Iron-catalyzed allylic amination by nitroorganics

Article abstract of DOI:10.1039/a807248e

[CpFe(CO)₂]₂ catalyzes the reaction of nitroaromatics with olefins under CO to produce allyl amines regioselectively; a coordinated organonitrogen species is implicated as the active aminating agent.

Full text of DOI:10.1039/a807248e

Iron-catalyzed allylic amination by nitroorganics
Radhey S. Srivastava† and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, USA.
E-mail: knicholas@ou.edu
Received (in Corvallis, OR, USA) 15th September 1998, Accepted 28th October 1998
[CpFe(CO)2]2 catalyzes the reaction of nitroaromatics with
olefins under CO to produce allyl amines regioselectively; a
coordinated organonitrogen species is implicated as the
active aminating agent.
(entries 1, 6–9), nitrobenzene gave a nearly quantitative yield,
nitroarenes bearing electron-withdrawing groups gave moder-
ate yields, and nitroanisole afforded a poor yield of the
corresponding allyl amine.
Initial screening experiments demonstrated that the dinuclear
1 is the most active among several related complexes tested as
amination catalysts. In the reaction of nitrobenzene with AMS
(dioxane, 160 °C, 70 atm CO) neither bimetallic Cp2Mo2(CO)4
nor other mono-iron complexes, e.g. Fe(CO)5, CpFe(CO)2Br or
CpFe(CO)2(THF)+, produced appreciable quantities of allyl
amine. However, a 52% yield of the allyl amine from AMS was
In contrast to oxygenation, synthetically useful methods for the
direct introduction of nitrogen into hydrocarbons (nitrogen-
ation) are few. Metal-mediated nitrogenation reactions have
been receiving increasing attention, as evidenced by recent
reports of metal-catalyzed aziridinations1 and hydroaminations2
of olefins, both of which involve addition to the double bond.
Processes for the direct allylic nitrogenation of hydrocarbons,
which could expand the scope of the commercially important
ammoxidation of propylene to acrylonitrile,3 also hold con-
siderable appeal. Recent efforts in this laboratory and others
have led to the development of Mo-4 and Fe-catalyzed5
reactions of olefins with aryl hydroxylamines, which provide N-
aryl-N-allyl amines in generally moderate yields and with
excellent regioselectivity. A novel azodioxide–iron complex
has been implicated as the active aminating agent in the allylic
aminations catalyzed by iron salts.5b
In order to enhance the synthetic utility of metal-catalyzed
allylic aminations and to further explore the reactivity of
coordinated organonitrogen species, we have been interested in
the use of more readily available prospective aminating agents,
including amines6 and, here, nitroorganics. The metal-catalyzed
reactions of nitroarenes and CO can produce various useful
organonitrogen compounds, including amines, urethanes, ureas
and isocyanates.7 We now report the discovery of a new allylic
amination system, which employs nitroarenes as aminating
1
obtained using CpFe(CO)2(h -allyl) (2, 5 mol%) as (pre)-
catalyst. The documented ability of the two active (pre)-
catalysts, 1 and 2, to generate the CpFe(CO)2 radical10,11
suggests that this species may be involved in the catalytic
mechanism.
Given the unprecedented catalytic activity of 1 and the
distinctive regioselectivity of the amination reaction several
experiments were conducted to probe the nature of the actual
aminating species. Firstly, evidence was gathered indicating
that two potential reactive free organonitrogen species are not
involved. Nitrosobenzene, an established enophile,12 was
excluded as an intermediate since the amination of AMS by
nitrobenzene proceeded cleanly in the presence of the PhNO-
trapping agent, 2,3-dimethylbutadiene [eqn. (2)],13 with none of
the corresponding hetero-Diels–Alder adduct being produced.¶
Table 1 Allylic amination by nitroarenes catalyzed by [CpFe(CO)2]2
Yielda,b
Entry
Alkene
Nitroarene
Allylamine
(%)
agents and inexpensive [CpFe(CO)2]2 as the catalyst [eqn. (1)].
Initial experiments indicate that this system is distinct, both
mechanistically and chemoselectively, from the recently re-
ported Ru3(CO)12/(a-diimine)-catalyzed system for allylic
amination.8
The reactions of olefins with nitroarenes catalyzed by
[CpFe(CO)2]2 (5 mol%) were carried out in dioxane at 150–180
°C under 50–75 atm of CO.‡ Representative results for a
number of olefins and nitro compounds are summarized in
Table 1. The desired allyl amines were the only olefin-derived
products detected, generally constituting 75–95% of the volatile
N-containing products; N,N-diaryl ureas are the principal
byproducts.§ The yield of allyl amine depends markedly on the
structure of both the olefin and the nitro compound. Among the
small set of test olefins 1,1-disubstituted derivatives were
