tryptophan has been developed, both as a single amino acid residue and within peptides. Important mechanistic insight into this process has been gained by characterising a Pd catalytically competent nanoparticle phase which evolves during the early stages of reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc48481e

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
A mild and selective Pd-mediated methodology
for the synthesis of highly fluorescent 2-arylated
tryptophans and tryptophan-containing peptides:
a catalytic role for Pd0 nanoparticles?†
Cite this: Chem. Commun., 2014,
50, 3052
Received 6th November 2013,
Accepted 25th January 2014
Thomas J. Williams, Alan J. Reay, Adrian C. Whitwood and Ian J. S. Fairlamb*
DOI: 10.1039/c3cc48481e
www.rsc.org/chemcomm
A Pd-mediated direct C–H bond functionalisation of tryptophan
has been developed, both as a single amino acid residue and within
peptides. Important mechanistic insight into this process has been
gained by characterising a Pd catalytically competent nanoparticle
phase which evolves during the early stages of reaction.
The functionalisation and modification of complex biomolecules
by the formation of C–C bonds has benefited significantly from the
development of mild conditions for Pd-mediated cross-coupling
reactions.1 Pioneering work by Davis and co-workers has, for
Fig. 1 Direct arylation of tryptophan under mild conditions.
Sanford’s reaction conditions were initially applied to
example, demonstrated that it is possible to perform Suzuki– N-acetyl, O-methyl protected-tryptophan 1 to afford, after column
Miyaura reactions on proteins containing genetically incorporated chromatography, 2-phenyltryptophan 2 in 56% yield (Fig. 1).
aryl iodides.2 Metal-mediated direct functionalisation of C–H Changes to the catalytic conditions included modifying the
bonds in complex molecule synthesis is a rapidly expanding field, Pd catalyst and loading, solvent, inert atmosphere and reaction
including in the total synthesis of natural compounds.3 We have temperature, but no increase in yield was recorded. Optimal
reported the mild and selective direct C–H functionalisation of highly turnover numbers and yields were seen between 2.5–5 mol% Pd.
sensitive purine nucleosides, e.g. adenosine and 20-deoxyadenosine.4
Intriguingly, reaction of 1 with PhB(OH)2 and PhI(OAc)2 led to
As part of these studies we have been able to gain valuable mecha- the rapid formation of Pd0 nanoparticles (PdNPs) during the first
nistic insight, most notably characterizing the formation of Pd/Cu- few minutes of the reaction. This finding is in keeping with
containing nanoparticles during substrate turnover. In this study, another Pd-mediated C–H bond functionalisation of benzoxazoles
we have turned our attention to the mild5 C–H bond function- using PhI(OAc)2, which we recently reported upon.11 The in situ
alisation of amino acids,6 specifically tryptophan, a hydrophobic generated PdNPs from reaction 1 - 2a were encapsulated by
indole-containing amino acid which alters the structure and addition of exogenous polymer stabilizer (i.e. polyvinylpyrrol-
function of proteins, and is a fluorescent marker.7
idinone, PVP), which was added to an aliquot of the reaction
The C–H bond functionalisation of indoles mediated by Pd is mixture after 1 h (2.5 mL, containing B4.8 mmol Pd; 10 equiv.
well established.8 We recognised that Sanford’s methodology8d PVP added and AcOH removed at 40 1C and ca. 0.750 mmHg).
could be applied to selectively functionalise tryptophan, which This allows reliable analysis of the size and distribution of the
employs PhB(OH)2 and PhI(OAc)2 as reagents, forming diaryl- PdNPs without concern that solvent removal leads to metal
iodonium salts in situ,9 and catalytic Pd(OAc)2. These reactions aggregation. Transmission electron microscopy (TEM) confirmed
could proceed via a PdII/IV catalytic manifold.10 In this paper we the presence of the encapsulated PdNPs (n = 100; average PdNP
detail the selective arylation of tryptophan and tryptophan- size is 2.52 nm, represented in Fig. 2).12
containing peptides under mild catalytic conditions.
