C-O Bond Cleavage of Alcohols via Visible Light Activation of Cobalt Alkoxycarbonyls

Article abstract of DOI:10.1021/acs.organomet.9b00552

A strategy for the activation of C-O bonds in alcohols via a carbonylation-homolysis-decarboxylation process is described. Using readily available cobalt(II) porphyrin precursors, carbonylation of simple alcohols provides access to alkoxycarbonyl cobalt(III) complexes. Spectroscopic, crystallographic, and computational methods are used to provide structural details and an estimate for the Co-C bond dissociation energy of an alkoxycarbonylcobalt(III) complex of 39.8 kcal/mol for the first time. Visible light irradiation in the presence of the radical trapping agent TEMPO and a thiol reducing agent demonstrates the cleavage of the alcohol C-O bond under oxidative and reductive conditions, respectively. Addition of a stoichiometric reducing agent allows the use of a catalytic amount of hindered thiol for the reduction of a benzylic alcohol to the corresponding hydrocarbon.

Full text of DOI:10.1021/acs.organomet.9b00552

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
C−O Bond Cleavage of Alcohols via Visible Light Activation of
Cobalt Alkoxycarbonyls
Dana R. Chambers,† Antoine Juneau,‡ Cory T. Ludwig,† Mathieu Frenette,‡ and David B. C. Martin*,†
†Department of Chemistry, University of California Riverside, Riverside, California 92521, United States
‡
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Departement de Chimie, Universite du Quebec a Montreal, Case Postale 8888, Succursale Centre-Ville, Montreal, Quebec H3C
3P8, Canada
S
* Supporting Information
ABSTRACT: A strategy for the activation of C−O bonds in
alcohols via a carbonylation−homolysis−decarboxylation
process is described. Using readily available cobalt(II)
porphyrin precursors, carbonylation of simple alcohols
provides access to alkoxycarbonyl cobalt(III) complexes.
Spectroscopic, crystallographic, and computational methods
are used to provide structural details and an estimate for the Co−C bond dissociation energy of an alkoxycarbonylcobalt(III)
complex of 39.8 kcal/mol for the first time. Visible light irradiation in the presence of the radical trapping agent TEMPO and a
thiol reducing agent demonstrates the cleavage of the alcohol C−O bond under oxidative and reductive conditions, respectively.
Addition of a stoichiometric reducing agent allows the use of a catalytic amount of hindered thiol for the reduction of a benzylic
alcohol to the corresponding hydrocarbon.
Scheme 1. Strategies for C−O Bond Activation of Alcohols
Utilizing Cobalt Porphyrins
INTRODUCTION
■
Alcohols are one of the most abundant classes of organic
molecules, with the hydroxyl group being prevalent in naturally
occurring feedstocks such as sugars and complex natural
products. Alcohols are also valuable synthetic intermediates,
being readily introduced and converted to a myriad of other
functional groups. Most transformations, however, require
preactivation of the hydroxyl group due to the relatively inert
nature of the C−OH bond.1 In particular, radical chemistry
relies on conversion to a halide or xanthate ester as a means to
overcome the strong C−O bond, which can be weakened by
the formation of an adjacent radical (Scheme 1A).2 Trans-
formations such as the Barton−McCombie deoxygenation
facilitate cleavage of the strong C−O bond through the
reaction of a xanthate ester with a tin radical species following
the mechanistic pathway shown in Scheme 1B.3 More recently,
iridium photocatalysis were utilized to generate radical (or
radical anion) intermediates that fragment4 and in 2018, Rovis,
Doyle, and co-workers reported a unique direct deoxygenation
via C−O homolysis of phosphoranyl radicals (Scheme 1C).5 It
remains an important goal to identify systems that avoid
stoichiometric preactivation and generate benign, low-molec-
ular-weight byproducts. The ability to directly activate a wide
variety of alcohols using an earth-abundant metal catalyst
would represent a very valuable advance in the efficient use of
alcohols in radical chemistry.
cobalt complexes with salen, salophen, and dimethylgyloxime
ligands.7 Alkylcobalt derivatives of these complexes are
typically synthesized from alkyl halides, and thermal- or
Coenzymes B12 (adenosylcobalamin, methylcobalamin, etc.)
