Inherent stability limits of intramolecular boron nitrogen lewis acid-base pairs

Article abstract of DOI:10.1002/chem.201200442

The reaction of (C₆F₅)₂BH (1) with N,N-dimethylallylamine (2), N,N-diethylallylamine (3) and 1-allylpiperidine (4) afforded the five-membered ring systems (C₆F₅) ₂B(CH₂)₃NR₂ (R=Me (5), Et (6)) and (C₆F₅)₂B(CH₂)₃N(CH ₂)₅ (7) with an intramolecular dative B-N bond. A different product was obtained from the reaction of (C₆F ₅)₂BH (1) with N,N-diisopropylallylamine (8), which afforded the seven-membered ring system (C₆F₅) ₂B(CH₂)₃N(iPr)CH(Me)CH₂ (9) under extrusion of dihydrogen. All compounds were characterised by elemental analysis, NMR spectroscopy and single-crystal X-ray diffraction experiments. Density functional theory (DFT) studies were performed to rationalise the different reaction mechanism for the formation of products 6 and 9. The bonding situation of compound 9 was analysed in terms of its electron density topology to describe the delocalised nature of a borane- enamine adduct. Two roads diverged: Two different reaction pathways in the hydroboration of allylamines R ₂NCH₂CHCH₂ with (C₆F ₅)₂BH lead to the expected heterocycles with B-N bonds for small R substituents, but to hydride abstraction and evolution of dihydrogen for R=iPr (see scheme). Copyright

Full text of DOI:10.1002/chem.201200442

FULL PAPER
DOI: 10.1002/chem.201200442
Inherent Stability Limits of Intramolecular Boron Nitrogen Lewis Acid–Base
Pairs
Daniel Winkelhaus, Beate Neumann, Hans-Georg Stammler, Raphael J. F. Berger,
Yuri V. Vishnevskiy, and Norbert W. Mitzel*[a]
Abstract: The reaction of (C6F5)2BH
(1) with N,N-dimethylallylamine (2),
N,N-diethylallylamine (3) and 1-allylpi-
peridine (4) afforded the five-mem-
seven-membered ring system (C6F5)2B-
(CH2)3N(iPr)CH(Me)CH2 (9) under
extrusion of dihydrogen. All com-
pounds were characterised by elemen-
tal analysis, NMR spectroscopy and
single-crystal X-ray diffraction experi-
ments. Density functional theory
(DFT) studies were performed to ra-
tionalise the different reaction mecha-
nism for the formation of products 6
and 9. The bonding situation of com-
pound 9 was analysed in terms of its
electron density topology to describe
the delocalised nature of a borane–
enamine adduct.
G
ACHTUNGTRENNUNG
bered ring systems (C6F5)2B
(R=Me (5), Et (6)) and (C6F5)2B-
(CH2)3N(CH2)5 (7) with an intramolec-
ACHTNUGTERN(NUGN CH2)3NR2
A
ACHTUNGTRENNUNG
ꢀ
ular dative B N bond. A different
product was obtained from the reaction
of (C6F5)2BH (1) with N,N-diisoprop-
Keywords: boron · heterocycles ·
hydroboration · Lewis acids · nitro-
gen
ACHTUNGTRENNUNGylallylamine (8), which afforded the
ꢀ
Introduction
ethylene-linked P B species capable of easily activating di-
hydrogen.[5] Likewise, a linked boron nitrogen system de-
ꢀ
Initiated by the establishment of the concept of frustrated
Lewis pairs (FLP),[1] recent developments have been direct-
ed towards molecules that contain Lewis acidic and basic
sites at a distance, which establishes an electronic push–pull
situation for substrates.[2] This is an intermolecular variant
of the classical FLP concept, which requires steric over-
crowding at the Lewis acidic and basic sites and prevents
direct adduct formation. Thus an unquenched reaction po-
tential is left to be exerted on substrates small enough to
bind to both sites of contrasting reactivity. Particular interest
was given to systems that reversibly activate dihydrogen,[3]
because this sets the scene for metal-free hydrogenation cat-
alysis. Due to toxicity concerns, the need to synthesise prod-
ucts free of traces of transition metals is of high interest to
the pharmaceutical industry.[4] Whereas the first FLP sys-
tems tested were composed of separate Lewis acid and base
molecules, later investigations also turned to molecules with
Lewis acidic and basic sites that were connected by a cova-
lent linker. Important contributions to this chemistry stem
from Erker et al., who investigated cyclic, intramolecular
duced from tetramethylpiperidine was applied by Rieger
et al. for the catalytic hydrogenation of imines, enamines
and silyl enol ethers.[6]
Recently, Lammertsma and co-workers presented an arti-
cle on pre-organised FLPs in which R2PCH2BR’2 systems—
including fluorine-free tBu2PCH2BPh2—with geminal donor
and acceptor sites were shown to be capable of dihydrogen
activation. The latter was attributed to an optimised align-
ment of the acceptor and donor site.[7]
We have recently investigated a BCN system previously
established by Bꢀrger and Pawelke et al.[8] with highly elec-
tronegative substituents, (F3C)2BCHACHTNURGTNE(NUG SiMe3)NMe2, but
found that it did not activate dihydrogen. This is probably
due to the fact that this compound as well as
(F3C)2BCH2NMe2 adopts the form of three-membered BCN
ꢀ
rings in solution and the solid state, with a B N bond being
too strong despite the ring strain involved in these systems.
However, they show a highly unusual electron-density distri-
bution. According to Baderꢁs quantum theory of atoms in
molecules (QTAIM),[9] its charge density topology is that of
ꢀ
an open-chain system, as no bond critical point for the N B
interaction is found and accordingly no ring critical point. In
this sense the system behaves similarly to compounds with
SiON (X3Si-O-NR2),[10] SnON[11] and SiNN units (X3Si-
[a] D. Winkelhaus, B. Neumann, Dr. H.-G. Stammler, Dr. R. J. F. Berger,
Dr. Y. V. Vishnevskiy, Prof. Dr. N. W. Mitzel
Lehrstuhl fꢀr Anorganische Chemie und Strukturchemie
Universitꢂt Bielefeld
N(R’)-NR2,[12] which show short Si N/Sn N distances and
small Si-O-N, Sn-O-N and Si-N-N angles, respectively. We
have studied such compounds intensely in the course of our
investigations on the a effect in silicon chemistry.[13] The
most extreme examples in this context are F3CSiF2ONMe2
ꢀ
ꢀ
Universitꢂtsstrasse 25, 33615 Bielefeld (Germany)
Fax : (+49)(0)521-106-6182
E-mail: mitzel@uni-bielefeld.de
Homepage: http://www.uni-bielefeld.de/chemie/ac3/ak-mitzel/
start.html
ꢀ
and F3SiONMe2. Both show neither a bond path nor an Si
N bond-critical point in the topology analysis of the electron
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200442.
