Isotopic labeling for determination of enantiomeric purity by 2H NMR spectroscopy

Article abstract of DOI:10.1021/ol702225r

(Chemical Equation Presented) The use of ²H NMR spectroscopy as a tool for the analysis of enantiomeric purity is reported. Enantiopure isotopically chiral substrates bearing a monodeuterated methylene unit were prepared; introduction of an add

Full text of DOI:10.1021/ol702225r

ORGANIC
LETTERS
2007
Vol. 9, No. 25
5179-5182
Isotopic Labeling for Determination of
Enantiomeric Purity by H NMR
2
Spectroscopy
Hayley Jackman, Stephen P. Marsden,* Peter Shapland,† and Simon Barrett
School of Chemistry, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom
s.p.marsden@leeds.ac.uk
Received September 11, 2007
ABSTRACT
The use of 2H NMR spectroscopy as a tool for the analysis of enantiomeric purity is reported. Enantiopure isotopically chiral substrates
bearing a monodeuterated methylene unit were prepared; introduction of an additional asymmetric center leads to diastereomers which can
be distinguished by 2H NMR on a standard spectrometer. The assays allow for simple semiquantitative analysis of asymmetric transformations.
As screening-based approaches to the discovery and opti-
mization of new catalytic asymmetric processes become more
widespread, there has been a concomitant growth in interest
in the development of methods for the high-throughput
and/or in situ analysis of reaction conversion and enantio-
meric purity. Methods which avoid the use of chromato-
graphic techniques include mass spectrometry,1 IR thermog-
raphy,2 UV-visible3 or fluorescence spectroscopy,4 and
colorimetric5 and biochemical (antibody6 or enzyme-based7)
methods. Given the near-ubiquitous availability of NMR
spectrometers to research chemists, new methods based
around this technique would be expected to gain wide-
spread uptake;8-10 in addition, used in conjunction with
linked autosamplers, this would facilitate automated parallel
screening programmes and the accumulation of real-time
kinetic data.
Morken has elegantly demonstrated the use of a 13C-
labeled isotopically chiral ketone for direct analysis of
asymmetric induction in enantioselective reductions.8 Initial
enantiomeric ratios were determined by a single FT pulse,
while more accurate experiments (8 FT pulses) revealed an
average error of (3% by comparison with chiral GC experi-
ment in a data acquisition time of only 4 min. The use of
† Current address: GlaxoSmithKline, Medicines Research Center, Gun-
nels Wood Road, Stevenage SG1 2NY, U.K.
(1) (a) Guo, J.; Wu, J.; Siuzdak, G.; Finn, M. G. Angew. Chem., Int. Ed.
1999, 38, 1755-1758. (b) Reetz, M. T.; Becker, M. H.; Klein, H.-W.;
Stockigt, D. Angew. Chem., Int. Ed. 1999, 38, 1758-1761. (c) Markert,
C.; Pfaltz, A. Angew. Chem., Int. Ed. 2004, 43, 2498-2500. (d) Brewer,
B. N.; Zu, C.; Koscho, M. E. Chirality 2005, 17, 456-463.
(2) Reetz, M. T.; Becker, M. H.; Kuhling, K. M.; Holzwarth, A. Angew.
Chem., Int. Ed. 1998, 37, 2647-2650.
(6) (a) Matsushita, M.; Yoshida, K.; Yamamoto, N.; Wirsching, P.;
Lerner, R. A.; Janda, K. D. Angew. Chem., Int. Ed. 2003, 42, 5984-5987.
(b) Taran, F.; Gauchet, C.; Mohar, B.; Meunier, S.; Valleix, A.; Renard, P.
Y.; Creminon, C.; Grassi, J.; Wagner, A.; Mioskowski, C. Angew. Chem.,
Int. Ed. 2002, 41, 124-127.
(3) (a) Zhu, L.; Anslyn, E. V. J. Am. Chem. Soc. 2004, 126, 3676-
3677. (b) Zhu, L.; Zhong, Z.; Anslyn, E. V. J. Am. Chem. Soc. 2005, 127,
4260-4269. (c) Folmer-Anderson, J. F.; Lynch, V. M.; Anslyn, E. V. J.
Am. Chem. Soc. 2005, 127, 7986-7987. (d) Mei, X.; Wolf, C. J. Am. Chem.
Soc. 2006, 128, 13326-13327.
(4) (a) Pu, L. Chem. ReV. 2004, 104, 1687-1716 and references therein.
(b) Zhao, J.; Fyles, T. M.; James, T. D. Angew. Chem., Int. Ed. 2004, 43,
3461-3464. (c) Li, Z.-B.; Lin, J.; Qin, Y.-C.; Pu, L. Org. Lett. 2005, 7,
3441-3444. (d) Li, Z.-B.; Lin, J.; Pu, L. Angew. Chem., Int. Ed. 2005, 44,
1690-1693. (e) Tumambac, G. E.; Wolf, C. Org. Lett. 2005, 7, 4045-
4048.
(7) (a) Abato, P.; Seto, C. T. J. Am. Chem. Soc. 2001, 123, 9206-9207.
(b) Li, Z.; Buetikofer, L.; Witholt, B. Angew. Chem., Int. Ed. 2004, 43,
1698-1702. (c) Dey, S.; Karukurichi, K. R.; Shen, W.; Berkowitz, D. B.
J. Am. Chem. Soc. 2005, 127, 8610-8611. (d) Dey, S.; Powell, D. R.; Hu,
C.; Berkowitz, D. B. Angew. Chem, Int. Ed. 2007, 46, 7010-7014.
(8) Evans, M. A.; Morken, J. P. J. Am. Chem. Soc. 2002, 124, 9020-
9021.
(9) (a) Reetz, M. T.; Eipper, A.; Tielmann, P.; Mynott, R. AdV. Synth.
Catal. 2002, 344, 1008-1016. (b) Reetz, M. T.; Tielmann, P.; Eipper, A.;
Ross, A.; Schlotterbeck, G. Chem. Commun. 2004, 1366-1367.
(10) (a) Canet, I.; Meddour, A.; Courtieu, J.; Canet, J. L.; Salau¨n, J. J.
Am. Chem. Soc. 1994, 116, 2155-2156. (b) Phillips, A. R.; Sharman, G. J.
Chem. Commun. 2004, 1330-1331.
(5) Eelkema, R.; van Delden, R. A.; Feringa, B. L. Angew. Chem., Int.
Ed. 2004, 43, 5013-5016.
10.1021/ol702225r CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/15/2007

