Frustrated lewis pair chemistry of chiral (+)-camphor-based aminoboranes

Article abstract of DOI:10.1002/chem.201300462

Dimethylamino-(+)-camphorenamine reacted with an equimolar amount of Piers' borane, HB(C₆F₅)₂, to give the corresponding iminium-hydroborate zwitterionic salt. Being in equilibrium with the parent enamine-HB(C₆F₅)₂ N-B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine-HB(C ₆F₅)₂ adduct. When the starting enamine is present in excess over HB(C₆F₅)₂, the produced bornylamine-HB(C₆F₅)₂ adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine- HB(C ₆F₅)₂ pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion. Copyright

Full text of DOI:10.1002/chem.201300462

DOI: 10.1002/chem.201300462
Frustrated Lewis Pair Chemistry of Chiral (+)-Camphor-Based
Aminoboranes
Markus Lindqvist, Kirill Axenov, Martin Nieger, Minna Rꢀisꢀnen, Markku Leskelꢀ, and
Timo Repo*[a]
Abstract:
Dimethylamino-(+)-cam-
group in the molecule. Detailed studies
revealed that the hydrogen splitting in
this reaction proceeded through an in-
bornyl
the starting enamine is present in
excess over HB(C6F5)2, the produced
bornylamine–HB(C6F5)2 adduct breaks
ACHUTGTNRNEaNUG mine–HBACHTUNGTREN(NNGU C6F5)2 adduct. When
phorenamine reacted with an equimo-
lar
amount
of
Piersꢀ
borane,
ACHTUNGTRENNUNG
HB
A
termolecular pathway leading to
a
A
ACHTUNGTRENNUNG
iminium–hydroborate zwitterionic salt.
Being in equilibrium with the parent
up, eliminating free bornylamine and
forming the initial camphorenamine–
HBACHTUGNTREN(NUG C6F5)2 pair. This results in hydro-
genation of the camphorenamine
framework in a catalytic fashion.
Keywords: boron · frustrated Lewis
pairs · hydrogenation · nitrogen ·
reaction mechanism
enamine–HB
was able to split hydrogen heterolyt-
ACHTUNGTRENNUNGically, hydrogenating the iminium
(C6F5)2 N–B pair, this salt
Introduction
linked aminoborane system facilitates hydrogen splitting
and catalytic hydrogenation reactions.[19] In such ansa-
Frustrated Lewis pairs (FLP) are combinations of sterically
demanding strong Lewis acids and Lewis bases that are not
able to form classical adducts.[1] Lewis acidic and basic cen-
ters in FLPs have unquenched reactivity that can be used in
activation and splitting of small molecules, for example,
CO2,[2] carbonyl compounds,[3] unsaturated hydrocarbons,[4]
N2O,[5] and many others.[6] FLPs are able to split hydrogen
heterolytically,[7] opening a way to catalytic hydrogenation
of unsaturated organic substrates in the absence of metals.[8]
Most of the FLPs studied so far employ substituted boranes
and boryl groups as Lewis acidic counterparts.[1,9] From
aminoACTHGNUTERNNUbG oranes, the steric size, basicity and nucleophilicity of
the nitrogen counterpart can be tuned in such a way that hy-
drogen splitting takes place very fast and reversibly so that
the ammonium–borate species can catalyze hydrogenations
of a large variety of unsaturated substrates, regardless of
their steric hindrance.[20] Recently, it has been shown that
o-aminoborylphenylenes also split hydrogen, even at room
temperature.[21] Alternatively, 1,2-aminoboranes with amino
and boron centers in neighboring positions can be prepared
by selective syn hydroboration of enamines. Such N–B sys-
tems often decompose, releasing olefins and aminoboranes
(Scheme 1).[22] However, Erker and co-workers recently syn-
thesized stable 1,2-aminoboranes from enamines and
[10]
[11]
those, BACHTUNGTRENNUNG(C6F5)3 and HBAHCTUNGTERN(NUGN C6F5)2 are particularly impor-
tant. Conversely, Lewis basic centers have been shown to be
varied more readily without compromising their reactivity.
