Copper-catalyzed allylic amination of olefins with nitrosoarenes

Article abstract of DOI:10.1016/S0040-4039(03)00628-2

The transition metal catalyzed allylic amination of olefins are studied. A screening of catalysts known for the intermediacy of PhNO in the amination of alkene with phenylhydroxylamine reveals that the hydrated copper salt in conjugation with copper powde

Full text of DOI:10.1016/S0040-4039(03)00628-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3271–3274
Copper-catalyzed allylic amination of olefins with nitrosoarenes
Radhey S. Srivastava
Department of Chemistry, University of Louisiana at Lafayette, LA 70504, USA
Received 7 February 2003; revised 27 February 2003; accepted 5 March 2003
Abstract—The transition metal catalyzed allylic amination of olefins are studied. A screening of catalysts known for the
intermediacy of PhNO in the amination of alkene with phenylhydroxylamine reveals that the hydrated copper salt in conjugation
with copper powder as reductant has the best catalytic properties using nitrosobenzene as nitrogen-fragment donor. The catalytic
system has been studied for a variety of alkenes. Unsymmetrical alkenes react with high regioselectivity with N-functionalization
at the less substituted vinylic carbon. © 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
of molybdooxaziridine complexes [MoLn(h2-RNO)]
with alkenes, producing allyl amines.6 The ene reactions
of azo-,7 nitroso-8 and N-sulfinylcarbamate9 derivatives
exhibit high regioselectivity with double bond transposi-
tion, but require additional NꢀN or NꢀO reduction steps
to produce allyl amines.
The method for new, selective, and direct synthesis of
organonitrogen compounds from hydrocarbons remains
a desirable goal in both industry and academie due to the
profit potential. Considerable research has been done
with transition metal promoted allylic amination of
alkenes and alkynes, which may offer an appealing route
to functionalized amines via CꢀN bond formation.1 The
addition of a NꢀH bond to an alkene is a noteworthy
chemical transformation because it leads to the forma-
tion of a CꢀN bond, and therefore to the functionaliza-
tion of the hydrocarbon. In contrast to the considerably
explored areas of homogeneous metal-catalyzed olefin
oxygenation, studied metal-promoted nitrogenation
reactions are few. Metal-mediated nitrogenation reac-
tions have received growing attention, as evidenced by
recent reports on metal-catalyzed aziridination2 and
hydroamination3 of olefins, both of which involve addi-
tion to double bonds.
Metal-catalyzed allylic amination of olefins, which pro-
duces allyl amines with high regioselectivity, has been
investigated by several research groups including our
own (Eq. (1)). Initially, molybdenum-10 and iron11-cata-
lyzed amination of olefins employing arylhydroxyl-
amines was developed. These reactions proceed regiose-
lectively with N-functionalization at the less substituted
olefinic carbon (Eq. (1)). Subsequently, metal-promoted
allylic aminations of olefins using the more readily
available nitroarenes as aminating agents, carbon
monoxide as reductant and Ru3(CO12)/diimine12 or
[Cp(*)Fe(CO)2]213 as catalysts have been discovered (Eq.
(2)). These reactions require demanding reaction condi-
tions (150–200°C, 25–50 atm) and proceed regioselec-
tively with N-functionalization at the less substituted
olefinic carbon. Our recent discovery14 of photo-assisted
allylic aminations proceeds smoothly under much milder
conditions, allowing the reaction to be carried out
expediently. Allylic aminations of olefins using
CuCl2·2H2O as catalyst and phenylhydroxylamine as
aminating agent have been reported recently.15,16
The transformation of olefin to allyl amine has been
accomplished stoichiometrically with group 16 imido
compounds (R%NꢁXꢁNR%; X=S4, Se5) as aminating
agents. This reaction is regioselective for retention of the
double bond position as it proceeds via an ene reaction
followed by [2,3]-sigmatropic rearrangement. Sharpless
has reported the stoichiometric imido-transfer reactions
(1)
E-mail: rss1805@louisiana.edu
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00628-2

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R. S. Sri6asta6a / Tetrahedron Letters 44 (2003) 3271–3274
(2)
Two-aminating agents for allylic amination have been
extensively used. Nitroarene (Eq. (2)) requires rigorous
reaction conditions and phenylhydroxylamine (Eq. (1))
is unavailable commercially and must be synthesized. In
order to enhance the synthetic utility of metal-catalyzed
allylic amination and to explore further the significant
advancement in the synthetic utility of allylic amina-
tion, new and more stable potential aminating agents
such as nitrosoarenes have been studied. Since PhNO is
stable, inexpensive, and commercially available, it is
Table 1. Allylic amination catalyzed by CuCl2·2H2O and Cu-powder

