Oxo/imido heterometathesis between N-sulfinylamines and ketones catalyzed by a silica-supported molybdenum imido complex

Article abstract of DOI:10.1016/j.mencom.2012.03.003

Grafting the molecular complex (MesN)2Mo(CH2CMe2Ph)2 onto the surface of silica dramatically enhances its catalytic activity in the oxo/imido heterometathesis reaction of N-sulfinylamines with carbonyl compounds.

Full text of DOI:10.1016/j.mencom.2012.03.003

Mendeleev
Communications
Mendeleev Commun., 2012, 22, 64–66
Oxo/imido heterometathesis between N-sulfinylamines and ketones
catalyzed by a silica-supported molybdenum imido complex
Pavel A. Zhizhko, Anton A. Zhizhin, Dmitry N. Zarubin* and Nikolai A. Ustynyuk
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5085; e-mail: zaroubine@ineos.ac.ru
DOI: 10.1016/j.mencom.2012.03.003
Grafting the molecular complex (MesN)2Mo(CH2CMe2Ph)2 onto the surface of silica dramatically enhances its catalytic activity in
the oxo/imido heterometathesis reaction of N-sulfinylamines with carbonyl compounds.
The transition metal-mediated metathesis of nitrogen–carbon
OH
Si
MesN
NMes
Me
MesN
NMes
and nitrogen–heteroatom multiple bonds (heterometathesis) is a
quite novel type of nonoxidative nitrogen transfer reactions with
exciting prospects for organic synthesis. The reports on such type
of reactions are still sparse and include the metathesis of imines,1,2
carbodiimides3 and nitriles;4 the condensation of isocyanates and
N-sulfinylamines to carbodiimides5 and sulfurdiimines,6 respec-
tively;andtheimido-deoxygenationofaldehydes with isocyanates,2
N-sulfinylamines7 and iminophosphoranes.8
PhMe, room
temperature
Mo
Mo
+
O
O
O
Me
Ph
Me
Ph
O
PhCMe3
Me
O
Me Ph
Si
Me
O
O
SiO2-300
1
We found previously that vanadium and molybdenum oxo
chlorides, as well as bis(imido)molybdenum diaroxides, exhibit
catalytic properties in the heterometathetical imidation of alde-
hydes with N-sulfinylamines.7 The immobilization of VOCl3 onto
silica dramatically enhances its catalytic activity, which allowed
us not only to successfully develop a water-free express method
for the preparation of imines of a wide range of aldehydes but
also for the first time to introduce some ketones into the reaction.9
However, aside from N-sulfinyl-2-trifluoromethylaniline, the scope
of N-sulfinylanilines reactive towards ketones turned out to be
limited to those lacking o-substituents as a consequence of a
steric factor. So far as molybdenum oxo chlorides were earlier
noted to be superior to VOCl3 as homogeneous catalysts for the
oxo/imido heterometathesis of sterically hindered substrates,
silica-supported oxo or imido molybdenum complexes can be
expected to be more efficientcatalyststhantheirvanadiumanalogue
with an expanded range of applicability in the imido-transfer
reactions.
Here we report preliminary results on the preparation of a
silica-supported molybdenum imido complex that acts as a hetero-
geneous catalyst for the oxo/imido heterometathesis between a
wide range of N-sulfinylamines and simple ketones.
Since molybdenum oxochlorides are inappropriate precursors
for immobilization because of their low solubility that prevents
uniform dispersion of catalytic sites on the surface, we chose
the toluene-soluble molybdenum bis(imido) dialkyl complex
(MesN)2Mo(CH2CMe2Ph)2 (Mes = 2,4,6-Me3C6H2) for this
purpose being guided by the methodology of grafting transition
metal alkyl complexes onto inorganic supports.10
Scheme 1
To estimate the catalytic activity of surface complex 1, it was
tested in the oxo/imido heterometathesis of N-sulfinyl-p-toluidine
with acetophenone (Table 1) and compared with its molecular
precursor and other homogeneous and heterogeneous catalysts.
Itwasfoundthatgraftingthecomplex(MesN)2Mo(CH2CMe2Ph)2
onto silica dramatically enhanced its catalytic activity (Table 1,
entries 1 and 3), likewise it had been earlier noted for VOCl3.9 At
Table 1 Comparison of the catalytic activities of 1 and some homogeneous
and heterogeneous catalysts.
Me
Me
Ph
cat. (5 mol%)
heptane,
SO2
+
TolN
S
O + O
TolN
Ph
Entry
Catalyst
Time/h
Conversion (%)a
1
2
3
4
Mo/SiO2-300 (1)
2V/SiO2-500 (2)b
1
5
5
5
³95
90
37
(MesN)2Mo(CH2CMe2Ph)2
MOCl3 (M = V, Mo)
No reaction
aSpectroscopically estimated conversion. b Obtained by grafting 2 mol%
VOCl3 onto SiO2-500 according to the procedure described earlier.9
† All manipulations were carried out under an argon atmosphere using
standard Schlenk techniques. The NMR spectra were recorded using Bruker
AMX 400 and Avance 300 spectrometers in CDCl3. Chemical shifts were
1
measured relative to residual solvent H (7.24 ppm) and 13C (77 ppm)
resonances. The 19F NMR spectra were referenced externally to TFA.
The IR spectra were measured with a Specord M82 spectrophotometer in
CHCl3. Elemental analyses were performed in the Laboratory of Micro-
analysis of INEOS RAS.
The immobilized complex Mo/SiO2-300 1 was prepared by
grafting (MesN)2Mo(CH2CMe2Ph)2 onto the surface of silica
(partially dehydroxylated at 300°C) from a toluene solution at
ambient temperature.† The elemental analysis of 1 revealed
0.32 mmol Mo per gram of the solid (3.1 wt%), whereas the
value of 0.40 mmol Mo g–1 was expected at the total chemisorp-
tion of the complex added. The Mo:N:C ratio was approximately
1:2:32, which is close to 1:2:28 corresponding to the monografted
surface species with two imido and one alkyl ligands (Scheme 1).
6
Preparation of Mo/SiO2-300 1. (MesN)2Mo(CH2CMe2Ph)2 (1.36 g,
2.16 mmol) was dissolved in 15 ml of toluene and added slowly to a stirred
suspension of silica (4.30 g, 71.60 mmol) in 20 ml of toluene. The mixture
was stirred overnight at room temperature and then filtered. The residue was
washed with toluene (3×10 ml) until the filtrate became colourless and
then dried in vacuo to give 5.30 g of an orange-red solid. Found (%):
C, 12.40; N, 0.89; Mo, 3.1.
© 2012 Mendeleev Communications. All rights reserved.
– 64 –

