Aromatic oxidative decompositions of copper Schiff base complexes

Article abstract of DOI:10.1016/j.tetlet.2009.04.100

Copper Schiff base complexes used in enantioselective aziridination reactions were shown to possess a proclivity for aromatic oxidative coupling reactions.

Full text of DOI:10.1016/j.tetlet.2009.04.100

Tetrahedron Letters 50 (2009) 4225–4228
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Tetrahedron Letters
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Aromatic oxidative decompositions of copper Schiff base complexes
*
Jean-Christophe Andrez
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper Schiff base complexes used in enantioselective aziridination reactions were shown to possess a
proclivity for aromatic oxidative coupling reactions.
Received 29 March 2009
Revised 14 April 2009
Accepted 24 April 2009
Available online 3 May 2009
Ó 2009 Elsevier Ltd. All rights reserved.
The aziridine functionality is present in a number of naturally
occurring molecules and the biological properties of aziridine-con-
taining compounds such as mitomycins, azinomycins, FR-900482,
maduropeptin, and azicemicins are of significant medicinal inter-
est. In addition, aziridines are useful synthetic entities since they
can give rapid access to a variety of functional groups.1 Extensive
studies were conducted to develop aziridination catalysts giving
high enantioselectivities and turnover numbers (TONs).2 However,
aziridinations of challenging substrates still often require extended
reaction times and high catalyst loading which is incompatible for
manufacturing scale syntheses. The understanding of the catalysts
decomposition pathways becomes crucial for designing a catalyst
with longer lifetime and higher turnover numbers.
Ongoing research in our laboratory has provided us with an
opportunity to affect a Jacobsen-type aziridination of an olefinic
substrate.3 Experimental and theoretical studies of this reaction
suggest that the catalytically active species emerging from the
interaction of a Cu(I)-bis-imine complex, 1 (derived from nonrace-
mic 1,2-cyclohexane diamine) with imidoiodinane, 2,4 is a Cu(III)
complex of structure 3 (Scheme 1).5
The organochemistry of copper with oxidation state greater
than (+I) is not well known and only a few tetradentate copper(III)
species were characterized (Scheme 2).7 The literature records no
direct observation of species 3, and of course, a number of mecha-
nistic issues pertaining to the aziridination reaction as well as to
the decomposition of 3 remain to be clarified. Herein, we report
the first characterization of complexes 3 by mass spectrometry
(MS-ESI+). We also bring an explanation based on the decomposi-
tion products formed through the interaction of complexes 4 with
imidoiodinane 2 as to why certain catalysts give high selectivity
and TON and others fail in those regards.
Complexes 4 were prepared in acetonitrile solution by the cus-
tomary method (Scheme 3). Copper(I) being d10 and diamagnetic,
standard NMR experiments can be performed.
The best diimine catalysts described for the aziridination reac-
tion, 4c and 4h, possess ortho chloro substituents on Ar1. Addition
X
X
Ar2-SO2-N=I-Ph
2
The exact oxidation state of copper during the reaction has been
the subject of intensive investigations.6 On the basis that complex
1 gave the same enantioselectivity for the aziridination of styrene
using imidoiodinane 2 or the corresponding photogenerated tosyl-
nitrene obtained from tosylazide, Jacobsen has concluded that the
reaction involved the same Cu(III)-nitrene species.3b More recently,
high-level quantum chemical calculations confirmed this result
and suggested a Cu(III)-nitrene species to be the reactive interme-
diate in a Cu(I)/Cu(III) catalytic cycle.3a In these studies, the phenyl
moiety of PhI in PhI@NSO2Ar2 was shown to have no influence on
the reactivity and selectivity. Therefore, a redox pathway where
PhI is fully dissociated from the complex to form a discreet
Cu(III)-nitrene species prevails as the accepted mechanism.
N
N
N
N
Cu
N
Cu
Ar1
(III)
Ar1
(I)
Ar1
Ar1
3
1
S
Ar2
O2
Scheme 1. Presumed copper(III) complex 3 formed during the interaction of 1 with
imidoiodinane 2.
C6F5
H
N
N
CF3
CF3
S
S
EtO
Et2N
Cu
Cu
H N Cu N H
N
C6F5
C6F5
N
N
F3C CF3
Cu
CF3
F3C
Ph3P=N=PPh3
C6F5
* Tel.: +1 778 552 8649.
