A novel reductive dimerization/oxidative dehydrogenation of aldimines mediated by lanthanoid metals

Article abstract of DOI:10.1021/jo001328d

A lanthanoid metal-mediated novel reductive dimerization/oxidative dehydrogenation of a variety of aldimines has been achieved. Aromatic aldimines (1) were dimerized in the presence of 0.5 mol of ytterbium metal (Yb) and 1-naphthaldehyde (1-NpCHO) to give

Full text of DOI:10.1021/jo001328d

514
J . Org. Chem. 2001, 66, 514-520
A Novel Red u ctive Dim er iza tion /Oxid a tive Deh yd r ogen a tion of
Ald im in es Med ia ted by La n th a n oid Meta ls
Wusong J in, Yoshikazu Makioka, Tsugio Kitamura, and Yuzo Fujiwara*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
6-10-1 Hakozaki, Higashiku, Fukuoka 812-8581, J apan
yfujitcf@mbox.nc.kyushu-u.ac.jp
Received September 5, 2000
A lanthanoid metal-mediated novel reductive dimerization/oxidative dehydrogenation of a variety
of aldimines has been achieved. Aromatic aldimines (1) were dimerized in the presence of 0.5 mol
of ytterbium metal (Yb) and 1-naphthaldehyde (1-NpCHO) to give the corresponding vicinal diimines
(2) in good to high yields. Samarium metal (Sm) or samarium(II) diiodide (SmI2) gives unsatisfied
yields under the same reaction conditions. As an oxidant, 1-NpCHO gives the best result. In addition,
HMPA plays an important role as a cosolvent in this reaction. Reaction of various aldimines with
Yb metal is disscussed in detail. Moreover, it has been found that the electron-donating substitutents
on the benzene ring promote the reaction and that the electron-withdrawing substitutents retard
the reaction. The reaction of deuterated N-benzylideneaniline with Yb metal confirmed that
transformation of the hydride occurred from aldimines to the oxidant aldehyde. The mechanism of
the reaction was discussed.
In tr od u ction
direct reaction of ytterbium metal with triphenylimine
and characterized by X-ray analysis.6 It is interesting
that the ytterbium-imine complexes have unique chemi-
cal properties such as strong basicity and little nucleo-
philicity, which are different from those of lanthanoid-
diaryl ketone complexes.7 We report here a lanthanoid
metal mediated novel reductive dimerization/oxidative
dehydrogenation of aldimines to give vicinal diimines.
The vicinal diimine compounds are useful substrates
in organic reactions. Diimines such as 1,4-diazabutadiene
(t-BuDAB) and their derivatives have been applied in
coordination chemistry and asymmetric synthesis as
excellent ligands (or co-ligands).8 Despite the importance
of those functional units, few general methods for the
synthesis of diimines are reported. So far, most of the
vicinal diimines are still preparaed from the reactions
of R-dicarbonyl compounds with amines.9 Recently, Ito
et al. reported that vicinal diimines could be obtained
Carbon-carbon bond formation via coupling reaction
is one of the most important methods in synthetic organic
chemistry. Recently, lanthanoid reagents have been
developed for organic synthetic reactions.1 Among lan-
thanoid reagents, lanthanoid metals themselves have
been also utilized for various transformations of organic
functional groups or carbon-carbon bond formations via
coupling reactions.2 In our studies of lanthanoid chem-
istry, we have succeeded in using the zerovalent lantha-
noid metals as reagents for organic reactions. For in-
stance, we have demonstrated that diaryl ketones are
“umpoled” by ytterbium metal and react with various
electrophiles3 and that imine compounds undergo a
similar reaction to give 1,2-diamines4 or R-amino acids.5
Recently, a triphenylimine dianion complex [Yb(η2-Ph2-
CNPh)-(HMPA)3] has been successfully isolated from the
* To whom correspondence should be addressed. Phone: +81-92-
642-3548. Fax: +81-92-642-3548.
(4) (a) Taniguchi, Y.; Kuno, T.; Nakahashi, M.; Takaki, K.; Fujiwara,
Y. J . Alloys Compd. 1994, 216, L9. (b) Taniguchi, Y.; Kuno, T.;
Nakahashi, M.; Takaki, K.; Fujiwara, Y. Appl. Organomet. Chem. 1995,
9, 491.
(5) Takaki, K.; Tanaka, S.; Fujiwara, Y. Chem. Lett. 1991, 493.
