Visible Light Mediated Generation of trans-Arylcyclohexenes and Their Utilization in the Synthesis of Cyclic Bridged Ethers

Article abstract of DOI:10.1021/jacs.8b04642

While accessible via UV-irradiation of cis-cyclohexene, trans-cyclohexene has thus far been an investigation driven by curiosity, and due primarily to its short lifespan, has until recently not been employed for productive synthesis. Herein, we present st

Full text of DOI:10.1021/jacs.8b04642

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Article
Visible light mediated generation of trans-arylcyclohexenes
and their utilization in the synthesis of cyclic bridged ethers
Jon I. Day, Kamaljeet Singh, Winston Trinh, and Jimmie D. Weaver
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Jul 2018
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Visible light mediated generation of trans-arylcyclohexenes
and their utilization in the synthesis of cyclic bridged ethers
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Jon I. Day, Kamaljeet Singh, Winston Trinh, and Jimmie D. Weaver III*
107 Physical Science, Department of Chemistry, Oklahoma State University, Stillwater, OK 74078.
ABSTRACT: While accessible via UVꢀirradiation of cisꢀcyclohexene, transꢀcyclohexene has thus far been an investigation driven
by curiosity, and due primarily to its short lifespan, has until recently not been employed for productive synthesis. Herein, we preꢀ
sent straightforward conditions that provide access to a class of transꢀarylcyclohexenes and demonstrate their utility in the forꢀ
mation of oxabicyclic ethers, which are otherwise inaccessible from the corresponding cisꢀcyclohexene. A key challenge to utilizꢀ
ing the incredible ca. 52 kcal/mol strain energy of transꢀcyclohexene to drive synthesis was overcoming its short lifetime. Herein,
we show that preꢀorganization via hydrogen bonding between the substrate and the reaction partner prior to isomerization is a viaꢀ
ble strategy to overcome the inherently short lifetime of transꢀcyclohexene.
reactive trans-cycloalkene in a controlled manner. For this,
we were drawn to trans-cyclohexenes.
INTRODUCTION
The release of strain energy to facilitate chemical synꢀ
thesis includes standouts such as Danishefsky’s work with
cyclobutenone and cyclopentadiene [4+2] cycloadducts,1
that of Garg with cycloarynes,2 or that of Fox in which
trans cyclooctenes react with tetrazines,3 though there are
many others.4 The release of this built in or chemically
triggered strain can make otherwise challenging transforꢀ
mations possible. In contrast to a oneꢀtime event, an attracꢀ
tive alternative approach to building in triggers or integral
molecular strain is to capture energy from visible light, as
this could alleviate the difficulty associated with performꢀ
ing chemistry on prestrained molecules, or the difficulty
associated with the installation of functional groups that
serve as the trigger. With this in mind, we were drawn to
E.J. Corey’s communication from 1965 reporting the forꢀ
mation of transꢀ2ꢀcycloheptenone,5 in which the cisꢀ
cycloheptenone was photoexcited with UV light in situ
with cyclopentadiene, which yielded only a [4+2] type
product that appeared to be generated through a pericyclic
reaction with the trans-isomer, elegantly demonstrating the
potential of even transient strain to promote synthesis.
In our previous work,6 we used visible light and Irꢀbased
photocatalysts to dynamically isomerize styrenoids, and
generate a photostationary state of alkenes highly enriched
in the Zꢀconfiguration (Scheme 1A). While works have
demonstrated the diverse utility of these and transforꢀ
mations, such as the Noël group’s trifluoromethylation,
hydrotrifluoromethylation, and difluoromethylation,7 the
Qing group’s stereoselective trifluoromethylation of styꢀ
renes,8 the Gilmour group’s work with riboflavin mediated
isomerizations9 and corresponding synthetic routes to couꢀ
marins,10 the Rueping11 and Reiser12 groups’ works toward
applications in process, we sought to further build on this
foundation, suspecting that it would be possible to use visiꢀ
ble light photocatalysis to generate a strained and therefore
Larger, less strained trans-cycloalkenes can be generated
by UV photolysis, and are isolable,13 or at least readily
observable.14 Evidence for transꢀcyclohexene was less
forthcoming, however, presumably due to its high strain
(ca. 52 kcal/mol), and correspondingly short lifetime. Durꢀ
ing pulsed photolysis experiments, JoussotꢀDubien15 and
Dauben15ꢀ16 reported having observed a transient species
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Attempts to use a stronger Brønsted acid, such as HCl,
simply led to elimination of water (entry 3).