have been extensively studied for their ability to undergo
reversible Co−C bond cleavage to generate carbon-centered
radicals in numerous important enzymatic processes.6 Many
model systems have been developed to mimic this homolytic
bond cleavage that are more synthetically available, such as
Received: August 15, 2019
© XXXX American Chemical Society
A
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light-driven homolysis8 leads to radical cyclizations9 and
initiation of living radicals for polymerizations.10,11 Pattenden
and co-workers have demonstrated stoichiometric radical
cyclizations12 and intermolecular Michael additions with cobalt
salophen pyridine complexes13 while Carreira and co-workers
used cobaloximes in a catalytic Heck-type coupling.14
Table 1. Oxidative Carbonylation of a PMP-porphyrin
Complex
a
While the synthesis and reactivity of the analogous
alkylcobalt porphyrin complexes have been studied,15−17
including determining their Co−C bond dissociation ener-
gies,18,19 much less is known about the acyl and alkoxycarbonyl
complexes. Chan and co-workers reported isolable acyl
Co(por) complexes (por = porphyrinato dianion) as
intermediates in a photochemical C−C bond activation of
ketones.20 Only one alkoxycarbonyl derivative has been
reported by Fu and co-workers for the initiation of living
radical polymerization.21 Development of a general synthesis of
alkoxycarbonyl porphyrin complexes would facilitate further
structure and reactivity studies.
We envisioned that the C−O bond cleavage of alcohols
could be achieved via decarboxylation of alkoxycarbonyl
radicals generated via alcohol carbonylation (Scheme 1D).22
In this way, a classic organometallic process could be merged
with cobalt-mediated radical chemistry to ultimately develop a
catalytic C−O bond cleavage via a polar/radical crossover
process. Herein, we describe the efficient synthesis of
porphyrin-derived cobalt alkoxycarbonyl complexes and the
study of the radical generation with visible light. This strategy
enables the C−O bond activation of alcohols and alkyl radical
generation with carbon dioxide as the only byproduct and
establishes the fundamental processes for a cobalt-mediated
C−O bond activation strategy.
a
Reaction conditions: Co(por) (0.25 mmol), alcohol (1.25 mmol),
DDQ (0.25 mmol), toluene (20 mL), 7 atm of CO, room
temperature. DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
RESULTS AND DISCUSSION
■
We first turned our attention to the synthesis of alkoxycarbonyl
cobalt porphyrin complexes with representative alcohols. Fu
and co-workers have demonstrated the synthesis of a
methoxycarbonylcobalt tetramesitylporphyrin (TMP) using
methanol and silver(I) triflate under CO.21 Unfortunately,
this synthesis was not effective with more complex alcohols in
our hands, giving only undesired side products. As such, we
have developed a method informed by previous work from our
group on the carbonylation of salen and salophen cobalt
complexes and other related reports.22−24 Both methods are
quite general, tolerating a wide range of alcohols and ligand
substitution patterns.
electron-deficient porphyrin TFP was achieved, albeit in
lower yield (12, 56%).
Structural and Spectroscopic Studies. In order to
obtain detailed structural information, we carried out X-ray
diffraction studies. Crystals of complex 9 were grown by slow
diffusion of pentane into CHCl3, resulting in long thin red
crystals with the complex adopting a square-pyramidal
geometry (Figure 1). This pentacoordinate form is commonly
observed in similar porphyrin systems that contain acyl20 and
vinyl25 substituents. The alkoxycarbonyl complex has a slightly
shorter Co−C1 bond length in comparison to the acyl
substituents (1.906 Å vs 1.922−1.926 Å) and comparable Co−
N bond lengths. The cobalt is displaced above the porphyrin
plane toward the apical carbonyl group (C1), which is also
observed in the other acyl and amine26 complexes but not in
the instance with an apical vinyl group. This may result from a
greater degree of back-bonding into the π* orbital of the
alkoxycarbonyl substituent that is less pronounced in the vinyl
counterpart.
