N-terminus to C-terminus metallacyclicpeptides employing tungsten-alkyne coordination

Article abstract of DOI:10.1016/j.jorganchem.2004.09.062

The capacity of using tungsten-alkyne coordination to form metallacyclicpeptides was examined. Dialkynyl species 1, 2 and 3 were prepared. 1 is a dialkyne derivative of ammonia, 2 is a dialkyne derivative of alanine, while 3 is a dialkyne derivative of the dipeptide alanylalanine. In 2 and 3 the two alkyne groups were appended at the N- and C-termini. The N-terminal alkyne was prepared by acylating the N-terminal amine with propargylchloroformate. The C-terminal alkyne was introduced by acylating the C-terminal carboxylic acid with propargylamine. Outcomes of metallacyclization reactions were assessed using ¹H NMR spectroscopy and electrospray positive ion mass spectrometry. Both 2 and 3 underwent successful cyclization to yield the metallacyclicpeptides 16 and 17, respectively. However, 1 did not cyclize; instead, it formed a variety of acyclic and cyclic oligomeric tungsten-bis(alkyne) species. The failure of 1 to cyclize is attributed to its inability to position its two alkyne groups parallel to each other and spaced 6.5 ? apart. The ¹H NMR spectra for 16 and 17 show that these metallacyclicpeptides exist as a complex mixture of isomers that differ in how the ligands are positioned around the tungsten center. At elevated temperatures metallcyclicpeptide 17 will rapidly interconvert between the various isomers. In contrast, 16 does not readily interconvert between isomers, even at elevated temperatures. That 16 does not rapidly interconvert between isomers, even at 90°C, is attributed to the tight packing and limited conformational freedom of this metallacyclicpeptide.

Full text of DOI:10.1016/j.jorganchem.2004.09.062

Journal of Organometallic Chemistry 689 (2004) 4837–4847
www.elsevier.com/locate/jorganchem
N-terminus to C-terminus metallacyclicpeptides employing
tungsten–alkyne coordination
*
Timothy P. Curran , Richard S.H. Yoon, Brian R. Volk
Department of Chemistry, Trinity College, 300 Summit Street, Hartford, CT 06106-3100, USA
Received 29 July 2004; accepted 27 September 2004
Available online 5 November 2004
Abstract
The capacity of using tungsten–alkyne coordination to form metallacyclicpeptides was examined. Dialkynyl species 1, 2 and 3
were prepared. 1 is a dialkyne derivative of ammonia, 2 is a dialkyne derivative of alanine, while 3 is a dialkyne derivative of
the dipeptide alanylalanine. In 2 and 3 the two alkyne groups were appended at the N- and C-termini. The N-terminal alkyne
was prepared by acylating the N-terminal amine with propargylchloroformate. The C-terminal alkyne was introduced by acylating
the C-terminal carboxylic acid with propargylamine. Outcomes of metallacyclization reactions were assessed using 1H NMR
spectroscopy and electrospray positive ion mass spectrometry. Both 2 and 3 underwent successful cyclization to yield the metallacy-
clicpeptides 16 and 17, respectively. However, 1 did not cyclize; instead, it formed a variety of acyclic and cyclic oligomeric tungsten–
bis(alkyne) species. The failure of 1 to cyclize is attributed to its inability to position its two alkyne groups parallel to each other and
1
˚
spaced 6.5 A apart. The H NMR spectra for 16 and 17 show that these metallacyclicpeptides exist as a complex mixture of isomers
that differ in how the ligands are positioned around the tungsten center. At elevated temperatures metallcyclicpeptide 17 will rapidly
interconvert between the various isomers. In contrast, 16 does not readily interconvert between isomers, even at elevated tempera-
tures. That 16 does not rapidly interconvert between isomers, even at 90 ꢀC, is attributed to the tight packing and limited conform-
ational freedom of this metallacyclicpeptide.
ꢁ 2004 Elsevier B.V. All rights reserved.
Keywords: Tungsten; Alkynes; Metallacyclicpeptide; Conformation
1. Introduction
Another possible use of the tungsten–alkyne coordi-
nation chemistry with peptides is the formation of met-
allacyclicpeptides. In this approach, a peptide bearing
two alkyne groups is joined to the tungsten to form a
macrocyclic ring (Scheme 2).
In order to determine whether this second approach
would work, the dialkynyl compounds 1–3 (Scheme 3)
were prepared and reacted with W(CO)3(dmtc)2. The
common element in 1–3 is the presence of a terminal
alkyne group at both ends of the molecule. With 2 and
3, the alkyne group appears at the N- and C-termini
of the peptide chain. We report here that 1 does not
form the desired complex; instead it forms three other
bis(alkyne) complexes. However, both 2 and 3 do form
Uncovering methods for coordination of amino acids
and peptides to transition metals is a growing research
area in bioorganometallic chemistry [1]. As part of this
effort, we have been exploring the use of alkyne ligands
[2] for coordinating tungsten to amino acids and pep-
tides [3]. In this approach, the alkyne is introduced onto
the amino acid or peptide, and two of the resulting alky-
nylamino acids or alkynylpeptides are then coordinated
to tungsten (Scheme 1).
*
Corresponding author. Tel.: +1 8602975276; fax: +1 8602975129.
E-mail address: timothy.curran@trincoll.edu (T.P. Curran).
