An enamine/HB(C6F5)2 adduct as a dormant state in frustrated lewis pair chemistry

Article abstract of DOI:10.1021/om4004225

The enamine piperidinocyclopentene reacts with HB(C₆F ₅)₂ by formation of the C-Lewis base/B-Lewis acid adduct 10. It shows a zwitterionic iminium ion/hydridoborate structure. However, this adduct formation is apparently reversible and may generate the "invisible" frustrated Lewis pair 11 as a reactive intermediate by hydroboration of the enamine C=C bond in an equilibrium situation at room temperature. Consequently, the FLP 11 was trapped by typical FLP reactions, namely by the reaction with dihydrogen to give the ammonium/hydridoborate 12, the acetylene deprotonation products 13 and 14, and simple borane adducts with pyridine (15) and with an isonitrile (17). The products 10 and 12-15 and the isonitrile adduct 17 were characterized by X-ray diffraction. A DFT study determined the thermodynamic features of the 10 a? 11 equilibrium and of a previously discussed reference system (18 a? 19) derived by reacting piperidinocyclohexene with HB(C₆F₅)₂.

Full text of DOI:10.1021/om4004225

Article
pubs.acs.org/Organometallics
An Enamine/HB(C6F5)2 Adduct as a Dormant State in Frustrated Lewis
Pair Chemistry
Bao-Hua Xu,†,§ Kathrin Bussmann,† Roland Frohlich,†,∥ Constantin G. Daniliuc,†,∥
̈
Jan Gerit Brandenburg,‡,⊥ Stefan Grimme,‡,⊥ Gerald Kehr,† and Gerhard Erker*,†
†Organisch-chemisches Institut der Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
‡Mulliken Center for Theoretical Chemistry, Institut fur Physikalische und Theoretische Chemie, Universitat Bonn, Beringstraße 4,
53115 Bonn, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The enamine piperidinocyclopentene reacts
with HB(C6F5)2 by formation of the C-Lewis base/B-Lewis
acid adduct 10. It shows a zwitterionic iminium ion/
hydridoborate structure. However, this adduct formation is
apparently reversible and may generate the “invisible”
frustrated Lewis pair 11 as a reactive intermediate by
hydroboration of the enamine CC bond in an equilibrium situation at room temperature. Consequently, the FLP 11 was
trapped by typical FLP reactions, namely by the reaction with dihydrogen to give the ammonium/hydridoborate 12, the
acetylene deprotonation products 13 and 14, and simple borane adducts with pyridine (15) and with an isonitrile (17). The
products 10 and 12−15 and the isonitrile adduct 17 were characterized by X-ray diffraction. A DFT study determined the
thermodynamic features of the 10 ⇄ 11 equilibrium and of a previously discussed reference system (18 ⇄ 19) derived by
reacting piperidinocyclohexene with HB(C6F5)2.
Scheme 1
INTRODUCTION
■
Frustrated Lewis pair chemistry1 has made some remarkable
achievements in metal-free small-molecule activation (e.g.,
heterolytic dihydrogen splitting2,3 and catalytic hydrogena-
tion4,5) and small-molecule binding (e.g., CO2,6 SO2,7 nitrogen
oxides8,9). Some rather unusual reactions have been carried out
at intramolecular frustrated Lewis pair templates (e.g.,
anomalous Staudinger reaction,10 formylborane formation11).
Frustrated Lewis pairs (FLP) are comprised of pairs of Lewis
acids and bases that are hindered from efficient neutralization
by steric1 or electronic means.12 This situation often allows for
finding interesting synergistic or cooperative reactions with
added substrates. However, this does not mean that FLPs are
completely devoid of any interaction between their Lewis acid
(LA) and base (LB) components. In the reactions of
intermolecular FLPs with small molecules, some interaction
between the components may even be necessary to avoid the
termolecularity “trap”. In many vicinal P/B and N/B FLPs we
had found weak intramolecular interactions between the Lewis
acid and base components. For the examples 1 and 3 (in
Scheme 1) we determined the Gibbs activation energies of the
reversible rupture of the LA···LB interaction by dynamic 19F
NMR spectroscopy (ca. 12−14 kcal mol−1).13−15 The typical
FLP reactions of these systems are likely to proceed via the
open isomers (1′, 3′) out of these equilibria.
relatively mild reaction conditions (2.5 bar, room temperature)
it cleanly gave the dihydrogen splitting product 8 that was
derived from the “invisible” FLP 7, which had probably been
available from the equilibrium situation.
We have now found another (simpler) example for such a
behavior, indicating that the role of preceding LA−LB adduct
formation needs to be given proper attention in the design of
frustrated Lewis pair systems and their reactions.
The adduct situation can be quite extreme in a few FLP
cases. We recently found that the camphor-derived enamine 5
selectively formed the apparently stable adduct 616 upon
treatment with Piers’ borane [HB(C6F5)2] (Scheme 2).17
Nevertheless, when this was exposed to dihydrogen under
Special Issue: Applications of Electrophilic Main Group Organo-
metallic Molecules
Received: May 14, 2013
Published: July 22, 2013
© 2013 American Chemical Society
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Lewis base adduct 10 and not to the anticipated formation of
the respective hydroboration product 11. This behavior is
different from that of other enamine/HB(C6F5)2 reactions (see
above) that in many cases cleanly resulted in the formation of
the vicinal N/B FLPs under comparable reaction conditions.
Nevertheless, we decided to expose the adduct 10 to
dihydrogen. For that purpose the adduct 10 was generated in
situ in pentane solution from the enamine 9 and HB(C6F5)2
(15 min, room temperature). Then the system was exposed to
dihydrogen (2.5 bar, 30 min). After 5 min a white precipitate
began to appear. After a 30 min reaction time the precipitate
was collected to give a 87% yield of the product 12 (see
Scheme 3 and Figure 2). The 1H NMR spectrum of compound
Scheme 2
Scheme 3
RESULTS AND DISCUSSION
■
The special FLP system that we have investigated here is
derived from cyclopentanone. It was converted to the enamine
piperidinocyclopentene (9). This enamine was then treated
with 1 molar equiv of HB(C6F5)2. Workup after 2 h of stirring
the dichloromethane solution at room temperature eventually
gave the product 10, which was isolated as a white solid
material in 94% yield.