The C−Co bond dissociation energies (BDE) of a variety of
alkyl cobalt complexes have been measured experimentally
through the extensive studies of Halpern and others, and more
recently DFT calculations have provided estimates for many
more complexes.27−29 The C−Co BDEs for alkoxycarbonyl
complexes, on the other hand, have not been reported to the
best of our knowledge. We calculated complex 9’s C−Co BDE
using the BP86 functional that was found to correctly predict
BDEs for alkyl C−Co BDEs (see the Supporting Information
for more details).19,30,31 The predicted structure using DFT
We first evaluated the oxidant required for this trans-
formation, finding many inorganic oxidants and weaker
benzoquinones ineffective. A strong oxidant such as DDQ
was optimal for the in situ oxidation without interfering with
the carbonylation process. As shown in Table 1, optimized
conditions using 1 equiv of DDQ and 5 equiv of alcohol
produced complexes 5−8 in good yields following purification
by recrystallization. This method was efficient for 1°, 2°, and
3° alcohols. These complexes are air stable at room
temperature for months; however, they are highly labile on
either silica gel or alumina, resulting in decomposition to the
Co(II) porphyrin starting material. This method of carbon-
ylation was then tested on a wider range of porphyrin ligands
with differing electronics (Table 2). Oxidation/carbonylation
of 1-phenethanol with electron-rich porphyrin systems such as
TAP, TPP, and OEP gave excellent conversion to complexes
9−11, respectively. The same transformation with the
B
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Table 2. Oxidative Carbonylation with Substituted
Porphyrins
a
Reaction conditions: Co(por) (0.25 mmol), alcohol (1.25 mmol),
DDQ (0.25 mmol), toluene (20 mL), 7 atm of CO, room
b
temperature. Reaction conditions: Co(OEP) (0.25 mmol), alcohol
Figure 2. Overlay of the optimized structure at the BP86/LanL2DZ
level of theory (green) and X-ray crystal structure of compound 9
(red).
(0.75 mmol), DDQ (0.25 mmol), toluene (15 mL), 7 atm of CO,
room temperature. Reduced amount of alcohol to facilitate isolation.
Figure 1. X-ray crystal structure of 9 with thermal ellipsoids at the
50% probability level. The compound crystallizes as a racemate; only
one enantiomer is shown for clarity. Selected bond lengths (Å) and
angles (deg): Co−C1 1.906(2), Co−N1 1.963(2), Co−N2 1.958(2),
Co−N3 1.945(2), Co−N4 1.956(2); C1−Co−N3 95.45, C1−Co−
N4 88.17, C1−Co−N1 96.45, C1−Co−N2 90.87, N3−Co−N4
89.79, N4−Co−N1 90.41, N3−Co−N2 89.76, N1−Co−N2 90.32.
Figure 3. Absorption spectra of 9 in CHCl3.
Reactivity Studies. We next examined the reactivity of the
alkoxycarbonyl compounds under activation with visible light
to determine if they retain behavior similar to that of vitamin
B12 and their alkylcobalt counterparts. We anticipated that
visible light corresponding to the absorption bands above
would be sufficient to homolyze the Co−C bond. To compare,
the energy of green photons at the emission maximum of a
simple LED source (515 nm) is 55.5 kcal/mol, more than
sufficient to cleave the C−Co bond.
Radical trapping experiments were performed with benzylic
alcohol derivatives, TEMPO, and various light sources (Table
3).32 On the basis of studies by Newcomb and Pattenden, we
expected to observe the decarboxylated products due to the
rapid formation of stabilized benzylic radicals from the
intermediate acyl radical.33,34 Efficient radical generation
could be achieved with a 390 nm LED source, and green
showed excellent agreement with the crystal structure (Figure
2) with a calculated C−Co bond length of 1.892 vs 1.906 Å in
the crystal structure. In comparison to other Co−alkyl
complexes, the calculated bond dissociation energy was fairly
high at 39.8 kcal/mol.
With the structure of the alkoxycarbonyl complexes
confirmed, we further examined their spectroscopic character-
istics. The UV−vis absorption spectrum for compound 9
(Figure 3) exhibited a strong Soret band at 424 nm and
another maximum at 540 nm. The absorption maxima are well-
suited for photochemical activation using green or violet light
(vide infra). Excitation at either band gave no measurable
fluorescence for compound 9.