with a 9 ꢀs lifetime at room temperature and a ca. 7.5
kcal/mol barrier to isomerization back to the starting mateꢀ
rial. While irradiation with UV light led to multiple prodꢀ
ucts, it has also been shown that if excited in acidic methaꢀ
Next, we evaluated the effect of the photocatalyst on the
reaction (entries 4ꢀ6). We found, as previously demonꢀ
strated,6a that both the steric volume and the emissive enerꢀ
gy of the catalyst affect the rate of the reaction. Sterically
larger photocatalysts such as PC 2 with its much larger
molecular radius effectively suppressed reactivity (entry 4
vs. 5), despite very similar emissive energy to PC 1 (54.5
vs. 55.2 kcal/mol), indicating the likelihood of an energy
transfer pathway. The most effective catalysts were relaꢀ
tively smaller (entries 5 and 6). The optimal catalyst was
found to be PC 4, whose emissive energy is 58.6 kcal/mol,
but is only slightly larger than standard PC 1. PC 4 was
used for further optimization. Evaluation of several carꢀ
boxylic acids (entries 7ꢀ10), revealed that a 90% solution of
formic acid was optimal, where both weaker and stronger
acids were detrimental to the reaction.
nol, the transꢀphenylcyclohexene underwent methanolysis
17
of the olefin14b,
(Scheme 1B). Additionally, Schuster18
proposed a similar intermediate for the rearrangement of
cyclohexenones. Further studies from the McClelland
group with UV flash photolysis have demonstrated the
presence of a transient cationic intermediate, which in some
cases could be intercepted with a solvent.19 Importantly,
these precedents demonstrated the differential reactivity
between the cis- and trans-cyclohexenes, and the potential
for productive syntheses.
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Despite these compelling early results, while larger
trans-cycloalkenes have been deployed in synthesis,20 the
application of transꢀcyclohexene in synthesis is still almost
nonꢀexistent, with the exception of the recent report from
the Larionov group21 of an elegant exploitation of UV light
mediated carboborative ring contraction. This sparsity is
likely due, in part, to the use of UV light and historically
strongly acidic conditions (i.e. H2SO4),17b which limit the
functional group tolerance. A more challenging aspect in
performing synthesis, however, is contending with the
small energetic barrier to isomerization (ca. 7.5 kcal/mol).
In order to accomplish this, we reasoned that precoordinaꢀ
tion of the reactive partner would be the key to overcoming
the difficulties of the short lifetime of the transꢀalkene,
because it would allow the entropic penalty to be paid beꢀ
fore the transition state, thus bringing down the energy
barrier.
We postulated that an alcohol group could hydrogen
bond with an acid prior to isomerization (Scheme 1C),
preparing ahead of time for the short lived, highꢀenergy
transꢀcyclohexene, ultimately facilitating the protonation of
the alkene by preꢀpositioning the acid near the alkene. The
generated carbenium ion and the internal alcohol could then
undergo ring closure to form the ether product. In addition,
use of a tertiary benzylic alcohol, prone to acid catalyzed
elimination, would demonstrate the increased reactivity of
the trans-cyclohexene. The expected reactivity of trans-
cyclohexene in relation to its cis-isomer would be orthogoꢀ
nal because it would circumvent the expected elimination
of the tertiary alcohol and allow reaction to take place only
when stimulated by light. Further, we reasoned that the use
of a photocatalyst would allow us to generate the requisite
trans-cyclohexene indirectly with visible light, preventing
side reactions that typically accompany UV photolysis.
EXPERIMENTAL
Analysis of the optimal temperature revealed maximal
reaction rate at 45 °C (entry 15). Curiously, examination at
early timepoints revealed a bifurcation of the rate constant,
which increased with temperatures both higher and lower
than 0 °C (entries 11ꢀ16). This unusual result is consistent
with a highly reactive intermediate, in which lower temperꢀ
atures extend the lifetime of the reactive species by reducꢀ
ing the free energy available for thermal reversion to the
To begin our investigation, we subjected cyclohexenol
1a to catalytic amounts of PC1 (Ir(ppy)3), formic acid
which we anticipated would not induce elimination, due to
its weak acidity, and blue LEDs (Table 1, entry 1). We
were pleased to observe the formation of the desired bicyꢀ
clic ether in good conversion (78%). Control reactions
established the necessity of all three components (entry 2).
