Metal-free conversion of methane and cycloalkanes to amines and amides by employing a borylnitrene

Article abstract of DOI:10.1002/anie.200705936

(Chemical Equation Presented) C-H insertion: Borylnitrenes, which are generated in situ by photoylsis of azides, convert unactivated alkanes by intermolecular C-H insertion into aminoboranes (see scheme), which in turn can be reacted further to give amines or amides. The boryl group serves two purposes: it converts the nitrene into a highly reactive BN vinylidene analogue, and it is easily cleaved from the product.

Full text of DOI:10.1002/anie.200705936

Communications
DOI: 10.1002/anie.200705936
Alkane Activation
Metal-Free Conversion of Methane and Cycloalkanes to Amines and
Amides by Employing a Borylnitrene**
Holger F. Bettinger,* Matthias Filthaus, Holger Bornemann, and Iris M. Oppel
The selective transformation of methane into more reactive
molecules, considered to be one of the “holy grails” of
chemistry,[1] lies at the heart of our understanding of chemical
reactivity and has potentially far-reaching practical implica-
3
ꢀ
tions. The challenge in achieving this transformation arises
unactivated C(sp ) H bonds of hydrocarbons, including
methane, according to Equation (3).
from the low reactivity of methane.[2] An economically
ꢀ
feasible process for C H activation is not available, but is
highly desirable owing to the abundance of methane as the
major constituent of natural gas. Although natural gas is the
most abundant, low-cost, carbon-based feedstock, most basic
chemicals are produced today indirectly from petroleum in
energy-extensive processes.[3]
While superacids,[4] free radicals and radical cations,[2a]
and enzymatic systems[2a] can be used to functionalize simple
hydrocarbons, much success has been achieved in the field of
transition-metal chemistry.[2c,5] Atypical theme of transition-
Nitrenes, short-lived reactive intermediates, can be gen-
erated from azides[8] and are known to undergo C H bond-
ꢀ
insertion reactions.[9] The intermolecular insertion of free
ꢀ
nitrenes into C H bonds, however, is usually not of synthetic
ꢀ
metal-mediated C H bond activation is the oxidative addi-
value: for example, photolysis of phenyl azide in hydrocarbon
tion of an alkane to a coordinatively unsaturated metal center
[LnMx] [Eq. (1)], which is usually generated in situ by thermal
or photochemical decomposition of a suitable precursor.[5b]
The alkane RH acts then as a nucleophile towards the
electrophilic metal center [LnMx] [Eq. (1)].
solvents produces primarily polymeric materials.[10] Only very
ꢀ
electrophilic nitrenes yield the C H insertion product with
hydrocarbon solvents in appreciable amounts.[11–14]
The current state of the art[15] in the amidation of C H
ꢀ
bonds thus relies on nitrene surrogates, for example iminoio-
[16]
=
danes PhI NR
and aryl azides,[17] in transition-metal-
mediated reactions, although metal-free conversions are
also known.[18] The groundbreaking work goes back to
Breslow and co-workers,[19] who demonstrated in 1982 that
cyclohexane can be amidated in 3–6% yield based on the
iminoiodane in the presence of metal porphyrins. Although
Frey et al. recently recognized the similarity of certain
stable carbenes and coordinatively unsaturated metal centers
in the splitting of dihydrogen and ammonia [Eq. (2)].[6]
This analogy may be extended to subvalent nitrogen
compounds: the single nitrogen center of an electrophilic
singlet borylnitrene 1 has a low-lying unoccupied and a high-
lying occupied molecular orbital.[7] Here we show that certain
borylnitrenes 1 are good reagents for the transformation of
since then considerable achievements have been made,[15]
3
ꢀ
only a few examples exist in which the C(sp ) H bonds of
saturated unactivated hydrocarbons can be functionalized in
good yields in intermolecular reactions.[20–23]
Borylnitrenes 1 are transient species, which have been
trapped successfully.[24] The recently characterized catechol
derivative 1a (Scheme 1), a triplet-ground-state nitrene
obtained photochemically from the corresponding azidobor-
ane 3a under matrix-isolation conditions,[25] showed unusually
high reactivity. We ascribed this to the electronic similarity
between 1a in its singlet state and difluorovinylidene, a
“superelectrophilic” carbene that inserts into methane and
dihydrogen at 20–40 K.[26]
[*] Dr. H. F. Bettinger, M. Filthaus, Dr. H. Bornemann
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum
Universitätsstrasse 150, 44780 Bochum (Germany)
