The synthesis and structural characterization of novel N-meta-ferrocenyl benzoyl amino acid esters

Article abstract of DOI:10.1016/j.inoche.2005.01.031

A series of N-meta-ferrocenyl benzoyl amino acid esters 3-10 have been prepared by coupling meta-ferrocenyl benzoic acid 2 to the amino acid esters using the conventional 1,3-dicyclohexylcarbodiimide (DCC), 1- hydroxybenzotriazole (HOBt) protocol. The amino acids employed in the synthesis were glycine, l-alanine, l-leucine, l-phenylalanine, β-alanine, 4-aminobutyric acid, 2-aminobutyric acid and 2-aminoisobutyric acid. The compounds were fully characterized by a range of NMR spectroscopic techniques and mass spectrometry (MALDI-MS, ESI-MS).

Full text of DOI:10.1016/j.inoche.2005.01.031

Inorganic Chemistry Communications 8 (2005) 429–432
www.elsevier.com/locate/inoche
The synthesis and structural characterization of novel
N-meta-ferrocenyl benzoyl amino acid esters
a
David Savage , Nicola Neary , Gwen Malone , Steven R. Alley ,
a
a
b
b
John F. Gallagher , Peter T.M. Kenny
a,b,
*
a
School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
National Institute for Cellular Biotechnology, Dublin City University, Dublin 9, Ireland
b
Received 7 April 2004; accepted 31 January 2005
Available online 14 March 2005
Abstract
A series of N-meta-ferrocenyl benzoyl amino acid esters 3–10 have been prepared by coupling meta-ferrocenyl benzoic acid 2 to
the amino acid esters using the conventional 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) protocol. The
amino acids employed in the synthesis were glycine, L-alanine, L-leucine, L-phenylalanine, b-alanine, 4-aminobutyric acid, 2-amino-
butyric acid and 2-aminoisobutyric acid. The compounds were fully characterized by a range of NMR spectroscopic techniques and
mass spectrometry (MALDI-MS, ESI-MS).
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Bioorganometallic chemistry; MALDI; Amino acid esters
The organometallic compound ferrocene is a promis-
ing candidate for incorporation in novel materials due
to its stability, spectroscopic properties, electrochemical
properties and ease of use. As a direct consequence of
these factors, research in the area of ferrocenyl deriva-
tives has seen a dramatic increase in attention over the
past decade, primarily for the ultimate goals of achiev-
ing novel sensor compounds, peptide mimetic models
and unnatural drugs [1–10]. The incorporation of a fer-
rocene group onto proteins has shown the mediation of
electron transfer between electrodes and the protein re-
dox site [3,5]. The synthesis and structural characteriza-
tion of novel N-ferrocenoyl and N-ferrocenyl amino acid
and peptide derivatives has been reported [11–28]. The
aim of this research is to incorporate three key moieties
in the synthesis of unusual biological materials, namely
(i) an electroactive core, (ii) a conjugated linker that can
act as a chromophore and (iii) an amino acid derivative
that can interact with other molecules via hydrogen
bonding. In this communication, we report the synthesis
and structural characterization of a series of novel N-
meta-ferrocenyl benzoyl amino acid ester derivatives.
We recently reported the synthesis and structural char-
acterization of a series of N-para-ferrocenyl benzoyl
amino acid ester derivatives [28] and the N-ortho-ferr-
ocenyl benzoyl amino acid derivatives will be reported
in due course.
The meta-ferrocenyl benzoic acid 2 was prepared
using conventional diazonium salt chemistry. Treatment
of ethyl-3-aminobenzoate with sodium nitrite in the
presence of hydrochloric acid yielded the diazonium
salt, which was then reacted with ferrocene in situ to
yield the meta-substituted ferrocenyl ethyl benzoate 1.
The ester group was cleanly cleaved by treatment with
10% sodium hydroxide to yield meta-ferrocenyl benzoic
1
acid 2. The H NMR spectrum showed signals for the
aromatic ring protons at d 8.03 (s), d 7.8 (d), d 7.77
(d) and d 7.43 (t), integrating for one proton each, char-
acteristic of a meta-disubstituted aromatic ring. The
*
Corresponding author. Tel.: +353 1 7005 689; fax: +353 1 7005 503.
E-mail address: peter.kenny@dcu.ie (P.T.M. Kenny).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.01.031

