Reaction of iron aminocarbene complexes with electronically deficient alkenes

Article abstract of DOI:10.1016/S0022-328X(00)00707-5

Thermal reaction of iron aminocarbene complexes bearing aromatic substituent (CO)₄Fe=C(Ar)N(CH₃)₂ with methyl acrylate, acrylonitrile and methyl vinyl ketone furnishes 1,4-dicarbonyl compounds - formal products of Michael

Full text of DOI:10.1016/S0022-328X(00)00707-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 617–618 (2001) 329–333
Reaction of iron aminocarbene complexes with electronically
deficient alkenes
Ivan Rotrekl, Libor Vyklicky´, Dalimil Dvora´k *
'
Prague Institute of Chemical Technology, Technicka´ 5, 166 28 Prague 6, Czech Republic
Received 17 July 2000; accepted 26 September 2000
Abstract
Thermal reaction of iron aminocarbene complexes bearing aromatic substituent (CO)4FeꢀC(Ar)N(CH3)2 with methyl acrylate,
acrylonitrile and methyl vinyl ketone furnishes 1,4-dicarbonyl compounds — formal products of Michael addition of an acyl
anion and the alkene. Nonactivated, di- and trisubstituted alkenes are unreactive under the same conditions. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Iron; Acyl anion equivalent; Aminocarbene; C–H insertion
1. Introduction
Alkoxycarbenes of Group 6 elements usually form
cyclopropanes as the main product in a reaction with
electron deficient alkenes. Aminocarbene complexes
have been studied to a much lesser extent in this regard.
It was demonstrated that aminocarbenes of Group 6
elements form cyclopropanes only in the intramolecular
manner [2]. Intermolecular reactions give different
products. Thus, thermal reaction of N-arylamino-sub-
stituted Fischer chromium carbenes having a pendant
2-(1-alkenyl) substituent at the benzene ring affords
either substituted indoles or quinolines, depending on
the starting material [3]. Pentacarbonyl[(N,N-dimethyl-
amino)methylene]chromium forms products of formal
[2+2+1] cycloaddition, [4+1] cycloaddition or CꢁH
insertion [4], which depends on the substrate structure.
Reaction of (CO)5CrꢀC(CH3)N(CH3)2 (1) with methyl
acrylate has been reported to afford 1,4-dicarbonyl
substituted compounds (formal Michael addition prod-
ucts of an acyl anion and the alkene) as a result of CꢁH
insertion and subsequent hydrolysis (Scheme 2) [4].
Recently it has been shown that the same reaction of
analogous aryl-substituted chromium aminocarbenes is
catalyzed by Pd(OAc)2, presumably via an unstable
palladium carbene intermediate [5]. Thermal and pho-
tolytic reactions of lithium- [6] or tetramethylammo-
nium pentacarbonyl(1-oxidoalkylidene)chromate(0) [7]
salts with various electron-deficient alkenes also give
products of formal Michael addition.
Alkoxycarbene complexes of Group 6 elements are
known to form cyclopropanes via metallacyclobutane
intermediate [1] (Scheme 1, path a). These products are
often accompanied with derivatives arising from the
olefinic CꢁH insertion reaction (Scheme 1, path b).
Scheme 1.
Scheme 2.
* Corresponding author.
E-mail address: dvorakd@vscht.cz (D. Dvor' a´k).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00707-5

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I. Rotrekl et al. / Journal of Organometallic Chemistry 617–618 (2001) 329–333
tion in 1,2-dichloromethane and in acetonitrile did not
reach completion even in 80 h. Increasing the tempera-
ture resulted in formation of inseparable mixtures of
products. Therefore THF and toluene at 50°C were
used for the next experiments. While the reaction itself
was very smooth and clean, work-up proved difficult,
since iron is very efficiently complexed by the product.
Many attempts to oxidize iron compounds (FeIII and
CeIV salts, organic peroxides, aminooxides, molecular
oxygen in aqueous and non-aqueous solutions) have
been tried without success. Finally simple exposure of
the dilute ethereal solution of the reaction mixture to
light (sunlight gave the best results) in the presence of
FeCl3·6H2O on air, followed by filtration of the precip-
itated iron oxides and chromatography, gave the best
results. However, this process is very slow and complete
precipitation of the iron oxides takes about 2 weeks.
