Iron-catalyzed intramolecular allylic C-H amination

Article abstract of DOI:10.1021/ja211600g

A highly selective C-H amination reaction under iron catalysis has been developed. This novel system, which employs an inexpensive, nontoxic [Fe ^IIIPc] catalyst (typically used as an industrial ink additive), displays a strong preference for allylic C-H amination over aziridination and all other C-H bond types (i.e., allylic > benzylic > ethereal > 3° > 2° ? 1°). Moreover, in polyolefinic substrates, the site selectivity can be controlled by the electronic and steric character of the allylic C-H bond. Although this reaction is shown to proceed via a stepwise mechanism, the stereoretentive nature of C-H amination for 3° aliphatic C-H bonds suggests a very rapid radical rebound step.

Full text of DOI:10.1021/ja211600g

Communication
pubs.acs.org/JACS
Iron-Catalyzed Intramolecular Allylic C−H Amination
Shauna M. Paradine and M. Christina White*
Roger Adams Laboratory, Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States
S
* Supporting Information
Scheme 1. Reactivity Trends for Fe-Catalyzed
Intramolecular C−H Amination
ABSTRACT: A highly selective C−H amination reaction
under iron catalysis has been developed. This novel
system, which employs an inexpensive, nontoxic [FeIIIPc]
catalyst (typically used as an industrial ink additive),
displays a strong preference for allylic C−H amination
over aziridination and all other C−H bond types (i.e.,
allylic > benzylic > ethereal > 3° > 2° ≫ 1°). Moreover, in
polyolefinic substrates, the site selectivity can be controlled
by the electronic and steric character of the allylic C−H
bond. Although this reaction is shown to proceed via a
stepwise mechanism, the stereoretentive nature of C−H
amination for 3° aliphatic C−H bonds suggests a very
rapid radical rebound step.
ethods that enable the selective oxidation of C−H bonds
M
are a powerful means of rapidly introducing function-
ality into complex molecules, obviating the need to carry
sensitive groups through multiple manipulations and stream-
lining synthetic efforts.1 C−H amination in particular is of
interest because of the prevalence of nitrogen functionalities in
biologically active molecules and the relative difficulty of
incorporating nitrogen into molecular frameworks. The field of
metal nitrenoid-based C−H amination was pioneered by
Breslow, who demonstrated that Fe(TPP)Cl (TPP =
tetraphenylporphyrinato) could aminate aliphatic and benzylic
C−H bonds.2 Despite the abundance and nontoxicity3 of iron,
the emerging use of selective iron catalysis,4,5 and the possibility
of orthogonal reactivity, very few C−H amination methods
employing iron catalysts have been published since these
seminal reports.6 In this paper, we describe the first general and
highly selective C−H amination reaction under iron catalysis.
Metal nitrenoid-based C−H amination methods have been
most extensively studied with rhodium catalysts, which are
thought to react via a concerted asynchronous mechanism.7
The observed intramolecular reactivity trends indicate that C−
H amination occurs preferentially at electron-rich C−H bonds
(3° > ethereal ≈ benzylic > 2° ≫ 1°) and that alkene
aziridination competes favorably with allylic C−H amination.8
Conversely, nitrenoid-based catalysis with first-row metals such
as copper, cobalt, manganese, and iron is thought to proceed
via single-electron pathways.9 We hypothesized that the
reactivity trends with first-row metals should follow the trend
in homolytic bond dissociation energies (BDEs), as in the case
of ruthenium-based catalysis,10 making orthogonal C−H
amination reactivity possible. Under this type of reaction
manifold, for example, allylic C−H amination should be
strongly preferred (Scheme 1B).11 The development of a
general, highly selective allylic C−H amination reaction for
internal olefins would be particularly significant.12 Additionally,
if the rebound rates of the iron amido species are tuned with
the appropriate ligand environment, the resultant short-lived
carbon radicals would allow for highly selective aminations.13
We report herein the realization of these goals with an
[FeIIIPc]-catalyzed allylic C−H amination reaction, which
demonstrates the highest chemo- and site-selectivities reported
to date for nitrenoid-based amination. This reaction employs
[FePc]Cl (Pc = phthalocyaninato), an inexpensive commercial
compound that is typically used as an industrial additive for ink
and rubber manufacturing. [FeIIIPc] catalysis strongly favors
allylic C−H amination over aziridination and amination of all
other C−H bond types (Scheme 1A); moreover, high levels of
site selectivity for polyolefins are demonstrated on the basis of
electronics and sterics. Reactivity trends and mechanistic
studies support a stepwise process for functionalization that
occurs via initial homolytic C−H bond abstraction followed by
a rapid radical rebound.
