Cu(I) β-diketiminates for alkene aziridination: Reversible Cu-arene binding and catalytic nitrene transfer from PhI=NTs

Article abstract of DOI:10.1021/ic048968+

β-Diketiminato Cu(I)-lutidine complexes [RMeNN]Cu(2,4-lutidine) (R = Me (4a), ⁱPr (4b)) were prepared in high yield from Tl[RMeNN] and [CuBr(2,4-lutidine)₂]₂. Both 4a and 4b reversibly dissociate lutidine base in toluene to give [RMeNN]Cu(toluene) solvento complexes. A related base-free dicopper species {[Me₂NN]Cu} ₂ (6) bridged via η²-binding of opposing N-aryl rings could be isolated by the addition of Tl[Me₂NN] to CuBr. The lutidine precursors serve as precatalysts for the aziridination of alkenes with PhI=NTs. Styrene, β-methylstyrene, and cyclooctene gave the highest yields (59-96%) with a low olefin to PhI=NTs ratio (3:1) and 5 mol % catalyst loading.

Full text of DOI:10.1021/ic048968+

Inorg. Chem. 2004, 43, 6537−6539
Cu(I)
â
-Diketiminates for Alkene Aziridination: Reversible Cu
NTs
sArene
Binding and Catalytic Nitrene Transfer from PhI
d
LaTasha D. Amisial, Xuliang Dai, R. Adam Kinney, Ammani Krishnaswamy, and Timothy H. Warren*
Department of Chemistry, Box 571227, Georgetown UniVersity, Washington, D.C. 20057
Received July 30, 2004
â
(R
-Diketiminato Cu(I)
)
−
lutidine complexes [RMeNN]Cu(2,4-lutidine)
of alkene.9 In cases where the alkene is valuable or difficult
to separate from the aziridine product, the 10-100-fold
excess of alkene used with most catalysts becomes impracti-
cal. Copper catalysts employing monoanionic ligands such
as bis- or tris(pyrazolyl)borates are promising,4,7 such as the
recent report of alkene/PhIdNTs ratios as low as 1:1 in the
aziridination of styrene, cyclooctene, and 1-hexene with
electron-poor tris(pyrazolyl)borates.6,7
We are attracted to monoanionic â-diketiminates in
copper-catalyzed aziridination due to the steric and electronic
tunability these ancillary ligands afford via modifications to
the N-aryl and backbone substituents.10 Our recent isolation
of the three-coordinate â-diketiminato Co11 and Ni12 com-
plexes [NN]MdNAd has further piqued our interest in Cu-
(I) â-diketiminates13-17 for aziridination, as species with Md
NR groups have been long proposed as active intermediates
in metal-catalyzed aziridination.2c,3b,f
Me (4a), iPr (4b)) were prepared in high yield from Tl[RMeNN]
and [CuBr(2,4-lutidine)2]2. Both 4a and 4b reversibly dissociate
lutidine base in toluene to give [RMeNN]Cu(toluene) solvento
complexes. A related base-free dicopper species
(6) bridged via
2-binding of opposing N-aryl rings could be isolated
by the addition of Tl[Me2NN] to CuBr. The lutidine precursors serve
as precatalysts for the aziridination of alkenes with PhI NTs.
Styrene, -methylstyrene, and cyclooctene gave the highest yields
(59 96%) with a low olefin to PhI NTs ratio (3:1) and 5 mol %
catalyst loading.
{[Me2NN]Cu}
2
η
d
â
−
d
Since Kwart and Kahn’s seminal discovery in 1967 that
copper promoted the decomposition of benzenesulfonyl azide
to give aziridines in the presence of alkenes,1 copper
complexes have held prominence in metal-catalyzed aziri-
dination. A wide variety of chelating N-donor ligands with
Cu(I) and Cu(II) salts catalyze the aziridination of alkenes
with the imidoiodane PhIdNTs,2-7 the most commonly used
nitrene transfer reagent. Variants employing chiral bis-
(oxazolines)2 or related chiral diimine ligands3 can give
enantioselectivities as high as 98% ee.8
Two slightly different â-diketiminate ligands were ex-
plored in this investigation. The first is the symmetric
â-diketiminate [Me2NN]- possessing two 2,6-dimethylphenyl
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In addition to expanding the range of useful nitrene transfer
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substituents, a challenge in catalytic aziridination remains
the development of systems that do not require a large excess
* To whom correspondence should be addressed. E-mail: thw@
georgetown.edu.
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10.1021/ic048968+ CCC: $27.50
Published on Web 09/21/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 21, 2004 6537