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density according to QTAIM theory despite the observed
The H and 13C NMR spectra of compounds 6 and 7 ex-
ꢀ
short Si N distances in the range of the sum of the covalent
hibited characteristics similar to those of 5. In the 11B NMR
spectra of 6 and 7 resonances are observed at d=2.2 (n1/2 =
radii. By contrast, for the systems with SiCN and SiCO units
(e.g., F3SiCH2NMe2 or F3SiCH2OCH3) no significant donor–
acceptor interactions were found.[14]
141 Hz) and 1.64 ppm (n1/2 =126 Hz), respectively. The
19F NMR spectra show signals similar to those of compound
Due to the fact that the chemistry of pentafluorophenyl[15]
(C6F5)-substituted boron compounds is well developed and
their easier accessibility relative to analogous trifluoro-
5 with a Dd
[(m-F)
G
ꢀ
compounds. The described NMR spectroscopic data are con-
sistent with five-membered ring structures of 5, 6 and 7, that
is, propylene-bridged B/N ring systems with dative B N
bonds. This was subsequently confirmed by X-ray diffrac-
tion.
Single crystals of 5, 6 and 7 were obtained from concen-
trated hexane solutions. The ring structures of molecules of
5 and 7 are very similar and their parameters almost indis-
tinguishable within experimental error. The structures of 5,
6 and 7 are displayed in Figure 1, Figure 2, and Figure 3; se-
lected structural parameters are listed in their captions.
ꢀ
A
theses and properties of compounds that contain (C6F5)2B
and NR2 groups linked by a C3 chain. In this contribution
we report the syntheses of five-membered ring systems with
pentafluorophenyl-substituted boron atoms and varying
alkyl groups at the nitrogen atom that yield the intended ef-
ꢀ
fects on the charge distribution in the B N region of inter-
est.
Results and Discussion
Reactions of (C6F5)2BH with allylamines with small N sub-
stituents: Piersꢁ borane, (C6F5)2BH (1),[17] was treated with
N,N-dimethylallylamine (2), N,N-diethylallylamine (3)[18]
and 1-allylpiperidine (4), respectively, in toluene at 1008C.
Clean regioselective hydroboration resulted in the product
5,
6 and 7 in 77–80% yield as colourless powders
(Scheme 1).
Figure 1. Molecular structure of 5 in the crystal. Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: N1 B1
ꢀ
ꢀ
ꢀ
ꢀ
1.672(1), B1 C13 1.637(1), C13 C14 1.552(1), C14 C15 1.525(1), N1
ꢀ
ꢀ
ꢀ
ꢀ
C15 1.507(1), N1 C16 1.502(1), N1 C17 1.493(1), B1 C1 1.659(1), B1
C7 1.649(1); C17-N1-C16 108.6(1), C17-N1-C15 107.9(1), C16-N1-C15
107.4(1), C17-N1-B1 119.1(1), C16-N1-B1 112.3(1), C15-N1-B1 100.7(1),
C13-B1-C7 111.2(1), C13-B1-C1 115.3(1), C7-B1-C1 108.4(1), C13-B1-N1
98.0(1), C7-B1-N1 116.5(1), C1-B1-N1 107.4(1), C14-C13-B1 107.0(1),
C15-C14-C13 105.9(1), N1-C15-C14 106.9(1).
Scheme 1. Reactions of (C6F5)2BH (1) with allylamines 2, 3 and 4.
The new compounds were characterised by elemental
analyses, single-crystal X-ray diffraction and NMR spectros-
copy of the nuclei H, 11B, 13C and 19F. The H NMR spec-
trum of compound 5 shows three multiplets at d=1.48, 1.55
and 1.94 ppm caused by the six protons of the propylene
bridge. The corresponding signals in the 13C NMR spectrum
are observed at d=66.9, 20.0 and 17.5 ppm, the latter indi-
1
1
In the crystal structures of 5, 6 and 7 the five-membered
rings exhibit the typical envelope conformations. The three
carbon atoms C13, C14 and C15 and the boron atom define
an almost perfect plane, whereas the nitrogen atom is
placed above this plane. The torsion angle C15-C14-C13-B1
is 1.18 for 5, 0.28 for 6 and ꢀ1.58 for 7. The dihedral angles
between the (mean) planes B1-C13-C14-C15 and B1-N1-
C15 measure 135.2 (5), 134.8 (6) and 133.48 (7). The lengths
of the bonds created by the strong Lewis acidic boron atoms
and the nitrogen atoms for compounds 5–7 are 1.672(2),
1.685(2) and 1.676(2) ꢄ, respectively. This can be interpret-
ꢀ
cating by its broadness the formation of a B C bond. In the
11B NMR spectrum of 5 a single resonance at d=0.2 ppm
(n1/2 =97 Hz) is observed. The 19F NMR signals of the C6F5
groups are detected at d=ꢀ128.6 (o-C6F5), ꢀ157.3 (p-C6F5)
and ꢀ163.7 ppm (m-C6F5). The small separation of the p-
and m-C6F5 19F resonances is indicative of the presence of
a tetra-coordinate boron atom.[19]
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Lewis Acid–Base Pairs
FULL PAPER
ed as a strong dative bond, especially considering the ring
strain of the five-membered ring system. Due to the increase
of Lewis acidity of the boron atom by C6F5 substituents, the
ꢀ
B N bond lengths in these compounds are shorter com-
pared to the known related ring system [9-(2-dimethylami-
nomethyl)phenyl]-9-borabicyclo
and similar to (CH2)5B
(CH2)3NMe2 (1.682(4) ꢄ)[21] The B
N bond lengths in compounds 5–7 are nearly the same as in
the comparable five-membered BC3N ring of
(C6F5)2BC6H4(Me)NMe2 with 1.666(3) ꢄ.[22]
[3.3.1]nonane (1.746(3) ꢄ)[20]
ACHTUNGTRENNUNG
ꢀ
AHCTUNGTRENNUNG
ꢀ
ꢀ
The endocyclic B C bond lengths (B1 C13) have values
of 1.637(2) (5), 1.636(2) (6) and 1.634(2) ꢄ (7). This is in the
range of related five-membered ring systems with a C6F5-
substituted boron atom (1.646 ꢄ in (C6F5)2B
(tBu)2).[2b]
The coordination geometry around the boron and nitro-
ACHTUNGTRENNUNG(CH2)3P-
AHCTUNGTRENNUNG
gen atoms in compounds 5, 6 and 7 is distorted tetrahedral.
