Nitrene-transfer to olefins catalyzed by methyltrioxorhenium: a universal
catalyst for the [1+2] cycloaddition of C-, N-, and O-atom fragments to olefins
Hee-Joo Jeon and SonBinh T. Nguyen*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
Received (in Corvalis, OR) 28th September 2000, Accepted 5th December 2000
First published as an Advance Article on the web 16th January 2001
Methyltrioxorhenium (MTO) was found to catalyze the
transfer of the nitrene unit of [N-(p-tolylsulfonyl)imino]-
iodobenzene to a number of olefins providing aziridines in
moderate to good yields, establishing MTO as a universal
catalyst for the [1 + 2] cycloaddition of carbene, nitrene, and
oxo units to olefins.
aziridination of styrene. In general, higher substrate concentra-
tions gave better yields in shorter reaction times. A reaction of
40 equiv. of styrene relative to the nitrene source resulted in the
highest yield of aziridine (45%, Table 1, entry 5). In refluxing
MeCN (ca. 82 °C, Table 1, entries 6 and 7), the reaction was
essentially instantaneous. There was no noticeable change in
yield compared to those at room temperature.
The metal-catalyzed [1 + 2] cycloaddition of small functional
groups to olefins to yield 3-membered rings comprises a very
important class of reactions in organic syntheses.1,2 Most
important among this class of reactions is the addition of
isoelectronic second row (ISR) fragments, e.g. carbene, nitrene,
and oxo species, to olefins to produce cyclopropanes, aziridines,
and epoxides, respectively. From a valence-shell electron
consideration, it is quite reasonable to propose that these ISR
moieties may behave similarly toward olefins in the presence of
the same metal catalyst. In particular, we were intrigued by the
possibility that there may exist a “universal” catalyst which can
transfer all three of these ISR species to olefins. The
development of a common catalyst for multiple organic
reactions is a powerful concept in synthesis. Its significance lies
in the possibility that the ligand framework developed for one
reaction may be applied to other reactions without further
modification of the catalyst system. The advantages are most
apparent in the case of enantioselective reactions catalyzed by
soluble metal complexes, where the most labor-intensive
research is the development of the required chiral ligands.
Perhaps the most prominent example of this strategy is the work
by Sharpless and coworkers on the osmium-catalyzed asym-
metric olefin aminohydroxylation,3–6 where a chiral alkaloid
ligand used in the asymmetric olefin dihydroxylation reaction is
applied to the synthesis of chiral b-aminoalcohols. In the area of
[1 + 2] cycloadditions, the developers of Cu2,7–9 and Rh2,10
catalysts have utilized this strategy to some degree in that both
aziridination and cyclopropanation can be carried out by the
same metal catalyst.
Nitrile solvents (both MeCN and benzonitrile) were the most
suitable for this system. In most other solvents, such as Et2O,
CH2Cl2 PhMe, THF, and pyridine, aziridine did not form, and
all of the nitrene precursor was converted to TsNH2. In the
absence of nitrile solvents (neat conditions), the yield of
aziridine was 27% based on PhINTs.
In all cases, TsNH2 was the major side product of the
aziridination reaction. To determine the hydrogen atom sources
for TsNH2 formation, a series of labeling experiments was
conducted. A 10+1 mixture of styrene and PhINTs with MTO
(10 mol% relative to PhINTs) in deuterated solvent (CD3CN,
rigorously dried and distilled) was allowed to react for 4 h, and
the product mixture was analyzed by GC–MS. If the solvent was
the major hydrogen source, TsND2 or TsNDH would have been
expected to form. However, the product from hydrogen
abstraction was mostly TsNH2 with a small amount of TsNDH.
When styrene-d8 in MeCN was used, the byproduct consisted
mostly of TsND2 (m/z 173) and TsNDH (m/z 172). Although
both MeCN and styrene can supply hydrogen atoms for the
TsNH2 formation, the labeling experiments indicate that the
substrate is the major hydrogen source leading to the formation
of the TsNH2 side product.
To investigate the scope of the MTO catalyzed aziridination
reaction, several substituted olefins were examined using the
conditions optimized for the aziridination of styrene (Table 2).
In general, electron-withdrawing substituents at the para
position of styrenes slowed down the overall reaction although
yields remained essentially the same (Table 2, entries 1–4). For
conjugated aromatic olefins, substitution patterns affected both
We report herein the aziridination of olefins catalyzed by
methyltrioxorhenium11 (MTO). Combined with the known
olefin epoxidation12–14 and cyclopropanation15 catalyzed by the
same metal complex, our work establishes the unique activity of
MTO as a ‘universal’ catalyst for the [1 + 2] cycloaddition of
carbene, nitrene, and oxo units to olefins. To our knowledge,
this is the first instance where a single catalyst can be used for
three different [1 + 2] cycloaddition reactions.
Table 1 Substrate concentration effect in the aziridination of styrene
catalyzed by MTO
In the olefin aziridination experiments, we utilized [N-(p-
tolylsulfonyl)imino]iodobenzene (PhINTs) as the nitrene
source. General experimental conditions involved the mixing of
PhINTs (1 equiv.), olefin (5 equiv.), and MTO (10 mol%
relative to PhINTs) in MeCN at the appropriate temperature
(Table 1). Although the polymeric PhINTs was initially
insoluble, the mixture became homogeneous as the reaction
proceeded. The complete consumption of the nitrene source
signaled the end of the reaction. In the absence of MTO no
reaction occurred, as evidenced both by the lack of dissolution
of PhINTs as well as by GC analysis of the reaction solution
which shows trace TsNH2 as the only nitrogen-containing
product.
As depicted in Table 1, reaction conditions were varied to
elucidate their effect on the catalyst activity with respect to the
DOI: 10.1039/b007906p
Chem. Commun., 2001, 235–236
This journal is © The Royal Society of Chemistry 2001
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