Preparation of cyclohexene isotopologues and stereoisotopomers from benzene

Article abstract of DOI:10.1038/s41586-020-2268-y

The hydrogen isotopes deuterium (D) and tritium (T) have become essential tools in chemistry, biology and medicine¹. Beyond their widespread use in spectroscopy, mass spectrometry and mechanistic and pharmacokinetic studies, there has been considerable interest in incorporating deuterium into drug molecules¹. Deutetrabenazine, a deuterated drug that is promising for the treatment of Huntington’s disease², was recently approved by the United States’ Food and Drug Administration. The deuterium kinetic isotope effect, which compares the rate of a chemical reaction for a compound with that for its deuterated counterpart, can be substantial^1,3,4. The strategic replacement of hydrogen with deuterium can affect both the rate of metabolism and the distribution of metabolites for a compound⁵, improving the efficacy and safety of a drug. The pharmacokinetics of a deuterated compound depends on the location(s) of deuterium. Although methods are available for deuterium incorporation at both early and late stages of the synthesis of a drug^6,7, these processes are often unselective and the stereoisotopic purity can be difficult to measure^7,8. Here we describe the preparation of stereoselectively deuterated building blocks for pharmaceutical research. As a proof of concept, we demonstrate a four-step conversion of benzene to cyclohexene with varying degrees of deuterium incorporation, via binding to a tungsten complex. Using different combinations of deuterated and proteated acid and hydride reagents, the deuterated positions on the cyclohexene ring can be controlled precisely. In total, 52 unique stereoisotopomers of cyclohexene are available, in the form of ten different isotopologues. This concept can be extended to prepare discrete stereoisotopomers of functionalized cyclohexenes. Such systematic methods for the preparation of pharmacologically active compounds as discrete stereoisotopomers could improve the pharmacological and toxicological properties of drugs and provide mechanistic information related to their distribution and metabolism in the body.

Full text of DOI:10.1038/s41586-020-2268-y

Article
Preparation of cyclohexene isotopologues
and stereoisotopomers from benzene
https://doi.org/10.1038/s41586-020-2268-y
Received: 31 March 2019
Jacob A. Smith1, Katy B. Wilson1, Reilly E. Sonstrom1, Patrick J. Kelleher1, Kevin D. Welch1,
Emmit K. Pert1, Karl S. Westendorff1, Diane A. Dickie1, Xiaoping Wang2, Brooks H. Pate1 &
W. Dean Harman
1
ꢀ✉
Accepted: 11 March 2020
Published online: 20 May 2020
Check for updates
The hydrogen isotopes deuterium (D) and tritium (T) have become essential tools in
chemistry, biology and medicine1. Beyond their widespread use in spectroscopy,
mass spectrometry and mechanistic and pharmacokinetic studies, there has been
considerable interest in incorporating deuterium into drug molecules1.
Deutetrabenazine, a deuterated drug that is promising for the treatment of
Huntington’s disease2, was recently approved by the United States’ Food and Drug
Administration. The deuterium kinetic isotope efect, which compares the rate of a
chemical reaction for a compound with that for its deuterated counterpart, can be
substantial1,3,4. The strategic replacement of hydrogen with deuterium can afect both
the rate of metabolism and the distribution of metabolites for a compound5,
improving the efcacy and safety of a drug. The pharmacokinetics of a deuterated
compound depends on the location(s) of deuterium. Although methods are available
for deuterium incorporation at both early and late stages of the synthesis of a drug6,7
,
these processes are often unselective and the stereoisotopic purity can be difcult to
measure7,8. Here we describe the preparation of stereoselectively deuterated building
blocks for pharmaceutical research. As a proof of concept, we demonstrate a
four-step conversion of benzene to cyclohexene with varying degrees of deuterium
incorporation, via binding to a tungsten complex. Using diferent combinations of
deuterated and proteated acid and hydride reagents, the deuterated positions on the
cyclohexene ring can be controlled precisely. In total, 52 unique stereoisotopomers of
cyclohexene are available, in the form of ten diferent isotopologues. This concept can
be extended to prepare discrete stereoisotopomers of functionalized cyclohexenes.
