C-H bond amination by photochemically generated transient borylnitrenes at room temperature: A combined experimental and theoretical investigation of the insertion mechanism and influence of substituents

Article abstract of DOI:10.1021/om300063y

A number of azidoboranes having substitution patterns that are derived from catechol (3), pinacol (4a), 1,2-diaminoethane (4b,c), 1,2-ethanedithiol (4d), and 1,2,4,5-tetrahydroxybenzene as well as acyclic dialkoxy species (5) were synthesized and, in the case of 4c (N,N′-ditosyl-2-azido-1,3,2- diazaborolane), also structurally characterized. The azidoboranes were photolyzed in cyclohexane solvent in order to investigate the tendency of the generated borylnitrenes to undergo intermolecular C-H insertion reactions. The yields of intermolecular insertion products ranged from very good (4a) to vanishingly small, depending on the substitution of the azidoborane. For a number of borylnitrenes the zero-field splitting parameter D was measured in organic glasses at 4 K. The small primary kinetic isotope effect (k _H/k_D = 1.35) measured for 4a in mixtures of [H ₁₂]cyclohexane and [D₁₂]cyclohexane suggests that the insertion reaction is concerted and involves the singlet state of the borylnitrene. Computations at the CBS-QB3 and CCSD(T)/TZ2P levels of theory show that the relative energies of singlet and triplet states of a wide variety of borylnitrenes and even their nature as minima or saddle points depend strongly on the substituents. Photolysis of the most reactive azidoborane, 4a, in methane in a flow reactor at atmospheric pressure produces an intermolecular insertion product in low yields, in agreement with the expectation of intersystem crossing to the less reactive triplet state of the borylnitrene.

Full text of DOI:10.1021/om300063y

Article
pubs.acs.org/Organometallics
C−H Bond Amination by Photochemically Generated Transient
Borylnitrenes at Room Temperature: A Combined Experimental and
Theoretical Investigation of the Insertion Mechanism and Influence
of Substituents
Matthias Filthaus,†,‡ Larissa Schwertmann,‡ Patrik Neuhaus,‡ Rudiger W. Seidel,§ Iris M. Oppel,∥
̈
and Holger F. Bettinger*,†,‡
†Institut fur Organische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
̈
̈
̈
̈
§
‡Lehrstuhl fur Organische Chemie II and Lehrstuhl fur Analytische Chemie, Ruhr-Universitat Bochum, Universitatsstraße 150,
̈
̈
̈
̈
44780 Bochum, Germany
∥Institut fur Anorganische Chemie, RWTH Aachen, Landoltweg 1, 52074 Aachen, Germany
̈
S
* Supporting Information
ABSTRACT: A number of azidoboranes having substitution patterns that are
derived from catechol (3), pinacol (4a), 1,2-diaminoethane (4b,c), 1,2-ethanedithiol
(4d), and 1,2,4,5-tetrahydroxybenzene as well as acyclic dialkoxy species (5) were
synthesized and, in the case of 4c (N,N′-ditosyl-2-azido-1,3,2-diazaborolane), also
structurally characterized. The azidoboranes were photolyzed in cyclohexane solvent
in order to investigate the tendency of the generated borylnitrenes to undergo intermolecular C−H insertion reactions. The
yields of intermolecular insertion products ranged from very good (4a) to vanishingly small, depending on the substitution of the
azidoborane. For a number of borylnitrenes the zero-field splitting parameter D was measured in organic glasses at 4 K. The small
primary kinetic isotope effect (kH/kD = 1.35) measured for 4a in mixtures of [H12]cyclohexane and [D12]cyclohexane suggests
that the insertion reaction is concerted and involves the singlet state of the borylnitrene. Computations at the CBS-QB3 and
CCSD(T)/TZ2P levels of theory show that the relative energies of singlet and triplet states of a wide variety of borylnitrenes and
even their nature as minima or saddle points depend strongly on the substituents. Photolysis of the most reactive azidoborane,
4a, in methane in a flow reactor at atmospheric pressure produces an intermolecular insertion product in low yields, in agreement
with the expectation of intersystem crossing to the less reactive triplet state of the borylnitrene.
In our previous work we demonstrated that oxygen-
substituted borylnitrene 1a, photochemically accessible from
the corresponding azide 3a, is also highly reactive under matrix
isolation conditions.1,20−23 In contrast to 2b, however, 1a
requires photochemical excitation using visible light to undergo
insertion reactions into CH4 or H2 if isolated in solid argon.21,22
Further investigations in solution at room temperature revealed
that the pinacol derivative 1b is a ferocious intermediate that
inserts into the C−H bonds of hydrocarbon solvents very
efficiently without selectivity after statistical correction.21
While the intramolecular C−H insertion of transient nitrenes
is a well-known reaction,24−28 efficient intermolecular C−H
insertion reactions are limited to highly electrophilic nitrenes.