In initial reaction development, we tested several iron
catalysts known to support high-valent metal oxidants for their
efficacy in catalyzing the desired transformation (Table 1). The
non-heme Fe(PDP) catalyst previously developed by our lab
for aliphatic C−H hydroxylations4e−h was able to effect C−H
amination with no observed aziridination, but the yields were
poor (entry 1). The reactivity improved with the use of the
heme iron catalyst Fe(TPP)Cl (entry 2). Interestingly, the iron
complex of salen, known to be an effective heme ligand mimic
for epoxidations,14 showed no reactivity (entry 3). The iron
Received: December 12, 2011
Published: January 18, 2012
© 2012 American Chemical Society
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Communication
Table 1. Development of the Fe-Catalyzed Intramolecular
Allylic C−H Amination Reaction
Table 2. Scope of Fe-Catalyzed C−H Amination
a
Isolated yields (sum of syn + anti; d.r. ∼3:1 syn:anti); % recovered
b
starting material (rsm) values are given parentheses. Determined by
c
1H NMR analysis of the crude reaction mixture. Concentration = 0.1
d
e
f
M. d.r. =1:1. PhI(OPiv)2 was used as the oxidant. [FePc]Cl and
AgSbF6 were added together in three 3.3 mol % portions at 2 h
intervals.
a
b
Isolated yields (syn + anti); % rsm in parentheses. All product ratios
1
were determined by H NMR analysis of the crude reaction mixture.
c
Conditions: 4 × (0.03 equiv of [FePc]Cl, 0.03 equiv of AgSbF6), 2
d
phthalocyanine complex [FePc]Cl was superior to Fe(TPP)Cl
in terms of reactivity (entry 4). This is likely due to the
increased electron-withdrawing nature of phthalocyanine
ligands relative to their porphyrin counterparts, which results
in a more electrophilic metal center.15 Addition of a
noncoordinating silver salt (entry 5), the use of a mixed
solvent system (entry 6), and switching to the more soluble
PhI(OPiv)2 oxidant (entry 7) all led to a significant
improvement in the overall reactivity. Notably, the AgSbF6
additive could be eliminated from the optimized conditions
with only a slight decrease in reactivity (entry 8). A silver-free
catalyst system could be particularly beneficial in large-scale
applications where cost and procedural simplicity are key
considerations. Iterative addition of a solid mixture of [FePc]Cl
and AgSbF6 (three or four 3.3 mol % portions) at 2 h intervals,
although not always necessary (entry 9), was shown to improve
the yields with poorly converting substrates, whereas simply
increasing the catalyst loading was not beneficial (see below).
Under the optimized conditions, good yields of allylic C−H
amination product 1 were obtained with only trace aziridine
(>20:1 ins./azir.).