E-mail address: jc.andrez@gmail.com
Scheme 2. Examples of tetradentate copper(III) complexes.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.100

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J.-C. Andrez / Tetrahedron Letters 50 (2009) 4225–4228
Z
Y
Z
Y
Cl
O
H
O
PF
PF
6
R
6
Y
R
Z
Y
PF
H
6
X
X
Cl
N
N
Z
(I)
Cu
HN
SO
Cl
1) 2
W
W
1
1
+
N
N
N
N
O
S
O
W
W
W
W
2
(III)
2
3
2
3
2
(I)
2) HCl
Cu
Cu N
Y
OMe
Cl
(Ar2= 4-MeO-C6H4)
O N
2
12-19%
7-11%
Y
W
W
1
1
Z
Z
4a
O
H
7
6
X
X
Cl
2: Ar2= 2-NO2-C6H4
Y
R
Y
R
5
Z
4
Z
Scheme 4. Decomposition products 6 and 7 from reaction of 4a with 2.
a R = Cl; W1 = c-C4H8; W2 = β-H; W3 = α−H; X = Y = Z = H
b R = Cl; W1 = c-C4H8; W2 = β-H; W3 = α−H; X = H; Y = Z = D
c R = X = Z = H; W1 = c-C4H8; W2 = β-H; W3 = α−H; Y = Cl
d R = Cl; W1 = W2 = W3 = X = Y = Z = H
Alternatively, conducting the reaction with complex 4a and imi-
doiodinane 2, Ar2 = 2-NO2–C6H4 for 1 h without addition of styrene
followed by hydrolysis with hydrochloric acid provided the
decomposition products 6 (7–11% yield) and 7 (12–19% yield)
(Scheme 4).10 The remaining of the hydrolyzed complex was recov-
ered as p-chlorobenzaldehyde and o-nitrosulfonamide. An X-ray
structure of compound 6 was obtained and confirmed the ortho
relationship of the two aromatics relative to the chlorine atoms.11
Compound 7 was unambiguously assigned by NMR analysis as the
isomer with the sulfonamide ortho to the aldehyde.12
Next, we explored on a synthetic scale the effect of quenching
the reaction with deuterated hydrazine. Hydrazine could act either
as a competing amine for the formation of 7 or as a reducing agent
for copper salts.13 Complex 4a, dissolved in acetonitrile-d3, was re-
acted for 1 h with a stoichiometric amount of imidoiodinane 2,
Ar2 = 4-OMe–C6H4, and then quenched with anhydrous hydra-
zine-d4, dideuterochloride. After chromatography, diimine-hydra-
zine 8 was isolated as the only oxidized product (Scheme 5).14
The formation of compounds 7 and 8 involves the formal oxida-
tive insertion of a nitrogen into a C–H aromatic bond. This process
can entail aromatic hydrogen abstraction followed by trapping the
e R = Cl; W1 = W2 = W3 = Y = Z = H; X = D
f R = Cl; W1 = W2 = W3 = D; Y = Z = X = H
g R = Cl; W1 = W2 = W3 = X = H; Y = Z = D
h R = X = Z = W1 = W2 = W3 = H; Y = Cl
Scheme 3. Copper(I) and copper(III) complexes 4 and 5.
of 2, Ar2 = 4-MeO–C6H4, to an acetonitrile solution of complex 4h,
Ar1 = 2,6-dichlorophenyl, caused a color change from yellow to
blue-green. An electrospray mass spectrum of this blue–green
solution displayed an isotopic cluster corresponding to the molec-
ular mass of the intact cationic portion of complexes 5h (Fig. 1).8
The simultaneous presence of 4 chlorines and 1 copper atom gen-
erated an isotopic pattern for 5h (m/z = 620, 622, 624, 626, 628,
and 630), whose intensity ratios were in accord with predictions.
The complex emerging from the interaction of 4h with 2, Ar2 = 4-
MeO–C6H4, was also soluble and stable in different solvents
(MeOH, DCM).9
In order to determine the factors that influence the reactivity
and selectivity of the catalysts, specifically the ortho substitution
of the aromatic Ar1, known complex 4a and its deuterated non-cyc-
lic equivalent 4g was synthesized. Jacobsen showed that com-
plexes 4 lacking ortho substituents on Ar1 (e.g., 4a) gave low
enantiomeric excess and low TON for both aziridination and cyclo-
propanation reactions. Addition of 1 equiv of imidoiodinane 2,
Ar2 = 4-MeO–C6H4, to a solution of 4a in acetonitrile resulted in
an immediate color change from yellow to green and formation
of a catalytically active species, presumed to possess structure
5a. Addition of excess styrene (1.9 equiv) to this solution caused
formation of the expected aziridine in 1 h (yield = 82%).
