Palladium asymmetric reduction of β-carboline imines mediated by chiral auxiliaries assisted by microwave irradiation

Article abstract of DOI:10.1016/j.tetlet.2009.09.181

An alternative synthetic approach for the introduction of chirality in β-carboline moiety through in situ reduction of N-acyliminium ion intermediates generated from imine 2 and chloroformate of 8-phenylmenthyl as chiral auxiliary was achieved. The method applied microwave-assisted irradiation and used PdCl₂/Et₃SiH protocol as a mild reducing agent, which decreased reaction times to minutes when compared to the conventional thermal reactions. The diastereoselectivity (4-12:1) of the reduction produced R-amines, which were assigned after chiral auxiliary removal and spectroscopic data compared to products obtained from Noyori asymmetric hydrogenation catalyst.

Full text of DOI:10.1016/j.tetlet.2009.09.181

Tetrahedron Letters 50 (2009) 7059–7061
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium asymmetric reduction of b-carboline imines mediated by chiral
auxiliaries assisted by microwave irradiation
*
Marlene Espinoza-Moraga, Ana Gloria Caceres, Leonardo Silva Santos
Laboratory of Asymmetric Synthesis, Chemistry Institute of Natural Resources, Talca University, Talca, PO Box-747, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
An alternative synthetic approach for the introduction of chirality in b-carboline moiety through in situ
reduction of N-acyliminium ion intermediates generated from imine 2 and chloroformate of 8-phenyl-
menthyl as chiral auxiliary was achieved. The method applied microwave-assisted irradiation and used
PdCl2/Et3SiH protocol as a mild reducing agent, which decreased reaction times to minutes when com-
pared to the conventional thermal reactions. The diastereoselectivity (4–12:1) of the reduction produced
R-amines, which were assigned after chiral auxiliary removal and spectroscopic data compared to prod-
ucts obtained from Noyori asymmetric hydrogenation catalyst.
Received 17 September 2009
Revised 25 September 2009
Accepted 30 September 2009
Available online 4 October 2009
Keywords:
Palladium asymmetric reduction
Tetrahydro-b-carboline
Enantioselective synthesis
Chiral auxiliaries
Ó 2009 Elsevier Ltd. All rights reserved.
Despite great developments in synthetic organic chemistry,
there are still few methodologies that allow the stereoselective
construction of pre-determined moieties in some classes of com-
pounds. In this context, particular attention in efficient synthetic
routes for novel chemotypes is actively being pursued when stere-
oselectivity is required. Chiral auxiliaries based upon the cyclohex-
ane frame have been explored in several asymmetric reactions. A
large number of reaction types have been examined using chiral
auxiliaries of this type, although those that have been explored
with the readily available menthol rarely provide what would be
considered practical levels of control. On the other hand, auxilia-
ries that bear phenyl substituents such as Corey’s 8-phenylmen-
thol (1a) and Whitesell trans-2-phenylcyclohexanol (1b) have
provided excellent levels of control (Scheme 1).1 As part of our ef-
forts in the field of biologically relevant b-carbolines, we turned
our attention toward an alternative synthetic route for the prepa-
ration of chiral tetrahydro-b-carbolines such as 4.2
Numerous methods for the synthesis of optically active amines
are known, few being based on catalytic asymmetric synthesis.
Among the most popular is the asymmetric hydrogenation of keti-
mines or enamides using chiral Rh(I), Ir(I), or Ru(II) complexes.2
Thus, we investigated the scope of reduction of imines such as 2
mediated by chiral auxiliaries in a one-pot manner. The methodol-
ogy is based on the use of chloroformates 1a,b as chiral auxiliaries
for the reduction of dihydro-b-carbolines.3 Furthermore, the PdCl2/
Et3SiH protocol3,4 was employed as palladium hydride source and
O
O
Cl
+
O
PdCl2
Et3SiH
Ph
1a
1b
NH
N
N
or
N
MW
H
H
R
R
Cl
O
4
2
Ph
Scheme 1. Chiral auxiliary-mediated Pd-H reduction of imines
2 assisted by
microwave irradiation.
the reactions were assisted by microwave irradiation (MWA) in or-
der to accelerate reactions.