COMMUNICATION
Scheme 1. Synthesis of the Unsymmetric â-Diketiminates 2b and 3ba
146.44(18)° [4b]) is considerably more obtuse that the N2-
Cu-N3 angle (114.93(6)° [4a] and 115.85(17)° [4b]). This
is also reflected in the Cu-N1 bond (1.912(2) Å [4a] and
1.923(4) Å [4b]) and Cu-N2 bond (1.987(2) Å [4a] and
1.985(4) Å [4b]) distances that indicate unsymmetric che-
lation of the â-diketiminate ancillary ligand. These param-
eters stand in contrast to the symmetrically chelated
[iPr2NN]Cu(py) possessing two o-iPr substituted N-aryl rings
which exhibits a coplanar orientation of the pyridine ring
with the â-diketiminate backbone.15d In both 4a and 4b, the
lutidine ring is twisted away from the plane of the â-diket-
iminato chelate, and the “bent” trigonal geometry likely
results from repulsions between the lutidine o-Me group and
one N-aryl ring. In agreement, DFT calculations on a
simplified system indicate a soft potential for bending of a
pyridine ligand in the absence of steric effects.
a Conditions: (i) 1 equiv of diisoproylaniline, cat. TSA, reflux in toluene
3 h; (ii) 1.5 equiv of dimethylaniline, 1 equiv of TSA, reflux toluene 6 h,
Na2CO3 workup; (iii) KH in THF, then TlOAc.
N-aryl substituents. We recently reported the Cu(I)-alkene
adduct [Me2NN]Cu(η2-ethylene) of this ligand which was
found to cleave dioxygen13 as well as react with N2CPh2 to
give the structurally characterized dicopper carbene {[Me2NN]-
Cu}2(µ-CPh2) that is reactive toward alkene cyclopropan-
ation.18 The second ligand we used derives from the new
â-diketimine H[iPrMeNN] (2b) prepared in a two-step
process (Scheme 1). Condensation of 1 equiv of 2,6-
diisopropylaniline with 2,4-pentanedione under acid catalysis
afforded the corresponding iminoketone 1 as an oil. Further
condensation of crude 1 with 1.5 equiv of 2,6-dimethylaniline
and 1.0 equiv of toluenesulfonic acid allowed the isolation
of H[iPrMeNN] (2b) in 61% overall yield after basic workup.
A thallium(I) derivative was prepared by deprotonation in
THF by KH followed by reaction with TlOAc to give
Tl[iPrMeNN] (3b) in 77% yield.13
1
The H NMR spectrum of 4a at -80 °C in toluene-d8
reveals only one Ar-Me resonance at δ 2.425 ppm,
indicating that rotation about the Cu-lutidine bond and/or
lutidine dissociation/reassociation is still fast on the NMR
time scale at this temperature. As the temperature is
increased, the lutidine o-H resonance in 4a shifts from δ
7.407 ppm at -80 °C to δ 7.589 ppm at 70 °C. Since this
signal appears at δ 8.319 ppm for free lutidine in toluene-
d8, this suggests that an increasing equilibrium amount of
free lutidine is formed with increasing temperature. Similar
1
behavior is observed in VT H NMR spectra of 4b; the
lutidine o-H resonance moves downfield from δ 7.159 ppm
at -60 °C to δ 7.384 ppm at 80 °C.
Assuming that lutidine is not appreciably dissociated at
the low-temperature limit, the equilibrium constant Keq for
lutidine dissociation from 4a and 4b can be derived from
Although scouting reactions indicated that [Me2NN]Cu-
(ethylene) catalyzes the aziridination of styrene with PhIdNTs,
a Cu catalyst precursor lacking any olefin was desired to
eliminate contamination of the alkene substrate. Addition of
Tl[RMeNN] (R ) Me (3a), iPr (3b)) to 0.5 equiv of {CuBr-
1
the lutidine o-H H NMR chemical shift. van’t Hoff plots
of ln Keq versus 1/T over a wide temperature range (ca. -70
to 70 °C) allowed for the determination of ∆H (2.3(3) and
5.6(3) kcal/mol) and ∆S (3(1) and 7(1) cal/mol‚K) for the
dissociation of lutidine from 4a and 4b, respectively (Figures
S1 and S2). The low ∆H and ∆S values suggest that a
molecule of toluene-d8 solvent likely coordinates to the Cu-
center in the lutidine-free form (eq 2).
19
(2,4-lutidine)2}2 (prepared by addition of anhydrous cop-
per(I) bromide to excess 2,4-lutidine with heating) in toluene
results in the immediate precipitation of TlBr and isolation
i
of [RMeNN]Cu(2,4-lutidine) (R) Me (4a); Pr (4b)) as
yellow crystals from pentane in 87% and 85% yields,
respectively (eq 1).
A related lutidine-free complex {[Me2NN]Cu}2 (6) could
be isolated in moderate yield by chemical removal of lutidine
from 3a with BF3‚OEt2 in pentane, or, more conveniently,
via the addition of Tl[Me2NN] to CuBr in toluene followed
by crystallization from pentane in 53% yield. The asymmetric
unit in the X-ray structure of 6 (Figure 1) consists of 1.5
molecules of {[Me2NN]Cu}2 of which one set of bridged
[Me2NN]Cu fragments is related by inversion. (Disorder in
3 of the 6 unique N-aryl rings is observed.) Complementary
η2-arene interactions with an opposing N-aryl group link each
The X-ray crystal structures of 4a and 4b revealed three-
coordinate copper centers in which the 2,4-lutidine ligand
is sandwiched between two â-diketiminato N-aryl groups
(Figure 1). The most striking feature in these structures is
the considerable “bending” of the lutidine toward one N-aryl
ring such that the N1-Cu-N3 angle (145.80(6)° [4a] and
(16) Laitar, D. S.; Mathison, C. J. N.; Davis, W. M.; Sadighi, J. P. Inorg.
Chem. 2003, 42, 7354.
(17) (a) Yokota, S.; Tachi, Y.; Nishiwaki, N.; Ariga, M.; Itoh, S. Inorg.
Chem. 2001, 40, 5316. (b) Brown, E. C.; Aboelella, N. W.; Reynolds,
A. M.; Aullo´n, G.; Alvarez, S.; Tolman, W. B. Inorg. Chem. 2004,
43, 3335.
(18) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085.
(19) Dyason, J. C.; Engelhardt, L. M.; Healy, P. C.; Pakawatchai, C. P.;
White, A. H. Inorg. Chem. 1985, 24, 1950.
6538 Inorganic Chemistry, Vol. 43, No. 21, 2004