The adduct 10 was characterized by an X-ray crystal structure
analysis (Figure 1). The structure determination showed that
Figure 1. Molecular structure of the zwitterionic product 10.
compound 10 was formed by addition of the intact HB(C6F5)2
Lewis acid to the nucleophilic β-carbon atom of the enamine
CC double bond (B1−C2 = 1.692(4) Å, sum of the CBC
angles at boron ∑BCCC = 334.3(2)°, angles C1−C2−C3 =
102.6(2)°, C1−C2−B1 = 104.2(2)°, C3−C2−B1 = 111.4(2)°).
This has resulted in the formation of an iminium ion unit (C1−
N1 = 1.301(3) Å, ∑C1CCN = 360.1°, ∑N1CCC = 359.8(2)°).
In solution compound 10 shows a typical iminium ion 13C
NMR resonance at δ 206.2 and a 11B NMR doublet at δ −19.9
(1JBH ≈ 95 Hz). Due to the chiral carbon center in the five-
membered carbocyclic ring system the C6F5 substituents at the
pseudotetrahedral boron center of 10 are diastereotopic. They,
consequently, give rise to two equal-intensity sets of 19F NMR
Figure 2. Molecular structure of the zwitterionic ammonium/hydrido
borate dihydrogen activation product 12.
12 features a broad [N]−H resonance at δ 6.75 and a [B]−H
1
signal at δ 2.63 (1:1:1:1 quartet, JBH ≈ 76 Hz). We have also
identified the NCH (δ 2.61) and BCH (δ 1.56) resonances
(corresponding 13C NMR signals at δ 74.0 (NCH) and δ 33.1
(BCH), respectively). Compound 12 shows a 11B NMR
resonance at δ −21.6 (1JBH ≈ 78 Hz), and it features the 19F
NMR signals of a pair of diastereotopic C6F5 groups at the
boron atom (for further details see the Supporting
Information).
a
resonances (δ −132.6 (o), −162.5 (p), −166.3 (m, C6F5 ); δ
b
1
−132.8 (o), −161.2 (p), −165.6 (m, C6F5 )). In the H NMR
spectrum of 10 we have observed the [B]−H resonance as a
broad signal at δ 2.79.
The X-ray crystal structure analysis of compound 12 shows a
central five-membered carbocyclic ring that bears the N-
protonated piperidino substituent and the B(H)(C6F5)2 group
trans-1,2-attached to it (C1−N1 = 1.516(3) Å, C1−C2 =
We conclude that the exposure of the enamine 9 to the
reactive borane HB(C6F5)2 at room temperature apparently
leads to the formation of the boron Lewis acid/enamine-carbon
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1.541(3) Å, C2−B1 = 1.633(4) Å, dihedral angle N1−C1−
C2−B1 = −66.6(3)°). The protonated nitrogen atom shows a
sum of its CNC angles of ∑N1CCC = 337.1°. The borate B
atom likewise has a ∑B1CCC = 338.0° value that indicates a
distorted pseudotetrahedral coordination geometry. The five-
membered core ring shows a distorted envelope-like con-
formation,18 and the piperidinium moiety has an equatorially
substituted chair conformation.
It appears that the ammonium/borate product 12 was
formed by means of heterolytic dihydrogen cleavage by the not
directly observed FLP 11. It seems to have become populated
to some extent, probably by a pathway involving reversibility of
the enamine/HB(C6F5)2 adduct formation followed by a regio-
and stereoselective hydroboration reaction of the enamine C
C bond of 9. Consequently, it might be possible to trap the
alleged reactive FLP intermediate 11 by other typical FLP
reactions as well. This was actually the case. We could show
that the adduct 10 reacted slowly with phenylacetylene to
eventually yield the typical FLP reaction product 13. It had
often been observed that reactive FLPs tend to deprotonate 1-
alkynes to form ammonium (or phosphonium) alkynyl borate
products.19 In our case the respective zwitterionic ammonium/
alkynyl borate product 13 was isolated in 80% yield. It shows a
[N]−H 1H NMR resonance at δ 8.41 and the 13C NMR signals
of the [B]−CC−Ph unit at δ 110.1 (1JBC ≈ 70 Hz) and δ
99.2, respectively. The NCH and the BCH 1H NMR
resonances were located at δ 3.12 (δ(13C) 74.5) and δ 1.91
(δ(13C) 33.4 (1JBC ≈ 55 Hz), respectively. Compound 13
shows a typical borate anion 11B NMR signal at δ −18.1. Again,
the attached C6F5 groups are diastereotopic.
Figure 4. Molecular structure of compound 14.
added to the boron Lewis acid (B1−C11 = 1.603(3) Å, C11−
C12 = 1.208(3) Å, C12−C13 = 1.435(3) Å, C13−C14 =
1.331(3) Å). We find the usual trans-1,2-N,B-substitution
pattern at the central five-membered ring of compound 14
(C1−N1 = 1.523(2) Å, C2−B1 = 1.651(3) Å, dihedral angle
N1−C1−C2−B1 = 81.9(2)°).
1
Compound 14 features a H NMR [N]−H signal at δ 8.34
and a 11B NMR signal at δ −18.2. The 13C NMR NCH signal
occurs at δ 74.4 and that of the adjacent BCH at δ 33.3. The
conjugated ene-acetylide ligand shows a set of 13C NMR
resonances at δ 109.4 (BC), 100.6 (C), 129.2 (C), and
120.1 (CH2).
The adduct 10 also reacts with simple donor ligands via
equilibration with the hydroboration isomer 11 to yield typical
FLP adducts.20 With pyridine it forms the [B]−pyridine adduct
15 (see Scheme 4), which is characterized by a 11B NMR
In the X-ray crystal structure analysis of 13 (see Figure 3) we
find the −CCPh group attached at the boron atom (B1−
Scheme 4
Figure 3. Projection of the molecular structure of compound 13.
resonance at δ 1.4 and the occurrence of a pair of diastereotopic
C6F5 groups at boron. In this case some of the HB(C6F5)2 has
also become trapped by the pyridine donor to form the
pyridine−HB(C6F5)2 adduct 16 (ca. 3 mol %).