430
D. Savage et al. / Inorganic Chemistry Communications 8 (2005) 429–432
carboxylic acid proton was present at d 13.2. The ferr-
ocenyl ortho and meta protons on the (g5-C5H4) ring
were observed at d 4.84 and d 4.39, respectively, and
an intense singlet was present at d 4.03 for the (g5-
C5H5) ring. The condensation of meta-ferrocenyl ben-
zoic acid with the free N-terminal amino acid methyl
esters of glycine, L-alanine, L-leucine, L-phenylalanine,
and the free N-terminal amino acid ethyl esters of b-ala-
nine, 4-aminobutyric acid, 2-aminobutyric acid and
2-aminoisobutyric acid under basic conditions in the
presence of dicyclohexylcarbodiimide (DCC) and 1-
hydroxybenzotriazole (HOBt) yielded N-meta-ferroce-
nyl benzoyl amino acid esters 3–10, as yellow/orange
colored crystals (Scheme 1). The yields obtained ranged
between 55% and 68% and all gave analytical and spec-
troscopic data in accordance with the proposed struc-
ture [29]. The N-meta-ferrocenyl benzoyl derivatives
and ortho protons on the (g5-C5H4) ring are present at
d 4.39 and 4.85, respectively. The NH proton appears
as a triplet at d 8.62 and a quartet at d 3.51 corresponds
to the –NHCH2CH2CO– protons. The triplet at d 2.61 is
due to the second methylene group adjacent to the car-
bonyl group.
The 13C NMR spectra of compounds 3–10 show sig-
nals in the region d 66.7–84.6 indicative of a monosub-
stituted ferrocene subunit. The ipso carbon of the
(g5-C5H4) ring appears in a very narrow range of d
84.3–84.6. This signal is absent in the DEPT 135 spectra.
The carbon atoms of the aromatic ring are non-equiva-
lent and therefore six signals are visible in the region d
124.2–140.7. The methylene carbon atoms of derivatives
3, 5, 6, 7, 8, 9 and 10 were identified by DEPT-135. A
1
complete assignment of the H and 13C NMR spectra
of compound 7 is presented in Table 1.
1
were characterized by a combination of H NMR, 13C
Since the introduction of soft ionization techniques,
such as matrix assisted laser desorption ionization (MA-
DLI) and electrospray ionization (ESI), a wide range of
thermolabile and non-volatile compounds can be sub-
jected to mass spectrometric analysis [30–32]. As the
compounds were not amenable to electron ionization
studies, MADLI was employed in the analysis of com-
pounds 3, 5, 7, 8, 9, and 10, whereas compounds 4
and 6 were analyzed by electrospray ionization mass
spectrometry (ESI-MS). MALDI confirmed the correct
relative molecular mass for all the compounds and
examination of the mass spectra revealed the presence
of intense radical-cations, with cation adducts due to so-
dium and potassium adducts also present. The vast
majority of analytes subjected to analysis by soft ioniza-
tion techniques such as MALDI furnish protonated
molecular ion species as a result of proton transfer reac-
tions between the analyte and matrix or cation adduc-
tion. It has been reported that the molecular radical
cation of ferrocene and not the protonated molecular
ion is generated during MALDI analysis [33]. Fragment
ions were not observed or were of very low intensity in
1
NMR, DEPT-135 and H–13C COSY (HMQC) spec-
troscopy, matrix assisted laser desorption ionization
mass spectrometry (MALDI) and electrospray ioniza-
tion mass spectrometry (ESI).
All the proton and carbon chemical shifts for com-
pounds 3–10 were unambiguously assigned by a combi-
1
nation of DEPT-135 and H–13C COSY (HMQC). The
1H and 13C NMR spectra for compounds 3–10 showed
peaks in the ferrocene region characteristic of a mono
substituted ferrocene moiety [11,12,28]. The protons in
the ortho position of the (g5-C5H4) ring appear in the re-
gion d 4.63–4.96, whereas the protons in the meta posi-
tion occur in the range d 4.28–4.48. The (g5-C5H5) ring
appears in the region d 3.97–4.04. The protons of the
meta-disubstituted benzoyl group appear as a triplet,
doublet, doublet and singlet in the region d 7.2–8.00.
For example, in the case of the b-alanine derivative 7,
the aromatic protons are present as a triplet, two dou-
blets and a singlet at d 7.39, d 7.64, d 7.71 and d 7.89,
respectively. The (g5-C5H5) ring appears as a singlet in
the 1H NMR spectrum at d 4.03, whereas the meta
i
H N
2
Fe
Fe
OCH CH
2
3
OCH CH
2
3
O
O
ii
1
iii
Fe
Fe
H
COOR'
N
OH
O
O
R
3—10
2
Scheme 1. Synthesis of the N-meta-ferrocenyl benzoyl amino acid esters 3–10; Gly(OMe) 3, Ala(OMe) 4, Leu(OMe) 5, Phe(OMe) 6, b-Ala(OEt) 7, 4-
Aba(OEt) 8, 2-Aba(OEt) 9, Aib(OEt) 10: (i) NaNO2, HCl, 5 °C; (ii) NaOH/MeOH, H2O; (iii) DCC, HOBt, Et3N, amino acid ester.

