Chiral carbene-borane adducts: Precursors for borenium catalysts for asymmetric FLP hydrogenations

Article abstract of DOI:10.1039/c6dt02202b

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc₂BH) gave chiral adducts: (IMe)(Ipc₂BH) 24, (IPr)(Ipc₂BH) 25, (IBnMe)(Ipc₂BH) 26, (IPhMe)(Ipc₂BH) 27, and (ItBuMe)(Ipc₂BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2′-methoxy-1,1′-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

Full text of DOI:10.1039/c6dt02202b

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Guenther, J. Farrell, P. Eisenberger, B. Bestvater, P. D. Newman, R. Melen, C. Crudden and D. W. Stephan,
Dalton Trans., 2016, DOI: 10.1039/C6DT02202B.
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DOI: 10.1039/C6DT02202B
Journal Name
ARTICLE
Chiral Carbene–Borane Adducts: Precursors for Borenium
Catalysts for Asymmetric FLP Hydrogenations
Received 00th January
a
ab
a
c
c
Jolie Lam, Benjamin A. R. Günther, Jeffrey M. Farrell, Patrick Eisenberger*, Brian P. Bestvater,
2
0xx,
Accepted 00th January
0xx
b
b
cd
a
Paul D. Newman, Rebecca L. Melen,* Cathleen M. Crudden,* Douglas W. Stephan*
2
The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers’
borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the
corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-
substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared.
These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene
ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from
amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23.
Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr)
diisopinocampheylborane 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-
DOI: 10.1039/x0xx00000x
www.rsc.org/
ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with (Ipc BH) gave chiral adducts:
2
(
IMe)(Ipc
Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-
H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-
2 2 2 2 2
BH) 24, (IPr)(Ipc BH) 25, (IBnMe)(Ipc BH) 26, (IPhMe)(Ipc BH) 27, and (ItBuMe)(Ipc BH) 28.
1
triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were
competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were
obtained. The implications for chiral borenium cation catalyst design are considered.
charge, enhancing the Lewis acidity at boron without the need for
electron withdrawing fluorinated groups. Such systems have been
Introduction
3
4
35
36
Following the advent of the concept of “frustrated Lewis pairs” employed in 1,n-carboboration, hydroboration, haloboration
1
-2
37
(
FLPs) a decade ago, the initial application that emerged from the and alkyne/arene borylation reactions. In addition, borenium
metal-free activation of H was the use of these in hydrogenation cations have been found to act as the Lewis acid constituent of FLPs
2
3
-7
38-40
catalysis, resulting in a burgeoning and exciting area of new for catalytic hydrogenation.
For example, borenium cations of
8
-13
developments.
The scope of substrates that can be reduced by the form [(L)BBN][B(C F ) ] (L = N-heterocyclic carbene (NHC), BBN
6
5 4
an FLP protocol has broadened dramatically from the original = 9-boreniumbicyclo[3.3.1]-nonane), were shown to be effective in
7
38-39
application to imines, protected nitriles and aziridines. Indeed the hydrogenation of imines and enamines.
These catalysts
offer the advantage of being readily derived from robust and easily
have emerged. prepared carbene adducts of 9-BBN. In a subsequent effort, we
1
4
15
protocols for the reduction of enamines, silylenol-ethers,
1
6-18
19
20-23
alkenes,
alkynes, ketones and aldehydes
8
-10, 13, 24-26
These advances have been reviewed in several places.
showed that these particular systems could be optimised for
reactivity by modification of the carbene. Subsequently Crudden
and Eisenberger employed related triazolium-borenium cations in
Analogous to the development of homogeneous transition-
metal based catalysts, following the discovery of FLP catalysts,
attention has turned to the development of stereoselective
systems. The first to report such an advance was Klankermayer,
40
hydrogenation catalysis.
Imine reductions by borenium catalysts typically occur by
who described the use of a chiral electrophilic borane derived from delivery of hydride from the borane adduct to the iminium cation
3
8-39
(
+)-α-pinene in ketimine reduction affording 13% ee in the reduced (Scheme 1).
Thus, introducing chirality in the borane adduct
2
7
amine product. Since then, other chiral catalyst systems have provides potential for controlling stereochemistry in the products.
emerged (Figure 1). In some cases, stereoselectivity has climbed to The use of NHCs and triazolylidene derivatives as supporting
2
8-32
give ee’s as high as 99%.
electron withdrawing pentafluorophenyl-groups to enhance the incorporate chirality through this ligand. In this manuscript, we
Lewis acidity at the boron center that enables H activation and describe the synthesis of chiral carbene adducts of borenium ions
catalytic turnover in imine reduction. Recently however, the and preliminary evaluation of these systems in hydrogenation
These systems (Figure 1) all employ ligands for borenium ions provides a straightforward approach to
2
33
chemistry of borocations has gained significant momentum. In catalysis.
these systems the tri-coordinate boron possesses a formal positive
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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DOI: 10.1039/C6DT02202B
3
h. The reaction mixture was stirred at 55 °C for 18 h while gas
evolution was observed. After cooling to 25 °C, 1 L of water was
added and the aqueous phase was isolated. The aqueous phase
was then made strongly basic by the addition of NaOH and the
product was extracted with CH
organic layers were dried over MgSO
2
Cl
2
(3 x 500 mL). The combined
and the solvent was removed
4
in vacuo to provide the product 2 as a colourless solid. Yield: 7.77 g
1
(
(
(
2
(
54.62 mmol, 58%) H NMR (CDCl
3
): 0.77 (s, 3H), 0.79 (s, 3H), 1.00
s, 3H), 1.24-1.32 (m, 1H), 1.54-1.66 (m, 6H), 1.96-2.04 (m, 1H), 2.96
3
3
13
1
dd, JHH = 9 Hz, JHH = 7 Hz, 1H). C{ H} NMR (CDCl
6.0, 30.4, 38.5, 46.3, 60.9, 61.1. HRMS (ESI+): C
calc.): 143.1543, m/z (obs.): 143.1546.
3
): 16.4, 22.3,
+
8 19 2
H N , [M+H ] m/z
Synthesis of (1R,5S)-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]-
oct-2-ene tetrafluoroborate (3): A suspension of 2.00 g camphoric
Figure 1 Selected chiral electrophilic boranes used in FLP hydrogenations.
The highest enantioselectivities reported for each catalyst are shown.
diamine 2 (14.06 mmol, 1 equiv.) and 1.62 g NH
1
1
4
BF
.1 equiv.) in 15 mL triethylorthoformate (excess) was stirred at
10 °C for 2 h. Upon cooling to 25 °C the precipitate was isolated
O (3 x 30 mL). Removal of the
solvent in vacuo afforded the product 3 as a colourless solid. Yield:
4
(15.45 mmol,
by filtration and washed with Et
2
1
1
3
=
1
.84 g (10.84 mmol, 77%). H NMR (CD
3
CN): 0.98 (s, 3H), 1.11 (s,
3
H), 1.22 (s, 3H), 2.02-2.17 (m, 3H), 2.29-2.35 (m, 1H), 3.42 (d, JHH
5 Hz, 1H), 7.61 (s, 1H), 7.84 (bs, 2H). C{ H} NMR (CD
6.8, 20.6, 34.0, 39.5, 42.7, 62.5, 66.6, 152.4. HRMS (ESI+): C
1
3
1
3
CN): 16.2,
9 17 2
H N ,
+
[
M ] m/z (calc.): 153.1386, m/z (obs.): 153.1388.
Synthesis of (1R,5S)-1,2,4,8,8-pentamethyl-4-aza-2-diazoniabi-
cyclo[3.2.1]oct-2-ene tetrafluoroborate (4), (1R,5S)-2,4-diethyl-
1
,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-2-ene tetrafluoro-
borate (5), (1R,5S)-2,4-diisopropyl-1,8,8-trimethyl-4-aza-2-azonia-
bicyclo[3.2.1]oct-2-ene tetrafluoroborate (6): These compounds
were prepared in a similar fashion from the corresponding
iodoalkane and thus one preparation is detailed. 1.31 mL
iodomethane (20.83 mmol, 5 equiv.) were added to a suspension of
Scheme 1 Mechanism of borenium-based hydrogenation of imines.
1
.00 g diamine
2 3
3 (4.17 mmol, 1 equiv.) and 1.44 g K CO
Experimental Section
(
10.41 mmol, 2.5 equiv.) in MeCN (25 mL). The reaction mixture
General Remarks Unless otherwise stated, all reactions and
manipulations were performed under an atmosphere of dry,
oxygen-free, nitrogen in a glove box (Innovative Technology or
MBraun). All solvents (including deuterated) were dried and stored
was stirred under reflux for 44 h. Upon cooling to 25 °C the solvent
and remaining iodomethane were removed in vacuo. The residual
solid was dissolved in CH
2 2
Cl and then filtered. The resulting filtrate
was concentrated under reduced pressure to give the product 4a as
1
1
over molecular sieves under a nitrogen atmosphere before use. H,
a white solid. Yield: 0.98 g (4.07 mmol, 86%). H NMR (CDCl
3
): 1.08
s, 3H), 1.13 (s, 3H), 1.27 (s, 3H), 1.87-1.93 (m, 1H), 2.05-2.13 (m,
1
3
11
19
31
C, B, F and P NMR data were recorded on a Bruker Avance III
(
3
or Bruker 500 MHz spectrometer. All NMR spectra were recorded
at 298 K unless otherwise stated and these data are reported in
ppm. NMR assignments are further detailed in the SI. A Perkin-
Elmer analyser was used for carbon, hydrogen, and nitrogen
elemental analysis. (3S,7S)-3,7-diisopropyl-2,3,7,8-tetrahydro-
imidazo[4,3-b:5,1-b’]-bis (oxazole)-4-ium triflate (18), and (3S,7S)-
1
3
1
H), 2.31-2.37 (m, 1H), 2.53-2.59 (m, 1H), 3.12 (d, J = 5 Hz, 1H),
HH
1
3
1
.24 (s, 3H), 3.32 (s, 3H), 9.02 (s, 1H). C{ H} NMR (CDCl
6.9, 21.8, 30.6, 37.0, 38.5, 40.8, 41.1, 68.8, 69.5, 153.7.
3
): 14.2,
+
HRMS (ESI+): C11
H
21
N
2
, [M ] m/z (calc.): 181.1699, m/z (obs.):
1
81.1701.