The FLP−pyridine adduct 15 was characterized by X-ray
diffraction (Figure 5). It features the trans-1,2-attachment of
the piperidino substitutent at C1 and the B(C6F5)2(pyridine)
group at C2 of the central saturated five-membered carbocycle
(N1−C1 = 1.479(2) Å, C2−B1 = 1.639(2) Å, dihedral angle
N1−C1−C2−B1 = 99.1(1)°). The boron atom is four-
coordinated, and it has the pyridine ligand bonded to it
(B1−N2 = 1.644(2) Å).
C11 = 1.598(8) Å, C11−C12 = 1.200(6) Å, C12−C13 =
1.437(8) Å, angles B1−C11−C12 = 176.5(5)°, C11−C12−
C13 = 175.3(5)°). The boron and the nitrogen atom are again
trans-1,2-attached at the central five-membered ring (B1−C2 =
1.668(7) Å, C1−N1 = 1.511(7) Å, dihedral angle B1−C2−
C1−N1 = 76.7(5)°), and the nitrogen atom is found to be
protonated (∑N1CCC = 338.3°).
The enamine/HB(C6F5)2 adduct 10 reacts similarly with 2-
methylbutenyne. In this case the reaction was facilitated by the
addition of a catalytic amount of pyridine. Stirring of a solution
of 10 with the enyne overnight at room temperature resulted in
the formation of the formal trapping product 14 of the alleged
FLP 11 with the terminal acetylene. The X-ray crystal structure
analysis (see Figure 4) shows that the conjugated acetylide
anion, formed by deprotonation by the amine base of 11, was
We have also treated the in situ formed pyridine adduct 15
with tert-butyl isocyanide. This leads to the formation of the
new FLP−isonitrile adduct 17 (admixed with the pyridine−
HB(C6F5)2 adduct 16 (ca. 6 mol %) and the enamine 9 (ca. 6
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effect. We calculated the equilibrium energies at the PW6B95-
D3//TPSS-D3 level utilizing def2-QZVP AO basis sets and
included corrections to free enthalpies in solution with the
COSMO-RS model (for details see the Supporting Informa-
tion). In the case of the cyclopentane-derived system, adduct
formation (giving 10) is favored by ca. 15 kcal mol−1 over the
hydroboration reaction, giving the FLP 11. In contrast, in case
of the cyclohexane-derived system hydroboration of the
cyclohexenylamine to give the FLP 18 is slightly favored (by
ca. 1.5 kcal mol−1) over the simple Lewis acid−Lewis base
adduct 19 formation.
This amazing thermodynamic difference is probably due to a
sum of effects, but the internal stabilization of the FLP (or the
lack of it) is likely a major factor. We had previously shown that
the piperidinocyclohexene-derived FLP 18 shows a weak
internal amine−borane interaction.15 We determined the
Gibbs activation energy of the N···B dissociation of this system
by dynamic 19F NMR spectroscopy as ΔG⧧diss =13.2 0.2 kcal
mol−1, which marks an upper limit of the N···B bond strength
in 18. In the piperidino−cyclopentene hydroboration product
11 the specific geometry probably prohibits any strong internal
N···B interaction so that this thermodynamic stabilizing feature
is missing for the 10 ⇄ 11 equilibrium system (similar to what
must be assumed for the previously described 6 ⇄ 7
equilibrium16 (see Scheme 2)).
Figure 5. Molecular structure of the FLP−pyridine adduct 15.
mol %)). We obtained crystals of compound 17 that were
suitable for an X-ray crystal structure analysis (see Figure 6). It
CONCLUSION
■
We have shown that the enamine piperidino−cyclopentene
forms the Lewis acid−Lewis base adduct 10 upon treatment
with HB(C6F5)2. The formation of this adduct by attack of the
nucleophilic enamino carbon at the strongly electrophilic boron
center is thermodynamically favorable (by >15 kcal mol−1
according to our DFT calculations). Nevertheless, the LA−
LB adduct formation is reversible. The FLP 11 can be formed
in an equilibrium situation. This was shown by the respective
trapping reactions that made use of some typical FLP reactions.
The situation is different in the piperidino−cyclohexene/
HB(C6F5)2 case. There we have not observed the formation of
the respective iminium−hydridoborate LA···LB adduct but
instead hydroboration of the enamino CC bond to yield the
isolated FLP 18. The main difference between these two
superficially closely related systems seems to be that the FLP 18
featuring the chair-shaped cyclohexylene backbone is geometri-
cally ideally set to undergo a stabilizing internal amine−borane
LA···LB interaction that is apparently much less favorable for
the cyclopentylene-derived analogue 11. It seems that this
internal N···B interaction in 18 is important, since it is of a
sufficient magnitude to bring some thermodynamic stabilization
that helps to shift the equilibrium (19 ⇄ 18) to the FLP side,
but it is weak enough still to allow for a high FLP reactivity.
These systems underline the importance of weak Lewis acid−
Lewis base interactions in frustrated Lewis pair chemistry and
show us that a suitable balance of interaction vs free Lewis
acid−base systems can very favorably determine the chemistry
of specific FLP systems.
Figure 6. Molecular structure of compound 17.
shows that we have trapped the FLP 11 from the unfavorable
10 ⇄ 11 equilibrium by forming the isonitrile addition product
to the boron Lewis acid (B1−C11 = 1.616(3) Å, C11−N2 =
1.141(2) Å, angles B1−C11−N2 = 175.4(2)°, C11−N2−C12
= 172.2(2)°). The free nitrogen base of the piperidino unit
(∑N1CCC = 336.8°) and the boron substituent are again trans-
1,2-attached at the central five-membered ring (N1−C1 =
1.466(3) Å, C2−B1 = 1.630(3) Å, dihedral angle N1−C1−
C2−B1 = 70.2(2)°).