Such systematic methods for the preparation of pharmacologically active
compounds as discrete stereoisotopomers could improve the pharmacological and
toxicological properties of drugs and provide mechanistic information related to
their distribution and metabolism in the body.
Typically, hydrogenation of benzene using D2 gas leads to isotopologue and resistant to substitution13. Furthermore, this system displays con-
mixtures of cyclohexane9–11. However, Taube et al.12 demonstrated that siderable electronic asymmetry, and the benzene complex WTp(NO)
the complex [Os(NH3)5(η2-benzene)]2+ could be deuterated to form a (PMe3)(η2-benzene) (1) can be prepared on a multi-gram scale14 and in
single stereoisotopomer of [Os(NH3)5(η2-cyclohexene-d4)]2+ using enantioenriched form15. Treatment of an acetone-d6 solution of 1 with
D2 and a Pd/C catalyst. We posited that benzene bound in this man- diphenylammonium triflate (DPhAT; pKa ≈ 0) at −30ꢀ°C affords its clean
ner could also be converted to cyclohexene using four well defined conversion to the η2-benzenium complex [WTp(NO)(PMe3)(η2-C6H7)]
additions of two protons and two hydrides, passing through an η2- (OTf) (2; Fig. 2). Using chilled diethyl ether as a precipitating solvent,
1,3-cyclohexadiene intermediate (Fig. 1). If these reactions could be 2 can be isolated from dichloromethane in 86% yield (1.9 g). As an ace-
performed regio- and stereoselectively, one could access a diverse set tonitrile solution, the η2-benzenium complex 2 is moderately stable
of isotopologues and even stereoisotopomers of cyclohexene using at room temperature but soon decomposes (half-life, t1/2 ≈ 6 min). At
various combinations of proteated and deuterated reagents.
0ꢀ°C, however, 2 exists in equilibrium with its diastereomer 3 in a 10:1
The dearomatization agent {WTp(NO)(PMe3)} is considerably more ratio (Fig. 2) and persists for three hours without substantial decom-
activating than its osmium predecessor13. Strong π-backbonding position. The major isomer (2) is formed with the metal binding two
renders arene and diene complexes of this system highly nucleophilic internal carbons of the five-carbon π system, and with the newly formed
1
2
✉
University of Virginia Department of Chemistry, Charlottesville, VA, USA. Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA. e-mail: wdh5z@virginia.edu
288 | Nature | Vol 581 | 21 May 2020

D2
D2
a
Dn
Dn
Deuterium studies
cat.
cat.
With all hydrogen resonances for the methylated η2-cyclohexene com-
plex 9 fully assigned, we investigated the regio- and stereochemical
fidelity of the reaction sequence (Fig. 3). When the η2-benzenium com-
D
D
D
D2
Regio- and
stereoselective
[Os]
[Os]
+
plex 11 was prepared from 1 using [MeOD2 ]OTf, a loss of signal inten-
sity was observed, corresponding to the methylene endo proton. This
indicates that protonation of the η2-benzene occurs syn to the metal
( Fig. 3). A complementary experiment was next performed, starting
with the fully deuterated benzene complex, 17, in which MeOH2+ was
used as the acid source. In this case, protonation led to a single broad
proton resonance for the deuterated η2-benzenium complex 18.
This proton signal is ~0.03 ppm upfield from its proteo counterpart,
consistent with a primary H/D isotopic shift19. The endo-selective pro-
tonation of the benzene ligand in 1 is in stark contrast to the addition
of carbon and heteroatom electrophiles, which have been observed
to add anti to η2-arene and η2-diene ligands of tungsten complexes13.
When η2-benzenium complexes 11 and 18 were treated with NaBD4 or
NaBH4, respectively, the complementary cyclohexadiene complexes
12 and 19 were formed (Fig. 3). A comparison of NOESY data for
all three isotopologues of the cyclohexadiene complex (4, 12, 19)
confirmed that the proton delivered from the borohydride reagent
was anti to the metal (Fig. 3). The cyclohexadiene complexes 12 and
19 were then taken forward to their π-allyl analogues 13, 15 and 20
(Fig. 3). In contrast to protonation of the η2-benzene ligand of 1, the
acidic hydrogen was delivered predominantly antito the metal (Fig. 3).
