Photoassisted, iron-catalyzed allylic amination of olefins with nitroarenes

Article abstract of DOI:10.1016/S0040-4039(01)02308-5

Olefins react with nitroarenes and carbon monoxide photochemically in the presence of [Cp*Fe(CO)₂]₂ (Cp=η⁵-C₅Me₅) to produce N-aryl-N-allyl amines in moderate to good yields. Unsymmetrical olefins react highly regioselectively with N-functionalization at the less substituted vinylic carbon. Mechanistic probes using 2′-nitrobiphenyl, 2,3-dimethylbutadiene and nitrosobenzene suggest that neither aryl nitrenes nor nitrosoarenes are the active aminating agents in these reactions.

Full text of DOI:10.1016/S0040-4039(01)02308-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 931–934
Photoassisted, iron-catalyzed allylic amination of olefins with
nitroarenes
a
b
b,
Radhey S. Srivastava, Manoj Kolel-Veetil and Kenneth M. Nicholas *
a
Department of Chemistry, University of Louisiana at Lafayette, Lafayette, LA 70504, USA
b
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, USA
Received 19 November 2001; revised 3 December 2001; accepted 4 December 2001
5
Abstract—Olefins react with nitroarenes and carbon monoxide photochemically in the presence of [Cp*Fe(CO)2]2 (Cp*=h -
C Me ) to produce N-aryl-N-allyl amines in moderate to good yields. Unsymmetrical olefins react highly regioselectively with
5
5
N-functionalization at the less substituted vinylic carbon. Mechanistic probes using 2%-nitrobiphenyl, 2,3-dimethylbutadiene and
nitrosobenzene suggest that neither aryl nitrenes nor nitrosoarenes are the active aminating agents in these reactions. © 2002
Elsevier Science Ltd. All rights reserved.
1
. Introduction
less substituted olefinic carbon. Recently, metal-pro-
moted allylic aminations using the more readily avail-
able nitroarenes as aminating agents, carbon monoxide
The direct synthesis of allyl amines from olefins is an
attractive goal since these compounds are both versatile
11
as reductant, and Ru (CO) /diimine or [Cp(*)Fe-
3
12
1
2
12
synthetic intermediates and valuable end products but
their preparation typically involves nucleophilic substi-
tution of allylic electrophiles which can suffer from
(
CO)2]2 as catalysts have been discovered (Eq. (1));
the latter catalyst system also displays high ene reac-
tion-type regioselectivity. Although these reactions
employ neutral conditions, their high operating temper-
ature and pressure (150–200°C, 25–50 atm) are disad-
vantages that could their limit widespread utilization.
We describe herein photoassisted nitroarene-based
allylic aminations which proceed efficiently under much
milder conditions, allowing the reactions to be carried
out conveniently in glass reaction vessels. The results of
preliminary mechanistic probes are also presented.
3
poor regioselectivity. The alkene to allyl amine trans-
formation has been accomplished stoichiometrically
with group 16 imido compounds (R%N=X=NR%; X=
S, Se ) as aminating agents, which display moderate
regioselectively for retention of the position of unsatu-
ration. Ene reactions of azo-, nitroso- and N-
4
5
6
7
8
sulfinylcarbamate derivatives display high regio-
selectivity with double bond migration but require addi-
tional NꢀN or NꢀO reduction steps to produce the allyl
amines.
2
. Results and discussion
We and others have investigated metal-catalyzed allylic
nitrogenation reactions which produce amines directly
from olefins and with exceptional regioselectivity. Ini-
tially, molybdenum and iron -catalyzed reactions of
olefins with aryl hydroxylamines were developed which
proceed regioselectively with N-functionalization at the
While mechanistic studies of the thermal, iron-catalyzed
process are ongoing, we suspected that either CO disso-
ciation or homolysis of the FeꢀFe bond could be
important steps in generating the catalytically active
9
10
(1)
*
0
Corresponding author. E-mail: knicholas@ou.edu
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02308-5