N
N
1) 2
4a
O
Cl
N
Cl
S
2) N2D4, 2 DCl
8%
H
O
8
MeO
2 : Ar2= 4-MeO-C6H4
Scheme 5. Decomposition product 8 from reaction of 4a with 2 and quenching
with N2D4, 2DCl.
Intens.
x104
All, 0.1-0.7min (#3-#18)
622.3
Cl
621.9
2.0
O
S
O
N
N
Cl
623.9
OMe
Cu
N
1.5
1.0
0.5
0.0
Cl
619.9
625.9
624.9
622.9
Cl
620.9
626.9
5
620
625
630
200
300
400
500
600
700
m/z
Figure 1. Positive-ion electrospray mass spectrum of the cationic portion of complex 5h.

J.-C. Andrez / Tetrahedron Letters 50 (2009) 4225–4228
4227
resulting electrophilic radical with a nucleophilic nitrogen.15 Alter-
natively, an aryl-copper complex intermediate can form and trig-
ger a copper-mediated amination.16 The formation of product 6
is more difficult to explain and might involve a Nazarov-type cycli-
zation with a highly specific geometry round the metal center to
promote the coupling. In another experiment, complex 4a was dis-
solved in acetonitrile-d3 and reacted for 1 h with stoichiometric
amount of imidoiodinane 2, Ar2 = 4-OMe–C6H4, followed by addi-
tion of different electrophiles (methyl iodide, allyl bromide, or
methyl acrylate, 10 equiv). No product corresponding to the addi-
tion of iodide, bromide, methyl, allyl, or methyl acrylate or to the
formation of deuterated 4-chlorobenzaldehyde was detected by
MS-ESI+ and 1H NMR of the crude material.17 However, the reac-
tion still provided compounds 6 and 7 (8% and 14% yields, respec-
tively) after hydrolysis with HCl. Therefore, a free radical process is
unlikely to occur. The aromatic oxidative couplings found in com-
pounds 6, 7, and 8 are most likely the result of the formation of an
aryl–copper complex as an intermediate. The following experi-
ments provide more evidences for this hypothesis.
Results obtained from mass spectroscopy analyses confirmed
that the aromatic portion Ar1 of the ligand was oxidized during
the interaction of complexes 4a, 4b, and 4d–g with imidoiodinane
2.18 For the first part of this study, non-chiral ligands 4d–g were
used for commodity of deuterium labeling.19 A mass spectrum of
a solution of 4d (electrospray ionization, ESI+) exhibited an isotopic
cluster arising from its intact cationic portion. Specifically, M+ sig-
nals appeared at m/z = 367, 369, 371, and 373 in a ratio of
36.1:45.3:16.3:2.3, in complete accord with predictions. No signals
arising from doubly charged ions were observed, indicating that
passage of the analyte through the ionizing sector of the mass
spectrometer does not promote oxidation of Cu(I) to Cu(II). The
ESI spectrum of an acetonitrile solution containing equimolar
amounts of 4d and the imidoiodinane 2, Ar2 = 4-MeO–C6H4, was
quite simple, displaying an isotopic cluster at m/z = 551, 553,
555, and 557. The intensity ratios of these four signals were in ac-
cord with the presence of 1 Cu and 2 Cl atoms. These masses cor-
respond to the cationic portion of 5d (nominal masses 552, 554,
556, and 558) minus one hydrogen (Fig. 2). Signals were also ob-
served at m/z = 429 and 431 corresponding to a fragment of the ori-
ginal complex 5d in which one of the imine group was cleaved. No
signals belonging to the cationic portion of 4d or to the intact cat-
ionic portion of 5d were apparent. Additionally, MS/MS experi-
ment of the peaks centered at m/z = 551 provided a cluster of
peaks at m/z = 382. This fragment was assigned as [Cu(L)N]+ based
on the excellent matching of the observed and simulated isotope
distributions. This fragment corresponds to the cleavage of the
N–Ts bond and loss of a sulfinic radical (ꢀ171). The facile forma-
tion of a nitrido–copper complex [Cu(L)N]+ in the gas phase for
5d is very interesting. This suggests that this nitrido-copper com-
plex possesses a great thermal stability and the cleavage of the
N–Ts bound in 5d is much easier than that in 5h.