0022-328X/$ - see front matter ꢁ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.09.062

4838
T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
2.2. Reaction of 1 with W(CO)3(dmtc)2
H
H
W(CO)3(dmtc)2
+
S
W
S
MeOH,∆
S
S
Compound 1 was reacted with W(CO)3(dmtc)2 in an
effort to form the desired metallacyclic product, 12. In or-
der to minimize the chance for oligomer formation, 1 was
reacted with W(CO)3(dmtc)2 under high dilution (1 mM)
conditions. Thus, 1 was dissolved in MeOH at a concen-
tration of 1 mM, the solution brought to reflux, and a
solution of W(CO)3(dmtc)2 dissolved in CH2Cl2 was
added slowly to the MeOH solution of 1. Addition of
W(CO)3(dmtc)2 to the colorless solution of 1 initially pro-
duced a light green solution. The green color indicated
formation of a monoalkyne complex, which presumably
has the formula W(1)(CO)(dmtc)2 [4]. This green color
slowly changed to a yellow color during the next 2 h of
reflux. The yellow color indicated formation of a tung-
sten–bis(alkyne) complex [5]. Evaporation of the solvents
produced a crude, yellow solid. Analysis of the solid by
TLC showed three major spots that possessed a yellow
color. Flash chromatography was used to isolate the
three compounds that give rise to these spots.
2 Peptide
H
S
Peptide Peptide
S
Me
= dmtc =
N
S
S
Me
Scheme 1.
H
H
S
S
W
S
W(CO)3(dmtc)2
+
S
MeOH,∆
H
Peptide
H
Peptide
Scheme 2.
the desired metallacyclicpeptides, and the conforma-
tional behavior of these complexes is examined.
S
S
S
2. Results and discussion
W
S
2.1. Synthesis of 1–3
12
The syntheses of 1–3 are shown in Scheme 4A–C.
Treatment of propargylchloroformate (4) with propar-
gylamine hydrochloride (5) in the presence of a trialkyl-
amine base cleanly yields 1 (Scheme 4A). To prepare 2
(Scheme 4B), Boc–Ala–OSu (6) was reacted with prop-
argylamine hydrochloride (5) in the presence of a trial-
kylamine base to produce 7 [3]. Subsequent treatment
of 7 with trifluoroacetic acid (which removes the Boc
protecting group) yields the salt 8. Without further puri-
fication, 8 is reacted with propargylchloroformate (4) in
the presence of a trialkylamine base to produce 2
(Scheme 4B). The preparation of 3 (Scheme 4C) began
with the acylation of Boc–Ala–Ala–OH (9) with propar-
gylamine hydrochloride (5) using a carbodiimide cou-
pling reagent (EDC) to yield 10. Subsequent treatment
of 10 with trifluoroacetic acid (which removes the Boc
protecting group) yielded the salt 11, which was then re-
acted with propargylchloroformate (4) in the presence of
a trialkylamine base to produce 3.
O
NH
O
The three yellow products isolated from the reaction
were analyzed by positive ion ESMS [6]. Because tung-
sten possesses four major, naturally occurring isotopes,
the molecular ions for tungsten–bis(alkyne) complexes
generate unique and characteristic peak patterns [3].
Analysis of all three products showed that none were
the desired metallacyclic product, 12. The ESMS data,
however, could be used to ascertain the identities of
the three yellow products.
The product obtained in the highest, purified yield
(15%) gave an M + Na ion with peaks centered around
721 m/z. This M + Na ion is consistent with a structure
having one tungsten, two dmtc groups, and two mole-
cules of 1 [W(1)2(dmtc)2 = 13]. This bis(alkynylpeptide)
complex formed even with the concentrations of the
O
O
Me
O
Me
O
H
N
H
N
O
N
H
O
N
H
O
N
H
N
H
O
Me
O
1
2
3
Scheme 3.

T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
4839
O
O
DIEA,
O
N
H
+
ClH3N
O
Cl
A
CH2Cl2
4
5
1
O
Me
O
O
Me
TFA,
B
5, DIEA,
CH2Cl2
H
N
OSu
O
N
N
H
CH2Cl2
H
O
O
O
7
6
Me
Me
H
H
4, DIEA,
CH2Cl2
N
N
O
N
H
O2CF3C H3N
O
O
2
8
O
Me
O
O
Me
O
C
H
N
H
N
5, EDC,
O
N
H
OH
O
N
H
N
H
DIEA, CH2Cl2
O
Me
O
O
Me
10
9
Me
O
Me
O
H
N
H
N
4, DIEA,
TFA,
O
N
H
N
H
O2CF3C H3N
N
H
CH2Cl2
CH2Cl2
O
Me
O
Me
11
3
Scheme 4.
reactants being very low. One of the possible structures
for 13 is shown in Scheme 5.
indicates that the metallacyclization of 1 with tungsten
here is not possible. Using data from X-ray crystal struc-
tures of tungsten bis(alkyne) complexes [2], it can be
estimated that the two alkynes in these bis(alkyne) com-
plexes need to be positioned parallel to each other and
The second product isolated and purified from this
reaction (7%) produced molecular ion peaks in the
ESMS centered around 1145 m/z. This M + Na ion is
consistent with a structure having two tungstens, four
dmtc groups and two molecules of 1, [W2(1)2-
(dmtc)4 = 14]. This molecule has to possess a metallacy-
clic structure, with the tungstens bridged by two
molecules of 1; one of the possible structures for 14 is
shown in Scheme 5.