Examples are fluorinated phenyl (R = C6HnF5−n)-,29−34 cyano
(R = NC)-,35 carbonyl (R = R′CO)-,36,37 and oxycarbonylni-
trenes (R = R′OCON),38−47 as well as the “inorganic” sulfonyl
(R = R′S(O)2)-48,49 and phosphorylnitrenes (R = (R′O)2P-
(O)).50,51 Furthermore, transition-metal-mediated amination or
amidation of C−H bonds is currently receiving significant
attention.52−62
INTRODUCTION
■
Borylnitrenes 1 are related to vinylidenes 2 due to substitution
of the CC group by the isoelectronic BN unit (Chart 1).1 While
Chart 1. Structures of Borylnitrenes 1 and Vinylidenes 2
the parent vinylidene 2a is a transient reactive intermediate,2−12
the difluoro derivative 2b can be generated photochemically
from difluoroethyne and observed directly by spectroscopy
using matrix isolation techniques. Under matrix isolation
conditions high reactivity is observed for 2b upon annealing
toward a variety of substrates, including CH4, H2, and Xe.13−18
Therefore, 2b was termed “superelectrophilic” by Sander et
al.19
Received: January 26, 2012
Published: May 3, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the Azidoboranes 3−6 Investigated in the Present Work from the Corresponding Chloroboranes 7−10
Prior to our work, Paetzold and co-workers have shown that
thermolysis and photolysis of azidoboranes generally results in
iminoboranes RBNR without the involvement of transient
borylnitrenes.63−67 It was possible to trap borylnitrenes in
intramolecular and intermolecular reactions only in the case of
the thermolysis (gas phase) or photolysis (cyclohexane
solution) of diaminoazidoboranes (R2N)2BN3 with bulky
substituents (R = i-Pr, 2,6-dimethylpiperidino).66 Efficient
insertion into the C−H bonds of solvent molecules was not
observed by Paetzold and co-workers in their photolysis
experiments with (R2N)2BN3.66 Rather, products isolated from
the photolyses were explained to form by insertion of the
borylnitrene (R2N)2BN into one of the C−N (in the case of R
= i-Pr) or B−NR2 bonds (in the case of R = 2,6-piperidino) of
an unreacted azide.66 The nitrene that formed from this azide
upon further irradiation then stabilized itself by intramolecular
reactions with the bulky organic ligands via C−H insertion or
hydrogen abstraction.66
RESULTS AND DISCUSSION
Synthesis of Azidoboranes. The azidoboranes 3−6 were
synthesized from the corresponding known chloroboranes 7−
10 by reaction with trimethylsilyl azide in dichloromethane
solution (Scheme 1). All azides 3−6 are very moisture-
sensitive, potentially explosive compounds.
Solid-State Structure of Compound 4c. Experimental
structural data of azidoboranes with three-coordinate boron
atoms are scarce.68−70 We therefore attempted to obtain single
crystals of the new azidoboranes that are suitable for X-ray
analysis, and we succeeded in the case of 4c. Single crystals of
4c were obtained by slow evaporation of the dichloromethane
solvent. The compound crystallizes in the monoclinic space
group C2. The molecule lies on a crystallographic 2-fold
rotation axis, with the azide group being disordered by
symmetry (Figure 1). The 1,3,2-diazaborolane framework is
■
The factors that govern the reactivity of borylnitrenes in
intermolecular C−H insertion reactions are uncertain at this
time, as are mechanistic details. It is not understood why the
nitrogen and oxygen derivatives investigated by Paetzold’s
group do not undergo intermolecular C−H insertion reactions.
Is the high tendency toward intermolecular C−H insertion of
1b due to the oxygen donor atoms or the chelate nature of the
ligand, or are both essential? The limited knowledge of
borylnitrene chemistry has prompted us to investigate the
reactivity of borylnitrenes in more detail. The purpose of the
present paper is to
Figure 1. Molecular structure of 4c. Symmetry-related atoms are
denoted by “A”.
(i) evaluate the potential of various borylnitrenes in
intermolecular amination reactions using cyclohexane as
a model substrate
(ii) deduce the spin multiplicity of the reactive intermediate
by measurement of the primary kinetic isotope effects in
cyclohexane solution
(iii) demonstrate that the reactivity of 1b is sufficiently high
to aminate methane in a flow reactor at room
temperature and atmospheric pressure
(iv) establish a theoretical basis for understanding borylni-
trene properties with the help of computational
chemistry techniques
essentially planar. The azido group is slightly bent (N(2)−
N(3)−N(4) = 168.6(12)°), as is typically observed for covalent
azides. The B(1)−N(2)−N(3) angle is unusually large
(144.1(6)°) in comparison to those for related azides,
presumably for steric reasons related to the bulky sulfonyl
groups.
Solid-State Structures of PinBNH(cycloalkyl). We
reported in our preliminary investigation of the photo-
decomposition of 4a in cycloalkane solvents high yields of
C−H insertion products (see Scheme 2).21 The lability of the
B−N bond of the aminoborane 11 under hydrolytic conditions
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Scheme 2. Photochemical Reaction of 4a with Hydrocarbons
and Products Obtained from Workup of Aminoborane 11
Scheme 3. Stepwise vs Concerted Mechanisms for the
Insertion of Nitrene 1b into C−H Bonds
can be exploited for the transformation of (cyclo)alkanes into
primary amines 12 or into amides 13 after acetylation of the
aminoborane (Scheme 2).21
The aminoboranes obtained from reaction with cyclopentane
(11b) and cyclohexane (11a) could be structurally charac-
terized.21 The cycloheptane reaction product 11c could in the
meantime also be crystallized, and the structure was solved
(Figure 2).
radical pair RP to the product (pathway A) or a one-step
concerted insertion via transition structure TS into the C−H
bond of the hydrocarbon substrate (pathway B).
One way of determining which of the two mechanisms is at
work is the measurement of the kinetic isotope effect (KIE),
kH/kD. A small KIE (1−2) is in agreement with a concerted
reaction, while a large KIE is indicative of a hydrogen
abstraction reaction. For example, the KIE for the C−H
insertion of (diethoxyphosphoryl)nitrene was determined from
reaction with mixtures of [H12]- and [D12]cyclohexane to be
1.02(3).51 This is clearly supportive of the involvement of a
singlet nitrene. On the other hand, a large kinetic isotope effect
(kH/kD = 14.7(3)) was measured for the efficient intra-
molecular insertion of a photolytically generated (2-(2-
phenylethyl)phenyl)nitrene into a benzylic C−H bond, and it
was concluded to involve the triplet state of the nitrene.73,74 We
measured the isotope effect for the insertion reaction of 1b
using mixtures of [H12]- and [D12]cyclohexane and arrived at a
value of 1.35, indicative of a concerted insertion reaction
involving the singlet state of 1b. Indeed, efficient intermolecular
C−H insertion reactions are the hallmark of highly reactive
singlet nitrenes.