1
5
(
: Yield: 0.89 g (3.00 mmol, 96%). H NMR (CDCl
3
): 1.03 (s, 3H), 1.15
3
,7-di-tertbutyl-2,3,7,8-tetrahydroimidazo
[4,3-b:5,1-
[(3S,7S)-3,7-t-Bu -2,3,7,8-
N-(1-phenylethylidene) aniline, (+)-
3
3
s, 3H), 1.30 (t, J = 7 Hz, 3H), 1.34 (s, 3H), 1.38 (t, J = 7 Hz, 3H),
HH HH
b']bis(oxazole)-4-ium
C H O N ][O SCF ] (19),
diisopinocampheylborane and 1,3-dimethylimidazolium iodide,
1
triflate,
2
41-42
43
1.94-2.01 (m, 1H), 2.10-2.17 (m, 1H), 2.26-2.31 (m, 1H), 2.49-2.55
7
7
2
2
3
3
3
4
4
45
(
m, 1H), 3.19 (d, J = 5 Hz, 1H), 3.52-3.60 (m, 1H), 3.62-3.69 (m,
HH
13
1
4
6
1H), 3.70-3.77 (m, 1H), 3.79-3.86 (m, 1H), 9.05 (s, 1H). C{ H} NMR
-methyl-3-phenylimidazolium
iodide,
1-benzyl-3-
4
7
(CDCl ): 14.1, 14.3, 17.2, 17.3, 21.9, 32.1, 40.6, 40.8, 45.3, 48.7,
methylimidazolium iodide and (3S,7S)-3,7-di-isopropyl-2,3,7,8-
3
+
6
6.3, 69.9, 153.2. HRMS (ESI+): C13
H
25
N
2
, [M ] m/z (calc.): 209.2012,
tetrahydroimidazo[4,3-b:5,1-b’]yilidene-9-
4
8
m/z (obs.): 209.2018.
borabicyclo[3.3.1]nonane
were prepared by literature
procedures. In some cases repeated attempts to secure satisfactory
elemental analyses lead to low C content, despite the use of
additional oxidant, and is attributed to the formation of boron-
carbide during combustion.
1
6
(
3
1
(
: Yield: 2.46 g (6.43 mmol, 96%). H NMR (CDCl
3
): 0.90 (s, 3H), 1.12
3
3
s, 3H), 1.20 (d, J = 6.7 Hz, 3H), 1.27 (d, J = 7 Hz, 3H), 1.32 (s,
H), 1.36 (d, JHH = 7 Hz, 3H), 1.47 (d, JHH = 7 Hz, 3H), 1.92-1.98 (m,
H), 2.04-2.09 (m, 1H), 2.13-2.20 (m, 1H), 2.36-2.42 (m, 1H), 3.28
d, JHH = 5 Hz, 1H), 3.85 (sept, JHH = 7 Hz, 1H), 4.31-4.39 (m, 1H),
.13 (s, 1H). C{ H} NMR (CDCl ): 14.9, 17.7, 20.7, 21.7, 22.2, 23.6,
5.1, 33.4, 40.4, 41.1, 51.1, 55.9, 61.4, 71.5, 151.1. HRMS (ESI+):
C H N , [M ] m/z (calc.): 237.2325, m/z (obs.): 237.2330.
HH
HH
3
3
3
3
Synthesis of (1R,3S)-1,2,2-trimethylcyclopentane-1,3-diamine (2):
To a solution of 18.94 g (1R,3S)-camphoric acid 1 (94.95 mmol,
1
3
1
8
2
3
1
.0 equiv.) and 50 mL of conc. sulfuric acid in 300 mL chloroform
+
was added 17.99 g NaN (276.73 mmol, 2.9 equiv.) over a period of
3
15 29 2
2

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DOI: 10.1039/C6DT02202B
Synthesis
3.2.1]oct-3-ylidene-bis(pentafluorophenyl)borane
suspension of 107 mg of 4 (0.40 mmol, 1 equiv.) and 80 mg -14.2 (d, JBH = 83 Hz). C{ H} NMR (benzene-d
of
(1R,5S)-1,2,4,8,8-pentamethyl-2,4-diazobicyclo- 1H), 1.56-1.69 (m, 4H), 1.95-2.01 (m, 3H), 2.05-2.19 (m, 3H), 2.21-
1
1
[
(7):
A
2.30 (m, 2H), 2.37-2.57 (m, 3H), 3.04 (s, 6H). B NMR (benzene-d
6
):
1
13
1
6
): 15.4, 17.0, 22.2,
K[N(SiMe
3
)
2
] (0.40 mmol, 1 equiv.) in 2 mL THF were stirred at r.t. 23.2, 25.5, 28.7, 31.0, 31.4, 34.6, 35.6, 38.8, 39.0, 40.5, 41.5, 69.0,
+
for 60 h. After filtration over Celite and removal of the solvent in 70.2.1. HRMS(ES+): C19
vacuo, the free carbene was redissolved 2 mL toluene. A solution of (obs.): 302.3009.
H36BN
2
, [M ] m/z (calc.): 302.3008, m/z
1
38 mg of Piers borane (0.4 mmol, 1 equiv.) in 1 mL toluene was
1
1
3
7
0: Yield: 220 mg (0.67 mmol, 67%), H NMR (benzene-d
6
): 0.43 (s,
added. The reaction mixture was stirred at 25 °C for 2 h. The
volatiles were removed in vacuo and the product was extracted
with toluene to give the pure product 5d as an orange solid. Yield:
1
3
3
3
H), 0.69 (s, 3H), 0.74 (s, 3 H), 0.94 (t, JHH = 7 Hz, 3H), 1.05 (t, JHH
=
Hz, 3H), 1.22 (m, 2H), 1.35 (m, 1H), 1.49 (m, 3H), 1.67 (m, 1H),
1.93 (m, 1H), 2.09 (m, 6H), 2.27 (m, 3H), 2.41 (m, 1H), 2.54 (m, 2H),
.14 (m, 1H), 3.60 (sept, JHH = 7 Hz, 1H), 3.67 (sept, JHH = 7 Hz, 1H),
1
27 mg (0.24 mmol, 60%). H NMR (CD
2
Cl
2
): 0.88 (s, 3H), 0.89 (s,
3
3
3
H), 0.98 (s, 3H,), 1.59-1.68 (m, 1H), 1.84-1.90 (m, 1H), 2.20-2.27
11
1
3
11
4.30-4.39 (m, 1H). B NMR (benzene-d
6
): -14.3 (d, JBH = 84 Hz).
): 18.1, 18.3, 20.3, 20.9, 21.7, 22.6, 24.6,
5.4, 25.8, 30.8, 32.1, 32.8, 37.7, 39.7, 40.5, 40.6, 49.9, 52.8, 59.8,
(
(
m, 1H), 2.64 (s, 3H), 1.75 (d, JHH = 5 Hz, 1H), 2.88 (s, 3H). B NMR
CD
13
]
1
13
1
C{ H} NMR (benzene-d
6
2
Cl
2 2 2
): -23.4 (d, JBH = 91 Hz). C{ H} NMR (CD Cl ): 15.5, 16.8,
2
2
1
2.8, 29.6, 36.4, 36.9, 39.3, 41.6, 42.3, 70.7, 71.5, 136.3, 138.2,
+
1
9
3
69.7.2. HRMS (ESI+): C21
H40BN
2
, [M ] m/z (calc.): 330.3321, m/z
: C 76.35, H 11.40, N
2 2
40.2, 147.5, 149.4, 153.6. F NMR (CD Cl ): -133.3 (d, JFF = 22 Hz),
3
3
3
(obs.): 330.3325. Anal. Calcd. for C21H40BN
2
-
134.7 (d, JFF = 22 Hz), -161.57 (t, JFF = 20 Hz), -161.63 (t, JFF =
+
8.48%; Obs. C 76.53, H 10.57, N 8.00%.
2
2
0 Hz), -165.49 to -165.72 (m). HRMS (ESI+): C23H20BF10N , [M ] m/z
1
(
calc.): 524.1591, m/z (obs.): 524.1593. Anal. Calcd. for 11: Yield: 227 mg (0.63 mmol, 63%). H NMR (benzene-d ): 0.45 (s,
6
3
C
5
23
H
20BF10
N
2
: C 52.50, H 4.02, N 5.32%; Obs. C 53.24, H 3.33, N 3H), 0.66 (s, 3H), 0.93-0.97 (m, 9H), 1.15 (d, J = 7.2 Hz, 3H), 1.24
HH
3
.11%.
(d, J = 7.2 Hz, 3H), 1.27 (bs, 2H), 1.41 (bs, 1H), 1.45-1.52 (bm,
H), 1.93-2.04 (bm, 3H), 2.09-2.71 (m, 12H), 5.47 (sept, JHH = 7 Hz,
HH
3
2
1
=
2
3
[
C
7
Synthesis
3.2.1]oct-3-ylidene-bis(pentafluorophenyl)borenium
tetrakis(pentafluorophenyl)borate (8): solution of 287 mg
0.31 mmol, 1 equiv.) trityl tetrakis(pentafluorophenyl)borate
0.31 mmol, 1 equiv.) in toluene was added dropwise to a solution
of
(1R,5S)-1,2,4,8,8-pentamethyl-2,4-diazobicyclo-
3
11
1
H), 5.56 (sept, JHH = 7 Hz, 1H). B NMR (benzene-d
6
): -12.9 (d, JBH
): 18.2, 18.5, 20.5, 21.0, 21.9,
2.7, 24.8, 25.0 (bs), 25.6, 26.0, 27.1 (bs), 30.9, 32.3, 33.0, 37.8,
9.9, 40.7, 40.8, 50.1, 52.9, 59.9, 69. 8.3. HRMS(ESI+): C23
[
13
1
85 Hz). C{ H} NMR (benzene-d
6
A
(
(
H44BN ,
2
+
M ] m/z (calc.): 358.3634, m/z (obs.): 358.3624. Anal. Calcd. for
H44BN : C 77.08, H 12.09, N 7.82%; Obs. C 76.76, H 12.91, N
23 2
of 95 mg borane-adduct 7 (0.31 mmol, 1 equiv.) in toluene at -
4
2
0 °C. The reaction mixture was then allowed to warm slowly to
5 °C and was stirred for 18 h, while an insoluble oil was formed.
.76%.