The third product isolated and purified (2%) from the
reaction of 1 with W(CO)3(dmtc)2 gave M + Na peaks
in the ESMS centered around 1282 m/z. This M + Na
ion is consistent with a structure having two tungstens,
˚
approximately 6.5 A apart. A Dreiding model of 1 was
constructed and examined. At their closest possible ap-
proach, the two alkynes in the model were approxi-
˚
mately 7.2 A apart. Thus it appears that 1 cannot
position its two alkyne groups in close enough proximity
to effect cyclization. From the Dreiding model of 1, it is
clear that the structural rigidity imposed on 1 by the ur-
ethane group keeps the two alkynes from being able to
˚
get within 6.5 A of each other.
four dmtc groups and three molecules of
1
2.3. Cyclization of 2 with W(CO)3(dmtc)2
[W2(1)3(dmtc)4 = 15]. This species is a cross between 13
and 14. A possible structure for 15 is shown in Scheme 5.
The absence of the desired product from this reaction
mixture, and the observed formation of 13, 14, and 15,
Compound 2, the dialkynylalanine, was reacted with
W(CO)3(dmtc)2 under high dilution conditions in an
effort to form the desired metallacyclization product,

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T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
S
S
S
15%
W
S
O
O
N
H
O
N
H
O
+
13
H
N
O
7%
O
O
S
W
S
S
O
W(CO)3(dmtc)2
S
S
S
W
S
S
O
N
H
MeOH, ∆
High Dilution
1
N
H
O
14
+
H
N
O
2%
O
S
S
W
S
S
S
W
S
S
= Me2NCS2 = dmtc
S
S
S
HN
O
O
O
NH
O
15
Scheme 5.
16 (Scheme 6). As with the reaction of 1, addition of the
W(CO)3(dmtc)2 to a refluxing, 1 mM solution of 2 in
MeOH initially generated a light green solution, which
on further heating turned yellow. Workup of the reac-
tion and analysis by TLC showed the formation of
one major yellow band. This compound was isolated
and purified by flash chromatography. Analysis of the
amorphous, yellow solid showed it to be the desired met-
allacyclicpeptide, 16. The ESMS produced a series of
peaks centered around 655 m/z, which match the peaks
expected for the M + Na ion for 16. The 1H NMR spec-
trum also possessed all the resonances for the hydrogens
expected in 16, including the alkyne hydrogens at 11
ppm and the N-methyl hydrogens from the dithiocarba-
mates located in the 3.0–3.5 ppm region.
Because there are two different stereochemistries open
to the dithiocarbamate ligands, and because there are
two orientations open to each alkyne ligand, 16 appears
as a mixture of these isomers. The isomerism open to 16
arises from what von Zelewsky refers to as ‘‘fuzzy
S
O
Me
S
S
H
N
W
S
W(CO)3(dmtc)2
O
N
H
S
= Me2NCS2 = dmtc
S
O
MeOH, ∆
NH
O
High Dilution
O
N
O
2
H
Me
16
Scheme 6.

T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
4841
stereochemistry’’ [7]. Shown in Scheme 7 are the various
isomers open to 16. First, there are two possible orienta-
tions for the two alkyne groups in 16; they can be either
anti to each other (indicated by those isomers that have
their alkyne C–H bonds pointing in opposite directions,
A–H) or they can be anti to each other (indicated by
those isomers that have their alkyne C–H bonds point-
ing in the same direction, I–P). The syn- and anti-isomer
groupings can be further divided into two groups de-
fined by the two possible stereochemistries of the dith-
iocarbamate ligands around the tungsten; isomers A–
D and I–L have one of the stereochemisties, while iso-
mers E–H and M–P have the other. Finally, these last
groupings of isomers can be further divided into two
more groups that differ in the orientation of the peptide
chain (N-terminus to C-terminus) relative to the tung-
sten; A–B, E–F, I–J, M–N have one of the orientations,
while C–D, G–H, K–L, O–P have the other orientation.
Although it appears that there are 16 different iso-
mers possible for 16, in actuality there are eight possible
isomers. This is true because each isomer shown in
Scheme 7 has an identical twin. Thus, A is the same as
C, and B is the same as D. Likewise, E and G are the
same, as are F and H. Other identical pairs are I and
L, J and K, M and P, and N and O. Because of these
identical pairs, the actual number of possible isomers
for 16 is 8 rather than 16.
If interconversion between these isomers is slow, then
1
the H NMR spectrum will show resonances from each
isomer. Thus, the appearance of multiple peaks for a gi-
ven hydrogen type in 16 indicates the presence of multi-
ple isomers that either slowly interconvert or do not
interconvert at all.
One set of hydrogens that can reveal the conforma-
tional behavior of 16 are the alkyne hydrogens that ap-
pear around 11 ppm. If 16 adopts only one of the
O
O
O
O
O
O
HN
S
W
S
HN
S
W
S
Me
HN
S
S
HN
S
W
S
NH
Me
O
S
S
S
S
S
NH
S
S
O
O
W
S
O
O
O
NH
O
O
NH
E
F
A
B
NH
NH
O
O
NH
O
S
O
NH
S
W
S
S
S
S
S
S
W
S
W
S
S
S
Me
W
S
HN
S
S
Me
O
HN
N
H
S
O
O
N
H
O
O
O
G
H
C
D
O
O
S
S
W
S
O
NH
O
NH
S
S
S
S
W
S
O
O
O
O
S
S
W
S
O
O
N
N
S
S
NH
NH
H
H
W
S
HN
S
S
HN
O
O
I
J
M
N
O
O
S
HN
S
HN
S
NH
S
W
W
NH
H
S
O
S
NH
O
N
S
S
S
O
O
S
O
O
S
S
S
W
S
W
S
S
O
NH
O
NH
O
O
K
L
O
P
Scheme 7.

4842
T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
orientations shown in Scheme 7, then the 1H NMR spec-
trum should show two singlets of equal intensity, one for
each alkyne hydrogen. If 16 adopts two of the orienta-
tions, then the 1H NMR spectrum should show four sin-
glets, with pairs of the singlets having equal intensities.
