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NMR Study of Nuclear Spin Polarization during Chemical Reactions with Ortho Hydrogen.

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NMR Study of Nuclear Spin Polarization during
Chemical Reactions with Ortho Hydrogen **
By Joachim Bargon,* Jorg Kandels, and Klaus Woelk
Since the discovery of the CIDNP phenomenon,['. nuclear spin polarization effects in N M R spectra of chemical
reactions have traditionally been ascribed to radical intermeand Weitekarnpr4-51 showed, however, that
d i a t e ~ . [Bowers
very similar phenomena can also be observed in chemical
reactions involving spin-polarized hydrogen. Thus, the cause
of a nuclear spin polarization found by N M R spectroscopy
is not unambiguous. Indeed, N M R polarization signals have
been observed in the past during hydrogenation in the presence of organometallic catalysts[61and often wrongly interpreted as C I D N P effects.['] Here we describe the experimental elucidation of the cause of nuclear spin polarization by a
method involving the two isomers of molecular hydrogen
(i.e., ortho and para hydrogen).
So Par, nuclear spin polarizations during hydrogenations
have been investigated exclusively by using para hydrogen,
as reflected in the acronyms PASADENA (Parahydrogen
And Synthesis Allows Dynamically Enhanced Nuclear
Alignment)['] or PHIP (Para Hydrogen Induced Polarization).'8,9]We have now studied these effects with both ortho
and para hydrogen.["]
Molecular hydrogen contains two protons having nuclear
spin I = 1/2. These spins couple to give a total spin 1 (tripletstate nuclear spin in ortho hydrogen) or 0 (singlet-state nuclear spin in para hydrogen). At room temperature, the small
energy difference between the two spin isomers can be neglected. Owing to the threefold degeneracy of the triplet
state, the ratio of the two isomers at room temperature is
75% ortho to 25% para. Hydrogenation with H, in this
ratio leads to intermediates and final products whose N M R
signals are characterized by the slight difference in thermal
occupation (ANIN z lo-') of the nuclear spin states in the
magnetic field.
If, on the other hand, only one of the two spin isomers-for example, para hydrogen-is used in the hydrogenation,
then, in the intermediates and final products, the only nuclear spin states occupied are those exhibiting singlet character
Prof. Dr. J Bargon, DipLPhys. J. Kandels, DipLChem. K. Woelk
lnstitut fur Physikalische Chemie der Universitit
Wegelerstrasse 12, D-5300 Bonn 1 (FRG)
This work was supported by the Fonds der Chemischen Industrie. We
thank Prof. Dr. W Keim, Technische Hochschule Aachen, Prof Dr. U .
Nu&, Universitiit Tubingen, Prof. Dr. H . Hoherg, Max-Planck-Institut.
Mulheim a. d. Ruhr, and Prof. Dr. M . Dosrere, Universite Mons, Belgium,
for valuable discussions.
VCH Verlu~.~~e,sellschuft
nihH, D-6940 Wiwdieim, 1990
with respect to the protons undergoing transfer. All other
states remain unoccupied. This enormous difference in occupation leads to strong emission and absorption signals in the
N M R spectrum.
The mixture can be enriched in ortho hydrogen by exploiting the fact that, at low temperatures, the two spin isomers
are not equally adsorbed on surfaces (theory of the hindered
rotator["]). Gas chromatography o n alumina as a diamagnetic adsorber at 77 K thus affords a mixture enriched in
ortho hydrogen." This method yields a mixture containing
about 80% ortho and 20% para hydrogen.
A mixture enriched in para hydrogen can be obtained by
making use of the fact that, in the presence of a paramagnetic
adsorber (active charcoal), a thermal equilibrium is established between ortho and para hydrogen. At low temperatures, therefore, para hydrogen is favored
In this way, we obtained a mixture of approximately 46%
ortho and 54% para hydrogen at 77 K.
The isomer ratio in the mixture was determined by difference measurement with two thermal conductivity cells that
exploit the differing specific heats of the two spin isomers at
low temperatures (77 K).1141
The enriched hydrogen obtained in this way was directly
bubbled via a capillary into the sample tube of a 'H NMR
FT spectrometer (80 MHz) and used to hydrogenate acrylonitrile in the presence of Wilkinson catalyst, tris(tripheny1phosphane)chlororhodium(i), in [DJbenzene [Eq. (a)].
= CH
+ H2
For kinetic reasons, the polarization signal of the reaction
product propionitrile is maximal only after bubbling hydrogen into the reaction mixture at a constant rate for 30 s.
Successive N M R pulses must therefore be separated by a
corresponding period of time. The result of the N M R study
is summarized in Figure 1.
Figure 1 a shows the polarization signal of propionitrile
during the hydrogenation with enriched ortho hydrogen,
Figure 1 b the complementary signal observed with enriched
para hydrogen. Comparison of these two spectra reveals that
the sequence of emission and absorption is reversed. The
intensity ratio of the two spectra varies between 1 :4 and 1 : 5.
This finding is explained as follows: During the hydrogenation, ortho and para hydrogen participate independently in the reaction. In principle, therefore, the total intensity
I of the N M R signal of the reaction product should be obtainable by adding the intensities of ortho and para hydrogen weighted according to their relative amounts in the mixture. For hydrogenation with hydrogen in thermal equilibrium at room temperature (75 YOortho and 25 YOpara hydrogen), Equation (b) holds.