This Fe-based system displays the highest chemoselectivities
for intramolecular allylic C−H amination over aziridination
reported to date. The selectivities are strong for allylic
amination products with aliphatic (E)-olefins and styrenyl,
trisubstituted, terminal, and cyclic olefins (Table 2) as well as
α,β-unsaturated esters and allylic acetates (see Table 3, entries
9 and 10). In particular, the tolerance of terminal olefins is
quite notable. When a terminal olefin is present in the
bishomoallylic position, high selectivities are achieved for allylic
C−H amination product 4 over the aziridination product (12:1
ins./azir.). This is in stark contrast to both rhodium- and
ruthenium-catalyzed systems, where aziridination is generally
competitive and in some cases preferred for terminal olefins,
including those positioned remotely.11c,16 Cyclic olefins are also
viable substrates for C−H amination, even when the sulfamate
ester is in the homoallylic position. β-Cholesterol was readily
aminated via its sulfamate ester to afford a single diastereomer
equiv of PhI(OPiv)2, 4:1 PhMe/MeCN, rt, 8 h. Determined by GC
e
analysis of the crude mixture; starting d.r. = 97:3. Determined by GC
analysis of the crude mixture; starting d.r. = 5:95.
of the 1,2-difunctionalized product 5 in 58% yield. [FeIIIPc] is
also a competent catalyst for benzylic and 3° aliphatic C−H
aminations. It is significant to note that the aliphatic C−H
amination of a substrate containing a stereochemically defined
3° C−H center was stereospecific, affording (+)-(6R)-7 or
(−)-(6S)-7 with no loss of chiral information.
We next sought to determine the reactivity trends for
[FeIIIPc]-catalyzed amination of allylic C−H bonds relative to
other C−H bond types (Table 3). Accordingly, intramolecular
competition experiments were performed on sulfamate ester
compounds having an allylic C−H bond and a second,
energetically distinct type of C−H bond positioned at the β
and β′ carbons, respectively. Upon comparison of the [FeIIIPc]-
catalyzed method with the traditional Rh2(OAc)4-catalyzed
system, it is clear that the competing olefin aziridination
pathway is strongly suppressed in the iron system relative to
rhodium catalysis (entries 1−8). Additionally, the [FeIIIPc]-
catalyzed system is generally more selective in discriminating
between energetically distinct C−H bonds than the
Rh2(OAc)4-catalyzed system. For the iron-catalyzed reaction,
allylic C−H amination is exclusively preferred over both
aliphatic 2° and 3° C−H amination (β/β′ >20:1; entries 1 and
3). Although the same selectivity is observed for 2° C−H
bonds, rhodium nitrenes show essentially no preference for
allylic versus 3° C−H bond insertion (β/β′ = 1.3:1; entries 2
and 4). Ruthenium nitrenes have also been reported to have a
much lower preference (β/β′ = 5:1).11c Moreover, while iron
nitrenes demonstrate synthetically useful levels of selectivity for
allylic versus ethereal and benzylic C−H bond aminations (β/β′
= 7:1 and 5:1, respectively; entries 5 and 7), rhodium nitrenes
show only a slight preference for allylic versus ethereal and
benzylic C−H aminations (β/β′ = 4:1 and 2:1; entries 6 and 8).
In general, the following reactivity trend is observed for the
iron-catalyzed C−H amination: allylic > benzylic > ethereal >
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Communication
catalyst, the less sterically hindered allylic C−H bond of a
citral-derived substrate was functionalized with useful selectivity
(β/β′ = 7:1; entry 14), affording product 17 in good yield
(53%).
Table 3. Reactivity Trends for Allylic C−H Amination
To probe the mechanism of the [FeIIIPc]-catalyzed C−H
amination, we determined the effect of deuterium substitution
on the rate of benzylic amination. The measured kinetic isotope
effect (KIE) of 2.5 0.2 for the C−H amination of 18 under
iron-based catalysis is higher and statistically different from that
measured for the same substrate under rhodium-based catalysis
(kH/kD = 1.8
0.2) (eq 1).8a However, this value is much
lower than what has been previously measured for reactions
proceeding via a C−H abstraction/rebound amination
mechanism (kH/kD = 6−12).7c Given the modest KIE values
and the observed stereoretention in amination of tertiary C−H
centers, we performed an additional study on Z-olefins to
determine if scrambling of the double bond geometry occurred
during allylic C−H amination When sulfamate ester 20 (>20:1
Z/E) was subjected to [FeIIIPc]-catalyzed C−H amination, the
allylic-functionalized product 21 was obtained as a 9:1 Z/E
mixture, likely through the intermediacy of a stabilized carbon-
centered radical (eq 2). No isomerization was observed under
Rh2(OAc)4 catalysis, suggesting that different functionalization
mechanisms are operative in these two cases.