(6) Makioka, Y.; Taniguchi, Y.; Fujiwara, Y. Organometallics 1996,
15, 5476.
(1) (a) Kagan, H. B.; Namy, J . L. In Handbook on the Physics and
Chemistry of the Rare Earths; Gschneidner, K. A., Eyring, L., Eds.;
Elsevier: Amsterdam, 1984; p 525. (b) Watson, P. L.; Parshall, G. W.
Acc. Chem. Res. 1985, 18, 51. (c) Kagan, H. B.; Namy, J . L. Tetrahedron
1986, 42, 6573. (d) Molander, G. A. Chem. Rev. 1992, 92, 29. (e)
Imamoto, T. Lanthanides in Organic Synthesis; Academic Press:
Landon, 1994. (f) Appl. Organomet. Chem.; Craig, P. J ., Fujiwara, Y.,
Eds.; Wiley: Chichester, 1995; Vol. 9. (g) Molander, G. A.; Harris, C.
R. Chem. Rev. 1996, 96, 307. (h) Shibasaki, M.; Sasai, H.; Arai, T.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1236. (i) Kobayashi, S. Eur. J .
Org. Chem. 1999, 15.
(7) Makioka, Y.; Saiki, A.; Takaki, K.; Taniguchi, Y.; Kitamura, T.;
Fujiwara, Y. Chem. Lett. 1997, 27.
(8) (a) Anwander, R. In Topics in Current Chemistry 179 Organo-
lanthanoid Chemistry: Synthesis, Structure, Catalysis; Herrmann, W.
A., Vol. Eds.; Springer: New York, 1996; p 33. (b) Rix, F. C.; Brookhart,
M.; White, P. S. J . Am. Chem. Soc. 1996, 118, 4746. (c) Rix, F. C.;
Brookhart, M.; White, P. S. J . Am. Chem. Soc. 1996, 118, 2436. (d)
Klein, R. A.; Elsevier, C. J .; Hartl, F. Organometallics 1997, 16, 1284.
(e) Carmona, D.; Vega, C.; Lahoz, F. J .; Elipe, S.; Oro, L. A. Organo-
metallics 1999, 18, 3364. (f) Steinhuebel, D. P.; Lippard, S. J .
Organometallics 1999, 18, 3959. (g) Zuccaccia, C.; Macchioni, A.
Organometallics 1999, 18, 4367.
(9) (a) Scholz, A.; Thiele, K. H.; Scholz, J .; Weimann, R. J . Orga-
nomet. Chem. 1995, 501, 195. (b) Yang, K.; Lachicotte, R. J .; Eisenberg,
R. Organometallics 1997, 16, 5234. (c) Faust, R.; Go¨belt, B.; Weber, C.
Synlett 1998, 64. (d) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J .
Am. Chem. Soc. 1998, 120, 4049. (e) Faust, R.; Go¨belt, B.; Weber, C.;
Krieger, C.; Gross, M.; Gisselbrecht, J . P.; Boudon, C. Eur. J . Org.
Chem. 1999, 205.
(2) (a) Imamura, H.; Yamada, K.; Nukui, K.; Tsuchiya, S. J . Chem.
Soc., Chem. Commun. 1986, 367. (b) Fujiwara, Y.; Takaki, K.; Tan-
iguchi, Y. J . Alloys Compd. 1993, 192, 200.
(3) (a) Hou, Z.; Takamine, K., Fujiwara, Y.; Taniguchi, H. Chem.
Lett. 1987, 2061. (b) Hou, Z.; Takamine, K.; Aoki, K.; Shiraishi, O.;
Fujiwara, Y.; Taniguchi, H. J . Org. Chem. 1988, 53, 6077. (c) Takaki,
K.; Beppu, F.; Tanaka, S.; Tsubaki, Y.; J intoku, T. Fujiwara, Y. J .
Chem. Soc., Chem. Commun. 1990, 516. (d) Hou, Z.; Yamazaki, H.;
Kobayashi, K.; Fujiwara, Y.; Taniguchi, H. J . Chem. Soc., Chem.
Commun. 1992, 722. (e) Makioka, Y.; Uebori, S.; Tsuno, M.; Taniguchi,
Y.; Takaki, K.; Fujiwara, Y. Chem. Lett. 1994, 611. (f) Makioka, Y.;
Tsuno, M.; Ueboki, S.; Taniguchi, Y.; Takaki, K.; Fujiwara, Y. J . Org.