Among a huge variety of suitable Lewis bases, only phos-
phines[12] and amines[13] have undeniably displayed the most
versatility and usability for hydrogen activation. In addition,
splitting of hydrogen has also been performed with FLPs
based on Lewis basic carbenes,[14] compounds containing
low-valent carbon,[15] or carbonyl groups,[16] among others.[1]
HBACHTNURGTNE(NUG C6F5)2 linked through alkyl or cycloalkyl back-
Combinations of bulky amines and BACHTNURGTNEUNG(C6F5)3 have been
shown to split hydrogen reversibly,[17,18] and it has been dem-
onstrated that combining nitrogen and boron centers in one
[a] M. Lindqvist, Dr. K. Axenov, Dr. M. Nieger, Dr. M. Rꢁisꢁnen,
Prof. M. Leskelꢁ, Prof. T. Repo
Department of Chemistry
Laboratory of Inorganic Chemistry
University of Helsinki
P.O. Box 55, 00014 (Finland)
Fax : (+358)919150198
E-mail: timo.repo@helsinki.fi
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300462.
Scheme 1. Hydroboration of enamines with subsequent deboronation.
Examples of stable 1,2-aminoboranes.
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FULL PAPER
ACHTUNGTRENNUNG
bones.[23,24b] Most of the intramolecular B–N adducts formed
were able to activate hydrogen and even worked as catalysts
for reduction of enamine substrates. It is proposed that the
hydrogen splitting process is thermodynamically permitted
in this situation because of release of internal strain caused
by formation of a four-membered ring.[21,24b]
Stereoselective (induced by an optically active substrate)
syn hydroboration of chiral enamines could lead to the re-
spective enantiopure 1,2-aminoboranes. This offers the pos-
sibility of metal-free catalytic enantioselective hydrogena-
tion of organic compounds by chiral nitrogen–boron based
FLPs.[25] Inspired by these thoughts, we studied reactions be-
tween HBACHTUNGTRENNUNG(C6F5)2 and chiral enamines containing a (1R)-
(+)-camphor backbone. During the preparation of the pres-
ent work, Erker and co-workers published parallel studies
of a piperidine-substituted (1R)-(+)-camphor-1,2-aminobo-
ACHTUNGTRENNUNG
rane,[26] which was found to be in equilibrium between the
Scheme 3. Formation of the zwitterionic iminium–borohydrides 8 and 8’.
1,2-aminoborane and the zwitterionic iminium–hydroborate
forms. Our experiments confirm this. Moreover, as shown
here, the chemical behavior of these (1R)-(+)-camphorami-
noboranes can be manipulated by variation of substituents
at the nitrogen center. We report herein hydrogen splitting
reactions provided by a FLP of dimethyl-(1R)-(+)-camphor-
peaks for the diastereotopic methyls of the NMe2 groups
(dH =3.56/3.33 and 3.48/3.03 ppm) and for the methyl groups
of the camphor bridge (dH =0.77/0.73 and 0.94/0.90 ppm),
respectively. The 13C{1H} NMR spectra of 8a and 8’a fea-
tured signals at dC =215.7 and 216.6 ppm (13C{1H} NMR,
75 MHz, [D6]benzene, 278C), characteristic for iminium
carbon atoms (C=NMe2). Thus, the produced stereoisomers
8a and 8’a can be identified as zwitterionic iminium–hydro-
borates like their piperidine analogues (Scheme 3). The
enamine and Piersꢀ borane, HBACTHUNRGTNEUNG(C6F5)2.
Results and Discussion
Dimethylamino-(1R)-(+)-camphorenamine 6a was obtained
by using a procedure reported for its piperidine analogue 6b
by Carlson and Nilsson (Scheme 2).[27] (1R)-(+)-Camphor 4
was reacted with dimethylamine 5a in the presence of TiCl4,
and dimethylaminoenamine 6a was isolated in a good yield
from the reaction mixture by distillation under reduced
pressure.
anionic character of the HBACHTUNTRGNEUNG(C6F5)2 groups in 8a and 8’a was
elucidated by a characteristic shift of the corresponding
11B NMR signals to a negative region (11B NMR, 160 MHz,
[D6]benzene, 278C: dB =À19.2 and À22.0 ppm) and typical
diastereotopic 19F NMR patterns with Dd(F
ACTHNUGRTENUNG(m,p)) values
around 3.2 ppm for both isomers. The observed splitting of
the 11B NMR signals for 8a and 8’a into doublets with typi-
cal JACTHNUTRGENU(GN B,H) values (96 and 97 Hz, see the Supporting Infor-
mation for spectral details) indicates that hydrogen remains
connected to boron after the reaction. Additional support
for the structural identification of the isomers of 8a and 8’a
was gathered by single-crystal X-ray diffraction studies
(Figure 1 and Figure 2).[28] Single crystals suitable for struc-
tural analysis were grown by slow evaporation of hexane
into a dichloromethane solution of 8 at À308C. The ob-
tained crystals contained both diastereomers, 8a and 8’a.
