Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene-Ag(I) Complex

Article abstract of DOI:10.1021/acs.joc.9b00624

The unprecedented synthesis, single-crystal X-ray structure, and first catalytic application of a dicarbene-Ag(I) complex [Ag(BAC)₂][CO₂CF₃] (BAC = bis(diisopropyl)aminocyclopropenylidene) is reported. This novel complex provides a versatile catalytic platform for selective aerobic oxidation of benzylic alcohols to aldehyde or ketone products in high yields. Ease of experimental execution coupled with the use of abundant atmospheric molecular oxygen as an oxidant and low catalyst loading are inherit strengths of these oxidations.

Full text of DOI:10.1021/acs.joc.9b00624

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Article
Selective Aerobic Oxidation of Benzylic Alcohols
Catalyzed by a Dicyclopropenylidene-Ag(I) Complex
Roya Mir, Rozhin Rowshanpour, Katie Dempsey, Melanie Pilkington, and Travis Dudding
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00624 • Publication Date (Web): 21 Mar 2019
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The Journal of Organic Chemistry
Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by a Dicyclopropenylidene-Ag(I)
Complex
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Roya Mir, Rozhin Rowshanpour, Katie Dempsey, Melanie Pilkington and Travis Dudding*
Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario,
Canada, L2S 3A1
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(
iPr) N
N(iPr)2
CF CO
2
o
Ag
3
2
1.46 A
o
2
.65 A
o
1
.19 A
(iPr) N
N(iPr)2
2
via
o
(1 mol%)
2
.99 A
OH
O
toluene, tert-BuOK, rt, 4 - 8 h
dry air
R
R
ABSTRACT: The unprecedented synthesis, single-crystal X-ray structure and first catalytic application
of a dicarbene-Ag(I) complex [Ag(BAC) ][CO CF ] (BAC = bis(diisopropyl)aminocyclopropenylidene)
2
2
3
is reported. This novel complex provides a versatile catalytic platform for selective aerobic oxidation of
benzylic alcohols to aldehyde or ketone products in high yields. Ease of experimental execution coupled
with the use of abundant atmospheric molecular oxygen as an oxidant and low catalyst loading are
inherit strengths of these oxidations.
Introduction
Silver catalyzed processes currently find widespread use by a diverse range of scientists, although
1
not so long ago this group 11 metal was thought to have low catalytic activity. Past notions aside, the
advancement of homo- and heterogeneous silver catalyzed transformations continue to gain traction,2
e.g., in Ag(I)-catalyzed aldol and allylation reactions of carbonyls and imines, C‒ H activation enabling
carbon-carbon (C‒ C)/carbon-heteroatom (C‒ X) bond formation, intramolecular (hetero)cyclizations,
3
and cycloadditions. By the same token, Ag(I)-catalyzed alcohol oxidations furnishing carbonyls are
paramount for upgrading raw commodity chemicals to versatile synthetic building blocks and value-add
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materials. Often associated with these oxidations, however, are limitations including poor substrate
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scope, low catalyst efficiencies, as well as challenges related to experimental execution and product
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isolation (Scheme 1a). An additional obstacle to carbonyl synthesis from alcohols is selectivity. For
instance, the selective oxidation of benzylic alcohols can produce aldehyde, ketone and/or carboxylic
acid products.
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To overcome these limitations several Ag(I)-catalyzed protocols have emerged over the years,
including the use of SiO or CaO supported silver catalysts as oxidizing agents for the gas-phase
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oxidation of benzyl alcohols. Further the homogenous use of unsupported silver nitrate (AgNO ) at low
3
catalyst loadings (2 mol%) under reflux conditions was reported for the liquid-phase oxidation of
5
a
benzylic alcohols. Moreover, as a recent entry the catalytic use of N-heterocyclic carbene (NHC) Ag(I)
Ag(NHC) ][AgBr ] complexes and stoichiometric amounts of base (BnMe NOH) were employed for
[
2
2
3
the selective oxidation of alcohols to aldehydes.4d
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Results and Discussion
In reflecting upon these previous works and with the intent of expanding the scope of Ag(I)
catalyzed methodologies targeting selective alcohol oxidations we envisioned merging the redox
activity of Ag(I) with the strong donor ability of carbene
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bis(diisopropyl)aminocyclopropenylidene (BAC) as a stabilizing ligand to provide dicarbene-
+
Ag(I) complex [Ag(BAC) ] as a catalytic platform for selective alcohol oxidation (Scheme 1b).
