Efficient synthesis of chiral C2-symmetric diamines via allylboration of bis-N,N′-metallodiimines

Article abstract of DOI:10.1016/j.tetlet.2009.11.013

An enantioselective synthesis of C₂-symmetric bis-homoallylic aromatic and heteroaromatic diamines in 54-89% yields, in 73-94% de and ≥98% ee has been achieved via the allylboration of the corresponding N,N′-bis(trimethylsilyl)dialdimines and N,N′-bis(diisobutylalumino)dialdimines with B-allyldiisopinocampheylborane in the presence of methanol, followed by alkaline hydrogen peroxide workup. One-pot synthesis of stable N,N′-bis(benzaldimine-triethylborane) complexes and subsequent allylboration to afford benzene diamines is also described.

Full text of DOI:10.1016/j.tetlet.2009.11.013

Tetrahedron Letters 51 (2010) 332–336
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Tetrahedron Letters
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Efficient synthesis of chiral C2-symmetric diamines via allylboration
of bis-N,N0-metallodiimines
*
P. Veeraraghavan Ramachandran , Debanjan Biswas, Marek P. Krzeminski, Guang-Ming Chen
Department of Chemistry, Purdue University, 560 Oval Drive West Lafayette, IN 47907-2084, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
An enantioselective synthesis of C2-symmetric bis-homoallylic aromatic and heteroaromatic diamines
in 54–89% yields, in 73–94% de and P98% ee has been achieved via the allylboration of the
corresponding N,N0-bis(trimethylsilyl)dialdimines and N,N0-bis(diisobutylalumino)dialdimines with B-ally-
ldiisopinocampheylborane in the presence of methanol, followed by alkaline hydrogen peroxide workup.
One-pot synthesis of stable N,N0-bis(benzaldimine–triethylborane) complexes and subsequent allylboration
to afford benzene diamines is also described.
Received 8 August 2008
Revised 3 November 2009
Accepted 5 November 2009
Available online 10 November 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
C2-Symmetric diamines
Trimethylsilylimines
Aluminoimines
Imine–borane complex
Allylboration
1. Introduction
We had reported that N-trimethylsilylarylaldimines can be ally-
lborated, in the presence of stoichiometric amount of water or meth-
The development of new chiral ligands for stereoselective reac-
tions is of critical importance in the realm of asymmetric synthesis.1
A vast number of the successful ligands that are extensively used in
asymmetric reactions possess C2-symmetry since they reduce the
number of possible diastereomeric transition states and thus impart
a beneficial effect on stereocontrol by eliminating less selective
pathways.2,3 Chiral C2-symmetric ligands with oxygen-, nitrogen-,
and phosphorus electron-donor atoms are regarded as some of the
most effective ones in transition metal-catalyzed asymmetric reac-
tions.4 In our efforts to prepare chiral C2-symmetric ligands, we
had described the asymmetric reduction of diacylbenzenes and dia-
cylpyridines with (ꢀ)-B-chlorodiisopinocampheylborane [(ꢀ)-DIP-
ChlorideTM, 1] providing the corresponding C2-symmetric diols in
70% to P99% de and 81% to P99% ee (Scheme 1)5 and the allylbora-
tion of dicarboxaldehydes with (ꢀ)-B-allyldiisopinocampheylbora-
ne [(ꢀ)-Ipc2BAll, 2] affording C2-symmetric bis-homoallylic diols
in 88–98% de and P98% ee (Scheme 2).6
The extensive application of optically active C2-symmetric
diamines as chiral inducers in a vast number of asymmetric pro-
cesses7 prompted us to examine the allylboration of bis-N,N0-
metallodiimines for the synthesis of bis-homoallylic diamines.