Chem. 1996, 61, 372.
10.1021/jo001328d CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/28/2000

Dimerization/Oxidative Dehydrogenation of Aldimines
J . Org. Chem., Vol. 66, No. 2, 2001 515
Ta ble 1. Effect of Va r iou s Oxid a n ts on th e
with others (entries 1-5). Among various aromatic
aldehydes, the best results were obtained when 1-naph-
thaldehyde (1-NpCHO) was used (entries 1-3), which
implied that the structure of aromatic aldehydes is
important for the dehydrogenative coupling of aldimines.
In addition, the fact that bulky 1-NpCHO is appropriate
as an oxdant indicates that reaction is restricted by steric
factors to a greater or lesser extent. Moreover, the high
yield of 2a was obtained by using an equimolar amount
of 1-NpCHO and 1a in the reaction (entry 2). On the
other hand, the yield of the product is only 7% when an
aliphatic aldehyde is used (entry 6). In addition, it is
apparent that copper(II) chloride (CuCl2), peroxides (m-
ClC6H4CO3H; K2S2O3), and tert-butyl nitrite (t-BuONO)
give inferior results (entries 7-10) and that only 17%
yield is obtained in the case of oxygen (entry 11). It is
interesting that the desired reaction can also proceed
with carbon monoxide as an oxidant to give diimine 2a
in 17% yield (entry 12), and no insertion product was
observed.14
It is worthy to note that the reaction of 0.5 mol of Yb
metal with N-benzylideneaniline (1a ) gives the corre-
sponding coupling product, N,N′-diphenyl-1,2-diphenyle-
thane-1,2-diimine (2a ) in high yield (81%, entry 2, Table
1). When a small amount of the Yb metal (0.25 mmol)
was used in the present reaction, the dehydrogenative
coupling product 2a was obtained only in 39% yield, and
some starting material 1a remained unreacted (entry 3).
To find optimum reaction conditions, we studied the
effects of the solvents and reaction temperature on the
model reaction of N-benzylideneaniline (1a ) with Yb
metal. The presence of cosolvent is necessary in this
reaction since the reaction of 1a and Yb metal cannot
proceed in THF alone as a solvent. Several polar com-
pounds are chosen as cosolvents to study dehydrogenative
coupling of 1a , and hexamethylphosphoramide (HMPA)
was found to give the highest yield of 2a (CAUTION: see
the Experimental Section). Coordination of HMPA to Yb
would enhance the stability of a ytterbium-aldimine
complex.15 Although the N,N′-dimethyl-N,N′-propyle-
neurea can be also used, the product yield is low. Other
cosolvents such as N,N,N′N′-tetramethylurea, N,N,N′,N′-
tetramethylethylenediamine, and pyridine did not work.
The dehydrogenative coupling of 1a can also occur in the
absolute toluene to give diimine 2a in 62% yield. How-
ever, the reaction proceeded slowly and longer reaction
times were necessary. In the case of N-(p-dimethylami-
no)benzylideneaniline (1d ), no reaction was observed
with Yb metal in toluene/HMPA even if the reaction time
was extended from 2 to 24 h at room temperature, while
the reaction of 1d with Yb metal proceeded smoothly in
THF/HMPA (vide infra). This result indicates that the
polar solvent is the more suitable in this reaction.
Deh yd r ogen a tive Cou p lin g Rea ction of Ald im in e 1a a
entry
oxidant
yield (%) of 2a b
1
2
3
4
5
6
7
8
9
1-NpCHOc
1-NpCHO
1-NpCHO
p-MeC6H4CHO
o-MeOC6H4CHO
i-PrCHO
CuCl2
60
81d
39e
31
49
7
m-ClC6H4CO3H
K2S2O8
6
10
11
12
t-BuONO
O2
CO
1
17f
17f
a
Reaction conditions: 1a (1.0 mmol), Yb (0.5 mmol), THF (2
b
mL), HMPA (0.5 mL), MeI (2 µL), oxidant (2.0 mmol), rt, 2 h. GC
d
yield based on the aldimine 1a . c 1-Naphthaldehyde. 1-NpCHO
(1.0 mmol) was used. e Yb (0.25 mmol) and 1-NpCHO (1.0 mmol)
were used. f One atm, 24 h.