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R. S. Sri6asta6a et al. / Tetrahedron Letters 43 (2002) 931–934
species, either of which should be promoted photochem-
reactions are comparable to those observed for the
thermal reactions catalyzed by 1b.
1
3
ically. Indeed, experiments revealed that near UV
irradiation (>300 nm) of dioxane solutions containing
a-methyl styrene, nitrobenzene and dinuclear complexes
1
2
The similar reactivity/selectivity features of the thermal
1
a–c (10 mol%) at 80–120°C under 3–6 atm of CO in a
and photoassisted reactions catalyzed by 1b suggest the
intermediacy of a common aminating species. To address
this issue trapping experiments were conducted to assess
the role of reactive organonitrogen and organoiron
species in the reactions. First, irradiation of 2-nitro-
biphenyl with a-methyl styrene under catalytic condi-
tions produced the corresponding N-biphenyl-N-allyl
amine and 2-biphenylamine but no carbazole, which
rules against the intervention of a free aryl nitrene
1
4
Fisher–Porter bottle produced the corresponding allyl
15
amine (R%=Ph, R=H) with the Cp*–iron complex 1b
being the most effective (Table 1). Variable quantities of
aniline, azo- and azoxybenzene were formed as byprod-
ucts. No allyl amine was formed in the absence of CO
or catalyst.
[
Cp*Fe(CO) ] (1b) was then employed as the catalyst to
2
2
17
survey the scope and selectivity of the photoinduced
intermediate (Eq. (2)). To test for the intermediacy of
7c–e
amination with a representative set of olefins and
nitrosobenzene, a proven enophile,
the amination of
1
6
nitroarenes. The following features of these reactions
are illustrated in Table 1: (1) acyclic trisubstituted and
a-methyl styrene was conducted in the presence of
2,3-dimethylbutadiene, which can trap nitrosobenzene in
18
1,1-disubstituted olefins react most efficiently; (2) unsym-
a facile Diels–Alder reaction. The allyl amines derived
from the olefin and diene were produced along with a
substantial amount of N-phenyl-3,4-dimethylpyrrole,
but none of the Diels–Alder adduct (Eq. (3)). While this
pyrrole could derive from photoinduced dehydration of
metrical olefins react with excellent regioselectivity, with
N-incorporation occurring at the less substituted olefinic
carbon; and (3) nitrobenzene and electron deficient
nitroarenes give the highest yields. The yields and the
reactivity and selectivity features of these photoassisted
1
9
the adduct from the diene and free nitrosobenzene,
(2)
Table 1. Photoassisted, catalytic allylic amination by nitroarenes

R. S. Sri6asta6a et al. / Tetrahedron Letters 43 (2002) 931–934
933
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(
3)
when a dioxane solution of PhNO, a-methylstyrene and
complex 1b were irradiated under the usual conditions
little of the allyl amine (<5%) was detected. This finding
argues against the intermediacy of free nitrosoarenes in
the reactions catalyzed by 1b and suggests that the
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PhNO.
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3. Conclusions
The photoassisted allylic amination of olefins by
nitroarenes catalyzed by [Cp*Fe(CO) ] (1b) thus pro-
vides a direct, regioselective and convenient route to
N-aryl-N-allyl amines. Further studies of the synthetic
utilization and mechanism of these reactions are in
progress.
7. (a) Keck, G. E.; Webb, R. R.; Yates, J. B. Tetrahedron
1981, 37, 4007; (b) Ensley, H. E.; Mahadevan, S. Tetra-
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1
Acknowledgements
9
. (a) Srivastava, A.; Ma, Y.; Pankayatselvan, R.; Dinges,
W.; Nicholas, K. M. J. Chem. Soc., Chem. Commun.
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We are grateful for support provided by the National
Science Foundation.
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6. Experimental. A 125 mL Pyrex Fisher–Porter bottle
Ref. 14) was charged with 6.15 mmol of olefin, 1.17
mmol of nitroarene, 0.11 mmol of [Cp*Fe(CO)2]2 and
silica provided solution samples for GC and GC/MS
8
analysis. Products were isolated by concentrating the
hexane/dioxane solution and then chromatographing
over neutral alumina, the allyl amines being eluted with
1:10 hexane/ether. The products were identified by com-
parison of their NMR and mass spectra and GC reten-
tion times with those of authentic samples prepared
previously (Refs. 9, 10 and 12).
1
(
1
0
0 mL of dry dioxane; runs analyzed by GC contained
.15 mmol of naphthalene as an internal standard.
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with attached gauge, inlet valve and pressure relief
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adduct from 2,3-dimethylbutadiene and nitrosobenzene
to 1-phenyl-3,4-dimethyl pyrrole was also independently
demonstrated upon its irradiation under the present
reaction conditions.
valve) was attached and the vessel was purged three
times with carbon monoxide (toxic) in an efficient fume
hood. The vessel was pressurized with 100 psi of CO,
immersed in a preheated oil bath at 120°C, and then
irradiated while stirring in a Rayonett photoreactor
with 300 or 350 nm lamps (12×20 W) or with a 400 W
Hg vapor lamp for 9–10 h. The vessel was allowed to
cool and then was vented (toxic CO) in the fume hood.
Most of the iron-containing material was precipitated
by addition of hexane. Filtration through a plug of