Although a few racemic bis-homoallylic diamines have been re-
ported8, to the best of our knowledge, the chiral molecules have
not been reported.
anol, with 2 to provide the corresponding homoallylic amines in
69–90% yield and 81–94% ee.9 The reaction proceeds via the rapid
liberation of free NH aldimine intermediate as an allylborane com-
plex, followed by the allyl transfer via a six-membered chair-like
transition state to an amino-dialkylborane intermediate, which
upon workup provides the homoallylamine (Scheme 3). We also
synthesized a series of N-aluminoimines by the partial reduction
of a series of representative aromatic and aliphatic nitriles with DI-
BAL-H, followed by the allylboration with 2 to obtain the corre-
sponding homoallylic amines (Scheme 3).10,11 In this case also,
1 equiv of methanol was added as an indispensable ingredient for
themethanolysis ofN–Albond resultingintheformation ofthesame
intermediate for subsequent allylboration with Ipc2BAll. Recently,
we had also reported the preparation of stable aldimine–triethylbo-
rane complexes by the partial reduction of nitriles with Super-Hy-
drideÒ (LiEt3BH), followed by the addition of stoichiometric
methanol and subsequent one-pot allylboration with Ipc2BAll to af-
fordchiral homoallylicaminesin high yieldand enantioselectivity.12
We sought to extend these reactions to aromatic diimines.
Accordingly, we prepared a series of aromatic N,N0-bis(trimethyl-
silyl)dialdimines (4a–e) from benzene-1,2-dicarboxaldehyde (3a),
benzene-1,3-dicarboxaldehyde (3b), benzene-1,4-dicarboxaldehyde
(3c),2,5-thiophenedicarboxaldehyde(3d),and2,6-pyridinedicarbox-
aldehyde (3e) following the same procedure for the preparation of
N-trimethylsilylbenzaldimine.13 Treatment of benzene-1,4-dicar-
boxaldehyde 3c with lithium bis(trimethylsilyl)amide at ꢀ50 °C in
ether and subsequent purification by vacuum distillation afforded
* Corresponding author. Tel.: +1 765 494 5303; fax: +1 765 494 0239.
E-mail address: chandran@purdue.edu (P.V. Ramachandran).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.013

P. V. Ramachandran et al. / Tetrahedron Letters 51 (2010) 332–336
333
O
O
OH
OH
2BCl
N
N
O
-25 ºC, Et2O
12 h
+
(-)-DIP-Chloride (1)
82% de, >99% ee
Scheme 1. Asymmetric reduction of diacylpyridines with (ꢀ)-DIP-chloride.
2B
O
-100 ºC, THF
4 h
N
H
H
+
N
OH
OH
O
O
(-)-Ipc2BAll (2)
92% de, >98% ee
Scheme 2. Allylboration of pyridinedicarboxyaldehyde with (ꢀ)-Ipc2BAll.
LiN(SiMe3)2 (1.05 equiv)
Et2O, -50 ºC, 1 h
O
1. 2 (1.2 equiv)
R'
NH2
N
2. MeOH (1.0 equiv)
Ph
PhCN
DIBAL-H (1.0 equiv)
THF, 0 ºC, 1 h
Ph
THF, -100 ºC, 1-3 h
Ph
87%, 94% ee (from N-silylimine)
R' = SiMe3
Al(iBu)2
90%, 88% ee (from N-aluminoimine)
Ipc
OMe
Ipc
H
SiMe3
N
B
MeOH
Ipc
Me3Si
B
+
Ipc
N
R
2
R
11
δ
B NMR: 78
Ipc
Ipc
B
NH2
NH
B
Ipc
[O]
HN
Ipc
R
R
R
11
δ
B NMR: 47
Me3SiOMe
Scheme 3. Allylboration of N-trimethylsilyl- and N-aluminobenzaldimine with Ipc2BAll.