2
In acting upon this concept and furthering our program interest in cyclopropenium and
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cyclopropenylidene chemistries, herein we report the synthesis and first catalytic application
of homoleptic dicarbene-Ag(I) complex [Ag(BAC) ][CO CF ] (1). As reported vide infra
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dicarbene-Ag(I) complex 1 catalyzes the selective oxidation of benzylic alcohols to aldehydes or
ketones in the presence of abundant atmospheric molecular oxygen as an oxidant and
inexpensive base (Scheme 1c).
Scheme 1. Silver Catalyzed Oxidation of Alcohols
(a) Limitations
Poor catalyst efficiency
Limited substrate scope
Lack of product selectivity (aldehydes vs. carboxylic acids)
(b) Concept
+
(iPr) N
(iPr) N
2
N(iPr)2
2
+
Ag(I)
Ag(I)
(
iPr) N
(iPr) N
2
N(iPr)2
2
Stabilizing
Carbene Ligand
Redox Active
Ag(I)
Dicarbene-Ag(I) Complex
(c) Solution
[
Ag(BAC) ][CO CF ]
2
2
3
O
1
(1 mol%)
ArCH -OH
2
toluene, tert-BuOK, rt, 4 - 8 h Ar
H
dry air
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At the outset of our study, with ease of experiment and practicality in mind, the synthesis of
homoleptic dicarbene-Ag(I) complex [(BAC) Ag]CO CF (1) was realized by an inventive
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approach involving stirring known carbene precursor bis(diisopropylamino)cyclopropenium
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(
BAC)− H•Cl (2) with silver trifluoroacetate and K CO in acetone for 4 hours at reflux (Figure
2 3
1
a). Subsequent X-ray diffraction analysis of a single crystal of [Ag(BAC) ][CO CF ] obtained
2 2 3
by vapor diffusion of ethyl acetate into an acetonitrile solution of 1 revealed half of the molecule
was crystallographic unique with the Ag(I) cation sitting on an inversion centre, coordinated to
cyclopropenylidene ligands in a linear fashion (Figure 1b). Meanwhile, the triflate counterion
was highly disordered and was modeled over four positions. A summary of the experimental
crystal data for 1 together with an ORTEP plot of the cation can be found in the Supporting
Information (see Figure S1 and Table S1).
Figure 1. (a) Synthesis of [Ag(BAC) ][CO CF ] (1) and (b) ORTEP plot of the molecular
2
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structure of 1 with appropriate labelling scheme. Thermal ellipsoids are plotted at 50%.
Hydrogen atoms and disordered counterion are omitted for clarity)
(
Next, to better understand the underlying structural and electronic facets of
Ag(BAC) ][CO CF ] single point density functional theory (DFT) calculations at the uB3LYP-
[
2
2
3
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D3/LACV3P**++ level of theory were performed using the X-ray coordinates of the cation
+
[
Ag(BAC) ] of 1 to provide 1+DFT. Clear from the HOMO – 12 of this cationic complex was a
2
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stabilized in-phase bonding interaction between a d orbital of the two-coordinate silver metal
z
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ion and the carbenic carbon lone pairs of the cyclopropenylidene (BAC) ligands (Figure 2, top).
On the other hand, the plotted average local ionization energy and computed NBO charge of
0
.415 e at the silver atom of 1+
offered clues to applying 1 in catalysis (Figure 2, bottom),
DFT
namely, as a catalyst for oxidation-reduction (redox) reactions.
DFT Insight to Applications - oxidation-reduction (redox) reactivity
Stability
Ag-Carbene Stabilization
HOMO-12
Reactivity
local Ionization potential
NBO charge
Ag = 0.415 e
Figure 2. Single point uB3LYP-D3/LACV3P**++ computed HOMO-12 (isovalue = 0.04) of
cation [Ag(BAC) ] (1+DFT) and depicted electrostatic potential plotted onto the local ionization
+
2
(isovalue minimum = -0.09 and maximum = 0.1).