The resulting η2-allyl complexes (13, 15, 20) underwent a confor-
mational change (‘allyl shift’) such that the second proton added
D
b This work
H/D
H/D
1. H/D+
2. H/D–
1. H/D+
2. H/D–
H/D
H/D
[W]
[W]
[W]
H/D
H/D
• Selected isotopologues • Regio- and stereoselective
c
PMe3
NO
N
200 g
scale
30 g
scale
N
W
N
W(CO)6
N
WTp(NO)Br2
N
1
B
H
N
Fig. 1 | Methods for the deuteration of benzene. a, Existing methods for the
selective deuteration of benzene can lead to over-reduction and a mixture of
isotopologues. b, The current approach provides access to cyclohexene
isotopologues and stereoisotopomers. c, The dearomatized benzene complex
WTp(NO)(PMe3)(η2-benzene).
sp3 carbon distal to the PMe3 ligand. The minor isomer (3) is bound at became H6exo (conversion of 4 to 6; Fig. 2), while the first proton added
a terminus of the π system with the sp3 carbon proximal to the phos- became H5endo. For allyl complexes 13 and 20, full stereoselective pro-
phine. Proton nuclear magnetic resonance (NMR) data and density tonation was achieved. However, with the preparation of 15 or 26 we
functional theory (DFT) calculations (Supplementary Information; Sup- experienced difficulties in achieving full deuterium incorporation,
plementary Figs. 1–3) of these η2-benzenium complexes (2, 3) suggest owing to an unusually large deuterium kinetic isotope effect (DKIE)
that they are similar in structure to complexes of the form [WTp(NO) (kH/kD ≈ 37 at −30ꢀ°C for the deuteration of 12 or 4, where kH and kD are
(PMe3)(η2-allyl)]+ (ref. 16), where the allyl ligand is tightly bound to the specific rate constants for protonation and deuteration, respectively).
metal through only two carbons. A third carbon, weakly associated This DKIE was determined for 4 as the average value from three separate
to the metal, resembles a carbocation, and is indicated as such in the experiments in which 26 was generated from acidic solutions with dif-
figures (Fig. 2). Combining cold solutions of 2 and tetrabutylammo- ferent H/D ratios (Supplementary Information section K). This DKIE
nium borohydride generates WTp(NO)(PMe3)(η2-1,3-cyclohexadiene) could be decreased by raising the temperature to 22ꢀ°C; however, this
exclusively (4). Despite the coexistence of the allyl conformer 3 in compromised the stereofidelity of the resulting deuterated product
solution, the WTp(NO)(PMe3)(η2-1,4-cyclohexadiene) complex (8) is (15), with endodeuteration of the η2-diene 12, which competed with exo
undetected (Fig. 2) in the reaction mixture16. The η2-diene complex 4 deuteration. Consequently, stereoselective deuterium incorporation
is then treated with either DPhAT or HOTf/MeOH acids to generate the at the H6exo position of cyclohexene (that is, 16, 33–35, 41, 44, 49, 51;
η2-allyl complex (6)16. When 6 is subjected to base, it deprotonates to Fig. 3) could not be achieved above ~75–80%. A similar outcome was
form 5, a stereoisomer of 4 (ref. 16), in which the uncoordinated double observed when we tried to convert the d6-isotopologue diene 19 to
bond is now distal to the PMe3 (ref. 16). Combining the allyl complex 6 allyl 30. Finally, treatment of 13, 15 or 20 with a hydride or deuteride
with a hydride source produces the desired η2-cyclohexene complex 7 source again confirmed that the corresponding η2-cyclohexene prod-
(67%). Crystals suitable for X-ray structure determinations are grown ucts (14, 16, 21) are formed by nucleophilic addition anti to the metal
for complexes of cyclohexadiene 4, allyl complex 6, and cyclohexene 7, (Fig. 3). Similarly to the 1,3-diene complex 4, its isomer 5 undergoes
and a rendering of these structures, along with key nuclear Overhauser exoprotonation to form the allyl complex 24. Remarkably, treatment
effect (NOE) interactions are provided in Supplementary Information of the 1,4-cyclohexadiene complex (8) with D+ (D2NPh2+ in MeOD) also
(Supplementary Fig. 4). Overlapping signals in the 1H NMR spectrum undergoes direct exo protonation (Fig. 3), this time providing allyl
of cyclohexene complex 7 precludes unambiguous stereochemical 25. The direct exogenous protonation of the unconjugated C=C bond
assignments of some of the ring proton signals.
in 8 appears to result in a carbocation that can be stabilized by the
By methylating the nitrosyl ligand of 7 (CH3OTf) to generate participation of the nitrosyl ligand, as revealed by DFT calculations.