the 1,4-[N,N0-bis(trimethylsilyl)]dibenzaldimine 4c in high yield. The
allylboration of 4c was carried out with 2 at ꢀ78 °C in the presence of
2 equiv of methanol. The completion of the reaction was monitored
by 11B NMR spectroscopy, and alkaline oxidative workup , followed
by column chromatography provided the desired C2-symmetric dia-
mine 5c in 85% yield with 75% de and 99% ee (Scheme 4). The de
was determined by either the 1H or the 19F NMR spectroscopy of the
bis-MTPA amides derived from the diamines. Lowering the reaction
temperature to ꢀ100 °C improved the de to 85% establishing a direct
correlation between the temperature and the diastereoselectivity for
the allylboration of bis-(trimethylsilyl)diimines (Table 1, entries 4
and 5). Allylboration of 1,2-[N,N00-bis(trimethylsilyl)]dibenzaldimine
(4a) and 1,3-[N,N0-bis(trimethylsilyl)]dibenzaldimine (4b) with 2 at
ꢀ78 °C provided the corresponding bis-homoallylic diamines 5a
and 5b in 57% yield, 88% de, P99% ee and 88% yield, 75% de, 99.6%
ee, respectively (Table 1, entries 1 and 2).
The results are summarized in Table 1. Upon successful prepara-
tion of bis-homoallylic diamines by allylboration of pure 4a–e,
we also attempted a one-pot allylboration of the in situ-generated
1,4-[N,N0-bis(trimethylsilyl)]dibenzaldimine (4c) with 2. Prepara-
tion of 4c by the treatment of benzene-1,4-dicarboxaldehyde (3c)
with lithium bis(trimethylsilyl)amide, followed by the addition of
2 and methanol at ꢀ78 °C in the same pot afforded 5c in 54% yield,
72% de, and P99% ee (Table 1, entry 6). The low yield of the reac-
tion could probably be attributed to the presence of impurities in
the in situ-generated bis-(trimethylsilyl)diimine.
Allylboration of N-aluminoimines has significant practical
advantages over that of N-silylimines due to the ease of prepara-
tion, ready availability of the starting nitriles, and relatively higher
stability of both aromatic and aliphatic N-aluminoimines. In an ef-
fort to develop a generalized protocol for the preparation of C2-
symmetric bis-homoallylic diamines starting from different classes
of N-metaloimines, we extended our N-aluminoimine-based proto-
col11 for the preparation of selected diamines (5a, c, and e) by the
partial reduction of 1,2-dicyanobenzene (6a), 1,4-dicyanobenzene
Heteroaromatic bis-homoallylic diamines 5d and 5e were also
prepared from the corresponding dialdimines (4d and 4e) at
ꢀ78 °C and ꢀ100 °C in 74–81% yield, 72–94% de, and P98% ee.

334
P. V. Ramachandran et al. / Tetrahedron Letters 51 (2010) 332–336
SiMe3
Me3Si
O
O
2
1. (2.25 equiv.)
NH2
N
H2N
N
LiN(SiMe3)2
2. MeOH (2.0 equiv)
3. H2O2/NaOH
Ar
H
H
Ar
Ar
H
H
Et2O, -50 ºC, 1h
3a: 1,2-Ph
3b: 1,3-Ph
3c: 1,4-Ph
54-89%, 73-94% de, 99% ee
4a: 1,2-Ph
4b: 1,3-Ph
4c: 1,4-Ph
5a: 1,2-Ph
5b: 1,3-Ph
5c: 1,4-Ph
5d: 2,5-Thioph
5e: 2,6-Py
3d: 2,5-Thioph
3e: 2,6-Py
4d: 2,5-Thioph
4e: 2,6-Py
Scheme 4. Allylboration of N,N0-bis(trimethylsilyl)dibenzaldimines with (ꢀ)Ipc2BAll.
