Styrene Aziridination by Iron(IV) Nitrides

Article abstract of DOI:10.1002/anie.201503773

Thermolysis of the iron(IV) nitride complex [PhB(tBuIm)₃FeN] with styrene leads to formation of the high-spin iron(II) aziridino complex [PhB(tBuIm)₃Fe-N(CH₂CHPh)]. Similar aziridination occurs with both electron-rich and electron-poor styrenes, while bulky styrenes hinder the reaction. The aziridino complex [PhB(tBuIm)₃Fe-N(CH₂CHPh)] acts as a nitride synthon, reacting with electron-poor styrenes to generate their corresponding aziridino complexes, that is, aziridine cross-metathesis. Reaction of [PhB(tBuIm)₃Fe-N(CH₂CHPh)] with Me₃SiCl releases the N-functionalized aziridine Me₃SiN(CH₂CHPh) while simultaneously generating [PhB(tBuIm)₃FeCl]. This closes a synthetic cycle for styrene azirdination by a nitride complex. While the less hindered iron(IV) nitride complex [PhB(MesIm)₃FeN] reacts with styrenes below room temperature, only bulky styrenes lead to tractable aziridino products.

Full text of DOI:10.1002/anie.201503773

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International Edition: DOI: 10.1002/anie.201503773
German Edition: DOI: 10.1002/ange.201503773
Nitrogen Heterocycles
Styrene Aziridination by Iron(IV) Nitrides**
Salvador B. MuÇoz III, Wei-Tsung Lee, Diane A. Dickie, Jeremiah J. Scepaniak, Deepak Subedi,
Maren Pink, Michael D. Johnson, and Jeremy M. Smith*
[
4]
Abstract: Thermolysis of the iron(IV) nitride complex [PhB-
reported. These catalysts include many examples of iron-
based complexes that are catalysts for aziridine synthesis by
(
tBuIm) FeꢀN] with styrene leads to formation of the high-
3
[
5]
spin iron(II) aziridino complex [PhB(tBuIm) Fe-N-
sulfilimine transfer. More recently, the family of iron-based
catalysts has been expanded to include complexes that
facilitate alkyl- and arylnitrene transfer to styrenes and
3
(
CH CHPh)]. Similar aziridination occurs with both elec-
2
tron-rich and electron-poor styrenes, while bulky styrenes
[
6]
hinder the reaction. The aziridino complex [PhB(tBuIm) Fe-
aliphatic alkenes.
3
N(CH CHPh)] acts as a nitride synthon, reacting with
electron-poor styrenes to generate their corresponding azir-
idino complexes, that is, aziridine cross-metathesis. Reaction of
Transition metal nitrides might also be expected to show
similar reactivity, in which a two-electron nitrogen atom
transfer reaction from the nitride to the alkene leads to
formation of a coordinated aziridino ligand. A conceptually
attractive feature of such a reaction is that the newly formed
aziridino ligand provides a platform for accessing a wider
range of N-functionalized aziridines than is possible by
current nitrene transfer strategies. However, the majority of
nitride ligands are notoriously unreactive, and examples of
nitride ligands reacting with hydrocarbons are notable for
2
[
PhB(tBuIm) Fe-N(CH CHPh)] with Me SiCl releases the N-
3 2 3
functionalized aziridine Me SiN(CH CHPh) while simultane-
3
2
ously generating [PhB(tBuIm) FeCl]. This closes a synthetic
3
cycle for styrene azirdination by a nitride complex. While the
less hindered iron(IV) nitride complex [PhB(MesIm) FeꢀN]
3
reacts with styrenes below room temperature, only bulky
styrenes lead to tractable aziridino products.
[7]
their rarity. To the best of our knowledge, there is only one
example of alkene aziridination by a nitride complex, in which
T
he facile reactivity of aziridines, coupled with their ability
VI
to undergo highly regio- and stereoselective transformations,
makes them important and versatile synthons in the prepa-
ration of small molecules (e.g. pharmaceuticals) and materials
addition of pyridine to the Ru nitride complex [(sal-
+
chda)RuꢀN] (salchda = N,N’-bis(salicylidene)-o-cyclohexyl-
diamine dianion) activates the nitride ligand towards alkene
[1]
[8]
(
e.g. medical devices). The aziridine moiety is also found in
aziridination. A handful of other nitride complexes have
some natural products and synthetic compounds that have
interesting or useful biological properties, making them of
been reported to react with alkenes although aziridino ligand
formation does not occur, for example, cis-[(terpy)Os(N)Cl2]
+
[2]
interest as synthetic targets in their own right.
Given their importance, it is not surprising that numerous
inserts the nitrogen atom into the C=C bond of stilbene and
[9]
conjugated dienes, generating azaallenium products.
[
3]
schemes for the synthesis of aziridines have been devised.