Pre-synthesised PVP-PdNPs13 (average size B1.8 nm, Pd0,
5 mol%) were also found to be a viable catalytic species for the
tryptophan arylation, affording 2a in 57% yield (Fig. 3). This
result shows that PdNPs are catalytically competent under the
reaction conditions. In the example given in Fig. 1, we propose
that the PdNPs are acting as a reservoir for Pd0, akin to related
Heck arylation chemistry.14 Glorius and co-workers have nicely
Department of Chemistry, University of York, York, UK YO10 5DD.
E-mail: ian.fairlamb@york.ac.uk
† Electronic supplementary information (ESI) available: Experimental details and
crystallographic data. CCDC 968490 and 968491. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc48481e
3052 | Chem. Commun., 2014, 50, 3052--3054
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Fig. 2 (a) Histogram of particle size (diameter, nm) for a sample of PdNPs
(encapsulated by PVP); (b) TEM image of PVP-encapsulated PdNPs after
1 h heating; (c) appearance of PdNPs in reaction.
Fig. 5 Arylation of tryptophan using Cu(OAc)2 as a co-catalyst.
Following this study, some limitations were revealed in terms
of the substrate scope using Pd(OAc)2 as the precatalyst.‡ For
example, a simple switch from PhB(OH)2 to 4-MeC6H4B(OH)2 led
to the biaryl compound derived from the latter only {PhI(OAc)2
could be aiding oxidative homocoupling of the aryl boronic
acid}.16 Simply switching the oxidant to Cu(OAc)2 afforded 2a
in 93% yield (under the conditions described in Fig. 5). Here,
Cu(OAc)2 is assisting the reoxidation of Pd0 (with O2 from air).
This was confirmed by conducting the same reaction under an
Ar atmosphere, which showed B11% conversion to 2a, i.e. a
single turn-over of the CuII co-catalyst. A series of analogues
(2b–2d) were formed in good yields using this procedure. The
electron-rich p-anisoyl boronic acid was susceptible to oxidative
homocoupling, leading to 2e being formed in modest yield,
expected under oxidative conditions.
Fig. 3 Pre-synthesised PVP-PdNPs are catalytically competent species in
the arylation of 1 to afford 2a.
The specific optical rotations of the products, and analysis
by chiral HPLC, indicated that the arylated tryptophan products
maintained their stereochemical purity (see ESI†).
Scheme 1 PVP-encapsulation of in situ generated PdNPs from the reac-
tion detailed in Fig. 1 (TEM characterisation in Fig. 2).
Single crystal X-ray diffraction structures for analogues 2c and
2d (Fig. 6), confirm absolutely their structural connectivity. The
data shows in the solid-state that the aryl and indole groups
deviate from planarity, which is likely a crystal packing effect. The
conformational preference of the ester, amino and aryl moieties
could affect the intrinsic fluorescence properties and provide a
useful starting point for future TDDFT calculations.
UV-vis and fluorescence measurements reveal the effect of the
different aryl groups within the tryptophan framework (Fig. 7).
The most electronically distinct analogues, 2d (4-CF3) and 2e
(4-OMe), exhibit the highest absorption shifts, generating a
characteristic V-shape. Such a correlation has been reported by
Marder and co-workers in 1,4-bis( p-R-phenylethynyl)benzenes
Fig. 4 Changes in the IR spectra observed on the reaction of Pd(OAc)2
(1616 cmꢀ1) with 1, affording Pd(OAc)2(1)2 (1606 cmꢀ1).
shown that heterogeneous C–H bond activation is viable in
benzo[b]thiophene arylation (Scheme 1).15
The reaction of tryptophan 1 with Pd(OAc)2 (1 : 1) in THF at
ambient temperature (0.5 h) was monitored by in situ infrared
spectroscopy (Fig. 4). The carbonyl stretching band at 1616 cmꢀ1 is
Pd(OAc)2, which disappears on addition of 1, with the appearance
of a new band at 1606 cmꢀ1, proposed to be Pd(OAc)2(1)2. Only
small changes were noted by 1H NMR spectroscopic analysis,
which is in keeping with the tryptophan ligand being weakly
coordinated to PdII. Crucially, Pd(OAc)2(1)2 rapidly reduces to form
Pd0 and is the seed that leads to the generation of PdNPs under
the catalytic conditions – amine ligands lower the PdII reduction
potential to give Pd0 ions.4b
Fig. 6 Single crystal X-ray structures for 2c (left) and 2d (right); ellipsoids
shown at 50%, H-atoms omitted.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 3052--3054 | 3053

View Article Online
ChemComm
Communication
longer peptides, especially as it is known that certain peptides
template (by reduction) the formation of specifically-sized and
shaped PdNPs.18 These aspects pertaining to C–H bond function-
alisation are currently being investigated within our laboratories.
