Mild Isomerization of Conjugated Dienes Using Co-Mediated Hydrogen Atom Transfer

Article abstract of DOI:10.1021/acs.orglett.9b04594

A mild and high yielding rearrangement of 1,3-disubstituted-1,3-dienes to 1,1,4-trisubstituted-1,3-dienes using a cobaloxime catalyst and a silane cocatalyst is reported. Chiral centers near the conjugated diene were not racemized. Deuterium labeling studies are consistent with a hydrogen atom transfer mechanism, and radical intermediates were found to be accessible due to the observed ring opening of a cyclopropane ring.

Full text of DOI:10.1021/acs.orglett.9b04594

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Letter
Mild Isomerization of Conjugated Dienes Using Co-Mediated
Hydrogen Atom Transfer
Kyle R. Delgado, Dustin D. Youmans, and Steven T. Diver*
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ABSTRACT: A mild and high yielding rearrangement of 1,3-
disubstituted-1,3-dienes to 1,1,4-trisubstituted-1,3-dienes using a
cobaloxime catalyst and a silane cocatalyst is reported. Chiral
centers near the conjugated diene were not racemized. Deuterium
labeling studies are consistent with a hydrogen atom transfer
mechanism, and radical intermediates were found to be accessible due to the observed ring opening of a cyclopropane ring.
ydrogen atom transfer (HAT) has become a powerful
In this report, we find that HAT applied to 1,3-dienes results in
Hmethod for alkene isomerization. Mild access to alkyl a mild and selective isomerization of both π bonds to yield a
more highly substituted 1,1,4-trisubstituted-1,3-diene 2
(Scheme 1b). Instances of single isomerization to 3 were
also observed using Co catalysts Co1−Co4. The starting 1,3-
dienes 1 are easily prepared by ene-yne metathesis (EYM)
using Grubbs catalysts.
radicals provides routes to cycloisomerization and radical
functionalization as well.1 With Co−H catalysts, HAT
chemistry involves hydrogen atom transfer to produce a
radical pair,2 which can undergo isomerization, reduction, or
oxidation reactions.3 Advantages of this chemistry include its
mildness, functional group tolerance, and use of earth-
abundant transition metals such as Mn and Co. Shenvi et al.
developed this chemistry for practical alkene isomerization
(Scheme 1a)4a,b,1,4c with a similar system based on
cobaloximes (e.g., Co1) developed by Norton,5 but there are
few applications of HAT chemistry to conjugated dienes.6
With conjugated dienes 1, an allylic radical can isomerize in
two possible directions, toward R1, analogous to simple
alkenes,4a or toward R2 (Scheme 1b). Isomerization by the
latter pathway was observed previously with Ru(II) hydrides.
Controlled isomerization of conjugated dienes provides
access to highly functionalized 1,3-dienes which are found in
many natural products and can be readily functionalized.
Dienyl isomerization by a Ru(II) hydride was reported by us a
few years ago involving a mechanistically distinct process,
occurring via π-allyl Ru intermediates (Scheme 1d).7 The
process was found to be stereoconvergent since E/Z reactant
mixtures were taken to a single product. The π-allyl Ru
intermediate was thought to dictate the regiochemistry, giving
only 2. However, high temperatures were needed and some
side reactions such as oxidation by transfer hydrogenation were
observed. HAT chemistry is generally mild and offers
functional group tolerance due to radical intermediates.
Exposure of conjugated dienes to free radicals can be
problematic due to the potential for polymerization. If the
radical intermediates are well tolerated in conjugated dienes,
then HAT chemistry may offer mild reaction conditions that
could be beneficial for improving the substrate scope of dienyl
isomerization, allowing for applications to complex molecule
synthesis.8 We considered that it may further be possible for an
allylic radical intermediate to isomerize to the R1 position, an
alternative that was not previously observed with the RuH
catalyst.
