O2-Mediated Oxidation of Aminoboranes through 1,2-N Migration

Article abstract of DOI:10.1002/anie.201803168

In analogy to the classical reaction of C?B bonds with peroxides, the first oxidative functionalization of aminoboranes through a 1,2-N migration was realized. Readily available aliphatic nitro compounds are thereby transformed into N- and O-functionalized hydroxylamines in a single synthetic operation. Addition of hazardous peroxides is avoided. Instead, the insertion of O₂, as the terminal oxidant, into Zn?C bonds provides the necessary peroxides. The required zinc organyls, in turn, are formed through a boron-to-zinc exchange, from an organoboronic ester byproduct of the nitro-to-aminoborane transformation.

Full text of DOI:10.1002/anie.201803168

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Title: O2 Mediated Oxidation of Aminoboranes via 1,2-N Migration
Authors: Marian Rauser, Daniel P. Warzecha, and Meike Niggemann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803168
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10.1002/anie.201803168
Angewandte Chemie International Edition
COMMUNICATION
O2 Mediated Oxidation of Aminoboranes via 1,2-N Migration
Marian Rauser,[a] Daniel P. Warzecha[a] and Meike Niggemann*[a]
Abstract: In analogy to the classical reaction of C-B bonds with
peroxides, the first oxidative functionalization of aminoboranes via a
1,2-N migration was realized. Thereby, readily available aliphatic nitro
compounds are transformed into N- and O-functionalized hydroxyl-
amines in a single synthetic operation. Addition of hazardous
peroxides is avoided. Instead, the insertion of O2, as the terminal
oxidant, into Zn-C bonds provides the necessary peroxides. The
required zinc organyls, in turn, are formed via a boron to zinc
exchange, from an organoboronic ester byproduct of the nitro to
aminoborane transformation.
sustainable process we strove for an in-situ generation of the
oxidizing agent, using oxygen as the terminal oxidant.
Classical C-B bond oxidation
a)
H
[OH
]
H
O
O
H
2O2
O
Bpin
3C
O
B
R
3C
R
3C
R
Bpin
O
O
1,2-C
migration
of
byproduct
work:
This
N-B bond oxidation
b)
NO2 to N-B
transformation
FINISH
Nitrogen containing compounds are valuable and commercially
important chemicals. Thus, the search for the introduction of
nitrogen moieties or their functionalization has continuously
progressed.[1] Organic hydroxylamines have proven to be highly
valuable as structural motives / intermediates in drug synthesis,[2]
polymerization inhibitors[3] or nitric oxide donors.[4] Also, they are
highly efficient electrophilic amination agents. Especially
O-benzoylhydroxylamines (R2N-OBz) have gained attention over
the last decade.[5] Commonly, they are prepared by a radical one-
step addition of DBPO to the corresponding amine.[5a,6] Alternative
peroxides have also been used for amine oxidation in some
cases.[7] Orthogonal approaches are based on the reduction of
either nitro[8] or oxime[9] moieties generating primary hydroxyl-
amines. The direct reductive alkylation of aliphatic nitro com-
pounds via nitroso intermediates remains challenging.[10] Up to
date, the synthesis of functionalized hydroxylamines mostly re-
sorts to multistep syntheses, especially when the amine is not
commercially available and an O-functionalization other than
O-Bz is desired.[11]
Functionalization methods for C-B bonds are well established.
Among these, the oxidation with peroxides[12] has been known for
nearly a century and is frequently utilized since the development
of Brown´s hydroboration-oxidation sequence (Scheme 1, a).[13]
Conversely, despite the methodology for the synthesis of
aminoboranes has significantly advanced,[14] their application lags
disturbingly behind.[15] Recently, we realized a highly efficient
protocol for the preparation of aliphatic aminoboranes from nitro
compounds, enabled by a combination of B2pin2 and zinc
organyls.[16] Since the reaction showed a high generality for the
preparation of functionalized aminoboranes, we became
interested in their processing. We considered a transformation of
aminoboranes to the corresponding hydroxylamines via a 1,2-N
migration, in analogy to the classical oxidation of organoboranes,
an ideal starting point for the development of future applications
of this type (Scheme 1, b). Much to our surprise, we found such a
transformation unprecedented.
R2
N
R1
E
R2
N
O
R1
R2
Bpin
R2
E
pinB
O
Zn( R2
O
)
2
R2
O
pinB
R2 Zn
O
B to Zn
transmetalation
O
R2
O
R2 ZnR2
N
R1
B
O
O
O
O2
oxygen
insertion
1,2-N
migration
R2 Zn
OO
R2
O
ZnR2
B
2
R2
N
R1
START
R1 NO2
2pin2
Bpin
Scheme 1: Oxidation of organo- (a) and aminoboranes (b).