[WTp(NOMe)(PMe3)(η2-C6H10)]OTf (9)17, the chemical shifts of the A subsequent [1,2]-hydride shift results in the formation of the allyl
cyclohexene ring separate to the point that each proton can be assigned complex 25 (Supplementary Fig. 5). Unambiguous assignment of the
with high confidence (Supplementary Information sections G and H). deuterated hydrogen atom in 25 comes from its conversion to 9-d1
An X-ray structure determination of 9 provides conclusive evidence (via 39; Fig. 3). To demonstrate regio- and stereocontrol of deuterium
for methylation of the nitrosyl oxygen (Fig. 2), analogous to earlier incorporation, additional deuterated isotopomers of the allyl complex
literature reports18. Strong NOE interactions between the ring endo were prepared from the monodeuterated dienes 22 and 23 and from the
protons and the methylated nitrosyl ligand further facilitate these benzene-d6-derived allyls 30 and 31 (Fig. 3). The allyl complexes 24–31
assignments, and quantitative NOE experiments support the stereo- were then combined with deuteride or hydride to form 18 additional
chemical assignments of all diastereotopic protons on the cyclohexene cyclohexene complexes, 32–46, 49–51. In principle, one can selec-
ring (Supplementary Information section H).
tively make ten different isotopologues of the cyclohexene complex
Nature | Vol 581 | 21 May 2020 | 289

Article
a
dideuterated (14, 33, 35, 43–46), and four trideuterated (34, 49–51)
isotopomers of the cyclohexene complex were prepared using these
methods (Fig. 3).
OTf
PMe3
PMe3
PMe3
ON
H
H
ON
Tp
ON
Tp
+
H
Ph2NH2
Oxidation of the tungsten complex 7 with 2,3-dichloro-5,6-dicyano-
p-benzoquinone (DDQ) releases the free cyclohexene (Fig. 2; 10). Such
action on 32, 42, 45 and 46 confirmed the expected regiochemistry of
these d1 and d2 isotopomers of cyclohexene via 13C NMR. Introduction of
a single deuterium in 3-deuterocyclohexene or 4-deuterocyclohexene
allows one to distinguish all six of the carbons in the 13C NMR spectrum,
owing to isotopic shifting of the now asymmetric cyclohexene carbons
(Supplementary Fig. 7). Alternatively, solvent-free heating of various
isotopologues of the methylated complex 9 induce the release of the
cyclohexene ligand for analysis by molecular rotation resonance (MRR)
spectroscopy (Supplementary Information section L)20. These experi-
ments determined that: (1) over-deuteration is exceedingly low (<2%).
(2) The stereoselectivity is excellent when assessed by observation
of undesired cis/trans isomers, which in the worst case is 22:1 and in
other cases it is 40:1 or higher. (3) The dominant stereoisotopomers
in all cases are those predicted by the 1H NMR data. As a final check of
the stereochemical assignments, the locations of the deuterium atoms
H
W
W
W
Tp
–30 °C
H
H
H
H
3
2
1
NBu4BH4
1,4-cyclo-
hexadiene
1,3-cyclo-
hexadiene
–30 °C
MeOH
PMe3
ON
PMe3
PMe3
ON
ON
H
H
H
W
W
W
4
5
Tp
Tp
Tp
H
H
H
H
6
8
5
4
H
+
Ph2NH2
H+
Base
OTf
OTf
PMe3
PMe3
ON
ON
Tp
H
6
H
+
were confirmed for complex 45 using neutron diffraction measure
-
Ph2NH2
W
H
H
W
H
H
Tp
ments (Supplementary Information section I; TOPAZ at Oak Ridge
National Laboratory).