aminated most efficiently (entries 1,2) while 1,2-disubstituted,
trisubstituted and terminal olefins were aminated less efficiently
(entries 3–5). With unsymmetrical alkenes (entries 1–5) single
regioisomers were obtained which are derived from introduc-
tion of nitrogen at the less-substituted vinylic carbon with
double bond transposition, characteristic of ene reactions.9
Probing the structure/reactivity correlation of the nitroarene
component with a-methylstyrene (AMS) as the substrate
a
b
GC yield, naphthalene standard. New compounds were isolated and
characterized spectroscopically. c Isolated yield.
† Present address: Department of Chemistry, University of Southwestern
Louisiana, Lafayette, LA 70504, USA.
Chem. Commun., 1998, 2705–2706
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extracts were chromatographed on silica gel using light petroleum–Et2O as
eluent to afford the allyl amine. New compounds were characterized by
NMR and MS analysis.
§ Small amounts of aryl amine (1–20%), azoarene (1–5%), and azoxyarene
(1–5%) were also detected by GC–MS analysis. Non-volatile N,N-diaryl
urea by-products were isolated by crystallization/chromatography.
¶ The hetero-Diels–Alder adduct was shown to be stable under the
conditions of the catalytic reaction.
∑ Selected data for 3: nmax(KBr)/cm21 2033 and 1965; EPR (CH2Cl2)/G
3525 (bs); m/z (FAB) 628, 471, 396, 342, 288, 195; the elemental analysis
of 3, while indicating the presence of C, H, N and Fe, suggests
contamination by an organic impurity.
** In the Ru3(CO)12/diimine-catalyzed reactions, aryl amines are the major
byproducts (ref. 8) and amination in the presence of dimethylbutadiene
affords substantial quantities of the hetero-Diels–Alder adduct. F. Ragaini
and S. Cenini, personal communication, 1998.
Similarly, amination of AMS by 2-nitrobiphenyl afforded the
corresponding N-biphenyl-N-allyl amine exclusively [eqn. (3)],
but no detectable carbazole, the established product of intra-
molecular nitrene trapping.14 Together these results suggest that
a coordinated organonitrogen species serves as the active
aminating agent in the reactions catalyzed by 1.
More direct information on the iron species involved in the
catalytic reaction was obtained by examining a dark red
paramagnetic compound 3 which can be isolated from the
stoichiometric reaction of 1 with nitrobenzene in the absence of
olefin (dioxane, 100 °C, 75 atm CO, Scheme 1). Although we
have not yet established the structure of 3,∑ this compound
appears to be catalytically relevant since it both stoichiomet-
rically (in the absence of nitrobenzene) and catalytically (in the
presence of nitrobenzene) efficiently aminates AMS (dioxane,
160 °C, 60 atm CO, TON ca. 8). Moreover, 3 aminates AMS in
the presence of the nitrosobenzene trap, 2,3-dimethylbutadiene,
again suggesting that the nitrogen fragment is transferred in the
coordination sphere of the metal.
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Scheme 1
In summary, [CpFe(CO)2]2 has been found to be an effective
catalyst for the regioselective allylic amination of olefins by
nitroarenes. It is significant that iron-catalyzed reactions
involving nitroorganics and CO are rare.15 Moreover, the
differing chemoselectivity and trapping results between the
present system and the Ru3(CO)12/a-diimine-catalyzed amina-
tions8** suggests the involvement of distinctly different active
aminating agents. Further insight into the mechanism of this
new catalytic amination process, including the nature of the
active iron species involved, awaits the results of studies in
progress.
We are grateful for support provided by the National Science
Foundation (CHE 9610277). We also thank Professors F.
Ragaini and S. Cenini (Milan) for discussions and communica-
tion of their recent results prior to publication.
Notes and references
‡ General procedure: [CpFe(CO)2]2 (0.15 mmol), nitrobenzene (2.9 mmol),
olefin (3.8 mmol) and dioxane (10 ml) were placed in the glass liner of a
stainless steel autoclave under nitrogen. The autoclave was charged with
CO (900–1000 psi) and then heated at 160–180 °C for 22–24 h, during
which time aliquots were withdrawn via dip tube for GC analysis. The
autoclave was cooled, the solution was transferred into a Schlenk tube, and
the volatiles were removed in vacuo. The residue was triturated with light
petroleum–Et2O. The insoluble residue contained the N,N-diaryl ureas. The
15 H. Alper and K. E. Hashem, J. Am. Chem. Soc., 1981, 103, 6514 (?
carbamates); K. Cann, T. Cole, W. Sleigir and R. Pettit, J. Am. Chem.
Soc., 1978, 100, 3969 (? anilines); J.E. Kmiecik, J. Org. Chem., 1965,
30, 2014 (? azoarenes).
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