The solvent was evaporated in vacuo and the crude product Synthesis of (1R,5S)-1,2,4,8,8-pentamethyl-4-aza-2-diazoniabi-
washed with pentane (3 x 3 mL) to afford the desired product 6a as cyclo[3.2.1]oct-3-ylidene-(9-boreniumbicyclo[3.3.1]-nonane)
1
a white solid. Yield: 252 mg (0.26 mmol, 83%). H NMR (CD Cl ): tetrakis(pentafluorophenyl)borate (12), (1R,5S)-2,4-diethyl-1,8,8-
2
2
1
.10 (s, 3H), 1.15 (s, 3H), 1.34 (s, 3H), 1.63-1.72 (bm, 3H), 1.83-1.87 trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-3-ylidene-(9-borenium-
(
bm, 1H), 1.94-2.07 (bm, 6H), 2.15-2.23 (m, 3H), 2.23-2.31 (bm, 5H), bicyclo[3.3.1]-nonane) tetrakis(pentafluorophenyl)borate (13),
3
2
.37-2.44 (m, 1H), 2.93 (s, 3H), 3.05 (s, 3H), 3.15 (d, JHH = 5 Hz, 1H). (1R,5S)-2,4-diisopropyl-1,8,8-trimethyl-4-aza-2-azoniabicycle-
1
1
13 1
B NMR (CD Cl ): -16.8 (s), 89.1 (bs). C{ H} NMR (CD Cl ): 14.4, [3.2.1]oct-3-ylidene-(9-boreniumbicyclo[3.3.1]-nonane) tetrakis-
2
2
2
2
1
3
7.0, 21.8, 22.9, 30.7, 30.8, 31.2, 32.4 (bs), 33.2 (bs), 36.5, 36.7, (pentafluorophenyl)borate (14): These compounds were prepared
7.4, 38.0, 38.9, 41.0, 42.3, 71.1, 71.2, 124.0 (bs), 124.9 (bs), 136.4 in a similar fashion and thus one preparation is detailed. A solution
1
1
1
(
d, JFC = 243 Hz), 138.8 (d, JFC = 246 Hz), 148.7 (d, JFC = 241 Hz), of
287 mg
(0.31 mmol,
1 equiv.)
trityl
tetrakis-
19
1
73.2. F NMR (CD Cl ): -133.1 (bs), -163.7 (m), -167.6 (bs). HRMS (pentafluorophenyl)borate (0.31 mmol, 1 equiv.) in toluene was
2
2
+
(
2
ESI+): C19H35BN , [M+H ] m/z (calc.): 302.3002, m/z (obs.): added dropwise to
a solution of 95 mg borane-adduct 9
3
02.2999.
(0.31 mmol, 1 equiv.) at -40 °C. The reaction mixture was then
allowed to warm slowly to 25 °C and was stirred for 18 h, while an
insoluble oil was formed. The solvent was evaporated in vacuo and
the crude product washed with pentane (3 x 3 mL) to afford the
Synthesis of (1R,5S)-1,2,4,8,8-pentamethyl-4-aza-2-diazoniabi-
cyclo[3.2.1]oct-3-ylidene-(9-borabicyclo[3.3.1]-nonane) (9),
1R,5S)-2,4-diethyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]-
(
desired product 12 as a white solid. Yield: 252 mg (0.26 mmol,
oct-3-ylidene-(9-borabicyclo[3.3.1]-nonane) (10), (1R,5S)-2,4-
diisopropyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-3-ylid-
ene-(9-borabicyclo[3.3.1]-nonane) (11): These compounds were
prepared in a similar fashion and thus one preparation is detailed.
1
8
1
3%). H NMR (CD Cl ): 1.10 (s, 3H), 1.15 (s, 3H), 1.34 (s, 3H), 1.63-
2
2
.72 (bm, 3H), 1.83-1.87 (bm, 1H), 1.94-2.07 (bm, 6H), 2.15-2.23
(
m, 3H), 2.23-2.31 (bm, 5H), 2.37-2.44 (m, 1H), 2.93 (s, 3H), 3.05 (s,
3
11
3
H), 3.15 (d, JHH = 5 Hz, 1H). B NMR (CD
C{ H} NMR (CD Cl ): 14.4, 17.0, 21.8, 22.9, 30.7, 30.8, 31.2, 32.4
2
Cl
2
): -16.8 (s), 89.1 (bs).
1
07 mg of the carbene precursor 4 (0.40 mmol, 1 equiv.) and 80 mg
13
1
2
2
of K[N(SiMe ] (0.40 mmol, 1 equiv.) were suspended in 2 mL of
3 2
)
(
1
2
bs), 33.2 (bs), 36.5, 36.7, 37.4, 38.0, 38.9, 41.0, 42.3, 71.1, 71.2,
THF and stirred at 25 °C for 20 min. The reaction mixture was
filtered through celite and the volatiles were removed in vacuo to
give the free carbene. The carbene was redissolved in 2 mL toluene
and added to a suspension of 48 mg of 9-BBN dimer (0.20 mmol,
1
1
24.0 (bs), 124.9 (bs), 136.4 (d, JFC = 243 Hz), 138.8 (d, JFC
=
1
19
46 Hz), 148.7 (d, JFC = 241 Hz), 173.2. F NMR (CD
2
Cl
2
): 133.1 (bs),
+
-
163.7 (m), -167.6 (bs). HRMS(ESI+): C H BN , [M+H ] m/z (calc.):
19 35 2
3
02.3002, m/z (obs.): 302.2999.
0
.5 equiv.) in 2 mL of toluene. The mixture was stirred for 30 min
1
and the solvent was removed under reduced pressure. The residue 13: Yield: 299 mg (0.29 mmol, 98%). H NMR (CD Cl ): 1.05(s, 3H),
2
2
was extracted with pentane (4 x 2 mL), which was subsequently 1.16 (s, 3H), 1.27 (t, 3J = 7.1 Hz, 3H), 1.37 (t, 3J = 7.1 Hz, 3H), 1.41
removed in vacuo to provide the product 9 as a white solid. Yield: (s, 3H), 1.64-1.70 (m, 3H), 1.81 (s, br, 1H), 1.89-2.02 (m, 6H), 2.07-
1
9
(
8 mg (0.32 mmol, 81%). H NMR (benzene-d ): 0.36 (s, 3H), 0.58 2.12 (m, 1H), 2.14-2.18 (m, 1H), 2.21-2.25 (m, 1H), 2.27-2.37 (m,
6
3
2
s, 3H), 0.74 (s, 3H), 0.97-1.05 (m, 1H), 1.15-1.32 (m, 3H), 1.51 (bs, 5H), 2.42-2.47 (m, 1H), 2.75 (dq, J = 7 Hz, J = 16 Hz, 1H), 3.09
HH HH
3

Dalton Transactions
Page 4 of 14
View Article Online
DOI: 10.1039/C6DT02202B
3
2
(
dq, JHH = 7 Hz, JHH = 14.5 Hz, 1H), 3.22-3.29 (m, 2H), 3.53 (dq, b:5,1-b']bis[1,3]oxazol-4-ylidene-borane
(21),
(3S,7S)-3,7-di-
3
2
11
J
= 7 Hz, J = 16 Hz, 1H). B NMR (CD Cl ): -16.7 (s), 89.1 (bs). isopropyl-2,3,7,8-tetrahydroimidazo[4,3-b:5,1-b']bis[1,3]oxazol-4-
C{ H} NMR (CD
bs), 31.2 (bs), 31.5, 35.7, 36.0, 36.3, 36.4, 39.5, 40.0, 46.4, 48.6, 2,3,7,8-tetrahydroimidazo[4,3-b:5,1-b']bis[1,3]oxazol-4-ylidene-
6.2, 69.4, 136.5 (d, JCF = 237 Hz), 138.2 (d, JFC = 240 Hz), 148.4 (d, (9-borabicyclo[3.3.1]-nonane) (23): These compounds were
FC = 238 Hz) 171.3.4. F NMR (CD
HH
HH
2
2
1
3
1
2 2
Cl ): 13.3, 13.3, 16.3, 16.7, 20.4, 21.8, 22.0, 30.9 ylidene-(dicyclohexylchloroborane) (22), (3S,7S)-3,7-di-tertbutyl-
(
1
1
6
1
19
3
J
2
Cl
2
): -133.1 (bs), -163.7 (t, JFF
=
prepared in a similar fashion and thus one preparation is detailed.
3
+
2
0 Hz), -167.6 (t,
J
FF = 18 Hz). HRMS(ESI+): C21
H
38BN
2
, [M ] m/z Triflate salt 18 (110 mg, 0.285 mmol) and 9-BBN dimer (35 mg,
(
2 3 2
calc.): 329.3123, m/z (obs.): 329.2739. Anal. Calcd. for C21H38BN : C 0.1425 mmol) were dissolved in 2.5 mL THF at -35 °C. K[N(SiMe )
5
3.60, H 3.80, N 2.78%; Obs. C 54.21, H 2.60, N 2.50%.
(58 mg, 0.29 mmol) were dissolved in 2 mL THF at -35 °C and the
solution was added dropwise to the mixture. The mixture was
stirred overnight at room temperature and the volatiles were
removed in vacuo. The residue was extracted with pentane, filtered
through Celite, and dried to a light yellow powder. Crystals for a
1
1
1
1
1
1
4: Yield: 276 mg (0.26 mmol, 93 %). H NMR (CD
2
Cl
2
): 1.09 (s, 3H),
3
3
.15 (s, 3H), 1.26 (d, JHH = 6.5 Hz, 3H), 1.34 (d, JHH = 6.5 Hz, 3H),
.46 (d, JHH = 6.8 Hz, 3H), 1.53 (s, 3H), 1.61 (d, JHH = 6.8 Hz, 3H),
3
3
.64-1.70 (m, 1H), 1.78-1.81 (bm, 1H), 1.81-1.85 (bm, 2H), 1.87-
single crystal X-ray study were obtained by slow evaporation of
.89 (bm, 1H), 1.90-1.94 (bm, 2H), 2.03-2.13 (m, 3H), 2.23-2.39
1
3
pentane. Yield: 20 (60 mg, 0.167 mmol, 58%) H NMR (benzene-d
6
):
(
bm, 7H), 2.55-2.61 (m, 1H), 2.68 (sept,
J
HH = 6.8 Hz, 1H), 3.10
3
4
2
3
3
3
11
4.10 (ddd, JHH = 7 Hz, JHH = 3 Hz, JHH = 1 Hz, 2H), 3.92 (dd, JHH = 9
Hz, JHH = 1 Hz, 2H), 3.72 (ddd, JHH = 9 Hz, JHH = 7 Hz, JHH = 1 Hz,
H), 2.77-1.87 (m, 15H), 1.52-1.41 (m, 2H), 0.63 (d, JHH = 7 Hz, 6H),
0.44 (d, JHH = 7.0 Hz, 6H). B NMR (benzene-d
Hz). C NMR (benzene-d ): 124.0, 75.7, 62.3, 37.0, 36.6, 33.6, 30.0,
26.5, 26.0, 18.6, 14.6. MS (DART) m/z: 357 [M-H] . HRMS (ESI+):
(
sept, JHH = 7 Hz, 1H), 3.32 (d, JHH = 5 Hz, 1H). B NMR (CD
2
Cl
): 13.9, 16.2, 16.8, 19.3, 19.7,
0.3, 21.3, 21.6, 21.8, 29.2, 29.4, 33.0, 35.5, 35.7, 36.4, 36.5, 39.7,
2
): -
3
4
3
2
1
3
1
2
4
2
6.7 (s), 85.8 (bs). C NMR (CD
2
Cl
2
3
2
3
11
1
1
1
6
): -16.9 (d, JBH = 83
0.6, 55.9, 59.8, 61.0, 71.4, 136.1 (d, JCF = 242 Hz), 138.0 (d, JCF
=
13
1
19
6
40 Hz), 148.2 (d, JCF = 241 Hz), 171.6.5. F NMR (CD
2
Cl
2
): -133.1
+
3
3
(
bs), -163.8 (t, JFF = 21 Hz), -167.6 (t, JFF = 18 Hz). HRMS (ESI+):
+
+
[M ] m/z (calc.): C21
2 2
H34BN O 357.2713 (obs): 357.2724. Anal. Calc.