by SmI2-mediated double insertion of isocyanides into
organic halides.10 However, no reports on the direct
synthesis of vicinal diimines via dehydrogenative cou-
pling of aldimines are available to date. Previously, we
have reported that the reaction of ytterbium metal with
aldimines followed by hydrolysis of reaction mixture gives
diamines.11 Imamoto and Nishimura and others reported
that divalent samarium compounds are also capable of
the coupling imines to give 1,2-diamines.12 In our con-
tinuing studies on application of lanthanoid metals to
organic reactions, we have found that vicinal diimines
can be prepared from the reaction of aryl aldimines with
Yb metal followed by the treatment with an appropriate
oxidant (eq 1). Now we report these results.13
Resu lts a n d Discu ssion
It have been found that aldimines react smoothly with
Yb metal at room temperature in THF solvent to give a
dark-red suspension solution, which affords reductive
dimerization products, 1,2-diamines followed by treat-
ment of water.11 On the contrary, treatment with a proper
oxidant gives vicinal diimines. To investigate the suitable
oxidant for the reaction, we performed model reactions
using various oxidants and N-benzylideneaniline (1a ) in
the presence of Yb metal (eq 2).13 The results are
The effect of the reaction temperature was then
studied. The reaction was carried out as follows: first,
aldimine 1a was allowed to react with Yb metal at room
temperature for 2 h, and then the reaction temperature
was changed and 1-NpCHO was added with stirring
during a period of 2 h. The results are listed in Table 2.
summarized in Table 1. From the data of Table 1, one
can see that aromatic aldehydes are the best compared
(10) (a) Murakami, M.; Masuda, H.; Kawano, T.; Nakamura, H.; Ito,
Y. J . Org. Chem. 1991, 56, 2. (b) Murakami, M.; Ito, Y. J . Organomet.
Chem. 1994, 473, 93.
(11) Takaki, K.; Tsubaki, Y.; Tanaka, S.; Beppu, F.; Fujiwara, Y.
Chem. Lett. 1990, 203.
(12) (a) Imamoto, T.; Nishimura, S. Chem. Lett. 1990, 1141. (b)
Enholm, E. J .; Forbes, D. C.; Holub, D. P. Synth. Commun. 1990, 20,
981. (c) Lebrun, A.; Rantze, E.; Namy, J . L.; Kagan, H. B. New J . Chem.
1995, 19, 699.
(14) (a) Evans, W. J .; Drummond, D. K. J . Am. Chem. Soc. 1986,
108, 7440. (b) Evans, W. J .; Drummond, D. K. J . Am. Chem. Soc. 1988,
110, 2772. (c) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J . Am. Chem.
Soc. 1990, 112, 2011. (d) Deelman, B. J .; Stevels, W. M.; Teuben, J .
H.; Lakin, M. T.; Spek, A. L. Organometallics 1994, 13, 3881.
(15) (a) Hou, Z.; Kobayashi, K.; Yamazaki, H. Chem. Lett. 1991, 265.
(b) Hou, Z.; Wakatsuki, Y. J . Chem. Soc., Chem. Commun. 1994, 1205.
(13) J in, W.; Makioka, Y.; Taniguchi, Y.; Kitamura, T.; Fujiwara,
Y. Chem. Commun. 1998, 1101.

516 J . Org. Chem., Vol. 66, No. 2, 2001
J in et al.
Ta ble 2. Effect of Tem p er a tu r e on th e Deh yd r ogen a tive
Cou p lin g Rea ction of 1a a
(1e,f,j,n ) (entries 5, 6, 10, and 14), and reaction proceeded
slowly. For example, the color of the reaction mixture did
not change to blue-black until the mixture of 1f and Yb
metal was stirred for about 1 h under the present
conditions. These results suggest that the electron-
donating substituent on the benzene ring promotes the
reaction. In the case of 1g, a complex mixture was
formed. It might be due to the nitro group on the benzene
ring, which also reacts with Yb metal to make the
reaction complicated.16 It is noteworthy that aliphatic
aldimines are unsuitable to the reaction. For instance,
in the case of 1k (entry 11), the reaction proceeded slowly
and most of Yb metal remained unreacted in the reaction
mixture. The lower reactivity of aliphatic aldimines was
also observed in the reaction of aldimines with SmI2.12a
To investigate possible steric effects of substitutents on
the dehydrogenative coupling reaction, o- and m-substi-
tuted aldimines were examined (entries 15-21). As for
m-substituted aldimines (1o,p ,s,t) (entries 15, 16, 19, and
20), the reaction proceeded smoothly to give good yields
of diimines (2o,p ,s) except 1t. In the case of 1t, only 34%
yield of 2t was obtained due to coordination of methoxy
group on the benzene ring to the Yb, which would lower
the reactivity of a Yb-aldimine complex to 1-NpCHO.