Figure 2. Molecular structure of 11c.
Similarly to previously reported structures of pinBNH-
(R),71,72 the crystal packing of all cycloalkyl derivatives features
dimers that are bonded by two NH···O hydrogen bonds
(Figure 3). The N···O distances increase from 3.13 to 3.23 Å
Photogeneration of Borylnitrenes in Organic Glasses
and ESR Spectral Properties of Triplet Borylnitenes. We
measured the ESR spectra of the nitrenes generated photo-
chemically from the azides 3a,b, 4a,c, and 6 in organic glasses at
4 K (Table 1). The |D/hc| values that were observed between
1.48 and 1.62 cm−1 are typical for triplet nitrenes.75 The |E/hc|
values are 0 within the limited experimental accuracy of organic
glasses for compounds 4c and 6. Wentrup and co-workers have
established a linear correlation (D = 1.96673ρ − 2.0577 cm−1)
Table 1. ESR Parameters (in cm−1) Derived from Photolysis
of the Corresponding Azides in Solid Methylcyclohexane
(Unless Noted Otherwise) at 4 K and Values Computed
from the Correlation of Wentrup et al.75 Using the Spin
Density ρ on N Determined at the UB3LYP/EPR-III Level
of Theory
Figure 3. Hydrogen-bonded dimer of pinBN(H)cyclohexyl 11a and
the intramolecular N···O distance in the crystal structure.
with increasing size of the alicyclic ring. The strength of the
hydrogen bonds was computed at the SCS-RIMP2/def2-TZVP
level of theory. According to these calculations, the dimer of
11a is more stable than the monomer by 9.7 kcal mol−1.
Primary Kinetic Isotope Effect for CH Insertion of 1b.
In principle, two fundamentally different mechanisms are
conceivable for the amination of hydrocarbons by nitrenes
(Scheme 3): abstraction of a hydrogen atom from the
hydrocarbon substrate and rapid collapse of the resulting
b
azide
|D/hc|
|E/hc|
|D/hc| (computed)
3a
3b
4a
4c
6
1.505
1.483
1.573
0.0038
0.0027
0.0047
0
1.456
1.429
1.626
a
1.620
a
1.602
0
a
b
Measured in 2-methyltetrahydrofuran. Using D = 1.96673ρ −
2.0577 cm−1.
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between the spin density ρ on the nitrogen center computed at
the UB3LYP/EPR-III level and the D values deduced from the
spectra, including borylnitrene 1a.75 We observe qualitative
agreement between experiment and the correlation of Wentrup
et al.75 for compounds 3a,b and 4a, but we cannot compute the
spin densities for the nitrene derived from 4c, as the
corresponding basis set for sulfur is not available. Due to the
complicated electronic structure arising from the possible
formation of two nitrene centers from 6, UB3LYP
computations were not performed for this system.
Scheme 5. Suggested Formation of 14 from 5
Photolysis of Azidoboranes 3−6 in Hydrocarbon
Solvents. To evaluate the influence of the substituents at
boron on the ability of borylnitrenes generated photolytically
from azidoboranes to insert into unactivated C−H bonds of
hydrocarbons, the following procedure was typically applied.
The mixtures that were obtained from photolysis of
azidoboranes in cyclohexane solution using a 254 nm light
source were either reacted with alcohols (2-propanol or ethanol
depending on the substituent at boron) or were treated with
DMAP/AcCl followed by solid NaOH, as described previously
(see Scheme 2).21 The solutions thus obtained were analyzed
by GC-MS and GC after addition of external standards. In a
few cases hydrocarbons other than cyclohexane were used.
We have demonstrated previously that the pinacol derivative
4a inserts into a C−H bond of cyclohexane with a yield of
85%.21 To learn if the chelate nature of 4a is essential for
intermolecular C−H insertion, the acyclic derivative 5 was
investigated. Photolysis indeed also produced cyclohexylamine
after workup, but the yield was reduced considerably to 46%. As
an additional product, the amino alcohol 14 was obtained in
15−20% yield (Scheme 4).
thermolysis of 4b at 500 °C was investigated previously by
Paetzold et al.66 In addition to unreacted 4b, an HN3 adduct of
4b was identified as a reaction product.66 Photolysis of 4b
produced a dark solution that was treated with 2-propanol.
Among the many unidentified products, only trace amounts of
cyclohexylamine, B(OiPr)3, and bicyclohexyl were detected.
The additional product with m/z 103 that can be detected by
GC-MS may be due to amines 18−20, which can form by
intramolecular (18 or 20) or intermolecular (19) insertion
reactions (Chart 2). Note that photolysis of diaminoazidobor-
Chart 2. Possible Amines 18−20 with m/z 103 Formed
during Photolysis of 4b and Workup
Scheme 4. Photolysis of 5a,b in Cyclohexane Solution
anes (R2N)2BN3 with bulky substituents yields dimers by
insertion into the C−N or B−N bonds of an unreacted azide
molecule according to Paetzold et al. (vide infra).66
Neither 4c nor 4d is very soluble in cyclohexane. Photolysis
of suspensions of 4c,d followed by alcoholysis with 2-propanol
does not give any indication for intermolecular insertion into a
solvent C−H bond.