From this basis and in building on our interest in coinage metal catalysis we explored the use
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of homoleptic dicarbene-Ag(I) complex 1 as a catalyst for aerobic alcohol oxidation. To this
end, initial optimization studies employing benzyl alcohol (3a) in dichloromethane (DCM) with
dry atmospheric air as an oxidant and a low loading of 1 (1 mol%) in the absence of light
resulted in the recovery of unreacted starting alcohol (Table 1, entry 1). The addition of base tert-
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BuOK (2 equiv.) to assist in alcohol activation was then examined, which after 20 hours
generated oxidation product benzaldehyde (4a) in 10% yield along with unreacted alcohol 3a
(Table 1, entry 2). Motivated by the selective formation of aldehyde 4a and absence of over-
oxidation product benzoic acid the use of tetrahydrofuran (THF) as a solvent was examined
resulting in a two-fold increase in the yield of 4a (Table 1, entry 3). Next, increasing the
equivalents of base tert-BuOK (4 equiv.) led to the formation of aldehyde 4a in 67% yield (Table
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, entry 4). Meanwhile, the use of bases KOH or Na CO resulted in a dramatic decrease in
2 3
reaction yield (Table 1, entries 5 and 6). Finally, exchanging the solvent for toluene and using
base tert-BuOK (2 equiv.) to our delight afforded aldehyde 4a in 95% yield in 4 hours, while the
use of KOH as a base provided 4a in low 14% yield (Table 1, entries 7 and 8).
Table 1. Screening of Aerobic Oxidation Conditionsa,b
[
Ag(BAC) ][CO CF ]
2 2 3
1
(1 mol%)
OH
O
solvent, base, time, rt
dry air
3a
4a
time (h) yield (%)b
entry
solvent
DCM
DCM
THF
base (equiv.)
—
1
2
3
4
5
20
0
tert-BuOK (2 equiv.) 20
tert-BuOK (2 equiv.) 20
tert-BuOK (4 equiv.) 20
10
20
67
22
THF
THF
KOH (5 equiv.)
20
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THF
Na CO (5 equiv.)
20
<5
95
14
2
3
toluene tert-BuOK (2 equiv.) 4
toluene KOH (2 equiv.)
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aReaction conditions: 3a (1.2 mmol), solvent (5.0 mL) and 1 (1 mol%) at room temperature
b
1
under a dry air atmosphere in the absence of light. Yield was determined by H NMR based on
conversion of 3a to 4a.
With optimized conditions in hand, we examined the substrate scope of this oxidative
reaction. Accordingly, anisyl alcohol with a resonance electron donating p-methoxy substituent
provided 4b in 88% yield, while 3,5-dimethoxybenzyl alcohol gave 4c in 91% yield (Table 2).
Next, the reaction of the inductively electron-donating
Table 2. Aerobic Oxidation of Different Alcoholsa,b
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OH
[(BAC) Ag][CO CF ] (1 mol%)
O
2
2
3
toluene, tert-BuOK, rt, 4 - 8 h
R
R
3
dry air
4
MeO
O
O
O
O
O
O
MeO
Me
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OMe
4
a: 92%
4b: 88%
4f: 90%
4c: 91%
4d :86%
4h: 84%
Cl
O
O
O
O
F
Cl
Br
4e: 87%
4g: 90%
O
O
I
OH
HO
O
OMe
4l: 92%
4
i: 0%
4j: 90%
4k: 94%
O
O
O
O
O
Me
O
4
m: 90%
4n: 90%
4o: 85%
4p: 91%
non-benzylic
H
O
O
H
H
H
O
4q: 0%
4r: 0%
4s: 0%
aReaction conditions: 3 (1.2 mmol), solvent (5.0 mL) and 1 (1 mol%) at room temperature under
b
a dry air atmosphere in the absence of light. Yields refer to isolated material after column
chromatography purification.
p-methyl substituted benzyl alcohol afforded aldehyde product 4d in an 86% yield, whereas p-F,
Cl, Br substituted benzyl alcohols provided aldehyde products 4e-g in yields between 87 - 90%.
