Generation of reactivity from typically stable ligands: C-C bond-forming reductive elimination from aryl palladium(II) complexes of malonate anions

Article abstract of DOI:10.1002/1521-3773(20021115)41:22<4331::AID-ANIE4331>3.0.CO;2-J

Steric hindrance of the ancillary phosphane ligand triggers reductive elimination from aryl palladium complexes of typically unreactive ligands derived from 1,3-dicarbonyl anions [Eq. (1); FcPtBu₂ = di-tert-butylphosphanylferrocene]. This react

Full text of DOI:10.1002/1521-3773(20021115)41:22<4331::AID-ANIE4331>3.0.CO;2-J

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111, 3576; Angew. Chem. Int. Ed. 1999, 38, 3365; f) D.H.Camacho, I.
Nakamura, S.Saito, Y.Yamamoto, J. Org. Chem. 2001, 66, 270.
Monodisperse Surface Micelles of Nonpolar
Amphiphiles in Langmuir Monolayers**
[4] a) T.Mitsudo, Y.Hori, Y.Yamakawa, Y.Watanabe,
J. Org. Chem.
1987, 52, 2230; b) DM. T. .Chan, TB. .Marder, D.Milstein, N.J,
Taylor, J. Am. Chem. Soc. 1987, 109, 6385; c) B.M.Trost, W.Brieden,
K.H.Baringhaus, Angew. Chem. 1992, 104, 1392; Angew. Chem. Int.
Ed. Engl. 1992, 31, 1335; d) H.Doucet, B.Martin-Vaca, C.Bruneau,
P.H. Dixneuf, J. Org. Chem. 1995, 60, 7247; e) M.Al-Masum, Y.
Yamamoto, J. Am. Chem. Soc. 1998, 120, 3809.
Mounir Maaloum, Pierre Muller, and
Marie Pierre Krafft*
There are several examples of surface micelles (hemi-
micelles) formed by self-assembly of small molecules and
macromolecules that are adsorbed on solid surfaces and in
equilibrium with aqueous solutions.[1] On the other hand,
reports on surface micelles in Langmuir monolayers are
essentially limited to copolymers.[2] Although their existence
has long been predicted,[3,4] so far only one example of surface
micelles made from small amphiphilic molecules exists.[5]
These surface micelles were made from strongly polar
surfactants.Furthermore no rule for predicting the size of
the hemimicelles has been found.We report here on novel
surface micelles made of a nonpolar amphiphile, namely, the
semifluorinated alkane C8F17C16H33 (F8H16), their structure,
and a model that accounts for their size.
Since the pioneering work of Gaines demonstrated that
semifluorinated alkanes CnFnþ1CmH2mþ1 (FnHm diblocks)
form Langmuir monolayers,[6] their structure has remained
controversial.A primary issue was the orientation of the
FnHm molecules at the air water interface.Grazing-inci-
dence X-ray diffraction (GIXD) and X-ray reflectivity
(GIXR) studies on F12H18 concluded that the most probable
arrangement was a monolayer in which the Hm segments are
in contact with water and the Fn segments extend upwards
from the surface.[7] However, a bilayer structure in which the
diblocks are antiparallel, with tilted F8 segments outside and
interleaved H18 segments inside, was recently proposed on
the basis of X-ray reflectivity measurements.[8] In the bulk,
FnHm molecules crystallize in a large number of different
[5] Review: a) T.Kondo, T.Mitsudo, Chem. Rev. 2000, 100, 3205; for
hydrothiolation of alkynes, b) A.Ogawa, T.Ikeda, K.Kimura, T.
Hirao, J. Am. Chem. Soc. 1999, 121, 5108.
À
[6] Addition of P H bonds to alkynes: a) L.-B. Han, R. Hua, M. Tanaka,
Angew. Chem. 1998, 110, 98; Angew. Chem. Int. Ed. 1998, 37, 94;
b) M.A.Kazankova, I.V.Efimova, A.N.Kochetkov, V.V.Afanas
’ev,
I.P.Beletskaya, P.H.Dixneuf, Synlett 2001, 497.
[7] Review: a) I.Beletskaya, A.Pelter,
Tetrahedron 1997, 53, 4957;
b) ™Metal-catalyzed Hydroboration Reactions∫: G.C.Fu in Transition
Metals for OrganicSynthesis (Eds.: M. Beller, C. Bolm), Wiley-VCH,
Weinheim, 1998, pp.141 146.