Zinc organyls are known to form zinc alkyl peroxides upon
exposure to oxygen.[17] These have already been used as
oxidizing agents.[18] Also, zinc organyls are already present in the
reaction mixture, as they are an essential reagent for the nitro to
aminoborane transformation. Hence, the simple delivery of O2
after completion of the aminoborane formation should transform
residual zinc organyl into peroxide and thus provide the necessary
oxidizing agent. To further improve the efficiency of our strategy
and to keep the required amount of zinc organyl minimal, we
envisioned to recycle the organoboronic ester, which is formed as
a byproduct in the first reduction step and generate the zinc
organyl for the peroxide formation via a boron to zinc exchange.
This transmetalation is well established.[19] In summary, as shown
in Scheme 1, the reaction sequence was devised as follows:
1.) Nitro to aminoborane transformation with B2pin2 and ZnR2
and concomitant pinBR2 formation.
2.) PinBR2 to zinc transmetalation.
2
3.) Peroxide formation via O2 insertion into Zn-C
4.) Peroxide addition to aminoborane followed by 1,2-N migra-
tion generates the hydroxylamine.
The commonly employed reagents for the oxidation of
C-B bonds are peroxide based. To directly realize a more
5.) Hydroxylamine O-functionalization, via electrophile addition
during work up.
[a]
Marian Rauser, Daniel P. Warzecha, Prof. Dr. Meike Niggemann
Institute of Organic Chemistry, RWTH Aachen
Landoltweg 1, 52072 Aachen
Thereby, a simple protocol directly transforms aliphatic nitro
compounds into highly functionalized hydroxylamines with readily
available reagents in a single synthetic operation. No hazardous
peroxides need to be added. And most importantly, a precedent
is set for a new type of aminoborane transformation.
E-mail: niggemann@oc.rwth-aachen.de
Supporting information for this article is given via a link at the
end of the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201803168
Angewandte Chemie International Edition
COMMUNICATION
We started our investigation with phenylnitroethane 1a as a model
compound, which was treated with 1.2 equiv of ZnEt2 and
2.0 equiv B2pin2 in toluene. After full conversion of the starting
material to aminoborane 2a the inert atmosphere was substituted
with an oxygen atmosphere. To our delight hydroxylamine 3a was
formed with a moderate yield of 53% after a total reaction time of
12 h. Aminoboranes are air and moisture sensitive compounds.[20]
Thus, we assumed a lower oxygen concentration of air should
provide milder reaction conditions. After some optimization of the
oxygen delivery (see SI for details), we finally obtained
hydroxylamine 3a with an excellent yield of 89% using air as
oxygen source. Next, we tested, if a direct O-functionalization was
tolerated by the system. As mentioned above, the convenient and
direct preparation O-functionalized hydroxylamines as efficient
aminating agents is of high interest. Addition of Bz2O after
successful oxidation of aminoborane 2a gave O-benzoyl-
hydroxylamine 4a without losses and facilitated the isolation.
Having in hand an efficient protocol for a one-pot N- and O-func-
tionalization we examined the generality of the transformation
(Table 1). We started to explore the reaction scope with the
variation of the nitro compound. Arylnitroethanes with a tolyl- (4b),
and o-, m- and p-methoxy substitutions (4c-4e) reacted efficiently
(75-94% yield).
A thioether- (4f, 81%) or dimethylamine
functionality (4g, 64%) were well tolerated. Phenol substituents
required no protection (4h, 60%). Thiophene (4i, 87%) and even
halogen substituents such as fluorine (4j, 79%), chlorine (4k,
79%) and bromine (4l, 73%) were no limitation. Commercially
available nitromethane (4m, 91%) as well as -ethane (4n, 92%)
and -propane (4o, 94%) gave excellent yields. Finally, we
examined bulkier α-branched nitro compounds.
Table 1. Scope of the O2 mediated oxidation of aminbororanes.
Zn R2
1.2
equiv
or
2
Zn R2
2.5
X
equiv.
E
2.0
B Pin
O
equiv.