Imines 2a–k (Table 2) were obtained in yields around 75% from
the corresponding acids (3a–k) and tryptamine by coupling with
EDC/HOBt in CH2Cl2 at room temperature, which afforded the cor-
responding amides 5. The amides 5a–k were subjected to Bischler–
Napieralsky cyclization affording imines 2a–k.2b,3–5 Having pre-
pared imines 2 (Scheme 2), the next stage was set to introduce
the asymmetry through the chiral auxiliaries and Pd-H reduction
of the preformed N-acyliminium ions. It is well known that the
Noyori asymmetric hydrogen-transfer reaction of imines.6 Noyori
and co-workers have shown that p-cymene–Ru(II) complexes of
certain chiral 1,2-diamines are highly effective as catalysts for
the asymmetric reduction of imines. This method uses a HCO2H–
Et3N azeotropic mixture as the hydrogen source and provides a
convenient, general route to natural and unnatural b-carboline
alkaloids. However, few alternatives are found for asymmetric b-
carboline constructions based on imine systems.2
* Corresponding author. Tel.: +56 71 201575/574/573; fax: +56 71 200448.
E-mail address: lssantos@utalca.cl (L.S. Santos).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.181

7060
M. Espinoza-Moraga et al. / Tetrahedron Letters 50 (2009) 7059–7061
affording 6a in 90% yield and 50% dr (entry 5), and 7a in 80% yield,
O
EDC, HOBt
20 oC, 12 h
CH2Cl2
+
and 33% dr (entry 6), when CH2Cl2/THF (2:1) was used as solvent at
0 °C to 25 °C. The absolute configuration of the major compounds
was determined to be (R) after removal of the chiral auxiliaries
using CHCl3/HCl (2.0 M) and reflux, which gave (+)-4a in yields
around 90–95%. The enantiomeric excesses of major enantiomer
(R)-(+)-4a obtained from 6a and 7a were >99% ee, as determined
by HPLC analysis using a ChiralPack OD column, and it is in accor-
dance with pure (R)-(+)-4a data.3
NH2
NH
R
OH
N
H
3a-k
POCl3
N
C6H6
N
H
2a-k
N
O
reflux, 2 h
H
R
R
5a-k
Table 1 shows that these one-pot reduction reactions to proceed
efficiently by using CH2Cl2/THF (2:1) as solvent, when borohy-
drides are employed. The yields increased when the reaction was
performed using NaBH4 at ambient temperature, but with lower
% dr. Chiral auxiliaries were recovered in 95% yield with no de-
crease of its optical rotations. Selectivity and the chiral auxiliary-
mediated reduction of 2 was rationalized by the transition state
depicted in Figure 1, which is in accordance with the reduction
of the N-acyliminium ion 8 through its Si-face when 2 is used.
The same model depicted in Figure 1 can be applied using 1b as
chiral auxiliary.
Next, we set out to employ MWA in the asymmetric reduction
of 8.8 Microwave-assisted (MWA) conditions have been applied
in a variety of synthetic transformations for time as well as en-
ergy-saving aspects.9 In some cases, microwave-assisted irradia-
tion is more selective in comparison to thermal reactions by
favoring faster reactions and preventing decomposition of reac-
tants and products.10 Despite its increasing importance and recent
usage, there are few reports on the application of MWA for the
asymmetric reduction of imine groups.11 Therefore, the aim of
increasing the yields along with shortening of the reaction times,
we decided to explore the MWA irradiation process as the heating
source for the conversion of imines into respective chiral carba-
mates 6a–k employing 1a as chiral auxiliary. The same reaction
condition as depicted in Table 1 (entry 1) was carried out under
MWA irradiation in a sealed vessel. Reaction times were reduced
to nine minutes for imines 6a–k, the yields ranged from 65–90%
and were compared to the thermal conditions, as shown in Table
2. Further, it was also observed that when applying the described
protocol, the temperatures of the reaction mixture inside the reac-
tion vessel reached values of 60 °C (9 min) for MWA irradiation
reductions applying a potency of 90 W. Although these tempera-
tures were higher than those used in the thermal conditions, times
were shorter preventing in this way degradation of reactants and
products, and favouring the enhancement of yields. The selectivi-
ties observed using MWA reductions were compared to thermal
ones, and in some cases presented slight increase in the dr (Table
2, entries 2, 4, and 8).
