Iron(II)-catalyzed intramolecular aminohydroxylation of olefins with functionalized hydroxylamines

Article abstract of DOI:10.1021/ja311923z

A diastereoselective aminohydroxylation of olefins with a functionalized hydroxylamine is catalyzed by new iron(II) complexes. This efficient intramolecular process readily affords synthetically useful amino alcohols with excellent selectivity (dr up to > 20:1). Asymmetric catalysis with chiral iron(II) complexes and preliminary mechanistic studies reveal an iron nitrenoid is a possible intermediate that can undergo either aminohydroxylation or aziridination, and the selectivity can be controlled by careful selection of counteranion/ligand combinations.

Full text of DOI:10.1021/ja311923z

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Communication
Iron (II) Catalyzed Intramolecular Aminohydroxylation
of Olefins with Functionalized Hydroxylamines
Guan-Sai Liu, Yong-Qiang Zhang, Yong-An Yuan, and Hao Xu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja311923z • Publication Date (Web): 12 Feb 2013
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Iron (II) Catalyzed Intramolecular Aminohydroxylation of Olefins
with Functionalized Hydroxylamines
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Guan-Sai Liu, Yong-Qiang Zhang, Yong-An Yuan and Hao Xu*
Department of Chemistry, Georgia State University, 100 Piedmont Avenue SE, Atlanta, Georgia 30303, United States
Supporting Information Placeholder
Despite these important achievements, the direct conver-
ABSTRACT: A diastereoselective aminohydroxylation of
sion of a trans-olefin to an 1,2 anti-amino alcohol remains as
an unsolved synthetic problem. In addition, the iron cata-
lyzed aminohydroxylation is a less-explored process11. Re-
cently, Yoon discovered an iron-catalyzed olefin aminohy-
droxylation with sulfonyl oxaziridines12a and a highly enanti-
oselective oxaziridine-based aminohydroxylation of terminal
olefins was disclosed by the same author.12b We are interested
in discovering new iron-catalyzed olefin amination methods
that are unique in synthetic utility. Herein, we describe an
iron (II)-catalyzed diastereoselective aminohydroxylation of
an olefin to afford an 1,2 anti-amino alcohol with a function-
alized hydroxylamine, possibly via an iron nitrenoid13 inter-
mediate. The catalysts are able to transfer both the N and O
groups of the hydroxylamine intramolecularly to a variety of
olefins, affording amino alcohols with a stereochemical array
that is difficult to access with other known methods10
(Scheme 1). We envision this discovery offers an appealing
alternative for existing aminohydroxylation methods.
olefins with a functionalized hydroxylamine is catalyzed by
new iron (II) complexes. This efficient intramolecular pro-
cess readily affords synthetically useful amino alcohols with
excellent selectivity (dr up to > 20 : 1). Asymmetric catalysis
with chiral iron (II) complexes and preliminary mechanistic
studies reveal an iron-nitrenoid is a possible intermediate
that can undergo either aminohydroxylation or aziridination,
and the selectivity can be controlled by careful selection of
counter anion/ligand combinations.
Numerous biologically active alkaloids and pharmaceuti-
cals contain chiral amino alcohols, and therefore direct ami-
nohydroxylation of olefins is among the most utilized meth-
ods to introduce stereogenic nitrogen atoms in organic syn-
thesis. The pioneering Sharpless asymmetric aminohydrox-
ylation (AA)1 remains a powerful method for synthesis of
enantio-enriched syn-vicinal amino alcohols. This reaction
has also inspired extensive efforts to discover alternative
approaches to address its inherent limitations. Those discov-
eries include aminohydroxylations mediated by copper2, pal-
ladium3, platinum4, rhodium5, gold6, hypervalent iodine7, N-
phenyl hydroxamic acids8, and through electrochemistry9.
Among those discoveries, Yoon2b-g reported copper is able to
catalyze aminohydroxylation of an olefin with a sulfonyl oxa-
ziridine; Chemler2h-k discovered copper can catalyze intramo-
lecular olefin aminohydroxylation in the presence of
TEMPO; Göttlich2a disclosed copper can co-catalyze intra-
molecular olefin aminohydroxylation with a stoichiometric
amount of BF3·Et2O; and Donohoe10 developed Os-based
tethered strategy to solve the regioselectivity problem in
racemic olefin aminohydroxylation.
Table 1. Catalyst discovery for the aminohydroxylation
Scheme 1. Iron catalyzed aminohydroxylation
aReactions were carried out under argon at 23 ºC, unless
b
1
stated otherwise. Conversions were determined by H NMR
analysis with 1,3,5-trimethoxybenzene as an internal stand-
c
d
ard. Isolated yield. Catalyst was prepared in situ by stirring
FeBr2 or K4Fe(CN)6 with freshly prepared potassium salt of
e
the ligand 4. The reaction was performed at 45 ºC. fAn aziri-
dine was isolated. gThe reaction was performed at 70 ºC for 4
h
h. Conversion was less than 5% without ligands. TMEDA =
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tetramethylethylenediamine, OTf
fonate, NTf2 = trifluoromethanesulfonimide.