Formation of 14 is most readily explained by an intra-
molecular insertion into the β-C−H bond of the alkoxy
substituent and cleavage of the resulting five-membered
intermediate 15 by the alcohol. Insertion into an α-C−H
bond would yield the four-membered ring 16, which should
produce acetone and ammonia via hemiaminal 17 upon
alcoholysis (Scheme 5). Despite careful searching, no products
expected from α-C−H insertion could be detected. Also, the
acetylation of primary photoproducts did not give any
indication for α-C−H insertion. Formation of the less strained
five-membered-ring species 15 is obviously favored.
The photolysis of (CH3O)2BN3 in cyclohexane yields a
colorless insoluble solid that can be degraded to hydrazine
under basic conditions and to dinitrogen under acidic
conditions according to Paetzold and Maier.65 Thermolysis of
(CH3O)2BN3 in a closed tube, on the other hand, was reported
to yield methoxyamine after saponification.65 We were not able
to detect any hydrazine derivatives or alkoxyamines from the
photolysis of 5.
We next consider azidoboranes 3 and 6 of the catechol type
to investigate the influence of an aromatic ring on borylnitrene
reactivity. Photolysis of the catechol derivative 3a under the
conditions of matrix isolation (i.e., solid argon at 10 K) yields
the corresponding nitrene 1a in its triplet ground state. This
can photochemically react with methane by C−H insertion.21
In cyclohexane solution, on the other hand, no product of
intermolecular C−H insertion could be identified. Irradiation of
3a in cyclohexane results in a yellow-brown solution and
formation of a resin. Isolation of this resin indicates that it does
not include any unreacted azide, as the typical azide band
cannot be detected by IR spectroscopy. The resin is rather
inert: treatment with alcohol, acid, or base at elevated
temperatures or attempted acetylation did not give any
indication for the formation of cyclohexylamine or amide.
From the supernatant obtained after the photolysis a small
amount of yellow solid could be isolated. Its alcoholysis or
acetylation similarly did not produce any indication for the
formation of functionalized cyclohexane. Compounds identified
by GC-MS were B(OiPr)3, 1,2-bis(acetyloxy)benzene, and
bicyclohexyl, but acetamide or acetylated hydrazines were not
We next consider azides of the chelate type with nitrogen or
sulfur bound to boron: i.e., the 2-azido-1,3,2-diazaborolanes
(4b,c) and 2-azido-1,3,2-dithiaborolane (4d). The gas-phase
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detected. Photolysis of the tert-butylcatechol derivative 3b was
similarly disappointing, as no indication for cyclohexane
functionalization could be found. Running similar experiments
in cyclooctane also produced negative results for 3a,b.
well as enzymatic systems can activate simple hydrocarbons,
including methane.81−84 However, as the primary derivatization
product generally has weaker C−H bonds than methane itself, a
further complication lies in achieving the selective trans-
formation of a single C−H bond of methane. Much success
in methane transformation has been achieved in the field of
transition-metal-mediated C−H functionalization chemistry
over the years.56,85−90
Bubbling dry methane through a pure sample of liquid 4a at
room temperature and atmospheric pressure yields a gas
mixture that is passed through the quartz tube and continuously
photolyzed using the output of the low-pressure mercury lamp
(see Figure 4). Formation of a solid product from the gaseous
As described above, formation of resins was observed earlier
in the photochemistry of dialkoxyazidoboranes.65 It is none-
theless puzzling that the photodecomposition of 3a does not
produce cyclohexylamine derivatives. As the C−H insertion
works even with CH4 under cryogenic conditions, it may be
speculated that the observations in solution are due to the
instability of a possible aminoborane photoproduct under the
reaction conditions. It is known that the expected primary
photoproducts, CatBNHR (R = Me, Et, iPr), are oligomers (R
= Me), dimers (R = Et), or mixtures of dimers and monomers
(R = iPr).76 Indeed, different properties of Bcat and Bpin have
been noted and discussed previously.72,77,78 Attempts to
investigate the photochemistry of independently synthesized
CatBNHCy (Cy = cyclohexyl) failed, as we could not obtain
the compound in a pure state from the reaction of CatBCl and
cyclohexylamine in the presence of an amine base under a
variety of experimental conditions.
Considering that detection of functionalized cyclohexane was
not possible in the photochemistry of 3a,b in cyclohexane
solution, it is surprising that the diazide 6 produces cyclohexyl-
amine in 10−15% yield per azide group. Whether this result is
due to the higher stability of the aminoborane expected to form
by C−H insertion is unclear at this time.
Finally, we consider the photochemistry of 3c. Due to its
insolubility in cyclohexane the photodecomposition was
performed in benzene. After alcoholysis or acetylation of the
crude product mixture that contained insoluble resin-like
materials, aniline or N-phenylacetamide was observed (GC-
MS) in 11% or 15% yield. The mechanism of product
formation is unclear, but Abramovitch et al.79 concluded that
arylnitrenes form adducts with benzene by interaction with its π
system and that these adducts form aniline derivatives in dark
reactions.31 Due to the low yield of C−H functionalized
products, the reaction was not investigated further.
In summary, the pinacol system 4a is most efficient in the
intermolecular insertion reaction into a C−H bond of
cyclohexane solvent. The acyclic alkoxy compound 5 is less
efficient, due to substantial intramolecular insertion reaction.