Scheme 2.
We first investigated the scope of reduction of imine 2a medi-
ated by chiral auxiliaries in an one-pot manner. The tested chiral
auxiliaries were chloroformates of 8-phenylmenthyl (1a), and
trans-phenylcyclohexanyl (1b), as depicted in Scheme 3. In situ for-
mation of the corresponding N-acyliminium ions 8a and 8b from
chloroformates of chiral auxiliary 1a,b, and imine 2a, followed by
reduction afforded 6a (R = 8-phenylmenthyl) and 7a (R = trans-
phenylcyclohexyl) in moderate to good yields (Table 1). Pd-H
reductions of 8a and 8b were carried out using PdCl2/Et3SiH proto-
col7 at ꢀ78 °C, and compounds 6a and 7a were obtained in 87% and
75% yield, respectively (Table 1, entries 1 and 2). The diastereo-
meric ratio obtained employing palladium protocol were 6:1 for
6a (Table 1, entry 1) and 4:1 for 7a (Table 1, entry 2).
*
*
NaCNBH3 was also employed as reducing agent in CH2Cl2/THF
(2:1)1g as a solvent at ꢀ78 °C. Using NaCNBH3, 6a was obtained
in 88% yield and 66% dr (5:1 R/S, entry 3), and 7a in 73% yield
and 50% dr (3:1 R/S, entry 4), respectively. NaBH4 was also tested
O
R*
Cl
O
1
N
N
O
tables
1 and 2
R*
N
H
N
H
R
O
R
6a-k
7a-k
2a-k
R* = 8-phenylmenthyl, (1a)
trans-phenylcyclohexyl, (1b)
H
R
HCl (2.0 M)
NH
+
*R OH
95% recovery
N
H
CHCl3
reflux
4a-k
Scheme 3.
Table 1
Reduction of imine 2a to 6a/7a by using chloroformates of 8-phenylmenthyl (1a) and
trans-phenylcyclohexyl (1b) as chiral auxiliaries, and three different reducing agents
The absolute configurations of 6a–k were determined to be (R)
after chiral auxiliary removal using HCl/CHCl3 (2 M) that gave 4a–k
in 89–95% yields.12 As mentioned above, the ee% for all amines 4
was >99% ee as determined by HPLC using a ChiralPack OD column,
and it is in accordance with (R)-4.
Entry
Chiral
auxiliaries
Reducing agentc
(condition)
Yielda (%)
Product drb
1
2
3
4
5
6
1a
1b
1a
1b
1a
1b
PdCl2/Et3SiH
(A)
PdCl2/Et3SiH
(A)
NaCNBH3
(B)
NaCNBH3
(B)
NaBH4
(C)
NaBH4
(C)
87
75
88
73
90
80
6a
6:1 (R,S)
7a
4:1 (R,S)
6a
5:1 (R,S)
7a
3:1 (R,S)
6a
3:1 (R,S)
7a
In conclusion, a mild and efficient MWA methodology employ-
ing PdCl2/Et3SiH for the preparation of b-carboline amines through
Si face
Nu
Me
O
H
N
N
O
2:1 (R,S)
+
a
b
c
Isolated yields.
Diastereomeric ratio (dr) calculated based on HPLC analysis of product 6a.
A: Chiral auxiliary, CH2Cl2, rt, 30 min, then PdCl2/Et3SiH, Et3N, ꢀ78 °C, 1 h; B:
8a
chiral auxiliary, CH2Cl2, rt, 30 min, then NaCNBH3/THF:CH2Cl2 (1:2), ꢀ78 °C, 1 h; C:
chiral auxiliary, CH2Cl2, rt, 30 min, NaBH4/THF:CH2Cl2 (1:2), 0 °C to rt, 1 h.
Figure 1. Proposed facial discrimination in the addition of Pd-H to the N-
acyliminium ion derived from 2a and 1a.