[8] Review: a) K.A. Horn, Chem. Rev. 1995, 95, 1317; b) ™Hydrosilyl-
ation of Olefins∫: K.Yamamoto, T.Hayashi in Transition Metals for
OrganicSynthesis (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim,
1998, pp.120 129.
[9] Review: N.D.Smith, J.Mancuso, M.Lautens,
3257.
Chem. Rev. 2000, 100,
[10] Diboration of alkynes: a) T.Ishiyama, N.Matsuda, N.Miyaura, A.
Suzuki, J. Am. Chem. Soc. 1993, 115, 11018; b) T.Ishiyama, N.
Matsuda, M.Murata, F.Ozawa, A.Suzuki, N.Miyaura,
metallics 1996, 15, 713; c) G.Lesley, P.Nguyen, N.J.Taylor, T.B.
Marder, A.J.Scott, W.Clegg, N.C.Norman, Organometallics 1996,
Organo-
15, 5137; d) C.N.Iverson, M.R.Smith III, Organometallics 1996, 15,
5155.
[11] Review: M.Suginome, Y.Ito, Chem. Rev. 2000, 100, 3221.
[12] Silaboration of alkynes, a) M.Suginome, H.Nakamura, Y.Ito, Chem.
Commun. 1996, 2777; b) M.Suginome, T.Matsuda, H.Nakamura, Y.
Ito, Tetrahedron 1999, 55, 8787.
[13] Disilylation of alkynes: a) H.Sakurai, Y.Kamiyama, Y.Nakadaira, J.
Am. Chem. Soc. 1975, 97, 931; b) F.Ozawa, M.Sugawara, T.Hayashi,
Organometallics 1994, 13, 3237.
[14] Silastannylation of alkynes: M.Hada, Y.Tanaka, M.Ito, M.
Murakami, H.Amii, Y.Ito, H.Nakatsuji,
J. Am. Chem. Soc. 1994,
stable smectic phases, depending on temperature and on n and
116, 8754.
11]
m block lengths.[9
FnHm were instrumental in allowing
[15] E.Piers, R.T.Skerlj, J. Chem. Soc. Chem. Commun. 1986, 626.
[16] a) H.Nakamura, M.Sekido, M.Ito, Y.Yamamoto, J. Am. Chem. Soc.
1998, 120, 6838; b) N.Moneiro, G.Balme, Synlett 1998, 746; c) A.
F¸rstner, H.Szillat, F.Stelzer, J. Am. Chem. Soc. 2000, 122, 6785.
[17] Catalytic skeletal rearrangement: a) T.J.Katz, T.M.Sivavec, J. Am.
Chem. Soc. 1985, 107, 737; b) A.Kinoshita, M.Mori, Synlett 1994,
1020; c) B.M.Trost, G.J.Tanoury, J. Am. Chem. Soc. 1988, 110, 1636;
d) B.M.Trost, M.K.Trost, J. Am. Chem. Soc. 1991, 113, 1850; e) B.M.
Trost, A.S.K.Hashmi, Angew. Chem. 1993, 105, 1130; Angew. Chem.
Int. Ed. 1993, 32, 1085; f) N.Chatani, T.Morimoto, T.Muto, S.Murai,
J. Am. Chem. Soc. 1994, 116, 6049; g) N.Chatani, N.Furukawa, H.
Sakurai, S.Murai, Organometallics 1996, 15, 901; h) N.Chatani, K.
Kataoka, S.Murai, N.Furukawa, Y.Seki, J. Am. Chem. Soc. 1998, 120,
9104; i) ref.[16c].
reversible vertical phase separation from phospholipids upon
compression of Langmuir monolayers.[12] They allowed sub-
stantial stabilization of fluorocarbon-in-water emulsions and
control over particle size.[13,14]
F8H16[15] was thoroughly purified by column chromatog-
raphy, and its purity (> 99%) determined by GC, TLC, NMR
spectroscopy, and elemental analysis.Monolayers were
spread from a 1 mm solution of F8H16 in chloroform onto
pure water from a milli-Q system.Surface pressure Ps
(Wilhelmy plate method) versus molecular area isotherms
were recorded at 22.0 Æ 0.58C on a Langmuir trough (Riegler
& Kirstein, Germany) equipped with two movable barriers.