Likewise, if 16 adopts three of the orientations then
there should be six singlets with three pairs of singlets
having equal intensities. For every orientation adopted
by 16, another two signals for the alkyne hydrogens
should be seen.
adopted by 16, another four N-methyl singlets should
be seen.
1
A quick inspection of the H NMR spectrum of 16
shows that this molecule exists as more than one of
the isomeric structures shown in Scheme 7, as there
are multiple signals for all the hydrogen types present.
Fig. 1(a) shows the alkyne hydrogen region (10.7–11.7
ppm), and Fig. 1(b) shows the N-methyl region (2.8–
3.8 ppm) for 16. The alkyne hydrogen region shows four
distinct singlets, each possessing different intensities. The
spectrum also shows that some of these singlets have
shoulders, indicating the presence of other, overlapping
singlets. The N-methyl region also shows overlapping
singlets, with at least 12 distinct singlets visible. Taken
together, the appearance of these resonances supports
the conclusion that 16 adopts a number of the orienta-
tions shown in Scheme 7. Because the peaks overlap, it
is not possible to count the exact number of orientations
for 16. At a minimum 16 assumes three of the eight
A similar type of analysis can be applied to the N-me-
thyl hydrogens that appear between 3.0 and 3.5 ppm. If
16 adopts only one of the orientations in Scheme 7, then
1
the H NMR spectrum should show four N-methyl sin-
glets. If 16 adopts only two of the orientations in
Scheme 7, then the 1H NMR spectrum should show
eight N-methyl singlets. Likewise, if three of the orienta-
tions are adopted, then 12 N-methyl singlets will be vis-
1
ible in the H NMR spectrum. For every orientation
Fig. 1. (a) The region between 10.7 and 11.7 ppm in the 1H NMR spectrum of 16 in CDCl3. The peaks in this region arise from the terminal alkyne
hydrogens. (b) The region between 2.8 and 3.8 ppm in the 1H NMR spectrum of 16 in CDCl3. The peaks in this region arise from the N-methyl
hydrogens on the dmtc groups.

T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
4843
orientations shown in Scheme 7; it is possible that 16 as-
sumes more than three of these orientations.
Inspection of a Dreiding model of 2 supports this
conclusion. Compound 2 can adopt conformations in
which the two alkynes are parallel to each other and sit-
aligned in a syn-orientation (S, T, W and X), while the
other four have the two alkynes arranged in the anti-ori-
entation (Q, R, U and V). Finally, four of the isomers
have the peptide in one orientation (Q, S, U and W),
while the other four isomers have the opposite peptide
orientation (R, T, V and X).
˚
uated approximately 6.5 A apart. The model shows that
this can occur if the two alkynes are aligned with each
other (leading to the syn-isomers I, J, M and N shown
in Scheme 7) or if the alkynes are not aligned with each
other (leading to the anti-isomers A, B, E and F shown
in Scheme 7). The model indicates that none of the eight
possible orientations for 16 can be ruled out.
Like 16, the conformational behavior of metallacy-
clicpeptide 17 can be assessed by examination of the al-
kyne hydrogen and N-methyl resonances in the 1H
NMR spectrum. These two regions are shown in Fig.
2(a) and (b), respectively. The alkyne hydrogen region
shows at least 10 different singlets (more signals than
seen for 16 (Fig. 1(a))), indicating that 17 adopts a large
number of the isomers shown in Scheme 9. The N-me-
thyl region for 17 closely resembles the N-methyl region
for 16 (Fig. 1(b)), except for the addition of several small
singlets in the spectrum of 17. The additional signals
seen for 17 compared to 16 suggests that 17 populates
more of its possible isomeric forms than does 16. At a
minimum 17 assumes three of the orientations shown
in Scheme 9; it also is possible that 17 assumes all eight
of the orientations shown in Scheme 9.
2.4. Cyclization of 3 with W(CO)3(dmtc)2
Compound 3, the dialkynylalanine dipeptide, was re-
acted with W(CO)3(dmtc)2 under high dilution condi-
tions in an effort to form the cyclic dipeptide, 17
(Scheme 8). As with the similar reactions of 1 and 2, addi-
tion of the W(CO)3(dmtc)2 to a refluxing, 1 mM solution
of 3 in MeOH initially generated a light green solution,
which on further heating turned yellow. Workup of the
reaction and analysis by TLC showed the formation of
one major yellow band. This compound was isolated
and purified by flash chromatography. Analysis of the
amorphous, yellow solid showed it to be the desired prod-
uct, 17. The ESMS produced a series of peaks centered
around 726 m/z, which match the peaks expected for
the M + Na ion for 17. The 1H NMR spectrum also pos-
sessed all the resonances for the hydrogens expected in
17, including the alkyne hydrogens at 11 ppm and the
N-methyl hydrogens from the dithiocarbamates located
in the 3.0–3.5 ppm region, see Scheme 8.
Because there are two stereochemical orientations
available to the dithiocarbamate ligands, and because
the two alkyne ligands can be syn or anti with respect
to each other, and because there are two ways to orient
the peptide around the tungsten, like 16 there are eight
possible isomers open to metallacyclicpeptide 17. The
possible isomers of 17 are shown in Scheme 9. Four of
these isomers have one of the dithiocarbamate stereo-
chemistries (Q, R, S and T), while the other four isomers
have the opposite dithiocarbamate stereochemistry (U,
V, W and X). Four of the isomers have the two alkynes
2.5. Variable temperature NMR
To explore the conformational flexibililty of 16 and
1
17, their H NMR spectra were recorded in d6-DMSO
at temperatures ranging from 21 to 90 ꢀC. Recording
the spectra in d6-DMSO, an aggressive hydrogen bond
acceptor solvent, should break up any intramolecular
hydrogen bonding interactions, if there are any, in 16
and 17. Based on the literature of bis(alkyne) complexes
of molybdenum and tungsten [5a] raising the tempera-
ture should allow for interconversion between the iso-
mers of 16 and 17 and a simplification of their NMR
spectra.