=
trifluoromethanesul-
based bi-dentate and tri-dentate ligands (Table 1): ligands 3
and 4 (with π-acceptor character) facilitate much efficient
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reactions than TMEDA and sparteine (entries 1−4). While
FeII/3 complexes are able to catalyze the aminohydroxylation,
FeIII/3 complexes are inactive (entry 5). In addition, we ob-
served a significant counter ion effect: cyanide (entries 9 and
10) with strong π-acceptor character proves superior to bro-
mide (entries 3 and 4), triflate (entry 6), carboxylate (entry
7), and triflimide (entry 8). Re-examination of FeII cya-
nide/ligand combinations revealed that K4Fe(CN)6/3 can
catalyze a highly diastereoselective aminohydroxylation14,
delivering an anti-amino alcohol with a stereochemistry
complementary to the one obtained through Os-based
methods. We discovered that acyl hydroxyl carbamates prove
uniquely effective for the desired reaction: both alkyl and
sulfonyl hydroxyl carbamates are not viable substrates.15
Table 2. Substrate scope of iron catalyzed aminohydrox-
ylation
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To explore the scope and limitation of this reaction, we
have applied the method to a variety of olefins (Table 2).
trans-Di-substituted styrenyl olefins are excellent substrates
(dr > 20 : 1) (entries 1−4). Unlike Os-based methods that are
not tolerant to basic-nitrogen functional groups16, the olefin
containing a pyridine ring is a viable substrate (entry 4). A
para-methyl-styrenyl substrate suffers from a total loss of dr
(entry 5); however, electronic tuning on the benzoyl group
increased the dr to 3.1 : 1 (entry 6)17. trans-Di-substituted
olefins with 1-naphthyl, alkyl, and alkynyl substituents can
participate in the reaction with acceptable yields but dimin-
ished diastereoselectivity (entries 7−9, dr varies from 2.5 : 1 to
5 : 1). We note that the cis-di-substituted olefins also afford
anti-amino alcohols in this reaction (entries 10−11). The ste-
reochemical convergence (entries 1 and 10) and a related
crossover experiment18 suggest the step-wise nature of the
aminohydroxylation. In addition, tri-substituted olefins (en-
tries 12−13) are suitable substrates (dr varies from 2.2 : 1 to >
20 : 1). Di-substituted terminal olefins are decent substrates
(entry 14), while mono-substituted olefins (entry 15) suffer
from lower yield19. We also discovered a cyclohexanol-
derived substrate (entry 16) readily participates in the reac-
tion to afford an anti-hydroxyl oxazolidinone that was diffi-
cult to access with Os-based strategies.10f Further investiga-
tion reveals that non-allylic alcohol based substrates, includ-
ing an olefin containing hydroxamate, can also undergo
smooth reactions (entry 17).
Since the asymmetric synthesis of hydroxyl oxazolidinones
by catalytic aminohydroxylation has not been reported, we
explored the asymmetric induction with FeII–chiral bisoxazo-
line (BOX)20 complexes (Scheme 2A). Extensive optimiza-
tion21 revealed that Fe(NTf2)220e–chiral BOX ligand 10 is effec-
tive for enantio-induction. It converts both 9a and 9b to syn-
hydroxyl oxazolidinone 11 with the same ee and dr (82 % ee,
dr > 20 : 1)22. A more synthetically useful amino alcohol triad
13 was subsequently obtained without stereochemistry ero-
sion with a known method23. To our surprise, the diastere-
oselectivity observed here is different from the one in
K4Fe(CN)6 catalyzed reaction (Table 2). We also observed
that Fe(OAc)2–chiral BOX ligand 10 complex catalyzes a con-
vergent aminohydroxylation of both 9a and 9b, affording 11
with diminished selectivities (71 % ee, dr = 5 : 1). In addition
to 11, a chiral aziridine 12 (65 % ee, dr > 20 : 1) was obtained
from both 9a and 9b.21 It is important to note that the sense
of enantioinduction for 11 and 12 is the same; and they can-
not interconvert under reaction conditions.24 We were curi-
aCombined isolated yield. bDetermined by 1H NMR.
cPhenyl vinyl ketone was also isolated as a side product.
We initiated the catalyst discovery with a cinnamyl alco-
hol-derived substrate 1 that incorporates both a trans-
disubstituted olefin and a functionalized hydroxylamine.