The difference in occupation of the nuclear spin levels,
obeys the Boltzwhich determines the signal intensity Ilherma,,
mann distribution and thus is several orders of magnitude
smaller (ca.
than the difference in occupation deterTherefore, it can
mining the signal intensities Z,, and Zorlho.
be neglected (see Fig. 1 c). Equation (c) is thereby obtained.
0570-08331Y0/0101-0058$ 02.50!0
A n z m . Chem. In!. Ed. Engl. 29 I 1 990) N o . 1
R. U. Kirss, T. C. Eisenschmid. R. Eisenberg. J. Am. Chem Soc. I f 0 (1988)
We therefore propose that the acronym PHIP, introduced by €isenher,?, be
reinterpreted as “Polarized Hydrogen Induced Polarization”.
Y. L. Sandler, J. Phvs. Chem. 58 (1945) 58.
M. Dosrkre. J. Chem. Educ. 62 (1985) 891.
Durrant, B. Durrant: Introduclion in Advunced Inorgunk. Chemisrry,
P. .I.
Longmans, Green, London 1962, pp. 368-371.
T. W. Bradshaw, J. 0. W Norris, Res. Sci. Instrum. S8 (1987) 83.
M. G. Pravica. D. P. Weitekamp, Chem. Phys. Lett. 145 (1988) 255.
A New Route to Highly Enantiomerically Enriched
(Z)-(l-Methyl-2-butenyl)boronic Esters **
By Rainer Stiirrner*
The (Z)-(1-methyl-2-butenyl)boronic ester I is a reliable
reagent for chain lengthening of aldehydes to homoallyl alcohols 2 with full reagent control of the diastereoselectivity.“. 21
2 , 71 %. 9 4 % e.e
Fig. I . Wilkinson catalyst (20 mg) and acrylonitrile (200 pL) dissolved in 1 mL
of C‘,D,: NMR spectra at 80 MHz with a 45“ pulse; A = acrylonitrile;
B = propionitrile; NMR pulse after hydrogenation for 30 s with (a) enriched
c) NMR puke
ortho hydrogen (/o,tbo) and (b) enriched para hydrogen
several minutes after termination of the hydrogenation. The propionitrile nuremains.
clear spin system is relaxed; the weak thermal signal I,,,,,,,
This equation holds only for pure ortho o r pure para hydrogen. Adjustment for the experimentally accessible isomer
ratios (thermal conductivity measurement) gives an intensity
ratio between I : 5 and 1 :6. This result agrees, within the
limits of error, with the intensity ratio obtained from the
experimental spectra.
According to the considerations above, the complernentary behavior of the two spin isomers in hydrogenations is
independent of the strength of the applied magnetic field and
thus applicable to hydrogenations performed outside the
N M R spectrometer, though these reactions display different
polarization patterns.“ ’1
Here we describe a new, simple route to the boronic esters
1 from the Grignard reagent 3 and the borate 4; this ap-
proach combines, for the first time, a racemization due to
1,3-metallotropic shift [31 with a kinetic resolution of a race-
Received: June 20, 1989 [Z 3403 I€]
German version: Angew. Chem. 102 (1990) 70
CAS Registry numbers.
CH,=CHCN, 107-13-1, H,, 1333-74.0
[I] J. Bargon, H. Fischer, 2. Nururforsch. A22 (1967) 1551
[2] H. R. Ward. R. G . Lawler, J. Am. Chem. Soc. 89 (1967) 5518.
[3] A. R. Lepley. G. L. Closs: Chemrcull~:Induced Magnetic Polurizulion,
Wiley, New York 1973
[4] C. R. Bowers, D. P Weitekamp, Phys. Rev. Lett. 57 (1986) 2645.
I09 (1987) 5541.
[5] C R. Bowers, D. P Weitekamp, J. Am. Chem. SOC.
[6] R . L Sweany, J. Halpern, J. Am. Chem. Soc. 99 (1977) 8335.
[7] The interpretation of the data of P. F. Seidler. H. E.Bryndza. J. E. Frommer. L. S Stuhl. R. G. Bergman, Orpmomrtullic.72 (1983) 1701, must he
corrected accordingly.
[8] T C Eisenschmid, R. U. Kirss, P. P Deutsch, S. I . Hommeltoft. R. Eisenberg. J. Bargon, R. G. Lawler, A. L. Balch, J Am. Chern. Soc. 109 (1987)
Angrit. Chmz. / I / [ . Ed. @I.
29 11990) N O . I
[*] DipLChem. R. Stiirmer
Fachbereich Chemie der Universitit
Hans-Meerwern-Strasse, D-3550 Marburg (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft
(SFB260, partial project with Prof. Dr. R. W Hoffmunn) and by the
“Graduiertes Kolleg Metallorganische Chemie”. Universitzt Marburg. I
thank Prof. Dr. R. M.: H c f j h m n for his friendly support of this work.
$> VCH Verlug.s~esell.schufimhH, 0.6940 Wernheim, 1990
US70-0X33IY0j010l-ooSY $02.SO/O
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hydrogen, spina, chemical, nuclear, nmr, reaction, stud, polarization, ortho
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