The 1H NMR spectra of 16 and 17 in d6-DMSO at 21
ꢀC are nearly identical to their spectra in CDCl3 at 21
ꢀC. The spectra in d6-DMSO show multiple peaks for
each resonance. Also, the chemical shifts of each reso-
nance are nearly identical in both d6-DMSO and CDCl3.
This indicates that the solution structures of 16 and 17
are essentially the same in both solvents and do not pos-
sess any intramolecular hydrogen bonds.
S
S
S
O
Me
O
W
S
H
N
W(CO)3(dmtc)2
S
=
Me2NCS2 = dmtc
O
N
H
N
H
S
O
NH
MeOH, ∆
O
Me
Me
High Dilution
O
NH
O
21% Yield
NH
3
Me
O
17
Scheme 8.

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T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
O
O
O
Me
H
O
O
N
HN
O
O
H
N
HN
S
W
S
Me
O
O
HN
S
S
S
W
S
HN
S
W
S
S
S
HN
S
W
S
HN
O
S
S
O
S
S
HN
H
N
Me
O
HN
O
O
O
O
H
N
Me
Q
R
U
V
Me
Me
O
O
O
S
W
S
O
N
H
O
S
S
S
O
N
S
S
O
H
NH
Me
W
S
O
NH
O
NH
Me
O
NH
H
S
W
S
N
H
N
S
W
S
S
S
S
S
HN
O
N
H
O
HN
O
N
O
H
O
O
S
T
W
X
Scheme 9.
Fig. 2. (a) The region between 10.7 and 11.7 ppm in the 1H NMR spectrum of 17 in CDCl3. The peaks in this region arise from the terminal alkyne
hydrogens. (b) The region between 2.8 and 3.8 ppm in the 1H NMR spectrum of 17 in CDCl3. The peaks in this region arise from the N-methyl
hydrogens on the dmtc groups.

T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
4845
The behavior of 16 and 17 in d6-DMSO differ as the
temperature is raised. With 17, the spectrum simplifies,
with the alkyne hydrogens coalescing to one singlet
somewhere between 60 and 70 ꢀC. Coalescence in this
temperature range is similar to the coalescence tempera-
tures observed for acyclic bis(alkyne) complexes of
molybdenum [5a]. In contrast, the spectrum of 16 does
not show any simplification, even when the temperature
of the solution is raised to 90 ꢀC; in addition, heating of
16 to 90 ꢀC leads to significant decomposition of the
sample. These results show that the smaller ring in 16
is tightly confined and is unable to easily switch from
one conformation to another. This indicates that the iso-
mers formed in the synthesis of 16 are kinetic products.
A Dreiding model of 2 in which the two alkynes are held
Cambridge Isotope Labs. Silica gel for flash chromatog-
raphy was purchased from Silicycle. NMR spectra were
obtained on a GE Omega 300 instrument. Electrospray
mass spectra were obtained on a LCQ APCI/Electro-
spray LC MS-MS. Samples for mass spectral analysis
were dissolved in MeOH (approximately 1 mg/mL) in
borosilicate glass test tubes.
3.2. Preparation of 1
To a stirred suspension of 300 mg (3.28 mmol, 1.0
equiv.) of propargylamine hydrochloride in 30 mL
CH2Cl2 was added 2.17 mL (13.1 mmol, 4.0 equiv.) of
DIEA. Once all the propargylamine hydrochloride dis-
solved the solution was treated with 0.257 mL (3.11
mmol, 1.0 equiv.) of propargylchlorformate. After stir-
ring at 23 ꢀC for 1 h the solvents were evaporated and
the residue redissolved in 30 mL EtOAc, transferred to
a separatory funnel and washed: 3 · 25 mL 1 M HCl,
3 · 25 mL saturated NaHCO3 and 1 · 25 mL brine.
The EtOAc layer was dried (MgSO4), filtered and evap-
orated to yield 254 mg (64%) of pure 1 as a white solid:
˚
parallel to each other and spaced 6.5 A apart does show
that the molecule has little or no conformational free-
dom. With the larger ring found in 17 comes more con-
formational freedom. A Dreiding model of 3 in which
the two alkynes are held parallel to each other and
˚
spaced 6.5 A apart shows that this molecule will have
more freedom of motion when complexed to the tung-
1
sten. This is the likely reason why the peaks in the H
1
m.p. 35–38 ꢀC; TLC, Rf 0.55 (1:1 EtOAc/hexanes); H
NMR spectrum of 17 in d6-DMSO coalesce between
60 and 70 ꢀC, but the same peaks in 16 do not.
NMR (CDCl3) d 5.40 (1H, br s), 4.72 (2H, d, J = 2.4
Hz), 4.00 (2H, dd, J = 2.4, 5.4 Hz), 2.53 (1H, t, J = 2.4
Hz), 2.28 (1H, t, J = 2.4 Hz).
2.6. Summary
3.3. Attempted cyclization of 1; preparation of 13, 14 and
15
The results from this study show that metallacyclic-
peptides can be obtained by reaction of W(CO)3(dmtc)2
with a peptide bearing alkyne groups at both the N- and
C-termini. Limitations on the ability to cyclize reside in
dialkynes that are shorter than a dialkynylamino acid.
