Ö. Karaca et al. / Journal of Catalysis 344 (2016) 213–220
215
2.4. Synthesis of compound 5
allowed to warm to r.t. and stirred for 1 h. For qualitative analysis
00 L of the reaction solution was diluted with 1500 L MeCN
1
l
l
To a solution of complex 2 (10 mg, 13
5 mL) ethyl diazoacetate (3.2 lL, 26 lmol, 2.0 equiv.) in MeCN
l
mol, 1.0 equiv.) in MeCN
and characterized by GC-MS. Next to the main product 1-carbe-
thoxy-2-phenylcyclopropane, dimerization products of EDA, ethyl
maleate and ethyl fumarate, were detected (see SI for spectra).
(
(
2 mL) was slowly added at À30 °C. The mixture was slowly
warmed to r.t. and stirred for 4 h. After the mixture was concen-
trated to 3 mL at r.t., 7 mL of diethyl ether was added. The mixture
3
. Results and discussion
was filtered and the residue was dried in vacuo to give 5 as a gray
1
solid. H NMR (400 MHz, CD
3
CN, 296 K): d 8.34 (s, 2H, NCHN), 7.39
The catalytic performance of complexes 1 and 2 as potential
2
(
5
4
[
m, 4H, ACH), 6.83 (s, 4H, ACH), 6.82 (d, JHAH = 14.7 Hz, 4H, ACH
.89 (d, JHAH = 14.7 Hz, 4H, ACH
2
),
)),
catalysts for aldehyde olefination using benzaldehyde, ethyl dia-
zoacetate (EDA) and triphenylphosphine is evaluated. Time-
dependent yield studies show high aldehyde consumption for both
complexes. Application of 4 mol% of 1 affords E-ethyl cinnamate in
2
2 2 2 3
), 4.27 (s, 2H, @CH(CO CH CH
.11 (q, 4H, ACH CH ), 1.25 (t, 6H, ACH
2
3
2
CH
3
). ESI-MS: m/z 930.60
+
À +
+ +
À +
6
] .
5 + 2H + PF
6
] , 784.89 [5 + H ] , 638.90 [5 À PF
4
2
6% yield and 20% azine, PhCH@NAN@CHCO Et, after 4 h at 70 °C.
2
.5. Catalytic synthesis of Ph P@CH(CO Et)
3
2
In contrast, no olefin formation takes place in the presence of com-
plex 2, leading to the selective accumulation of (usually unwanted)
A solution of PPh
.17 mmol, 10 mol%) in MeCN (12 mL) was heated at 70 °C. Subse-
quent addition of ethyl diazoacetate (0.2 mL, 1.64 mmol,
.0 equiv.) resulted in gas evolution for approximately ten minutes.
The reaction mixture was then stirred for 4 h at this temperature.
The solvent was evaporated and the residue was dissolved in
3
(431 mg, 1.64 mmol, 1.0 equiv.) and 1 (0.1 g,
azine (63%; Fig. 2) by
cycloaddition reaction between aldehyde and phosphazine,
Ph P@NAN@CHCO Et, an intermediate which is formed from
EDA and PPh in situ, leads to azine as (by-)product 4 (Scheme 2)
19–42,50].
In other catalytic reactions, e.g. epoxidations, complexes 1 and 2
a non-catalytic reaction. A [2 + 2]-
0
3
2
1
3
[
1
2 3
0 mL of EtOAc. After the solution was filtered over Al O and
do not differ so noticeably in their reactivity, prompting further
investigations into their difference in performance in aldehyde ole-
fination [43–49]. Reaction of 2 solely with EDA reveals decomposi-
tion of 2 to a defined metal-free species 5, which can be identified
washed using EtOAc as eluent, the solvent was removed at 40 °C.