23 2
C H42BN , [M ] m/z (calc.): 356.3472, m/z (obs.): 356.3477.
for C21 : C 70.39, H 9.85, N 7.82%. Found: C 69.30, H 9.34,
2 2
H35BN O
Synthesis of (1S,4R,6R)-2,5-dialkyl-6,9,9-trimethyl-2,5-diaza-3- N 7.42%
borabicyclo[4.2.1]nonanes (alkyl = Me: 15, Et: 16, iPr: 17): These
2
1: Crystals for a single crystal X-ray study were obtained by slow
compounds were prepared in a similar fashion and thus one
preparation is detailed. 50 mg of the borane adduct 12 (0.17 mmol)
were dissolved in 4 mL pentane and heated to 50-110 °C for 4-16 h.
The solvent was then removed in vacuo to provide the pure
evaporation from toluene. Yield: (44 mg, 0.176 mmol, 62%).
1
6
H NMR (benzene-d ): 4.06-3.91 (m, 4H), 3.86-3.74 (m, 2H), 2.89
3
4
3
(
ddq, JHH = 10 Hz, JHH = 7 Hz, JHH = 4 Hz, 2H), 2.40-1.65 (m, 1H),
3
3
11
0
d
2
C
.65 (d, JHH = 7 Hz, 6H), 0.44 (d, JHH = 7 Hz, 6H). B NMR (benzene-
product 15 in quantitative yield as a white solid. Yield: 50 mg
1
13
1
6
): -35.6 (q, JBH = 88 Hz). C NMR (benzene-d
6
): 76.1, 61.1, 29.0,
2.7, 18.3, 14.3. MS (DART) m/z: 249 [M-H] . HRMS (ESI): (calc.). for
2 2
249.1774, (obs): 249.1771. Anal. Calc. for C13H23BN O :
(
0.17 mmol, >99%). H NMR (benzene-d
6
): 0.78 (s, 3H), 0.97 (s, 3H),
+
1
1
2
.15 (s, 3H), 1.35-1.42 (m, 1H), 1.53-1.60 (m, 4H), 1.63-1.95 (m,
3H), 2.09-2.25 (m, 3H), 2.38 (bs, 1H), 2.53 (bs, 1H), 2.59 (s, 3H),
13
H
22BN
2
O
2
3
11
C 62.42, H 9.27, N 11.20%. Found: C 62.02, H 9.27, N 11.10%.
.62 (d, JHH = 5 Hz, 1H), 2.64 (s, 3H). B NMR (benzene-d
6
): 44.2
1
3
1
(
bs). C{ H} NMR (benzene-d
6
): 5.0, 16.6, 20.6, 21.8, 22.7, 24.9 (bs), 22: Crystals for a single crystal X-ray study were obtained by slow
5.6, 28.4, 30.5, 30.8, 34.2, 35.2, 38.3, 38.5, 40.1, 41.0, 44.5, 67.6, evaporation of toluene. Yield: (50 mg, 0.111 mmol, 35%). H NMR
1
2
6
3
+
3
2
3
8.7 (bs), 69.8, 79.2. HRMS(ESI+): C19
02.3002, m/z (obs.): 302.3003.
2 6
H36BN , [M ] m/z (calc.): (benzene-d ): 4.56 (dd, JHH = 7 Hz, JHH = 2 Hz, 2H), 3.91 (dd, JHH = 9
3
4
3
2
Hz, JHH = 1 Hz, 2H), 3.71 (ddd, JHH = 9 Hz, JHH = 7 Hz, JHH = 1 Hz,
2H), 3.02 (dtd, JHH = 14 Hz, JHH = 7 Hz, JHH = 3 Hz, 2H), 2.39 (bs,
1H), 2.36 (bs, 1H), 2.09-0.95 (m, 18H), 0.68 (d, JHH = 7 Hz, 6H), 0.55
3
3
4
1
1
3
1
2
3
6: Yield: 20 mg (0.06 mmol, >99%). H NMR (benzene-d
6
): 0.81 (s,
3
3
3
H), 0.85 (s, 3H), 0.92 (t, JHH = 7 Hz, 3H), 1.20 (t, JHH = 7 Hz, 3H),
.22 (s, 3H), 1.36-1.43 (m, 1H), 1.53-2.01 (m, 15H), 2.12-2.24 (m,
H), 2.43-2.58 (m, 3H), 2.64 (d, J = 5 Hz, 1H), 2.69 (s, 1H), 3.20-
.35 (m, 2H). B NMR (benzene-d
3
11
13
(
d,
(benzene-d
29.6, 29.5, 28.4, 28.3, 28.2, 28.0, 18.7, 14.4. MS (DART) m/z: 413
J
HH = 7 Hz, 6H). B NMR (benzene-d
6
): 3.2 (bs). C NMR
3
6
): 75.9, 64.5, 32.8, 32.4, 31.4, 31.2, 31.1, 30.2, 29.7,
HH
11
13
1
6
): 44.0 (bs). C{ H} NMR
+
[
M-Cl] . HRMS (ESI): calc. for C H BN O 413.33393, found
25 42 2 2
(
benzene-d ): 14.3, 15.7, 17.1, 18.2, 21.7, 29.8, 30.6, 30.8, 31.0,
6
4
13.33381.
3
7
1.5, 37.6, 38.8, 39.0, 39.3, 41.8, 42.5, 46.9, 47.3, 50.4, 51.9, 66.3,
+
6.2. HRMS (ESI+): C H BN , [M ] m/z (calc.): 328.3164, m/z 23: Crystals for a single crystal X-ray study were obtained by slow
2
1
38
2
1
(
2
obs.): 328.3175. Anal. Calcd. for C21H38BN : C 76.35, H 11.90, N evaporation of pentane. Yield: (88 mg, 0.228 mmol, 80%). H NMR
3
3
8
.48%; Obs. C 76.57, H 11.63, N 8.09%.
(benzene-d ): 4.15 (d, J
3.78 (dd, JHH = 9 Hz, JHH = 6 Hz, 2H), 2.82-1.74 (m, 14H), 1.55 (bs,
= 5 Hz, 2H), 4.04 (d, J
= 9 Hz, 2H),
6
HH
HH
3
3
1
1
3
1
7: Yield: 45 mg (0.13 mmol, >99%). H NMR (benzene-d
6
): 0.74 (s,
11
3
3
1H), 1.16 (bs, 1H), 0.88 (s, 18H). B NMR (benzene-d
6
): -16.4 (d,
): 124.3, 78.6, 68.3, 39.4, 35.3,
33.1, 32.4, 27.5, 26.6, 25.9. MS (ESI+) m/z: 385 [M-H]+. HRMS (ESI):
(calc.) for C23 + 385.3026, (obs): 386.3032. Anal. Calc. for
H), 0.85 (s, 3H), 0.86 (d, J =7 Hz, 3H), 1.00 (d, J = 7 Hz, 3H),
.22 (d, JHH = 7 Hz, 6H), 1.25 (s, 3H), 1.49-1.55 (m, 2H), 1.71-1.81
m, 6H), 1.86-1.97 (m, 6H), 2.23-2.32 (m, 3H), 2.94 (d, J = 6 Hz,
HH
H), 3.31 (s, 1H), 3.49 (sept, JHH = 7 Hz, 1H), 3.93 (sept, JHH = 7 Hz,
H). B NMR (benzene-d ): 44.0 (bs). C{ H} NMR (benzene-d ):
0.7, 21.3, 21.9, 22.1, 23.8, 25.0 (bs), 25.4, 27.1, 28.2, 29.0, 31.3,
1
13
HH
HH
3
JBH = 86 Hz), C NMR (benzene-d
6
3
(
3
3
2 2
H38BN O
1
1
2
3
1
1
13
1
C
6
23
H
39BN
2
O
2
: C 71.50, H 10.17, N 7.25%. Found: C 70.45, H 10.16, N
6
6
.79%
2.2, 33.2, 33.4, 42.0, 47.2, 48.1, 48.6, 49.6, 54.6 (bs), 66.9, 70.0. Synthesis of
1,3-dimethylimidazol-2-ylidene-di-(1S,2R,3S,5S)-
+
HRMS (ESI+): C23
H42BN
2
,
[M ] m/z (calc.): 356.3477, (obs.): isopinocampheylborane (24), 1,3-diisopropylimidazol-2-ylidene-
(25), 1-benzyl-3-
3
56.3484. Anal. Calcd. for C H BN : C 77.08, H 12.04, N 7.82%; di-(1S,2R,3S,5S)-isopinocampheylborane
2
3
42
2
Obs. C 77.10, H 12.36, N 7.75%.
methylimidazol-2-ylidene-di-(1S,2R,3S,5S)-isopinocampheylbor-
ane (26), 1-methyl-3-phenylimidazol-2-ylidene-di-(1S,2R,3S,5S)-
isopinocampheylborane (27), 1-tert-butyl-3-methylimidazol-2-
Synthesis of (3S,7S)-3,7-di-isopropyl-2,3,7,8-tetrahydroimidazo-
[
4,3-b:5,1-b']bis[1,3]oxazol-4-ylidene-(9-borabicyclo[3.3.1]-no-
ylidene-di-(1S,2R,3S,5S)-isopinocampheylborane
(28):
These
nane) (20), (3S,7S)-3,7-di-isopropyl-2,3,7,8-tetrahydroimidazo[4,3-
4

Page 5 of 14
Dalton Transactions
View Article Online
DOI: 10.1039/C6DT02202B
.