D. Savage et al. / Inorganic Chemistry Communications 8 (2005) 429–432
[3] V. Degani, A. Heller, J. Am. Chem. Soc. 110 (1988) 2615.
431
Table 1
1H and 13C spectroscopic data for 7
[4] A. Nomoto, T. Moriuchi, S. Yamazaki, A. Ogawa, T. Hirao,
J. Chem. Soc., Chem. Commun. (1998) 1963.
12
13
2
1
11
4
14
[5] M. Kira, T. Matsubara, H. Shinohara, M. Sisido, Chem. Lett.
(1997) 89.
5
3
15
17
O
20
16
Fe
6
18
7
21
[6] H. Plenio, C. Aberle, Organometallics 16 (1997) 5950.
[7] P.D. Beer, F. Szemes, V. Balzani, C.M. Sala, M.G.B. Drew, S.W.
Dent, M. Maestri, J. Am. Chem. Soc. 119 (1997) 11864.
[8] T. Moriuchi, A. Nomoto, K. Yoshida, T. Hirao, Organometallics
20 (2001) 1008.
N
H
O
8
10
19
22
O
9
Site
1H NMR
13C NMR
84.6
HMQC
1
2, 3
4, 5
6 to 10
11
4.85
4.39
4.03
67.1
69.7
70.1
[9] T. Itoh, S. Shirakami, N. Ishida, Y. Yamashita, T. Yoshida,
H.-S. Kim, Y. Wataya, Bioorg. Med. Chem. Lett. 10 (2000) 1657.
[10] H. Fink, N.J. Long, A.J. Martin, G. Opromolla, A.J.P. White,
D.J. Williams, P. Zanello, Organometallics 16 (1997) 2646.
[11] H.-B. Kraatz, J. Lusztyk, G.D. Enright, Inorg. Chem. 36 (1997)
2400.
134.9
12
13
7.64
7.39
7.71
129.5
128.9
124.3
14
15
[12] J.F. Gallagher, P.T.M. Kenny, M.J. Sheehy, Inorg. Chem.
Commun. 2 (1999) 200.
140.6
167.8
16
17
7.89
125.2
[13] J.F. Gallagher, P.T.M. Kenny, M.J. Sheehy, Inorg. Chem.
Commun. 2 (1999) 327.
18
19
3.51
2.61
35.7
34.3
[14] H.-B. Kraatz, D.M. Leek, A. Houmam, G.D. Enright, J. Lusztyk,
D.D.M. Wayner, J. Organomet. Chem. 589 (1999) 38.
[15] T. Moriuchi, A. Nomoto, K. Yoshida, T. Hirao, J. Organomet.
Chem. 589 (1999) 50.
20
21
173.4
4.08
1.19
61.3
14.6
22
[16] P. Saweczko, H.-B. Kraatz, Coord. Chem. Rev. 192 (1999) 185.
[17] O. Brosch, T. Weyhermuller, N. Metzler-Nolte, Eur. J. Inorg.
Chem. 2 (2000) 323.
[18] A. Hess, J. Sehnert, T. Weyhermuller, N. Metzler-Nolte, Inorg.
Chem. 39 (2000) 5437.
the MALDI spectra. This is in contrast to the analysis of
the related para-ferrocenyl benzoyl amino acid deriva-
tives by FABMS where important fragment ions were
observed [28]. The ESI mass spectra displayed
[M + H]+ species and intense adducts due to sodium
and potassium were present 22 and 38 Da higher than
the protonated molecular ion species. A fragment ion
was observed at m/z 261 due to the ferrocenylphenyl
subunit at the N-terminal.
[19] T. Moriuchi, K. Yoshida, T. Hirao, Organometallics 20 (2001)
3101.
[20] Y.M. Xu, H.-B. Kraatz, Tetrahedron Lett. 42 (2001) 2601.
[21] T. Moriuchi, K. Yoshida, T. Hirao, J. Organomet. Chem. 637
(2001) 75.
[22] A. Wieckowska, R. Bilewicz, A. Misicka, M. Pietraszkiewicz, K.
Bajdor, L. Piela, Chem. Phys. Lett. 350 (2001) 447.
[23] H.-B. Kraatz, Y.M. Xu, P. Saweczko, J. Organomet. Chem. 637
(2001) 335.
[24] T. Moriuchi, A. Nomoto, K. Yoshida, A. Ogawa, T. Hirao,
J. Am. Chem. Soc. 123 (2001) 68.
In conclusion, the novel N-meta-ferrocenyl benzoyl
amino acid esters 3–10 were prepared in good yields
using organic peptide synthetic protocols. The com-