Fax: (+49)234-321-4353
E-mail: Holger.Bettinger@rub.de
In order to investigate the reaction of 1a with methane,
we isolated azide 3a in argon doped with methane (1–2%
CH4 or CD4) at 10 K. Photolysis of 3a using UV irradiation
(l = 254 nm) resulted in the complete disappearance of 3a
and the concomitant formation of nitrene 1a according to the
IR spectra. In addition, a set of new IR signals appears during
the photochemical decomposition of 3a (see Figure 1 as well
as Tables S1 and S2, and Figure S2 in the Supporting
Information).
Dr. I. M. Oppel
Lehrstuhl für Analytische Chemie, Ruhr-Universität Bochum
Universitätsstrasse 150, 44780 Bochum (Germany)
[**] This work was supported by the DFG and the Fonds der Chemischen
Industrie. We thank Professor Wolfram Sander for his interest and
support of this work, and Patrik Neuhaus for the ESR measure-
ments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4744
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Chemie
Scheme 2. Synthesis of azidoborane 3b (2-azido-4,4,5,5-tetramethyl-
1,3-dioxaborolane, pinBN3).
Information).[28] The identity of the cyclohexyl (2b-cy6) and
cyclopentyl (2b-cy5) products was confirmed further by
single-crystal X-ray analysis (Figure 2).[29]
Scheme 1. Photochemistry of azidoborane 3a in an argon matrix
doped with 1–2% methane.
Figure 1. IR spectra obtained from the photolysis of azide 3a in solid
ꢀ
argon at 10 K. a) IR spectrum computed for the C H insertion product
2a at the B3LYP/6-311+G** level of theory. The numbers of the
assigned vibrational modes (see Table S1 in the Supporting Informa-
tion) are given on top. b) IR spectrum obtained after three photolysis
cycles (254 nm followed by 550 nm) of 3a in the presence of CH4 (2%
in Ar). c) IR spectrum of borylnitrene 1a obtained in Ar in the absence
of methane.
Figure 2. Single-crystal structure of 2b-cy6 (top) and 2b-cy5 (bottom)
determined at 108 K and 113 K, respectively.[29] BN bond lengths:
1.386(4) Š (2b-cy6) and 1.393(2) Š (2b-cy5).
The good yields of up to 85% (Table 1) are remarkable in
view of the lower reactivity of most other free nitrenes in
[11–13]
ꢀ
intermolecular C H bond-insertion reactions.
Phospho-
ꢀ
Based on the signal in the NH-stretching region (n(N H):
ryl nitrenes show reactivity similar to that of borylnitrene
ꢀ1
3480 cm , n(N D): 2572 cmꢀ1) and the computed IR spec-
1b.[14]
ꢀ
trum, the new spectral features are assigned to the amino-
borane 2a. A s2a is known to be oligomeric in the solid state
and in solution,[27] we have made no attempts to synthesize it
independently.
The aminoboranes of type 2b can conveniently be trans-
formed into primary amines RNH2 4 by alcoholysis or into
amides RNHCOAc 5 by acylation (Scheme 3). The amidation
of the reaction products 2b to give 5 allows the investigation
ꢀ
Annealing of the matrix to 35 K did not increase the yield
of 2a, but upon irradiation with visible light (l > 550 nm)
nitrene 1a reacted with methane at 10 K to give 2a. As a side
reaction, formation of azide 3a was also observed under these
conditions (Scheme 1).
of the selectivity of the C H transformation by GC–MS
analysis using 2,3-dimethylbutane as the substrate. Compar-
ison with an authentic sample of N-(1,1,2-trimethylpropyl)-
acetamide shows that the combined yield of C H insertion
products is 74%. The relative amount of insertion is statistical
ꢀ
ꢀ
ꢀ
The high tendency of 1a to insert into the unreactive C H
and thus the reaction at the stronger primary C H bonds
yields the major product (Table 1). The observation of the
bond of methane suggests that this reaction should also be
observable under photochemical conditions in solution. To
avoid problems associated with the possible oligomerization
of catecholates,[27] we investigated the pinacolate system 3b,
which was synthesized as outlined in Scheme 2. Photolysis
(l = 254 nm) of the novel azide 3b in (cyclo)alkane solutions
at room temperature indeed yielded the expected amino-
boranes according to spectral information (see the Supporting
ꢀ
insertion into the primary C H bonds demonstrates the
unusually high reactivity of borylnitrene 1b. For comparison,
the very reactive pentafluorophenyl nitrene inserts exclu-
sively into the tertiary C H bonds of 2,3-dimethylbutane
ꢀ
under similar conditions.[11c]
In summary, this investigation shows that borylnitrenes
are very active reagents for intermolecular amination and
Angew. Chem. Int. Ed. 2008, 47, 4744 –4747
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4745