Table 1
Allylboration of N,N0-bis(trimethylsilyl)dialdimines with (-)-Ipc2BAll
Entry
Dialdimines
Bis-homoallylic diamines
Temp (°C)
#
Yielda (%)
S,S
R,R
R,S
deb (%)
eeb (%)
1
2
3
4
5
6
7
8
9
4a
4b
4b
4c
4c
4c
4d
4d
4e
4e
1,2-Ph
1,3-Ph
1,3-Ph
1,4-Ph
1,4-Ph
1,4-Ph
2,5-Thioph
2,5-Thioph
2,6-Py
ꢀ78
5a
5b
5b
5c
5c
5c
5d
5d
5e
5e
57
89
82
85
78
54
81
79
77
74
94
87.5
97
87.3
92.3
86.3
86.2
95
0
0.2
0
0.5
0
0
0.7
0
0
6
12.3
3
12.2
7.7
13.7
13.1
5
88
P99
P98
P99
99
P99
P99
P98
P99
P99
P99
ꢀ78
ꢁ75
ꢀ100
ꢀ78
94
ꢁ75
ꢁ85
73
72
90
ꢀ100
ꢀ78c
ꢀ78
ꢀ100
ꢀ78
89.4
97
10.6
3
ꢁ79
10
2,6-Py
ꢀ100
0
94
a
b
c
Isolated yield after chromatography.
Determined by analysis of 1H or 19F NMR of bis-Mosher amide.
One-pot reaction without purification of 4c.
1. 2 (2.25 equiv.)
2. MeOH (2.0 equiv)
-78 ºC, 3 h
Al(i-Bu)2
H
(i-Bu)2Al
N
NH2
H2N
N
DIBAL-H
CN
NC
3. H2O2/NaOH
H
Et2O, 0 ºC, 1h
65-82%, 78-89% de, 85-98% ee
6a
: 1,2-Ph
7a c e
,
,
6c
6e
: 1,4-Ph
: 2,6-Py
5a
: 1,2-Ph
5c
: 1,4-Ph
5e
: 2,6-Py
Scheme 5. Allylboration of N,N0-bis(diisobutylalumino)dialdimines with (ꢀ)-Ipc2BAll.
(6c), and 2,6-pyridinedicarbonitrile (6e) with DIBAL-H to obtain
the corresponding N,N0-bis(diisobutylalumino)dialdimines (7a, c,
and e), followed by the allylboration with 2 in the presence of
2 equiv of methanol at ꢀ78 °C for 3 h (Scheme 5). The reaction pro-
ceeded well and the desired bis-homoallylic diamines (5a, c, and e)
were obtained in 69–82% yield with 80–90% de and P98% ee. The
results are summarized in Table 2. Due to our interest in fluoro-or-
ganic chemistry, we included a fluorinated diimine as well. Thus,
the preparation of 1,4-[N,N0-bis(diisobutylalumino)]-2,3,5,6-tetra-
fluorobenzaldimine (7f) from 1,4-dicyano-2,3,5,6-tetrafluoroben-
zene (6f) and allylboration with 2 provided the corresponding
bis-homoallylic amine (5f) in 94% de and the major diastereomer
was obtained in P99% ee (Scheme 6).
Subsequently, we also developed a methodology to prepare dii-
mines from imine–borane adducts, N,N0-bis(benzaldimine–tri-
ethylborane) complexes (8b and c), derived by the partial
reduction of 1,3-dicyanobenzene (6b) and 1,4-dicyanobenzene
(6c), respectively, with 2 equiv of LiEt3BH, followed by the addition
of 2 equiv of methanol. One-pot allylboration of these in situ-gen-
erated N,N0-bis(benzaldimine–triethylborane) complexes with 2 at
ꢀ78 °C for 6 h afforded the desired diamines (5b and c) in 72–79%
yield, 78–82% de, and P99% ee (Scheme 7).