Further, the oxidation of m-Cl substituted benzyl alcohol afforded 4h in slightly lower 84%
yield. Demonstrating the impact of sterics and/or electronegative groups nearby the alcohol
functionality 2-iodobenzaldehdye (4i) was subjected to the reaction conditions resulting in the
recovery of unreacted starting material. By contrast, salicyl alcohol provided product
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salicylaldehyde (4j) in 90% yield. Similarly, benzoin provided diketo product benzil (4k) in 94%
yield and vanillyl alcohol afforded vanillin (4l) in 92% yield. Moreover, 1,1-diphenylmethanol
and 1-naphthalenemethanol were oxidized to products 4m and 4n in 90% yields. Likewise,
piperonal (4o) was formed in 90% yield. Next, expansion of this reaction methodology to
secondary benzylic alcohol substrate 1-phenylethanol afforded acetophenone (4p) in 91% yield.
By comparison, 2-phenylethanol, cinnamyl alcohol and cholesterol substrates 3q-s did not react,
clearly demonstrating the chemoselectivity of this catalytic procedure for benzylic alcohols.
Moreover, in showing the chemoselectivity of this methodology the oxidation of benzyl alcohol
1
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(3a) in the presence of an equimolar amount of non-benzylic alcohol 2-phenylethanol (3q)
provided exclusively benzaldehyde (4a) with recovery of unreacted 3q.
Finally, to demonstrate the scalability of this catalytic methodology, the reaction of benzyl
alcohol was performed on a 1 gram scale to provide product 4a in 90% isolated yield (Scheme
2
).
Scheme 2. Gram-scale Catalysis by [Ag(BAC) ][CO CF ]
2
2
3
OH
[(BAC) Ag][CO CF ] (1 mol%)
O
2
2
3
toluene, tert-BuOK, rt, 4 h
3a
dry air
4a
(0.9 g, 92% yield)
(9.2 mmol, 1.0 g)
1 gram scale reaction
To investigate the catalytic role of homoleptic dicarbene-Ag(I) complex
Ag(BAC) ][CO CF ] in these oxidations, several control reactions were preformed. Preforming
[
2
2
3
the reaction with substrate 3a in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
led to low conversion, thereby suggesting the necessary role of radical species in these
oxidations (Table 3, entry 1). Next, to probe the importance of ligand effects, catalytic Ag(I)
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salts, namely, silver chloride (AgCl) and silver triflouroacetate (CF CO Ag) were employed
3
2
(Table 3, entries 2 and 3). In both cases, product conversion was halted, clearly demonstrating
the beneficial role of cyclopropylidene ligand coordination of the Ag(I) metal ion in catalyst 1.
Further, after stirring 1 in a basic toluene solution under dry atmospheric air for 72 hours there
was no visible degradation of the complex, thus making the catalytic role of in situ derived
decomposition product(s) of 1 improbable (Table 3, entry 4). This last result, coupled with ease
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of synthesis and isolation of complex 1 is remarkable, especially, as isoelectronic [(NHC) Ag]X
2
−
−
−
−
(
X = NO , OTf , BF , PF ) complexes typically equilibrate between mono- and bis-NHC
3 4 6
coordinated Ag(I) species.10
Table 3. Mechanistic Control Studies a,b,c,d
entry
reaction conditions
time (h) yield (%)d
1
2
3
4
1 (1 mol %) + TEMPO (5 mol%)a
AgOTf (1 mol%) a
24
24
24
5
<5
AgCl (1 mol%) a
<5
1 (1 mol %) + tert-BuOK (5 equiv.)b 72
—c
a
tert-BuOK (2 equiv.), benzyl alcohol (3a) (1.2 mmol) and toluene (5 mL) were used.
b
c
1
Experiment performed in absence of benzyl alcohol (3a). No decomposition of 1 based on H
d
1
NMR was observed. Yield was determined by H NMR.
As for the mechanism of these oxidations, the role of radical intermediates is likely, yet the
underlying mechanistic details nebulous. Indeed, single-electron transfer (SET) or hydrogen
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atom transfer (HAT) involving non-ligated reactive triplet oxygen vs. mediated by a silver η - or
2
η -O ligand bound complex of 1 might be involved.