C
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a
Table 3. TEMPO Trapping Experiments
Table 4. Hydrogen Atom Transfer Studies Using a Sterically
Hindered Thiol
with an excess of hindered thiol 15, only moderate yields were
achieved along with full consumption of thiol to disulfide. We
explored Hantzsch ester 19 as a stoichiometric reducing agent
to regenerate the thiol from the thiyl radical 18 (Scheme 2),
Scheme 2. Proposed Turnover of Thiol with Hantzsch Ester
19
a
Reaction conditions: 8, 10, or 11 (0.02 mmol), TEMPO (0.04
mmol), CHCl3 (2 mL), room temperature. TEMPO = (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl.
LEDs (510−520 nm) also gave full conversion, however with a
longer reaction time (16 h). A higher intensity LED lamp with
emission maxima at 450 and 550 nm achieved the same
conversion in 4 h.35 Some homolysis was observed with 440
and 456 nm light sources; however, these mismatched
wavelengths were much less effective, as expected from the
observed absorption spectra.
Under these conditions, efficient homolysis and loss of CO2
occurred and TEMPO adducts 13 and 14 were isolated in 58−
80% yield (Table 3); as expected, only decarboxylated
products were observed. Co(TAP), Co(TPP), and Co(OEP)
were also isolated from these reactions, indicating that no
major decomposition of the Co-porphyrin complexes took
place under these reaction conditions. Interestingly, when the
p-bromobenzyloxycarbonyl compound was subjected to
irradiation conditions, the TEMPO-trapped product was
isolated without loss of the bromine atom. This outcome
demonstrates that this method of radical generation tolerates
aryl bromides, a reactive source of radicals under traditional
initiation conditions.
To further explore the utility of this strategy to
deoxygenation, irradiation in the presence of a hydrogen
atom donor was performed. We focused on thiols on the basis
of the favorable thermodynamics of H atom transfer between
thiols and the transient benzylic radical (S−H BDE = 82 kcal/
mol for thiophenol, C−H BDE = 86 kcal/mol for ethyl-
benzene).36−39 Initial studies found that the thiophenol
underwent rapid dimerization to disulfide under our reaction
conditions; therefore, more sterically hindered thiols were
examined (Table 4). Introducing methyl or isopropyl
substituents at the ortho positions slowed dimerization and
increased the yield of ethylbenzene (entry 1, 43% yield). Even
which could address the issue of disulfide formation.
Gratifyingly, inclusion of Hantzsch ester increased the yield
of ethylbenzene to 94% (entry 2). Although Hantzsch ester
was not effective in the absence of thiol (<5% yield), it was
highly efficient at turning over the thiyl radical and allowed the
H atom donor to be reduced to substoichiometric quantities,
providing ethylbenzene in 86% yield (entry 3). These studies
demonstrate that decarboxylation and reductive radical
trapping can be achieved with these complexes under mild
conditions.
CONCLUSIONS
■
We have synthesized several new alkoxycarbonyl complexes of
cobalt porphyrins and examined their structure and reactivity.
Alcohols undergo efficient carbonylation with Co-porphyrins
to produce alkoxycarbonyl cobalt species featuring different
substitution patterns. These complexes are estimated to have
Co−C BDEs of 39.8 kcal/mol by computational methods and
absorb visible light to produce acyl radicals that readily
decarboxylate. Trapping of the alkyl radicals with TEMPO and
a thiol reducing agent provides a proof of concept for
decarboxylation and reduction protocols in the presence of
cobalt porphyrins. This approach enables the C−O bond
activation of alcohols via alkyl radical generation with carbon
dioxide as the only byproduct and establishes the fundamental
D
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porphyrin (0.25 mmol, 1 equiv), 130 μL of benzyl alcohol (1.25
mmol, 5 equiv), and 56.7 mg of DDQ (0.25 mmol, 1 equiv). After 16
h, the reaction mixture was worked up as outlined in the general
procedure. The reaction afforded a red solid (215 mg, 0.23 mmol,
processes for a cobalt-mediated C−O bond activation strategy.