The angles around the boron atom lie in the ranges 98.0(1)–
116.5(1)8 for 5, 98.3(2)–116.6(2)8 for 6 and 97.7(1)–118.1(1)8
for 7. The values around the nitrogen atom are in the ranges
100.7(1)–119.1(1)8 for 5, 99.9(1)–115.1(1)8 for 6 and 99.7(1)–
116.3(1)8 for 7. For a detailed comparison of structural pa-
rameter values, refer to Figures 1–3.
It should be mentioned that we have attempted reactions
of 5, 6 and 7 with dihydrogen to evaluate their potential in
the sense of frustrated Lewis base pairs (although a “frustra-
tion” is not present in the ring structures determined in the
solid state); however, we did not observe such reactions.
Figure 2. Molecular structure of 6 in the crystal. Hydrogen atoms are
ꢀ
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: N1 B1
ꢀ
ꢀ
ꢀ
ꢀ
1.685(2), B1 C13 1.636(2), C13 C14 1.550(2), C14 C15 1.518(2), N1
ꢀ
ꢀ
ꢀ
ꢀ
C15 1.508(2), N1 C16 1.512(2), N1 C18 1.525(2), B1 C1 1.657(3), B1
ꢀ
ꢀ
C7 1.655(2), C16 C17 1.512(2), C18 C19 1.531(2); C15-N1-C16 109.3(1),
C15-N1-C18 110.9(1), C16-N1-C18 111.2(1), C15-N1-B1 99.9(1), C16-N1-
B1 115.1(1), C18-N1-B1 110.0(1), C13-B1-C7 110.6(1), C13-B1-C1
115.4(1), C7-B1-C1 107.9(1), C13-B1-N1 98.3(1), C7-B1-N1 116.6(1), C1-
B1-N1 108.2(1), C14-C13-B1 106.5(1), C15-C14-C13 106.4(1), N1-C15-
C14 106.8(1), N1-C16-C17 116.5(1), N1-C18-C19 118.0(2).
Reactions of (C6F5)2BH with N,N-diisopropylallylamine:
The reactions described above of (C6F5)2BH with allyl-
AHCTUNGTREGaNNUN mines with different nitrogen alkyl substituents led to the
ꢀ
ring systems 5, 6 and 7 with dative B N bonds. By contrast,
reaction of (C6F5)2BH (1) with N,N-diisopropylallylamine
(8)[18] at 1008C in toluene afforded a product with a seven-
membered ring system under extrusion of hydrogen
(Scheme 2). The product could be isolated as a colourless
powder in 75% yield.
Scheme 2. Reaction of (C6F5)2BH (1) with N,N-diisopropylallylamine (8).
Compound 9 was characterised by elemental analyses, by
Figure 3. Molecular structure of 7 in the crystal. Hydrogen atoms are
single-crystal X-ray diffraction and by 1H, 11B, 13C and
1
19F NMR spectroscopy. The H NMR spectrum of 9 shows
ꢀ
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: N1 B1
ꢀ
ꢀ
ꢀ
ꢀ
1.676(2), B1 C13 1.634(2), C13 C14 1.549(2), C14 C15 1.528(2), C16
three broad signals with the same integral at d=2.86, 1.61
and 1.34 ppm. They can be assigned to the six protons of the
propylene bridge. The B-CH2-CH(Me) moiety is observed
as a broad signal at d=2.90 ppm. The signals at d=3.02
(sept) and 0.41 ppm (d) result from the isopropyl group. A
singlet for the methyl group is observed at d=1.06 ppm.
Corresponding signals can be detected in the 13C NMR spec-
trum of 9. Due to the quadrupole broadening by the boron
ꢀ
ꢀ
ꢀ
C17 1.552(2), C17 C18 1.526(2), C18 C19 1.527(2), C19 C20 1.524(2),
N1 C15 1.515(2), N1 C16 1.518(2), N1 C20 1.505(2), B1 C1 1.651(2),
B1 C7 1.657(2); C20-N1-C15 110.3(1), C20-N1-C16 109.21, C15-N1-C16
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
108.9(2), C20-N1-B1 116.3(1), C15-N1-B1 99.7(1), C16-N1-B1 111.9(1),
C13-B1-C7 115.5(1), C13-B1-C1 110.3(1), C7-B1-C1 107.3(1), C13-B1-N1
97.7(1), C7-B1-N1 108.0(1), C1-B1-N1 118.1(1), C14-C13-B1 106.9(1),
C15-C14-C13 105.8(1), N1-C15-C14 106.4(1), N1-C16-C17 113.4(1), N1-
C20-C19 113.5(1), C16-C17-C18 111.3(1), C17-C18-C1 110.8(1), C18-C19-
C20 111.6(1).
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N. W. Mitzel et al.
nuclei, both aliphatic carbon atoms directly bonded to
boron give broad signals at d=42.7 and 24.3 ppm, respec-
tively. The resonance for the quaternary carbon atom is de-
tected at d=197.4 ppm. 1H/1H-COSY, 1H/13C-HMQC and
1H/13C-HMBC data confirm this assignment. The 11B NMR
spectroscopic data show a single resonance at d=ꢀ14.4 ppm
(n1/2 =52 Hz). 19F NMR spectroscopic signals can be ob-
served at d=ꢀ133.5 (o-), ꢀ161.0 (p-) and ꢀ164.8 (m-C6F5).
ꢀ
has been reported in the literature. However, Stephan and
co-workers described a system with a related N=C-C-B
moiety,
[tBuNC4H
(3-PhC=C(H)
E
(C6F5)2)]
(10)
(Scheme 4).[23]
ꢀ
The B1 C13 bond of compound 9 at 1.630(2) ꢄ is of simi-
lar length to the boron carbon bond in the five-membered
ꢀ
ring systems 5–7. The B1 C17 distance, which is the one de-
scribed as partially bonded in the mesomeric Lewis formu-
lae in Scheme 3, is nearly 0.06 ꢄ longer (1.688(2) ꢄ) than
The small DdACHTUNGTRENNUNG[(m-F) ACHTUNTGREN(NUGN p-F)] separation of approximately
Dd=4 ppm is indicative of a tetra-coordinate boron atom.