The alkyne group can readily be appended at the N-ter-
minus by reaction of the peptide with propargylchloro-
formate, while an alkyne group can be readily
appended at the C-terminus by acylation of the carboxyl
group with propargylamine hydrochloride using a car-
bodiimide coupling reagent. The metallacyclicpeptides
studied here (16 and 17) both assume multiple confor-
mations in solution. The two metallacyclicpeptides differ
in their ability to interconvert between conformational
isomers at higher temperatures; 17 can interconvert be-
tween the isomers available to it, while 16 appears una-
ble to interconvert.
A 1000 mL, 3-neck, round-bottom flask equipped
with a stir bar, an addition funnel and a reflux condenser
was placed under an N2 atmosphere. To the flask was
added a degassed solution of 1 in 450 mL MeOH. To
the addition funnel was added a degassed solution of
W(CO)3(dmtc)2 [8] in 75 mL CH2Cl2. The MeOH solu-
tion was brought to reflux and the W(CO)3(dmtc)2 solu-
tion was added to the flask dropwise over the course of 1
h. After 2 h at reflux the solution had turned a light yel-
low color. Heating was halted and the solvents evapo-
rated. TLC analysis of the crude product showed three
major yellow bands. The crude product was subjected
to flash chromatography (2 cm silica gel column, 10
mL fractions, 3:2 EtOAc/hexanes eluant). After 60 frac-
tions had been collected the eluant was changed to pure
EtOAc, and an additional 80 fractions were collected.
From these fractions 13 (fractions 41–55, 61 mg, 15%),
14 (fractions 119–126, 28 mg, 7%) and 15 (fractions
81–88, 7 mg, 2%) were isolated pure.
3. Experimental
3.1. General procedures
13: amorphous yellow solid; TLC, Rf 0.45 (1:1
EtOAc/hexanes); ESMS, M + Na ion theoretical isotope
pattern calculated for WC20H26O4N4S4Na [9]: 717
(0.3%), 718 (0.1%), 719 (65.7%), 720 (53.0%), 721
(100%), 722 (31.7%), 723 (90.6%), 724 (23.8%), 725
(17.6%), 726 (4.1%), 727 (1.6%), 728 (0.3%), 729
(0.1%). Found: 717 (1.5%), 718 (1.1%), 719 (72.9%),
Boc–Ala–OSu and Boc–Ala–Ala–OH were pur-
chased from Bachem. Propargylchloroformate and
propargylamine hydrochloride were purchased from
Aldrich. DIEA and EDC were purchased from Acros
Organics. CDCl3 and d6-DMSO were purchased from

4846
T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
720 (61.1%), 721 (100%), 722 (33.9%), 723 (89.6%), 724
(21.5%), 725 (19.1%), 726 (4.3%), 727 (1.7%), 728
(1.3%).
mmol, 15 equiv.) DIEA and 2.0 mL (24 mmol, 8 equiv.)
of propargylchloroformate. After 18 h the solvents were
evaporated and the residue redissolved in 50 mL EtOAc,
transferred to a separatory funnel and washed: 3 · 50
mL 1 M HCl, 3 · 50 mL saturated NaHCO3 and
1 · 50 mL brine. The EtOAc layer was dried (MgSO4),
filtered and evaporated to yield the crude product.
TLC (EtOAc) showed the crude product to be impure,
so it was subjected to flash chromatography (3 cm silica
gel column, 20 mL fractions, eluant 2:1 hexanes/EtOAc).
Evaporation of fractions 31–44 provided 428 mg (67%)
14: amorphous yellow solid; TLC, Rf 0.12 (1:1
EtOAc/hexanes); ESMS, M + Na ion theoretical isotope
pattern calculated for W2C26H38O4N6S8Na [9]: 1139
(0.2%), 1140 (0.1%), 1141 (19.1%), 1142 (27.4%), 1143
(64.8%), 1144 (53.3%), 1145 (100.0%), 1146 (65.6%),
1147 (91.5%), 1148 (39.8%), 1149 (51.9%), 1150
(18.7%), 1151 (14.8%), 1152 (4.7%), 1153 (2.5%), 1154
(0.7%), 1155 (0.3%), 1156 (0.1%). Found: 1139 (0.8%),
1140 (2.1%), 1141 (25.5%), 1142 (29.4%), 1143 (67.0%),
1144 (55.2%), 1145 (100.0%), 1146 (64.9%), 1147
(86.7%), 1148 (42.4%), 1149 (49.6%), 1150 (15.9%),
1151 (14.0%), 1152 (2.6%), 1153 (1.1%), 1154 (0.5%),
1155 (0.4%), 1156 (1.8%).
1
of pure 2 as a white solid: TLC, Rf 0.74 (EtOAc); H
NMR (CDCl3) d 6.58 (1H, br s), 5.58 (1H, br s), 4.76
(2H, s), 4.31 (1H, m), 4.09 (2H, dd, J = 7.4, 2.4 Hz),
2.54 (1H, t, J = 2.4 Hz), 2.28 (1H, t, J = 2.4 Hz), 1.45
(3H, d, J = 6.8 Hz).
15: amorphous yellow solid; TLC, Rf 0.33 (1:1
EtOAc/hexanes); ESMS, M + Na ion theoretical isotope
pattern calculated for W2C33H45O6N7S8Na [9]:1276
(0.2%), 1277 (0.1%), 1278 (18.2%), 1279 (27.6%), 1280
(64.1%), 1281 (56.1%), 1282 (100.0%), 1283 (70.8%),
1284 (93.1%), 1285 (45.6%), 1286 (53.3%), 1287
(22.2%), 1288 (15.9%), 1289 (5.8%), 1290 (2.8%), 1291
(0.9%), 1292 (0.3%), 1293 (0.1%); Found: 1276 (2.3%),
1277 (2.0%), 1278 (19.6%), 1279 (29.0%), 1280 (64.3%),
1281 (56.9%), 1282 (100.0%), 1283 (67.1%), 1284
(89.3%), 1285 (41.3%), 1286 (48.0%), 1287 (19.1%),
1288 (15.1%), 1289 (4.9%), 1290 (2.6%), 1291 (1.4%),
1292 (1.3%), 1293 (0.8%).