A
In contrast to the low reactivity of most transition metal
particularly appealing strategy involves the catalytic transfer
of nitrenes to alkenes, which has been extensively investigated
and a wide range of metal-based catalysts have been
nitride complexes, the iron(IV) complexes [PhB(RIm) FeꢀN]
3
(R = tBu 1, Mes 2) which are supported by bulky tris-
[10,11]
(carbene)borate ligands (Figure 1),
are able to access
[
+]
[+]
[
*] Dr. S. B. MuÇoz III, Dr. W.-T. Lee, Dr. M. Pink, Prof. J. M. Smith
Department of Chemistry, Indiana University
8
00 E. Kirkwood Avenue, Bloomington, IN 47403 (USA)
E-mail: smith962@indiana.edu
Dr. D. A. Dickie
Department of Chemistry and Chemical Biology
The University of New Mexico
Albuquerque, NM 87131 (USA)
Figure 1. The iron(IV) nitrides [PhB(RIm) FeꢀN] (R=tBu 1, Mes 2).
3
Dr. J. J. Scepaniak, D. Subedi, Prof. M. D. Johnson
Department of Chemistry and Biochemistry
New Mexico State University
diverse reaction pathways. Thus, we have chronicled two-
electron nitrogen atom transfer reactions to substrates such as
Las Cruces, NM 88003 (USA)
[10,11a,12,13]
CꢀO, CꢀNR and PR ,
one-electron reactions with
3
+
[
] These authors contributed equally to this work.
[11b,14]
hydrogen atom donors and organic radicals,
additions with conjugated dienes.
and cyclo-
[
**] Funding from the DOE-BES (DE-FG02-08ER15996) and Indiana
University is gratefully acknowledged. J.M.S. is a Dreyfus Teacher-
Scholar. S.B.M. thanks NSF-AMP (HRD 0929343) and NIH-RISE
[14]
A detailed experimental and computational investigation
into the reaction of 2 with phosphines has provided evidence
(
R25 GM061222-11) for financial support. The Bruker X8 X-ray
[13]
for the ambiphilic nature of the nitride ligand.
An
diffractometer was purchased via an NSF CRIF:MU award to The
University of New Mexico, CHE-0443580.
important observation from this study is that the transition
state for nitrogen atom transfer involves a s-symmetry
interaction between the nitride LUMO and phosphine
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503773.
1
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HOMO as well as a p-symmetry interaction between the
nitride HOMO and the phosphine LUMO. Since this orbital
interaction is reminiscent of bonding between low-valent
metals and unsaturated hydrocarbons, we were stimulated to
investigate the reactivity of iron(IV) nitrides towards alkenes.
Here we report the reactions of 1 and 2 with a range of
styrenes, which leads to formation of the corresponding
iron(II) aziridino complexes. Evidence for the reversibility of
aziridino ligand formation as well as the ability to release an
N-functionalized aziridine from the metal is also demon-
strated.
styrene 2,4,6-(CH ) C H C(H)=CH . In addition, no reaction
3 2 6 3 2
between 1 and b-substituted styrenes, for example, C H -
6
5
C(H)=C(H)CH , is observed.
3
While the reactions of nitride complex 1 generally require
heat, complex 2 reacts with styrenes under ambient con-
ditions. Thus, 2 reacts at room temperature with 3,5-
(CF ) C H CH=CH and C H C(CH )=CH to afford the
3
2
6
3
2
10
7
3
2
corresponding high-spin iron(II) aziridino complexes 4 and 5
as the sole reaction products (Scheme 2). These complexes
Heating a solution of complex 1 with excess styrene leads
to formation of the high-spin (S = 2) iron(II) aziridino
complex [PhB(tBuIm) Fe-N(CH CHPh)] (3) in high yield
3
2
(
Scheme 1). The molecular structure of 3 has been deter-
Scheme 2. Reaction of 2 with bulky styrenes to afford the correspond-
ing aziridino complexes 4 and 5. Inset: X-ray crystal structures of 4
(left) and 5 (right). Thermal ellipsoids at 50%, hydrogen atoms and
most of the tris(carbene)borate ligand omitted for clarity.
Scheme 1. Reaction of 1 with a range of styrenes, leading to formation
of the corresponding aziridino complexes. Inset: X-ray crystal structure
of 3, where Ar=C H . Thermal ellipsoids at 50%, hydrogen atoms and
have also been structurally characterized, showing similar
metrical parameters to 3, except for significant pyramidaliza-
tion of the aziridino nitrogen atom (sum of angles around
N(7) = 336.2(9)8 and 348.2(4)8 for 4 and 5, respectively).
While 2 reacts with these bulky styrenes under very mild
conditions, even below room temperature, the reaction scope
of this complex is generally more limited than that of 1, with
less bulky styrene substrates leading to the formation of
intractable products.