In order to determine which hydrogen was lost from the pre-
sumed 5d, we examined the behavior of deuterated complexes
4d–g. The ESI+-MS of a solution containing equimolar amounts of
4e and 2, Ar2 = 4-MeO–C6H4, or of 4f and the same imidoiodinane,
again exhibited an isotopic cluster at [Mꢀ1]+ relative to 5e and 5f.
Thus, the species arising from 4e produced signals at m/z = 555,
557, 559, and 561, and that obtained from 4f at m/z = 553, 555,
557, and 559. A high-resolution MS measurement of the ion of
63-
m/z = 553 confirmed its composition to be C23H18D2N3O3S35Cl2
Cu. However, reaction of complex 4g with 2, Ar2 = 4-MeO–C6H4,
yielded a species that displayed ESI+-MS signals two mass units
lower than expected for 5g, signaling loss of deuterium. These
[MꢀD]+ signals appeared at m/z = 558, 560, 562, and 564. One must
thus conclude that the formation of the [MꢀH]+ species in all such
experiments is due to loss of an aromatic H.
Because the lost of the H atom is part of the strongest C–H bond
present in the molecule, it is improbable that such a loss is due to
direct fragmentation. In fact, varying the ionization potential and
the inlet temperature of the ESI mass spectrometer had virtually
no effect on relative signal intensities. Moreover, removal of one
electron from any kind of Cu(III) complex, is likely to be more dif-
ficult than removal of an electron from Cu(I) complexes 4. But as
detailed earlier, no such evidence of oxidative events appear in
the mass spectra of 4, suggesting that the oxidation does not take
place during passage through the mass spectrometer; but that the
redox event occurs beforehand. These complexes (5d–g–Hꢀ) were
also soluble and stable in different solvents (DCM, MeOH, and
acetonitrile). The formal aryl radical emerging from the removal
of a hydrogen is somehow highly stabilized. It is known that
a
rC–H–Cu interaction can significantly reduce the pKa of an aryl
C–H group and therefore could explain the lost of the hydrogen
atom.20 A base-assisted C–Harom bond cleavage mechanism that re-
tains the formal oxidation state of the metal atom is likely for this
Figure 2. Positive-ion electrospray mass spectrum of the cationic portion of complex 5d–Hꢁ and in the inset MS/MS spectrum of the peak at m/z = 551.

4228
J.-C. Andrez / Tetrahedron Letters 50 (2009) 4225–4228
6. (a) Zhang, W.; Lee, N. H.; Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116, 425–426;
PF
6
(b) Daz-Requejo, M. M.; Prez, P. J.; Brookhart, M.; Templeton, J. L.
Organometallics 1997, 16, 4399–4402; (c) Comba, P.; Lang, C.; Lopez de
Laorden, C.; Muruganantham, A.; Rajaraman, G.; Wadepohl, H.; Zajaczkowski,
M. Chem. Eur. J. 2008, 14, 5313–5328.
N (III)
Cu
N
Cl
1/2 PhINSO Ar
2
2
5a
N
SO -Ar
2
2
7. (a) Furuta, H.; Maeda, H.; Osuka, A. J. Am. Chem. Soc. 2000, 122, 803–807; (b)
Naumann, D.; Roy, T.; Tebbe, K.-F.; Crump, W. Angew. Chem., Int. Ed. 1993, 32,
1482–1483; (c) Ribas, X.; Jackson, D. A.; Donnadieu, B.; MahÌa, J.; Parella, T.;
Xifra, R.; Hedman, B.; Hodgson, K. O.; Llobet, A.; Stack, T. D. P. Angew. Chem., Int.
Ed. 2002, 41, 2991–2994; (d) Hanss, J.; Beckmann, A.; Kruger, H. J. Eur. J. Inorg.
Chem. 1999, 163–172.
1/2 PhI +
9
Cl
1/2 NH SO Ar
2
2
2
Ar2= 4-MeO-C6H4
Scheme 6. Possible C–H activation product.
8. General procedure for mass spectrometry analysis of complex 5:
tetrakis(acetonitrile)copper hexafluorophosphate (93 mg, 0.25 mmol) was
added to a solution of the Schiff base (0.25 mmol) in 2 mL of acetonitrile or
DCM. After the solution was stirred for 15 min, a homogeneous yellow solution
formed. Imidoiodinane 2 (0.25 mmol) was then added at room temperature.
