Nitrosoarene-Cu(I) complexes are intermediates in copper-catalyzed allylic amination

Article abstract of DOI:10.1021/ja044093m

Reactions of nitrosobenzene and N,N′-diethyl-4-nitrosoaniline with [Cu(CH₃CN)₄]PF₆ produce novel homoleptic Cu(I)-nitrosoarene complexes, [Cu(ArNO)₃]PF₆, 1 (Ar = Ph) and 2 (Ar = 4-Et₂NC₆H₄NO). The X-ray structure of 2 reveals that the copper is coordinated in a severely distorted trigonal planar geometry to the N-atom of the nitrosoarene ligand. Reactions of the PhNO complex 1 with olefins and an olefin/diene mixture provide evidence for its role as an intermediate and possibly the active nitrogen transfer agent in the Cu-catalyzed allylic amination of olefins by aryl hydroxylamines. Copyright

Full text of DOI:10.1021/ja044093m

Published on Web 04/27/2005
Nitrosoarene-Cu(I) Complexes Are Intermediates in Copper-Catalyzed Allylic
Amination
Radhey S. Srivastava,*,† Masood A. Khan,‡ and Kenneth M. Nicholas‡
Department of Chemistry, UniVersity of Louisiana at Lafayette, Lafayette, Louisiana 70504, and
Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019
Received September 28, 2004; E-mail: rss1805@louisiana.edu
The direct synthesis of organonitrogen compounds from hydro-
carbons (nitrogenation) is a practically attractive but chemically
challenging goal. Besides the rapidly expanding range of N-func-
tionalization options involving addition to C-C unsaturation,1
allylic nitrogenation of unsaturated hydrocarbons offers a de-
sirable route to polyfunctional amines.2 We and others have
described allylic aminations catalyzed by Mo(VI),3 Fe(II,III),4,5 and
Cu(I,II)6,7 salts and complexes, using aryl hydroxylamines as ami-
nating agents (eq 1). More recently, nitroarene-8 and aminoarene9-
based, metal-catalyzed allylic aminations have also been developed.
Mechanistic studies of the hydroxylamine/olefin reactions cata-
lyzed by LMo(VI)O2,3b (phthalocyanine)Fe(II),5 and hydrated
Figure 1. X-ray structure of the cation of 2. Selected bond lengths (Å)
CuCl27 revealed the intervention of free PhNO, a proven enophile,10
as the active aminating agent. However, the aminations catalyzed
by Fe(II,III) salts involve a novel iron-azodioxide complex4b,c as
the active PhN transfer agent. Our preliminary study on the
Cu(I)-catalyzed allylic amination6 excluded the intermediacy of free
nitrosoarene and aryl nitrene from trapping experiments, suggesting
that a coordinated organonitrogen species could be the active
aminating agent. Relatedly, group 10 metal complexes of ni-
trosoarenes have been proposed as intermediates in enantioselective
hetero-Diels-Alder11 and O-nitroso aldol reactions.12 To explore
further the nature of the reactive intermediates in these reactions,
we report herein (1) the isolation and structure determination of
the first homoleptic nitrosoarene-metal complexes, and (2) evi-
dence that such complexes are intermediates and likely N-transfer
agents in Cu(I)-catalyzed allylic aminations.
and bond angles (deg): Cu(1)-N(4) 1.8983(15), Cu(1)-N(6) 1.9296(19),
Cu(1)-N(2) 1.9701(18), O(1)-N(2) 1.2646(16), N(4)-Cu(1)-N(6) 133.81-
(6), N(4)-Cu(1)-N(2) 120.96(6), N(6)-Cu(1)-N(2) 104.96(4).
obtained from the reaction of [Cu(CH3CN)4]PF6 with excess N,N′-
diethyl-4-nitrosoaniline (1:4.3, CH2Cl2, rt). The resulting greenish-
red solid 2 (70% yield) was identified with the aid of IR, NMR,
MS (FAB), and X-ray crystallography. As shown in Figure 1, the
cation of 2 consists of a distorted trigonal planar, 16-electron
Cu(ArNO)3+ moiety. The copper atom is coordinated to the nitro-
soarene ligands through the N-atom with the NO units directed
out of the CuN3 plane. The Cu-N bond lengths (av. 1.933 Å) are
markedly varied, ranging from 1.898 to 1.970 Å, as are the
N-Cu-N bond angles, which range from 105 to 134°. These dis-
tortions from ideal trigonal planarity are among the most severe
found in d10 ML3 complexes14 and are presumed to be sterically
derived.15 The N-O bond lengths, however, are relatively uniform
(av. 1.258 Å) and are comparable to those in the free ligand (1.252