F8H16 formed a monolayer that remained stable up to
about 8 mNmÀ1 (Figure 1) with a limiting area of about 30 ä2
that corresponds to the cross section of a perfluorinated chain,
which is larger than that of a typical hydrocarbon chain (ca.
[18] a) W.A.Herrmann, C.-P.Reisinger, M.Spiegler, J. Organomet. Chem.
1998, 557, 93; b) K.L. Greenman, D.S. Carter, D.L. Van Vranken,
Tetrahedron 2001, 57, 5219; c) K.Miki, F.Nishino, K.Ohe, U.Sakae, J.
Am. Chem. Soc. 2002, 124, 5260.
[19] B.H.Lipshutz, D.Pollart, J.Monforte, H.Kotsuki,
Tetrahedron Lett.
1985, 26, 705.
[20] T.Soga, H.Takenoshita, M.Yamada, T.Mukaiyama, Bull. Chem. Soc.
Jpn. 1990, 63, 3122.
[*] Dr.M.P.Krafft, Dr.M.Maaloum, Dr.P.Muller
Institut Charles Sadron (CNRS)
[21] a) T.Kumamoto, N.Tabe, K.Yamaguchi, H.Yagishita, H.Iwasa, T.
Ishikawa, Tetrahedron 2001, 57, 2717; b) V.M.Marathias, P.H.Bolton,
Biochemistry 1999, 38, 4355; c) W.M.Clark, A.M.Ticker-Eldridge,
G.Kris Huang, L.N.Pridgen, M.A.Olsen, R.J.Mills, I.Lantos, N.H.
Baine, J. Am. Chem. Soc. 1998, 120, 4550.
6, rue Boussingault, 67083 Strasbourg Cedex (France)
Fax : (þ 33)3-88-41-4099
E-mail: krafft@ics.u-strasbg.fr
[**] This work was supported by the CNRS.We thank AtoFina (Pierre-
Bÿnite, France) for the gift of fluorinated precursors.
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(0.5 and 7 mNmÀ1) investigated, the surface micelles have a
characteristic diameter of 300 ä.The size distribution of the
micelles was highly monodisperse.The average height H of a
micelle was assessed by a series of AFM line scans to be 23 Æ
5 ä. At 7 mNmÀ1, the surface micelles pack in an hexagonal
arrangement, as assessed by the position of the Bragg peaks
(see the two rings in Figure 2b).
When surface pressure was increased from 0 to 7 mNmÀ1,
the density of the micelles increased progressively.However,
the size of the micelles was independent of surface density.
When the pressure was further increased to 9 mNmÀ1, the
micelle arrangement became even more compact, but no
coalescence between domains was observed.
The F8H16 monolayers were also analyzed by X-ray
specular reflectivity.Figure 3 shows X-ray data obtained from
a monolayer compressed and transferred at 7 mNmÀ1.The
Figure 1.Isotherm of surface pressure Ps versus molecular area A,
obtained by compressing F8H16 at an air water interface at 228C.The
two arrows indicate the surface pressures of transfer for AFM imaging
(Figure 2).
20 ä2).[16] Because the van der Waals radius of fluorine is
À
larger than half the C C bond length, steric hindrance causes
the chain to adopt a (15/7) helical conformation, which
renders the carbon backbone rigid.[17] The isotherm was
reversible, without hysteresis, in compression/expansion cy-
cles.The monolayers were transferred onto a silicon wafer,
previously treated with a ™piranha solution∫ (conc.H SO4/
2
30% H2O2 3/1) by using the Langmuir Blodgett technique.
The transferred films were analyzed with an atomic force
microscope (AFM, NanoScopeIII) in tapping mode with a
nitride cantilever.All scans were performed in the repulsive
regime.
Figure 2a shows an AFM image of the F8H16 monolayer
transferred at 7 mNmÀ1 onto a silicon wafer.It shows the
presence of two-dimensional micelles that form a nanoscale
structure on the surface.The AFM images of these surface
micelles were analyzed quantitatively by two-dimensional
Fourier analysis (Figure 2b).Diffusion rings were observed
that originate from hard-core repulsions.For both pressures
Figure 3.Experimental curve of X-ray reflectivity obtained from the
monolayer of F8H16 transferred to a silicon wafer at 7 mNmÀ1.The dotted
curve is the fit obtained with the model in Figure 4a.
experimental curve was fitted to single- and two-layer models
with Parratt 32 software.The single-layer model gave poor
results, because the corresponding fit always had its second
minimum at three times the first minimum, which is clearly
not the case in the experimental curve.A two-layer model
provided good agreement with the experimental data.