1
3
compounds were prepared in a similar fashion and thus one 28: (498 mg, 60% yield) H NMR (CD
2
Cl
2
): 7.09 (d, JHH = 2 Hz, 1H),
3
preparation is detailed. In a glovebox, (+)-diisopinocampheylborane 6.73 (d, JHH = 2 Hz, 1H), 3.87 (s, 6H), 2.41-2.32 (m, 1H), 2.27-2.15
396 mg, 1.38 mmol, 1 equiv.), K[N(SiMe ] (290 mg, 1.45 mmol, (m, 1H), 2.11-1.95 (m, 3H), 1.82 (s, 9H), 1.77-1.60 (m, 3H), 1.58-1.48
.05 equiv.) and 1,3-dimethylimidazolium iodide (310 mg, (m, 2H), 1.22-1.00 (br, 20H), 0.63 (d,
(
3 2
)
3
11
1
1
J
HH = 7 Hz, 3H). B NMR
1
13
1
.34 mmol, 1 equiv.) were weighed into a flame-dried nitrogen- (CD
2
Cl
2
): –6.85 (d, JBH = 87 Hz). C{ H} NMR (CDCl , partial): 119.5,
3
cooled Schlenk flask. A magnetic stir bar was added and the flask 118.1, 60.9, 52.0, 50.2, 43.5, 43.2, 43.0, 42.6, 39.7, 39.5, 38.6, 38.6,
was sealed with a rubber septum. The flask was removed from the 34.1, 32.8, 32.2, 32.2, 28.5, 28.3, 24.7, 24.2, 23.5, 23.4. Anal. Calc.
glovebox and connected to a Schlenk line where it was stirred for C28H49BN : C 79.22, H 11.63, N 6.20%. Found: C 78.64, H 11.51,
2
under nitrogen. Dry THF (20 mL) was added via cannula and the N 6.65%.
solution was stirred for 20 h at 25 °C. The solvent was removed in
Synthesis of rac-9-(1,3-diphenyl-1,2,3-triazol-5-ylidene)-B-H-(10)-
phenyl-9-borabicyclo[3.3.2]decane (29): In a glove box a 4 dram
vacuo and the flask was reintroduced to the glovebox. The white
residue was washed with pentane (3 x 2 mL). These washings were
filtered through a plug of Celite and the residue washed with
toluene (3 x 2 mL). The washings were filtered through the same
Celite plug into a separate vial. The vials was capped and put in a
freezer (-35 °C) where colourless crystals formed. The crystals were
2
vial with a stir bar was charged with lithium-B-H -(10)-phenyl-9-
49
borabicyclo[3.3.2]decane (62 mg, 0.25 mmol) and THF (2 mL) was
added. To this solution iodomethane (0.03 mL, 0.5 mmol) was
added dropwise with stirring and after 1 h reaction at ambient
temperature the mixture turns a white milky colour. The reaction
mixture was filtered through a plug of Celite and concentrated in
vacuo to dryness. To the remaining solid was added 1,3-diphenyl-
washed with cold pentane (3 x 1 mL) and dried in vacuo to give 24
1
(
(
198 mg, 37% yield). H NMR (CDCl
3
): 6.81 (s, 1H), 6.74 (s, 1H), 3.88
br, 6H), 2.18 (m, 1H), 2.13-2.07 (m, 2H), 2.05-1.96 (m, 1H), 1.92-
1
H-1,2,3-triazolium tetrafluoroborate (77 mg, 0.25 mmol) as well as
a stir bar. The contents of the vial were dissolved in THF (4 mL) and
a solution of Na[N(SiMe ] (46 mg, 0.25 mmol) in THF (1 mL) was
1
1
3
=
.83 (m, 2H), 1.75-1.63 (m, 3H), 1.59-1.52 (m, 1H), 1.43-1.34 (m,
H), 1.32-1.23 (m, 1H), 1.19-1.02 (m, 2H), 1.122 (s, 3H) 1.117 (s,
H), 1.09 (overlapping s, 6H), 1.08 (d, JHH = 7 Hz, 3H), 0.95 (bd, JHH
3
3
3 2
)
3
3
added dropwise with stirring. The reaction mixture is then capped
and allowed to stir overnight. The reaction mixture was filtered
through a syringe filter and all volatiles removed in vacuo.
9 Hz, 2H), 0.71 (bd, JHH = 9 Hz, 2H), 0.59 (d, JHH = 7 Hz, 3H), (No
1
1
1
B-H peak found). B NMR (toluene-d
8
): –9.1 (d,
) partial: 121.1, 120.0, 50.8, 49.9, 45.0, 43.4,
3.1, 42.7, 39.3, 39.1, 37.8, 37.5, 36.2, 35.3, 33.6, 33.3, 28.7, 28.5,
JBH = 86 Hz).
1
3
1
C{ H} NMR (CDCl
3
Crystallisation from toluene/pentane at –25 °C gave compound 27
4
2
3
1
+
15 mg (0.034 mmol, 13%). H NMR (benzene-d
(
6
): 7.90 (s, 1H), 7.75
3.8, 23.4, 23.13, 23.11. HRMS-EI (m/z): [M-H] calc. for C25
H
42BN
2
:
3
m, 2H), 7.42 (m, 2H), 7.31 (t, JHH = 8 Hz, 2H), 7.10 (m, 1H), 7.06 –
81.34410, found: 381.34507.
6
.82 (m, 6H), 6.80 – 6.76 (m, 2H), 3.17 – 2.48 (m, 4H), 2.38 – 2.05
1
3
11
2
5: Yield: (198 mg, 34%). H NMR (CDCl
3
): 6.97 (d, JHH = 2 Hz, 1H), (m, 11H), 1.94 (m, 1H), 1.47 (m, 1H). B NMR (benzene-d
6
):
̶1 5.06
3
3
1
13
6
.92 (d, JHH = 2 Hz, 1H), 5.84 (septet, JHH = 7 Hz, 1H), 5.13 (septet, (d, JBH = 86 Hz). C NMR (benzene-d ): 172.9, 157.5, 138.0, 135.3,
HH = 7 Hz,1H), 2.25 (m, 1H), 2.19-2.04 (m, 4H), 1.85, (m, 1H), 1.75- 130.4, 130.1, 129.8, 129.7, 128.8, 128.4, 128.1, 127.3, 126.1, 123.4,
6
3
J
3
1
.67 (m, 2H), 1.65-1.54 (m, 3H), 1.46 (d, J = 7 Hz, 3H), 1.459 (d, 121.0, 48.9 (BCHPh), 44.2, 37.8, 36.2, 32.5, 30.9, 28.6, 26.1, 25.3.
HH
3
3
3
J
HH = 7 Hz, 3H), 1.459 (d, JHH = 7 Hz, 3H), 1.37 (d, JHH = 7 Hz, 3H),
3
3
Synthesis of (S)-2-amino-2'-methoxy-1,1'-binaphthalene: From (S)-
1
3
.36 (d, J = 7 Hz, 3H), 1.12 (s, 3H), 1.11(s, 3H), 1.10 (d, J = 7 Hz,
HH HH
3
3
2'-methoxy-1,1'-binaphthyl-2-carboxylic acid (5.45 g, 16.6 mmol) as
H), 1.08 (s, 3H), 1.06 (s, 3H), 0.95 (d, JHH = 9 Hz, 1H), 0.84 (d, JHH
Hz, 1H) 0.48 (d, J = 7 Hz, 3H). B NMR (toluene-d ): –8.2 (d,
BH = 86 Hz). C{ H} NMR (CDCl
=
50
3
11
described in the literature via a Curtius rearrangement. The crude
9
HH
8
1
13
1
product was used directly without any further purification in the
synthesis of 30.
J
3
, partial): 115.8, 115.4, 50.6, 49.9,
4
2
9.3, 48.9, 44.9, 43.3, 43.1, 42.1, 39.0, 38.9, 37.1, 35.4, 33.8, 32.7,
8.38, 28.36, 24.10, 24.08, 23.82, 23.70, 23.38, 23.33, 23.03, 22.9. Synthesis of (S)-2-azido-2'-methoxy-1,1'-binaphthalene (30).
51
Anal. Calc. for C H BN : C 79.43, H 11.72, N 6.39%. Found: C Synthesised using a modified literature procedure. In a 100 mL
2
9
51
2
7
9.30, H 11.90, N 6.39%.
round bottom flask open to air (S)-2-amino-2'-methoxy-1,1'-
binaphthalene (16.6 mmol) was dissolved in MeCN (60 mL) and
tert-butylnitrite (90%, 3.3 mL, 24.9 mmol) followed by Me SiN
(94%, 2.8 mL, 20.0 mmol) were added drop-wise with ice-cooling.
After stirring with ice-cooling for 2 h the mixture was warmed to
ambient temperature and stirred for 24 h. The precipitate was
filtered and washed with a minimal amount of MeCN followed by
copious amounts of pentanes. The title compound was obtained as
an ochre powder. Yield: 3.36 g (10.31 mmol, 62%). All volatiles
were removed in vacuo from the mother liquor and another 1.09 g
1
2
6
6: Yield: (667 mg, 83% yield). H NMR (CD Cl ): 7.42-7.21 (bm, 5H),
.88 (bs, 1H, minor), 6.80(bs, 1H, major), 6.76 (bs, 1H, major), 6.66
2
2
3
3
2
2
(
bs, 1H, minor), 5.74 (d, J = 15 Hz, 1H, minor), 5.66 (d, J = 15
HH HH
2
2
Hz 1H, major), 5.57 (d, JHH = 15 Hz, 1H, major), 5.17 (d, JHH = 15 Hz,
1
0
H, minor), 3.94-3.88 (overlapping singlets, 3H, minor/major), 2.25-
.60 (br, 34H), (No B-H peak found). B NMR (CD Cl ): –8.7
2 2
1
1
1
13
1
(
obscured, minor), 9.11 (d, J = 85 Hz, major). C{ H} NMR (CDCl ,
BH 3
partial, major only): 136.6, 129.0, 128.5, 128.2, 120.6, 119.8, 52.8,
5
3
0.8, 49.94, 44.7, 43.4, 43.0, 42.7, 39.4, 39.1, 37.6, 36.3, 35.3, 33.5,
3.4, 28.6, 28.5, 23.9, 23.4, 23.4. Anal. Calc. for C31 : C 81.20,
(
3.34 mmol, 20%) of the title compound was isolated after flash
column chromatography (silica 10 cm x 4 cm, CH Cl /MeOH 50:1).
Combined yield: 4.44 g, (13.7 mmol, 82%). H NMR (CDCl ): 8.06 (d,
H
47BN
2
2
2
H 10.33, N 6.11%. Found: C 80.72, H 10.81, N 6.16%.
1
3
1
3
3
3
2
6
1
7: Yield: (407 mg , 56% yield). H NMR (CDCl
3
): 7.45-7.40 (bm, 5H),
JHH = 9 Hz, 1H), 8.03 (d, JHH = 9 Hz, 1H), 7.93 (d, JHH = 8 Hz, 2H),
3
3
3
3
.95 (d, J = 2 Hz, 1H,), 6.90 (br, 1H), 4.01 (s, 3H) 2.15-1.55 (br, 7.53 (d, J = 9 Hz, 1H), 7.50 (d, J = 9 Hz, 1H), 7.45 (t, J = 7 Hz,
HH
HH
HH
HH
3
3
0H), 1.15-0.60 (br, 15H), 1.11 (s, 3H), 1.08 (s, 3H), 0.72 (d, JHH = 8 1H), 7.38 (t, JHH = 7 Hz, 1H), 7.34 – 7.25 (m, 2H), 7.34 – 7.25 (m,
11
1
13
1
3
13
1
Hz, 3H). B NMR (CD Cl ): δ –9.74 (d, J = 75 Hz). C{ H} NMR 2H), 7.09 (d, J = 8 Hz, 1H), 3.83 (s, 3H). C{ H} NMR (CDCl ):
2
2
BH
HH
3
(
CD
2
2
Cl , partial): 140.6, 129.0, 128.8, 128.4, 122.4, 121.2, 51.2, 155.0, 136.0, 134.0, 133.8, 131.2, 130.4, 129.8, 129.1, 128.3, 128.2,
5
2
0.3, 44.5, 43.7, 43.5, 42.8, 39.6, 39.4, 38.3, 36.4, 35.3, 33.6, 33.3, 127.1, 126.9, 126.0, 125.3, 125.0, 124.1, 123.9, 118.7, 117.8, 113.7,
8.7, 28.5, 23.9, 23.8, 23.2, 23.1. Anal. Calc. for C30H45BN : C 81.06, 56.8. HRMS(EI ): (calc.) for C21H N O 325.1215, (obs.): 325.1206.