We thank Dr Meg Stark for TEM measurements, Mr Henry
Durant for exploratory work. EPSRC funded T.J.W. (DTA PhD, grant
code EP/P505178/1) and Royal Society partly-funded I.J.S.F. (URF).
This article is published in celebration of the 50th Anniversary of
the opening of the Chemistry Department at the University of York.
Fig. 7 Fluorescence spectra for 2-aryl-tryptophan 2a–e (the red dotted
line denotes the cut-off from amino acid residues in proteins).
Notes and references
‡ As PdNPs are formed rapidly in this chemistry, we define Pd(OAc)2 as
a precatalyst.
Table 1 UV-vis data and Stokes shifts for 2-aryl-tryptophans 2a–ea
Compound Abs. lmax (nm) Em. (nm) Stokes shift e/cmꢀ1 molꢀ1 dmꢀ3
1 S. De Ornellas, T. J. Williams, C. G. Baumann and I. J. S. Fairlamb, in
Catalytic C-H/C-X Bond Activation, ed. X. Ribas, RSC Publishing,
Cambridge, 2013.
2 (a) J. M. Chalker, C. S. C. Wood and B. G. Davis, J. Am. Chem. Soc.,
2009, 131, 16346; (b) C. D. Spicer and B. G. Davis, Chem. Commun.,
2011, 47, 1698; (c) C. D. Spicer, T. Triemer and B. G. Davis, J. Am.
Chem. Soc., 2012, 134, 800.
2a
2b
2c
2d
2e
308
310
306
318
320
370
375
416
368
368
62
65
110
50
9120
8893
14 684
10 297
11 644
48
3 (a) L. Ackermann, R. Vicente and A. R. Kapdi, Angew. Chem., Int. Ed.,
2009, 48, 9792. For a beautiful example, see: (b) M. E. Kieffer,
K. V. Chuang and S. E. Reisman, J. Am. Chem. Soc., 2013, 135, 5557.
4 (a) T. E. Storr, A. G. Firth, K. Wilson, K. Darley, C. G. Baumann and
I. J. S. Fairlamb, Tetrahedron, 2008, 64, 6125; (b) T. E. Storr, C. G.
Baumann, R. J. Thatcher, S. De Ornellas, A. C. Whitwood and I. J. S.
Fairlamb, J. Org. Chem., 2009, 74, 5810; (c) T. E. Storr, J. A.
Strohmeier, C. G. Baumann and I. J. S. Fairlamb, Chem. Commun.,
2010, 46, 6470. For the characterisation of Pd nanoparticles in
Sonogashira reactions of bromo-purines, see: (d) A. G. Firth,
K. Wilson, C. G. Baumann and I. J. S. Fairlamb, Nucleosides,
Nucleotides Nucleic Acids, 2011, 30, 168.
a
Solutions of 2a–e in CH2Cl2.
¨
5 J. Wencel-Delord, T. Droge, F. Liu and F. Glorius, Chem. Soc. Rev.,
2011, 40, 4740.
6 2-Arylated tryptophans can be accessed by C–H bond activation using
cationic Pd, microwave heating and reaction temperatures ranging
from 80–150 1C, typically 90 1C, see: (a) S. Preciado, L. Mendive-Tapia,
F. Albericio and R. Lavilla, J. Org. Chem., 2013, 78, 8129; (b) J. Ruiz-
´
Rodrıguez, F. Albericio and R. Lavilla, Chem.–Eur. J., 2010, 16, 1124.
The selective C-2 alkylation of tryptophan by PdII/norbornene-
promoted C–H activation has been reported, see: (c) H. K. Potukuchi
and T. Bach, J. Org. Chem., 2013, 78, 12263.