Efforts to extend this carbonylation−homolysis−decarboxyla-
tion process to a catalytic method analogous to the Barton−
McCombie deoxygenation are ongoing and will be reported in
due course.
1
93%): IR (film) 2933, 1688, 1606, 1246, 1025 cm−1; H NMR (400
MHz, CDCl3) δ 8.89 (s, 8H, por-H), 7.92 (s, br, 8H, PMP), 7.24 (d, J
= 8.0 Hz, 8H, PMP), 7.03 (t, J = 7.4 Hz, 1H, Ph), 6.86 (t, J = 7.5 Hz,
2H, Ph), 5.34 (d, J = 7.5 Hz, 2H, Ph), 4.07 (s, 12H, OCH3), 2.63 (s,
2H, OCH2Ph); 13C NMR (10 MHz, CDCl3) δ 159.4, 145.9, 134.7,
134.3, 132.7, 127.8, 127.2, 126.3, 121.8, 112.4, 67.9, 55.7; HRMS
(ESI) m/z calcd for C56H43CoN4O6 M+ 926.2509, found 926.252
tert-Butyloxycarbonylcobalt meso-(4-Methoxyphenyl)-
porphyrin (7). This compound was prepared according to the
general procedure A using 198 mg of cobalt meso-(4-methoxyphenyl)-
porphyrin (0.25 mmol, 1 equiv), 119 μL of tert-butyl alcohol (1.25
mmol, 5 equiv), and 56.7 mg of DDQ (0.25 mmol, 1 equiv). After 16
h, the reaction mixture was worked up as outlined in the general
procedure. The reaction afforded a red solid (134 mg, 0.15 mmol,
EXPERIMENTAL SECTION
■
1H and 13C NMR spectra were recorded on a Bruker Avance Neo 400
MHz spectrometer unless otherwise indicated and were internally
referenced to residual protio solvent signal (note: DMSO-d6
referenced at δ 2.50 and 39.52 ppm, respectively; CDCl3 referenced
at δ 7.26 and 77.16 ppm, respectively). Data for 1H NMR are
reported as follows: chemical shift (δ, ppm), integration, multiplicity
(s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, app =
apparent), and coupling constant (Hz). Data for 13C NMR are
reported in terms of chemical shift, and no special nomenclature is
used for equivalent carbons. IR spectra were recorded on a Bruker
Alpha FT-IR spectrometer. Exact masses were recorded on an Agilent
LCTOF instrument using direct injection of samples in methanol into
the electrospray source (ESI) with positive ionization. LEDs were
purchased from Kessil (http://www.kessil.com/photoredox/
Products.php).
1
60%): IR (film) 2928, 1692, 1507, 1247, 1027 cm−1; H NMR (500
MHz, CDCl3) δ 8.88 (s, 8H, por-H), 8.03 (s, 8H, PMP), 7.25 (d, J =
8.8 Hz, 8H, PMP), 4.07 (s, 12H, OCH3), −0.75 (s, 9H, t-Bu); 13C
NMR (125 MHz, CDCl3) δ 159.3, 145.6, 134.7, 134.4, 132.5, 121.0,
112.3, 55.6, 25.8; HRMS (ESI) m/z calcd for C53H45CoN4NaO6Na
(M + Na)+ 915.2569, found 915.2567.
General Methods. All reactions were carried out in oven-dried or
flame-dried glassware charged with a magnetic stir bar, were prepared
under an inert nitrogen atmosphere, and were subsequently degassed
with carbon monoxide. Solvents were dried by passage through
alumina columns. Commercially available alcohols were distilled from
calcium hydride prior to use. Co(II) porphyrins were prepared
according to known literature procedures or purchased and used as
received.40−43
General Procedures. Procedure A. To a mixture of Co(II)
porphyrin and DDQ were added 20 mL of toluene and the respective
alcohol. The reaction mixture was degassed and pressurized to 7 atm
of CO using a Parr pressure vessel and stirred for the indicated time,
in the dark, at room temperature. Upon completion of the reaction,
the solvent was removed and the crude reaction mixture was
recrystallized from CH2Cl2 and heptane. The red precipitate was
filtered through Celite and washed several times with heptane to
remove excess unreacted alcohol. Subsequent washing with 2/1
hexanes/CH2Cl2 separated the product from the remaining Co(II)
starting material. The red Co(III) filtrate was concentrated to yield a
red solid. No further purification was required.