Å),16 suggesting the absence of significant back-bonding from
Cu(I) to the nitrosoarene ligand. Compound 2 is the first crystal-
lographically characterized, homoleptic nitrosoarene-metal com-
plex and the first copper complex bearing a simple C-nitroso
ligand.17
Seeking to produce copper complexes that could be potential
intermediates in Cu(I)-catalyzed allylic aminations, [Cu(CH3CN)4]-
PF6 was treated with 4.2 equiv of PhNO (CHCl3, rt) to produce a
dark red, labile complex 1 which is formulated as [Cu(PhNO)3]-
PF6 based on its Cu content, the amount of recovered PhNO, and
1
its H NMR spectrum, which shows a 2:1:2 set of aromatic reso-
nances downfield from PhNO (Scheme 1). Unfortunately, the
Scheme 1
Most importantly, complex 1 is an intermediate, and possibly
the active N-transfer agent, in the Cu-catalyzed amination reactions
with PhNHOH based on the following observations. First, 1 was
isolable (47%) in the reaction of PhNHOH with R-methylstyrene
(AMS) in the presence of [Cu(CH3CN)4]PF6 (90 °C, dioxane, 3 h;
Scheme 1). Second, the reaction of nitrosobenzene complex 1 with
excess AMS (90 °C, dioxane, 20 h) produced the corresponding
allylamine 3 cleanly (40%, Scheme 2),18 whereas the more electron-
rich nitrosoaniline complex 2, like its free ligand, was unreactive
toward olefins. Third, slow addition of PhNHOH to a mixture of
either 1 or 2 (8 mol %) and AMS at 90-95 °C (24 h) produces
allylamine 3 (36% from 1 and 15% from 2), indicating that 1 and
sensitivity and lability of 1 has thus far thwarted its detailed struc-
ture determination.13 A more tractable Cu-ArNO derivative was
† University of Louisiana at Lafayette.
‡ University of Oklahoma.
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J. AM. CHEM. SOC. 2005, 127, 7278-7279
10.1021/ja044093m CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
investigations of the synthetic and mechanistic aspects of
Cu-catalyzed allylic amination are underway.
Acknowledgment. Financial support provided by the National
Science Foundation and helpful discussions with Prof. G. Richter-
Addo are appreciated.
Supporting Information Available: Preparative and characteriza-
tional data for 1 and 2, including the X-ray crystallographic data for 2
and the PM3(TM) computational output for 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
Scheme 3
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2 are (pre)catalysts for the amination of AMS by PhNHOH.
Significantly, the stoichiometric amination by complex 1 does not
involve the intermediacy of free PhNO since heating 1 with a 1:1
mixture of 2-methyl-2-pentene and 2,3-dimethylbutadiene (a trap-
ping agent for free PhNO19) gave allylic amination products 4 and
5 exclusively (8 and 18%, respectively, based on 1), rather than
the Diels-Alder trapping product 6, the same result found
previously in the Cu(I)-catalyzed aminations with ArNHOH.6
The above observations, together with the distinctive regiose-
lectivity of the Cu-catalyzed reactions, lead us to suggest as a
plausible reaction pathway the one shown in Scheme 3. Initially,
some Cu(II) is likely generated by Cu(I) reduction of ArNHOH
(PhNH2 detected).20 The resulting Cu(II) then oxidizes PhNHOH
to PhNO with formation of the C-nitroso complex 1. This likely
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to a free or coordinated olefin by a metal-mediated ene-type
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(21) The active N-transfer agent 1 could also have L ) dioxane and/or olefin.
The apparent N-centered, enophilicity of 1 parallels that found in some
nitrosoaldol reactions proposed to involve Ag(I)-ArNO complexes but
contrasts with the O-based reactivity of others (ref 12) and with the N-O-
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We have thus established here the first structurally verified
Cu(I) complex of a C-nitroso compound, demonstrated its inter-
mediacy in the Cu(I)-catalyzed allylic amination of olefins, and
suggested its role as the active group transfer agent. Further
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