Other typical Michael acceptors — acrylonitrile and
methyl vinyl ketone also react with 2a, giving the
corresponding 1,4-dicarbonyl compounds 4 and 5, re-
spectively (Scheme 3, Table 1). Substrates with di- or
trisubstituted double bonds (methyl crotonate, methyl
cinnamate, methyl methacrylate, cinnamaldehyde, cro-
tonaldehyde, mesityl oxide and maleic anhydride) were
unreactive, probably due to steric reasons. Iron
aminocarbene complexes 2b–e, bearing heteroaromatic
substituents, reacted in a similar manner, giving the
corresponding 4-substituted 4-oxobutanoates 3b–e in
acceptable yields (Scheme 3, Table 1). In most cases
reaction in toluene gave better results compared to
THF. The only exceptions were the reactions of 2-
thienylcarbene (2c) and 3-pyridylcarbene (2e), which
gave slightly better results when THF was used as the
solvent. 2-Pyridyl complex 2d, for reasons that are
unclear, reacted with methyl acrylate only in toluene.
Complexes (CO)4FeꢀC(H)N(CH3)2 (2f) and (CO)4Feꢀ
C(CO2-t-Bu)N(CH3)2 (2g) gave only inseparable mix-
tures of products under identical conditions.
Scheme 3.
But only very little is known about reactivity of iron
carbene complexes toward alkenes. Alkoxycarbene
complexes of iron are known to afford products of
CꢁH insertion exclusively in the reaction with alkenes
[8] and dienes [9]. However, nothing is known about
aminocarbenes. Therefore we aimed our studies at this
class of compounds, and herein we wish to report the
results of our study of the reaction of iron aminocarbe-
nes bearing aromatic substituent with electron deficient
alkenes. Since iron alkyl substituted aminocarbenes are
not accessible directly from tertiary amides [10] and
alkoxy–amine exchange is reported to give very poor
results with iron alkoxycarbene complexes [11], these
compounds were not included in the study.
2. Results and discussion
Reaction of [(N,N-dimethylamino)phenylmethylene]-
tetracarbonyliron(0) (2a) with excess methyl acrylate in
THF at 50°C furnished methyl-4-oxo-4-phenylbutan-
oate (3a) [12] as the single organic product. The struc-
ture of this compound was established unambiguously
1
by H-NMR spectrum (see Section 3), which was iden-
tical with that reported in the literature [13]. This is a
difference from the reaction of iron alkoxycarbenes
with alkenes, where only formation of alkoxyalkenes
was reported [8].
Further experiments showed that while reaction at
50°C in THF or toluene was completed in 48 h, reac-
Analogous formation of 1,4-dicarbonyl compounds
has been reported recently from the reaction of aryl
substituted chromium aminocarbenes with methyl acryl-
ate and acrylonitrile catalyzed with Pd(OAc)2 in the
Table 1
Carbene complex
Alkene
Product
Yield (%)
Reaction in toluene
Reaction in THF
2a
2a
2a
2b
2c
2d
2e
Methyl acrylate
Acrylonitrile
3a
4
5
3b
3c
3d
3e
77
33
46
48
56
44
27
30
29
34
26
Methyl vinyl ketone
Methyl acrylate
Methyl acrylate
Methylacrylate
Methyl acrylate
61
No reaction
32

I. Rotrekl et al. / Journal of Organometallic Chemistry 617–618 (2001) 329–333
331
used. Neutral aluminum oxide (activity II–III) was
obtained from Merck. All prepared 1,4-dicarbonyl
compounds are known compounds.
3.1. [(N,N-Dimethylamino)-2-thienylmethylene]-
tetracarbonyliron(0) (2c)
Iron pentacarbonyl (1.4 ml, 10 mmol) was added at
−78°C via syringe to a suspension of C8K [14], pre-
pared from potassium (1.06 g, 27 mmol) and graphite
(2.49 g, 207 mmol) in THF (40 ml). The resulting
mixture was stirred for 10 min at −78°C, then a
solution of N,N-dimethylthiophene-2-carboxamide
(1.11 g, 8 mmol) in THF (10 ml) was added via syringe,
and stirring was continued at this temperature for
another 10 min. The reaction vessel was then trans-
ferred to a cooling bath adjusted to −45°C, and
stirring was continued at this temperature. (At higher
temperature, insoluble and therefore unreactive solvate
of K2Fe(CO)4 precipitates! [10]) After 20 min, the mix-
ture was cooled again to −78°C and chlorotrimethyl-
silane (2.5 ml, 20 mmol) was added. Stirring was con-
tinued for 20 min at −78°C, and the mixture was
allowed to warm to 10°C. Alumina (8 g) was added,
THF was evaporated at diminished pressure (bath tem-
peratureB30°C), and the residue was dried under high
vacuum for several hours. Light petroleum (50 ml) was
added, and the mixture was stirred vigorously for sev-
eral minutes under argon atmosphere. The suspension
formed was then transferred onto a column filled with
neutral alumina (60 g). Elution with light petroleum–
dichloromethane 5:1 and crystallization from n-hep-
tane–dichloromethane furnished 0.785 g (45%) of pure
Scheme 4.