At first sight it may seem surprising that treatment of
piperidinocyclohexene with HB(C6F5)2 gives the hydroboration
product 1815 whereas the piperidinocyclopentene−HB(C6F5)2
reaction leads to a completely different result, namely to the
formation of the Lewis acid−Lewis base adduct 10 (see Scheme
5). A DFT analysis21 showed that this is a thermodynamic
Scheme 5
EXPERIMENTAL SECTION
■
General Procedure. Reactions with air- and moisture-sensitive
compounds were carried out under argon using Schlenk-type glassware
or in a glovebox. Solvents and the commercially available substrate
were dried and distilled under argon prior to use. Bis-
(pentafluorophenyl)borane (HB(C6F5)2) was prepared as described
in the literature.22 The following instruments were used for physical
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characterization of the compounds. Elemental analyses: Foss-Heraeus
CHNO-Rapid. ESI mass spectra: Bruker Daltonics MicroTof. NMR:
Varian UNITY plus 600 NMR spectrometer (1H, 599.9 MHz; 13C,
150.8 MHz; 19F, 564.4 MHz; 11B, 192.4 MHz), Varian 500 MHz
INOVA (1H, 499.9 MHz; 13C, 125.7 MHz; 19F, 470.3 MHz; 11B, 160.4
MHz); Bruker AV 300 (1H, 300 MHz; 13C, 75 MHz, 31P, 122 MHz;
19F, 282 MHz, 11B, 96 MHz). In 1H NMR and 13C NMR, the chemical
shift δ is given relative to TMS and referenced to the solvent signal;
19F NMR, chemical shift δ is given relative to CFCl3 (external
reference); 11B NMR, chemical shift δ is given relative to BF3·Et2O
(external reference). Assignments of the resonances were supported by
2D experiments. In the lists of NMR data, a superscript “t” indicates a
tentative assignment. X-ray diffraction: Data sets were collected with a
Nonius KappaCCD diffractometer. Programs used: data collection,
COLLECT;23 data reduction, Denzo-SMN;24 absorption correction,
Denzo;25 structure solution, SHELXS-97;26 structure refinement,
SHELXL-97;27 graphics, XP.28 Thermal ellipsoids are shown with
30% (10, 12, 14, 15, 17) or 15% (13) probability, R1 values are given
for observed reflections, and wR2 values are given for all reflections.
Preparation of Enamine 9. The reactions were carried out under
argon. A mixture of cyclopentanone (1.6 mL, 18.2 mmol) and
piperidine (5.3 mL, 54.6 mmol, 3 equiv) in pentane was stirred at
room temperature for 1 h and was cooled to 0 °C. Then a solution of
titanium tetrachloride (1 mL, 9.1 mmol, 0.5 equiv) in pentane was
added dropwise over 30 min. The reaction mixture was stirred for 1 h
before it was warmed to room temperature and stirred for another 5 h.
The resulting gray suspension was filtered under argon, and the
residue was washed with pentane (2 × 30 mL). The organic phases
(pentane) were combined, and all volatile components were removed
under reduced pressure. Pure enamine 9 (2.3 g, 83%) was obtained by
distillation of the crude product at 53−55 °C under 2 mbar. 1H NMR
(600 MHz, C6D6, 299 K): δ 4.49 (m, 1H, CH), 2.76 (m, 4H,
NCH2), 2.47 (m, 2H, 3-H), 2.31 (m, 2H, 5-H), 1.85 (m, 2H, 4-H),
1.40 (m, 4H, 7-H), 1.28 (m, 2H, 8-H). 13C{1H} NMR (151 MHz,
C6D6, 298 K): δ 152.5 (CN), 96.9 (CH), 49.7 (NCH2), 32.3
(C5), 31.0 (C3), 25.9 (C7), 24.8 (C8), 23.2 (C4).
0.31 (−0.21) e Å−3. The hydrogen atom at B1 was refined freely;
others were calculated and refined as riding atoms.
Preparation of Compound 12. Enamine 9 (30 mg, 0.20 mmol)
and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7
mL), and the mixture was stirred for 15 min. After the solution was
degassed, H2 (2.5 bar) was introduced for 30 min, whereupon a white
powder precipitated after 5 min. The precipitate was collected by
cannula filtration after 2 h and washed with pentane (2 × 2 mL). All
volatile components were removed in vacuo to yield compound 12 (87
mg, 87%). Crystals suitable for X-ray crystal structure analysis were
grown by a saturated CH2Cl2/cyclopentane (1/3 v/v) solution of 12
at −30 °C. Anal. Calcd for C22H20BF10N: C, 52.93; H, 4.04; N, 2.81.
Found: C, 52.95; H, 4.13; N, 2.87. 1H NMR (600 MHz, CD2Cl2, 299
K): δ 6.75 (br m, 1H, NH), 3.49, 2.76 (each m, each 1H, 10-H), 3.38,
3.05 (each m, each 1H, 6-H), 2.63 (1:1:1:1 q, 1JHB ≈ 76 Hz, 1H, BH),
2.61 (m, 1H, NCH), 2.19, 0.94 (each m, each 1H, 5-H)t, 2.10, 1.90
(each m, each 1H, 9-H)t, 2.06, 1.72 (each m, each 1H, 7-H)t, 1.96,
1.47 (each m, each 1H, 8-H)t, 1.77, 1.57 (each m, each 1H, 3-H)t,
1.71, 1.52 (each m, each 1H, 4-H)t, 1.56 (m, 1H, BCH)g (a superscript
“g” indicates a ghsqc experiment). 13C{1H} NMR (151 MHz, CD2Cl2,
299 K): δ 74.0 (NCH), 55.9 (C6), 46.4 (C10), 33.1 (br 1:1:1:1 q, 1JBC
≈ 52 Hz, BCH), 28.2 (m, C5)t, 25.2 (C9)t, 24.7 (C7)t, 23.3 (br, C3)t,
22.7 (C8)t, 22.0 (br, C4)t (C6F5 not listed). 11B NMR (192 MHz,
1
CD2Cl2, 299 K) δ −21.6 (d, JBH = 77.8 Hz). 19F NMR (564 MHz,
a
b
CD2Cl2, 299 K): δ −133.5 (2F, o-C6F5 ), −133.7 (2F, o-C6F5 ),
a
−162.6 (t, 3JFF = 20.0 Hz, 1F, p-C6F5 ), −164.0 (t, 3JFF = 20.1 Hz, 1F,
p-C6F5 ),−165.5 (m, 2F, m-C6F5 ), −167.1 (m, 2F, m-C6F5 ).