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cisꢀcyclohexene species. Conversely, we expected that
tocatalyst.
increased temperatures increase the rate either by increasꢀ
ing the probability of the cisꢀcyclohexene encountering an
excited photocatalyst to generate the reactive transꢀ
cyclohexene, or alternatively, could populate higher energy
conformations, which would be more capable of energy
transfer. We explore these possibilities below.
At this point in the investigation, several key assumpꢀ
tions about the reaction had been made. It was assumed
that, as established previously in related transformations,6,
11ꢀ12
photochemical energy from a photon in the visible
light region is absorbed by a photosensitizer, which after
photoexcitation to a singlet, ultimately leads to a longꢀlived
triplet excited state.25 Because the catalyst is in a triplet
state, and photocatalyst radii dependency is observed (Taꢀ
ble 1, entry 4), then the energy transfer most likely occurs
via a Dexter26 mechanism, exciting the substrate to a triplet
biradical. This triplet biradical is capable of bond rotation
about the former double bond. The triplet biradical rotates
to an orthogonal position because not only do the formerly
π electrons now repel each other, but rotation also serves to
relieve 1,2ꢀeclipsing interactions. Upon returning to the
ground state from the triplet landscape via a nonꢀradiative
intersystem crossing event (ISC; conical intersection) at, or
very near the transition state for rotation about the double
bond, the former alkene can then further rotate and relax to
form either the transꢀcyclohexene which can engage in
subsequent reactivity, or unproductively reform the cis-
alkene, evolving its energy as heat.
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We initiated our exploration of the scope of the reaction
by varying the substituent attached at the carbinol carbon.
The reaction was remarkably tolerant to various substituꢀ
tion at this position, giving the cyclized product in very
good yields for electron poor arenes (2aꢀ2d), electron neuꢀ
tral arenes (2e and 2f), alkynyl (2g), and alkyl substituents
(2h). The propensity of some of the carbinols to ionize
under acidic conditions (i.e. tertiary benzylic22 or proparꢀ
gylic alcohols23), highlights the importance of using a weak
acid, and demonstrates the enhanced basicity of transꢀ
cyclohexene. We next explored the vinyl arene substituent
(2a vs. 2iꢀ2n). The arene is an essential component of the
substrate as it makes the triplet state of the alkene energetiꢀ
cally accessible through visible light photocatalysis.32
Because the conjugation is an important facet to triplet
stabilization,24 it is unlikely that the reaction would proceed
with ortho substituents which dramatically disrupt the conꢀ
jugation both in the ground state and the vertically excited
triplet. Within this limitation, the arene was quite amenaꢀ
ble to substitution with electron rich (2a), neutral (2i), and
moderately electron poor (2j and 2k) arenes. However,
strongly electron deficient arenes (2l) failed to give any
cyclized product. This failure may result from very slugꢀ
gish protonation of the transꢀalkene, which instead underꢀ
goes thermal reversion to the relaxed cis- starting material,
although electronic modulation of the HOMO–LUMO gap
is possible as well, and may prevent excitation by the phoꢀ
There were, however, several mechanistic quandaries,
such as what purpose does the acid serve? Does the acid
serve only as a proton source, or does it actually precoordiꢀ
nate as we have supposed? These questions ultimately culꢀ
minate in the penultimate question: what does the transiꢀ
tion state look like? We therefore sought to probe these
questions, beginning by investigating the geometry of the
transient reactive species.
MECHANISTIC INVESTIGATION
To gain further insight to the reaction, and determine if
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the geometries we were envisioning were feasible, we set
out to perform conformational analyses computationally.
Since triplet state energies would be of interest here, we
settled on using MøllerꢀPlesset (MP2)27 theory, ultimately
with large, correlation consistent basis sets.28 First, we
minimized the starting alcohol and performed two dihedral
drivers at the freely rotatable C–C bonds connecting the
three rings (Figure 1A). The carbinol phenyl (C1) preferred
to rotate 90o from the adjacent ring, presumably to avoid
allylic strain. Looking at the styrenyl phenyl revealed a
more complex landscape, in which the lowest energy conꢀ
former has the two rings 28o out of planarity. We subjected
the 90° cross section to an increased number of scan points
for rotational angles (Figure 1B). Next, we subjected select
structures to a higher level of computational theory, in orꢀ
der to provide insight into the relationship between the
thermal and the excited state landscapes.