M. Espinoza-Moraga et al. / Tetrahedron Letters 50 (2009) 7059–7061
7061
Table 2
Microwave-assisted irradiation (90 W) and thermal reduction of imine 2 to 6 by using chloroformates of 8-phenylmenthyl (1a) as chiral auxiliary and PdCl2/Et3SiH as reducing
agent in CH2Cl2
Entry
Imine X (R)
MWA reaction
6, dra (yield %)b
Thermal reaction
6, dra (yield %)b
Time (°C)
Time (°C)
1
2
3
4
5
6
7
8
9
Me, 2a
Et, 2b
iso-Pr, 2c
1-Pentenyl, 2d
Ph, 2e
Bn, 2f
(CH2)2CO2Me, 2g
(CH2)3CO2Me, 2h
(CH2)7CH@CH(CH2)7Me, (Z)-2i
(CH2)7(CH@CHCH2)4(CH2)3Me, (Z,Z)-2j
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
9 min (60 °C)
6a, 5:1 (90%)
6b, 7:1 (85%)
6c, 9:1 (82%)
6d, 11:1 (78%)
6e, 1:1 (65%)
6f, 4:1 (75%)
6g, 5:1 (77%)
6h, 5.5:1 (80%)
6i, 7:1 (89%)
6j, 9:1 (88%)
0.5 (ꢀ78 °C)
1 (ꢀ78 °C)
6a, 6:1 (87%)
6b, 6:1 (87%)
6c, 12:1 (88%)1c
6d, 10:1 (85%)
6e, 1.5:1 (85%)
6f, 3.5:1 (90%)
6g, 5:1 (91%)
6h, 5:1 (89%)
6i, 8:1 (70%)
3 (ꢀ78 °C)
3 h (ꢀ78 °C)
1 h (ꢀ78 °C)
1 h (ꢀ78 °C)
2 h (ꢀ78 °C)
2 h (ꢀ78 °C)
2 h (ꢀ78 °C)
2 h (ꢀ78 °C)
10
6j, 10:1 (75%)
11
9 min (60 °C)
6k, 12:1 (88%)
1 h (ꢀ78 °C)
6k, 13:1 (85%)2b
O
, 2k
a
Diastereomeric ratio (dr) calculated based on HPLC of product 6a–k.
Isolated yields.
b
3233–3236; (b) Ferreri, C.; Costantino, C.; Chatgilialoglu, C.; Boukherroub, R.;
Manuel, G. J. Organomet. Chem. 1998, 554, 135–137; (c) Mirza-Aghayan, M.;
Boukherroub, R.; Bolourtchian, M.; Rahimifard, M. J. Organomet. Chem. 2007,
692, 5113–5116; (d) Mirza-Aghayan, M.; Boukherroub, R.; Rahimifard, M. J.
Organomet. Chem. 2008, 693, 3567–3570; (e) Mirza-Aghayan, M.; Boukherroub,
R.; Rahimifard, M. Tetrahedron Lett. 2009, 50, 5930–5932.
chiral auxiliary induction has been developed. The PdCl2/Et3SiH
protocol is easy to manage, is inexpensive, safe to handle, stable,
and is not pyrophoric, and no inert atmosphere is required. Inter-
estingly, the MWA irradiation reactions presented similar yields
with very short reaction times in contrast to the conventional ther-
mal reactions. The reaction conditions are particularly attractive
and are an example of a green chemistry approach due to perform-
ing reactions in a very short time period by MWA thereby reducing
energy consumption, time savings, and increasing efficiency. If one
compares the energy efficiency of conventional synthesis (heating/
cooling by conduction and convection currents), and microwave-
assisted reactions, it can be noted that for most chemical transfor-
mations a significant energy-saving (up to 80-fold) can be expected
using microwaves as an energy source on a laboratory scale.13
8. All the microwave reactions were performed in CEM Discover LabMate
equipment in a closed vessel (built-in infrared sensor) with cooling system.
9. (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259–281. and references
cited therein; (b) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284.
10. Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225–
9283.