2
2
air
Bpin
NO2
N
N
R1
R1 R2
R1 R2
toluene,
rt, 4 h
toluene,
rt,
h
8
1
2
5
7
6
/ /
then
E
LG
OBz
N
OBz
OBz
OBz
N
OBz
N
or
or
OH
Bz
Bz
2O
Bz
Cl
Bz
O
N
N
N
E+
=
Et
Et
Et
Bn
4a
Et
Et
MeO
Et
MeS
N
BzO
89% Bz O)
,
o
m
(
2
4c,
4d,
4e,
-OMe 94%
-OMe 87%
p-OMe 87%
[a]
4b
75%
,
4f
4g
4h,
1
in
)
81%
64%
,
60%
Et
,
(OH
4a'
Bz
Cl)
75%
,
(
OBz
N
[c]
4a''
Bz
OH)
88
%
(
Et
OBz
N
OBz
N
OBz
OBz
OBz
N
OBz
OBz
OBz
N
X
N
N
N
N
Me
4m
Et
Et
Et
Et
Et
Et
Et
Et
p-F 79%
-Cl 79%
p-Br 73%
4j,
S
o
4k,
5i, 87%
4n
91%
92%
,
,
4o
,
94%
4p
91%
,
4q
4s
4l,
95%
95%
,
4r
,
90%
,
Zn(nBu
or
Ac
2O
or
Ac
)
2
E+
Ac
E+ Piv
E+
=
=
=
Boc2O
Zn(nHex
2O
)
2
IZnnBu
Cl
Et
N
Et
N
Et
N
OBz
N
OBz
Bn
Bn
Bn
Bn
Bn
N
Piv
O
O
OBoc
5
6c
95%
,
6a 77% Ac
6b
82%
,
,
72% Zn(nBu)
(
(
2O)
5a
,
5b
84%
,
)
2
[b]
6a'
Ac
Cl)
71%
(
n
5a'
68% Zn Bu
(I
)
E+
H
=
=
=
E+
H
E+
H
=
RCO2
Cl
RCO2
RCO2
NEt2
Et
Et
O
I
O
Et
N
O
O
Zn(secBu
BrZncycHex
)
2
Bn
N
Bn
N
Bn
OBz
N
OBz
N
O
O
Bn
Bn
Cl
[c]
[c]
6e
[c]
84%
6f
91%
6d
86%
76%
62%[b]
5c
5d
,
,
=
E+
H
E+
H
E+
H
Zn
RCO2
=
RCO2
RCO2
OMe
Et
N
O
Et
N
Et
N
O
O
BrZnBn
2
OBz
N
OBz
N
Bn
Bn
Bn
Bn
Bn
O
Ph
O
O
Bn
[c]
[c]
[c]
6g
88%
6h
6i
89%
79%
5e
81%[b]
70%
,
5f
,
OMe
Conditions: 1.2 equiv zinc organyl were added to 2.0 equiv of B2pin2 and 1.0 equiv of the corresponding nitro compound 1 (0.2 mmol) in 1 mL toluene and
stirred for 12 h at rt; The hydroxylamine 3 was subsequently benzoylated (3.0 equiv) with the corresponding electrophile; [a] 2.0 equiv of ZnEt2 used. [b]
2.5 equiv zinc halide; 1.1 equiv corresponding acid were added as electrophile after its activation with CDI (1.1 equiv).
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10.1002/anie.201803168
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COMMUNICATION
Secondary nitro compounds like 2-nitropropane (4p, 91%),
phenyl-2-nitropropane (4q, 95%), nitrocyclopentane (4r, 90%),
and even the tertiary tert-nitrobutane (4s, 95%) all reacted with
excellent yields. Aminoboranes derived from aromatic nitro
compounds were not oxidized.
mechanism very unlikely. The product of the oxidation with lithium
tert-butyl hydroperoxide (Scheme 2, e), is consistent with the
oxidation mechanism via an ionic addition - 1,2-migration
sequence.
Next, we varied the zinc organyl. Phenyl nitroethane 1a reacted
efficiently with primary zinc organyls, such as butyl (5a, 72%) and
hexyl (5b, 84%) followed by oxidation to the hydroxylamines.
Despite their lower reactivity,[21] efficiency is high also with zinc
halides (5a’, 68%). More bulky secondary (5c, 76%) or cyclic (5d,
62%) zinc organyls showed no limitations for the overall reaction
sequence. Allylic (5e, 70%) and benzylic organyls (5f, 81%)
reacted readily. No oxidation occurred with aryl zinc organyls,
presumably due to inefficient peroxide formation.[17]
To complete the scope, we tested the compatibility with different
O-functionalization reagents. Esterification of hydroxylamine 3a
with acid chlorides (BzCl 4a’, 75%, AcCl 6a’ 71%) proved slightly
less efficient than the corresponding anhydrides (Bz2O, 4a 89%,
Ac2O 6a 77%). Sterically more demanding pivalic (6b, 82%) or
Boc (6c, 95%) anhydride reacted readily. Reaction with benzoic
acid via the coupling agent CDI proceeded efficiently (4a’’, 88%).