Procedure B. To a mixture of alkoxycarbonylcobalt(II) porphyrin
and TEMPO was added 1.5 mL of CHCl3, and the reaction mixture
was degassed by three cycles of freeze−pump−thaw. The reaction
mixture was stirred for 4 h at room temperature and irradiated by
450/550 nm LEDs. Upon completion, the Co(II) product was
precipitated by addition of heptane. Filtration through Celite on
washing several times with heptane separated the TEMPO-trapped
product, and subsequent washing with CH2Cl2 isolated the Co(II)
porphyrin. Concentration of the filtrates gave the desired products
with no further purification required.
4 - B r o m o b e n z y l o x y c a r b o n y l c o b a l t m e s o - ( 4 -
Methoxyphenyl)porphyrin (8). This compound was prepared
according to the general procedure A using 198 mg of cobalt meso-(4-
methoxyphenyl)porphyrin (0.25 mmol, 1 equiv), 234 mg of 4-
bromobenzyl alcohol (1.25 mmol, 5 equiv), and 56.7 mg of DDQ
(0.25 mmol, 1 equiv). After 16 h, the reaction mixture was worked up
as outlined in the general procedure. The reaction afforded a red solid
(226 mg, 0.23 mmol, 94%): IR (film) 2954, 1671, 1607, 1247, 1001
cm−1; 1H NMR (400 MHz, CDCl3) δ 8.89 (s, 8H, por-H), 7.89 (s, br,
8H, PMP), 7.23 (d, J = 8.8 Hz, 8H, PMP), 6.97 (d, J = 8.2 Hz, 2H,
Ph), 5.19 (d, J = 8.2 Hz, 2H, Ph), 4.07 (s, 12H, OCH3), 2.53 (s, 2H,
OCH2Ph); 13C NMR (125 MHz, CDCl3) δ 159.4, 145.8, 134.7,
134.4, 134.1, 132.8, 130.9, 128.3, 121.8, 112.5, 67.0, 55.7; HRMS
(ESI) m/z calcd for C56H42BrCoN4ONa+ 1027.1512, found
1027.1534.
1-Phenethyloxycarbonylcobalt meso-(4-Methoxyphenyl)-
porphyrin (9). This compound was prepared according to the
general procedure A using 198 mg of cobalt meso-(4-methoxyphenyl)-
porphyrin (0.25 mmol, 1 equiv), 151 μL of 1-phenylethanol (1.25
mmol, 5 equiv), and 56.7 mg of DDQ (0.25 mmol, 1 equiv). After 16
h, the reaction mixture was worked up as outlined in the general
procedure. The reaction afforded a red solid (216 mg, 0.23 mmol,
1
92%): IR (film) 2957, 2833, 1690, 1504, 1241, 996 cm−1; H NMR
(400 MHz, DMSO-d6) δ 8.76 (s, 8H, por-H), 7.90 (s, br, 8H, PMP),
7.33 (d, J = 8.8 Hz, 8H, PMP), 6.85 (t, J = 7.4 Hz, 1H, Ph), 6.70 (t, J
= 7.7 Hz, 2H, Ph), 4.95 (d, J = 7.3 Hz, 2H, Ph), 4.03 (s, 12H, OCH3),
2.90 (q, J = 6.2 Hz, 1H, OCHCH3), −0.75 (d, J = 6.5 Hz, 3H,
OCHCH3); 13C NMR (100 MHz, DMSO-d6) δ 158.8, 143.8, 143.7,
141.5, 134.5, 133.9, 132.1, 127.1, 126.1, 123.2, 119.0, 112.4, 69.2,
55.4, 21.3; HRMS (ESI) m/z calcd for C57H45CoN4O6Na (M + Na)+
963.2563, found 963.2533.