11. Kappe, C. O.; Dallinger, D. Mol. Div. 2009, 71–193.
12. The NMR spectroscopic data for all compounds are in accordance with
previously described for 4a–k. Optical rotations obtained for majoritary R
compounds are as follows: Compound 4a: ½a D
4b: ½a D +62.0 (c 1.0, MeOH). Compound 4c: ½a D
4d was not previously described: ½a D
ꢁ
+51 (c 1.0, MeOH). Compound
+66 (c 1.0, MeOH). Compound
ꢁ
ꢁ
ꢁ
ꢀ25.0 (c 1.0, CHCl3). FT-IR (KBr film)
cmꢀ1: 3409, 3218, 3062, 2929, 2844, 2744, 1641, 1562, 1452, 1343, 1317,
1288, 1155, 1108, 1002, 909, 744. 1H NMR (400 MHz, CDCl3) d: 1.52–1.70 (3H,
m), 1.84–1.91 (1H, m), 2.09–2.19 (2H, m), 2.68–2.80 (1H, m), 2.75 (1H, dq, J 8.0,
1.9), 3.03 (1H, ddd, J 15.5, 8.0, 5.5), 3.34 (1H, dt, J 14.5, 4.5), 4.07 (1H, br s), 4.98
(1H, br d, J 10.2), 5.03 (1H, dd, J 17.1, 1.6), 5.80 (1H, ddt, J 17.1, 10.2, 6.7), 7.09
(1H, dt, J 7.6, 0.7), 7.14 (1H, dt, J 7.6, 0.7), 7.30 (1H, d, J 7.8), 7,47 (1H, d, J 7.8),
7.84 (1H, br s, NH). 13C NMR (100 MHz, CDCl3) d: 22.6, 25.0, 33.7, 34.3, 42.5,
52.5, 109.0, 110.7, 115.0, 118.0, 119.3, 121.5, 127.5, 135.6, 136.1, 138.3. HRMS,
ESI(+)-MS: m/z calcd for [C16H20N2+H]+: 241.1705, found: 241.1701. Compound
Acknowledgments
FONDECYT (Project 1085308) is great acknowledged for finan-
cial support to Laboratory of Asymmetric Synthesis (LAS). M.E.M.
thanks Programa de Doctorado en Productos Bioactivos-UTalca
for fellowship.
4e: ½a D
4g (tetracyclic lactam obtained after chiral auxiliary removal):3
1.0, CHCl3). Compound 4h (tetracyclic lactam obtained after chiral auxiliary
ꢁ
ꢀ4.0 (c 1.0, CHCl3). Compound 4f: ½a D
ꢁ
ꢀ55.0 (c 1.0, MeOH). Compound
½aꢁD
+240.5 (c
References and notes
removal):3
described: ½a D
½
a D
ꢁ
ꢁ
+260.5 (c 1.0, CHCl3). Compound 4i was not previously
+20.0 (c 1.0, MeOH). 1H NMR (400 MHz, CDCl3) d: 0.90 (3H, t,
1. (a) Whitesell, J. K. Chem. Rev. 1992, 92, 953–964; (b) Blaser, H. U. Chem. Rev.
1992, 92, 835–852; (c) Shankaraiah, N.; da Silva, W. A.; Andrade, C. K. Z.;
Santos, L. S. Tetrahedron Lett. 2008, 49, 4289–4291; (d) Wanner, K. T.; Kartner,
A. Heterocycles 1987, 26, 921–924; (e) D’Oca, M. G. M.; Pilli, R. A.; Vencato, I.
Tetrahedron Lett. 2000, 41, 9709–9712; (f) Shankaraiah, N.; Pilli, R. A.; Santos, L.
S. Tetrahedron Lett. 2008, 49, 5098–5100; (g) de Oliveira, M. C. F.; Santos, L. S.;
Pilli, R. A. Tetrahedron Lett. 2001, 42, 6995–6997; (h) Santos, L. S.; Pilli, R. A. J.
Braz. Chem. Soc. 2003, 14, 982–993.
2. (a) Kobayashi, S.; Ishitani, H. Chem. Rev. 1999, 99, 1069–1094; (b) Shankaraiah,
N.; Santos, L. S. Tetrahedron Lett. 2009, 50, 520–523; (c) Shankaraiah, N.; Santos,
L. S. Tetrahedron Lett. 2009, 50, 2700.
3. da Silva, W. A.; Rodrigues, M. T., Jr.; Shankaraiah, N.; Ferreira, R. B.; Andrade, C.