2 15 3
+
H 10.20, N 6.30%. Found: C 80.63, H 10.66, N 6.22%.
5

Dalton Transactions
Page 6 of 14
View Article Online
DOI: 10.1039/C6DT02202B
Synthesis
of
(S)-2-(4-(Trimethylsilyl)-1H-1,2,3-triazolyl)-2'- 9 Hz, 1H), 7.68 (m, 1H), 7.52 (s, 1H), 7.50 – 7.21 (m, 10H), 7.08 (d, J
methoxy-1,1'-binaphthalene, (31): In a glove box a 4-dram vial was = 9 Hz, 1H), 3.80 (s, 3H), 1.96 – 1.24 (m, 10H), 1.06 (m, 3H), 0.54 (s,
1
1
1
charged with (S)-2-azido-2'-methoxy-1,1'-binaphthalene (30, 1H), 0.16 (s, 1H). B NMR (CD
2
Cl
): 155.2, 138.3, 134.8, 133.8, 133.4, 133.3,
(2 mL) and ethynyltrimethylsilane 132.4, 132.0, 131.8, 130.4, 130.2, 129.3, 128.9, 128.9, 128.7, 128.6,
1 mL, 7.08 mmol) was added. The reaction mixture was stirred at 128.3, 127.7, 127.5, 125.5, 124.5, 124.4, 122.5, 116.0, 113.1, 56.5,
ambient temperature for 3 weeks. The product was isolated by 36.6, 36.2, 31.6, 31.6, 26.4, 26.0, 22.9.
flash column chromatography (silica 20 cm x ø 4 cm, CH Cl /MeOH
00:0 → 100:5). Yield: 589 mg (1.39 mmol, 90%). H NMR (CDCl ):
.15 (d, JHH = 9 Hz, 1H), 8.05 (d, JHH = 8 Hz, 1H), 8.00 (d, JHH = 9 Hz,
2
):
̶
18.4 (d, JBH = 84 Hz).
1
3
1
5
00 mg, 1.54 mmol) and [Cu(PPh
3
)Br] (32.0 mg, 0.078 mmol). The
2 2
C{ H} NMR (CD Cl
solids were dissolved in CH Cl
2
2
(
2
2
1
General procedure for high pressure hydrogenation: In an inert
atmosphere glovebox, NHC-borane (0.0261 mmol, 1 equiv.), trityl
tetrakis(pentafluorophenyl)borate (24 mg, 0.0261 mmol, 1 equiv.)
and substrate (0.522 mmol, 20 equiv.) were weighed into vials.
0.4 mL of solvent was used to dissolve the trityl
tetrakis(pentafluorophenyl)borate and the orange solution was
transferred to the vial with the NHC-borane, which turns colourless
upon mixing. The colourless solution was transferred to the vial
with the substrate with an additional 0.2 mL solvent and equipped
1
8
1
7
3
1
1
1
3
3
3
3
3
3
3
H), 7.99 (d, JHH = 9 Hz, 1H), 7.86 (d, JHH = 8 Hz, 1H
Hz, 1H), 7.26 – 7.44 (m, 5H
.
), 7.58 (t, JHH
), 7.12 (d, JHH = 8 Hz, 1H), 7.05 (s, 1H),
.67 (s, 3H), 0.10 (s, 9H). C{ H} NMR (CDCl ): 155.0, 145.0, 134.5,
3
33.8, 133.5, 133.1, 130.8, 130.2, 129.5, 128.8, 128.4, 128.3, 128.1,
=
3
.
13
1
27.4, 127.2, 127.1, 126.9, 124.5, 124.0, 123.4, 117.6, 113.0, 56.4, –
+
.4. HRMS(EI ): (calc.) for C26
H
25
N
3
OSi 423.1767, (obs.): 423.1771.
Synthesis
of
(S)-2-(1H-1,2,3-triazolyl)-2'-methoxy-1,1'- with a stir bar and placed in a Parr pressure reactor. The reactor
binaphthalene, (32): A 50 mL round bottom flask was charged with was sealed, removed from the glovebox, and purged 10 times with
a solution of 31 (1.20 g, 2.83 mmol) in THF (15 mL). A solution of hydrogen and ultimate pressurised to 102 atm. The reactor was
TBAF (3.2 mL, 1.0
stirring the reaction mixture at ambient temperature for 2 h H
30 mL) was added and the aqueous phase was extracted with Conversions were determined by H NMR spectroscopy from an
CH Cl (3x). The combined organic phases were washed with aliquot. The sample was then concentrated in vacuo, dissolved in
saturated brine, dried over MgSO , filtered, and all volatiles were 9:1 hexanes:ethyl acetate and passed through a short silica plug.
M
in THF, 3.2 mmol) was added dropwise. After stirred magnetically at the specified temperature for the specified
2
O
time and then vented slowly. NMR samples were taken in CDCl .
3
1
(
2
2
4
removed on a rotary evaporator. The product was obtained as a The sample was concentrated in vacuo and enantiomeric excess
slightly brown solid. The crude product 32 was identified by was determined by chiral HPLC (Chiralcel OD-H, 98.0 hexanes, 1.0
1
H NMR spectroscopy and used without further manipulations in isopropanol) with comparison to a racemic sample prepared by 1,3-
the next step.
dimethylimidazol-2-ylidene-9-borabicyclo[3.3.1]nonane in the
same hydrogenation process.
Synthesis of (S)-1-(2'-Methoxy-1,1'-binaphthyl-2-yl)-3-phenyl-1H-
1
,2,3-triazolium tetrafluoroborate, (33): Synthesised following a Racemization experiment: In a glove box 9-(1,3-diphenyl-1,2,3-
5
2
modified published method A Schlenk tube was charged with triazol-5-ylidene)-9-borabicyclo[3.3.1]nonane (21 mg, 0.060 mmol)
crude 32 (855 mg, 2.43 mmol) from the previous reaction, and trityl tetrakis(pentafluorophenyl)borate (55 mg, 0.060 mmol)
5
3
diphenyliodonium tetrafluoroborate (1.39 g, 3.76 mmol), CuSO
(
4
were placed in a vial and dissolved in CD
2
Cl
2
(0.5 mL) generating
40
22 mg, 0.138 mmol) in DMF (5 mL) and was stirred at 95 °C for borenium salt 36. The resulting solution was transferred to a J.
1
6 h. The mixture was cooled to ambient temperature and all Young NMR tube equipped with a Teflon stopper. To this solution
volatiles removed in vacuo. The resulting oil was subjected to flash- (+)-bis[(R)-1-phenylethyl]amine (35, 140 µL, 0.612 mmol) was
column chromatography (silica, 4 cm x 8 cm, CH Cl /MeOH 100:1 → added by syringe and the NMR tube was capped and shaken. A
0:1) to give a slightly ochre solid. Yield: 1.121 g (2.17 mmol, 89%). separate NMR tube was charged with a mixture of B(C (31 mg,
): 8.87 (s, 1H), 8.51 (s, 1H), 8.23 (d, JHH = 9 Hz, 1H), 0.060 mmol) and (+)-bis[(R)-1-phenylethyl]amine (35, 140 µL,
2
2
2
6 5 3
F )
1
3
H NMR (CDCl
8
3
3
3
2 2
.08 (d, JHH = 8 Hz, 1H), 8.00 (m, 2H), 7.85 (d, JHH = 8 Hz, 1H), 7.68 0.612 mmol) in CD Cl (0.5 mL). The reaction progress was
3
3
1
(
t, J = 8 Hz, 1H), 7.54 – 7.24 (m, 10H), 7.02 (d, J = 8.4 Hz, 1H), monitored by H NMR spectroscopy.
HH HH
1
3
1
3
1
1
5
.75 (s, 3H). C{ H} NMR (CDCl
3
): 154.7, 134.8, 134.4, 133.5, 132.7,
32.6, 132.1, 132.0, 131.9, 131.8, 130.6, 130.3, 128.7, 128.6, 128.6,
28.4, 128.3, 127.7, 127.3, 124.1, 124.1, 121.8, 121.3, 114.7, 112.9,
X-ray Diffraction Studies Crystals were coated in paratone oil and
mounted in a cryo-loop. Data were collected on a Bruker APEX2 X-
ray diffractometer using graphite monochromated Mo-Kα radiation
+
6.3. HRMS(EI ): (calc.) for C H N O 428.1757, (obs.) 428.1739.
2
9 22 3
(
0.71073 Å). The temperature was maintained at 150(2) K using an
Synthesis of (S)-9-(1-(2'-Methoxy-1,1'-binaphthyl-2-yl)-3-phenyl- Oxford cryo-stream cooler for both, initial indexing and full data
H-1,2,3-triazol-4-yl)-9-borabicyclo[3.3.1]nonane, (34a/b): In collection. Data were collected using Bruker APEX-2 software and
glove box a 2 dram vial was charged with triazolium salt 31 processed using SHELX and Olex2 an absorption correction applied
1
a
(
0
0
129.0 mg, 0.25 mmol), Na[N(SiMe ) ] (95% pure, 48.5 mg, using multi-scan within the APEX-2 program. All structures were
3 2
5
4-55
.25 mmol),
9-borabicyclo[3.3.1]nonane-dimer
(30.5 mg, solved by direct methods within the SHELXTL package
and
5
6-57
.125 mmol). At ambient temperature THF (2 mL) was added and refined with Olex2.
the reaction mixture was stirred at ambient temperature overnight
1
1
(
20 h). B NMR spectroscopy of an aliquot in THF confirmed the
clean formation of the product. The reaction mixture was filtered Results and Discussion
through a syringe filter and all volatiles were removed in vacuo. The
For the synthesis of our first, camphor-derived borenium ion,
the commercially available, enantiomerically pure (1R,3S)-
camphoric acid was converted to the corresponding diamine 2 in an
acid-mediated reaction with sodium azide to give the product in a
yield of 58% (Scheme 2). Diamine 2 was then transformed into
carbene precursor 3 in a ring-closing reaction with triethyl
orthoformate (HC(OEt)3) and ammonium tetrafluoroborate
reminder was taken up in a minimal amount of toluene, layered
with pentanes, and kept at –25 °C. The supernatant was decanted
and the solid was washed with pentanes and dried in vacuo to give
the title compound as ochre an ochre solid. Yield: 114 mg
(
0.207 mmol, 82%). NMR data only for the major isomer 34a
1
3
3
reported. H NMR (CD Cl ): 8.23 (d, J = 9 Hz, 1H), 8.12 (d, J = 8
2
2
HH
HH
3
3
Hz, 1H), 8.05 (d, JHH = 9 Hz, 1H), 7.99 – 7.86 (m, 1H), 7.83 (d, JHH
=
6

Page 7 of 14
Dalton Transactions
View Article Online
DOI: 10.1039/C6DT02202B
(
4 4
NH BF ) in a yield of 79%. Finally, the introduction of R-groups by
nucleophilic substitution reactions with halogenated alkanes
affords carbene precursors salts 4-6 in yields ranging from 60 to
9
5% (Scheme 2).