The color of the solution changed from yellow to green. The reaction mixture
process: (Mn+ + R ꢀ Harom ? Mn+ ꢀ R + H+). No C–H activation of
Cu(III) complexes has been described in the literature whereas
few reports detail the reaction for Cu(II) complexes. Thus, this
would be the first example of this type of C–H activation on a
Cu(III) complex. The oxidation of metal amine complexes readily
gives metal aminyls or metal amides depending on the metal
and ligands.21 Further oxidation of complex 5a–Hꢀ with excess imi-
doiodinane 2 or with complex 5a, or by dismutation is therefore
expected to give aminyl radical complex 9 whose cationic portion
is observed in mass spectrometry (Scheme 6).
was stirred for 10 min and 50 lL of the solution was withdrawn and diluted
with 2 mL of methanol. Mass spectrometry analyses were obtained on this
sample.
9. Performing the run in acetonitrile, MeOH, or DCM did not change the result of
the MS experiment.
10. Longer reaction times (12 h) do not change the yield of 6 and 7.
11. Crystallographic data (excluding structure factors) for the structures in this
Letter have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no CCDC 705130.
12. 6,60-Dichloro-biphenyl-3,30-dicarbaldehyde (6) and N-(5-chloro-2-formyl-phenyl)-
2-nitro-benzenesulfonamide (7): To complex 4a (142 mg, 0.25 mmol) in
acetonitrile (2 mL) was added iminoiodine 2 (Ar2 = 2-NO2–C6H4), (101 mg,
0.25 mmol) at 0 °C whereupon the color of the solution changed from yellow to
green. The reaction was stirred for 1 h at room temperature then cooled down
again to 0 °C and quenched with concentrated aqueous HCl or DCl (2 mL). The
reaction was stirred for 20 minutes at room temperature followed by addition of
EtOAc (15 mL)and H2O(10 mL). Thelayers were separated and theaqueous layer
was extracted with EtOAc (15 mL). The combined organic extracts were dried
over anhydrous MgSO4, filtered, and concentrated under reduced pressure to
give 165 mg of a white solid containing mainly p-chlorobenzaldehyde, o-
nitrobenzensulfonamide, and iodobenzene. Careful TLC chromatography using
EtOAc/hexanes (1/9) allowed the separation and the isolation of the biphenyl
compound 6 as transparent crystals (7.6 mg, 11% yield). 1H NMR (300 MHz,
CDCl3): d = 10.03 (2H, s), 7.91 (2H, dd, J = 8.1, 1.9), 7.81 (2H, d, J = 1.9), 7.69 (2H, d,
J = 8.1) ppm. 13C NMR (75 MHz, CDCl3): d = 190.4, 140.1, 138.1, 134.9, 132.1,
130.6, 130.5 ppm. IR (KBr): 1687, 1199 cmꢀ1. Mp: 43–45 °C. MS (EI): m/z = 278
(M+ꢁ), 249 (–CO). From the crude material obtained in the previous experiment,
compound 7 was isolated as an oil (16.5 mg, 19% yield) by TLC chromatography
using a gradient of solvent starting with EtOAc/hexanes (2/8) and finishing with
EtOAc/hexanes (4/6). 1H NMR (300 MHz, CDCl3): d = 11.50 (1H, br), 8.23 (1H, dd,
J = 9.1, 3.6), 7.87 (2H, m), 7.76 (2H, m), 7.59 (1H, d, J = 8.2), 7.19 (1H, dd, J = 8.2,
1.8) ppm. MS (ESI+): m/z = 363.0 (M+Na+). HRMS (ESI+): calcd for C13H9ClN2O5SN
362.9818; found 362.9816.
In conclusion, we have demonstrated by mass spectrometry
that the aryl moiety Ar1 of complexes 4 was oxidized with an imi-
doiodinane if no ortho substituents are present. The reactivity of
such a species differs greatly from the non-oxidized one and opens
the door to decomposition pathways (formation of compounds 6
and 7) that significantly shorten the lifetime of the complex. In-
deed, Jacobsen observed that Cu(I) complexes of the type 1, where-
in the aryl segments Ar1 carried only 1 ortho-substituent, afforded
both low TONs (63.6) and moderate ees.22 In contrast, changing
Ar1 to 2,6-dichlorophenyl 4c resulted in a particularly active aziri-
dination catalyst, in terms of both TON (ꢂ16) and enantioselectiv-
ity (ee >98%). A similar trend was also observed by P. Scott for the
enantioselective aziridination using copper complexes of biaryl
Schiff bases.23 Details of the mechanism of formation of compound
6 and synthetic applications of this C–H activation are currently
being investigated in our laboratory and will be reported in due
course.