The electron density distribution of the two-layer model
(fitted curve in Figure 3) is given in Figure 4a.It consists of an
upper layer (in contact with air) with a thickness of 10.0 ä and
an electron density of 487 enmÀ3, and a lower layer with a
thickness of 19.3 ä and an electron density of 290 enmÀ3.The
total thickness is 29.3 ä. It is noteworthy that the height of the
micelle, as measured by X-ray reflectivity, is in close agree-
ment with the length lC of a fully extended F8H16 molecule, as
calculated[18,19] from lC ¼ 1.3 n þ 1.265m þ 2.58 ¼ 33.2 ä for
n ¼ 8 and m ¼ 16).Integration over the whole layer gives a
total electron density of D ¼ 1050 enmÀ2.F8H16 consists of
an F8 segment bearing 201 electrons and an H16 segment with
129 electrons.The mean area per molecule is obtained by
dividing the total number of electrons of a molecule by D,
which gives a molecular area of 31.5 ä2, which is the value
expected if the density is preserved during transfer onto the
wafer.However, note that the proportion of electrons in the
upper layer is only 46%, while a simple model in which the
fluorinated segments of all molecules point towards air would
give an upper layer representing 61% of the total.Likewise,
Figure 2. a) AFM image (0.5 î 0.5 mm) of surface micelles in a F8H16
monolayer transferred to a silicon wafer at 7 mNmÀ1.b) Two-dimensional
Fourier transform of (a), showing two rings.
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ing the volumes of the two layers.If we set the thickness of the
hydrocarbon layer to the measured X-ray value, the mean
electron density of the internal hydrocarbon layer becomes a
function of the radius R of the micelle.An estimation of the
density of the hydrocarbon part can be obtained by using the
van der Waals volumes of CH3 and CH2 [Eq.(1)] which leads
to an electron density of 282 enmÀ3.For the disklike model we
find R ¼ 13.0 nm, while the ellipsoid model gives R ¼ 15.5 nm.
Varying the density by Æ 5% leads to variation of R from 11.5
to 14.8 nm and from 13.5 to 17.8 nm, respectively. This shows
that closing of the domains to reduce the density mismatch
provides a reasonable explanation for the observed micelle
size.
ꢀ3
V ¼ VCH þ 15 VCH ¼ 54:3 þ 15 Â 26:9 ¼ 457:8 A
ð1Þ
3
2
We then calculated the theoretical electron density profiles
for the two models (Figure 4b).Only the disklike model
shows an upper region of higher electron density.Moreover,
we can also understand the apparent electron deficiency of
the upper layer by comparing the density profile of the
disklike model to that of a homogeneous layer.The hydro-
carbon volume fraction fH of the disklike surface micelle can
be calculated by using Equation (2) where VH is the hydro-
carbon volume of the surface micelle, VF þ VH is the total
volume, HH and HF are the lengths of the fluorinated and
hydrocarbon segments of the semifluorinated alkane mole-
cule, and R is the radius of the surface micelle.
Figure 4.a) Variation of the electron density D as a function of height Z
corresponding to the fit of the experimental reflectivity curve (Figure 3).
This clearly shows that the first layer is less dense than the second.
b) Theoretical electron density profiles (D) for the disklike model (full
line) and ellipsoidal model (dashed line) depicted in Figure 5.The density
of a homogeneous layer is represented by a dotted line.
the density of the lower layer is larger than that expected for a
alkane hydrocarbon layer.
VH
¼
ꢀH
VH þ VF
ꢀ
ꢁ
Given the AFM and X-ray data, we tested two models for
the shape of the micelles.As the domains are roughly circular,
we assumed a circular shape.The X-ray data suggest that the
micelle height is close to the length of an extended molecule,
but a homogeneous layer of extended molecules whose
fluorinated parts point towards air would lead to an unrealisti-
cally low density for the hydrocarbon part.Bending the
molecules towards the surface at the edge of the domain can
substantially reduce this mismatch.The two models that we
considered are a disklike shape with quarter circles at the
borders and an ellipsoidal shape (Figure 5).