Inconclusion,wehavedevelopedaconvenient,highlystereoselec-
tivegeneralprocedureforthepreparationofC2-symmetricbis-homo-
allylic diamines via the allylboration of a series of aromatic
dialdimines. We successfully prepared N,N0-bis(trimethylsilyl)dialdi-
mines from dialdehydes, N,N0-bis(diisobutylalumino)dialdimines,
and N,N0-bis(aldimine–triethylborane) complexes from dinitriles
Table 2
Allylboration of N,N0-bis(diisobutylalumino)dialdimines with (ꢀ)-Ipc2BAll
Entry
Dialdimines
Bis-homoallylic diamines
#
Yielda (%) S,S R,R R,S deb (%) eeb (%)
1
2
3
7a
7c
7e
1,2-Ph
1,4-Ph
2,6-Py
5a 79
5c 82
5e 69
95
90
91
0
1
0
5
9
9
90
80
82
P99
P98
P99
a
Isolated yield after chromatography.
Determined by analysis of 1H or 19F NMR of bis-Mosher amide.
b

P. V. Ramachandran et al. / Tetrahedron Letters 51 (2010) 332–336
335
F
F
F
F
F
F
F
N
H
(i-Bu)2Al
N
H
Al(i-Bu)2
DIBAL-H
NC
CN
Et2O, 0 ºC, 1h
F
6f
7f
F
F
2
1. (2.25 equiv.)
NH2
H2N
2. MeOH (2.0 equiv)
-78 ºC, 3 h
F
F
3. H2O2/NaOH
5f
65%, 94% de, >99% ee
Scheme 6. Allylboration of 1,4-[N,N0-bis(diisobutylalumino)-2,3,5,6-tetrafluoro-benzaldimine with (ꢀ)-Ipc2BAll.
NH2
NH2
1. 2, -78 ºC, 6h
2. H2O2/NaOH
Et3B
BEt3
H
1. LiEt3BH, THF
CN
0 ºC, 4 h
H
N
N
NC
H
H
2. MeOH, 0 ºC
30 min
5b
6b
>98
72%, 78% de,
% ee
NH2
8b
8c
H
H
N
H2N
1. LiEt3BH, THF
0 ºC, 4 h
2
1. , -78 ºC, 6h
2. H2O2/NaOH
BEt3
N
H
Et3B
NC
CN
2. MeOH, 0 ºC
30 min
H
5c
6c
79%, 82% de, >98% ee
Scheme 7. Allylboration of N,N0-bis(benzaldimine–triethylborane) complexes.
and achieved the allylboration of both isolated and in situ-generated
dialdimines. We are currently exploring to extend this methodology
to synthesize aliphatic diamines.
spectroscopy showed the diamine to be of 75% de. The major
diastereomer was obtained in 99% ee. 1H NMR: (300 MHz, CDCl3)
d 7.41–7.27 (m, 4H), 5.79–5.64 (m, 2H), 5.12–4.99 (m, 4H), 3.96
(dd, J = 5.2 Hz, J = 7.7 Hz, 2H), 2.47–2.28 (m, 4H), 1.80 (br s, 4H);
13C NMR: (75 MHz, CDCl3) d 144.4, 135.5, 126.5, 117.7, 55.1, 44.1.