5
(or HOTf/MeOH)
4
H
H
6 (via 8)
6 (via 4)
Mechanistic considerations
NBu4BH4
The reaction of 1 with D+ to form 11 results in deuterium incorporation
exclusively endoto the metal, but this does not conclusively show which
carbon is initially protonated (Supplementary Fig. 8). Given that the
endoproton of the benzene ligand in 1 completely preempts protona-
tion from an exogenous acid (exo), we propose that the protonation
must be concerted—that C–H bond formation is intramolecular and
simultaneous with electronic changes at the metal—which could lower
the activation barrier for this process relative to protonation by an
external acid. Such a mechanism could occur via a hydride intermedi-
ate, but this seems sterically untenable. Instead, we propose a mecha-
nism (Supplementary Information section M) in which the nitrosyl
ligand is first protonated to form a hydroxylimido ligand analogous to
that reported by Legzdins et al.21. This action is followed by a concerted
proton transfer in which a gamma carbon of the benzene is protonated
simultaneously with the release of electron density back into the tung-
sten through the NO group. The role of nitrosyl ligands in intramolecu-
lar proton transfer has been previously documented22. By contrast, the
stereochemistry and kinetics of η2-diene protonation (for example,
4; Fig. 2) indicates that the hydrogen is delivered exogenously, anti
to the tungsten (Fig. 1). We speculate that whereas endo protonation
may still be accessible for these 1,3-cyclohexadiene complexes, the
less-delocalized diene ligand is probably more basic than its η2-benzene
predecessor, and its direct exo protonation apparently preempts the
purported endo mechanism at −30ꢀ°C.
OTf
O
PMe3
Me3P
H
H
H
6
N
ON
H
H
H
H
H
Heat
W
H
W
CH3OTf
6
H
Tp
H
Tp
5
5
H
H
H
4
H
4
H
H
H
H
(H = 2H or 1H)
10
9
7
DDQ
b
H3C
H3C
NOE
CH3
P
O
H6endo
CH3
N
H
N
H1
W
N
H6exo
N
N
H4endo
N
H2
HB
N
H
H3endo
9
Fig. 2 | Formation of tungsten-bound cyclohexene from benzene.
a, Sequential reduction of benzene to cyclohexene bound to tungsten (by
addition of 1H or 2H). 10 was confirmed by 13C-NMR and MRR spectroscopy and
9 was confirmed by quantitative NOE. b, Solid-state molecular structure from a
single-crystal X-ray diffraction study and relevant NOE interactions (red
arrows) for the methylated cyclohexene complex 9 (Ph2NH2+ as OTf salt).
Transition-metal-promoted endo deuteration of benzene was
using the procedures outlined above (d0–d4; d6–d10), eight of which observed in the η4-benzene complexes Cr(CO)3(η4-benzene)2− and
(7, 16, 32–38) are reported herein.
Mn(CO)3(η4-benzene)− by Cooper et al.23,24, and was proposed to occur
Levels of isotopic purity for the cyclohexene ligand isotopologues via hydride intermediates23,24. More recently, Chirik et al. explored the
were determined by recording high-resolution mass spectrometry molybdenum-catalysed reduction of benzene and cyclohexadiene, with
(HRMS) data for the corresponding complexes as their methylated D2 (g), which resulted in mixtures of isotopologues of cyclohexane11.
adducts (Fig. 2.; 9-dn) to create a suitable cation for electrospray ioni- However, reduction of cyclohexene with D2 produced a single cis iso-
zation mass analysis. Using the isotope envelope of 9-d0 as a refer- topomer of 1,2-dideuterocyclohexane using the molybdenum catalyst.
ence (Supplementary Fig. 6), the isotopic purity of 7, 16 and 32–38
The high stereoselectivity enabled by the tungsten system provides
(as converted to 9-dn) was estimated to be >90%, with the exception of unprecedented control over the preparation of specific isotopologues
16 (79%), for which the high DKIE of the second protonation prevented and isotopomers of cyclohexene, starting from either benzene com-
complete deuteration at the H6exo position (see above). Finally, as a plex 1 or its deuterated analogue 17, and using either proteated or
demonstration of how the {WTp(NO)(PMe3)} system precisely governs deuterated sources of acids and hydrides (Supplementary Table 1).