While the nitrogen- and sulfur-substituted systems 4b−d as
well as the aromatic compounds 3a,b give at most trace
amounts of intermolecular insertion products, the diazide 6
performs better, for unknown reasons. Photodecomposition of
3c in benzene yields aniline, but the yield of functionalized
benzene is lower than that observed for pentafluorophenylni-
trene.31
Photolysis of 4a in Methane Gas at Room Temper-
ature. The screening of a number of borylnitrenes toward the
ability to insert into C−H bonds identified 1b as the most
efficient one. We were therefore intrigued by the question if
C−H transformation of methane is possible with 1b under
conventional laboratory conditions, especially in view of the
limited examples of nitrenes inserting into C−H bonds of
methane.80 Methane is a particularly challenging alkane to
functionalize. The low reactivity of methane is reflected by its
high first C−H bond dissociation energy (438.8 kJ/mol), high
ionization potential (12.5 eV), low proton affinity (4.4 eV), low
acidity (pKa = 48), and a large HOMO−LUMO gap.81,82 The
first step of any methane derivatization process requires the
activation of an “inert” C−H bond. Highly reactive species as
Figure 4. Schematic setup of the flow system used for the photolytic
gas-phase transformation of methane into methylamine.
reactants is observed in the exposition zone and behind it
during photolysis. Multinuclear NMR (1H, 13C, and 11B)
investigations of the crude reaction products indicate the
formation of the desired methane derivative PinBNHMe (11d)
along with byproduct. The 11B shift at 24.4 ppm is in the typical
region of aminoboranes of type 11 and is in good agreement
with our previous results for cycloalkane derivatization.21 The
pinacol Me groups (δ(1H) 1.22; δ(13C) 24.8) and NMe (δ(1H)
2.51; δ(13C) 23.3) are also observable by NMR of the crude
products. The desired product 11d cannot be separated from
the byproduct by crystallization or sublimation, due to its
extreme air and moisture sensitivity. Nevertheless, the expected
monomethylamine CH3NH2 can be detected by GC analyses
after hydrolysis or alcoholysis with 2-propanol, along with
ammonia (NH3) and hydrazine (H2NNH2). The photolysis
reaction was also conducted under neat argon gas at room
temperature. Under these reaction conditions no methylamine
was detected after workup. Thus, the intramolecular rearrange-
ment of a methyl group from the pinacol ligand to the nitrene
center in 1b can be excluded as a possible source of
methylamine.
Quantification of the reaction yield using GC-MS and GC on
the basis of authentic samples could be achieved by acetylation
of the crude aminoborane (Scheme 1). For this purpose the
photolysis reaction was carried out until the azide 4a was
completely consumed. The amount of unreacted azide 4a in the
product mixture after photolysis could not be determined in
our experiments, as it cannot be detected by GC due to
decomposition. The reaction products were dissolved in diethyl
ether and reacted with acetyl chloride (CH3COCl) in the
presence of N,N-dimethylaminopyridine (DMAP). The
amount of derivatization product 13 (R = CH3) was
determined after addition of an internal standard by GC to
be 6−8% based on consumed 4a, assuming quantitative
acetylation. In addition, small amounts of AcNH2 (via H
abstraction) and the hydrazine derivatives AcNHNH2 (via H
abstraction/dimerization) were obtained.
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Table 4. Energy (in kcal mol−1) of the Closed-Shell Singlet
State of Unsaturated Cyclic Borylnitrenes and Their
On the basis of the KIE measurements in solution described
above, it is expected that the singlet nitrene is responsible for
the C−H insertion reaction also in the gas phase. The yield of
C−H insertion product then depends decisively on the
unknown lifetime of the singlet nitrene and the length of the
mean free path. In the gas phase the mean free path is much
longer than in the condensed phase, resulting in the “loss” of
some singlet nitrene by formation of triplet nitrene. Indeed, the
formation of AcNH2 and AcNHNH2 can be ascribed to the
involvement of the triplet nitrene. In addition, the unknown
quantum yield of the photochemical nitrene generation has a
decisive influence on the overall reaction yield. It may be
expected that reducing the mean free path by increasing the
methane pressure will increase the yield further.
Theoretical Investigation of Borylnitrene Structures
and Singlet−Triplet Energy Gaps. In order to better
understand the properties of borylnitrenes and their involve-
ment in the thermal or photochemical decomposition of
azidoboranes, we investigated the influence of substituents on
the borylnitrenes using computational chemistry techniques
(see Tables 2−4 for energy data). We chose the complete basis
Corresponding Cyclic Iminoboranes Relative to the Lowest
Energy Triplet State (3A2) of the Respective Borylnitrene, As
c
Computed at the CBS-QB3 Level of Theory
a
Using int(grid=99590) in B3LYP geometry and vibrational frequency
computation. The default integration grid results in Nimag = 1 (9i
b
cm−1). Strongly distorted with a long N−C distance (1.782 Å); see
c
also ref 132. A negative sign indicates that the singlet is lower in
energy than the triplet state. The triplet corresponds to a minimum in
all cases, while the singlet can be a higher order stationary point, as
d
given in brackets. See also ref 133.
higher level single energy points up to the CCSD(T) level to
extrapolate to a complete basis set estimate of the energy.
For the parent borylnitrene, H2BN, of C2v symmetry the
3
open-shell configuration ...(1b1)1(2b2)1 can give rise to the A2
1
and A2 states, while the ...(1b1)2(2b2)0 configuration gives rise
Table 2. Energy (in kcal mol−1) of the Singlet States of X2BN
1
1
(1A1 in C2v and A in C1) and of XBNX Relative to the
to the closed-shell A1 state. It was shown previously that the
20
3
1
1A1 state is lower in energy than the A2 and A2 states.
Lowest Energy Triplet State (3A2) of X2BN, As Computed at
a
1
the CBS-QB3 Level of Theory
However, the A1 singlet H2BN corresponds to a first-order
saddle point, and releasing the symmetry constraint during
geometry optimization results in linear iminoborane,
HBNH.20,93 A similar instability of singlet X2BN toward
rearrangement to XBNX is computed for X = CH3. Thus
singlet dialkylboryl nitrenes do not correspond to reactive
intermediates and should not be observable directly or
indirectly by trapping experiments. This conclusion is in
agreement with observations made in the decomposition of
dialkyl and diaryl boron azides, where borylnitrene-derived
products have never been observed.