b
a
b
X-ray crystal structure analysis of compound 12: formula
C22H20BF10N, M = 499.20, colorless crystals, 0.17 × 0.13 × 0.03
mm, a = 11.9859(8) Å, b = 10.9740(7) Å, c = 16.7187(6) Å, β =
102.097(3)°, V = 2150.2(2) Å3, ρcalcd = 1.542 g cm−3, μ = 1.337 mm−1,
empirical absorption correction (0.804 ≤ T ≤ 0.961), Z = 4,
monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2)
K, ω and φ scans, 17568 reflections collected ( h, k, l), (sin θ)/λ =
0.60 Å−1, 3654 independent (Rint = 0.063) and 2780 observed
reflections (I > 2σ(I)), 315 refined parameters, R1 = 0.054, wR2 =
0.147, maximum (minimum) residual electron density 0.24(−0.20) e
Å−3. Hydrogen atoms at B1 and N1 were refined freely; others were
calculated and refined as riding atoms.
Preparation of Compound 10. HB(C6F5)2 (79 mg, 0.20 mmol)
was dissolved in CH2Cl2 (6 mL) and the solution stirred for 5 min. A
solution of 9 (30 mg, 0.20 mmol) in CH2Cl2 (2 mL) was added at
room temperature. After the reaction mixture had been stirred for 2 h,
all volatile components were removed under reduced pressure. The
resulting residue was washed with pentane (2 × 30 mL) and dried
under vacuum to give a white solid of 10 (93 mg, 94%). Crystals
suitable for X-ray crystal structure analysis were grown by diffusion of
cyclopentane into a saturated CH2Cl2 solution of 10 at −30 °C. Anal.
Calcd for C22H18BF10N: C, 53.15; H, 3.65; N, 2.82. Found: C, 53.16;
Preparation of Compound 13. Enamine 9 (30 mg, 0.20 mmol)
and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in CH2Cl2 (7
mL), and the mixture was stirred for 15 min. Then a solution of
phenylacetylene (20 mg, 0.20 mmol) in pentane (3 mL) was added.
After the reaction mixture had been stirred for 12 h, the solvent and
volatile components were removed under reduced pressure. The
resulting residue was washed with pentane (2 × 30 mL) and dried
under vacuum to give a light yellow solid (13; 96 mg, 80%). Crystals
suitable for X-ray crystal structure analysis were grown by a saturated
CH2Cl2/cyclopentane (1/5 v/v) solution of 13 at −30 °C. Anal. Calcd
for C30H24BF110N: C, 60.12; H, 4.04; N, 2.34. Found: C, 60.32; H,
4.01; N, 2.15. H NMR (500 MHz, CD2Cl2, 299 K): δ 8.41 (br, 1H,
NH), 7.36 (m, 2H, o-Ph), 7.29 (m, 2H, m-Ph), 7.26 (m, 1H, p-Ph),
3.75, 2.74 (each m, each 1H, 10-H), 3.40, 2.91 (each m, each 1H, 6-
H), 3.12 (m, 1H, NCH), 2.08, 1.01 (each m, each 1H, 5-H)t, 1.91 (br,
1H, BCH), 1.87, 1.49 (each m, each 1H, 9-H)t, 1.83, 1.37 (each m,
each 1H, 7-H)t, 1.70 (m, 2H, 4-H)t, 1.58, 1.42 (each m, each 1H, 3-
H)t, 1.57, 1.30 (each m, each 1H, 8-H)t. 13C{1H} NMR (126 MHz,
CD2Cl2, 299 K): δ 131.7 (o-Ph), 128.9 (m-Ph), 128.0 (p-Ph), 125.2 (i-
Ph), 123.7 (br, i-C6F5), 110.1 (br 1:1:1:1 q, 1JBC ≈ 70 Hz, BC), 99.2
(PhC), 74.5 (NCH), 55.7 (C6), 46.4 (C10), 33.4 (br 1:1:1:1 q, 1JBC
≈ 55 Hz, BCH), 29.4 (C5)t, 24.9 (C7)t, 24.6 (C4)t, 23.7 (C3, C9)t,
22.5 (C8)t (C6F5 not listed). 11B{1H} NMR (160 MHz, CD2Cl2, 299
K) δ −18.1 (ν1/2 ≈ 40 Hz). 19F NMR (470 MHz, CD2Cl2, 299 K): δ
1
H, 3.81; N, 2.88. H NMR (600 MHz, CD2Cl2, 299 K): δ 3.76, 3.41
(each m, each 1H, 6-H), 3.63, 2.76 (each m, each 1H, 10-H), 3.44 (br,
1H, 2-H), 3.02, 2.52 (each m, each 1H, 5-H), 2.79 (br m, 1H, BH),
2.07, 1.84 (each m, each 1H, 4-H)t, 2.06, 1.96 (each m, each 1H, 3-
H)t, 1.77, 1.68 (each m, each 1H, 7-H)t, 1.72, 1.56 (each m, each 1H,
8-H)t, 1.54, 1.43 (each m, each 1H, 9-H)t. 13C{1H} NMR (151 MHz,
CD2Cl2, 299 K): δ 206.2 (NC), 123.5 (br, i-C6F5), 54.4 (C10), 54.0
(C6), 46.1 (br, C2), 34.8 (C5), 31.5 (C3)t, 26.5 (C9)t, 26.3 (C7)t,
23.2 (C8)t, 21.1 (C4)t (C6F5 not listed). 11B NMR (192 MHz,
1
CD2Cl2, 299 K): δ −19.9 (d, JBH ≈ 95 Hz). 19F NMR (564 MHz,
a
b
CD2Cl2, 299 K): δ −132.6 (m, 2F, o-C6F5 ), −132.8 (m, 2F, o-C6F5 ),
b
−161.2 (t, 3JFF = 20.1 Hz, 1F, p-C6F5 ), −162.5 (t, 3JFF = 20.2 Hz, 1F,
a
b
a
p-C6F5 ), −165.6 (m, 2F, m-C6F5 ), −166.3 (m, 2F, m-C6F5 ).