both the stereochemistry and the regiochemistry of protonation and As an illustration, consider the d2 isotopologue of the cyclohexene
hydride addition, a series of five monodeuterated (32, 39–42), seven complex, 7-d2. Given that the {WTp(NO)(PMe3)} system is available in
290 | Nature | Vol 581 | 21 May 2020

a
b
c
D
PMe3
H
W
D
D
+
–
D
D
D
D
D
ON
Tp
H2NPh2
BD4
[W]
[W]
[W]
[W]
[W]
[W]
15
H
H
D
H
D
D
D
42
6
23
D
+
=
[W]
[W]
[W]
14
1.D2NPh2
17
–
–
BH4
–
1
+
2. BD4
BH4
+
D2NPh2
5
MeOH2
Me3HP
[W]
NO
D
D
D
D
D
D
+
W
24
MeOD2
32
D
D
D
Tp
–
H
BD4
D
D
D
[W]
49
[W]
[W]
[W]
H
+
D
–
D
D2NPh2
BH4
[W]
45
D
H
[W]
[W]
39
D
[W]
D
51
D
D
11
18
8
25
D
–
D
–
–
D
D
BH4
H
BD4
BD4
D
[W]
[W]
D
H
D
+
D
D
[W]
34
–
D
MeOD2
H
BH4
D
43
DOTf
D
D
D
H
D
[W]
D
H
D
D
[W]
D
[W]
41
[W]
[W]
12
50
[W]
D
4
D
26
From 13
D
From 29
D
D
–
H
BD4
D
H
19
D
30
[W]
d
H
+
D
H
+
–
D
H
Ph2NH2
44
D
H
H
D
H2NPh2
BH4
+
D
DOTf
Ph2NH2
D
H
–
D
[W]
D
H
D
BH4
[W]
40
H
[W]
[W]
D
H
D
H
27
D
D
D
H
D
D
D
–
D
D
H
BD4
D
D
D
[W]
D
20
From 19
H
H
[W]
[W]
D
D
[W]
D
20
[W]
13
36
D
46
D
22
D
D
D
15
D
H
H
From 11
D
D
D
–
–
D
D
[W]
+
BH4
D
BH4
–
D2NPh2
–
–
BD4
BH4
BH4
[W]
[W]
D
D
H
D
H
29
35
H
D
D
H
D
H
H
37
D
[W]
D
H
D
D
D
D
D
D
D
H
D
D
D
D
H
–
D
H
+
BH4
[W]
–
D2NPh2
D
[W]
16
[W]
14
BD4
[W]
D
[W]
D
D
[W]
28
D
[W]
31
From 18
23
D
D
D
D
H
21
D
33
D
H
D
H
D
38
From 2
D
Fig. 3 | Synthesis of isotopologues and stereoisotopomers of the cyclohexene complex 7. a, Detailed syntheses of d2, d4 and d6 isotopologues. b, Synthesis of d1
and d2 isotopologues. c, Synthesis of d3 isotopologues. d, Synthesis of d6, d7 and d8 isotopologues.
enantioenriched form15, one has access to 14 different isotopomers deuterium into the active pharmaceutical ingredient2. Given that each
(individual enantiomers of 14, 33, 35, 43–46; Supplementary Table 2). stereoisotopomer of a biologically active substance will have its own
The cyclohexene-d2 ligand of these complexes, once removed from the unique pharmacokinetic profile, the ability to stereoselectively deuter-
metal by oxidative decomplexation, would be available as 11 individual ate cyclohexene or other MedChem building blocks could enable the
isotopomers: both enantiomers of cis-3,4-, trans-3,4-, cis-3,5-, trans-3,5-, development of new probes, fragment libraries and leads for medicinal
trans-4,5- and the meso compound cis-3,6-dideuterocycloohexene. chemists, as well as provide a new tool for organic and organometal-
Similarly, 11 distinct isotopomers of cyclohexene-d8 should be avail- lic mechanistic studies. Cyclohexene can be readily converted into
able using this methodology starting from benzene-d6. Regarding perhydroindoles25, perhydroisoquinolines26 and azepines27. How-
cyclohexene complexes 7-d3 and 7-d7, eight isotopomers of each would ever, the inability to chemically differentiate the two alkene carbons
be available, and all 16 of these complexes would yield a unique, chiral or the enantioface of the deuterated cyclohexene limits its potential.