Scheme 2. Condensation reaction of (1R)-(+)-camphor 4 with amines.
Reaction of enamine 6a and Piersꢀ borane (7; HB
(C6F5)2)
The salts 8a/8’a feature a camphoriminium core with a hy-
À
gives the zwitterionic diastereomers 8a and 8’a in a 1:1
ratio. Similar behavior was observed by Erker and co-work-
ers for the piperidine analogue 6b (Scheme 3).[26] The zwit-
terionic diastereomers 8a and 8’a had spectral features that
were almost the same as those observed for the 8b/8’b pair.
dridoborate group, HB
N
À
À
(8a, C(26) B(2): 1.698(6) ꢃ; 8’a, C(1) B(1): 1.686(6) ꢃ).
The distance between the iminium nitrogen and a-carbon
À
atoms is typical for C=N double bonds (8a, C(31) N(36):
[29]
À
1.312(6) ꢃ; 8’a, C(6) N(11): 1.302(6) ꢃ).
The sum of
1
The H NMR spectrum of 8a in [D6]benzene featured two
angles around the a-carbon atom is close to 3608, pointing
main series of signals, corresponding to the two formed
out its planar configuration and sp2 character. In diastereo-
stereo
N
ACHTUNGTRENiNGNU somers 8a and 8’a, the hydridoborate group is oriented
in the region dH =1.4–3.82 ppm attributed to CH and CH2
protons of the camphor core (1H NMR, 300 MHz,
[D6]benzene, 278C), each stereoisomer produced 1H NMR
either endo or exo, respectively. Formation of diastereomeric
1:1 mixtures of 8a and 8’a excludes a reaction pathway in-
volving stereoselective syn hydroboration followed by the
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10413

T. Repo et al.
spectrum as a signal at dB =À8.5 ppm (11B NMR, 160 MHz,
[D6]benzene, 508C), characteristic for B–N adducts. Addi-
tionally, 1H NMR resonances corresponding to the C=C
double bond proton at dH =6.02 ppm (vs. dH =4.77 ppm for
the parent enamine 6a) and for the NMe2 groups at dH =
3.00 ppm (1H NMR, 300 MHz, [D6]benzene, 278C) were ob-
1
served. Unfortunately, H NMR signals of the camphor pro-
tons could not be distinguished from those displayed by the
major components of the system, the iminium–borohydrides
8a and 8’a. In addition to 19F NMR signals for the iminium–
borohydrides 8a and 8’a, 19F NMR peaks corresponding to
the 6a–HB
G
À129.2 ppm, d(Fp)=À157.4, À158.3 ppm and d(Fm)=
À164.0, À164.4 ppm (19F NMR, 282 MHz, CDCl3, 278C). By
performing variable-temperature H NMR and 19F NMR ex-
periments in [D8]toluene, we were able to shift the equilibri-
um between the 6a–HBACHTUNRGTNE(UNG C6F5)2 adduct and the iminium–bor-
ohydrides 8a and 8’a from a ratio of 1:8 at 278C to 1:5 at
758C.
Figure 1. Experimental X-ray structure of 8a.
The diastereomeric pair 8b and 8’b splits hydrogen
through intermediate formation of 1,2-aminoboranes 9b and
9’b, leading to the hydrogenated ammonium–borohydride
diastereomers 10b and 10’b (Scheme 4).[26] Surprisingly, the
Figure 2. Experimental X-ray structure of 8’a.
potentially reversible migration of a hydride anion from the
C-1 atom back to boron (Scheme 3, path A).[30] Selectivity in
the reaction presented in Scheme 3 is lost due to the fact
that it is not a concerted syn hydroboration but, rather, an
Scheme 4. Formation of the zwitterionic ammonium–hydroborates 10b
and 10’b.[26]
electrophilic attack by HBACHTUNRGTEN(UNG C6F5)2 to the polarized enamine
double bond. Electrophilic addition to 1-aminocyclohexenes
places a new substituent in an axial orientation favoring the
chair intermediate over the twist boat conformation.[31] The
camphor bridge locks the conformation of the cyclohexene
ring into the boat form, leading to the loss of selectivity.