ꢁ
2
2
ðRÀðHF þ HHÞÞ Hh þ 2 ðRÀðHF þ HHÞÞH2H þ 3 HH3
ꢀ
ꢁ
¼
ꢁ
2
2
2
3
ðRÀðHF þ HHÞÞ ðHF þ HHÞ þ 2 ðRÀðHF þ HHÞÞðHF þ HHÞ þ 3 ðHF þ HHÞ
ð2Þ
Since fH must be equal to the hydrocarbon volume fraction
of the semifluorinated alkane molecule and VH, VF, HH, and
HF depend on m and n, it appears that R is a simple function of
n and m.This means that we can predict the size of the surface
micelles for a given semifluorinated alkane molecule.
In conclusion, we have found that the nonpolar semi-
fluorinated alkane F8H16 self-assembles into Langmuir
monolayers which, transferred to silicon wafers, consist of
highly monodisperse stable surface micelles.The hydrocarbon
segments of the F8H16 molecules are directed toward the
substrate, while the fluorinated segments point outwards
toward the air.We demonstrated that the size of these
micelles is controlled by the density mismatch between the
fluorinated and hydrogenated segments.The radius of the
surface micelle is a function of the lengths of the hydrocarbon
and fluorinated segments of the FnHm molecules.These
results demonstrate the possibility of decorating surfaces with
molecular clusters of predetermined size in the nanometer
range.
The fluorinated external layer is considered to be composed
of rigid F8 rods with a thickness of 11.5 ä and and an area per
chain of 30 ä2.The density of the hydrocarbon part can then
be deduced from the molecular constraint by simply calculat-
Received: June 21, 2002 [Z19585]
Figure 5.Schematic representation of the cross section of a surface micelle
of F8H16 after transfer of the monolayer onto a silicon wafer: a) disklike
model; b) ellipsoidal model.The gray area represents the region of
fluorinated segments F8, and the dotted area the region of hydrocarbon
segments H16.
[1] G.G.Warr, Curr. Opin. Colloid Interface Sci. 2000, 5, 80 94.
[2] J.Zhu, A.Eisenberg, R.B.Lennox,
5583 5588.
J. Am. Chem. Soc. 1991, 113,
Angew. Chem. Int. Ed. 2002, 41, No.22
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COMMUNICATIONS
[3] O.Albrecht, H.Gruler, E.Sackmann, J. Phys. (Paris) 1978, 39, 301
313.
[4] M.Flˆrsheimer, H.Mˆhwald, Colloids Surf. 1991, 55, 173 189.
improvement in such ratiometric methods would be obtained
if the two proton-exchanging pools were parts of the same
molecule (single-molecule CEST procedure).Paramagnetic
[Ln(dotamGly)]À complexes (dotam ¼ 1,4,7,10-tetrakis(carb-
amoylmethyl)-1,4,7,10-tetrazacyclododecane) would be ap-
[5] T.Kato, M.Kameyama, M.Ehara, K.Iimura,
1786 1786.
Langmuir 1998, 14,
[6] G.L.Gaines, Jr., Langmuir 1991, 7, 3054 3056.
[7] Z.Huang, AA. .Acero, N.Lei, SA. .Rice, Z.Zhang, ML. .
Schlossmann, J. Chem. Soc. Faraday Trans. 1996, 92, 545 552.
[8] A.El Abed, E.Pouzet, M.-C.Faurÿ, M.Saniõre, O.Abillon, Phys. Rev.
E 2000, 62, R5895 R5898.
[9] J.F.Rabolt, T.P Russell, R.Twieg, Macromolecules 1984, 17, 2786
2794.
[10] TP. .Russell, JF. .Rabolt, R.Twieg, RL. .Siemens, BL. .Farmer,
Macromolecules 1986, 19, 1135 1143.
[11] P.Marczuk, P.Lang, M.Mˆller, Colloids Surf. A 2000, 163, 103 113.
[12] M.P.Krafft, F.Giulieri, P.Fontaine, M.Goldmann, Langmuir 2001, 17,
6577 6584.
propriate for this purpose since they are characterized by the
presence of two kinds of mobile protons, namely, the four
equivalent amide protons and the two protons of the water
molecule coordinated to the LnIII ion.It has been previously
shown that the use of a cocktail formed by [Yb(dotamGly)]À
and [Eu(dotamGly)]À allows the set-up of a ratiometric
method for pH measurements by making use of the two
exchanging pools provided by the amide protons of the
Yb derivative and by the metal-coordinated water protons of
the Eu complex, respectively.Herein we demonstrate that the
paramagnetic dotamGly complexes of the lighter LnIII ions
(Pr, Nd, and Eu) behave as pH-responsive ™single-molecule
CEST∫ agents.The efficiency of saturation transfer (ST), once
the irradiation time is sufficiently long to reach a steady-state
ST value,[3] is directly related to the exchange rate of the
irradiated protons (kex), their molar concentration (defined by
the product n[C], where n is the number of irradiated protons
and [C] is the molar concentration of the agent), and inversely
related to the longitudinal relaxation rate of the bulk water
protons during the irradiation R1,irr [Eq.(1)].