The molecular structure of compound 9 was determined
by single-crystal X-ray crystallography (single crystals were
obtained from a concentrated solution in toluene). Com-
pound
9 crystallises in the monoclinic space group
P21/c with four molecules in the unit cell. The structure is
displayed in Figure 4; selected structural parameters are
listed in the caption.
Scheme 3. Mesomeric structures of product 9.
ꢀ
the latter B C bonds and also
ꢀ
longer than the B C bond of
Stephanꢁs
compound 10
(1.671(5) ꢄ).[23] Due to this fact,
ꢀ
this boron carbon bond can be
assigned a (partial) dative char-
acter. The coordination around
the boron atom is distorted tet-
rahedral with the largest angle
Scheme 4. tBuNC4H
C(H)(C6F5)B(C6F5)2] (10) with
a N=C-C-B moiety.[23]
3ACHTUNGTRENNUNG[3-PhC=
G
ACHTUNGTRENNUNG
between
117.2(1)8. Consequently, the
angle C13-B1-C17 adopts
a smaller value of 106.6(1)8.
C13-B1-C7
at
ꢀ
ꢀ
The N1 C15 and N1 C19 bonds exhibit lengths of
1.486(2) and 1.495(2) ꢄ, respectively. This is slightly larger
Figure 4. Molecular structure of 9 in the crystal. Hydrogen atoms are
[24]
ꢀ
ꢀ
than normal N C bonds (1.47 ꢄ). By contrast, the N1
C16 bond has a length of 1.304(2) ꢄ and is therefore slightly
longer than a nitrogen–carbon double bond (1.22 ꢄ).[24] It is
ꢀ
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: N1 C16
ꢀ
ꢀ
ꢀ
ꢀ
1.304(2), N1 C15 1.486(2), N1 C19 1.496(2), B1 C13 1.630(2), B1 C17
ꢀ
ꢀ
ꢀ
ꢀ
1.689(2), C13 C14 1.537(2), C14 C15 1.522(2), C16 C17 1.467(2), C16
ꢀ
ꢀ
C18 1.499(2), B1 C1 1.665(2), B1 C7 1.661(2); C16-N1-C15 120.6(2),
C16-N1-C19 123.3(1), C15-N1-C19 116.1(1), C13-B1-C7 117.2(1), C13-
B1-C1 106.7(1), C7-B1-C1 103.9(1), C13-B1-C17 106.6(1), C7-B1-C17
110.8(1), C1-B1-C17 111.7(1), C14-C13-B1 118.4(1), C15-C14-C13
115.5(1), N1-C15-C14 113.4(1), N1-C16-C17 122.4(1), N1-C16-C18
120.5(1), C17-C16-C18 117.1(1), C16-C17-B1 113.0(1), N1-C19-C21
110.3(1), N1-C19-C20 111.1(1), C21-C19-C20 113.0(1).
ꢀ
also longer than the corresponding N C bond in 10
(1.295(4) ꢄ).[23] The C16 C17 distance (1.467(2) ꢄ) is re-
ꢀ
markably shorter than the carbon single bonds of the pro-
ꢀ
ꢀ
pylene bridge, C13 C14 at 1.537(2) and C14 C15 at
ꢀ
1.522(2) ꢄ, which represent standard C C bond lengths
(1.54 ꢄ).[24] However, C16 C17 is also longer than a typical
ꢀ
double bond (1.33 ꢄ).[24] The related carbon C C bond in
ꢀ
10 has a length of 1.470(5) ꢄ.[23] The mesomeric enamine
structure contribution to the bonding situation of 9 explains
Three meaningful mesomeric structures have to be taken
into consideration for compound 9: besides the seven-mem-
bered ring system, two open forms—an enamine and an imi-
nium ion form—can contribute to bonding in the Lewis
structure formula picture.
The structure exhibits a seven-membered cyclo-BC3NC2
ring system. No structure of another heterocyclic system of
this ring size that contains one boron and one nitrogen atom
ꢀ
the length of the single bond C16 C17 and the notably
ꢀ
shortened length of the C16 C18 bond with a value of
1.499(2) ꢄ is a result of the almost planar coordination ge-
ometry of C16. Consequently, the nitrogen atom features
a nearly perfect trigonal planar coordination with bond
angles of 120.6(1) (C16-N1-C15), 123.3(1)8 (C16-N1-C19)
and 116.1(1)8 (C15-N1-C19).
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Lewis Acid–Base Pairs
FULL PAPER
Proposed mechanism of formation for 6 and 9: It is obvious
that the first step of the reactions of N,N-diethylallylamine
(3) and N,N-diisopropylallylamine (8) with (C6F5)2BH is hy-
droboration. The product in the case of 3 cyclises to give 6,
but in the case of 8 dihydrogen is extruded and 9 is formed.
To rationalise differences in the mechanisms behind these
reactions, we present some mechanistic considerations in
Scheme 5. A full evaluation of all possible intermediates
gen by to yield an iminium cation (15). Hydride abstractions
from bulky amines with BACHTUNRGTNE(NUG C6F5)3 have already been report-
ed.[26] Calculations for the isolated molecules predict an in-
creasing energy when transforming 12 into 15 by +23.7 kcal
molꢀ1, whereas the corresponding process 11!14 is more
endothermic at +30.9 kcalmolꢀ1. A conceivable 1,3-hydro-
gen shift of the iminium ion 15 can then lead to the forma-
tion of a vinyl ammonium salt (17); this process is also pre-
dicted to be endothermic by +3.0 kcalmolꢀ1 (whereas the
corresponding step for R=H: 14!16 is slightly exothermic
and yields ꢀ3.1 kcalmolꢀ1).
ꢀ
Note that a structure related to 17 in the sense that N H
ꢀ
and B H functions come close together and form
a BH···HN interaction between inversely polarised hydrogen
atoms (dihydrogen bonding) has recently been structurally
characterised in the form of 1-N-TMPH-CH2-2-[HB-
AHCTNUERTG(GNNUN C6F5)2]C6H4 by neutron diffraction; the H···H separation in
this compound is 1.674(8) ꢄ.[27]
Our DFT calculations of the structure of 17 result in a re-
lated structure and give a more unusual value of 1.383 ꢄ for
a contact of two formally unbound hydrogen atoms, but still
underline the similarity in bonding. Either the intermediate
15—or more likely 17—eliminates dihydrogen to yield the
seven-membered heterocycle 9. These processes are pre-
dicted to be exothermic by ꢀ11.4 or ꢀ14.4 kcalmolꢀ1, re-
spectively (similar to 16!18 by ꢀ12.3 kcalmolꢀ1). The en-
amine structure 9a can thereby serve as an intermediate of
slightly different structure geometrically that relaxes into 9
(where it serves as description of one possible resonance
form). The whole process of H2 elimination starting from 12
and ending at 9 is accompanied by an increase in energy by
+12.3 kcalmolꢀ1 (and +15.5 kcalmolꢀ1 for 11!18).