Thus, electrophilic attack proceeds either from above or
below to the electron-rich p orbital on the C2-carbon
(Scheme 3, path B).
In addition to the iminium–borohydrides 8a and 8’a, the
NMR spectra revealed signals belonging to a minor product.
Even though the presence of the parent aminoborane 9a
(Scheme 3) was expected, conversely, these data correspond-
dimethyliminium–hydroborates 8a and 8’a reacted with hy-
drogen in a different way to the recently described piperidi-
nium analogues 8b and 8’b.
Conversely, upon stirring the 8a/8’a mixture for 2 h in tol-
uene at 808C under an atmosphere of hydrogen (2 atm), ad-
ducts of N,N-dimethylisobornylamine and N,N-dimethylbor-
nylamine with HBACHTUNRGTNEUNG(C6F5)2 (11a and 11’a, respectively) were
obtained in a 1:1 ratio (Scheme 5). In their 11B NMR spec-
trum, compounds 11a and 11’a display a broad signal at
dB =À7.8 ppm (11B NMR, 160 MHz, CDCl3, 278C), which is
characteristic for boron–nitrogen adducts (see the Support-
1
ing Information for further details). H NMR signals of the
ed to the B–N adduct of enamine 6a and HB
(C6F5)2. This
CH protons at the C-1 carbon atoms are shifted downfield:
dH =3.59 (11a) and 3.79 (11’a) ppm. To confirm the assigned
minor component of the system featured in the 11B NMR
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Camphor-Based Aminoboranes
FULL PAPER
Figure 3. 1H NMR spectra (300 MHz, CDCl3, 278C) of hydrogen splitting
products obtained from 8a/8’a (top) and adduct 11’a (bottom), synthe-
Scheme 5. Hydrogen splitting by iminium–hydroborates 8a/8’a.
sized by mixing amine 12’a and HBACTHUNRGTNEUNG(C6F5)2.
structures, adduct 11’a was synthesized in an alternative
way. Enamine 6a was hydrogenated by heating with formic
acid.[27] This reaction gave bornylamine 12’a selectively.
the hydrogen splitting products, 11a and 11’a, being a result
of HÀ or DÀ attack with an equal probability in the species
[D]-13a and [D]-13’a (Scheme 6).[32] The 13C{1H} NMR sig-
nals of the C-2 carbon atoms in [D]-11a and [D]-11’a were
recorded as a combination of several triplets at dC =31.3
Treatment of 12’a with HBACHTUNTRGNEUNG(C6F5)2 led to the endo isomer
11’a (Scheme 5). This product has exactly the same NMR
spectral characteristics as those obtained independently
from hydrogen splitting reactions (Figure 3).
and 34.4 ppm (13C{1H} NMR,126 MHz, CDCl3, 278C), which
[33]
À
À
is characteristic for monodeuterated CHD groups. The
deuterated C-1 carbon atom gave low intensity triplets at
dC =76.1 and 72.4 ppm, whereas sharp singlets at dC =76.4
and 72.9 ppm were attributed to the protonated C-1 carbon
atoms (see spectral details in the Supporting Information).
These results indicated that hydrogen splitting reactions
with isomers 8a and 8’a and their piperidinium analogues
8b and 8’b occurred through different reaction pathways.
Whereas piperidine-derived iminium–hydroborates 8b and
8’b gave the corresponding ammonium–hydroborate prod-
ucts 10b and 10’b (Scheme 4), their dimethyliminium ana-
logues 8a and 8’a transformed into amino–borane adducts
11a and 11’a. As shown by Erker et al.,[26] the former reac-
tion proceeded through an intermediate rearrangement of
8b and 8’b salts into the corresponding aminoboranes 9b
To determine the internal process involved in the hydro-
gen splitting reaction, we studied splitting of deuterium with
the 8a/8’a pair (conditions: toluene, 2 atm of deuterium, 2 h
at 808C). This reaction led to the corresponding deuterated
exo/endo adducts [D]-11a and [D]-11’a (Scheme 6). Stereo-
2
isomers [D]-11 featured H NMR signals of exo/endo deute-
rium atoms connected to the C-1 and to C-2 carbon atoms
of the bornyl/isobornyl core (2H NMR, 77 MHz, CDCl3,
278C: dD =3.64, 3.35 and 1.90–1.33 ppm, respectively).