13C labels does, however, require the presence of prochiral
carbon substituents (such as a geminal dimethylalkyl group)
which necessarily limits the substrate scope of the method.
Much greater substrate scope could be achieved by an
alternative isotopic substitution, namely that of a deuteron
2
for a proton in a labeled methylene unit. The H nucleus is
NMR active with spin +1. Moreover, analysis can be
performed on standard spectrometers by utilizing the deu-
terium lock channel as the observation channel; this makes
the method available to most workers. Herein we disclose
2
proof-of-principle studies which verify that H NMR of
isotopically chiral substrates is a viable method for analysis
of enantiomeric purity.
To demonstrate the feasibility of the method, we chose
the isotopically chiral 2-phenylcyclohexanone 1 as our target
system. This was prepared in isotopically enantiopure fashion
according to Scheme 1.
Figure 1. 2H NMR spectra of (a) pseudoracemic (2RS) and (b)
scalemic (enriched in 2S) ketone 1.
Scheme 1. Synthesis of Isotopically Enantiopure Ketone 1
second signal at 2.20 ppm, in a 1:1 ratio with the signal at
1.96 ppm, which was assigned as belonging to (2R)-1 (Figure
1a). The absolute chemical shift values were calculated by
reference to added C6D6, but for most experiments the
reference was omitted; although the chemical shifts of the
signals drifted slightly between runs, this did not interfere
with the crucial data, namely the integration of the signals.
Given that an aim of the study was to develop a method
capable of facilitating in situ monitoring of conversion and
ee, it was important to demonstrate that the signals could be
differentiated in a wide range of solvents. Pleasingly, clear
signal separation was observed in acetone, benzene, carbon
tetrachloride, chloroform, ether, and THF, with ∆δD values
of between 0.17 and 0.24 ppm.13 This bodes well for the
application of the technique in reaction screening across a
range of conditions.
Known 2-phenylcyclohexenone (prepared according to
Wender11) was subjected to asymmetric reduction using the
CBS catalyst to give alcohol 2 in 98% yield (93% ee).
Directed epoxidation of the allylic alcohol with vanadyl
acetoacetonate and tert-butyl hydroperoxide gave the epoxy
alcohol 3 in quantitative yield as a single diastereoisomer.
The deuterium label was then introduced by SN2 ring-opening
of the epoxide with lithium aluminum deuteride. This reac-
tion produced an approximately equimolar mixture of regio-
isomeric ring-opening products that was subjected to ste-
reoselective reduction of the benzylic alcohol according to
Sharpless.12 The desired alcohol 4 was then isolated in 36%
yield over two steps. This material (93% ee) could be recrys-
tallized to optical purity from pentane if desired. Finally,
oxidation to cyclohexanone (2S)-1 was achieved with PCC
in 90% yield. This route gave (2S)-1 in 32% yield over five
steps and was found to be reliable on a scale up to 2.5 g.
2H NMR analysis of (2S)-1 in CH2Cl2 showed a single
signal, as expected, at 1.96 ppm. Conversion of diastereo-
merically pure 1 to a pseudo-racemic mixture of stereoiso-
mers at the 2-position was achieved by equilibration via the
pyrrolidinyl enamine. Analysis by 2H NMR now revealed a
Having demonstrated that differentiation of the two
pseudoenantiomers was possible, we next investigated the
performance of the assay in a quantitative sense. A series of
admixtures was therefore created by mixing different quanti-
ties of pseudoracemic (2RS)-1 and stereochemically pure
2
Table 1. Comparison of HPLC vs H NMR for Determination
of Enantiopurity of Ketone 1
entry
% ee (HPLC)
% ee (2H NMR)
1
2
3
4
5
6
7
8
9
0.2
17.5
54.5
58.4
71.3
80.9
82.3
83.3
90.0
0.6
18.8
57.0
67.6
75.0
85.2
87.0
87.0
>95a
(11) Wender, P. A.; Erhardt, J. M.; Letendre, L. J. J. Am. Chem. Soc.
1987, 105, 2114-2116.
a Minor isotopic diastereomer not detected.
(12) King, S. B.; Sharpless, K. B. Tetrahedron Lett. 1994, 35, 5611-
5612.
5180
Org. Lett., Vol. 9, No. 25, 2007