Scheme 1. HAT Isomerization of 1,3-Dienes
Optimization of the diene isomerization using a variety of
Co precatalysts and hydride promoters are summarized in
Received: December 21, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04594
© XXXX American Chemical Society
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Table 1. Initially, conditions were adapted from Shenvi’s
Scheme 2. Chemoselectivity of in Situ Prepared Co
Hydrides
alkene isomerization using CoIII(salen)Cl9 as the precatalyst.
a
Table 1. Optimization of HAT Promoted Isomerization
isomerization catalyst system (salen)CoIIICl and 0.5 equiv of
PhSiH3, 1-decene was completely isomerized after 3 h at 60
°C. We infer that the cobaloxime-derived putative hydride has
optimum reactivity with 1,3-dienes.11
The scope of the Co-promoted dienyl isomerization is
shown in Scheme 3. The starting materials were 1,3-dienes
entry
catalyst
5% CoIII(salen)Cl
5% Co1
5% Co1
2% Co1
1% Co1
5% Co2
5% Co3
5% Co3
5% Co4
5% Co1
5% Co1
5% Co1
5% Co1
5% Co1
5% Co1
solvent
temp, time
yield
3E/3Z
1
2
3
4
5
6
7
8
9
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
THF
acetone
CH2Cl2
MeOH
C6H5CF3
benzene
benzene
60 °C, 3 h
60 °C, 3 h
rt, 12 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
rt, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
60 °C, 3 h
0
−
91
88
96
75
75
93
91
84
88
78
80
81
67
92
82
0
4:1
2.5:1
1.1:1
1:1
4:1
6:1
4:1
4:1
4:1
3:1
4:1
2.4:1
2:1
2:1
3:1
0
Scheme 3. Scope of Co-Promoted Dienyl Isomerization
b
10
11
c
12
13
14
15
16
17
18
5% Co1
5% CoII(salen)
5% Co(TPP)
trace
1:1
a
b
Major 3E-stereoisomer shown. Reaction run in the dark.
c
Substituted PMHS (6 equiv) for PhSiH3. PMHS = polymethyl-
hydrosiloxane; TPP = meso-tetraphenylporphyrin.
With substrate 1a, no conversion was observed (entry 1).
Combining PhSiH34a with Norton’s cobaloxime catalysts,3a,10,5
we identified conditions for dienyl isomerization (entry 2).
The reaction could be run at room temperature though longer
times were required for full conversion (entry 3). Decreased
loading of Co1 was tolerated, but the 3E/3Z ratio decreased to
1:1 E/Z (entries 4, 5). Other cobaloximes were effective (entry
6). The more air-sensitive Co3 showed the highest yield and
had good reactivity at room temperature (entries 7, 8).
Subsequently, we focused on Co1 due to its ease of
preparation. Reaction occurred in the dark and with alternative
reducing agents such as polymethylhydrosiloxane (PMHS,
entries 10, 11). A solvent screen showed that dienyl
isomerization occurred in most common organic solvents
(entries 12−16). Interestingly, the Co(II) species Co(II)-
(salen) and Co(TPP) did not catalyze the isomerization
(entries 17, 18). Though reactions were performed at 3 h
reaction times to make comparisons, it is noted that some were
faster (vide infra) and most could be run at room temperature
over longer periods of time. Higher temperature (60 °C) or
longer reaction times (12 h) also helped improve the E/Z
ratio, favoring the 3E-isomer of 2a.