Scheme 5. Proposed mechanism for the formation of 6 and 9.
would require extensive work, because numerous conforma-
tional possibilities would have to be considered. Moreover,
due to the fact that many dative bonds and highly polar spe-
cies are involved, neglecting solvent polarity and concentra-
tion effects would lead to a rather incomplete description.
Consequently, we have restricted a computational explora-
tion to some key species.
These considerations do not take into account reaction
barriers; this is due to the complicated conformational be-
haviour, which makes a search for the reaction pathway ex-
tremely costly. Furthermore, the proposed intermediates,
and consequently transition states, involve betaine struc-
tures, dative bonds and even BH···HN hydrogen interac-
tions, which make a further rigorous quantumchemical treat-
ment a worthwhile goal and indicate that solvent and con-
centration effects might play a very significant role. In this
sense, the energies discussed above should not be overinter-
preted.
The first question to be answered was whether the forma-
ꢀ
tion of a ring with a B N bond following the hydroboration
reaction would be hindered in the case of the more bulky
isopropyl substituents in 8.
DFT (RI-BP86/def2-SVP) calculations show that in the
case of the ethyl-substituted compound the five-membered
ring (6) is 10.3 kcalmolꢀ1 lower in energy than the open-
chain form with three-coordinate boron and nitrogen atoms
(11). However, the situation is reversed for the isopropyl-
substituted compound, in which the open form (12) is
10.7 kcalmolꢀ1 lower in energy than the closed ring (13).
This is consistent with the known fact that isopropyl groups
lead to a steric overcrowding at nitrogen as is well estab-
lished in Hꢀnigꢁs base (iPr2EtN) and the planarised nitrogen
atom in triisopropylamine.[25]
However, we may conclude that the crucial difference be-
tween the behaviour of the two systems is the hindrance of
ꢀ
B N bond formation for 13. This helps to rationalise why
no further reaction is observed and 6 is isolated as the prod-
uct.
On the other hand, the proposed intermediate 14 is
higher in energy than 15, which provides some indication of
why this process might be disfavoured for R=H. In this
context it should be mentioned that 6 (as well as 5 and 7)
have been heated with the intention to convert them into
seven-membered ring products analogous to 9, but these ex-
perimental attempts remained fruitless.
This indicates that 6 has found a resting state in this cy-
clised form, whereas for 12 the unquenched reaction poten-
tial of free Lewis acid and base remains. This seems to ini-
ꢀ
tiate an alternative reactive pathway. The
BACHTUNGTERNU(NG C6F5)2 moiety
of the open intermediate 12 can react by means of an a-hy-
dride abstraction from one of the isopropyl groups at nitro-
Chem. Eur. J. 2012, 00, 0 – 0
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N. W. Mitzel et al.
ellipticities of these bonds (0.09 and 0.14), which do not
reach the values of isolated double bonds such as in ethene
(0.45) or even benzene (0.23).
ꢀ
By contrast, the B1 C17 bond has the lowest electron
density within the seven-membered ring with a value of
(BCP)=0.77 eꢄꢀ3, which is in the region of a dative
ACTHUNGTRENNUG1AHCTUNGTRENNNUG
bond.[28] This assumption is confirmed by the Laplacian
2
value, which is ꢀ5 1
(BCP)=0.41 eꢄꢀ5. The other three
ACHTUNGTRENNUNG
carbon boron bonds all have higher values for the electron
(BCP)=1.06, 0.96 and 0.94 eꢄꢀ3 for B1 C13,
ACHTUNGTRENNUNGB1
ꢀ
ꢀ
density (1
ꢀ
C1 and B1 C7, respectively) and their Laplacian values are
2
(BCP)=3.28, 0.14 and 0.51 eꢄꢀ5 for
ACHTUNGTRENNUNG
ꢀ
always positive ( 5 1
ꢀ
ꢀ
ꢀ
B1 C13, B1 C1 and B1 C7, respectively), thus indicating
their covalent nature. Likewise, the remaining bonds of the
methylene bridge within the seven-membered ring exhibit
electron-density values of 1ACHTNUGRTNEUNG
(BCP)=1.57, 1.61 and 1.64 eꢄꢀ3
ꢀ
ꢀ
ꢀ
for C13 C14, C14 C15 and N1 C15. The Laplacian param-
eters of these bonds are higher than the values of the previ-
2
ꢀ
ꢀ
ous described boron carbon bonds with 5 1
ACHTUNGTREN(NUNG BCP)=11.91,
12.60 and 14.03 eꢄꢀ5, respectively, which is in the region of
a typical covalent bond.
Figure 5. Electron-density plots of 9 obtained at the MP2/TZVPP level of
theory in the planes a) C16-C17-B1 and b) C17-C16-N1 with atom posi-
tions, atomic interaction lines and bond-critical points and corresponding
Laplacian plots (c and d; negative values as dotted lines). The labelling
scheme is shown below.
ꢀ
The ellipticity at the BCP of the bond B1 C17 bond has
a value of 0.09, which indicates a deformation along the
dative bond. This also reflects the partial enamine resonance
structure in the Lewis formula description.
In a related sense, the electron-density topology of 17
ꢀ
ꢀ
(Figure 6) is interesting because it contains an N H···H B
Electron-density topologies of 9 and 17: To get a better de-
scription of the nature of the chemical bonding in the seven-
membered ring system 9 we calculated the electron density
of this molecule at the MP2/TZVPP level of theory (see the
Experimental Section) and analysed the charge density by
means of QTAIM.[8] The electron density and the Laplacian
maps are displayed in Figure 5; selected results are listed in
Table 1.