2
On the basis of the findings the tentative DFT supported catalytic cycle depicted in Scheme 3
is offered (see Supporting Information, Scheme S1). Initiating from dicarbene-Ag(I) complex
1
1
1
1
1
1
1
1
1
1
2
2
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[
Ag(BAC) ][CO CF ] (1) alkoxide-coordination provides argenate(I) complex 6 (6DFT) featuring
2 2 3
a T-shaped geometry at the tri-coordinate silver atom as revealed by DFT calculations.
≠
−1
Subsequent low barrier (G = 5.7 kcal mol ) reaction in the presence of ground state triplet
3
dioxygen ( O ) then occurs by “loose” pseudo-six-membered H-atom transfer transition state
2
TSDFT1. Dominant facets of this triplet state structure C···H and O···H bond-making and bond-
breaking distances of 1.46 Å and 1.19 Å, as well as, localized single electron spin density at
dioxygen and emerging aldehyde carbonyl (Scheme 3, bottom). From TSDFT1, the exergonic
formation of peroxy-Ag(I) complex 7 (7DFT) having a singlet state electron configuration and
−
1
free energy that is -19.8 kcal mol lower than 6
manifest. Catalyst turnover by ligand
DFT
exchange at silver providing (BAC) Ag-OR (6) then affords 4a.
2
Scheme 3. Tentative Catalytic Cycle
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(
iPr)2N
N(iPr)2
N(iPr)2
Ph
O
O-
Ag
3O2
5
1
(
iPr)2N
O
6
Ph
O-
Ph
5
tert-BuO-
o
1.46 A
o
2
.65 A
H
Ph
o
1.19 A
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4a
tert-BuOH
o
2.99 A
HO O-
Ph
OH
3a
Base
TSDFT
1
HO- + 1/2O2
(iPr)2N
iPr)2N
N(iPr)2
N(iPr)2
Ag
O
(
O
HO
7
H
Ph
(
iPr)2N
N(iPr)2
Ag
(
iPr)2N
O
N(iPr)2
O
O
H
Ph
TSDFT
1
spin density
Conclusion
To conclude, the novel synthesis and X-ray structure of a stable dicarbene-Ag(I) complex
[Ag(BAC) ][CO CF ] applied as a catalyst for selective aerobic oxidations of alcohols furnishing
2
2
3
aldehyde and ketone products was disclosed. Notably, this unprecedented oxidative technology
utilizes abundant atmospheric molecular oxygen as a terminal oxidant and low catalyst loadings.
Further, the stability of this Ag(I) complex and evolving congeners is impressive and bodes well
for future uses of cyclopropenylidene metal complexes as catalytic manifolds for a range of
applications, which is currently underway in our lab.
Experimental Section
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General information
Materials were obtained from commercial suppliers and were used without further purification.
The reactions monitored by thin layer chromatography (TLC) using silica gel 60 F254 EMD
Merck whilst visualized with a hand-held UV lamp. Flash column chromatography was
performed over Silicycle ultrapure silica gel (230-400 mesh). NMR spectra were recorded on a
1
1
1
1
1
1
1
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13
19
Bruker DPX-300 spectrometer ( H 300 MHz, C 75.5 MHz, F 292 MHz) in CDCl unless. The
3
observed chemical shifts are reported as 훿-values (ppm) relative to tetramethylsilane (TMS).
Mass analyzer double focusing sector used for the HRMS measurements.
‒
Procedure for synthesis of (BAC) Ag•CF CO 1. To a flame dried flask covered from light
2
3
2
−
8
were added bis(diisopropylamino)cyclopropenium (BAC)−H•Cl (2) (162.0 mg, 0.6 mmol),
silver trifluoroacetate (132.5 mg, 0.6 mmol), K CO (82.9 mg, 0.6 mmol), and dry acetone (6
2
3
mL) under N atmosphere. This mixture was refluxed with stirring for 4 hours. Thereafter, the
2
resulting suspension was filtered and the solvent removed by reduced pressure rotary
evaporation. The remaining solid was washed with ethyl acetate (1 mL) to obtain 1 as a white
1
solid (165 mg, 90%). H NMR (300 MHz, CDCl ) δ 3.64-3.83 (m, 4 H), 1.41 (d, J = 5.5, 12 H),
3
1
3
1
19
1
.21 (d, J = 12, 2 H); C{ H} NMR (75.5 MHz, CDCl ) δ 148.9, 55.5, 48.4, 21.2.; F NMR
3
+
+
(
282.4 MHz, CDCl ) δ -151 ; HRMS (FAB) m/z: [M ] calcd for C H N Ag was 579.3545,
3 30 56 4
Found 579.3544.