The catalyst formed from cobaloxime and PhSiH3 showed
surprisingly different isomerization reactivity with alkenes
versus 1,3-dienes. The standard conditions of entry 2 in
Table 1, catalyst Co1 (benzene 60 °C, 3 h), did not isomerize
1-decene to its internal isomer. Under the same conditions,
catalysts Co2 and Co4 also did not isomerize 1-decene. A
competition experiment where equimolar amounts of 1a and
1-decene were present with Co1 under the standard conditions
only resulted in isomerization of the 1,3-diene, but not 1-
decene (Scheme 2). In contrast, using Shenvi’s alkene
obtained directly by ene-yne metathesis, used as E/Z
mixtures.12 The standard conditions were adapted from entry
2, Table 1, and the reaction time was set at 3 h. Similar to our
previous study employing a Ru−H catalyst, the isomerization
gave dienes 2. Oxygen functionality was generally well
tolerated and protection was not necessary as alcohols were
found to perform well. With substrates containing alcohols,
none of the corresponding aldehyde was detected in the crude
1H NMR spectrum. Oxidation can be a side reaction using Ru
hydride catalysts. Isomerization occurred into substituted
positions without difficulty. Last, an aldehyde substrate was
tolerated showing compatibility of carbonyl functionality with
the catalyst system. The trisubstituted alkene was formed as an
E/Z mixture, and in select cases when longer reaction times
were used, an increase in E/Z ratio occurred. In three cases, E-
selectivity was observed and often the E/Z isomers proved
separable by flash chromatography.
Though the Co-promoted dienyl isomerization was stereo-
convergent, the E and Z reactants displayed different reactivity
profiles and resulted in different kinetic E/Z ratios of
products.13 The time course for reaction starting with pure E
or Z isomers is shown in Figure 1. First, the E-diene E-1j was
about three times more reactive than the Z isomer Z-1j.
Second, the conversion of E-1j showed that a secondary alkene
isomerization occurs during the reaction. At 10 min, complete
consumption of E-1j is found resulting in a 1:1 3E/3Z ratio
(trisubstituted alkene) of 2j. After another 20 min, the 3E/3Z
ratio had upgraded to 2:1. Given these remarkably fast diene
B
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needed to address the interplay of the two regiochemical
pathways.
We investigated additional substrates to verify that the
stereochemistry near the reactive sites would not be affected
(Scheme 5). Because of our interest in using this mild Co-
Scheme 5. Stereochemistry Probes
promoted dienyl isomerization in natural product total
synthesis, we were particularly concerned about the integrity
of enantiomerically enriched chiral centers near the reactive
sites. Chiral centers of secondary alcohols are vulnerable to
oxidation in Ru- and RuH-promoted reactions.14 When highly
enantiomerically enriched 1e and 1n were subject to Co-
promoted dienyl isomerization, no loss of stereochemical
integrity at either center was observed.
Several experiments shed light on the reaction mechanism.
Given the precedents by Shenvi4a and Norton,3a,5 we expected
radical intermediates. Accordingly, cyclopropyl-substituted 1,3-
diene 1t was prepared by ene-yne metathesis and subject to the
standard conditions at room temperature (Scheme 6). Triene
Figure 1. Contrasting reactivity and different initial product E/Z
ratios starting with pure E or Z isomers. (a) E-1j. (b) Z-1j.
Concentrations were determined by H NMR spectroscopy (CDCl3,
1
rt).
position isomerizations, the longer reaction times in Table 1
serve to upgrade the product stereochemistry through a
secondary Z to E isomerization. Third, the slower conversion
of the Z-isomer results in stereoselective product formation.
After only 50% conversion of starting Z-1j, a 4.5:1 product
ratio of 3E-2j/3Z-2j is obtained.