presence of triethylamine. A non-catalyzed reaction
afforded only a low yield of 1,4-dicarbonyl product
after prolonged reaction time [5]. In our case, however,
the reaction of iron carbene 2a with methyl acrylate
was not influenced by the presence of a palladium
catalyst (Pd(OAc)2 or Pd(PPh3)4) under the conditions
described in the Ref. [5]. The chromium aminocarbene
(CO)5CrꢀC(Ph)N(CH3)2 did not react with Michael ac-
ceptors under the above conditions used for its iron
analogue 2a.
We suppose that the mechanism of this reaction is
the same as was suggested previously for similar reac-
tions of chromiun carbenes [1,4]. This involves forma-
tion of a metallacyclobutane with subsequent b-
elimination and reductive elimination followed by hy-
drolysis during work-up (Scheme 4).
In conclusion, reaction of iron aminocarbene com-
plexes with non-hindered electron deficient alkenes pro-
ceeds with olefinic CꢁH insertion under formation of
1,4-dicarbonyl compounds. This is the first example of
iron carbenes giving such a type of product. The reac-
tion is limited in scope (only aromatic iron aminocarbe-
nes can be used) and the isolation of the product is time
consuming. However, since this reaction proceeds un-
der very mild and neutral conditions, we believe that in
some cases iron aminocarbene complexes can be used
as synthetic equivalents of an acyl anion.
1
carbene complex: M.p. (dec.) 82°C; H-NMR l 3.23 (s,
3H, anti-NCH3), 4.00 (s, 3H, syn-NCH3), 6.58 (m, 1H,
ArH), 6.98 (m, 1H, ArH), 7.45 (m, 1H, ArH); IR
(CHCl3) w=2043 (s), 1968 (m), 1940 (s), 1919 (s) cm−1
.
Anal. Calc. for C11H9FeNO4S: C, 43.02; H, 2.95; N,
4.56. Found: C, 42.99; H, 3.16; N, 4.53%.
3. Experimental
3.2. [(N,N-Dimethylamino)-2-pyridylmethylene]-
tetracarbonyliron(0) (2d)
1H-NMR spectral data were recorded on a Varian
Gemini spectrometer at 300 MHz in CDCl3, and chem-
ical shifts are referenced to TMS. IR spectra were
recorded on a Nicolet 750FT-IR spectrometer. Tetrahy-
drofuran was distilled from benzophenone ketyl under
argon prior to use. Toluene was distilled from sodium.
Non-dried diethyl ether was used for work-up of reac-
tion mixtures. Michael acceptors were obtained from
Aldrich and Fluka and used as purchased. Iron
aminocarbene complexes were prepared according to
published procedures [10]. For the purification of crude
reaction mixtures, radial-layer chromatography (Chro-
matotron model 8924 and Merck silica gel 60 PF) was
The same procedure as above starting with N,N-
dimethyl-2-pyridylcarboxamide (1.20 g, 8 mmol), after
chromatography (light petroleum–dichloromethane
3:2) and crystallization from n-heptane–dichloro-
methane, afforded 0.95 g (39%) of yellow solid. 1H-
NMR l 3.09 (s, 3H, anti-NCH3), 4.01(s, 3H, syn-
NCH3), 7.08 (m, 2H, ArH), 7.73 (m, 1H, ArH), 8.57
(m, 1H, ArH); IR (CHCl3): w=2045 (m), 1969 (m),
1941 (s), 1923 (s) cm−1. Anal. Calc. for C12H11FeN2O4:
C, 47.68; H, 3.34; N, 9.27. Found: C, 47.57; H, 3.55; N,
9.33%.