Mendeleev Commun., 2012, 22, 64–66
[Mo]=NMes
ArNSO
the same time, a comparison of both silica-supported complexes
convincingly shows that molybdenum catalyst 1 is superior to
vanadium one 2 (Table 1, entries 1 and 2) in the heterometathetical
imidation of ketones. Note that no reaction was observed in the
presence of vanadium and molybdenum oxochlorides (Table 1,
entry 4) or in the absence of any catalyst.
a
MesNSO
R
[Mo]=NAr
c
b
O
I
ArN
ArN
S
S
O
R'
R
To reveal the peculiarities and a scope of application of the
heterometathetical imidation of ketones with N-sulfinylamines
catalyzed by surface complex 1, we studied the interaction of
various reactants using 5 mol% of the catalyst (Table 2).‡ The
reactions were carried out in n-heptane as a solvent under reflux
and monitored by IR spectroscopy. Final conversion was estimated
SO2
ArN
d
S
O
NAr
ArN
[Mo]=O
R'
II
[Mo] = (MesN)Mo(CH2CMe2Ph)/SiO2-300
1
by H NMR spectroscopy. The reaction of acetophenone and
2-acetylfurane with sterically unhindered o-unsubstituted N-sul-
finylanilines or o-monosubstituted ones with small electron-with-
drawing groups like the fluorine atom proceeds smoothly reaching
a nearly quantitative conversion within 1–2 h (Table 2, entries
1–3, 6 and 7), whereas an introduction of electron-donating groups
in both ketone and N-sulfinylaniline slows down the reaction
(cf. entries 1, 3, 4 and 8). However, the reaction is more sensitive
to steric factor rather than to electronic effects (cf. entries 3 and 4;
2 and 9; 1, 4 and 5; 5 and 10). Thus, acetophenone and its deriva-
tives give quantitative conversions in several hours, whereas the
reactions of benzophenone with the same N-sulfinylanilines are
much slower. For example, the reaction of the former with 2-tri-
fluoromethyl-N-sulfinylaniline (entry 10) did not reach quanti-
tative conversion even within 10 h. o-Disubstituted derivatives,
e.g., N-sulfinyl-2,4,6-trichloroaniline (entry 11), do not afford
the ketimine in a reasonable yield even after prolonged heating.
Interestingly, in this case, apart from N-(1-phenylethylidene)-
2,4,6-trichloroaniline, up to 10% di(2,4,6-trichlorophenyl)sulfur-
diimine as a by-product was spectroscopically detected. This fact
points out that the alternative oxo/imido heterometathesis occurs,
in which N-sulfinylaniline is both imidating and oxo component
as it was earlier found in a similar reaction of the imidation of
dimethylformamide (DMF) with N-sulfinylamines homogeneously
Scheme 2
silica-supported complex 1 with N-sulfinylamine(stepa)affording
respective arylimide I and N-sulfinylmesitylamine.¶ The further
oxo-deimidation of complex I proceeds under the action of ketone
(route b) to lead to surface oxide II and ketimine. For poorly
reactive substrates, the competitive route to II can be observed
(route c) involving the second molecule of N-sulfinylamine instead
of ketone to produce sulfurdiimine. The final imido-deoxygena-
tion of II with N-sulfinylamine (step d) regenerates surface
complex I. Both oxo-deimidation11 and imido-deoxygenation12
processes are well-documented for molecular oxo and imido
molybdenum complexes.
In summary, grafting the molecular complex (MesN)2Mo-
(CH2CMe2Ph)2 onto the surface of silica dramatically enhances
its catalytic activity in the oxo/imido heterometathesis reaction
of N-sulfinylamines with carbonyl compounds. As compared with
silica-supported vanadium oxochloride, the application of silica-
supported molybdenum imide allows expanding the scope of
reactive N-sulfinylamines for the imidation of ketones.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2012.03.003.
catalyzed by (MesN)2Mo(CH2CMe2Ph)2.6 Indeed, as it was exem
-
plified by N-sulfinyl-2-fluoroaniline, in the absence of ketone,
the condensation of N-sulfinylamine results in di(2-fluorophenyl)-
sulfurdiimine in 57% yield within 5 h.
The proposed scheme of the imido-transfer reaction of N-sul-
finylamines with ketones§ (Scheme 2) is analogous to that for
the related reactions of N-sulfinylamines with DMF and aldehydes
catalyzed by (MesN)2Mo(CH2CMe2Ph)26 and MOCl3 (M =V, Mo),7
respectively. It includes the initial step of transimidation of starting
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49
‡
For experimental details, see Online Supplementary Materials.
The mechanism of the reaction is now under investigation and will be
§
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25
¶
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– 65 –

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– 66 –