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Beginning from the minimized geometry with the methꢀ
oxyphenyl ring canted 28° from planarity with the cis-
cyclohexene ring, a vertical excitation from the singlet to
the triplet state was calculated using the ccꢀPVTZ basis
set.28b The energy difference for this excitation was found
to be in excess of the emissive energy of the photocatalyst.
This is because the minimized geometry favors deconjugaꢀ
tion of the methoxyphenyl dihedral angle at C4 of the cis-
cyclohexene, and therefore puts the singlet to triplet excitaꢀ
tion energy out of reach of the photocatalyst. At ca. 1
kcal/mol higher energy, rotation of the methoxyphenyl ring
into planarity with the cis-cyclohexene double bond is easiꢀ
ly thermally accessible at a reaction temperature of 45 °C.
Based on a Boltzmann distribution population analysis,29
increasing reaction temperature from 0 °C to 45 °C increasꢀ
es the frequency of observing a molecule in the fully conꢀ
jugated conformer by ca. 30%, based on the relative enerꢀ
gies of these conformers. This provides some insight to
support our prior supposition as to the bifurcation of reacꢀ
tion rate in relation to reaction temperature (Table 2, entries
13ꢀ16).
Vertical excitations from the singlet to the triplet state
were calculated at minima and maxima of this landscape, as
well as 0o. In lieu of performing an exhaustive and correꢀ
spondingly computationally expensive ring conformer
search on the cyclohexene ring, dihedral angles for rotation
about the C4–methoxyphenyl ring were frozen, and a simꢀ
ple geometry optimization was performed on the ground
state singlet, in the gas phase. While absolute energy difꢀ
ferences for singlet to triplet excitation are greater than
expected, the trend of the gap demonstrates the importance
of conjugation. Conjugation of the alkene with the methoxꢀ
yphenyl π cloud brings the calculated excitation energy to a
minimum, which is known experimentally to be within the
range of the emission energy of the photocatalyst (58.6
kcal/mol).30
Next, we considered the strained trans-cyclohexene speꢀ
cies. Importantly, upon ISC, the formation of the trans-
cyclohexene generates four potential diastereomers, of
which only two could lead to ring closure (axialꢀOH). We
have termed these the synꢀboat and the antiꢀchair (Figure
2). Calculations of the ground state energies of these speꢀ
cies indicated that the antiꢀchair would be lower in energy
by ca. 5.5 kcal/mol, which is consistent with the work of
Johnson.31 This energy difference cannot be used to anticiꢀ
pate the reactive conformer, however, since both are lower
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than the triplet state energy and thus accessible therefrom.
In order to discern the reaction pathway, we would need
stereochemical evidence. Since the upper limit for the enꢀ
ergy to excite the cis-cyclohexane is the emission energy of
the photocatalyst, then the maximum height conical interꢀ
section on the singlet state landscape must also be bounded
by this restriction. It is of note that the difference between
the ground state energy of the antiꢀchair trans-cyclohexene
and the λmax emissive energy of the photocatalyst is approxꢀ
imately the same as not only the value that Dauben found
for reversion of transꢀphenylcyclohexene to the cis- startꢀ
ing material, but also the computationally determined value
determined by Johnson for simple cyclohexene.