Following the same procedure benzoic acids with EWG or EDG
substituents in the o- and/ or p-position provided excellent yields
(EWG: 6d, 86%, 6e 84%; EDG: 6f 91%, 6g 88%). Unsaturated
(6h, 79%) and aliphatic acids (6i 89%) reacted readily.
a)
exclusion of O2
Bpin
1.2
2.0
ZnEt
B
equiv
equiv
2
N
Et
N
2pin2
Ph
Et
air
NO2
2a
,
,
Ph
Ph
OM
D6
C6
D6
C6
rt, 8 h
confirmed by
1H- 11B-NMR
4a
1a
rt, 4 h
/
b)
+
+
EtZnOEt
pinBEt
pinBOEt
Zn(OEt)2
D6
3h
C6
rt,
7
10
9
8
c)
ZnEt2 air
/
0%
d)[a]
HBpin
LHMDS
Et
Et
Et
OBz
ZnEt2 air
/
Bpin
NH
Et
N
N
11n
2n
4n
Et
Et
76%
LiO tBu
O
[a]
e)
f)
67%
Apart from providing a very straightforward access to function-
nalized hydroxylamines we were mostly interested in the
development of an oxidative 1,2-migration based transformation
of aminoboranes. To harness this reactivity for future applications
a distinct knowledge of the reaction mechanism is mandatory.
Thus, several control experiments were run to gain more insight
into the reaction pathway (see SI for details). Examination of the
BzO tBu 0%
O
Scheme 2: Control experiments; [a] The hydroxylamine 4 was subsequently
benzoylated (Bz2O, 3.0 equiv).
1
reaction mixture by both H- and 11B-NMR spectroscopy clearly
identified aminoborane 2 as the starting material of the oxidation
In summary, we developed an efficient and general protocol for
the preparation of N- and O- functionalized hydroxylamines from
readily available aliphatic nitro compounds in a single synthetic
operation. Preliminary mechanistic investigations confirm the
oxidation of the aminoborane intermediate via a 1,2-N migration.
Oxygen insertion into zinc organyls provides a convenient access
to zinc peroxides. Further increasing the efficiency of the protocol
and keeping the amount of required zinc organyl minimal, a boron
to zinc transmetalation is used to regenerate zinc organyls from
an organoboronic ester byproduct of the nitro to aminoborane
transformation.
reaction (Scheme 2, a). Treatment of pinBEt 7 with ZnOEt2
8
resulted in the formation of pinBOEt 9 and EtZnOEt 10, confirming
the feasibility of the boron to zinc transmetallation (Scheme 2, b).
Exposure of diethylamine 11n to ZnEt2/air or ZnEt2/O2 did not
oxidize the amine (Scheme 2, c). This excludes both the amine
11 and the corresponding zinc amide as intermediates.
To further emphasize the essential role of the B-N bond in the
aminoborane for the observed reactivity, while excluding
alternative reaction mechanisms via byproducts and reagents
remaining from its formation, we generated the aminoborane 2n
via a dehydrocoupling protocol. This also allowed for a more in-
depth analysis of the nature of the oxidizing agent. The addition
of ZnEt2/air (Scheme 2, d) indeed let to the formation of the
hydroxylamine 4n. Alternatively, lithium tert-butylperoxide
(Scheme 2, e) also oxidized aminoborane 2n. Addition of tert-
butylperoxybenzoate did not result in the formation of any
oxidation product (Scheme 2, f). In both cases no O-tert-butyl
hydroxylamine was detected. This confirms a peroxide as the
oxidizing agent, while showing that the boron substitution at the
N-atom efficiently suppresses an N-radical mediated reaction
Keywords: aminoborane functionalization • direct nitro
transformation • zinc peroxide • oxidation • oxygen
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COMMUNICATION
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10.1002/anie.201803168
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
From B to O. The first oxidative
functionalization of aminoboranes via a
1,2-N migration is presented. Thereby,
aliphatic nitro compounds are trans-
formed to a broad range of N- and O-
functionalized hydroxylamines, in a
single synthetic operation.
Marian Rauser, Daniel P. Warzecha,
Meike Niggemann*
Page No. – Page No.
O2 Mediated Oxidation of
Aminoboranes via 1,2-N Migration
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