3 2
Reaction of the salt 4 with K[N(SiMe ) ] and subsequent
6 5 2
addition of Piers' borane (HB(C F ) ) permitted the formation and
isolation of adduct 7 in 60% yield. Recrystallisation by slow
evaporation of the solvent afforded crystals suitable for an X-ray
diffraction study. These data confirmed the structure of 7 (Figure
2
). As expected, the structure shows a pseudo tetrahedral
geometry at the boron center with a B-CNHC bond distance of
.654(2) Å. Subsequent treatment of with trityl
1
7
tetrakis(pentafluorophenyl)borate resulted in hydride abstraction
from boron and the isolation of the corresponding borenium cation
1
9
8
in 72% yield.
diastereotopic C
16.7 and 56.0 ppm is consistent with the presence of the anionic
F
NMR resonances are1 consistent with
1
6 5
F rings, and the appearance of B NMR signals at
-
and cationic boron centres, respectively. Unfortunately, attempts
to employ 8 in the catalytic hydrogenation of N-benzylidene-tert-
butylamine using a 5 mol% catalyst loading at 25 °C and 4 atm H
2
were unsuccessful. Indeed, analysis of the reaction by NMR
spectroscopy was consistent with the stoichiometric formation of 7
and the iminium cation. These data demonstrate that while the FLP
derived from the borenium cation of 8 and imine effectively split
H
2
, the carbene adduct 7 is insufficiently hydridic to deliver hydride
to the iminium cation, thus inhibiting catalysis.
Scheme 2 Synthesis of 2-17.
To circumvent this issue, carbene adducts of the weaker Lewis
acid 9-BBN were targeted. The reaction of the salts 4-6 with
K[N(SiMe
3 2
) ] and 9-BBN affords the corresponding adducts 9-11
1
1
which give rise to B NMR signals that are sharp doublets at
1
1
1
1
8
8
4.2 ppm ( JBH = 83Hz), 14.3 ppm ( JBH = 84 Hz) and 12.9 ppm ( JBH =
4 Hz), respectively. These compounds were isolated in yields of
1% (9), 67% (10) and 63% (11) and gave the expected H NMR
1
spectra. The structure of 10 was also confirmed by X-ray diffraction
(
Figure 3). The B-CNHC distance is 1.693(5) Å which is significantly
3
8-39
larger than those seen in NHC-borane adducts (ca. 1.63-1.65 Å).
3
9
By analogy to previous reports, reaction of adducts 9-11 with
trityl tetrakis(pentafluorophenyl)borate gave the corresponding
borenium cations 12-14, which were isolated in 83-93% yields.
1
H NMR data show the concurrent formation of Ph CH while the
B NMR spectra show signals attributable to the cation as broad
3
1
1
peaks at 89.1 ppm (12, 13) and 85.8 ppm (14). Crystals of 13
suitable for X-ray diffraction (Figure 4) revealed an average B-CNHC
bond length of 1.609(4) Å. This is slightly longer than the analogous
B-C distances previously described in previously characterized
38
39
borenium cations (1.5768(3) °A, 1.580(3) Å ). This may be caused
by the increase in steric demand of the present carbene relative to
less demanding NHC ligands. Although the borenium centres in
compounds 12-14 were expected to be more active than that in 8,
exposure of a range of imines to 100 bar of H at 25 and 110 °C
2
3 2
gave no reduction. Equimolar reactions of 12-14 with t-Bu P and H
also showed no reactivity at 25 °C, although upon heating to 110 °C
7

Dalton Transactions
Page 8 of 14
View Article Online
DOI: 10.1039/C6DT02202B
for 24 h a slow formation of [HPt-Bu
3
][B(C
6 5 4
F ) ] in 26% yield was
3
1
107
observed as evidenced by P NMR spectroscopy. Interestingly,
1
1
the corresponding B NMR spectrum did not show the expected
formation of the borane adduct but rather a broad signal at 44.0
ppm. These observations suggest the possibility of a thermal
rearrangement of the borane adducts 9-11 (Scheme 2).
Figure 4 POV-ray depictions of the cation of 13 (one of the two molecules in
the asymmetric unit is shown). H-atoms and the anion are omitted for
clarity. C: black; N: blue; B: yellow-green.
Figure 2 POV-ray depictions of X-ray structure of 7. H-atoms are omitted for
clarity. C: black; N: blue; B: yellow-green, F: pink.
Figure 5 POV-ray depiction of 17, one of the molecules in the asymmetric
unit is shown. C: black; N: blue; B: yellow-green, H: gray. H-atoms, except
those on chiral carbons are omitted for clarity.
Figure 3 POV-ray depiction of 10. One of the two molecules in the
asymmetric unit is shown.) H-atoms have been omitted for clarity. C: black;
N: blue; B: yellow-green, H: gray.
The formation of 15-17 suggests that thermal degradation is a
significant factor in explaining the inability of 9-11 to act as the
Lewis acid component of an FLP in the activation of dihydrogen.
To examine this question, compounds 9-11 were allowed to While activation of hydrogen may proceed to some extent, ring-expansion
may out-compete hydride delivery and thus inhibit catalytic activity.
stand in solution for 24 h. Each species was observed to react
1
1
Alternatively, the substituents on the carbene may act to sterically
inhibit access to the vacant p-orbital on boron. This latter view is
also consistent with the well-known increase in steric demands
resulting from the expanded NCN-angle in carbene ligands in 9-11
compared to those seen in the imidazole-based NHC analogues.
further, affording a new signal in the B NMR spectra at ca. 44
ppm. In the case of 9, mild heating at 50 °C for a few hours led to
complete conversion to compound 15. Interestingly, the analogous
diethyl-substituted derivative gave 16, only after overnight heating
whereas the bulkiest adduct 11 was fully converted to 17 upon
heating to 110 °C overnight. NMR data were consistent with three-
coordinated boron centres in 15-17. An X-ray structure of 17
In an effort to circumvent the steric demands in the above
borenium cations, a second group of chiral borane adducts were
selected. In this case, fused oxazole rings were employed to deter
ring expansion and hopefully provide viable borenium catalysts for
FLP hydrogenations. The triflate salts 18 and 19 were prepared by
(Figure 5) confirmed that this compound results from cleavage of a
C-N-bond in the carbene and formation of a new B-N-bond with a
hydride-shift from boron to the carbene carbon atom. This affects
the net ring expansion of the carbene by the B-C link originating
from the 9-BBN framework. The C-B-bonds were found to average
literature methods. Reaction of the former species with 9-BBN, BH
and Cy BCl afforded the anticipated carbene adducts 20-22
Scheme 3). In a similar fashion, reaction of the tert-butyl-
3
2
1
.581(3)
in the precursor adduct. The newly formed B-N-bonds average
.401(3) Å in the two molecules in the asymmetric unit. These are
Å, which is significantly shorter than the carbene C-B-bond
(
substituted ligand 19 and 9-BBN afforded adduct 23. The
1
11
spectroscopic data were as expected. The B NMR signals of 20
5
8
considerably shorter than typical B-N bonds. It is interesting to
note that a single diastereomer of 17 was obtained demonstrating
that the ring exapansion is stereoselective, presumably a result of
the existing chiral centers in the carbene in 11 en route to 17.
and 23 were found to be similar, appearing at -16.9 and -16.4 ppm
1
as doublets with
JBH of 83 and 86 Hz, respectively. These
resonances are consistent with four coordinate boron centers and
thus are consistent with the formation of 20-23 as chiral carbene
adducts. It is noteworthy that compounds 20, 21 and 23 have been
exploited by Lindsay and McArthur as reagents for the
5
9
stoichiometric, enantioselective reduction of ketones.
8

Page 9 of 14
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DOI: 10.1039/C6DT02202B
R
R
1.610(3) Å, 1.593(2) Å, 1.647(2) Å and 1.641(2) Å, respectively.
These data reflect the steric impact of the substituents at the boron
centre. The shortest B-C bond distance is seen for the borane
adduct 21, while the longest of these four derivatives occurs for the
R
R
O
O
O
O
H
B
9
-BBN
N
N
N
N
[
O SCF ]
3 3
K[N(SiMe
)
]
3
2
Cy BCl derivative 22. Comparing the difference between 20 and 23
2
indicates that steric conflict between the t-butyl substituents and
the 9-BBN fragment results in a slight lengthening of the B-C bond.
R = i-Pr 20, t-Bu 23
R = i-Pr 18, t-Bu 19
n-BuLi
Cy BCl
2
i-Pr
Me SBH3 K[N(SiMe3)2]
2
O
O
Cl
N
i-Pr
B
Cy
Cy
O
N
N
BH3
i-Pr
N
O
22
i-Pr
21
Scheme 3 Synthesis of 20-23.
Figure 9 POV-ray depiction of 23, C: black; N: blue; B: yellow-green; O: red,
H: gray.. H-atoms except the BH are omitted for clarity.
Chiral borenium cations derived from chiral boranes were also
2
prepared. To this end, (+)-diisopinocampheylborane (Ipc BH) which
is accessible from the hydroboration of α-pinene, was combined
with the carbenes 1,3-dimethylimidazol-2-ylidene and 1,3-di-iso-
propylimidazol-2-ylidene to give 24 and 25 which could be isolated
in 34% and 37% yield, respectively (Scheme 3). Similarly, reactions
of unsymmetrically substituted NHCs 1-benzyl-3-methylimidazol-2-
ylidene, 1-methyl-3-phenylimidazol-2-ylidene and 1-tert-butyl-3-
Figure 6 POV-ray depiction of 20, C: black; N: blue; B: yellow-green; O: red,
H: gray. H-atoms except the BH are omitted for clarity.
methylimidazol-2-ylidene with Ipc BH gave the corresponding
2
adducts 26-28 in 83%, 56% and 60% yield, respectively (Scheme 4).
1
1
B NMR spectra of each of these products show doublet
resonances ranging from –6.8 to 9.7 ppm with B-H coupling
constants of 77-86 Hz. Interestingly, 26 degrades in chloroform,
presumably a result of the steric conflict between the NHC and
isopinocampheyl groups. Crystals suitable for X-ray diffraction
crystallography could be obtained for 24, 26 and 27 (Figures 10-12).