‒
General procedure for (BAC) Ag•CF CO catalyzed aerobic oxidation of alcohols 3a-h, 3j-
2
3
2
p.
A solution of benzyl alcohol 3a (129.8 mg, 1.2 mmol) in toluene (5 mL) was added to a mixture
‒
of silver catalyst (BAC) Ag•CF CO 1 (8.3 mg, 1 mol %) and t-BuOK (269.3 mg, 2.4 mmol) in
2
3
2
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a 10 mL round bottom flask. The flask was kept in the absence of light using aluminum foil. The
contents of the flask were placed under vacuum and an atmosphere of dry air reintroduced three
times. The reaction was then stirred at ambient temperature for 4 hours. The reaction mixture
was then filtered through Celite® S and the resulting filtrate concentrated by reduced pressure
rotary evaporation. The mixture was purified by flash silica gel column chromatography using
hexane: ethyl acetate (9:1) as an eluent to afford benzaldehyde 4a (117.2 mg) in 92% yield.
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Benzaldehyde (4a): (117.2 mg, 92% yield). Colorless liquid. H NMR (300 MHz, CDCl ) δ
3
1
0.03 (s, 1 H), 7.88-7.91 (m, 2 H), 7.62-7.67 (m, 1H), 7.51-7.57 (m, 2H).
1
2
1
4
-Methoxybenzaldehyde (4b): (143.7 mg, 88% yield). Colorless liquid. H NMR (300 MHz,
CDCl ) δ 9.83 (s, 1H), 7.78 (d, J = 8.8 Hz, 2H), 6.94 (d, J = 8.8 Hz, 2H), 3.82 (s, 3H).
3
1
3
1
3,5-dimethoxybenzaldehyde (4c): (181.5 mg, 91% yield).White solid. m.p. 45-48 ⁰C. H
NMR (300 MHz, CDCl ) δ, 9.92 (s, 1H), 7.03 (d, 2H, J = 2.3 Hz), 6.72 (t, 1H, J = 2.3 Hz), 3.86
3
(s, 6H).
1
1
1
4
-Methylbenzaldehyde (4d): (124.0 mg, 86% yield). Light yellow liquid. H NMR (300 MHz,
CDCl ) δ 9.95 (s, 1H), 7.76 (d, J = 8.1 Hz, 2H), 7.31 (d, J = 7.9 Hz, 2H), 2.42 (s, 3H).
3
1
1
1
4-Fluorobenzaldehyde (4e): (129.6 mg, 87% yield). Pale yellow liquid. H NMR (300 MHz,
CDCl ) δ 9.97 (s, 1H), 7.93 -7.89 (m, 2H), 7.21 (t, J = 8.6 Hz, 2H).
3
1
1
1
4
-Chlorobenzaldehyde (4f): (151.8 mg, 90% yield). White solid. m.p. 45-48 °C. H NMR
(
300 MHz, CDCl ) δ 10.00 (s, 1H), 7.84 (d, J = 8.5 Hz, 2H), 7.53 (d, J = 8.4 Hz, 2H).
3
1
1
1
4-Bromobenzaldehyde (4g): (200.0 mg, 90% yield). White solid. m.p. 54-57 °C. H NMR
(
300 MHz, CDCl ) δ 10.00 (s, 1H), 7.70-7.79 (m, 4H).
3
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3
-Chlorobenzaldehyde (4h): (141.7 mg, 84% yield). Light yellow liquid. H NMR (300 MHz,
CDCl ) δ 9.94 (s, 1H), 7.80 (t, J = 1.7 Hz, 1H), 7.71-7.75 (m, 1H), 7.54-7.58 (m, 1H), 7.45 (t, J =
3
7.7 Hz, 1H).
1
1
1
1
1
1
1
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1
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2
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2
2
2
2
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4
1
2-hydroxybenzaldehyde (4j): (131.9 mg, 90% yield). Colorless liquid. H NMR (300 MHz,
CDCl ) δ 11.03 (s, 1H), 9.99 (s, 1H), 7.51-7.57 (m, 2H), 6.98-7.05 (m, 2H).