The products [D]-11 also displayed 1H NMR signals of
exo/endo hydrogen atoms bound to C-1 carbon atoms
(1H NMR, 300 MHz, CDCl3, 278C: dH =3.64 and 3.35 ppm).
The total integration of intensity of these signals was only
50%, in comparison to the otherwise identical spectrum of
Scheme 6. Deuterium splitting with the system 8a/8’a.
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T. Repo et al.
and 9’b that were responsible for hydrogen splitting
droborate form is, in fact, in equilibrium with the adduct of
(Scheme 4). In our case, it seems that splitting of hydrogen
the starting enamine 6a and HB
ACHTUNGTRENNUNG
takes place through the 6a–HB
been observed that the zwitterionic system 8a and 8’a is in
equilibrium with 6a–HB(C6F5)2, which is a minor compo-
nent in solution (see above). This situation changes remark-
ably in the presence of hydrogen. Despite its coordinative
nature, 6a–HBACHTUNGTRENNUNG(C6F5)2 might behave as an active FLP and
split hydrogen or deuterium (Scheme 6). This should move
the reaction equilibrium from the zwitterionic form 8a and
8’a to the side of the 6a–HBACHTUNRGTEN(UNG C6F5)2 adduct and then further
(C6F5)2 N–B adduct. It has
nent of the equilibrium, 6a–HBACHTNUGTRENNUNG
elevated temperatures as an active FLP, performing splitting
of hydrogen and in this way moving the equilibrium from
the iminium–hydridoborate side to that of the hydrogenated
products. The size of the nitrogen substituent in the cam-
phor-based aminoboranes has a remarkable influence on the
structure of the equilibrium products and, consequently, on
the result of the hydrogen splitting process. In the presence
of hydrogen, bulky piperidine groups stabilize the 1,2-ami-
noborane framework against the dehydroboration process.
As a result, piperidine derivatives were hydrogenated into
the corresponding camphor-linked ammonium–hydrobo-
rates. Conversely, small methyl substituents facilitate inter-
mediate formation of an adduct between the starting cam-
AHCTUNGTRENNUNG
towards 11a and 11’a products (for the deuterium case
[D]-11a and [D]-11’a, respectively). In the proposed se-
quence (Scheme 6), addition of the [HDBACTHNUTRGNEUNG
(C6F5)2]À anion to
the iminium cation in the intermediately formed [D]-13 salts
should lead to [D]-11a and [D]-11’a isomers, for which the
C-1 carbon atoms should be connected with an equal proba-
bility either with deuterium or hydrogen.[32] All these
[D]-11a and [D]-11’a products, deuterated and hydrogenat-
ed at the C-1 carbon atoms, were detected in NMR studies
(see above).
phorenamine and HBACHTUNRGTNE(UNG C6F5)2 borane, thus providing an
active intermolecular frustrated Lewis pair, which is respon-
sible for splitting of hydrogen and formation of bornyl-
AHCTUNGTRENNaGUN mines as products of hydrogenation.
Starting from the unsaturated camphor framework 6a, hy-
drogen splitting reactions with zwitterionic salts 8a and 8’a
led to the fully hydrogenated N,N-dimethylbornyl- and iso-
bornylamines. This process could also be carried out in a
Experimental Section
General: All experiments were performed either by using a dual-mani-
fold gas-inlet/vacuum line or in a glove box (MBraun Unilab) under an
argon atmosphere. H2 was purchased from Oy AGA Ab as Scientific Hy-
drogen 6.0 grade and used without further purification. All reagents were
purchased from Sigma–Aldrich or Strem and purified by conventional
methods. Solvents were dried according to published procedures and dis-
tilled under an argon atmosphere. NMR experiments were performed
with a Varian Mercury 300 MHz spectrometer.