[13] J.G.Riess, Chem. Rev. 2001, 101, 2797 2919.
[14] J.G.Riess, Tetrahedron 2002, 58, 4113 4131.
[15] N.O.Brace, J. Org. Chem. 1973, 38, 3167 3172.
[16] A.A.Acero, M.Li, B.Lin, S.A.Rice, M.Goldmann, I.Z.Azouz, A.
Goudot, F.Rondelez, J. Chem. Phys. 1993, 99, 7214 7220.
[17] C.W.Bunn, E.R.Howell, Nature 1954, 174, 549 541.
[18] C.Tanford, The Hydrophobic Effect: Formation of Micelles and
Biological Membranes, Wiley, New York, 1973.
[19] P.Lo Nostro, SH. .Chen,
J. Phys. Chem. 1993, 97, 6535 6540.
Novel pH-Reporter MRI Contrast Agents**
Silvio Aime,* Daniela Delli Castelli, and
Enzo Terreno
The contrast obtained in magnetic resonance imaging
(MRI) relies essentially on differences in the intensity of the
1H water signal.Therefore, the contrast can be augmented by
the use of chemicals (contrast agents, CAs) able to enhance
the relaxation properties of water protons.[1] Recently, a novel
class of paramagnetic CAs has been proposed for MRI
applications.[2,3] Such chemicals contain paramagnetically
shifted mobile protons whose exchange with the bulk water
is slow on the NMR timescale (j kex j < j Dw j ).Thus, irradi-
ation of the mobile protons of the agent determines a decrease
of the 1H water signal through the so-called chemical
exchange saturation transfer (CEST) effect.[4,5]
One of the major advantages of the CEST agents relies on
the possibility of designing responsive agents in which the
effectiveness of the CA is not dependent on its concentration.
This goal can be pursued when the CEST properties of two
independent exchanging pools are monitored in the same
experiment (ratiometric method).[6] This possibility has been
demonstrated using a mixture of two compounds.[3] An
ꢂ
ꢃ
IS
I0
kex n½Cꢁ
ST % ¼ 1À
100 ¼
100
ð1Þ
111:2 R1;irr þ kex n½Cꢁ
The latter parameter, in the presence of a paramagnetic
LnIII complex, is mainly determined by the relaxation rate of
the metal-coordinated water protons, and this is directly
dependent on the intrinsic paramagnetism of the metal ion
(meff).[7]
IS and I0 refer to the intensity of the bulk water signal when
the irradiation pulse is set on-resonance (frequency non, signal
intensity IS) and off-resonance with respect to the frequency
of the bulk water protons (noff ¼ Ànon, I0).The off-resonance
measurement is necessary to take into account the direct
saturation effect on the bulk water signal which is determined
by the irradiation pulse.
The data reported in Figure 1 clearly show that all the three
complexes ([C] ¼ 30 mm) yield significant STeffects (at 312 K
and pH 7.4) upon selective irradiation of their pools of mobile
protons.In the case of [Eu(dotamGly)] À, the ST effect from
the coordinated water is so efficient that a 10% effect is still
detectable when the concentration of the complex is as low as
1 mm.This result makes this system the most efficient CEST
agent so far reported if the ST effect is normalized to the
number of irradiated spins.[2,4,8] The ST effect from the amide
protons (ST)NH follows a reverse order with respect to the
[*] Prof.S.Aime, Dr.D.Delli Castelli, Dr.E.Terreno
Dipartimento di Chimica I.F.M.
Via P.Giuria 7, 10125 Torino (Italy)
Fax : (þ 39)011-670-7855
E-mail: silvio.aime@unito.it
[**] Financial support from Bracco Imaging S.p.A., MIUR (PRIN), and
CNR (PF Oncology, L.95/95) are gratefully acknowledged.This work
has been carried out under the framework of the EU-COST D 18
action.
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