3.6. Cyclization of 2; preparation of 16
A 500 mL, three-neck, round-bottom flask equipped
with a stir bar, an addition funnel and a reflux condenser
was placed under an N2 atmosphere. To the flask was
added a degassed solution of 2 in 270 mL MeOH. To
the addition funnel was added a degassed solution of
W(CO)3(dmtc)2 [8] in 32 mL CH2Cl2. The MeOH solu-
tion was brought to reflux and the W(CO)3(dmtc)2 solu-
tion was added to the flask dropwise over the course of 1
h. After 18 h at reflux the solution had turned a light yel-
low color. Reflux was halted and the solvents evapo-
rated. TLC analysis of the crude product showed one
major yellow band. The crude product was subjected
to flash chromatography (2 cm silica gel column, 10
mL fractions, EtOAc eluant). The desired product, 16,
eluted pure in fractions 29–38. These fractions were
pooled and evaporated to yield 73 mg (29%) of pure
16 as an amorphous, yellow solid: TLC, Rf 0.20
3.4. Preparation of 7
To a suspension of 0.538 g (5.88 mmol, 1.6 equiv.) of
propargylamine hydrochloride in 35 mL CH2Cl2 was
added 8.0 mL (46 mmol, 12 equiv.) of DIEA. After stir-
ring for 5 min the solution was treated with 1.08 g (3.77
mmol, 1.0 equiv.) of Boc–Ala–OSu (6). After 18 h the
solvents were evaporated and the residue redissolved
in 50 mL EtOAc, transferred to a separatory funnel
and washed: 3 · 50 mL 1 M HCl, 3 · 50 mL saturated
NaHCO3, and 1 · 50 mL brine. The EtOAc layer was
dried (MgSO4), filtered, and evaporated to yield 717
mg (81%) of pure 7 as a white solid: TLC, Rf 0.66 (1:2
1
(EtOAc); H NMR d 11.3–10.9 (2H, m), 8.4–8.2, 7.5–
7.3, 6.7–6.5, 6.0–5.7, 5.4–4.9 (4H, m), 4.4–3.9 (3H, m),
3.5–3.3 (6H, m), 3.2–3.0 (6H, m), 1.5–1.3 (3H, m);
ESMS: M + Na theoretical isotope pattern for
WC16H24N4S4O3Na [9]: 651 (0.3%), 652 (0.1%), 653
(67.3%), 654 (51.4%), 655 (100.0%), 656 (27.9%), 657
(91.3%), 658 (20.3%), 659 (16.8%), 660 (3.4%), 661
(1.4%), 662 (0.2%), 663 (0.1%); Found: 651 (0.6%), 652
(1.3%), 653 (75.6%), 654 (53.5%), 655 (100.0%), 656
(25.0%), 657 (93.1%), 658 (18.0%), 659 (13.9%), 660
(3.0%), 661 (2.4%), 662 (0.8%), 663 (0.1%).
1
EtOAc/hexanes); H NMR (CDCl3) d 6.43 (1H, br s),
4.92 (1H, br s), 4.16 (1H, m), 4.05 (2H, m), 2.23 (1H,
t, J = 2.4 Hz), 1.46 (9H, s), 1.37 (3H, d, J = 6.8 Hz).
3.5. Preparation of 2
3.7. Preparation of 10
A solution of 717 mg (3.06 mmol, 1.0 equiv.) of 7 dis-
solved in 8 mL CH2Cl2 and chilled to 0 ꢀC was added 0.5
mL anisole and 8 mL CF3CO2H. The resulting solution
stirred for 1 h while warming to 23 ꢀC. The solvents
were evaporated in vacuo. The clear oil that remained
(the trifluoroacetate salt of 8) was redissolved in 30
mL CH2Cl2 and treated sequentially with 8.0 mL (46
Four hundred and fifty-eight milligrams (1.76 mmol,
1.0 equiv.) of Boc–Ala–Ala–OH (9) and 71 mg (1.78
mmol, 1.0 equiv.) of propargylamine hydrochloride
was suspended in 17 mL CH2Cl2. To the suspension
was then added 0.178 mL (1.08 mmol, 1.40 equiv.) of
DIEA and and 152 mg (0.791 mmol, 1.03 equiv.) of

T.P. Curran et al. / Journal of Organometallic Chemistry 689 (2004) 4837–4847
4847
EDC. After stirring for 18 h at 23 ꢀC the solvents were
evaporated and the residue redissolved in 50 mL EtOAc.
The EtOAc solution was transferred to a separatory fun-
nel and washed: 3 · 50 mL 1 M HCl, 3 · 50 mL satu-
rated NaHCO3, and 1 · 50 mL brine. The EtOAc was
dried (MgSO4), filtered, and evaporated to yield 141
mg (27%) of pure 10 as a white solid: TLC, Rf 0.31
tion had turned a light yellow color. Reflux was halted
and the solvents evaporated. TLC analysis of the crude
product showed one major yellow band. The crude
product was subjected to flash chromatography (2 cm
silica gel column, 10 mL fractions, EtOAc eluant). The
desired product, 17, eluted pure in fractions 26–43.