catalytic fashion.[24] By adding 10 mol% HB
ACTHNUTRGNEN(UG C6F5)2 to 6a
and heating the resulting mixture in [D6]benzene at 808C
for 16 h under 2 atm H2, enamine 6a was hydrogenated
(with 98% conversion verified by NMR spectroscopic analy-
sis) into the corresponding N,N-dimethylbornyl- and isobor-
nylamines (with a bornyl- and isobornyldimethylamine ratio
of 1:2). Quite surprisingly, this hydrogenation reaction pro-
duced isobornyldimethylamine as a major product, in con-
trast to reduction with formic acid for which it was obtained
in a negligible amount. As a result of hydrogenation, an
amine–borane adduct 11a/11’a is formed that reacts with an
excess of the starting enamine 6a giving the initial iminium–
borohydride system 8a and 8’a, which keeps the hydrogena-
tion process running in a catalytic mode.[8d,20] It was ob-
served that at 278C in a 1:1:1 mixture of isobornylamine
1-(1,7,7-Trimethylbicyclo
solution of TiCl4 (3.6 mL, 33 mmol) in hexane (10 mL) was added drop-
wise at À308C to a solution of dimethylamine (14 mL, 207 mmol) in
hexane (25 mL). Subsequently, solution of (+)-camphor (3.7 g,
[2.2.1]hept-2-en-2-yl)dimethylamine (6a):
A
AHCTUNGTRENNUNG
a
25 mmol) in hexane (15 mL) was added at such a rate that the reaction
mixture did not reach reflux. The reaction mixture was stirred overnight
at 258C, then the resulting suspension was filtered twice through a sin-
tered glass filter (porosity grade 4 and 3). The filtered clear solution was
collected and the remaining volatiles were removed under reduced pres-
sure. The product was isolated by distillation under vacuum (b.p. 468C/
3 mmHg) as a colorless oil (3.3 g, 18.2 mol, 72%). 1H NMR (300 MHz,
12’a, enamine 6a, and HB
HB(C6F5)2 amount was bound to amine 12’a and the rest re-
acted with 6a to give zwitterionic salts 8a and 8’a, 2% of
which was present in the form of the 6a–HB(C6F5)2 adduct.
These results suggest applicability for catalytic hydrogena-
tion of enamines with Piersꢀ borane when equilibrium be-
tween the resulting amine and the starting enamine–borane
adduct is achieved.
ACHTNUGTNER(UNNG C6F5)2, only 60% of total
CDCl3, 278C): d=4.77 (d, J=3.6 Hz, 1H; =CH), 2.48 (s, 6H; NACHTUNGTRENNUNG(CH3)2),
2.18 (m, 1H; CH), 1.83 (ddt, J=11.2, 9.0, 3.5 Hz, 1H; CH), 1.50 (ddd,
J=12.0, 8.9, 3.5 Hz, 1H; CH2), 1.15 (m, 1H; CH2), 1.01 (m, 1H; CH2),
1.09, 0.84, 0.72 ppm (each s, each 3H, CH3); 13C{H} NMR (75 MHz,
CDCl3, 278C): d=159.6, 108.0, 56.8, 54.0, 50.0, 42.6, 32.3, 27.5, 20.3, 20.1,
12.7 ppm.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Iminium–borohydrides 8a and 8’a: HBACHTNUTRGNE(UNG C6F5)2 (35 mg, 0.1 mmol) was dis-
solved in [D6]benzene and enamine 6a (18 mg, 0.1 mmol) was added to
the solution at RT. The suspension was stirred for approximately 5 min,
during which all solids dissolved. Quantitative conversion was verified by
1H and 19F NMR spectroscopy. Crystals were grown by slow evaporation
of hexane into a dichloromethane solution. Elemental analysis calcd (%)
for C27H26BF10N: C 54.88, H 4.22, N 2.67; found C 54.82, H 4.24, N 2.67.
Conclusion
Compound 8a: 1H NMR (300 MHz, CDCl3, 278C): d=3.56 (s, 3H;
NCH3), 3.33 (s, 3H; NCH3), 3.25–2.25 (brm, 1H; BH), 3.15 (brs, 1H;
CH), 2.04 (m, 3H; CH), 1.96 (m, 1H; CH2), 1.85 (m, 1H; CH2), 1.45 (m,
1H; CH2), 1.36 (m, 1H; CH2), 1.37 (s, 3H; CH3), 0.77 (s, 3H; CH3),
0.73 ppm (s, 3H; CH3); 13C{1H} NMR (75 MHz, CDCl3, 278C): d=
215.71, 149.8, 146.6, 138.7, 135.4, 61.4, 51.5, 50.9, 50.0, 48.4, 44.1, 33.9,
30.1, 22.2, 20.2, 15.1 ppm.