Scheme 2. Assay of Asymmetric Protonation of Silyl Enol
Ether 5a
Figure 2. 2H NMR spectra of compounds 6-9.
near identical values for the asymmetric induction. Addition-
ally, the major isomer was identified by 2H NMR as having
2S configuration, which is in agreement with the model
proposed by Yamamoto. This confirms that any secondary
kinetic isotope effect from the neighboring chiral methylene
unit is small in magnitude, as expected.
a Key: (a) ee by 2H NMR ) 38.5% (corrected for 93% ee of 5);
(b) ee by HPLC ) 37.4%.
We next sought to broaden the scope of chiral compounds
2
that could be assayed for enantiomeric purity by H NMR
2
(2S)-1. These were analyzed by both chiral HPLC and H
spectroscopy. Ketone 1 was therefore converted to its enol
triflate, then subjected to hydrogenolysis, leading to (isoto-
pically enantiopure) prochiral alkene 6 (Scheme 3). Alkene
6 was then converted by standard methods into pseudora-
cemic cis-diol 7, epoxide 8 and alcohol 9: these will be
valuable probes given that asymmetric dihydroxylation,
epoxidation and hydroboration/oxidation processes are well
documented and remain topics of active research.
NMR (in CH2Cl2) to assess the accuracy of the latter method
as a probe of enantiomeric purity (Figure 1b, Table 1). We
were gratified to find a good correlation between ee values
measured by both techniques, with all but one sample in the
range 0-83% ee giving an agreement (5% ee. For samples
of 90% ee and above (i.e., less than 5% of the minor isotopic
diastereomer), we were only able to reliably observe single
signals, setting an upper boundary to quantitative analysis.
Nevertheless, this still represents a qualitative indicator of a
highly enantioenriched sample.
We also confirmed that the method could be used to
accurately assay the outcome of an asymmetric reaction.
Thus, silyl enol ether 5 was prepared and subjected to
Yamamoto’s asymmetric protonation protocol, utilizing tin
tetrachloride/(R)-BINOL (Scheme 2).14 The sample produced
was analyzed by both chiral HPLC and 2H NMR, returning
Scheme 4. Assay of Asymmetric Dihydroxylation of Alkene
6a
2
Scheme 3. Synthesis of Additional Substrates for H NMR
Analysis
a Key: (a) ee by 2H NMR ) 41.6%; (b) ee by HPLC ) 40.0%.
Org. Lett., Vol. 9, No. 25, 2007
5181