The calculations show the highest values for the electron
densities within the seven-membered ring for the bonds
ꢀ
N1 C16 (1ACHTUNGTRENNUNG
(BCP)=2.25 eꢄꢀ3; BCP=bond critical point)
and C16 C17 (1ACHTUNGTRENNUNG
(BCP)=1.86 eꢄꢀ3). The Laplacian of these
ꢀ
bonds adopts values typical for covalent bonds:
2
2
ꢀ
ꢀ
ꢀ5 1
(BCP)=18.8 eꢄꢀ5 (N1 C16) and
5 1
ACHTUNGTRNE(NUNG BCP)=
15.7 eꢄꢀ5 (C16 C17). This is consistent with the interpreta-
tion as a partial doubly bonded system and supported by the
ꢀ
Figure 6. Electron density plots of 17 obtained at the RI-BP86/def2-SVP
level of theory in the planes a) B-H-H and b) N-H-H with atom positions,
atomic interaction lines and bond-critical points and corresponding Lap-
lacian plots (c and d; negative values of 5 1 as dotted lines).
Table 1. Parameters of the electron-density topology for selected bonds
2
in 9 in the calculated ground state: Electron densities 1ACTHNUTRGNEUNG
(BCP) [eꢄꢀ3],
2
Laplacians ꢀ5 1
Atoms
ACHTUNGTRENNUNG
(BCP) [eꢄꢀ5] and ellipticities e at the BCP.
2
1
G
ꢀ5 1
H
e
interaction. We discuss here only this part of the molecule.
A bond path between the two hydrogen atoms is found,
with an considerable electron-density value at the bond crit-
ꢀ
B1 C17
ꢀ
B1 C7
0.77
0.94
0.96
1.06
2.25
1.64
1.64
1.86
1.61
1.57
0.41
0.14
0.51
3.28
18.8
14.0
13.9
15.7
12.6
11.9
0.09
0.08
0.11
0.03
0.09
0.03
0.02
0.14
0.02
0.02
ꢀ
B1 C1
ꢀ
B1 C13
ical point (1
adopts a value of ꢀ5 1
(BCP)=ꢀ1.30 eꢄꢀ5. This is a value
(BCP)=0.35 eꢄꢀ3). The Laplacian at this point
ACHTUNGTRENNUNG
ꢀ
N1 C16
2
A
ꢀ
N1 C15
for a typical closed-shell interaction. These values compare
with those for the aminoborane dimer, (H3BNH3)2, in which
the H···H interactions are associated with distances between
1.9 and 2.4 ꢄ and with electron-density values at their BCPs
ꢀ
N1 C19
ꢀ
C16 C17
ꢀ
C15 C14
ꢀ
C14 C13
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Chem. Eur. J. 0000, 00, 0 – 0
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Lewis Acid–Base Pairs
FULL PAPER
between 0.05 and 0.12 eꢄꢀ3 and Laplacian values between
ꢀ5 1
purchased from ABCR. All amines were dried over BaO and freshly
condensed from 4 ꢄ molecular sieves. NMR spectroscopic measurements
were undertaken using Bruker DRX 500, Bruker Avance 500 and Bruker
Avance 600 instruments. NMR spectroscopic chemical shifts were refer-
enced to the residual peaks of the protons of the used solvents (1H, 13C)
or externally (11B, BF3·OEt2; 19F, CFCl3). Elemental analyses were per-
formed using a Leco CHNS 932 instrument.
2
ACHTUNGTRENNUNG
(BCP)=0.60 and 1.11 eꢄꢀ5.[29]
The H···H interaction energy for (H3BNH3)2 has been es-
timated to be 3.0 kcalmolꢀ1 in the solid state for each of the
twelve H···H (bifurcated) interactions to other molecules.[30]
Our estimate for the H···H interaction energy in 17 is
16.1 kcalmolꢀ1; this value is based on the comparison of the
cyclic molecule with a H···H dihydrogen bond (17) and an
open-chain conformation without a short H···H contact (ne-
glecting conformational contributions).
General procedure for the synthesis of 5, 6, 7, and 9: (C6F5)2BH (1)
(200 mg, 0.58 mmol) was suspended in toluene (3 mL) and the corre-
sponding allylamine (N,N-dimethylallylamine (2) (51 mg, 0.58 mmol),
N,N-diethylallylamine (3) (66 mg, 0.58 mmol), 1-allylpiperidine (72 mg,
0.58 mmol), or N,N-diisopropylallylamine (82 mg, 0.58 mmol)) was con-
densed onto it. After warming to room temperature, the clear solution
was heated to 1008C overnight and the volatiles removed under reduced
pressure to dryness. The residue was washed with pentane (2 mL) and
the solvent was removed under vacuum until dryness was achieved. In all
cases a colourless solid was obtained. Single crystals suitable for single-
crystal X-ray diffraction were obtained from a concentrated toluene or
hexanes solution, respectively, at ꢀ358C after several days.
ꢀ
ꢀ
The characteristics of the B H and N H bonds are as fol-
2
(BCP)=0.94 eꢄꢀ3, ꢀ5 1
(BCP)=1.82 eꢄꢀ3, ꢀ5 1(BCP)=28.7 eꢄꢀ5,
which compares well with Popelierꢁs values for the dimer of
(BCP)=0.48 eꢄꢀ5
ACHTUNGTRENNUNG
ꢀ
lows: B H
1
2
ꢀ
and N H
1
A
ACHTUNGTRENNUNG
2
H3BNH2: B H 1
(BCP)=1.07 eꢄꢀ3, ꢀ5 1
ACHTUNGTRENNUNG
ꢀ
2
and N H 1
(BCP)=2.42 eꢄꢀ3, ꢀ5 1
ꢀ
(CH2)3NMe2 (5): Yield: 195 mg (78%); 1H NMR (500.1 MHz,
ACHUTNGRENU(NG C6F5)2BACHUTNGTRENNGUN
[D8]toluene, 298 K): d=1.94 (m, 2H; NCH2), 1.88 (s, 6H; NCH3), 1.55
(m, 2H; BCH2 CH2), 1.48 ppm (br, 2H; B CH2); 11B{1H} NMR
(160.4 MHz, [D8]toluene, 298 K): d=0.2 ppm (n1/2 =97 Hz); 13C{1H} NMR
ꢀ
ꢀ
Conclusion
(125.7 MHz, [D8]toluene, 298 K): d=148.5 (dm, 1J
ACHTUNGTRENNUNG
C6F5), 140.1 (dm, 1J
A
ACHTUNGTRENNUNG
Hydroboration of a series of allylamines with Piersꢁ borane,
(C6F5)2BH, resulted in the formation of five-membered ring
systems with intramolecular dative B N bonds (5, 6 and 7).