1-Phenethyloxycarbonylcobalt meso-Tetraphenylporphyr-
in (10). This compound was prepared according to the general
procedure A using 167 mg of cobalt tetraphenylporphyrin (0.25
mmol, 1 equiv), 151 μL of 1-phenylethanol (1.25 mmol, 5 equiv) and
56.7 mg of DDQ (0.25 mmol, 1 equiv). After 16 h, the reaction
mixture was worked up as outlined in the general procedure. The
reaction afforded a red solid (232 mg, 0.23 mmol, 94%): IR (film)
3055, 2925, 1693, 1599, 1351, 1003 cm−1; 1H NMR (400 MHz,
DMSO-d6) δ 8.73 (s, 8H, por-H), 8.14 (s, br, 4H, meso-Ph), 7.98 (s,
br, 4H, meso-Ph), 7.79−7.74 (m, 12H, meso-Ph), 6.85 (t, J = 7.4 Hz,
1H, Ph), 6.70 (t, J = 7.7 Hz, 2H, Ph), 4.97 (d, J = 7.2 Hz, 2H, Ph),
2.92 (q, J = 6.4 Hz, 1H, OCHCH3), −0.73 (d, J = 6.5 Hz, 3H,
OCHCH3); 13C NMR (100 MHz, DMSO-d6) δ 143.5, 143.4, 141.6,
133.5, 132.2, 127.7, 127.1, 126.9, 126.1, 123.2, 119.4, 69.4, 21.2;
Ethoxycarbonylcobalt meso-(4-Methoxyphenyl)porphyrin
(5). This compound was prepared according to the general procedure
A using 198 mg of cobalt meso-(4-methoxyphenyl)porphyrin (0.25
mmol, 1 equiv), 73 μL of ethanol (1.25 mmol, 5 equiv), and 56.7 mg
of DDQ (0.25 mmol, 1 equiv). After 16 h, the reaction mixture was
worked up as outlined in the general procedure. The reaction afforded
a red solid (189 mg, 0.22 mmol, 87%): IR (film) 2931, 1691, 1570,
1
1287, 1001 cm−1; H NMR (400 MHz, CDCl3) δ 8.89 (s, 8H, por-
H), 8.04 (d, J = 8.0 Hz, 8H, PMP), 7.25 (d, J = 8.4 Hz, 8H, PMP),
4.07 (s, 12H, OCH3), 1.64 (q, J = 7.0 Hz, 2H, CH2CH3), −0.74 (t, J
= 7.0 Hz, 3H, CH2CH3); 13C NMR (100 MHz, CDCl3) δ 159.5,
145.9, 134.8, 134.3, 132.7, 121.8, 112.48, 62.67, 55.68, 13.01; HRMS
(ESI) m/z calcd for C51H42CoN4O6 (M+ H)+ 865.2431, found
865.2453.
Benzyloxycarbonylcobalt meso-(4-Methoxyphenyl)-
porphyrin (6). This compound was prepared according to the
general procedure A using 198 mg of cobalt meso-(4-methoxyphenyl)-
E
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HRMS (ESI) m/z calcd for C53H37CoN4O6Na+ 843.2141, found
843.2127.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00552.
■
S
1-Phenethyloxycarbonylcobalt Octaethylporphyrin (11).
This compound was prepared according to the general procedure A
using 59 mg of cobalt octaethylporphyrin (0.1 mmol, 1 equiv), 24 μL
of 1-phenylethanol (0.2 mmol, 2 equiv), and 22.7 mg of DDQ (0.1
mmol, 1 equiv). After 16 h, the reaction mixture was worked up as
outlined in the general procedure. The reaction afforded a red solid
(62.2 mg, 0.08 mmol, 84%): IR (film) 2964, 2870, 1697, 1263, 1021
cm−1; 1H NMR (400 MHz, CDCl3) δ 10.16 (s, 4H, meso), 6.79 (t, J
= 7.4 Hz, 1H, Ph), 6.67 (t, J = 7.6 Hz, 2H, Ph), 4.96 (d, J = 7.4 Hz,
2H, Ph), 4.17−3.97 (m, 16H, por-CH2CH3), 2.96 (q, J = 6.4 Hz, 1H,
OCHCH3), 1.93 (dd, J = 12.7, 7.4 Hz, 24H, por-CH2CH3), −0.82 (d,
J = 6.5 Hz, 3H, OCHCH3); 13C NMR (100 MHz, CDCl3) δ 143.2,
142.9, 143.1, 141.1, 127.1, 126.2, 123.6, 99.3, 72.3, 20.9, 20.1, 18.6;
HRMS (ESI) m/z calcd for C45H53CoN4O2Na (M + Na)+ 763.3393,
found 763.3426.