Pleasingly, analysis of compounds 7-9 by 2H NMR
confirmed that in each case the pair of isotopically diaster-
eomeric products could be distinguished with ∆δD values
in the range 0.20-0.31 ppm (Figure 2). Further, since the
signals for the diastereomers of 7-9 have chemical shifts
distinct from that of the starting alkene 6, the potential exists
to use the system for in situ analysis of both the rate and
enantioselectivity in new catalytic processes.
accurate for ketone 1 generally (5% ee up to the mid-80%
ee range and is qualitatively useful for higher ee samples.
The extension to other substrate classes for asymmetric
transformations has also been demonstrated. The method
offers the potential to monitor reaction progress in situ and
complements the existing methods for nonchromatographic
determination of enantiomeric purity.
Acknowledgment. We thank the EPSRC and Glaxo-
SmithKline for a CASE award (H.J.) and Dr. Stuart Warriner
(University of Leeds) for help in preparing the graphics.
Finally, we confirmed that semiquantitative analysis of
enantiomeric purity was possible by performing the dihy-
droxylation of alkene 6 under Sharpless’ conditions (Scheme
4).15 The resulting diol 7 was shown to be of 40.0% ee by
Supporting Information Available: Experimental pro-
1
cedures, compound characterization data, and H/13C NMR
2
chiral HPLC and 41.6% ee by H NMR.
2
spectra for compounds 1-8, plus H NMR/HPLC data for
In conclusion, we have demonstrated that substrates
containing an isotopically chiral methylene unit can be used
Table 1 and Schemes 2 and 4. This material is available free
of charge via the Internet at http://pubs.acs.org.
2
as convenient probes for enantiomeric purity by H NMR
on a standard spectrometer. The method is quantitatively
OL702225R
(13) No signal could be detected, however, in acetonitrile, methanol, and
DMSO.
(15) Sharpless, K. B.; Amberg, W.; Beller, M.; Chen, H.; Hartung, J.;
Kawanami, Y.; Lu¨bben, D.; Manoury, E.; Ogino, Y.; Shibata, T.; Ukita, T.
J. Org. Chem. 1991, 56, 4585-4588.
(14) Ishihara, K.; Kaneeda, M.; Yamamoto, H. J. Am. Chem. Soc. 1994,
116, 11179-11180.
5182
Org. Lett., Vol. 9, No. 25, 2007