Communications
novel compounds, and the photolysis experiments are given in the
Supporting Information.
Table 1: Yields (in % based on 3b) of aminoboranes 2b and organic
amides 5 obtained from the reaction of 3b (pinBN3) with a number of
hydrocarbons.
Caution: Boron azides may be explosive, and appropriate
precautions must be taken when handling these compounds.
X-ray diffraction: Single crystals of 2b-cy5 and 2b-cy6 were
grown by slow evaporation of the cycloalkane solvent. Intensity data
for 2b-cy5 and 2b-cy6 were both collected on an Oxford Diffraction
Xcalibur2 CCD employing the w scan method using CuKa radiation
for 2b-cy6 and MoKa radiation for 2b-cy5. The data were corrected
for Lorentz, polarization, and absorption (multiscan, compound 2b-
cy6 only) effects. 2b-cy6 and 2b-cy5 were solved by using direct
methods (SHELXS-97)[31a] and refined by using a full-matrix least-
squares refinement procedure (SHELXL-97).[31b] In both compounds,
the hydrogen atoms bonded to carbon atoms were placed at
geometrically estimated positions while those bonded to nitrogen
atoms were found in the Fourier difference synthesis and refined
freely with only the distance fixed to the literature value.
CCDC 671485 and 671486 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Photolysis in solution and workup: a) Photolysis of 3b: A
solution of 3b (250 mg, 1.48 mmol) in 30 mL of the cycloalkane was
photolyzed in quartz tubes for 16 h with a low-pressure mercury lamp
(l = 254 nm) under an argon atmosphere at room temperature. The
pale yellow reaction mixture was then filtered, and the solvent was
removed from the filtrate under reduced pressure. Sublimation
(708C, oil pump) yielded aminoborane 2b. b) Alcoholysis: A small
amount of the photoproduct was dissolved in 1.5 mL of dry
isopropanol. The reaction mixture was stirred for 30 min at room
temperature, and the formation of the free amine of 4 was confirmed
by GC–MS. c) Acylation: After photolysis the soluble and the
insoluble products were collected by removal of the hydrocarbon in
vacuo and then dissolved in 20 mL of anhydrous Et2O. The catalyst
N,N’-dimethylaminopyridine (10–20 mg) and acetyl chloride
(0.50 mL, 7.03 mmol) were added. The resulting suspension was
stirred for 18 h at room temperature. Solid NaOH (1.00 g, 25 mmol)
was added, and the reaction mixture was stirred for one more day.
Then a solution of hexamethylbenzene (internal standard; 0.240 g,
1.48 mmol) in 20 mL Et2O was added, the mixture was filtered, and
the solid was washed with Et2O (3 ” 10 mL). The filtrate was
separated and the mixture was analysed by GC–MS.
Entry
Substrate
pinBNHR 2b[a]
CH3CONHR 5[b]
1
84
77
2
85
83
3
4
79
75
77[c]
69[d]
[a] Yield of isolated product. [b] Amides were obtained from pinBNHR
and the RNHB(OH)2 precipitate[28] according to Scheme 3 and were
determined by GC–MS based on authentic samples. [c] Combined yield
of derivatization products from insertion into primary (63%) and tertiary
ꢀ
(11%) C H bonds. [d] Combined yield of derivatization products from
ꢀ
insertion into primary (58%) and tertiary (11%) C H bonds.
Scheme 3. Derivatization of aminoboranes 2b. DMAP=N,N’-dimethyl-
aminopyridine, Ac=acetyl.
Received: December 24, 2007
Published online: May 16, 2008
ꢀ
amidation of unactivated C H bonds. The high yields of
insertion products indicate that these insertion reactions are
fast relative to the rate of intersystem crossing to the triplet
ground state of borylnitrene 1b.[30] The boryl group fulfills two
purposes in this chemistry: 1) it transforms the nitrene into a
very active BN vinylidene analogue and 2) it is easily cleaved
to yield the desired organic substrate. Borylnitrene 1b thus
allows the efficient one-pot transformation of an alkane into a
primary amine or into an amide. We expect that modification
of the boryl group will allow utilization of visible-light
irradiation and regeneration of the borylazide, and will
possibly also provide increased selectivity in this transforma-
tion. Investigations on the transition-metal-catalyzed inter-
molecular aminations using azidoboranes are also underway
in our laboratory.
ꢀ
Keywords: C H activation · hydrocarbons · matrix isolation ·
methane · nitrenes
.
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¯
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79.999(7), b = 84.840(7), g = 75.462(7)8, V= 675.2(1) ꢀ3, 1calc
1.107 gcmꢀ3
2qmax = 119.988, l = 1.54178 ꢀ, T= 108 K, 2762
measured reflections, 1948 independent reflections (Rint
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=
,
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,
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.
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Angew. Chem. Int. Ed. 2008, 47, 4744 –4747
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4747