Regioisomeric dienyl isomerization was also possible. We
were curious to see whether isomerization could be induced
into dienes 3 if the preferred regiochemistry toward 2 was
blocked (Scheme 4). We were unable to observe this different
Scheme 6. Mechanistic Probe and Deuterium Incorporation
Study
Scheme 4. Alternative Regiochemistry
4t formed cleanly, showing that radical intermediates are
accessible under the standard reaction conditions.15 Next, two
deuterium labeling experiments were run with a substoichio-
metric Co catalyst prepared from PhSiD3.16 First, in the
isomerization of 1a, ca. 50% deuterium incorporation was
observed into the allylic methyl group of 2a, determined by the
diminution of the integral in the 1H NMR spectrum. Similarly,
we probed the mechanism of the Z to E isomerization, the
secondary isomerization observed in the time course graphs of
Figure 1. Starting from (3Z,5E)-2b, a high yield of isomerized
(3E,5E)-2b was observed with a diminution of the vinylic
proton at δ 5.5 ppm in the 1H NMR spectrum, suggesting 20%
deuterium incorporation. This established that the product of
Co-promoted isomerization was itself vulnerable to addition of
the putative Co−H species across the least substituted end of
regiochemistry with Ru hydrides previously.7 When an allylic
hydrogen was lacking, Co-promoted isomerization gave the
alternative regioisomer (1q to 3q). In a similar vein, the simple
diene 1r underwent clean isomerization to 3r showing that the
1,3-dienes do not need to be prepared from ene-yne
metathesis. Different regiochemistry was observed for 1s, a
substrate that possesses allylic hydrogens that can lead to either
regioisomer 2 or 3. Unlike the examples in Scheme 3, we
suggest that greater resonance stabilization by the ether oxygen
favors the regiosiomer 3s in this case. Further studies are
C
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the 1,3-diene product, explaining the Z to E isomerization
observed in the figures above.17
Based on the observations above, we suggest that a Co(III)
The synthesis of starting materials, the dienyl isomer-
ization procedure, reactivity profiles of pure stereo-
isomers, mechanistic studies and characterization data
for new compounds (PDF)
hydride is involved as proposed in Scheme 7.
Scheme 7. Proposed Mechanism
AUTHOR INFORMATION
Corresponding Author
■
Steven T. Diver − University at Buffalo, The State
University of New York, Amherst, New York;
orcid.org/0000-0003-2840-6726; Email: diver@
buffalo.edu
Other Authors
Kyle R. Delgado − University at Buffalo, The State
University of New York, Amherst, New York
Dustin D. Youmans − University at Buffalo, The State
University of New York, Amherst, New York
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04594
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. David Lacy (UB) for helpful
discussions. The authors gratefully acknowledge support
from the National Science Foundation through CHE-
1300702 and CHE-1566162.
This is a tentative hypothesis since none of the intermediates
proposed were observed and kinetics studies have not been
performed.18 Formation of the allylic radical B, shown also as
resonance structure B′, serves to weaken the adjacent CH
bond, giving the dienyl isomerization product and regenerating
the Co−H catalyst. Subsequent isomerization of the
trisubstituted alkene is possible through reversibility of the
last step, allowing isomerization via bond rotation in valence
isomer B. To explain the alternative regiochemistry, radical B
can eliminate into the R1 substituent if the primary pathway is
blocked or if greater resonance stabilization is achieved (panel
b). Last, the slower isomerization rate of Z-1j (seen above in
Figure 1B) may be due to steric strain in the resulting allylic
radical Z-B.19 From radical Z-B, direct allylic hydrogen atom
transfer by LnCo(II) may be possible, favoring the (3E,5E)
stereochemistry as observed in the time course graph of Figure
1.
In conclusion, a high yielding, stereoconvergent diene
isomerization has been reported using a putative Co hydride
catalyst. A wide reaction scope was observed, and stereocenters
near the diene were maintained. Unlike the Ru−H promoted
reaction, little over reduction and no dehydrogenation
byproducts were observed. Opening of a cyclopropane is
consistent with radical intermediates, and deuterium labeling is
consistent with Co-D addition to the geminal alkene of the
diene reactant (resulting in position isomerization) and
addition to the 1,2-disubstituted alkene of the product
(resulting in secondary alkene isomerization). The relative
mildness should make this a premier method for diene
isomerization.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
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[CoH] along with [CoD]. The [CoH] produced causes protium
incorporation competing with deuterium incorporation from [CoD].
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E
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