X-ray crystal structure analysis of compound 10: formula
C22H18BF10N, M = 497.18 colorless crystals, 0.48 × 0.42 × 0.10
mm, a = 9.4611(2) Å, b = 10.7218(2) Å, c = 11.6021(3) Å, α =
70.181(1)°, β = 82.793(1)°, γ = 81.050(1)°, V = 1090.37(4) Å3, ρcalcd
= 1.514 g cm−3, μ = 0.148 mm−1, empirical absorption correction
a
b
3
−130.5 (m, 2F, o-C6F5 ), −133.7 (m, 2F, o-C6F5 ), −162.0 (t, JFF
=
a
b
20.4 Hz, 1F, p-C6F5 ), −163.5 (t, 3JFF = 20.3 Hz, 1F, p-C6F5 ), −165.3
(m, 2F, m-C6F5 ), −166.4 (m, 2F, m-C6F5 ).
a
b
(0.932 ≤ T ≤ 0.985), Z = 2, triclinic, space group P1 (No. 2), λ =
̅
0.71073 Å, T = 223(2) K, ω and φ scans, 9370 reflections collected
( h, k, l), (sin θ)/λ = 0.60 Å−1, 3674 independent (Rint = 0.038)
and 3188 observed reflections (I > 2σ(I)), 311 refined parameters, R1
= 0.056, wR2 = 0.139, maximum (minimum) residual electron density
X-ray crystal structure analysis of compound 13: formula
C30H24BF10N, M = 599.31, colorless crystals, 0.28 × 0.16 × 0.13
mm, a = 14.2206(10) Å, b = 15.3184(12) Å, c = 16.8620(20) Å, β =
113.912(7)°, V = 3357.9(5) Å3, ρcalcd = 1.185 g cm−3, μ = 0.942 mm−1,
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Organometallics
Article
empirical absorption correction (0.778 ≤ T ≤ 0.887), Z = 4,
monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2)
K, ω and φ scans, 16908 reflections collected ( h, k, l), (sin θ)/λ =
0.60 Å−1, 4567 independent (Rint = 0.100) and 2344 observed
reflections (I > 2σ(I)), 382 refined parameters, R1 = 0.088, wR2 =
0.286, maximum (minimum) residual electron density 0.22(−0.18) e
Å−3. The hydrogen atom at N1 was refined freely, but with a fixed U
value; others were calculated and refined as riding atoms.
each 1H, 3-H)t, 1.51 (m, 4H, NCH2), 1.44 (m, 2H, 8-H), 1.38, 1.27
(each m, each 1H, 4-H)t (16: 8.57 (m, 2H, o-py), 8.17 (m, 1H, p-py),
7.69 (m, 2H, m-py)). 13C{1H} NMR (126 MHz, CD2Cl2, 299 K): δ
148.5 (o-py), 141.6 (p-py), 125.7 (m-py), 70.2 (NCH), 50.9 (br,
NCH2), 34.0 (br, BCH), 30.5 (C5)t, 27.5 (NCH2), 26.9 (C4)t, 25.5
(C8), 25.2 (C3)t, (16: 147.5 (o-py), 142.4 (p-py), 126.7 (m-py))
(C6F5 not listed). 11B NMR (160 MHz, CD2Cl2, 299 K) δ 1.4 (ν1/2
≈
1
180 Hz, 15), −8.3 (d, JBH ≈ 100 Hz, 16). 19F NMR (470 MHz,
Preparation of Compound 14. Enamine 9 (30 mg, 0.20 mmol)
and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7
mL), and the mixture was stirred for 30 min. Then a solution of
pyridine (ca. 3 mg, 0.04 mmol, 0.2 equiv) in pentane (3 mL) was
added and the reaction mixture was stirred for another 1 h before a
pentane solution (3 mL) of 2-methylbut-1-en-3-yne (20 mg, 0.30
mmol, 1.5 equiv) was added. Upon reaction overnight, the solvent and
volatile components were removed under reduced pressure to give a
white solid of 14 as a main product together with the pyridine−
HB(C6F5)2 adduct 16 in a ratio of 4/1a (overall yield 112 mg; ca. 84%
(14)). Anal. Calcd for C27H24BF10N: C, 57.57; H, 4.29; N, 2.49.
Found: C, 57.30; H, 4.30; N, 2.41. Crystals suitable for X-ray crystal
structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/
4 v/v) solution of the white solid (mixture of 14 and 16 in a ratio of 4/
a
b
CD2Cl2, 299 K): δ −129.7 (m, 2F, o-C6F5 ), −131.1 (m, 2F, o-C6F5 ),
b
−159.3 (t, 3JFF = 20.4 Hz, 1F, p-C6bF5 ), −159.9 (t, 3JFF = 20.5 Hz, 1F,
a
a
p-C6F5 ), −164.6 (m, 2F, m-C6F5 ), −164.9 (m, 2F, m-C6F5 ) (16:
3
−134.0 (m, 2F, o-C6F5), −158.8 (t, JFF = 20.1 Hz, 1F, p-C6F5),
−164.8 (m, 2F, m-C6F5)).
X-ray crystal structure analysis of 15: formula C27H23BF10N2, M =
576.28, colorless crystals, 0.40 × 0.32 × 0.10 mm, a = 10.6415(2) Å, b
= 10.9495(7) Å, c = 11.2153(7) Å, α = 86.286(3)°, β = 77.514(3)°, γ
= 87.659(3)°, V = 1272.73(12) Å3, ρcalcd = 1.504 g cm−3, μ = 1.227
mm−1, empirical absorption correction (0.639 ≤ T ≤ 0.887), Z = 2,
triclinic, space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and
̅
φ scans, 17127 reflections collected ( h, k, l), (sin θ)/λ = 0.60
Å−1, 4329 independent (Rint = 0.031) and 4148 observed reflections (I
> 2σ(I)), 361 refined parameters, R1 = 0.039, wR2 = 0.106, maximum
(minimum) residual electron density 0.19 (−0.20) e Å−3. Hydrogen
atoms were calculated and refined as riding atoms.