COMMUNICATION
Figure 1. X-ray structures of â-diketiminato Cu(I) complexes 4a, 4b, and 6 (only one unique {[Me2NN]Cu}2 dimer is shown).
Table 1. Alkene Aziridination with PhIdNTs Catalyzed by 4a and 4ba
[Me2NN]Cu fragment of 6. The inequivalent Cu-C(arene)
distances at each Cu center (Cu1-C30 2.103(5) Å, Cu1-
C29 2.204(5) Å; Cu2-C15 2.069(5) Å, Cu2-C14 2.242(5)
Å; Cu3-C57A′ 2.12(2) Å, Cu3-C58A′ 2.26(2) Å) indicate
a less-symmetric binding of the arene than observed in
Sadighi’s electron-poor [NN]Cu(η2-benzene) possessing 3,5-
(CF3)2C6H3 N-aryl and CF3 backbone substituents (Cu-C
2.117(4) and 2.102(4) Å).16 Dissolution of 6 in benezene-d6
gives a C2V-symmetric NMR spectrum identical to that of
[Me2NN]Cu(toluene)18 with the absence of resonances due
to free toluene.
Scouting runs of styrene aziridination with PhIdNTs
catalyzed by 5 mol % 4a in benzene, dichloromethane, and
acetonitrile identified the latter as the best solvent as judged
by the short time (ca. 20 min) required for consumption of
insoluble PhIdNTs. It should be noted that the solvento
species [RMeNN]Cu(NCMe)15 are likely present in room
temperature acetonitrile-d3 solutions of 4 as the lutidine o-H
resonance appears at δ 7.94 and 8.04 ppm for 4a and 4b,
respectively, approaching the expected value for free lutidine
in this solvent (δ 8.31 ppm).
a Conditions: 0.1393 mmol of PhIdNTs, 5 mol % 4a or 4b in 2 mL of
acetonitrile. Yields determined by GC.
effect of catalyst loading on styrene aziridination was also
investigated. Decreasing the amount of 4b to 2.5 and 1.25
mol % (styrene/PhIdNTs ) 1) resulted in a decrease in
aziridine yield to 61% and 28%, respectively. At the lowest
loading, the PhIdNTs reagent was no longer completely
consumed, indicating enhanced susceptibility to catalyst
decomposition pathways.
In the aziridination of eight different olefins with PhId
NTs using 3 equiv of olefin catalyzed by 5 mol % 4a in
acetonitrile, styrene, trans-â-methylstyrene, and cyclooctene
gave the highest yields (Table 1). The use of the somewhat
larger 4b marginally increased the yield of N-tosyl aziridines
derived from these three alkenes. Other substrates surveyed
under these conditions with 4a included the aromatic alkenes
R-methylstyrene and cis-stilbene which gave poor yields
(<10%), and the aliphatic alkenes 1-hexene, cis-2-octene,
and cyclohexene which gave marginally higher yields (10-
30%).
In summary, this system allows the use of a low alkene/
PhIdNTs ratio to achieve acceptable aziridination yields with
styrene, trans-â-methylstyrene, and cyclooctene. Further-
more, details involving the solution behavior of this â-diket-
iminato catalyst system may be important in understanding
interactions with alternative, soluble nitrene transfer reagents.
Studies with organoazides, which could potentially yield a
diverse range of N-substituted aziridines, are underway and
will be reported in due course.
Acknowledgment. T.H.W. thanks the ACS Petroleum
Research Fund (Type-G) and the NSF CAREER program
Further decreasing the alkene/PhIdNTs ratio to 1:1 with
styrene, trans-â-methylstyrene, and cyclooctene still resulted
in aziridine formation, albeit in lower yields (47-58%)
(Table 1). The decreased yield results in the increased
formation of the byproduct amine H2NTs which forms in
near quantitative yield in the absence of added olefin. The
(CHE-0135057) for generous support of this work.
Supporting Information Available: Experimental and com-
putational details (PDF) and X-ray crystallographic data (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC048968+
Inorganic Chemistry, Vol. 43, No. 21, 2004 6539