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I. Rotrekl et al. / Journal of Organometallic Chemistry 617–618 (2001) 329–333
3.3. General procedure for the reaction of iron
3.8. Methyl-4-oxo-4-(2-thienyl)butanoate (3c) [20]
aminocarbenes with electron deficient alkenes
Reaction in toluene, after chromatography (light
petroleum–diethyl ether–acetone–methanol, 50:30:
17:3) furnished 0.057 g (56%) of the product. The yield
in THF was 0.06 g (61%). 1H-NMR [21] l 2.76 (t,
J=13.7 Hz, 2H, CH2), 3.26 (t, J=13.7 Hz, 2H, CH2),
3.70 (s, 3H, COOCH3), 7.13 (m, 1H, ArH), 7.63 (d,
J=4.9 Hz, 1H, ArH), 7.76 (d, J=3.8 Hz, 1H, ArH).
Carbene complex (0.5 mmol) and unsaturated sub-
strate (5 mmol) were dissolved in THF or toluene (2
ml) and the mixture was heated to 50°C under argon
atmosphere for 48 h. The reaction mixture was than
diluted with diethylether, FeCl3·6H2O (0.032 g, 0.1
mmol) was added and the solution was allowed to sit in
sunlight until all iron compounds precipitated and a
clear solution was formed (usually 2 weeks were re-
quired). The solids were then filtered off, the solvents
were evaporated and the crude product was purified by
chromatography. Reaction products were characterized
by comparison of their 1H-NMR spectra with that
reported in the literature.
3.9. Methyl-4-oxo-4-(2-pyridyl)butanoate (3d) [22]
Reaction in THF and after chromatography (light
petroleum–diethyl ether–acetone–methanol, 50:30:
17:3) 0.042 g (44%) of the product. No product was
1
isolated from the reaction in toluene. H-NMR l 2.76
(t, J=13.2 Hz, 2H, CH2), 3.59 (t, J=17.0 Hz, 2H,
CH2), 3.69 (s, 3H, COOCH3), 7.47 (t, J=12.1 Hz, 1H,
ArH) 7.83 (t, J=14.8 Hz, 1H, ArH), 8.03 (d, J=7.7
Hz, 1H, ArH), 8.68 (d, J=3.9 Hz, 1H, ArH).
3.4. Methyl 4-oxo-4-phenylbutanoate (3a) [12]
Reaction in toluene, after chromatography (light
petroleum–diethyl ether–acetone, 80:10:10) furnished
0.074 g (77%) of the product. The yield in THF was
0.029 g (30%). 1H-NMR [13] l 2.75 (t, J=13.2 Hz, 2H,
CH2), 3.30 (t, J=13.2 Hz, 2H, CH2), 3.68 (s, 3H,
COOCH3), 7.44 (t, J=15.9 Hz, 2H, ArH), 7.55 (t,
J=14.8 Hz, 1H, ArH), 7.97 (d, J=8.7 Hz, 2H, ArH).
3.10. Methyl-4-oxo-4-(3-pyridyl)butanoate (3e) [23]
Reaction in toluene, after chromatography (light
petroleum–diethyl ether–acetone–methanol, 50:30:
17:3) furnished 0.026 g (27%) of the product. The yield
1
in THF was 0.03 g (32%). H-NMR l 2.79 (t, J=13.2
3.5. 4-Oxo-4-phenylbutyronitrile (4) [15]
Hz, 2H, CH2), 3.32 (t, J=13.2 Hz, 2H, CH2), 3.71 (s,
3H, COOCH3), 7.44 (q, J=16.5 Hz, 1H, ArH), 8.25
(dt, J=12.1 Hz, 1H, ArH), 8.79 (d, J=6.0 Hz, 1H,
ArH), 9.20 (d, J=1.6 Hz, 1H, ArH).
Reaction in toluene, after chromatography (light
petroleum–diethyl ether–acetone, 80:10:10) afforded
0.027 g (33%) of the product. The yield in THF was
1
0.023 g (29%). H-NMR [16] l 2.78 (t, J=7.1 Hz, 2H,
CH2), 3.39 (t, J=7.2 Hz, 2H, CH2), 7.50 (t, J=15.4
Hz, 2H, ArH), 7.61 (t, J=14.3 Hz, 1H, ArH), 7.97 (d,
J=8.2 Hz, 2H, ArH).