1
1a) at −30 °C (a determined by NMR in CD2Cl2 solution). H NMR
(500 MHz, CD2Cl2, 299 K): δ 8.34 (br, NH), 5.14 (dq, 2JHH = 2.2 Hz,
Z
2
4
4JHH = 1.6 Hz, 1H, CH2 ), 5.13 (dp, JHH = 2.2 Hz, JHH = 1.0 Hz,
E
1H, CH2 ), 3.75, 2.80 (each m, each 1H, 10-H), 3.44, 2.98 (each m,
Preparation of Compound 17. Caution! Many isocyanides are
toxic and must be handled with due care. Enamine 9 (30 mg, 0.20
mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane
(7 mL), and the mixture was stirred for 30 min. Then a solution of
pyridine (ca. 3 mg, 0.04 mmol, 0.2 equiv) in pentane (3 mL) was
added, and the reaction mixture was stirred for another 1 h before a
pentane solution (3 mL) of 2-isocyano-2-methylpropane (17 mg, 0.20
mmol) was added. Upon reaction overnight, the solvent and volatile
components were removed under reduced pressure to give a white
solid of 17 as the main product together with 16 in a ratio of 9/1a
(overall yield 95 mg; ca. 75% (17)). Crystals suitable for X-ray crystal
structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/
5 v/v) solution of the white solid (mixture of 17, 16 (6 mol %), and 9
(6 mol %)) at −30 °C (a determined by NMR in CD2Cl2 solution).
Anal. Calcd for C27H27BF10N2: C, 55.88; H, 4.69; N, 4.83. Found: C,
55.58; H, 4.45; N, 4.70. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 2.59
(m, 1H, NCH), 2.42, 2.08 (each m, each 2H, NCH2), 2.11 (m, 1H,
BCH), 2.01, 1.15 (each m, each 1H, 5-H)t, 1.66, 1.41 (each m, each
1H, 4-H)t, 1.64 (s, 9H, tBu), 1.59, 1.46 (each m, each 1H, 3-H)t, 1.24
(m, 2H, 8-H), 1.14, 1.09 (each br m, each 2H, NCH2) (16: 8.57 (m,
2H, o-py), 8.18 (m, 1H, p-py), 7.70 (m, 2H, m-py)). 13C{1H} NMR
(151 MHz, CD2Cl2, 299 K): δ 128.5 (br, CN), 73.3 (NCH), 60.4
(tBu), 50.8 (br, NCH2), 30.6 (C5)t, 29.6 (br, BCH), 29.4 (tBu), 27.0
(NCH2), 25.8 (C3)t, 25.4 (C8), 23.9 (C4)t (16: 147.5 (o-py), 142.4 (p-
py), 126.7 (m-py)) (C6F5 not listed). 11B NMR (192 MHz, CD2Cl2,
each 1H, 6-H), 3.06 (m, 1H, NCH), 2.04, 1.82 (each m, each 1H, 7-
H)t, 2.03, 1.79 (each m, each 1H, 9-H)t, 2.03, 0.95 (each m, each 1H,
5-H)t, 1.91, 1.44 (each m, each 1H, 8-H)t, 1.85 (dd, JHH = 1.6 Hz,
4
4JHH = 1.0 Hz, 3H, CH3), 1.82 (br m, 1H, BCH), 1.71 (m, 2H, 4-H)t,
1.57, 1.40 (each m, each 1H, 3-H)t, [16: 8.57 (m, 2H, o-py), 8.17 (m,
1H, p-py), 7.69 (m, 2H, m-py)]. 13C{1H} NMR (126 MHz, CD2Cl2,
299 K): δ 129.2 (C), 120.1 (CH2), 109.4 (br 1:1:1:1 q, 1JBC ≈ 72
Hz, BC), 100.6 (br, C), 74.4 (NCH), 55.8 (C6), 46.4 (C10),
33.3 (br 1:1:1:1 q, JBC ≈ 54 Hz, BCH), 29.3 (C5)t, 25.1 (C7)t, 24.6
1
(C4)t, 24.2 (CH3), 23.8 (C9)t, 23.7 (C3)t, 22.7 (C8)t (16: 147.5 (o-
py), 142.4 (p-py), 126.7 (m-py)) (C6F5 not listed). 11B NMR (160
1
MHz, CD2Cl2, 299 K) δ −8.3 (d, JBH ≈ 100 Hz, 16), −18.2 (ν1/2
≈
30 Hz, 14). 19F NMR (470 MHz, CD2Cl2, 299 K): δ −130.6 (m, 2F,
a
b
3
o-C6F5 ), −133.8 (m, 2F, o-C6F5 ), −162.2 (t, JFF = 20.3 Hz, 1F, p-
a
3
b
C6F5a), −163.7 (t, JFF = 20.3 Hz, 1F, p-C6F5 ), −165.4 (m, 2F, m-
b
C6F5 ), −166.5 (m, 2F, m-C6F5 ) (16: −133.9 (m, 2F, o-C6F5), −158.8
3
(t, JFF = 20.0 Hz, 1F, p-C6F5), −164.9 (m, 2F, m-C6F5)).
X-ray crystal structure analysis of compound 14: formula
C27H24BF10N, M = 563.28, colorless crystals, 0.35 × 0.18 × 0.12
mm, a = 8.8914(4) Å, b = 21.7999(5) Å, c = 13.4296(5) Å, β =
97.589(4)°, V = 2580.28(16) Å3, ρcalcd = 1.450 g cm−3, μ = 1.184
mm−1, empirical absorption correction (0.682 ≤ T ≤ 0.871), Z = 4,
monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2)
K, ω and φ scans, 19179 reflections collected ( h, k, l), (sin θ)/λ =
0.60 Å−1, 4460 independent (Rint = 0.041) and 3900 observed
reflections (I > 2σ(I)), 357 refined parameters, R1 = 0.046, wR2 =
0.122, maximum (minimum) residual electron density 0.18(−0.26) e
Å−3. The hydrogen atom at N1 was refined freely; others were
calculated and refined as riding atoms.