Preliminary inspection revealed that FeII salts alone are una-
ble to catalyze the reaction at room temperature; however,
the reaction is greatly accelerated by a variety of nitrogen-
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ous about this reactivity divergence and explored a series of
contrary, aziridination can effectively compete with the lig-
and transfer from B1, since the A (1,3) interaction is absent in
this conformation. Therefore, the aminohydroxylation
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ligands and discovered that the counter anion effect over the
product distribution is general.21 In literature, Bolm13i
demonstrated that Fe(OTf)2–chiral BOX can mediate asym-
metric styrene aziridination (up to 40 % ee). Lebel13j,k also
reported a copper catalyzed racemic aziridination from tosyl
hydroxyl carbamate 14. To gain further mechanistic insight,
we explored the reactivity of 14 under the optimized condi-
tions (Scheme 2B): Fe(NTf2)2 is unable to catalyze the cy-
clization of 14; Fe(OAc)2 can mediate the aziridination of 14,
affording 12 (the same ee with the one obtained from 9a and
9b), albeit in a low conversion. These results indicate that
the Fe(OAc)2 catalyzed aziridination of 9a, 9b, and 14 likely
go through the same intermediate.
should occur predominately from intermediate B2, leading to
an amino alcohol 11; while the cyclization from intermediate
B1 will mostly lead to a trans-aziridine 12. It is possible that
the mechanistic intricacy will vary with different counter
ion/ligand combinations, which will influence the relative
stability of B1 vs B2, as well as the rates of kA (aziridination)
vs kH (aminohydroxylation). Since the diastereoselectivity of
K4Fe(CN)6-catalyzed reactions is very different from the ones
catalyzed by Fe(NTf2)2 and Fe(OAc)2, it is less likely this
working hypothesis applies for the K4Fe(CN)6-catalyzed ra-
cemic reactions.
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Scheme 2. Iron catalyzed asymmetric intramolecular ole-
fin aminohydroxylation
Scheme 3. The mechanistic working hypothesis of
Fe(OAc)2 catalyzed asymmetric aminohydroxylation
In conclusion, we have discovered a new FeII catalyzed in-
tramolecular olefin aminohydroxylation with functionalized
hydroxylamines, where both the N and O functional groups
are efficiently transferred for olefin aminohydroxylation.
Preliminary mechanistic studies revealed an iron-nitrenoid is
possibly an intermediate that can undergo either olefin
aziridination or aminohydroxylation. This discovery opens
up the possibility of developing a unique and general ap-
proach for stereoselective olefin amination. Our current
effort focuses on better understanding the mechanistic de-
tails of this process and its application in complex-molecule
synthesis.
Reaction conditions: aFe(NTf2)2 (10 mol %), 10 (20 mol %),
b
MeCN, 0 °C, 8 h. Fe(OAc)2 (10 mol %), 10 (20 mol %), tolu-
ene/MeCN = 4 : 1, 0 °C, 4 h. cBoc2O, Et3N, DCM, RT, 3 h, then
LiOH, dioxane, RT, 4 h, 75 % for two steps. Boc2O = Di-tert-
butyl dicarbonate.
ASSOCIATED CONTENT
Supporting Information
The successful asymmetric induction provides further
mechanistic insights: (a) FeII–chiral ligand complexes are
involved both in rate and enantioselectivity-determining
steps; (b) olefin aziridination is likely a competing pathway
from a same intermediate. Presumably, two distinct types of
mechanistic pathways might operate: (a) Kharasch-type at-
om transfer radical addition25 or (b) pathways involving iron-
nitrenoid26 species. Based on the collected mechanistic evi-
dence, a working hypothesis that best corroborates the
Fe(OAc)2-catalzyed enantioselective reaction is shown in
Scheme 3. First, Fe(OAc)2–ligand complex can reductively
cleave the N–O sigma bond, and possibly convert the isomer-
ic cis and trans-styrenyl substrates 9a and 9b to iron-
nitrenoids A1 and A2, respectively. Then, a stepwise cy-
cloamination (from A1 and A2) will presumably occur, af-
fording two carbo-radical species–B1 and B2 that are in a fast
equilibrium. In principle, the intramolecular aziridination
should occur much slower than the ligand (OR) transfer27
from B2, because of the unfavorable A (1,3) interaction en-
countered in transition state leading to aziridines. On the
Experimental procedure, characterization data for all new
compounds, selected NMR spectra and HPLC traces. This
material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
hxu@gsu.edu
ACKNOWLEDGMENT
This work was supported by Georgia State University and the
American Chemical Society Petroleum Research Fund (ACS
PRF 51571-DNI1). We thank Chengliang Zhu for the synthesis
of a few substrates and Dr. Jillian Spangler in Prof. Huw Da-
vies’ group for helping us measure optical rotation.
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17207. (k) Roizen, J. L.; Harvey, M. E.; Du Bois, J. Acc. Chem. Res.
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tion product.
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