Transient terminal Cu-nitrene intermediates from discrete dicopper nitrenes

Article abstract of DOI:10.1021/ja065299l

Reaction of the copper(I) β-diketiminate {[Me₃NN]Cu}₂(μ-toluene) with the aryl azide N₃Ar (Ar = 3,5-Me₂C₆H₃) in toluene results in immediate effervescence and formation of the dicopper nitrene {[Me₃NN]Cu}₂(μ-NAr) (2) in 77% yield. The X-ray structure of 2 shows nearly symmetric bonding of the nitrene to two Cu centers separated by 2.911(1) A with Cu-N distances of 1.794(5) and 1.808(5) A along with a Cu-N-Cu angle of 107.8(2)°. This structure is conceptually related to the dicopper carbenes {[Me_xNN]Cu}₂(μ-CPh₂) (x = 2 or 3) (Dai, X.; Warren J. Am. Chem. Soc. 2004, 126, 10085. Badiei, Y. M.; Warren J. Organomet. Chem. 2005, 690, 5989.) which exhibit shorter Cu-Cu distances (2.4635(7) or 2.485(1) A) and acute Cu-C-Cu angles (79.51(14) or 80.1(2)°). Addition of the Cu(I) anilidoimine {[Me₂AI]Cu}₂ (prepared from CuO^tBu and the aniline-imine H[Me₂AI] in 77% yield) to a benzene-d₆ solution of 2 results in the formation of two new anilidoimine complexes {[Me₂AI]Cu(μ- NAr)Cu[Me₃NN] (5) and {[Me₂AI]Cu}₂(μ-NAr) (6) as well as [Me₃NN]Cu(benzene) over 3 h. These observations are consistent with the slow dissociation of a [Me₃NN]Cu fragment from 2 to generate the transient terminal nitrenes [Me₃NN]Cu=NAr and [Me₂AI]Cu=NAr quickly trapped by the [Me₂AI]Cu fragment to form the new unsymmetrical and symmetrical dicopper nitrenes 5 and 6. Preliminary reactivity studies indicate electrophilic reactivity at the nitrene moiety. Dicopper nitrene 2 reacts with 10 equiv PMe₃ and CN^tBu to give ArN=PMe₃ and ArN=C=N^tBu in 94% and 92% yields, respectively, with concomitant formation of [Me₃NN]Cu(L) (L = PMe₃ and CN_tBu). Reaction between 2 and 2 equiv PMe₃ allows for observation of the structurally characterized Cu(I) phosphaimide [Me₃NN]Cu(ArN=PMe₃) (7).Copyright

Full text of DOI:10.1021/ja065299l

Published on Web 11/08/2006
Transient Terminal Cu-Nitrene Intermediates from Discrete Dicopper Nitrenes
Yosra M. Badiei, Ammani Krishnaswamy, Marie M. Melzer, and Timothy H. Warren*
Georgetown UniVersity, Department of Chemistry, Box 571227, Washington, D.C. 20057-1227
Received July 24, 2006; E-mail: thw@georgetown.edu
Scheme 1. Synthesis and Structure of 2
Copper-based catalysts have held prominence in metal-catalyzed
aziridination since the discovery that copper promoted the decom-
position of benzensufonyl azide to give aziridines in the presence
of alkenes.1 Using PhIdNTs or related sulfonylnitrene precursors
in conjunction with a variety of copper complexes with N-donor
ligands has resulted in high aziridine yields as well as ee values up
to 98% in some cases.2 While both experimental3,4 and theoretical5,6
studies point to Cu-nitrene intermediates, direct evidence for these
elusive species is sparse. The use of either PhIdNTs or N3Ts/hν
with a C2-symmetric diimine Cu(I) catalyst gave identical ee values
in the aziridination of styrene derivatives, suggesting a common,
PhI-free active intermediate.3 Theoretical studies point to a κ2-N,O
binding mode of the NSO2R tosylnitrene moiety to a square planar
Cu center for which triplet and singlet states are comparable in
energy.5 As cationic7,8 and neutral9 catalyst systems also exhibit
nitrene insertion into C-H bonds to give sulfonylamines, the
synthetic exploration of copper nitrenes offers the potential to
understand factors that control these modes of reactivity and to
expand the range of nitrene transfer reagents.