Description of crystallographic methods and spectro-
scopic data (PDF)
Cartesian coordinates of DFT structures (XYZ)
Accession Codes
CCDC 1946972 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1 - P h e n e t h y l o x y c a r b o n y l c o b a l t T e t r a k i s -
(pentafluorophenyl)porphyrin (12). This compound was pre-
pared according to the general procedure A using 257 mg of cobalt
meso-tetrakis(pentafluorophenyl)porphyrin (0.25 mmol, 1 equiv), 53
μL of 1-phenylethanol (0.5 mmol, 2 equiv), and 56.7 mg of DDQ
(0.25 mmol, 1 equiv). After 16 h, the reaction mixture was worked up
as outlined in the general procedure. The reaction afforded a red solid
(165 mg, 0.14 mmol, 56%): IR (film) 2928, 1648, 1476 1217, 1007
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.B.C.M.: david-martin@uiowa.edu.
ORCID
David B. C. Martin: 0000-0002-8084-8225
Notes
The authors declare no competing financial interest.
■
1
cm−1; H NMR (400 MHz, CDCl3) δ 8.99 (d, J = 7.3 Hz, 8H, por-
H), 6.76 (d, J = 7.8 Hz, 1H, Ph), 6.63 (t, J = 7.6 Hz, 2H, Ph), 5.05 (d,
J = 7.7 Hz, 2H, Ph), 3.22 (q, J = 6.5 Hz, 1H, CHCH3), −0.57 (d, J =
6.5 Hz, 3H, CHCH3); 19F NMR (400 MHz, CDCl3) δ −133.93,
−137.47, −151.50; HRMS (ESI) m/z calcd for C53H17CoN4O2F20 Na
(M)+ 1180.0364, found 1180.0408.
1-((4-Bromobenzyl)oxy)-2,2,6,6-tetramethylpiperidine (13).
This compound was prepared according to the general procedure B
using 20 mg of 8 (0.02 mmol, 1 equiv) and 6.2 mg of TEMPO (0.04
mmol, 2 equiv). After 4 h, the reaction mixture was worked up as
outlined in the general procedure. The reaction afforded a clear oil
(4.8 mg, 0.015 mmol, 74%): IR (film) 2928, 1508, 1264, 734 cm−1;
1H NMR (400 MHz, CDCl3) δ 7.39 (d, J = 8.4 Hz, 2H, Ar), 7.16 (d, J
= 8.5 Hz, 2H, Ar), 4.69 (s, 2H, CH2Ar), 1.17 (d, J = 12.6 Hz, 12H),
1.07 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 137.3, 131.3, 129.1,
121.2, 78.0, 60.0, 39.7, 33.1, 29.7, 20.3, 17.1); HRMS (ESI) m/z calcd
for C16H25BrNO (M + H)+ 326.1120, found 326.1118.
ACKNOWLEDGMENTS
■
This work was supported by funding from the ACS Petroleum
Research Fund (57274-DNI1) and NSF (CHE-1751687).
Acknowledgment is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research. NMR instrumentation for this research was
supported by funding from the NSF (CHE-1626673) and the
U.S. Army (W911NF-16-1-0523). Mass spectrometry instru-
mentation for this research was supported by funding from the
NSF (CHE-0541848). We thank Prof. W. Hill Harman (UC
Riverside) for helpful discussions and Dr. Charlene Tsay for X-
ray crystallographic analysis. D.B.C.M. is a member of the UC
Riverside Center for Catalysis.
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outlined in the general procedure. The reaction afforded a clear oil (4
■
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