Efficient Aziridination of Olefins Catalyzed by a Unique Disilver(I) Compound

Article abstract of DOI:10.1021/ja038668b

A unique disilver(I) compound is an efficient catalyst for aziridination of olefins. Copyright

Full text of DOI:10.1021/ja038668b

Published on Web 12/05/2003
Efficient Aziridination of Olefins Catalyzed by a Unique Disilver(I) Compound
Yong Cui and Chuan He*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received September 22, 2003; E-mail: chuanhe@uchicago.edu
Silver(I) and silver(II) complexes are frequently used as stoichi-
ometeric oxidants for oxidation of various organic and inorganic
substrates. However, these metal ions are not often used as real
catalysts in oxidation or group transfer reactions that do not involve
Figure 1. Silver-catalyzed olefin aziridination.
radical chemistry. Despite the fact that the silver particle is an
efficient catalyst for olefin epoxidations in industry,1,2 efficient
epoxidation or aziridination reaction of unsaturated hydrocarbons
mediated by silver ions in solution has yet to be realized. The
nonradical oxidation reactions catalyzed by silver ions are limited,
although a carbene insertion chemistry was reported recently.3
Identifying new reactivity associated with silver ions may lead to
practical synthetic processes and better understanding of silver
chemistry.
We describe here an efficient olefin aziridination reaction
catalyzed by a novel disilver(I) compound. Aziridines are synthetic
intermediates that can be converted into important nitrogen-
containing functional groups.4 Although several efficient syntheses
of aziridines from olefins have been reported,5-14 none of these is
mediated by silver ions. During our investigation of silver chemistry,
we discovered that efficient aziridination of olefins could be
catalyzed by mixing 1 equiv of a tridentate 4,4′,4′′-tri-tert-butyl-
2,2′:6′,2′′-terpyridine (tBu3tpy) with 1 equiv of silver(I) salt and a
nitrenoid source PhIdNTs in acetonitrile at ambient temperature
(Figure 1). No reaction occurred in the absence of the silver(I) salt.
The reaction did not seem to be affected by the counteranions
Figure 2. (A) Formation of compound [Ag2(tBu3tpy)2(NO3)](NO3) 1. (B)
The molecular structure and space-filling model of 1.
because the use of AgNO3, AgOTf, AgClO4, and AgBF4 salts all
gave the same result. Surprisingly, clean production of aziridine
was only observed when tBu3tpy was used as the ligand to silver-
(I). With pyridine or 4-tert-butyl-pyridine as the ligand, only a trace
amount of aziridine product was observed. An increasing amount
of aziridine (35-50%, GC yield) could be prepared with bipyridine,
4,4′-di-tert-butyl-bipyridine, or 2,2′:6′,2′′-terpyridine (tpy) as the
ligand, but these catalytic reactions were accompanied by a variety
of byproducts. When tBu-bis(oxazoline) or Ph-bis(oxazoline)-
pyridine was used as the ligand, no product was observed under
the same reaction conditions.
To address the requirement of the tridentate ligand tBu3tpy, we
structurally characterized the catalyst or catalyst precursor from
the reaction medium. Equal molar amounts of AgNO3 and tBu3tpy
were mixed in acetonitrile and left to stand at room temperature
for 3 days. Pale yellow single crystals were obtained from the
solution. The structure was solved and revealed a dinuclear silver-
(I) compound [Ag2(tBu3tpy)2(NO3)](NO3) 1 (Figure 2). The disilver-
(I) center is stabilized by two tBu3tpy ligands with Ag(1) bound to
four nitrogen atoms and Ag(2) bound to two nitrogen atoms from
the ligands. Both silver ions are five-coordinate if the silver-silver
interaction is counted. Each ligand bridges two silver(I) ions. The
Ag(1)-Ag(2) distance is 2.842(2) Å, which may indicate a fairly
strong silver(I)-(I)silver interaction.15 The space-filling model
(Figure 2) shows that the two terminal positions of 1 are accessi-
ble, with one terminal side occupied by a weakly bound nitrate
anion.
1
To probe the solution structure of 1, we recorded the H NMR
of 1 in CD3CN and compared it to the spectrum of a mixture of
equal molar amounts of AgNO3 and tBu3tpy in the same solvent
(Figure S1). The two spectra were identical if the solution generated
in situ was allowed to stand for 5 min. If the spectrum was taken
immediately after mixing the ligand and silver salt, broader peaks
were observed, which may indicate that the system was still
equilibrating (Figure S1). Crystals of compound 1 were dissolved
in acetonitrile and exhibited the same activity in catalyzing the olefin
aziridination reaction as the mixed compound. The disilver
compound could also be isolated by flash chromatography after
completion of the catalytic reaction. Single-crystal X-ray structural
analysis of the isolated complex revealed a similar disilver(I) core
structure (Figure S4; OTf was used as the anion in this case). All
these results suggest that the disilver(I) compound is the precatalyst
present in the reaction solution and the structure is retained after
the reaction.
We also found that the same disilver(I) compound can epoxidize
olefins and oxidatively activate inert C-H bonds.16 Silver com-
pounds with other ligands either showed no activity or gave a
mixture of oxidized products.
Although the geometry of the disilver(I) compound could be a
factor in mediating the reactivity of the silver ions, we suspect that
the electronic communication between two silver ions may play
an important role in controlling the reactivity of compound 1. We
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10.1021/ja038668b CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Disilver(I)-Catalyzed Aziridination of Olefins
This silver-catalyzed aziridination reaction proceeds in good to
excellent isolated yields with a range of olefin substrates (Table
1). Terminal alkene substrates can be converted into aziridines in
good yields.
It is intriguing that the disilver(I) compound 1 exhibits the well-
controlled oxidation reactivity. A diRh(II) compound, Rh2(OAc)4,
was previously shown to catalyze similar olefin aziridination
reactions.6,14,18,19 The diRh(II) compound is used widely to catalyze
other types of oxidation reactions.6,20 The nature of the reactive
intermediates in the diRh(II) system remains unclear in these
reactions. It is interesting that both Rh2(OAc)4 and the disilver
compound 1 have short metal-metal distances and accessible
coordination sites at the terminal positions. The two systems may
share similar mechanistic characteristics. We are in the process of
exploring other catalytic reactivity and are engaged in performing
mechanistic investigations of the disilver compound described here.
The study may help advance oxidation catalysis in general.
Acknowledgment. This research was supported by the Uni-
versity of Chicago and an ACS Petroleum Research Fund 38848-
G3 to C. He. We thank Dr. Ian M. Steele for helping with the
X-ray crystallography.
Supporting Information Available: Experimental details, ORTEP
diagram of [Ag2(tBu3tpy)2(NO3)](NO3) 1 and [Ag2(tBu3tpy)2(OTf)]-
(OTf) 2 (PDF), and X-ray crystallographic data of 1 and 2 (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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a
All reactions were conducted at 0 °C to r.t. in CH3CN with 2 mol %
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