X
H
CH3
NH2
OH
F
X2BN, C2v −11.0 [1] −2.9 [1]
+31.4 [3] +27.2 [2] +25.7 [0]
X2BN, C1
not exist.
not exist. −0.4
−71.5 −2.1
+22.0
+24.2
not exist.
+47.0
b
XBNX
−89.0
a
A negative sign indicates that the singlet is lower in energy than the
triplet state. The triplet X2BN and singlet XBNX correspond to
minima in all cases, while the C2v-X2BN singlet can be a higher order
b
stationary point, as given in brackets. See also ref 130.
In contrast, the singlet difluoro derivative F2BN corresponds
to a minimum in C2v symmetry and is thus stable with respect
to rearrangement. The energetic ordering of singlet and triplet
states changes by fluorine substitution: the triplet state is lower
set extrapolation scheme CBS-QB3 of Petersson et al.91,92 This
technique employs B3LYP geometry optimization and
harmonic vibrational frequencies along with a number of
Table 3. Energy (in kcal mol−1) of the Closed-Shell Singlet State of Cyclic C2H4X2BN, C2H2X2BN, and Their Corresponding
Cyclic Iminoboranes Relative to the Lowest Energy Triplet State (3A2 Unless Noted Otherwise) of the Respective Borylnitrene,
d
As Computed at the CBS-QB3 Level of Theory
a
b
c
d
C2 symmetry for singlet and triplet (3A). C1 symmetry for singlet. C2 symmetry for singlet. A negative sign indicates that the singlet is lower in
energy than the triplet state. The triplet corresponds to a minimum in all cases, while the singlet can be a higher order stationary point, as given in
e
brackets. See also ref 131.
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in energy than the singlet state, indicating that the fluorine
atoms increase the electron density in the boron p orbital.
While the rearrangement from X2BN to XBNX is a highly
exothermic process for X = H, CH3, it is endothermic for X = F
(Table 2).
Carbocyclic singlet borylnitrenes do not correspond to
minima (see Tables 3 and 4), similar to acyclic (CH3)2BN.
They should thus not be trappable and transform to the
corresponding cyclic iminoboranes, in agreement with the
observations of Paetzold and co-workers.97 An exception is the
B-nitrenoborirene (Table 4). This has a singlet ground state,
and its rearrangement is only mildly exothermic. These
properties would make it a particularly attractive borylnitrene
for C−H insertion. Its synthesis, however, may be challenging.
The oxygen- or nitrogen-containing heterocyclic singlet
borylnitrenes are minima (Table 3). The 2-nitreno-1,3,2-
diazaborolane is exceptional: similar to the case for the acyclic
(H2N)2BN it prefers a distorted structure with an intra-
molecular stabilization of the nitrene center by one of the
nitrogen lone pairs (see Figure 5). Geometry optimization at
the CCSD(T)/TZ2P level of theory confirms the existence of
this minimum on the PES. The corresponding iminoboranes do
not exist as stationary points on the potential energy surface in
case of X = O, while their formation from the borylnitrene is
endothermic for X = NH. The triplet states, however, are lower
in energy than the singlets and therefore intersystem crossing is
in principle a reaction channel that can compete with singlet
insertion reactions.
Of particular interest are the diamino and dihydroxy
borylnitrenes. Their C2v-symmetric structures are higher order
stationary points on the singlet state potential energy surface.
The presence of donor groups (OH, NH2, as well as F
discussed above) destabilizes these singlet states such that the
triplet nitrenes are significantly lower in energy. As discussed
previously in the context of 3a,20 donors increase the electron
density in the boron π orbital and thus reduce its Lewis acidity
and its ability to accept π electrons from nitrogen. This results
in a destabilization of the singlet relative to the triplet state.
Relaxing the symmetry constraint results in asymmetric
structures of the singlet nitrenes with one “bridging” XH (X
= O, NH) group (Figure 5).
CONCLUSIONS
■
The combined experimental and computational investigation of
azidoborane photolysis in (cyclo)alkanes allows drawing the
following conclusions.
Figure 5. Structures of (HO)2BN, (H2N)2BN, and C2H4(NH)2BN as
computed at the B3LYP/6-311G(2d,d,p) and CCSD(T)/TZ2P (in
italics) levels of theory. Distances between non-hydrogen atoms are
given in Å; the angles are given in degrees.
1. The small kinetic isotope effect measured for the efficient
intermolecular insertion of borylnitrene 1b into a C−H
bond of cyclohexane is in agreement with a concerted
reaction involving the singlet state of the nitrene.
2. The substituents at the boron atoms have a significant
influence on the intermolecular C−H insertion chem-
istry. The pinacolato group is the most efficient and
results in efficient C−H insertion. This chelate precludes
the intramolecular C−H insertion that is an important
side reaction in the closely related acyclic dialkoxy
derivative. No or at most low yields of intermolecular C−
H insertion products are obtained for the cyclic aza
(4b,c) or thia (4d) as well as for the catecholato
derivatives (3). In particular, the failure of this last class
of borylnitrenes to efficiently produce C−H insertion
products in solution is surprising in view of the behavior
observed for 3a earlier in matrix isolation studies. On the
other hand, the 1,2,4,5-tetrahydroxybenzene-derived
diazide 6 yields intermolecular C−H insertion products
in modest yields.
3. The computations for acyclic and cyclic carbon
substituted singlet borylnitrenes show that these, with
the exception of experimentally unknown nitrenoborir-
ene, do not correspond to minima, in agreement with
numerous experimental studies. Heteroatom substitution
(NR2, OR, F) turns the singlet borylnitrenes into minima
and thus in principle they are experimentally observable,
in agreement with previous trapping and matrix isolation
studies. However, triplet states are lower than or very
close in energy to the singlet states and thus triplet state
chemistry, as well as rearrangement to acyclic iminobor-
anes, may compete with C−H insertion of the singlet
state.