Preparation of Compound 15. Enamine 9 (30 mg, 0.20 mmol)
and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7
mL), and the mixture was stirred for 30 min. Then a solution of
pyridine (16 mg, 0.20 mmol) in pentane (3 mL) was added to the
white suspension and a clear solution was formed immediately. After
the reaction mixture had been stirred for 2 h, the solvent and volatile
components were removed under reduced pressure to give a white
solid of 15 as the main product together with compound 16 (ca. 3 mol
%). Anal. Calcd for C27H23BF10N2: C, 56.27; H, 4.02; N, 4.86. Found:
C, 56.43; H, 3.98; N, 4.55. Crystals suitable for X-ray crystal structure
analysis were grown by a saturated cyclopentane solution of the white
solid (mixture of 15 and 16) at −30 °C. 1H NMR (500 MHz, CD2Cl2,
299 K): δ 9.32 (m, 2H, o-py), 8.07 (m, 1H, p-py), 7.59 (m, 2H, m-py),
2.68, 2.24 (each m, each 2H, NCH2), 2.65 (m, 1H, NCH), 2.47 (m,
1H, BCH), 1.80, 1.23 (each m, each 1H, 5-H)t, 1.77, 0.86 (each m,
299 K): δ −8.3 (d, 1JBH ≈ 100 Hz, 16), −16.9 (ν1/2 ≈ 120 Hz, 17). 19
F
a
NMR (564 MHz, CD2Cl2, 299 K): δ −132.6 (m, 2F, o-C6F5 ), −133.8
b
3
a
(m, 2F, o-C6F5 ), −159.3 (t, JFF = 20.2 Hz, 1F, p-C6F5 ), −162.1 (t,
b
a
3JFF = 20.3 Hz, 1F, p-C6F5 ), −164.9 (m, 2F, m-C6F5 ), −166.6 (m, 2F,
b
m-C6F5 ) (16: −134.0 (m, 2F, o-C6F5), −158.8 (t, 3JFF = 20.1 Hz, 1F,
p-C6F5), −164.8 (m, 2F, m-C6F5)).
X-ray crystal structure analysis of compound 17: formula
C27H27BF10N2, M = 580.32, colorless crystals, 0.28 × 0.22 × 0.15
mm, a = 14.8527(9) Å, b = 11.7383(4) Å, c = 15.9004(8) Å, β =
95.858(8)°, V = 2757.7(2) Å3, ρcalcd = 1.398 g cm−3, μ = 1.133 mm−1,
empirical absorption correction (0.742 ≤ T ≤ 0.848), Z = 4,
monoclinic, space group P21/c (No. 14), λ = 1.54178 Å, T = 223(2) K,
ω and φ scans, 22826 reflections collected ( h, k, l), (sin θ)/λ =
0.60 Å−1, 4781 independent (Rint = 0.042) and 3986 observed
reflections (I > 2σ(I)), 364 refined parameters, R1 = 0.048, wR2 =
0.136, maximum (minimum) residual electron density 0.23(−0.21) e
Å−3, hydrogen atoms calculated and refined as riding atoms.
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Article
(8) (a) Neu, R. C.; Otten, E.; Lough, A.; Stephan, D. W. Chem. Sci.
2011, 2, 170−176. (b) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am.
Chem. Soc. 2009, 131, 9918−9919.
ASSOCIATED CONTENT
* Supporting Information
Text and figures giving further experimental and spectroscopic
details, CIF files giving crystallographic data for 10, 12, 13−15,
and 17, and tables giving Cartesian coordinates. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(9) (a) Sajid, M.; Stute, A.; Cardenas, A. J. P.; Culotta, B. J.;
Hepperle, J. A. M.; Warren, T. H.; Schirmer, B.; Grimme, S.; Studer,
A.; Daniliuc, C. G.; Frohlich, R.; Petersen, J. L.; Kehr, G.; Erker, G. J.
̈
Am. Chem. Soc. 2012, 134, 10156−10168. (b) Cardenas, A. J. P.;
Culotta, B. J.; Warren, T. H.; Grimme, S.; Stute, A.; Frohlich, R.; Kehr,
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail for G.E.: erker@uni-muenster.de.
G.; Erker, G. Angew. Chem., Int. Ed. 2011, 50, 7567−7571.
■
(10) Stute, A.; Heletta, L.; Frohlich, R.; Daniliuc, C. G.; Kehr, G.;
̈
Erker, G. Chem. Commun. 2012, 48, 11739−11741.
(11) Sajid, M.; Elmer, L.-M.; Rosorius, C.; Daniliuc, C. G.; Grimme,
S.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2013, 52, 2243−2246.
Present Address
§State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Chinese Academy of Sciences,
No. 1, Beiertiao, Haidian District, Beijing 100190, People’s
Republic of China.
(12) (a) Stute, A.; Kehr, G.; Frohlich, R.; Erker, G. Chem. Commun.
̈
2011, 47, 4288−4290. (b) Rosorius, C.; Kehr, G.; Frohlich, R.;
̈
Grimme, S.; Erker, G. Organometallics 2011, 30, 4211−4219.
(c) Appelt, C.; Westenberg, H.; Bertini, F.; Ehlers, A. W.; Slootweg,
J. C.; Lammertsma, K.; Uhl, W. Angew. Chem., Int. Ed. 2011, 50, 3925−
3928.
Author Contributions
∥X-ray crystal structure analyses.
(13) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. Angew. Chem., Int.
Ed. 2010, 49, 1402−1405.
Author Contributions
⊥DFT calculations.
(14) Axenov, K. V.; Momming, C. M.; Kehr, G.; Frohlich, R.; Erker,
̈
̈
G. Chem. Eur. J. 2010, 16, 14069−14073.
Notes
(15) Schwendemann, S.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Sci.
̈
The authors declare no competing financial interest.
2011, 2, 1842−1849.
(16) Schwendemann, S.; Oishi, S.; Saito, S.; Frohlich, R.; Kehr, G.;
Erker, G. Chem. Asian J. 2013, 8, 212−217.
̈
ACKNOWLEDGMENTS
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
■
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MacGillivray, L. R.; Zaworotko, M. J. Organometallics 1998, 17, 2459−
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dx.doi.org/10.1021/om4004225 | Organometallics 2013, 32, 6745−6752

Organometallics
Article
NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published on the Web on July 22, 2013, with an
error in Scheme 1. The corrected version was reposted on July
23, 2012.
6752
dx.doi.org/10.1021/om4004225 | Organometallics 2013, 32, 6745−6752