Scheme 2 Direct C–H functionalisation of two selected peptides.
7 J. T. Vivian and P. R. Callis, Biophys. J., 2001, 80, 2093.
8 (a) N. Lebrasseur and I. Larrosa, J. Am. Chem. Soc., 2008, 130, 2926;
(b) S.-D. Yang, C.-L. Sun, Z. Fang, B.-H. Li, Y.-Z. Li and Z.-J. Shi,
Angew. Chem., Int. Ed., 2008, 47, 1473; (c) J. Zhao, Y. Zhang and
K. Cheng, J. Org. Chem., 2008, 73, 7428; (d) N. R. Deprez, D. Kalyani,
A. Krause and M. S. Sanford, J. Am. Chem. Soc., 2006, 128, 4972.
9 We have established that the reaction of 1 with d6-PhB(OH)2 and
PhI(OAc)2 affords a mixture of deuterated and non-deuterated
2-phenyl-tryptophan 2a in a ca. 1 : 1.5 ratio (by MS studies).
10 P. Sehnal, R. J. K. Taylor and I. J. S. Fairlamb, Chem. Rev., 2010, 110, 824.
and 2,5-bis(phenylethynyl) thiophenes, and could be an indication of
a push–pull type system within the series 2a–e.17 The largest
fluorescence intensity is seen for 2a – the other compounds 2b–e
exhibit lower fluorescence intensity. Within the series of 2a–e, 2c
(4-F) exhibits the largest Stokes shift. Moreover, the emission wave-
length is red-shifted relative to free tryptophan (ca. 360 nm) (Table 1).
Selective arylation of peptides. We have successfully applied 11 T. J. Williams and I. J. S. Fairlamb, Tetrahedron Lett., 2013, 54, 2906.
12 Unsupported PdNPs were characterised at a similar size (ꢁ0.3 nm) –
our best reaction conditions (given in Fig. 5) to the arylation of
dipeptide 3 (Scheme 2), which gave 4 in >95% conversion (by
see ESI† for TEM images.
13 (a) A. F. Lee, P. J. Ellis, I. J. S. Fairlamb and K. Wilson, Dalton Trans.,
HPLC analysis). When applied to a more complicated system –
the six residue peptide AcTrpLysLeuValGlyAlaOH 5 – in a
reaction with PhB(OH)2 to give 6 – it was necessary to increase
2010, 39, 10473; (b) P. J. Ellis, I. J. S. Fairlamb, S. F. J. Hackett,
K. Wilson and A. F. Lee, Angew. Chem., Int. Ed., 2010, 49, 1820.
14 (a) J. G. de Vries, Dalton Trans., 2006, 421; (b) M. Reetz and J. G.
de Vries, Chem. Commun., 2004, 1559.
catalyst loading to 30 mol% and 60 mol% for Pd and Cu 15 D.-T. D. Tang, K. Collins and F. Glorius, J. Am. Chem. Soc., 2013, 135, 7450.
16 J. M. Davidson and C. Triggs, J. Chem. Soc. A, 1968, 1324.
17 (a) P. Nguyen, Y. A. Zheng, L. Agocs, G. Lesley and T. B. Marder, Inorg.
respectively. This reaction proceeded in good conversion (86%).
In summary, we have reported a mild and selective direct C–H
Chim. Acta, 1994, 220, 289; (b) J. S. Siddle, R. M. Ward, J. C. Collings,
functionalisation reaction for the amino acid tryptophan, both as a
single residue and as a residue in short and longer-chained peptides.
It is tempting to suggest that the amino acid may play some role
in interacting with and stabilising the PdNPs, particularly for the
S. R. Rutter, L. Porres, L. Applegarth, A. Beeby, A. S. Batsanov, A. L.
Thompson, J. A. K. Howard, A. Boucekkine, K. Costuas, J. F. Halet and
T. B. Marder, New J. Chem., 2007, 31, 841.
18 C.-Y. Chiu, Y. Li and Y. Huang, Nanoscale, 2010, 2, 927; J. Cookson,
Platinum Met. Rev., 2012, 56, 83.
3054 | Chem. Commun., 2014, 50, 3052--3054
This journal is ©The Royal Society of Chemistry 2014