In conclusion, we have reported the first highly selective and
general C−H amination via iron catalysis. In addition to using
an economical and nontoxic metal source, [FeIIIPc]-catalyzed
intramolecular C−H amination exhibits the highest chemo- and
site selectivities reported to date for allylic C−H amination of
internal olefins. Allylic C−H amination is strongly preferred
over aziridination as well as over amination of stronger C−H
bonds (i.e., 3° and 2° aliphatic, ethereal, or benzylic).
Additionally, for polyolefin-containing substrates, allylic C−H
amination with this electrophilic, bulky oxidant occurs with
high selectivity at the most electron-rich, least sterically
hindered site. We anticipate that this reaction will find
widespread use in streamlining the synthesis of nitrogen-
containing complex molecules.
a
1
Determined by H NMR analysis of the crude reaction mixture (d.r.
b
≈ 3:1 unless otherwise noted). Isolated yields (syn + anti; E/Z > 20:1
in all cases); % rsm in parentheses. Conditions: 0.10 equiv of
c
[FePc]Cl, 0.10 equiv of PhI(OPiv)2, 4:1 PhMe/MeCN, rt, 6 h.
d
Conditions: 0.02 equiv of Rh2 (OAc)4, 1.1 equiv of PhI(OAc)2, 2.3
e
equiv of MgO, CH2Cl2, rt, 4 h. Conditions: 4 × (0.03 equiv of
[FePc]Cl, 0.03 equiv of AgSbF6), 2 equiv of PhI(OPiv)2, 4:1 PhMe/
MeCN, rt, 8 h.
3° > 2° ≫ 1°. Notably, this trend is in agreement with the C−
H bond dissociation energies (Scheme 1B).17
Similar to what has been shown with Fe(PDP)-catalyzed
aliphatic C−H oxidations, we hypothesized that allylic C−H
bonds in polyolefin compounds could be differentiated and
selectively functionalized by the bulky, electrophilic iron nitrene
oxidant on the basis of their electronic and steric characters. We
observed that electron-withdrawing groups proximal to the
allylic C−H bond, such as an α,β-unsaturated ester,
substantially decreased its reactivity toward C−H amination
(Table 3, entries 9 and 10). Consistent with this, amination of a
polyolefin-containing substrate occurred with high selectivity at
the more electron-rich allylic C−H bond (β/β′ = 14:1; entry
11) to afford monoaminated product 14 in a preparatively
useful yield (55%). Electron-withdrawing functionality remote
from the allylic C−H bond had no effect on the reactivity,
allowing acetoxy and ester moieties β to the sulfamate tether to
be well tolerated under the reaction conditions (entries 12 and
13). We next evaluated whether we could also differentiate
allylic C−H bonds in a polyolefinic substrate on the basis of
their steric environment. Gratifyingly, using the [FeIIIPc]
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures, compound characterization
data, and NMR spectra for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
white@scs.illinois.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.C.W. gratefully acknowledges Pfizer and Bristol-Myers
Squibb for generous gifts. S.M.P. is a recipient of the University
of Illinois Sylvia Stoesser and NSF Graduate Research
Fellowships. We thank Mr. Jeffrey Sayler for synthesizing the
alcohol starting material for Table 3, entry 11, and Dr. Tobias
Thaler for confirming the experimental procedure in entries 3,
4, 6, and 8 of Table 3.
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