2. Experimental
2.1. General procedure for the allylboration of N,N0-
bis(trimethylsilyl)dialdimines
2.2. General procedure for the allylboration of N,N0-
bis(diisobutylalumino)dialdimines
Compound 4c (1.41 g, 5.1 mmol) was added to a stirred solution
of (ꢀ)-B-allyldiisopinocampheylborane (2; 1 M in THF; 11.48 mL,
11.48 mmol), prepared from 1 and allylmagnesium bromide,14 di-
luted with THF (7 mL), and cooled to ꢀ78 °C, followed by the slow
addition of methanol (0.33 g, 10.2 mmol). The reaction mixture
was stirred at ꢀ78 °C for 3 h when the reaction was complete
DIBAL-H (1.44 g, 1.82 mL, and 10.2 mmol) was added to a solu-
tion of 1,4-dicyanobenzene (6c; 0.65 g, 5.1 mmol) in THF (5 mL)
cooled to 0 °C and the mixture was stirred for 1 h to obtain the cor-
responding N,N0-bis(alumino)diimine (7c). The reaction mixture
was cooled to ꢀ78 °C and was transferred via a cannula to a stirred
solution of (ꢀ)-B-allyldiisopinocampheylborane (2; 1 M in THF;
11.48 mL, and 11.48 mmol) diluted with THF (7 mL) and cooled to
ꢀ78 °C, followed by the slow addition of methanol (0.326 g,
10.2 mmol). The reaction mixture was stirred at ꢀ78 °C for 3 h when
the reaction was complete (11B NMR spectral peak shift from d
79 ppm to 47 ppm). The reaction was slowly oxidized with H2O2
(30% in H2O; 4.6 mL) in the presence of NaOH (3 M in H2O; 7.6 mL)
and was left stirred under positive N2 pressure while it slowly
warmed to rt. The product was extracted with EtOAc (3 ꢂ 15 mL),
the combined organic layer was washed with saturated brine solu-
(
11B NMR spectral peak shift from d 79 ppm to 47 ppm). The reac-
tion was slowly oxidized with H2O2 (30% in H2O; 4.6 mL) in the
presence of NaOH (3 M in H2O; 7.6 mL) and was left stirred under
positive N2 pressure while it slowly warmed to rt. The product was
extracted with EtOAc (3 ꢂ 15 mL), the combined organic layer was
washed with saturated brine solution and dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure, and
the crude product was purified on silica gel (dichloromethane/
methanol/triethylamine 95:4.5:0.5) to afford 0.94 g (4.33 mmol,
85% yield) of 5c. Analysis of the bis-MTPA amide using 19F NMR

336
P. V. Ramachandran et al. / Tetrahedron Letters 51 (2010) 332–336
tion and dried over anhydrous Na2SO4. The solvent was evaporated
under reduced pressure, and the crude product was purified on silica
gel (dichloromethane/methanol/triethylamine 95:4.5:0.5) to afford
0.902 g (4.18 mmol, 82% yield) of 5c. Analysis of the bis-MTPA amide
using 19F NMR spectroscopy showed the diamine to be of 80% de. The
major diastereomer was obtained in P98% ee.
Acknowledgment
The financial support from the Herbert C. Brown Center for Bor-
ane Research is gratefully acknowledged.
References and notes
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2.3. General procedure for the allylboration
of N,N0-bis(benzaldimine–triethylborane) complexes
Lithium triethylborohydride (1 M in THF; 10.2 mL, and
10.2 mmol) was added to a solution of 1,4-dicyanobenzene (6c;
0.65 g, 5.1 mmol) in THF (5 ml) at 0 °C and the solution was stirred
for 4 h. Methanol (0.326 g, 10.2 mmol) was added to the reaction
mixture and stirred for an additional 30 min. The reaction mixture
was cooled to ꢀ78 °C and slowly transferred via a cannula to a stir-
ring solution of (ꢀ)-B-allyldiisopinocampheylborane (2; 1 M in
THF; 11.48 mL, 11.48 mmol) diluted with THF (7 mL) and cooled
to ꢀ78 °C. The reaction mixture was stirred at ꢀ78 °C for 6 h when
the reaction was complete. The reaction was slowly oxidized with
H2O2 (30% in H2O; 4.6 mL) in the presence of NaOH (3 M in H2O;
7.6 mL) and was left stirred under positive N2 pressure while it
slowly warmed to rt. The product was extracted with EtOAc
(3 ꢂ 15 mL), the combined organic layer was washed with satu-
rated brine solution and dried over anhydrous Na2SO4. The solvent
was evaporated under reduced pressure, and the crude product
was purified on silica gel (dichloromethane/methanol/triethyl-
amine 95:4.5:0.5) to afford 0.87 mg (4.02 mmol, 79% yield) of 5c.
Analysis of the bis-MTPA amide using 19F NMR spectroscopy
showed the diamine to be of 82% de. The major diastereomer
was obtained in P99% ee.
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