Angewandte
Communications
Chemie
The HB(C6F5) catalyzed cyclotrimerization is not limited
= =
to the parent allene CH2 C CH2. Under similar conditions
we observed the formation of the cyclohexylallene trimeriza-
tion product 3b. The reaction was carried out at 1008C in
CD2Cl2 (24 h) in a sealed tube. Workup eventually gave the
tris-cyclohexyl substituted trimethylenecyclohexane product
3b as a colorless oil in 75% yield. The NMR analysis showed
that selectively the compound was formed that had all three
cyclohexyl substituents attached directly at the central six-
membered carbocyclic core in
(Scheme 4).
a cis,trans-arrangement
Scheme 4. HB(C6F5)2 catalyzed cyclotrimerization of cyclohexylallene.
We assume that a reaction pathway similar as depicted in
Scheme 3 is followed here, only that the hydroboration and
carboboration reactions all preferentially take place at the
Figure 2. A view of the molecular structure of compound 14 (ellipsoids
set at 30% probability; only one molecule of two found in the
asymmetric unit is discussed).[19] Selected bond lengths [ꢂ] and angles
[8]: B1–C11 1.556(7), B2–C18 1.546(6), B3–C19 1.553(6), C11–C12
1.543(6), C14–C18 1.545(5), C16–C19 1.538(5); B1-C11-C12 118.8(4),
B2-C18-C14 118.1(3), B3-C19-C16 116.5(3), ꢀB1ccc =359.6,
ꢀB2ccc =359.9, ꢀB3ccc =360.0.
=
unsubstituted allene CH2 terminals (for a respective reac-
tion Scheme see the Supporting Information). Consequently,
1
=
a pair of H NMR resonances were observed for the CH2
hydrogen atoms of C3,5 CH2 methylene groups (d = 4.71 and
=
1
=
4.58) and a singlet for the pair of symmetry-equivalent C
CH2 hydrogen atoms at d = 4.65 (13C: d = 152.3, 107.0 (C3,5
slightly in the conformational orientation of the planes of the
attached C6F5 substituents.
=
1
=
CH2), d = 149.9, 113.2 (C CH2)).
We have started to look for new applications of the now
easily available allene cyclotrimerization product 1,3,5-trime-
thylenecyclohexane (3a) and found a remarkable outcome of
its reaction with the hydroboration reagent HB(C6F5)2. For
this purpose we generated compound 3a by our catalytic
process in CD2Cl2 in situ (24 h, 608C) and then determined
the amount of formed product 3a by NMR spectroscopy with
an internal standard. Careful addition of three molar equiv-
alents of HB(C6F5)2 (in CD2Cl2 solution) at room temperature
gave the tris-hydroboration product 14, which we isolated as
a white solid in 59% yield.
The bulky Lewis acidic borane 14 serves as an active
=
hydrogenation catalyst for the imine PhCH NtBu under mild
conditions (RT, 1.5 bar H2). The reaction is a frustrated Lewis
pair reaction with the imine serving in both roles as a substrate
and the bulky base necessary for the FLP dihydrogen splitting
reaction.[16] The tris-borane also served as a catalyst for the
hydrogenation of the enamine 1-piperidinostyrene without an
additional Lewis base, although this reaction required more
forcing reaction conditions (for details, see the Supporting
Information).
The B(C6F5)2 groups in compound 14 are Lewis acidic.
Exposure to pivalonitrile resulted in the formation of the
respective tris-Lewis base adduct to the trivalent boron Lewis
acid functionalities of compound 14. In solution, the tris-
adduct 15 (Scheme 5) showed a single set of 13C NMR tBuCN
resonances at d = 121.5 (CN), and d = 29.7/26.8 (tBu), and
a single 11B NMR resonance at d = À5.5 (n1/2 ꢀ 600 Hz) (19F:
Dd19Fm,p = 5.4 ppm). Compound 15 was also characterized by
X-ray diffraction (see the Supporting Information for details,
including the depicted structure of the tris-nitrile adduct).
The NMR spectra of compound 14 indicated (on average)
a C3v-symmetric structure in solution, that is, an all-cis
arrangement of the three 1,3,5-attached CH2B(C6F5)2 sub-
stituents at the cyclohexane core (1H NMR: d = 2.01 (CH2-
[B]), d = 1.67 (CH), d = 1.51/1.01 (ring CH2); 13C: d = 41.3 (br,
CH2[B]), d = 37.6 (CH), d = 45.8 (ring CH2)). The 11B NMR
signal (d = 74.7) and the 19F NMR resonances (Dd19Fm,p
=
13.4) both indicated the presence of symmetry-equivalent
B(C6F5)2 groups with planar tricoordinate boron.
This was confirmed by the X-ray crystal structure analysis
of compound 14 (Figure 2). It shows a cyclohexane core in
À
a chair conformation with the three CH2B(C6F5)2 substitu-
ents attached in 1,3,5-positions (that is, at the carbon atoms
C12, C14, C16; see Figure 2 with unsystematical atom
numbering). These substituents are found in an all-cis
arrangement, and they are oriented in equatorial positions.
The boron atoms are all planar tricoordinate. They differ
Scheme 5. Selective tris-hydroboration reaction of compound 3a.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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