K. Z.; Pilli, R. A.; Santos, L. S. Org. Lett. 2009, 11, 3238–3241.
J 7.0 Hz), 1.20–1.41 (20H, m), 1.41–1.58 (2H, m), 1.64–1.76 (1H, m), 1.81–1.92
(1H, m), 1.93–2.02 (5H, m), 2.72–2.78 (2H, m), 3.02–3.07 (1H, m), 3.33–3.37
(1H, m), 4.05–4.09 (1H, m), 5.29–5.40 (2H, m), 7.09–7.15 (2H, m), 7.31 (1H, d, J
8.0 Hz), 7.49 (1H, d, J 8.0 Hz), 7.92 (1H, s). 13C NMR (100 MHz, CDCl3) d: 14.1,
22.3, 22.6, 25.8, 27.2, 27.25, 29.2, 29.3, 29.4, 29.5, 29.55, 29.7, 29.8, 29.85, 32.0,
34.9, 42.3, 52.8, 108.7, 110.8, 118.0, 119.4, 121.6, 127.3, 129.7, 130.0, 135.6,
135.70. HRMS, ESI(+)-MS: m/z calcd for [C28H44N2+H]+: 409.3583, found:
409.3578. Compound 4j was not previously described: ½a D
ꢁ
+12.0 (c 1.0, MeOH).
1H NMR (400 MHz, CDCl3) d: 0.88 (3H, t, J 7.0 Hz), 1.26–1.35 (6H, m), 1.52–1.66
(2H, m), 1.71–1.79 (1H, m), 1.86–1.94 (1H, m), 2.03–2.16 (5H, m), 2.75–2.88
(8H, m), 3.05–3.12 (1H, m), 3.34–3.39 (1H, m), 4.18 (1H, br s), 5.31–5.39 (8H,
m), 7.07–7.14 (2H, m), 7.31 (1H, d, J 8.0 Hz), 7.47 (1H, d, J 8.0 Hz), 7.48 (1H, d, J
8.0 Hz), 7.93 (1H, br s). 13C NMR (100 MHz, CDCl3) d: 14.1, 21.8, 22.7, 25.6, 25.7,
25.75, 25.76, 27.1, 27.2, 29.5, 31.5, 34.0, 42.1, 52.6, 108.5, 110.8, 118.1, 119.7,
121.7, 127.3, 127.5, 127.9, 128.1, 128.3, 128.6, 128.7, 129.4, 130.6, 135.7, 136.0.
HRMS, ESI(+)-MS: m/z calcd for [C30H42N2+H]+: 431.3426, found: 431.3430.
4. (a) Sakaitani, M.; Ohfune, Y. J. Org. Chem. 1990, 55, 870–876; (b) Coleman, R. S.;
Carpenter, A. J. J. Org. Chem. 1992, 57, 5813–5815.
5. Santos, L. S.; Pilli, R. A.; Rawal, V. H. J. Org. Chem. 2004, 69, 1283–1289.
6. (a) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc.
1996, 118, 4916–4917; (b) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc.
2000, 122, 1466–1478; (c) Mao, J. M.; Baker, D. C. Org. Lett. 1999, 1, 841–843;
(d) James, B. R. Catal. Today 1997, 37, 209–221.
Compound 4k: ½a D
ꢁ
+23.0 (c 1.0, MeOH).
13. (a) Gronnow, M. J.; White, R. J.; Clark, J. H.; Macquarrie, D. J. Org. Process Res.
Dev. 2005, 9, 516–518; (b) Ondruschka, B.; Bonrath, W.; Stuerga, D. In
Microwaves in Organic Synthesis; Loupy, A., Ed., 2nd ed.; Wiley-VCH: Weinheim,
2006; p 62. Chapter 2.
7. For the other uses of PdCl2/Et3SiH in reduction processes, see: (a) Kunai, A.;
Sakurai, T.; Toyoda, E.; Ishikawa, M.; Yamamoto, Y. Organometallics 1994, 13,