We recently reported the use of â-diketiminate ancillary ligands
to prepare doubly bridged [Co]2(µ-NR)2, singly bridged [Ni]2(µ-
NR), and terminal [M]dNR (M ) Co, Ni) imide (or nitrene)
complexes.10 Given our recent isolation of dicopper carbenes [Cu]2-
(µ-CPh2) which dissociate a â-diketiminato Cu(I) fragment in arene
solutions (eq 1),11 we reasoned that the electronic similarities
between singlet carbenes and nitrenes might allow similar stabiliza-
tion of a dicopper nitrene [Cu]2(µ-NR) from organoazides. In fact,
Vedernikov and Caulton have reported the generation of a deep
purple species identified as [{[NNN]Cu}2(µ-NTs)]2+ from NMR
and ESI-MS data in the reaction of a cationic Cu(I) catalyst bearing
a neutral tridentate ligand with PhIdNTs.7,12 Moreover, Cui and
He have shown that a unique terpyridine-based disilver system
serves as an efficient aziridination catalyst.13
range of Cu-N distances (1.842(5)-1.893(6) Å) reported for µ3-
NPh groups in Fenske’s [Cu24(NPh)14]4- and [Cu12(NPh)8]4-
clusters prepared from copper(I) salts and Li2NPh.15 The structure
of dicopper nitrene 2 is conceptually similar to {[Me2NN]Cu}2(µ-
CPh2) in which the two copper centers are separated by 2.4635(7)
Å.11a The longer Cu‚‚‚Cu separation in 2 is attributed to the ability
of an sp2-hybridized nitrene to σ-donate to the two copper centers
via two lone pairs; in contrast, the diphenylcarbene moiety can
donate only one lone pair shared between two copper fragments.
This difference in bonding is also reflected in the wider Cu1-
N5-Cu2 angle (107.8(2)°) in 2 than for the corresponding Cu-
C-Cu angle (79.51(14)°) in {[Me2NN]Cu}2(µ-CPh2).11a
Variable temperature 1H NMR (500 MHz) spectra of diamagnetic
2 in toluene-d8 reveal two separate fluxional processes. At -85
°C, four â-diketiminato Ar-H, six Ar-Me, and one broadened
backbone Me resonance are observed consistent with the ap-
proximate C2 symmetry observed in the solid-state that relates each
[Me3NN]Cu fragment. Upon warming to 0 °C, only two â-diketim-
inato Ar-H and o-Ar-Me resonances are observed owing to a
low-energy rocking motion about the Cu‚‚‚Cu vector with an
activation barrier ∆G* ) 9.4(2) kcal/mol at -72 °C. Further
N2CPh2
arene
2[Cu](arene)
8 [Cu]2(µ-CPh2) y z [Cu]dCPh2 + [Cu](arene)
-N2
[Cu] ) [MexNN]Cu (x ) 2 or 3) (1)
We initially explored the reactivity of organoazides with [Me2-
NN]Cu(2,4-lutidine), a competent catalyst for the aziridination of
olefins such as styrene and cyclooctene with PhIdNTs.14 No
interaction of the organoazides with the [Me2NN]Cu center could
1
1
be inferred by H NMR spectroscopy in benzene-d6. Presumably,
warming to 45 °C gives C2V-symmetric H NMR spectra due to
the weak σ-donor ability of the azide does not allow it to
successfully compete with 2,4-lutidine. Instead, the reaction between
{[Me3NN]Cu}2(toluene)11b (1) and 1 equiv NdNdNAr (Ar ) 3,5-
Me2C6H3) in toluene results in immediate effervescence and
dramatic coloration of the solution to dark violet from which crystals
of {[Me3NN]Cu}2(µ-NAr) (2) may be isolated from pentane at -35
°C in 77% yield. X-ray analysis shows that the nitrene moiety is
nearly symmetrically bound between two [Me3NN]Cu fragments
separated by 2.911(1) Å with Cu-N distances of 1.794(5) and
1.808(5) Å. The Cu-µ2-NAr distances in 2 are shorter than the
pairwise coalescence of these Ar-H and o-Ar-Me resonances with
an activation barrier ∆G* ) 14.7(2) kcal/mol at 29-34 °C. While
the higher energy process is consistent with rapid dissociation/
reassociation of a [Me3NN]Cu fragment from 2, we instead favor
complete rotation of the [Me3NN]Cu fragments along the individual
Cu-N bonds at the expense of the Cu‚‚‚Cu interaction. We see no
evidence for rapid exchange between [Me3NN]Cu(solvent) and 2
upon addition of 1 to solutions of 2 at room temperature. Moreover,
only one backbone C-H signal is observed for the dicopper nitrene
from -85 to +50 °C. This stands in contrast to arene solutions of
9
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J. AM. CHEM. SOC. 2006, 128, 15056-15057
10.1021/ja065299l CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of 3 and Chemical Exchange with 2
place with the anilidoimine-based dicopper nitrene 6. Reaction of
2 with only 2 equiv PMe3 leads to the formation of a third
P-containing substance 7 exhibiting a 31P NMR signal at δ 21.7
ppm in benzene-d6. This new species is [Me3NN]Cu(N(Ar)PMe3)
confirmed by its independent synthesis from 1 and Me3PdNAr.