Acknowledgements
Support of this research under Grants 203/95/0160
and 203/00/1240 from the Czech Grant Agency is grate-
fully acknowledged.
3.6. 1-Phenylpentane-1,4-dione (5) [17]
Reaction in toluene and chromatography (light
petroleum–diethyl ether–acetone, 80:10:10) furnished
0.041 g (46%) of the product. The yield in THF was
0.03 g (34%). 1H-NMR [18] l 2.26 (s, 3H, CH3), 2.89 (t,
J=6.3 Hz, 2H, CH2), 3.28 (t, J=6.3 Hz, 2H, CH2),
7.45 (m, 3H, ArH), 8.00 (d, J=7.1 Hz, 2H, ArH).
References
[1] For recent review see: L.S. Hegedus, Transition Metals in the
Synthesis of Complex Organic Molecules, University Science
Books, Sausalito, CA, 1999, pp. 151–157.
[2] (a) C.P. Casey, N.W. Vollendorf, K.J. Haller, J. Am. Chem. Soc.
106 (1984) 3754. (b) B.C. So¨derberg, L.S. Hegedus,
Organometallics 9 (1990) 3113.
3.7. Methyl-4-(2-furyl)-4-oxobutanoate (3b) [19]
[3] B.C. So¨derberg, E.S. Helton, L.R. Austin, H.H. Odens, J. Org.
Chem. 58 (1993) 5589.
[4] A.M. Sierra, B.C. So¨derberg, P.A. Lander, L.S. Hegedus,
Organometallics 12 (1993) 3769.
[5] M.A. Sierra, M.J. Manchen˜o, E. Sa´ez, J.C. del Amo, J. Am.
Chem. Soc. 120 (1998) 6812.
[6] B.C. So¨derberg, D.C. York, Organometallics 13 (1994) 4501.
[7] B.C. So¨derberg, D.C. York, E.A. Harriston, H.J. Caprara, A.H.
Flurry, Organometallics 14 (1995) 3712.
Reaction in toluene and chromatography (light
petroleum–diethyl ether–acetone–methanol, 50:30:
17:3) furnished 0.044 g (48%) of the product. The yield
1
in THF was 0.024 g (26%). H-NMR l 2.74 (t, J=13.2
Hz, 2H, CH2), 3.17 (t, J=13.7 Hz, 2H, CH2), 3.69 (s,
3H, COOCH3), 5.63 (m, 1H, ArH), 7.22 (d, J=3.8 Hz,
1H, ArH), 7.58 (d, J=1.1 Hz, 1H, ArH).

I. Rotrekl et al. / Journal of Organometallic Chemistry 617–618 (2001) 329–333
333
[8] M.F. Semmelhack, R. Tamura, J. Am. Chem. Soc. 105 (1983)
6750.
[16] I. Ben-David, E. Mishani, S. Royen, J. Org. Chem. 63 (1998)
4632.
[9] M.F. Semmelhack, J. Park, J. Am. Chem. Soc. 109 (1987) 935.
[10] D. Dvora´k, Organometallics 14 (1995) 570.
[11] M.F. Semmelhack, J. Park, Organometallics 5 (1986) 2550.
[12] M. Kugel, J. Liebigs, Ann. Chem. 299 (1898) 62.
[13] R. Ballini, M. Petrini, G. Rosini, Synthesis (1987) 711.
[14] M.A. Schwindt, T. Lejon, L.S. Hegedus, Organometallics 9
(1990) 2814.
[17] C. Paal, Chem. Ber. 16 (1883) 2865.
[18] S. Inaba, R.D. Rieke, J. Org. Chem. 50 (1985) 1373.
[19] W. Kern, G. Spiteller, Justus Liebigs Ann. Chem. (1985) 1168.
[20] M.P. Cagniant, M.P. Cagniant, Bull. Soc. Chim. Fr. (1953) 62.
[21] J. van Esch, F. Schoonbeek, M. de Loos, H. Kooijman, A.L.
Spek, R.M. Kellog, B.L. Feringa, Chem. Eur. J. 5 (1999) 937.
[22] S. Hauptmann, K. Hirschberg, J. Prakt. Chem. 34 (1966) 272.
[23] R.N. Castle, A. Burger, J. Am. Pharm. Assoc. 43 (1954)
163.
[15] C.F.H. Allen, M.R. Gilbert, D.M. Young, J. Org. Chem. 2
(1937) 227.
.