13. Hisashi, M. U.S. Patent 5741347, 1998.
14. Compound 6 (or the corresponding bis-hydrazone) may be present at the end
of the reaction but was not isolated.
Acknowledgments
I am grateful to M. A. Ciufolini who provided all the necessary
equipment and for helpful discussions. I also thank the University
of British Columbia, the Canada Research Chair program, NSERC,
Merck Frosst, the CNRS, the MRT (fellowship to J.C.A.), and the
Région Rhône-Alpes for support in this program. I am grateful to
D. Bouchu for assistance with the MS measurements.
15. Au, S.-M.; Huang, J.-S.; Yu, W.-Y.; Fung, W.-H.; Che, C.-M. J. Am. Chem. Soc. 1999,
121, 9120–9132.
16. (a) Klapars, A.; Antilla, J. C.; Huang, X.; Buchwald, S. L. J. Am. Chem. Soc. 2001,
123, 7727–7729; (b) Shafir, A.; Buchwald, S. L. J. Am. Chem. Soc. 2006, 128,
8742–8743; (c) Huffman, L. M.; Stahl, S. S. J. Am. Chem. Soc. 2008, 130, 9196–
9197.
17. Russell, G. A.. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 1, pp
275–331.
18. For a review on ESI of metalo complex see: Traeger, J. C. Int. J. Mass Spectrom.
2000, 200, 387–401.
Supplementary data
19. Chelation in the cyclohexyl-diamine system (compounds 4a–c) can impose
certain geometric restrictions on the possible modes of metal-substrate
interaction but a control ESI+-MS experiment showed that the original chiral
ligand 4a displayed the same pattern as ligand 4d upon addition of the
imidoiodinane 2: (a) Lavin, M.; Holt, E. M.; Crabtree, R. H. Organometallics
1989, 8, 99–104; (b) Toner, A. J.; Grundemann, S.; Clot, E.; Limbach, H. H.;
Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2000, 122,
6777–6778.
20. (a) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–513; (b) Lersch, M.; Tilset,
M. Chem. Rev. 2005, 105, 2471–2526; For examples of electrophilic activation,
see: (c) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37,
2180–2192; (d) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y.
Science 2000, 287, 1992–1995.
Supplementary data (detailed experimental procedures and
analytical data) associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2009.04.100.
References and notes
1. Yudin, A. K. Aziridines and Epoxydes in Organic Synthesis; Wiley-VCH: Weinheim,
Germany, 2006.
2. Müller, P.; Fruit, C. Chem. Rev. 2003, 103, 2905–2919.
3. (a) Li, Z.; Conser, K. R.; Jacobsen, E. J. J. Am. Chem. Soc. 1993, 115, 5326–5327; (b)
Li, Z.; Quan, R. W.; Jacobsen, E. J. J. Am. Chem. Soc. 1995, 117, 5889–5890; Other
Cu-mediated aziridination methods: (c) Evans, D. A.; Faul, M. M.; Bilodeau, M.
T. J. Am. Chem. Soc. 1994, 116, 2742–2753. and references cited therein.
4. (a) Brandt, P.; Södergreen, M. J.; Andersson, P. G.; Norrby, P.-O. J. Am. Chem. Soc.
2000, 122, 8013–8020; (b) Gillespie, K. M.; Crust, E. J.; Deeth, R. J.; Scott, P.
Chem. Commun. 2001, 785–786.
21. Hicks, R. G. Angew. Chem., Int. Ed. 2008, 47, 7393–7395.
22. Jacobsen also reports that complex 1 where Ar1 = phenyl displayed moderate
stereoselectivity (ees ꢂ50%) but good TON (ꢂ10), signifying that
cyclometallated species may still function as aziridination agents. This would
be in accord with the observation that the presumed 10 does aziridinate
styrene.
23. Gillespie, K. M.; Sanders, C. J.; O’Shaughnessy, P.; Westmoreland, I.; Thickitt, C.
P.; Scott, P. J. Org. Chem. 2002, 67, 3450–3458.
5. One of the oxygen atoms of the sulfonyl group may ligate the metal center: cf.
3
Ref. for a discussion.