Dimethylcamphorenamine 6a was prepared and reacted
with HBACHTUNGTRENNUNG(C6F5)2 to give stable respective iminium–hydrobo-
rate zwitterionic salts 8a/8’a. Surprisingly, the products were
able to split hydrogen heterolytically. Variable-temperature
NMR experiments show that, in solution, the iminium–hy-
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Camphor-Based Aminoboranes
FULL PAPER
Compound 8’a: 1H NMR (300 MHz, CDCl3, 278C): d=3.80 (brs, 1H;
CH), 3.48 (s, 3H; NCH3), 3.25–2.25 (brm, 1H; BH), 3.03 (s, 3H; NCH3),
2.00 (m, 1H; CH), 1.93 (m, 1H; CH2), 1.75 (m, 1H; CH2), 1.74 (m, 1H;
CH2), 1.50 (m, 1H; CH2), 1.28 (s, 3H; CH3), 0.94 (s, 3H; CH3), 0.90 ppm
(s, 3H; CH3); 13C{1H} NMR (75 MHz, CDCl3, 278C): d=216.7, 149.8,
146.6, 138.7, 135.4, 61.4, 56.2, 49.8, 46.8, 46.3, 44.7, 31.4, 23.4, 20.4, 19.4,
15.9 ppm.
Acknowledgements
We are grateful for the financial support from the Inorganic Materials
Chemistry Graduate Program and the Academy of Finland (139550).
Compound 8a/8’a: 19F NMR (282 MHz, CDCl3, 278C): d=À130.3 (m,
2F; o-C6F5A), À132.2 (m, 2F; o-C6F5B), À132.5 (m, 2F; o-C6F5C), À133.0
(brs, 2F; o-C6F5D), À161.5 (t, J=20.3 Hz, 1F; p-C6F5A), À161.6 (t, J=
20.4 Hz, 1F; p-C6F5C), À162.3 (t, J=20.4 Hz, 1F; p-C6F5D), À162.4 (t, J=
20.2 Hz, 1F; p-C6F5B), À164.9 (m, 2F; m-C6F5C), À165.3 (m, 2F; m-
C6F5D), À165.4 (m, 2F; m-C6F5A), À165.6 ppm (m, 2F; m-C6F5B);
11B NMR (160 MHz, [D6]benzene, 508C): d=À19.2 (d, J=97.1 Hz),
À22.0 (d, J=95.7 Hz).
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N,N-Dimethylbornyl-/isobornylamine–HBACHTNURGTNEUNG(C6F5)2 adducts 11a and 11’a:
A 25 mL Schlenk tube with a magnetic stirring bar, containing the in situ
formed iminium-borohydride 8a/8’a (0.4 mmol) in toluene (2 mL) was
pressurized to 2 atm with dihydrogen. The solution was stirred
(1000 rpm) at 808C for 2 h. The solvent was removed under reduced
pressure, leaving a sticky solid, which was further washed with hexane.
The white precipitation was collected and dried under vacuum. Yield:
180 mg (0.34 mmol, 85%). Elemental analysis calcd (%) for
C27H28BF10N: C 54.67, H 4.59, N 2.66; found C 54.24, H 4.27, N 2.67.
Compound 11a: 1H NMR (300 MHz, [D6]benzene, 278C): d=4.25–2.02
(brm, 1H; BH), 3.37 (t, J=9.0 Hz, 1H; CH), 2.37 (s, 3H; CH3), 2.07 (s,
3H; CH3), 1.77 (m, 1H; CH2), 1.32 (m, 1H; CH2), 1.28 (m, 1H; CH2),
1.14 (m, 1H; CH2), 1.11 (m, 1H; CH2), 0.80 (m, 1H; CH2), 0.77 (s, 3H;
CH3), 0.65 (m, 1H; CH2), 0.57 (s, 3H; CH3), 0.50 ppm (s, 3H; CH3);
13C{1H} NMR (75 MHz, [D6]benzene, 278C): d=150.4, 147.2, 141.8, 139.3,
138.7, 136.3, 76.5, 51.9, 51.2, 48.1, 47.4, 43.2, 41.9, 34.7, 26.2, 22.0, 21.5,
15.4 ppm.
Compound 11’a: 1H NMR (300 MHz, [D6]benzene, 278C): d=4.25–2.02
(brm, 1H; BH), 3.63 (dd, J=10.6, 5.7 Hz, 1H; CH), 2.40 (s, 3H; CH3),
2.23 (s, 3H; CH3), 1.70 (m, 1H; CH2), 1.47 (m, 1H; CH2), 1.35 (m, 1H;
CH2), 1.24 (m, 1H; CH2), 1.19 (m, 1H; CH2), 1.10 (m, 1H; CH2), 0.83
(m, 1H; CH2), 0.72 (s, 3H; CH3), 0.54 (s, 3H; CH3), 0.52 ppm (s, 3H;
CH3); 13C{1H} NMR (75 MHz, [D6]benzene, 278C): d=150.4, 147.2,
141.8, 139.3, 138.7, 136.3, 72.9, 51.9, 51.4, 48.1, 45.5, 43.2, 31.7, 28.8, 27.5,
22.0, 18.2, 17.5 ppm.