The yellow crude product was stirred in 50 mL of petroleum ether
and the resulting white solid was filtered. Residual solvent was
evaporated in vacuo, yielding the phosphorus ylide as a white solid
1
by H NMR spectroscopy and by ESI-MS analysis (Scheme 3). The
(
360 mg, 1.00 mmol, 63%). 1H NMR (400 MHz, CD
3
CN, 296 K): d
.67–7.60 (m, 9H, ACH), 7.53–7.49 (m, 6H, ACH), (3.87, major
3.69, minor) (b-s, 2H, ACH ), (2.80, major + 2.50, minor) (b-s,
H, @CH), (1.13, major + 0.67, minor) (b-s, 3H, ACH
CN, 296 K): d 17.7 (s, cisoid), 16.2 (s, transoid). ESI-
formation of this compound might take place via an unstable
iron(IV) carbene species as it is known for a series of iron com-
plexes [29–31,51,52]. Substrate olefination therefore does not take
place as complex 2 decomposes by ligand olefination. An analo-
gous decomposition product was not identified for 1.
7
+
1
2
3
1
3
). P NMR
(
162 MHz, CD
3
+
MS: m/z 349.3 [C22
22 2
H O P] .
3.1. Catalytic aldehyde olefination
2.6. Experimental procedure for the catalytic aldehyde olefination
Complex 1 was further investigated as olefination catalyst (see
Under standard conditions (4 mol% relative catalyst concentra-
Table S1, Supporting Info). Key goals for the optimization of cata-
lyst performance were high catalytic activity, e.g. high conversion
of aldehyde, and high selectivity, which in the following text is
indicated by the ratio of E-olefin to (unwanted) azine. Control
tion), benzaldehyde (7.6
amount of PPh were added to 0.4 mL of a 7.5 mM stock solution
of 1 or 2 in acetonitrile-d and dissolved completely in a J. Young
NMR tube. After the reaction was started by addition of ethyl dia-
zoacetate (11 L, 90 mol, 1.2 equiv.) under argon atmosphere the
lL, 75 lmol, 1.0 equiv.) and a respective
3
3
3
reactions lacking one of the reagents (catalyst 1, PPh , EDA,
l
l
sealed tube was shaken and immediately placed in the NMR spec-
trometer monitoring the reaction progress for several hours. For
catalyst concentrations 1 mol% and 10 mol%, 1.9 mM and
1
3
8.8 mM stock solutions of complex 1 in acetonitrile-d were pre-
pared, providing the same reaction volume for each reaction.
1
By means of H NMR the respective reactants and products
were quantified using the solvent of the diazo reagent (CH
.45) as internal standard. The following signals were used for
the quantification of the respective substrates in acetonitrile-d
benzaldehyde d 10.03 (1H, PhCHO), E-ethyl cinnamate d 6.50 (1H,
PhCH@CH(CO Et)), Z-ethyl cinnamate d 5.98 (1H, PhCH@CH(CO Et)),
azine 4 d 8.45 (1H, PhCH@NAN@CHCO Et), phosphorus ylide
d 3.89 (2H, Ph P@CH(CO CH CH )), 4-nitrobenzaldehyde d 10.15
1H, NO PhCHO), 4-anisaldehyde d 9.86 (1H, MeOPhCHO), ethyl
-nitrocinnamate 6.63 (1H, NO PhCH@CH(CO Et)), ethyl
Et)).
2 2
Cl d
5
3
:
2
2
2
3
2
2
3
(
2
4
4
d
2
2
-methoxycinnamate d 6.36 (1H, MeOPhCH@CH(CO
2
2.7. GC–MS analysis of cyclopropanation reaction
The experiment was conducted according to a modified proce-
dure by Woo et al. [30]. Under argon atmosphere ethyl diazoac-
etate (22.6 L, 0.02 mmol, 1.0 equiv.) was added to a solution of
(14.6 mg, 0.02 mmol, 1.0 equiv.) and styrene (256 L, 1.60 mmol,
0 equiv.) in MeCN (1 mL) at À40 °C. The reaction mixture was
Fig. 2. Performance of iron(II) NHC complexes 1 and 2 as aldehyde olefination
catalysts. Reaction conditions: benzaldehyde (75 mol, 1.0 equiv.), EDA (90 mol,
.2 equiv.), PPh (90 mol, 1.2 equiv.), catalyst (3 mol, 4 mol%) in CD CN at 70 °C.
Time-dependent yields were determined via H NMR spectroscopy.
l
l
l
l
1
8
l
1
3
l
3
1