3
1
5
1
Benzil (4k): (237.1 mg, 94% yield). Solid. m.p. 91-95 ⁰C. H NMR (300 MHz, CDCl ) δ 7⋅92-
3
8.01 (m, 4 H), 7⋅66-7.71 (m, 2 H), 7⋅53 (t, J = 7.9 Hz, 4 H).
1
6
4-Hydroxy-3-methoxybenzaldehyde (4l): (168.0 mg, 92% yield). Yellow solid. m.p. 79-83
1
⁰
C. H NMR (300 MHz, CDCl ) δ 9.85 (s, 1 H), 7.43-7.46 (m, 2 H), 7.06 (d, J = 8.5, 1 H), 6.21
3
(s, 1 H), 3.99 (s, 3 H).
1
7
1
Benzophenone (4m): (196.8 mg, 90% yield). White solid. m.p. 46-50 ⁰C. H NMR (300 MHz,
CDCl ) δ 7.80-7.84 (m, 4H), 7.57-7.63 (m, 2H), 7.47-7.51 (m, 4H).
3
11
1
2
-Naphthaldehyde (4n): (168.7 mg, 90% yield). White solid. m.p. 54-57 °C. H NMR (300
MHz, CDCl ) δ 10.43 (s, 1H), 9.30 (d, J = 8.5 Hz, 1H), 8.11 (d, J =8.2 Hz, 1H), 8.01 (dd, J =
3
1.1, 7.0 Hz, 1H), 7.94 (d, J = 8.2 Hz, 1H), 7.59-7.75 (m, 3H).
1
8
1,3-Benzodioxole-5-carboxaldehyde (4o): (153.1 mg, 85% yield). Pale yellow solid. m.p. 35-
1
3
6
8°C. H NMR (CDCl ) δ 9.82 (s, 1H,), 7.42 (dd, J=1.6, 7.9 Hz, 1H), 7.34 (d, J = 1.5 Hz, 1H),
3
.94 (d, J = 7.9 Hz, 1H), 6.09 (s, 2H).
1
6
1
Acetophenone (4p): (131.2 mg, 91% yield). Light yellow liquid. H NMR (300 MHz, CDCl )
3
δ 7.90-7.93 (m, 2H), 7.48-7.54 (m, 1H), 7.38-7.43 (m, 2H), 2.54 (s, 3H).
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ASSOCIATED CONTENT
Supporting Information
NMR spectra, computational calculations, and X-ray crystallographic analysis of 1.
The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION
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Corresponding Author
*
E-mail: tdudding@brocku.ca
ORCID
Travis Dudding: 0000-0002-2239-0818
Present Addresses
†
If an author’s address is different than the one given in the affiliation line, this information may
be included here.
Author Contributions
The manuscript was written through contributions of all authors. / All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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Financial support for this work was provided by the Natural Science and Engineering Research
Council (NSERC) Discovery grant (2014-04410), as well as an NSERC RTI grant (M.
Pilkington). Computations were carried out using facilities at SHARCNET (Shared Hierarchical
Academic Research Computing Network: www.sharcnet.ca) and Compute/Calcul Canada.
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by a Cyclopropenium Cation. J. Org. Chem. 2017, 82, 709. (c) Mir, R.; Dudding, T. A Au(I)-
Precatalyst with a Cyclopropenium Counterion: An Unusual Ion Pair. J. Org. Chem. 2016, 81,
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Bertrand, G. Cyclopropenylidenes: From interstellar space to an isolated derivative in the
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Acids: Proton-Catalyzed Hydroamination of Unactivated Alkenes and Activation of Au(I) for
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