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The presence of isotopic scrambling only on the formerly
vinyl carbon indicated that the protonation/deuteration was
essentially irreversible, and could have occurred in two
ways. Exo protonation of the synꢀboat can be ruled out,
since it would lead to incorporation of the deuteron on the
same side of the ring as the forming ether, not the experiꢀ
mentally observed product. Similarly, endo protonation of
the antiꢀchair conformer should be ruled out because it too
leads to the incorrect deuterated regioisomer. It is possible
that the protonation/deuteration could have occurred from
the endo face of the synꢀboat transꢀcyclohexene ring. Howꢀ
ever, because the endo face is the more sterically congestꢀ
ed, it seemed unlikely to be the face of protonation. Conꢀ
ceivably, the C1 hydroxyl could have been serving to proꢀ
tonate the endo face of the syn-boat conformer. Assessment
of this scenario reveals that the proton or deuteron would
subsequently have to travel through the van der Waals
cloud of the cyclohexane ring, likely requiring a tunneling
event to arrive at the proper orbital to generate the experiꢀ
mentally observed product. If this did occur, one could
It was envisioned that a deuterium label could provide
stereochemical evidence of the geometry of the bond which
undergoes hydroꢀalkoxylation, as the face of incorporation
and the number of sites deuterated would provide mechaꢀ
nistic insight with regard to the reversibility of the protonaꢀ
tion step. Submitting 1a to reaction conditions in which a
large excess of D2O was added resulted in deuterium incorꢀ
poration only on the face of the cyclohexene ring opposite
that of the forming ether, so that the deuterated product
shows signal attenuation equivalent to the incorporation of
1 hydrogen only in the axial position of the product (Figure
3A). This was affirmed by correlation with computational
NMR simulation (Figure 3B).29, 32
expect a very large kinetic isotope effect (KIE), (kH/kD
>
40),33 but it was not observed (vide infra). The simplest
explanation is that the deuteration event happens from the
exo face of the antiꢀchair. Although the exo protonation of
the antiꢀchair conformer seemed the most likely, the quesꢀ
tion still to be answered was how that exo protonation ocꢀ
curred.
In order to probe this, we next began investigating the
kinetics of the reaction. Initial rates experiments indicate
that the reaction is first order with respect to catalyst. With
the substrate, the reaction appeared to have zero order kiꢀ
netics.29 With formic acid, at low loadings there was a lineꢀ
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ar response to concentration, while at higher loadings it fell
point was that it further suggests that the protonation of the
alkene is not occurring from the endo face of the synꢀboat
conformer. If it was, it would have to travel through the van
der Waals cloud of the cyclohexene ring and would thereꢀ
fore likely have tunneling characteristics.33
out of the rate expression.29 This led us to suspect that the
acid may be undergoing a preꢀrate determining step equilibꢀ
rium, i.e. a precoordination event. Further support for the
latter idea was observed in 1H NMR via titration of the
tertiary alcohol starting material (1f) with formic acid under
anhydrous conditions, which resulted in a slight deshielding
of the hydroxyl signal in the reaction solvent, acetonitrile.29
This is consistent with hydrogen bonding of the proton with
an Hꢀbond acceptor, such as an oxygen in formic acid. It is
remarkable that this interaction can be observed at all, givꢀ
en that acetonitrile can compete with other hydrogen bond
acceptors, and overshadow many nonbonding or partially
bonding interactions.34
We then sought to combine the evidence of hydrogen
bonding between the hydroxyl proton and formic acid with
our conformational analysis of the transꢀcyclohexene.
Since exo protonation of the anti-chair trans-cyclohexene is
most likely, we sought to model a hydrogen bonded formic
acid molecule with the anti-chair geometry to see if a poꢀ
tential transition state would be geometrically feasible.
Optimization of the aforementioned geometries with a tranꢀ
sition state minimization at low basis set and theory level
(HF/3ꢀ21G) resulted in the geometry in Figure 4. This
model clearly demonstrates the possibility of such a transiꢀ
tion state in which a formic acid bridges the gap and allows
the exoꢀprotonation. While we are hesitant to assign the
identity of the transition state due to the inherent difficulꢀ
ties therein,35 this geometry explains a nonlinear transition
state and a corresponding smaller KIE.
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This precoordination of the substrate and acid serve to
reduce the entropy of the transition state. Importantly, we
anticipate such precoordination provides a proximal proton
to the trans-alkene upon its genesis, and that this is essenꢀ
tial to harnessing the strain energy. Similarly, while not
discussed in Larionov’s21 recent carboborative ring contracꢀ
tion, we suspect that the absence of dimerization products
or products of other reaction pathways is due to the precoꢀ
ordination of a Lewis acidic trialkyl borane with the alkene,
which serves a similar role to immediate protonation. We
conducted tandem rate experiments with phenylꢀ
cyclohexene, which is devoid of a hydroxyl group, and 1f29
under the standard reaction conditions in order to investiꢀ
gate the relative effect of the precoordination. These experꢀ
iments revealed much faster reactivity with the substrate
capable of precoordination (1f) than the phenylꢀ
cyclohexene could achieve without a hydrogen bond to the
formic acid, supportive of the assumption that the precoorꢀ
dination aids the reactivity.
When the aqueous environment of the reaction was reꢀ
placed with D2O, we observed a solvent KIE kH/kD of 1.5.