6
5
most of the tris(carbene)borate ligand omitted for clarity. Fe(1)–N(5)
.935(4); Fe(1)–C(1) 2.092(4); Fe(1)–C(8) 2.109(6);
1
N(5)–C(20) 1.417(11); N(5)–C(21) 1.317(14); C(20)–C(21) 1.476;
C(1)-Fe(1)-C(8) 90.58(16); Fe(1)-N(5)-C(20) 142.0(5);
Fe(1)-N(5)-C(21) 152.5(6); C(20)-N(5)-C(21) 65.3(6).
mined by single-crystal X-ray diffraction (Scheme 1, inset). To
the best of our knowledge, this is the first structurally
characterized complex featuring an anionic aziridino, rather
than neutral aziridine, ligand. The aziridino C(20)ÀN(5) and
C(21)ÀN(5) bonds of 3 are shorter and the C(20)-N(5)-C(21)
These synthetic results therefore establish an unusual
transformation for a nitride ligand, in which two-electron
nitrogen atom transfer to styrenes provides the corresponding
aziridino complexes. As mentioned above, there are a handful
of transition metal nitride complexes that react with
[
9]
bond angle larger than is observed for neutral aziridine
ligands, where the corresponding metrics average 1.486(1) Š
alkenes, and in only one instance does a two-electron
nitrogen atom transfer reaction occur to generate the three-
[15]
[8]
and 59.5(1.0)8, respectively. As expected from the anionic
nature of the aziridino ligand, the Fe(1)ÀN(5) bond is shorter
membered aziridine ring.
Intriguingly, the aziridino complex 3 can be used as an
[16]
than observed in iron aziridine complexes but similar to
iron(IV) nitride synthon. Specifically, heating
3 with
[17]
high-spin iron(II) amido complexes.
As with the latter
(p-NO C H )CH=CH leads to formation of the aziridino
2
6
4
2
complexes, the nitrogen atom of the aziridino ligand in 3 is
planar, with the sum of angles around N(5) = 359.9(6)8. The
other metrical parameters are unexceptional for a high-spin
complex [PhB(tBuIm) Fe-N(CH CH(p-(NO )C H ))] and
3 2 2 6 4
styrene, that is, aziridine cross-metathesis (Scheme 3). Similar
aziridine cross-metathesis occurs when 3 is treated with
electron-poor styrenes such as (p-ClC H )CH=CH and
[11–13,18]
iron(II) tris(carbene)borate complex.
6
4
2
Complex 1 reacts similarly with other styrenes, although
the reaction is sensitive to the size of the styrene substrate.
While all para-substituted styrenes investigated provide the
(p-CF C H )CH=CH , although no reaction occurs with
more electron-rich styrenes. While detailed studies aimed at
elucidating the aziridination mechanism are ongoing, this
3 6 4 2
corresponding aziridino complexes, as do o-(CH )C H C(H)= cross metathesis reactivity strongly suggests the aziridination
3
6
4
CH and C H CH=CH , there is no reaction with the bulkier
reaction to be reversible.
2
10
7
2
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silylated aziridines are not accessible by
[19]
current nitrene transfer strategies. In
addition, since the iron(II) chloride com-
[18]
plex is readily converted to 1,
this
reaction establishes a synthetic cycle for
styrene aziridination using an iron(IV)
nitride (Scheme 5). Preliminary results
suggest that access to other N-function-
alized aziridines will be similarly facile,
as illustrated by the synthesis of the N-
Scheme 3. Aziridine cross-metathesis.
methylated aziridine MeN(CH CHPh)
2
by reaction of 3 with MeI.
In summary, the reactivity of a suitable iron(IV)
nitrides allows a cycle for the synthesis of N-function-
alized aziridines to be developed. Work to investigate
the mechanism of the aziridination reaction is under-
way, as are efforts to develop catalytic cycles for the
synthesis of range of N-functionalized aziridines using
iron(IV) nitride complexes.
Scheme 4. Synthesis of [PhB(tBuIm) Fe-N(CH CHCH )] (6).
3
2
3
In contrast to the reactions with styrenes, which lead to
aziridino ligand formation, neither nitride complex reacts
with aliphatic alkenes. However, it is possible to independ-
ently prepare an iron(II) aziridino complex that would result
Keywords: alkenes · iron · nitrides
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10600–10603
Angew. Chem. 2015, 127, 10746–10749
from such a reaction. Treating [PhB(tBuIm) FeCl] with
3
[
LiN(CH CHCH )] yields the corresponding aziridino com-
2 3
plex [PhB(tBuIm) Fe-N(CH CHCH )] (6) in high yield
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3
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[
15]
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Scheme 5. Synthetic cycle for styrene aziridination by an iron(IV)
nitride complex.
1
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[
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