Compound 11a/11’a: 19F NMR (282 MHz, [D6]benzene, 278C): d=
À128.3 (m, 2F; o-C6F5A), À128.6 (m, 2F; o-C6F5B), À128.6 (m, 2F; o-
C6F5C), À128.6 (m, 2F; o-C6F5D), À156.6 (t, J=20.7 Hz, 1F; p-C6F5B),
À157.0 (t, J=20.7 Hz, 1F; p-C6F5C), À157.6 (t, J=20.7 Hz, 1F; p-C6F5D),
À158.1 (t, J=20.7 Hz, 1F; p-C6F5A), À163.7 (m, 2F; m-C6F5B), À163.8
(m, 2F; m-C6F5C), À163.9 (m, 2F; m-C6F5D), À164.0 ppm (m, 2F; m-
C6F5A); 11B NMR (160 MHz, [D6]benzene, 508C): d=À7.91 ppm.
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Bornyldimethylamine (12’a): Enamine 7a (360 mg, 2 mmol) was heated
under an argon atmosphere to 1008C. Formic acid (0.15 mL, 4 mmol)
was added at such a rate that foaming was kept under control. After ad-
dition, the reaction mixture was heated for 10 min at 1008C. 1 m HCl
(10 mL) was added and the aqueous phase was extracted with diethyl
ether (2ꢄ10 mL). The aqueous phase was collected and basified with 1m
NaOH, giving a milky emulsion, which was further extracted with diethyl
ether (3ꢄ10 mL). The organic phase was dried and all volatiles were re-
moved, leaving the product as a yellow oil (252 mg, 1.39 mmol, 70%).
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1H NMR (300 MHz, CDCl3, 278C): d=2.21 (s, 6H; N
ACTHUNGTRNEUNG(CH3)2), 2.05 ( m,
1H; NCH), 2.05 (m, 1H; CH2), 1.90 (m, 1H; CH2), 1.72 (m, 1H, CH2),
1.55 (m, 1H; CH), 1.28 (m, 2H; CH2), 1.05 (m, 1H; CH2), 0.96, 0.87,
0.82 ppm (each s, each 3H; CH3); 13C{1H} NMR (75 MHz, CDCl3, 278C):
d=72.8, 50.4, 48.5, 46.4, 44.5, 38.4, 29.0, 27.4, 20.4, 18.9, 17.0 ppm.
Catalytic hydrogenation of enamine 6a: A 25 mL Schlenk tube with a
magnetic stirring bar, containing a solution of enamine 6a (180 mg,
1 mmol) and HBACHTUNGTRENNUNG(C6F5)2 (35 mg, 0.1 mmol) in [D6]benzene (1 mL) was
pressurized to 2 atm H2. The solution was stirred (1000 rpm) at 808C for
16 h, then cooled and transferred to an NMR tube. Conversion was 98%
1
(determined by H NMR spectroscopic analysis). Ratio of bornyl and iso-
bornyl dimethylamines was 1:2.
Chem. Eur. J. 2013, 19, 10412 – 10418
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10417

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crystallized in single crystals containing both diastereomers. For the
crystal structure of 8a/8’a see the Supporting Information and
CCDC-921576. This data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. The crystal structure of 8b/8’b has been reported
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[32] It was shown that in the addition of a BH4 anion to a C=O carbon-
À
yl group the isotopic effect is quite close to 1; see: H. Yamataka, T.
Hanafusa, J. Am. Chem. Soc. 1986, 108, 6643–6646.
13
À
À
[33] Appearance of several triplets in the C NMR spectra for CHD
groups at the C-2 carbon atom of [D]-11a and [D]-11’a corresponds
to the formation of a diastereomeric mixture of isomers with endo
and exo positioning of deuterium at the C-2 center (see Scheme 6).
[19] a) V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger, M. Leske-
lꢁ, T. Repo, P. Pyykkç, B. Rieger, J. Am. Chem. Soc. 2008, 130,
Received: February 5, 2013
Published online: June 20, 2013
10418
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 10412 – 10418