The X-ray structure of 7 shows trigonal coordination at N with
Cu-N3 and N3-P distances of 2.010(3) and 1.586(3) Å, respec-
tively.17 Reaction of 2 with 10 equiv styrene in benzene over ca. 3
h results in ArNdNAr as the predominate nitrene-containing
product; no aziridination is observed. In the absence of styrene,
other nitrene-based decomposition products are also present which
include H2NAr and ArNHNHAr.
Scheme 3. Electrophilic Reactivity of 2 and X-ray Structure of 7
In summary, reaction of N3Ar with {[Me3NN]Cu}2(toluene) leads
to the dicopper nitrene {[Me3NN]Cu}2(µ-NAr) (2). Crossover
experiments with the Cu(I) anilidoimine 3 suggest slow dissociation
of a [Me3NN]Cu fragment from dicopper nitrenes 2 and 5 to give
terminal Cu-nitrene intermediates. While reaction of 2 with CNt-
Bu and PMe3 results in smooth group transfer, weaker nucleophiles
such as styrene result in the nitrene coupling product ArNdNAr.
Factors that govern this selectivity will be explored by the use of
electronically diverse nitrene substituents, especially since â-diketim-
inato Cu(I) complexes catalyze the aziridination of styrene with
PhIdNTs, a source of the particularly electron-poor tosylnitrene.
Acknowledgment. T.H.W. thanks the donors of the Petroleum
Research Fund administered by the ACS (PRF-G) and the NSF
CAREER program. M.M.M. is grateful to the Clare Booth Luce
Foundation for a predoctoral fellowship.
{[MexNN]Cu}2(µ-CPh2) which display signals for both the dicopper
carbene as well as [MexNN]CudCPh2 and [MexNN]Cu(solvent)
above 0 °C (eq 1).
Supporting Information Available: Experimental procedures with
characterization data as well as crystallographic information (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Use of a chemically distinct Cu(I) â-diketiminato analogue16a,b
{[Me2AI]Cu}2 (3) prepared from the corresponding free iminoani-
line16c and CuOtBu in 77% yield provides evidence for the terminal
nitrene intermediate [Me3NN]CudNAr (4) generated by slow,
reversible loss of a [Me3NN]Cu fragment from {[Me3NN]Cu}2(µ-
NAr) (2) (Scheme 2). Addition of 1 equiv 3 to room temperature
benzene-d6 solutions of the dicopper nitrene 2 results in the
formation of new species within 15 min. After 3 h, new anilidoimine
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Cu(µ-NAr)Cu[Me3NN] (5) and to a lesser extent, the symmetric
{[Me2AI]Cu}2(µ-NAr) (6) at δ 7.649 and 7.740 ppm are clearly
evident along with corresponding â-diketiminato C-H resonances
for 5 and [Me3NN]Cu(arene) at δ 4.874 and 4.808 ppm. The new
symmetric dicopper nitrene 6 may be isolated in 49% yield from
the reaction between {[Me2AI]Cu}2 and N3Ar, while 5 is formed
as the predominant species when a 1:1 mixture of {[Me3NN]Cu}2-
(toluene) and {[Me2AI]Cu}2 reacts with N3Ar. In the exchange
reactions shown in Scheme 2, the crowded nature of dicopper
nitrenes 2 and 5 strongly implicates the transient generation of the
terminal copper nitrenes [Me3NN]CudNAr (4) and [Me2AI]Cud
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formation of Me3PdNAr or ArNdCdNtBu in 94% and 92% yields
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