cyclohexene (eight cyclohexene-d3 and eight cyclohexene-d7). In total Nevertheless, by replacing the benzene ligand in Fig. 2 with a substituted
(Supplementary Table 2), the methodology outlined herein could pro- benzene, or by using a non-hydrogenic nucleophile in the conversion of
vide access to 52 unique isotopomers of cyclohexene derived from 6 to 7 (Fig. 2), one can envision a series of 3-substituted cyclohexenes
benzene and benzene-d6. For reference, the total number of isotopom- with highly defined isotopic patterns. As proof of concept, we pre-
ers for cyclohexene is 528.
pared the α,α,α-trifluorotoluene complex WTp(NO)(PMe3)(η2-CF3Ph)
The ability of {WTp(NO)(PMe3)} to be optically resolved on a practi- (ref. 28), which can be elaborated into a 3-(trifluoromethyl)cyclohexene
cal scale and to retain its stereochemical configuration, even when complex (47) analogous to the cyclohexene complex 7 (Supplementary
undergoing ligand displacement15, also makes it a valuable tool for Fig. 14). Liberation of the cyclohexene from {WTp(NO)(PMe3)} can be
determining the isotopic pattern of cyclohexene H/D isotopomers accomplished by a one-electron oxidant such as DDQ, Fe(III) or NOPF6
produced by other methods8. Consider, for example, a scenario in in yields of 70–75% (ref. 28). Oxidation of 47 would generate a cyclohex-
which an unknown isotopomer of cyclohexene-d1 is combined with ene that has been previously shown to undergo diastereoselective
the resolved form of benzene complex (R)-1 in solution and allowed to epoxidation, and would therefore be an attractive building block for
undergo ligand exchange. Even though the two faces of the cyclohexene medicinal chemistry29. Repeating the synthesis of 47 with deuteride in
ring will bind to tungsten with equal probability, the 1H NMR spectrum the final step yields the cis-6-deutero-3-(trifluoromethyl)cyclohexene
will be unique for each of the five possible isotopomers (Supplementary complex 52 in 95% yield. Various other isotopologues of 47 and 52 were
Information section C; Supplementary Fig. 11). A similar approach could also prepared (47, 52, 53, 54), and the reaction pattern was found to
be taken for any cyclic alkene (for example, dehydropiperidines, pyrro- be similar to that observed for benzene. The prepared compounds are
lines, cyclopentenes) for which a 1H NMR spectrum of a fully proteated summarized in Fig. 4, with synthetic details provided in Supplementary
species can be fully assigned (see above).
Information section B. Notably, in the syntheses of 47, 52, 53 and 54,
protonation at the carbon bearing the CF3 group ultimately occurs endo
to the metal, allowing the CF3 group to assume an exostereochemistry.