This is unusual because it is outside the ranges for what
would be expected for either a primary (greater than 2) or a
secondary normal KIE (1 to 1.4).33 Furthermore, in the case
of a rehybridization from an sp2 to sp3 carbon in the rate
determining step, one would expect to observe an inverse
KIE (0.8 to 1). Further obfuscating the interpretation of the
observed KIE is that there are multiple sites for deuterium
exchange, not only at the site observed in the final product,
but also in the hydroxyl, and in formic acid itself. Addiꢀ
tionally, if the proton/deuteron being transferred is from a
formic acid coordinated to the hydroxyl group, then a nonꢀ
linear transition state is expected, and thus a diminished
KIE would be expected as well. What could be said at this
DISCUSSION
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through an apparent exoꢀprotonation of the anti-chair diaꢀ
Combining all of these ideas into a cohesive, tentative
mechanistic proposal (Scheme 2), we believe key features
include the precoordination of a formic acid with the hyꢀ
droxyl proton (Scheme 2A). Upon excitation from the
ground state photocatalyst (G) an excited triplet state phoꢀ
tocatalyst (H) intercepts a cisꢀcyclohexene substrate, which
is in the proper conjugated conformer with an accessible
triplet state energy (F). The triplet state photocatalyst and
the singlet ground state substrate undergo Dexter energy
transfer to produce the excited triplet state cis-cyclohexene
biradical (I) and regenerate the ground state singlet photoꢀ
catalyst (G). The excited triplet state substrate biradical (I)
is then no longer bonding and undergoes rotation about the
remaining C–C σ bond so that the singly occupied orbitals
rotate orthogonal to each other (structures in Scheme 2B).
The substrate biradical is then close in both geometry and
energy to the thermal transition state for rotation, and unꢀ
dergoes an intersystem crossing event. Upon crossing from
the triplet back into the singlet energy landscape, it can
either relax to the cisꢀcyclohexene starting material (F) or
continue to rotate to the trans-cyclohexene antiꢀchair conꢀ
former (J), which is still coordinated with the formic acid
through a hydrogen bond (C). The protonation of the alkene
stereomer of the transꢀcyclohexene likely transfers more
than one hydrogen in the rateꢀdetermining step via a nonꢀ
linear transition state (Figure 4). Protonation of the alkene
generates a carbenium ion, which is rapidly attacked by the
alcohol oxygen to produce the oxabicyclic ether (D).
CONCLUSIONS
In conclusion, we have exploited the ability to dynamiꢀ
cally capture energy from visible light, and we have develꢀ
oped a strategy to exploit this energy to synthesize new
organic molecules. We have highlighted some of the intriꢀ
cacies that should be considered when thinking about enerꢀ
gy transfer photocatalysis, such as appropriate choice of
photocatalyst not only energetically, but also geometrically,
and in addition have highlighted the importance of the acꢀ
cessibility of the triplet state as a function of conformer
conjugation. A key hydrogen bonded precoordination event
makes possible the singular reactivity, and results in a
product that is not otherwise accessible. Its short lifetime is
a general and persistent issue that for many years has reꢀ
sulted in transꢀcyclohexene’s recalcitrance with regard to
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synthesis. We have demonstrated how precoordination with
The research results discussed in this publication were
made possible in part by funding through the award for
project number HRꢀ14ꢀ072, from the Oklahoma Center for
the Advancement of Science and Technology. Acknowlꢀ
edgment is made to the Donors of the American Chemical
Society Petroleum Research Fund for partial support of this
research with award ACSꢀPRF 56159ꢀDNI1. Quantum
mechanical calculations were performed on the supercomꢀ
puting cluster provided by the OSU High Performance
Computing Center at Oklahoma State University, which is
supported in part through the National Science Foundation
grant OCI–1126330. Some images were generated in part
through Chimera,36 which is developed by the Resource
for Biocomputing, Visualization, and Informatics at the
University of California, San Francisco (supported by
NIGMS P41ꢀGM103311). Special thanks to Sonal Priya for
help in editing this manuscript.
the reactive partner (an acid) can be used to overcome this
obstinacy without resorting to the use of UV light, and
provided a methodology that will hopefully make synthesis
involving trans-cyclohexene more tractable.
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ASSOCIATED CONTENT
Experimental details and product characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
*Jimmie.Weaver@okstate.edu
Funding Sources
ACKNOWLEDGMENT
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