In contrast to the ethyl-substituted amine, the reaction of
N,N-diisopropylallylamine with (C6F5)2BH led to a seven-
251.5 Hz; C6F5), 119.5 (br; i-C6F5), 66.9 (NCH2), 45.3 (NCH3), 20.0
(BCH2 CH2), 17.5 ppm (br; BCH2); 19F NMR (470.5 MHz, [D8]toluene,
ꢀ
ꢀ
298 K): d=ꢀ163.7 (m, 4F; m-C6F5), ꢀ157.3 (t, 3J
ACHTUNGTREN(NUGN F,F)=20.6 Hz, 2F;
p-C6F5), ꢀ128.6 ppm (m, 4F; o-C6F5); elemental analysis calcd (%) for
C17H12BF10N (431.08): C 47.37, H 2.81, N 3.25; found: C 47.45, H 2.73, N
3.26.
membered ring system (F5C6)2BACHTUNGTRENNUG(CH2)3NACHTUGNTERN(NUGN iPr)CMeCH2 (9)
A
ACHTUNGTRENNUNG
under dihydrogen formation. In the proposed mechanism,
A
ACHTUNGTRENNUNG
an a-hydride abstraction from one isopropyl group at N by
the BACHTUNGTRENNUNG(C6F5)2 moiety takes place, thus forming an iminium
cation, which eliminates dihydrogen to yield the product by
means of an enamine. The different reactivities obviously
depend on the steric demand of the nitrogen substituents.
Calculations confirm that two isopropyl groups prevent the
A
ACHTUNGTRENNUNG
ꢀ
NCH2CH3), 2.19 (m, 2H; NCH2), 1.55 (m, 2H; BCH2 CH2), 1.48 (br,
2H; B CH2) 0.48 ppm (t, 3J
(H,H)=7.4 Hz, 6H; NCH2CH3);
ꢀ
11B{1H} NMR (160.4 MHz, [D8]toluene, 298 K): d=2.2 ppm (n1/2
=
141 Hz); 13C{1H} NMR (125.7 MHz, [D8]toluene, 298 K): d=148.6 (dm,
1J(C,F)=237.2 Hz; C6F5), 140.1 (dm, 1J
(C,F)=250.1 Hz; C6F5), 137.7
(dm, 1J
(C,F)=248.4 Hz; C6F5), 120.5 (br; i-C6F5), 53.5 (NCH2), 50.0
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ꢀ
formation of a B N bond such as in 5, 6 and 7. In a proposed
ꢀ
(NCH2CH3), 21.1 (BCH2 CH2), 17.6 (br; BCH2), 11.0 ppm (CH3);
19F NMR (470.5 MHz, [D8]toluene, 298 K): d=ꢀ163.6 (m, 4F; m-C6F5),
mechanism, the unquenched reaction potential leads then to
a-hydride abstraction and follow-up reactions that finally
lead to 9. A key step is possibly the formation of a hydrido-
3
ꢀ157.2 (t, J
(F,F)=20.6 Hz, 2F, p-C6F5), ꢀ127.3 ppm (m, 4F; o-C6F5); el-
emental analysis calcd (%) for C19H16BF10N (459.14): C 49.70, H 3.51, N
3.05; found: C 49.59, H 3.81, N 3.04.
boranate–ammonium species with
a
relatively strong
1
(C6F5)2B
N
(CH2)5 (7): Yield: 219 mg (80%); H NMR (500.1 MHz,
ꢀ
ꢀ
B H···H N dihydrogen bond.
[D8]toluene, 298 K): d=3.18 (mt, 3J
ACHTUNGTRENNUNG
On the basis of the structural results from single-crystal
X-ray diffraction but also an electron-density topology anal-
ysis based on calculated charge-density distribution, this
compound (9) can be described as an electronically delocal-
ised system with a partially dative nature of the newly
2.26 (m, 2H; NCH2(B)), 2.09 (d, 3J
ACHTUNGTRENNUNG
ꢀ
ꢀ
1.56 (m, 2H; BCH2 CH2), 1.50 (m, 2H; BCH2 CH2), 1.22–1.20 (m, 1H;
(CH2CH2)2CH2), 1.02–0.85 ppm (m, 5H; (N
(N
G
(CH2CH2)2CH2);
11B{1H} NMR (160.4 MHz, [D8]toluene, 298 K): d=1.6 ppm (n1/2
120 Hz); 13C{1H} NMR (125.7 MHz, [D8]toluene, 298 K): d=148.6 (dm,
1J(C,F)=243.3 Hz; C6F5), 140.0 (dm, 1J
(C,F)=250.3 Hz; C6F5), 137.6
(dm, 1J
(C,F)=249.0 Hz; C6F5), 119.8 (br; i-C6F5), 53.7 (NCH2(B)), 50.0
(NCH2A(pip), 22.3 (N(CH2CH2)2CH2), 20.5 (N(CH2CH2)2CH2) 19.5
=
R
ACHTUNGTRENNUNG
ꢀ
ꢀ
formed boron carbon bond (B1 C17) and the double-bond
AHCTUNGTRENNUNG
ꢀ
character of C16 C17 within the seven-membered ring
G
G
ACHTUNGTRENNUNG
(BCH2 CH2), 16.2 ppm (br; BCH2); 19F NMR (470.5 MHz, [D8]toluene,
298 K): d=ꢀ163.6 (m, 4F; m-C6F5), ꢀ157.2 (m, 2F; p-C6F5), ꢀ126.9 ppm
(m, 4F; o-C6F5); elemental analysis calcd (%) for C20H16BF10N (471.14):
C 50.99, H 3.42, N 2.97; found: C 50.96, H 3.23, N 2.94.
ꢀ
system.