R. S. Sri6asta6a / Tetrahedron Letters 44 (2003) 3271–3274
3273
(3)
worthwhile to scrutinize the allylic amination of alkenes
using this nitrosoarene as aminating agent, as an alter-
native to allylic amine synthesis. Although only few
nitrosoarenes are commercially available, they can how-
ever be synthesized by the oxidation of anilines or the
nitrosation of arenes.17 To the best of our knowledge,
exploitation of nitrosoarenes as aminating agents in the
catalytic allylic amination of olefins is unexplored.
Trapping experiments exhibit that nitrene is not an
intermediate in these reactions.
8). However, the electron-rich alkene gave a lower yield
of product compared to the neutral parent compound
(run 4). Variable amounts of by-products (azobenzene
and azoxybenzene) were detected.
These catalytic reactions exhibit the same ene-type reac-
tion selectivity features as the other catalytic sys-
tems,10,11d,15 including double bond migration and
decreasing olefin reactivity with decreasing degree of
substitution. Hence they are likely to be closely related
mechanistically. To probe the possible intermediacy of
phenyl nitrene, a dioxane solution of o-(hydroxyl-
amino)biphenyl was heated in the presence of 121 and
no carbazole, which is a product of nitrene CꢀH inser-
tion, was detected. The formation of allylhydroxyl-
amine by the reaction of nitrosoarene and alkene has
been unequivocally established.10,11,15 The reaction
mechanism is unclear. Based on the previous studies
and our initial trapping experiments, however, the first
step in the catalytic cycle appears to be the ene reaction
of PhNO with alkene to produce an N-allylhydroxyl-
amine, which could deoxygenate by Cu(I) (Eq. (4))
(generated in situ) to yield N-aryl-N-allylamines. The
copper catalyst thus serves as a Cu(II)/Cu(I) redox
shuttle. The presence of copper-powder as reductant
may shift the equilibrium toward the right (Eq. (4)) to
produce Cu+, which is required for deoxygenation of
allylhydroxylamine.15 The relative stabilities of Cu+ and
Cu2+ depend on the nature of ligands and the solvent
system. The equilibrium can be readily shifted in either
direction.22
2. Results and discussion
Metal catalysts reported for PhNO intermediacy in the
catalytic allylic amination with phenylhydroxylamine
were tested. Thus, we have examined the
MoO2(dipic)HMPA,10
Fe-phthalocyanine11d
and
hydrated CuCl2·2H2O15 as catalysts for the reaction of
a-methylstyrene with PhNO. The results suggest that
CuCl2·2H2O is the best among these catalysts since it
produces 33% of the desired product. However, the
yield is further improved to 55% by using Cu-powder as
reductant. These results reveal that the mixture of
CuCl2·2H2O and Cu-powder (1:4 molar ratio) 1 has
comparable catalytic activity among the tested com-
pounds. Addition of nitrosobenzene to a dioxane solu-
tion (100°C) in an excess of olefin and 5-mol%
CuCl2·2H2O and Cu-powder produces the correspond-
ing N-aryl-N-allylamines in moderate to good yield
(Table 1) accompanied by small quantities of azoben-
zene, and azoxybenzene (Eq. (3)). A control experiment
in the absence of catalyst also produces 9% allyl amine,
probably due to the disproportionation of allylhydroxyl-
amine generated by the reaction of alkene and
PhNO.18,19
(4)
3. Summary
In summary, the advantages of the procedure for the
allylic amination of alkenes are (i) nitrosobenzene can
now be used as a the nitrogen source, (ii) the catalyst,
reductant and PhNO are inexpensive and commercially
available, (iii) the reaction is efficient and (iv) the
product yield is good.
Compound 1 was also employed as a catalyst to survey
the scope and selectivity of the amination with a repre-
sentative set of olefins. The yield of the products is
comparable to those reported earlier.10,11,15 2-Nitroso-
toluene is also effective as an aminating agent (run 9).
An attempt to use N,N-diethyl-4-nitrosoaniline, as an
aminating agent has been unsuccessful. The essential
findings of the reaction include: (a) trisubstituted and
1,1-disubstituted alkenes afford the best yields; (b)
unsymmetrical alkenes react with virtually complete
regioselectivity to produce the corresponding allyl
amine derived from double bond migration; (c) gener-
ally single isomers are observed. Features 1 and 2 are
typical of ene-type reactions.20 The yields, reactivity,
and chemoselectivity features of this reaction are
analogous to the Mo-10 and Cu(II)15,16 systems. Elec-
tronic variability on alkenes was also accessed with a
set of para-substituted a-methylstyrene derivatives.
Addition of an electron-withdrawing group (run 7) was
found to suppress the product yield considerably, while
the more electron rich groups produced higher yield of
product compared to electron withdrawing groups (run
4. Experimental
4.1. General
Anhydrous 1,4-dioxane, nitrosobenzene, CuCl2·2H2O
(Aldrich), and Cu-powder (Allied Chemicals) were
obtained commercially. GC–MS and MS spectra were
obtained at 70 eV (EI). GC analyses utilized a 3’
column packed with 3% OV 101.
4.2. Representative procedure
All reactions were performed under dry argon. The
olefin (7 mmol), CuCl2·2H2O (0.15 mmol) and Cu-pow-

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R. S. Sri6asta6a / Tetrahedron Letters 44 (2003) 3271–3274
der (0.61 mmol) were added to 1,4-dioxane (2 mL) and
heated to reflux (100°C). Nitrosobenzene (0.75 mmol)
was dissolved in 1,4-doixane (10 mL) and was slowly
added over a period of 6 h through a syringe pump.
After 8 h, the resulting dark red mixture was analyzed
by GC and GC–MS using naphthalene as the internal
standard. Comparisons with authentic samples pre-
pared as described in the literature were used to identify
allylamines.10,11,13,14
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