As the CBS-QB3 model relies on B3LYP geometries, we also
performed CCSD(T)/TZ2P geometry optimizations to con-
firm the existence of these stationary points on the PES. The
structure of (HO)2BN is characterized by a small O2−B−N
angle (79.1°) and a short distance (1.800 Å) between O2 and
N1. In (H2N)2BN, the corresponding angle and distance are
even smaller (68.1° and 1.614 Å). The structures of these
borylnitrenes are reminiscent of that reported for formylni-
trene, HC(O)N, which is intermediate between nitrene and
oxazirene.94−96 The dative in-plane interaction between an
oxygen lone pair and a vacant orbital at nitrogen provides the
driving force for the distortion of formylnitrene and the
stabilization of the singlet relative to the triplet state.94−96
Likewise, the distortion of (HO)2BN and (H2N)2BN are
indicative of dative interactions between the boron-bound
donor groups and the nitrene nitrogen atom. These
interactions are stabilizing the singlet relative to the triplet
state. While the triplet remains the ground state for (HO)2BN,
the singlet and triplet states are almost isoenergetic for
(H2N)2BN (Table 2).
The existence of these distorted borylnitrenes as minima on
the potential energy surface should make singlet (HO)2BN and
(H2N)2BN in principle observable. Their rearrangements to the
iminoboranes are close to thermoneutral, making the trapping
of the borylnitrene competitive. The results of the computa-
tions are in agreement with the report by Paetzold et al.66 for
photochemical decomposition of (R2N)2BN3, iminoborane
formation in case of (CH3O)2BN3 thermolysis, and with our
results for 5a,b described above.
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6: product washed with benzene and pentane; yield 1.53 g (95%) of
4. The observations under matrix isolation conditions and
in solution and the possible unproductive escape of the
singlet borylnitrene to the triplet state have prompted us
to investigate the reaction in the gas phase. Photolysis of
a mixture of azidoborane 4a and methane in a flow
reactor under atmospheric pressure results in a 6−8%
yield of intermolecular C−H insertion based on
consumed 4a. While this yield is low, the observation
of intermolecular C−H insertion products at all
demonstrates the high reactivity of 1b.
1
a faintly yellow solid. H NMR (CDCl3): δ 7.14 (s, 2 H). 13C{1H}
NMR (CDCl3): δ 98.8 (CH), 143.8 (OC). 11B{1H} NMR (CDCl3): δ
25.1. IR (KBr): 2149 cm−1.
Gas-Phase Functionalization of Methane. Methane (4.5) was
passed through two glass coils (d = 0.5 cm, l = 6 m) immersed in baths
held at −78 and 30 °C, and two drying columns loaded with molecular
sieves and silica gel in order to remove traces of water. Dry methane
was bubbled through a neat sample of liquid PinBN3 (4a; 3.00 g, 17.75
mmol) at room temperature and atmospheric pressure. The obtained
gas mixture was passed through a quartz tube (d = 1.7 cm, l = 55 cm)
and continuously photolyzed (36−48 h) using the output of a low-
pressure mercury lamp (length of photolysis zone approximately 25
cm). Formation of a solid product was observed in the exposition zone
and behind it during photolysis. After photolysis the apparatus was
flushed with argon and the photoproduct was removed by dissolution
in dry CH2Cl2 and washed into a Schlenk tube. After removal of the
CH2Cl2 in vacuo and dissolution in 15 mL of dry Et2O, the catalyst
DMAP (N,N-dimethylaminopyridine) and acetyl chloride (AcCl; 0.60
mL, 8.436 mmol) were added. The resulting suspension was stirred for
18 h at room temperature. Then solid NaOH (0.50 g, 12.50 mmol)
was added, and the reaction mixture was stirred for one more day.
Afterward cycloheptylamine (internal standard; 0.501 g, 1.7752 mmol)
in 15 mL of Et2O was added, the mixture was filtered, and the solid
was washed with Et2O (3 × 8 mL). The filtrate was separated, and the
mixture was analyzed by GC-MS and GC.
Single-Crystal X-ray Analysis. The X-ray intensity data for 4c
and 11c were measured with graphite-monochromated Mo Kα
radiation on an Oxford Diffraction Xcalibur2 diffractometer with a
Sapphire2 CCD. The CrysAlisPro software113 was used for the data
collection and data reduction. A semiempirical absorption correction
based on multiple-scanned reflections was applied.114 All crystal
structures were solved by direct methods using SHELXS-97 and
refined with SHELXL-97.115
Theory. The borylnitrenes were computed using the complete
basis set CBS-QB3 extrapolation scheme of Petersson et al.91,92 as
implemented in the Gaussian 09116 program. The CBS-QB3 scheme
employs geometry optimizations and harmonic vibrational frequency
computations using Becke’s117 three-parameter hybrid functional in
conjunction with the correlation functional of Lee, Yang, and Parr118
(B3LYP) along with higher level energy evaluations to extrapolate to
the complete basis set energy. For comparison, the geometries of the
three borylnitrenes given in Figure 5 were also computed using
coupled cluster theory with single, double, and a perturbative estimate
of triple excitations, CCSD(T),119,120 using analytic gradient
techniques.121 All electrons were considered in the correlation
treatment. The basis set employed was Dunning’s122 triple-ζ basis
set (10s6p contracted to 5s3p) with two sets of polarization
functions.123 The CCSD(T) calculations were performed with the
CFOUR program.124
EXPERIMENTAL AND THEORETICAL SECTION
■
General Considerations. All manipulations were carried out
under a dry argon atmosphere in oven-dried Schlenk type glassware or
in a glovebox. Solvents and substrates were dried before use
(cycloalkanes, Na; CH2Cl2, CaH2) and stored over activated molecular
sieves (3 Å). The chloroboranes 7b,98,99 7c,100 8b,101−103 8c,104,105
8d,106−108 9a,b,109,110 and 10111 and the azides 3a,68 4a,21 and 4b66
were prepared according to the literature. 1H and 13C{1H} NMR
spectra were recorded on a Bruker DRX 200 and 11B NMR spectra on
1
a Bruker DPX 250 spectrometer. H (200 MHz) and 13C (50 MHz)
chemical shifts are relative to tetramethylsilane, and 11B (80 MHz)
chemical shifts are relative to BF3·OEt2. GC/MS analyses used helium
gas and a Zebron ZB-5mS (Phenomenex) capillary column (length 30
m, i.d. 0.25 mm, film thickness 0.25 μm). GC analyses used hydrogen
gas and a CP-Wax 51 Amin FS (Varian) capillary column (length 25
m, i.d. 0.32 mm, film thickness 1.20 μm). The yields of the products
determined by GC are based on cycloheptylamine as an internal
standard and are response-corrected on the basis of authentic samples
where available. IR spectra were measured on a Bruker Equinox 55
spectrometer. EPR spectra were recorded on a Bruker Elexsys E500
spectrometer. The computer simulations of the EPR spectra were
performed by using the XSophe computer simulation software suite
(version 1.0.4),112 developed by the Centre for Magnetic Resonance
and Department of Mathematics, University of Queensland, Brisbane,
Australia, and Bruker Analytik GmbH, Rheinstetten, Germany.