However, if the purported diene intermediate is protonated under
Deuterated building blocks for the MedChem database
The development of deutetrabenazine, is considered by many as a prel- kinetic control, exo protonation forces the CF3 group endo, and the
ude to a new generation of medicines and therapies that incorporate result after a second hydride reduction is the cyclohexene complex
Nature | Vol 581 | 21 May 2020 | 291

Article
–
1. H/D+ 2. B(H/D)4 3. H/D+
b
PMe3
PMe3
PMe3
PMe3
a
H/D
D
CF3
NO
NO
NO
W
NO
H/D
H/D
H/D
D
[W]
W
W
W
[W]
+
Tp
Tp
Tp
Tp
[W]
[W]
1. H/D+
+
1. H+
2. BH4
3. H+
H/D
D
2. CN–
48
61
62
63
=
=
CN
CN
CN
3. H/D+
CN
Tp
CN
PMe3
PMe3
PMe3
–
–
CN–
BH4 or BD4
NO
NO
NO
W
4. BH4
D
W
W
D
Tp
Tp
H/D
CN
H/D
H/D
H/D
H/D
CF3
[W]
CF3
[W]
D
D
H/D
H/D
H/D
H/D
H/D
58
59
60
CN
CN
CN
[W]
[W]
or
H/D
PMe3
PMe3
PMe3
PMe3
CF3
NO
H/D
H/D
CN
CF3
CF3
CF3
H/D
NO
NO
NO
W
W
W
W
Tp
Tp
Tp
Tp
1. NOPf6
2. DMDO
1. NOPF6
2. Ref. 30
(ref. 29
)
D
54
47
52
53
D
D
D
H/D
H/D
Cl
H/D
CF3
H
CF3
H
Cl
H/D
PMe3
PMe3
PMe3
H/D
H/D
Cl
N
H/D
N
O
H/D
CF3
CF3
Cl
D
NO
NO
W
NO
CF3
D
O
O
W
W
Tp
Tp
Tp
H/D
H/D
O
H/D
H/D
H
H
COOH
H/D
COOH
H/D
D
O
O
R
R
55
56
57
D
D
Fig. 4 | Examples of functionalized cyclohexene isotopomer complexes.
a, Synthesis (speculated) of functionalized cyclohexenes; ref. 29 describes a
single-step reaction with DMDO (70%); ref. 30 describes a nine-step synthesis
that includes: (i) HCl/H2O, (ii) isobutylene, (iii) mCPBA, (iv) NaN3, (v) acetyl
chloride, (vi) H2/C, (vii) ethylene oxide, (viii) TsCl and (ix) TFA. b, Examples of
chemo- and stereoselectively deuterated cyclohexene complexes with CF3 and
CN groups on the cyclohexene. See Supplementary Information section B for
the full synthetic details of 47, 48, 52–63.
3.
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isotopologues of 55 with inversion of the stereocentre bearing the -CF3
substituent (Fig. 4, 56, 57; Supplementary Fig. 14).
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to selectively prepare isotopomers of functionalized cyclohex-
enes, we prepared the tungsten complex of cis,trans-3-cyano-4,
5-dideuterocyclohexene (58) by the addition of cyanide to the allyl
intermediate 13 (57%; diastereometric ratio >98%; Fig. 4.). Other
d1-isotopolouges were also prepared (Supplementary Fig. 15), and
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and then treating with NaCN (Supplementary Fig. 16). Conversely,
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each enantiomer). Using the tungsten dearomatization methodology,
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Nature | Vol 581 | 21 May 2020 | 293

Article
Author contributions W.D.H., J.A.S. and K.B.W. conceived the project. K.D.W., W.D.H. and J.A.S.
designed the experiments. J.A.S. and K.S.W. prepared the samples and collected NMR and
HRMS data. D.A.D. carried out X-ray molecular structure determinations. B.H.P., P.J.K. and R.E.S.
conceived and ran the rotational spectroscopy experiments. E.K.P. and K.S.W. carried out DFT
calculations. J.A.S. and W.D.H. wrote the manuscript. X.W. collected and processed neutron
diffraction data.
Data availability
All data are available in the main text and Supplementary Information,
including NMR spectra, experimental details, crystallographic infor-
mation, DFT calculations, rotational spectroscopy and HRMS data.
Supplementary crystallographic data for this paper (4, 7, 9 (X-ray) and
45 (neutron)) can be obtained from the Cambridge Crystallographic
Data Centre at www.ccdc.cam.ac.uk/structures (CCDC 1885723-1885725
and 1972890).
Competing interests The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41586-020-
2268-y.
Correspondence and requests for materials should be addressed to W.D.H.
Peer review information Nature thanks David Hesk and the other, anonymous, reviewer(s) for
their contribution to the peer review of this work.
Acknowledgements We acknowledge the assistance of E. Ashcraft in collecting HRMS
data. The work was funded by the National Institutes of Health (1R01GM132205-01) and the
University of Virginia. The single-crystal neutron diffraction experiment performed on
TOPAZ using resources at the Spallation Neutron Source, a DOE Office of Science User
Facility operated by the Oak Ridge National Laboratory, under contract number DE-AC05-
00OR22725 with UT-Battelle, LLC.
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