pounds were characterized by NMR spectroscopic tech-
niques and by mass spectrometry.
[25] S. Maricic, U. Berg, T. Frejd, Tetrahedron 58 (2002) 3085.
[26] S. Maricic, T. Frejd, J. Org. Chem. 67 (2002) 7600.
[27] D.R. van Staveren, T. Weyhermuller, N. Metzler-Nolte, J. Chem.
Soc., Dalton Trans. (2003) 210.
[28] D. Savage, J.F. Gallagher, Y. Ida, P.T.M. Kenny, Inorg. Chem.
Commun. 5 (2002) 1034.
[29] Synthesis of compound 7. b-Alanine ethyl ester hydrochloride
(0.3 g, 2.0 mmol) and triethylamine (0.5 ml) were added to a
solution of meta-ferrocenyl benzoic acid (0.5 g, 1.6 mmol), 1-
hydroxybenzotriazole (0.3 g, 2.2 mmol) and 1,3-dicyclohexylcar-
bodiimide (0.45 g, 2.2 mmol) in CH2Cl2 (50 ml) at 0 °C. After
30 min the solution was raised to room temperature and allowed
to proceed for 48 h. The precipitated N,N0-dicyclohexylurea was
removed by filtration and the filtrate was washed with water, 10%
potassium hydrogen carbonate, 5% citric acid and dried over
MgSO4. Recrystallization from petroleum ether (40–60 °C): ethyl
acetate gave 7 as orange needles, (0.44 g, 68%). m.p.104–106 °C.
Mass spectrum: Found: [M]+ 405.104, C22H23N1O3Fe requires:
405.103. IR mmax(KBr): 3629, 2933, 1740, 1625, 1548, 1455, 1261,
Acknowledgments
D.S. thanks the Irish American Partnership and Dub-
lin City University for the Funding of a Studentship
Award 1999-2002. This research was partly supported
by the National Institute for Cellular Biotechnology un-
der the Programme for Research in Third Level Institu-
tions (PRTLI, Round 3, 2001–2006). We also thank
John Kelly of Bruker Daltonics, UK for the MALDI
mass spectra.
1
1186 cmꢀ1. UV–vis kmax MeCN: 324 (e 1580), 448 (e 410) nm. H
NMR (400 MHz) d (DMSO): 8.62 (1H, t, J = 6.4 Hz, –CONH–),
7.89 (1H, s, ArH), 7.71 (1H, d, J = 7.6 Hz, ArH), 7.64 (1H, d,
J = 7.6 Hz, ArH), 7.39 (1H, t, J = 7.6 Hz, ArH), 4.85 {2H, t,
J = 1.2 Hz, ortho on (g5-C5H4)}, 4.39 {2H, t, J = 1.2 Hz, meta on
(g5-C5H4)}, 4.08 (2H, q, J = 7.2 Hz, –OCH2CH3), 4.03 {5H, s,
(g5-C5H5)}, 3.51 (2H, q, J = 6 Hz, –NHCH2CH2–), 2.61 (2H, t,
J = 7.2 Hz, –NHCH2CH2–), 1.19 (3H, t, J = 7.2 Hz, –OCH2CH3).
References
[1] G. Jaouen (Ed.), J. Organomet. Chem., Bioorganomet. Chem.
(special issue), 589 (1999) 1–126.
[2] R.D. Adams (Ed.), J. Organomet. Chem., Ferrocene Chem.
(special issue), 619 (1999) 1–875.

432
D. Savage et al. / Inorganic Chemistry Communications 8 (2005) 429–432
13C NMR (100 MHz) d (CDCl3): 173.4, 167.8, 140.6, 134.9, 129.5,
[31] K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshida,
Rapid Commun. Mass Spectrom. 2 (1988) 151.
128.9, 125.2, 124.3, 84.6, 70.1, 69.7, 67.1, 61.3 (ꢀve DEPT), 35.7
(ꢀve DEPT), 34.3 (ꢀve DEPT), 14.6.
[32] J.B. Fenn, J. Am. Soc., Mass Spectrom. 4 (1993) 524.
[33] T. Donovan McCarley, R.L. McCarley, P.A. Limbach, Anal.
Chem. 70 (1998) 4376.
[30] M. Karas, D. Bachmann, U. Bahr, F. Hillenkamp, Int. J. Mass
Spectrom. Ion Process. 78 (1987) 53.