Irradiations were carried out with a Grantzel low-pressure mercury
̈
lamp (254 nm).
General Procedure for Synthesis of Azides. To a solution of
the chloroborane in dichloromethane (20 mL per 1 g of chloroborane)
was added trimethylsilyl azide (1.0−1.5 equiv) at −80 °C. After slow
warming to room temperature overnight, the volatiles were removed
under reduced pressure. The azides were purified by recrystallization
or condensation and stored under argon in a refrigerator.
1
3b: recrystallization from hexane; yield 2.18 g (77%). H NMR
(CD2Cl2): δ 1.4 (s, 9 H, C(CH3)3), 7.1−7.3 (m, 3 H, ArH). 13C{1H}
NMR (CD2Cl2): δ 31.7 (ArC(CH3)3), 35.1 (ArC(CH3)3), 109.9
(ArC), 111.8 (ArC), 120.0 (ArC), 145.7 (CC(CH3)3), 147.2
(ArCOBN3)), 147.9 (ArCOBN3). 11B{1H} NMR (CD2Cl2): δ 26.2.
IR (NaBr): 2160 cm−1.
The hydrogen-bonded dimer of 13a was computed using second-
order Møller−Plesset (MP2) perturbation theory within the frozen
core approximation. The spin component scaling technique,
introduced by Grimme125 and known to improve MP2 energies, was
used. The resolution of the identity (RI) approximation was also
employed,126 as it reduces the time required for the MP2 treatment
while introducing only negligible errors. The def2-TZVP basis set was
employed, along with the recommended fitting basis set required for
the RI approximation.127 The SCS-MP2 computations were
performed with the Turbomole program.128,129
1
3c: product washed with pentane; yield 1.33 g (90%). H NMR
(CD2Cl2): δ 7.38 ppm (m, 2 H, ArH), 7.49 (s, 2 H, ArH), 7.75 (m, 2
H, ArH). 13C{1H} NMR (CD2Cl2): δ 108.9 (ArC), 125.4 (ArC), 127.8
(ArC), 130.4 (ArC), 147.4 (ArCOBN3). 11B{1H} NMR (CD2Cl2): δ
25.2.
1
4c: recrystallization from dichloromethane; yield 5.78 g (76%). H
NMR (CD2Cl2): δ 2.44 (s, 6 H, ArCH3), 3.54 (s, 4 H, CH2), 7.37 (m,
4 H, ArH), 7.75 (m, 4 H, ArH). 13C{1H} NMR (CD2Cl2): δ 21.1
(ArCH3), 44.6 (CH2), 126.8 (ArC), 129.7 (ArC), 135.5 (ArC), 144.6
(ArC). 11B{1H} NMR (CDCl3): δ 21.4. IR (Nujol): 2148 cm−1.
4d: crude product was used; yield 3.55 g (94%) of a slightly yellow
solid. 1H NMR (CDCl3): δ 3.47 (s, 4 H, CH2). 13C{1H} NMR
(CDCl3): δ 37.3 ppm (CH2). 11B{1H} NMR (CDCl3): δ 65.6.
5: no purification necessary; yield 8.32 g (96%) of a colorless solid.
1H NMR (CDCl3): δ 1.34 (d, 6.2 Hz, 12 H, OCH(CH3)2), 4.66 (sep,
6.2 Hz, 2 H, OCH(CH3)2). 13C{1H} NMR (CDCl3): δ 24.7
(OCH(CH3)2), 69.8 (OCH(CH3)2). 11B{1H} NMR (CDCl3): δ
20.5. IR (Nujol): 2151 cm−1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text giving the complete refs 113 and 121, tables giving
Cartesian coordinates and electronic energies for all computed
species, and CIF files giving X-ray crystallographic data for 4c,
11c, and tetrachlorocatechol trihydrate. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Article
(33) Young, M. J. T.; Platz, M. S. J. Org. Chem. 1991, 56, 6403.
AUTHOR INFORMATION
Corresponding Author
*E-mail: holger.bettinger@uni-tuebingen.de.
Notes
■
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the DFG and the Fonds der
Chemischen Industrie. We thank Dr. Gotz Bucher and Prof. Dr.
■
̈
Wolfram Sander for stimulating discussions.
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