These data confirmed the formulations and the expected
pseudotetrahedral geometry about boron. In the solid-state, B-CNHC
bond lengths are 1.638(2) Å, 1.648(4) Å and 1.636(2) Å for 24, 26
and 27, respectively.
Figure 7 POV-ray depiction of 21, C: black; N: blue; B: yellow-green; O: red.,
H: gray. H-atoms except the BH are omitted for clarity.
R
R'
N
N
R
X-
K[N(SiMe ) ]
N
3
2
BH
BH
THF, r.t., 24 h
KX
N
-
R'
R = R' = Me 24, i-Pr 25,
R = Me, R' = Bn 26, Ph 27, t-Bu 28
Scheme 4 Synthesis of 24-28.
Figure 8 POV-ray depiction of 22, C: black; N: blue; B: yellow-green; O: red,
Cl: green, H: gray.. H-atoms are omitted for clarity.
The structures of these compounds were subsequently
confirmed with crystallographic studies (Figures 6-9). The
corresponding B-Ccarbene bond distances in 20-23 were found to be
9

Dalton Transactions
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DOI: 10.1039/C6DT02202B
tBuONO
OMe
NH2
TMSN3
OMe
N3
MeCN
0
°C → r.t., 24 h
30, 82%
TMS
5
mol% [Cu(PPh )Br]
3
CH Cl
2
2
, r.t., 3 weeks
OMe
N
N
[Bu N]F
OMe
N
N
4
Figure 10 POV-ray depiction of 24. C: black; N: blue; B: yellow-green; O: red,
H: gray.. H-atoms except the BH are omitted for clarity.
N
N
THF, r.t., 2 h
SiMe3
32
31, 90%
[
Ph I][BF ]
2 4
5
mol% CuSO
4
DMF, 95 °C, 16 h
BF4
OMe
N
N
Ph
N
33, 89%
Na[N(SiMe ) ]
THF, r.t., 20 h
3
2
Figure 11 POV-ray depiction of 26. C: black; N: blue; B: yellow-green; O: red,
H: gray. H-atoms except the BH are omitted for clarity.
0.5 (9BBN)
2
OMe
N
OMe
N
N
Ph
Ph
N
N
+
N
BH
HB
3
4a
:
:
34b, 82%
1
10
Scheme 6 Synthesis of axial chiral carbene borane 34.
symmetric environment about boron might improve selectivities in
catalytic hydrogenations. To this end, lithium-B-H -(10)-phenyl-9-
2
borabicyclo [3.3.2]decane was prepared however formation of the
carbene adduct of the corresponding neutral borane (29) was
isolated in low yield (Scheme 5). Presumably these difficulties are
caused by the steric demands of both components. This view was
consistent with the observation that use of the more hindered
Figure 12 POV-ray depiction of 27. C: black; N: blue; B: yellow-green; O: red,
H: gray.. H-atoms except the BH are omitted for clarity.
Triazolium-based carbene adducts of chiral boranes developed
4
9
1
,3,4-triphenyl-1H-1,2,3-triazolium tetrafluoroborate failed to give
an adduct. Furthermore, when rac-29 precatalyst was activated by
Ph C][B(C in the hydrogenation of N-benzylidene tert-
butylamine at elevated H pressure, no hydrogenation was seen,
by Soderquist were also investigated as we speculated that the
C -
1
[
3
6 5 4
F ) ]
N
Ph
Ph
BF
N
2
N
N
Ph
Ph
N
N
Li
4
further indicating that excessive steric hindrance impedes effective
hydride delivery even to otherwise reactive substrates.
Ph
Ph
H
H
H
B
B
MeI, THF
Na[N(SiMe ) ].
An alternative strategy based on triazolium derivatives, was
based on use of a chiral triazolium derivatives. To this end, the
rac
3
2
5
0
known (S)-2-amino-2'-methoxy-1,1'-binaphthalene was reacted
with tert-butylnitrite and trimetylsilyl azide to give the
corresponding binaphthyl azide 30 in 82% yield. While sluggish, the
rac-29, 13%
Scheme 5 Synthesis of meso-ionic carbene 9-BBD adduct rac-29.
Cu-catalysed
ethynyltrimethylsilane was achieved in 90% yield of 32 using
Cu(PPh )Br] as the catalyst. Following deprotection, 32 was
Huisgen
cyclo-addition
of
31
with
[
3
10

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DOI: 10.1039/C6DT02202B
arylated to give the 1,2,3-triazolium tetrafluoroborate 33 in 89% of 55% and 47%, respectively, reflecting the observation that high
yield. Subsequent reaction with Na[N(SiMe ] and 0.5 equivalents steric demands inhibit hydrogenation catalysis for NHC-stabilised
of (9-BBN) gave a 10:1 mixture of the two regio-isomeric carbene borenium catalysts. Hydrogenation products of 24 and 26 showed
boranes 34a/b which was isolated in 82% combined yield (scheme enantiomeric excesses of 12% and 13%, respectively. Reduced
3 2
)
2
6
).
temperature (-30 °C) hydrogenation under the similar conditions
for 20 hours severely diminished conversion and offered only a
modest improvement in stereoselectivity (22% ee). Under these
latter conditions, 25 showed limited conversion and an
enantioselectivity of only 8%. (Table 1). While the two isomers of
Table 1 Hydrogenations of Imines by catalysts derived from 20, 21,23-28 and 34
3
4 could not be separated by crystallisation and were therefore
subsequently tested in hydrogenations as mixtures. At 10 mol%
catalyst loading and 90 atm H pressure in CH Cl , full conversion to
2
2
2
the corresponding amine was observed after less than 18 h reaction
time. However, the product was obtained in only 6% ee.
Entry Pre-catalyst t (h) T(°C) Yield (%) ee
While in some cases, the above borenium-based hydrogenation
catalysts were effective for reduction of ketamines, in general, poor
enantioelectivities were seen. In an effort to understand this we
speculated that the delivery of the hydride to the prochiral carbon,
maybe reversible leading to epimerisation of the resulting chiral
amine. To probe this question, the chiral, enantiopure secondary
amine (+)-bis[(R)-1-phenylethyl]amine (35) was exposed to a
catalytic amount of the previously reported achiral MIC-borenium
1
2
3
4
5
6
7
8
9
20
20
20
20
20
21
21
23
23
24
24
24
6
25
25
25
0
100
88
50
94
71
3
7
9
3
12
1
12
3
50
25
50
25
25
25
-30
25
25
-30
25
-30
-30
25
-30
25
8
24
3
5
6
7
4
0
ion 36. While at ambient temperature, only trace amounts of
24
48
4
0
-
1
meso-35 were detected by H NMR spectroscopy after 16 h, a
0
-
significant degree of epimerisation was seen after 42 h at 65 °C.
This stands in contrast, to the more rapid epimerisation of 35 with
1
0
1
2
3
4
5
6
7
8
9
0
55
5
12
20
-
1
1
1
1
1
1
1
1
1
2
24
24
24
24
4
60
a
6 5
B(C F ) and supports the view it should be possible to exploit that
24
24
0
b
borenium cations for asymmetric reductions.
12
<5
47
5
15
8
25
26
26
27
28
28
34
13
13
-
Conclusions
24
20
4
<5
0
In conclusion, exploiting both NHC and triazolium-based
carbenes, we have reported the synthesis of a number of novel
chiral carbene-borane adducts in which chirality is located either on
the carbene or alternatively on the borane. While some of these
species are effective precursors for imine reduction in
hydrogenation catalysis, in general these systems exhibit poor
enantioselectivity. Species substituted with C F groups on boron
-
20
18
0
-
100
6
a
1
b
toluene; chlorobenzene , all other catalysis done in CH
2
Cl
2
. Yield determined by
b
H NMR spectroscopy. Enantiomeric excess determined by chiral HPLC.
6
5
The potential of 20, 21, and 23 to act as precursors for preclude effective hydride delivery, while for 9-BBN and 9-BBD
borenium cation-based catalysts was probed via, a protocol in derivatives steric demands appear to be problematic under mild
which the borane precursors were treated with trityl conditions. In some cases, elevated temperatures were shown to
tetrakis(pentafluorophenyl)borate in CH
5 mol%) was exposed to a solution of the prochiral imine substrate cleavage in the carbene. Bisoxazoline-derived NHC borenium
Ph(Me)C=NPh and pressurised under 102 atm of H . Following the cations, the analogous axially chiral triazolium-borenium
2 2
Cl and the in situ catalyst afford a thermally induced ring-expansion resulting from C-N bond
(
2
reaction, the conversion and products were characterised by NMR derivatives as well as borenium cations derived from the chiral
spectroscopy and by chiral HPLC. Interestingly, 21 and 23 are borane, Ipc BH, provide active hydrogenation catalysts however
2
inactive giving 6% reduction at best even upon heating to 50 °C. In these systems provided limited stereoselectivity. Nonetheless, the
contrast, compound 20 serves as a competent catalyst precursor, low selectivities observed for these chiral borenium cation catalysts
affording effective imine reduction after 24 h at temperatures are not caused by selectivity erosion by epimerisation of the chiral
ranging from 0-50 °C. However, in all cases, the enantioselectivity product. This supports the notion that further catalyst design
for the ketamine reduction is low, ranging from 1-12% ee (Table 1). considerations may be fruitful. Our efforts to this end will be
In a similar trial, reduction of MeOC
catalyst precursor 20 occurred after 18 h giving the product in 91%
yield with 11% ee. In contrast, Ph(Me)C=NCH Ph as well as the
ketones Et CO and PhMeCO resulted in no reaction at all (See SI).
6 4
H (Me)C=NPh using the reported in due course.
2
2
Acknowledgements
Similarly catalytic studies of 24-28 and 34 these systems, the 5 BARG is grateful to ERASMUS+ for funding. RLM would like to
mol% catalyst loading resulted in no catalytic activity being acknowledge the EPSRC (grant number EP/N02320X/1) and
observed for 28 and only trace conversion of N-(1- Leverhulme Trust for funding. DWS is grateful for the support
phenylethylidene)aniline to N-phenyl-1-phenylethylamine was from the National Sciences and Engineering Research Council
1
observed using 27 by H-NMR spectroscopy after 4 h of reaction of Canada (NSERC, Canada). DWS is grateful for the award of a
under 102 atm hydrogen. Compounds 24 and 26 gave conversions Canada Research Chair. CMC acknowledges NSERC for
1
1

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DOI: 10.1039/C6DT02202B
Discovery and Accelerator Awards and the Canada Foundation 24. D. W. Stephan and G. Erker, eds., Frustrated Lewis Pairs II,
for Innovation for instrumental support. The authors wish to
acknowledge the Canadian Foundation for Innovation, project
number 19119, and the Ontario Research Fund for support of 25. D. W. Stephan and G. Erker, eds., Frustrated Lewis Pairs I
Expanding the Scope, Springer Press, Top. Curr. Chem., 334,
2013.
the Centre for Spectroscopic Investigation of Complex Organic
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TOC graphics
1
4