(iPr)C(Me)CH2 (9): Yield: 210 mg (75%); 1H NMR
AHCTNUTGERUNG(NN C6F5)2BACHTNURTGEG(NNUN CH2)3NACHTNUGTRENNGUN
Experimental Section
(500.1 MHz, [D8]toluene, 298 K): d=3.02 (sept, 3J
ACTHNUTRGNE(NUG H,H)=6.65 Hz, 1H;
ꢀ
NCH), 2.90 (brs, 2H; BCH2 Cq), 2.86 (brm, 2H; NCH2), 1.61 (brm, 2H;
General methods: All manipulations were performed under a rigorously
dry inert atmosphere of argon using standard Schlenk and glovebox tech-
niques. Toluene was dried with potassium and hexane was dried with
LiAlH4 before being employed in reactions. [D8]Toluene was dried with
Na/K alloy and degassed. Piersꢁ borane (C6F5)2BH,[17] N,N-diethylallyl-
ꢀ
BCH2 CH2), 1.34 (brs, 2H; BCH2), 1.06 (s, 3H; CqCH3), 0.41 ppm (d,
A
3J
(H,H)=6.65 Hz, 6H; CH3); 11B{1H} NMR (160.4 MHz, [D8]toluene,
298 K): d=ꢀ14.4 ppm (n1/2 =52 Hz); 13C{1H} NMR (125.7 MHz,
[D8]toluene, 298 K): d=197.4 (Cq), 148.1 (dm, 1J
ACHTNUGTRNEG(UN C,F)=236.6 Hz; C6F5),
A
138.6 (dm, 1J
(C,F)=246.7 Hz; C6F5), 137.1 (dm, 1J
ACHUTGTNRENNUG ACHUTNTGREN(NUGN C,F)=241.8 Hz;
ꢀ
C6F5), 53.9 (NCH), 47.4 (NCH2), 42.7 (br; BCH2 Cq), 24.4 (br and s;
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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N. W. Mitzel et al.
Table 2. Crystallographic data for compounds 5, 6, 7 and 9.
5
6
7
9
formula
Mr
C17H12BF10N
431.09
C19H16BF10N
459.14
C20H16BF10N
471.15
C21H18BF10N
485.17
T [K]
100(2)
100(2)
100(2)
100(2)
crystal size [mm]
crystal system
space group
a [ꢄ]
b [ꢄ]
c [ꢄ]
0.30ꢅ0.29ꢅ0.20
triclinic
P1
0.30ꢅ0.24ꢅ0.08
orthorhombic
Pbca
8.9409(3)
17.0929(7)
24.8217(8)
90
0.30ꢅ0.28ꢅ0.26
monoclinic
P21/c
11.4389(2)
16.4885(3)
19.9068(4)
90
90.421(1)
90
3754.5(1)
8
0.16ꢅ0.14ꢅ0.14
monoclinic
P21/c
14.7644(6)
10.9518(4)
13.4735(5)
90
111.586(1)
90
2025.8(1)
4
¯
7.4523(3)
7.6260(3)
16.6484(6)
91.9844(15)
96.0757(14)
118.9928(15)
818.90(5)
2
a [8]
b [8]
90
90
g [8]
V [ꢄ3]
Z
3793.4(2)
8
1.608
0.162
1856
1calcd [gcmꢀ3
]
1.748
0.182
432
1.667
0.166
1904
1.591
1.401
984
m [mmꢀ1
(000)
]
F
A
q range [8]
index range
3.4–30.0
ꢀ10ꢁhꢁ10
ꢀ10ꢁkꢁ10
ꢀ23ꢁlꢁ23
7614
3.1–27.5
ꢀ11ꢁhꢁ11
ꢀ22ꢁkꢁ22
ꢀ32ꢁlꢁ32
21893
3.0–30.0
0ꢁhꢁ16
0ꢁkꢁ23
ꢀ27ꢁlꢁ28
90771
3.2–72.0
ꢀ18ꢁhꢁ17
ꢀ13ꢁkꢁ13
ꢀ16ꢁlꢁ16
26213
reflns collected
unique reflns
4673
4315
10901
3951
observd reflns (2s)
Rint
4404
0.0251
3217
0.055
9140
0.043
3820
0.0277
data/restraints/params
4673/0/264
1.052
4315/0/282
1.014
10901/0/578
1.027
3951/0/301
1.050
GoF (F2)
R1, wR2 (I>2s(I))
R1, wR2 (all data)
0.0354, 0.0995
0.0373, 0.1014
0.44/ꢀ0.21
0.0386, 0.0879
0.0594, 0.0974
0.27/ꢀ0.24
0.0422, 0.1058
0.0549, 0.1147
0.32/ꢀ0.26
0.0327, 0.0851
0.0336, 0.0859
0.31/ꢀ0.33
D1(max/min) [eꢄꢀ3
]
ꢀ
BCH2 and BCH2 CH2), 23.1 (CqCH3), 18.3 ppm (CH3), quaternary
carbon of C6F5 ring was not observed; 19F NMR (470.5 MHz, [D8]toluene,
298 K): d=ꢀ164.8 (m, 4F; m-C6F5), ꢀ161.0 (m, 2F; p-C6F5), ꢀ133.5 ppm
(m, 4F; o-C6F5); elemental analysis calcd (%) for C21H18BF10N (485.17):
C 51.99, H 3.74, N 2.89; found: C 52.25, H 3.71, N 2.94.
We gratefully acknowledge the financial support of the Fonds der Chemi-
schen Industrie (stipend for D. W.) and the DFG within the priority pro-
gram SPP1178.
Crystallographic structure determinations: Single crystals of com-
pounds 5, 6, 7 and 9 suitable for X-ray diffraction measurement were
chosen inside a glovebox, suspended in a paratone-N/paraffin oil mixture,
mounted on a glass fibre and transferred onto the goniometer of the dif-
fractometer. The measurements were carried out with MoKa radiation
(l=0.71073 ꢄ) for 5, 6 and 7 and with CuKa radiation (l=1.54178 ꢄ) for
9. Crystallographic data of compounds 5, 6, 7 and 9 are listed in Table 2.
The structures were solved by direct methods and refined by full-matrix
least-squares cycles (program SHELX-97[31]). CCDC-865379 (5), 865380
(6), 865381 (7) and 865382 (9) contain the supplementary crystallographic
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.
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Acknowledgements
We thank Klaus-Peter Mester and Dr. Andreas Mix for recording the
NMR spectra and Brigitte Michel for performing the elemental analyses.
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Received: February 10, 2012
Revised: April 17, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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www.chemeurj.org
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N. W. Mitzel et al.
Hydroboration
D. Winkelhaus, B. Neumann,
H.-G. Stammler, R. J. F. Berger,
Y. V. Vishnevskiy,
ꢀ
Two roads diverged: Two different
reaction pathways in the hydrobora-
tion of allylamines R2NCH2CHCH2
with (C6F5)2BH lead to the expected
heterocycles with B N bonds for small
R substituents, but to hydride abstrac-
tion and evolution of dihydrogen for
R=iPr (see scheme).
N. W. Mitzel* ................. &&&& — &&&&
Inherent Stability Limits of Intramo-
lecular Boron Nitrogen Lewis Acid–
Base Pairs
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ÝÝ
These are not the final page numbers!