MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 2000; 38: 216–220
Rapid Communication
Examination of subsequent reaction products enhanced
through parahydrogen-induced nuclear polarization (PHIP)
∗
Andreas Koch and Joachim Bargon
Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
Received 22 September 1999; revised 15 November 1999; accepted 16 November 1999
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ABSTRACT: Homogeneous hydrogenation with parahydrogen yields strong nuclear spin polarization in the H NMR
spectra of the reaction products. This polarization can be transferred to subsequent reaction products and detected by in
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situ H NMR spectroscopy. As a typical example, the hydrogenation of 1-phenylpropyne with parahydrogen generates
the spin-polarized molecule 1-phenylpropene. Upon bromination of this product, its polarization is transferred to the
reaction product 1,2-dibromo-1-phenylpropane. As a second example, polarization was observed in the addition product
of DBr to spin-polarized dimethyl maleate which is generated from dimethyl acetylenedicarboxylate by addition of
parahydrogen. Copyright 2000 John Wiley & Sons, Ltd.
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KEYWORDS: NMR; H NMR; parahydrogen; polarization transfer
The use of parahydrogen might imply that this tech-
nique is restricted to hydrogenations. However, in this
study we demonstrate that the initially obtained spin polar-
ization can subsequently be transferred to the products of
an ensuing chemical reaction. For this purpose, at least
one reactant is generated by means of a preceding hydro-
genation using parahydrogen. If the hydrogenation product
undergoes a subsequent reaction with another compound
immediately after its formation, the derived product may
also display spin polarization.
INTRODUCTION
Parahydrogen-induced nuclear spin polarization (PHIP)
has turned out to be a versatile technique for investigating
transition metal-catalyzed hydrogenations using in situ
NMR spectroscopy.1,2 The reaction products generated in
this way show intense emission and absorption signals in
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their H NMR spectra. This effect is due to the selective
population of such energy levels of the spin states in
the product molecule which have some degree of singlet
character.3,4 The resulting deviation from the thermal
Boltzmann distribution of the spin system leads to a signal
enhancement of some orders of magnitude.
Accordingly, reaction intermediates and products in low
concentrations can be detected. Furthermore, the signal
patterns of the polarization spectrum yield information
about the stereo- and regioselectivity of hydrogenations.
This phenomenon has been termed PASADENA4 or
PHIP.5 In contrast to conventional NMR spectroscopy,
the maximum signal is obtained when a flip angle of ꢀ/4
is used.
The PHIP method has been extensively used to prove
the coherent or pairwise transfer of the two parahydrogen
nuclei in transition metal-catalyzed hydrogenations, i.e.,
both hydrogen atoms from the parahydrogen molecule
must be transferred simultaneously since only in this case
can polarization signals be expected.
RESULTS AND DISCUSSION
As a typical example, the addition of bromine to an alkene,
which is instantaneously generated via parahydrogen addi-
tion to an alkyne, was chosen. This reaction has the advan-
tage that it proceeds relatively fast so that the lifetime of
the initially formed hydrogenation product is shorter than
the corresponding spin–lattice relaxation time T1 of the
polarized protons in this intermediate.
As such a precursor, 1-phenylpropyne was hydro-
genated with parahydrogen in the magnetic field of an
80 MHz 1H NMR spectrometer using the rhodium(I) com-
plex [Rh(NBD)(PPh3)2]BF4 and benzene-d6 as the solvent.
For this purpose, the rhodium complex [Rh(NBD)(PPh3)2]
BF4 (2 mg) and 20 µl of 1-phenylpropyne were placed in
an NMR tube together with 700 µl of degassed benzene-
d6. Parahydrogen was then bubbled in situ through the
solution within the magnetic field of the spectrometer
at ambient temperature. The signal pattern of the spin-
polarized initial product, cis-1-phenylpropene, revealed
a cis addition of the parahydrogen atoms to the alkyne
(Fig. 1).
* Correspondence to: J. Bargon, Institute of Physical and Theoretical
Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn,
Germany.
Contract/grant sponsor: Volkswagen Foundation.
Contract/grant sponsor: Deutsche Forschungsgemeinschaft (DFG).
Contract/grant sponsor: German Federal Ministry for Science and
Technology (BMBF).
Contract/grant sponsor: Forschungsverbund Nordrhein-Westfalen.
Contract/grant sponsor: Fonds der Chemischen Industrie.
Copyright 2000 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2000; 38: 216–220