These fractions were pooled and evaporated to yield
28 mg (21%) of pure 17 as an amorphous, yellow solid:
1
(1:1 EtOAc/hexane); H NMR d 7.18 (1H, br s), 6.78
1
(1H, br s), 5.17 (1H, d, J = 5.9 Hz), 4.57 (1H, m),
4.20–3.90 (5H, m), 2.21 (1H, t, J = 2.4 Hz), 1.48 (9H,
s), 1.44 (3H, d, J = 6.8 Hz), 1.40 (3H, d, J = 7.3 Hz).
TLC, Rf 0.18 (EtOAc); H NMR 11.7–10.9 (2H, m),
8.0–3.8 (9H, m), 3.6–3.3 (6H, m), 3.2–3.0 (6H, m), 1.5–
1.2 (6H, m); ESMS M + Na theoretical isotope pattern
[9]: 722 (0.3%), 723 (0.1%), 724 (65.9%), 725 (52.8%),
726 (100.0%), 727 (31.1%), 728 (90.8%), 729 (23.3%),
730 (17.5%), 731 (4.0%), 732 (1.6%), 733 (0.3%), 734
(0.1%); Found: 722 (0.8%), 723 (0.9%), 724 (73.4%),
725 (55.4%), 726 (100.0%), 727 (31.4%), 728 (92.2%),
729 (19.2%), 730 (16.9%), 731 (4.3%), 732 (2.1%), 733
(0.4%), 734 (0.9%).
3.8. Preparation of 3
A solution of 141 mg (0.475 mmol, 1.0 equiv.) of 10
dissolved in 5 mL CH2Cl2 and chilled to 0 ꢀC was added
0.5 mL anisole and 5 mL CF3CO2H. The resulting solu-
tion stirred for 1 h while warming to 23 ꢀC. The solvents
were evaporated in vacuo. The clear oil that remained
(the trifluoroacetate salt of 11) was redissolved in 10
mL CH2Cl2 and treated sequentially with 1.26 mL
(7.24 mmol, 15 equiv.) DIEA and 0.313 mL (3.21 mmol,
6.8 equiv.) of propargylchloroformate. After 18 h the
solvents were evaporated and the residue redissolved
in 50 mL EtOAc, transferred to a separatory funnel
and washed: 3 · 50 mL 1M HCl, 3 · 50 mL saturated
NaHCO3, and 1 · 50 mL brine. The EtOAc layer was
dried (MgSO4), filtered, and evaporated to yield the
crude product. TLC (EtOAc) showed the crude product
to be impure, so it was subjected to flash chromatogra-
phy (2 cm silica gel column, 10 mL fractions, eluant 2:1
EtOAc/hexanes). Evaporation of fractions 39–46 pro-
vided 41 mg (31%) of pure 3 as a white solid: TLC,
Acknowledgments
This work was supported by a grant from the Na-
tional Science Foundation (NSF-RUI CHE-0305325),
and from a Student Research Assistant Grant from
Trinity College. We thank Profs. Nils Metzler-Nolte
and Margaret Harding for their helpful and insightful
questions at the Second International Symposium on
Bioorganometallic Chemistry, Zurich, Switzerland, July,
2004.
References
1
Rf0.24 (2:1 EtOAc/hexanes); H NMR (CDCl3) d 6.79
(1H, br s), 6.60 (1H, m), 5.49 (1H, d, J = 6.3 Hz), 4.8–
4.6 (2H, m), 4.49 (1H, pentet, J = 6.8 Hz), 4.3–3.9
(3H, m), 2.49 (1H, t, J = 2.4 Hz), 2.24 (1H, t, J = 2.4
Hz), 1.41 (6H, d, J = 6.8 Hz).
[1] K. Severin, R. Bergs, W. Beck, Angew. Chem., Int. Ed. Engl. 37
(1998) 1635.
[2] J.L. Templeton, Adv. Organomet. Chem. 29 (1989) 1.
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Org. Lett. 4 (2002) 2917.
[4] J.L. Templeton, R.S. Herrick, J.R. Morrow, Organometallics 3
(1984) 535.
3.9. Cyclization of 3; preparation of 17
[5] (a) R.S. Herrick, J.L. Templeton, Organometallics
842;
1 (1982)
A 500 mL, three-neck, round-bottom flask equipped
with a stir bar, an addition funnel and a reflux condenser
was placed under an N2 atmosphere. To the flask was
added a degassed solution of 53 mg (0.19 mmol, 1.0
equiv.) of 3 in 150 mL MeOH. To the addition funnel
was added a degassed solution of 97 mg (0.19 mmol,
1.0 equiv.) of W(CO)3(dmtc)2 [8] in 15 mL CH2Cl2.
The MeOH solution was brought to reflux and the
W(CO)3(dmtc)2 solution was added to the flask drop-
wise over the course of 1 h. After 18 h at reflux the solu-
(b) J.R. Morrow, T.L. Tonker, J.L. Templeton, J. Am. Chem. Soc.
107 (1985) 5004.
[6] (a) W. Henderson, B.K. Nicholson, L.J. McCaffrey, Polyhedron 17
(1998) 4291;
(b) J.C. Traeger, Int. J. Mass Spectrom. 200 (2000) 387.
[7] X. Hua, A. von Zelewsky, Inorg. Chem. 34 (1995) 5791.
[8] S.J.N. Burgmayer, J.L. Templeton, Inorg. Chem. 24 (1985)
2224.
[9] Theoretical isotope patterns were calculated using a program
available at a website provided by the University of Sheffield.
Available from: http://www.shef.ac.uk/chemistry/chemputer/.