However, all yields of diimines (1o,p ,s,t) (entries 15, 16,
19, and 20) are lower than those corresponding to
p-substituted aldimines (1b,c,h ,i) (entries 2, 3, and 8, 9).
This may be due to the less steric hindrance of p-
substituted aldimines (1b,c,h ,i). On the other hand, as
for o-substituted aldimines (1q,r ,u ), a great steric hin-
drance is observed. From the reactions of the o-methyl-
substituted aldimines 1q and 1u at room temperature
no diimines 2q and 2u were formed, but from the
hydrogenation product, diamines were formed as the
main products. In the cases of 1q and 1u , reflux tem-
perature was needed to cause the reactions (entries 17
and 21). In the case of o-methoxy-substituted aldimine
1r (entry 18), the complex mixture was obtained even at
reflux temperature.
entry
T (°C)
yield (%) of 2a b
1
2
3
4
5
rt
-35
0
40
66
81
34
51
67
66
a
Reaction conditions: 1a (1.0 mmol), Yb (0.5 mmol), THF (2
mL), HMPA (0.5 mL), MeI (2 µL), 1-NpCHO (1.0 mmol), rt, 2 h.
b
GC yield based on the aldimine 1a .
Ta ble 3. Deh yd r ogen a tive Cou p lin g Rea ction of Va r iou s
Ald im in es Med ia ted by Ytter biu m Meta la
yieldb
(%)
entry substrate
R1
R2
product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
Ph
p-MeC6H4
p-MeOC6H4 Ph
p-Me2NC6H4 Ph
p-ClC6H4
p-CNC6H4
p-NO2C6H4
Ph
Ph
Ph
Ph
p-MeC6H4
p-MeOC6H4 p-MeC6H4
p-MeOC6H4 p-ClC6H4
m-MeC6H4
m-MeOC6H4 Ph
o-MeC6H4
Ph
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
81
90
86
34c
45
5c
Ph
Ph
Ph
d
p-MeC6H4
p-MeOC6H4
p-ClC6H4
t-Bu
74
73
55
e
78
58c
16c
61
72
50f
d
p-MeC6H4
Ph
Ph
Ph
o-MeOC6H4
1s
1t
1u
Ph
Ph
Ph
m-MeC6H4
m-MeOC6H4
o-MeC6H4
2s
2t
2u
67
34
65f
a
Reaction conditions: 1 (1.0 mmol), Yb (0.5 mmol), THF (2 mL),
b
HMPA (0.5 mL), MeI (2 µL), 1-NpCHO (1.0 mmol), rt, 2 h. GC
yield based on the aldimine 1. c Isolated yield. Complex mixtures
d
were formed. e No reaction. f At reflux temperature.
The data in Table 2 show that room temperature is the
best to obtain the highest yield of diimine 2a in this
reaction (entry 1). Both lower and higher temperatures
result in the decrease of the yield of diimines (entries
2-5). The low temperature would decrease the reactivity
of the ytterbium-aldimine intermediate, while the high-
temperature accelerates the decomposition of the inter-
mediate.
Optimized reaction conditions are employed for the
dehydrogenative coupling of various aldimine derivatives
mediated by Yb metal (eq 3). The representative results
of the dehydrogenative coupling reactions are given in
Table 3.
The cross-coupling diimine could be obtained by the
reaction of two different aldimines with Yb metal. For
example, after Yb metal (0.5 mmol) had been allowed to
react with the mixture of aldimines 1b (0.5 mmol) and
1c (0.5 mmol) at room temperature for 2 h, 1-NpCHO (1
mmol) was added and the mixture continuously stirred
for 2 h, there was obtained the cross-coupling product,
N,N′-diphenyl-1-(p-methoxy)-2-(p-methyl)-ethane-1,2-di-
imine 2bc, in 23% yield together with homocoupling
products 2b (43%) and 2c (29%) (eq 4). This result
indicates that the unsymmetrical dimeric intermediate
would be formed in the reaction.
We also investigated dehydrogenative coupling of vari-
ous aldimines mediated by Sm metal. The reaction was
carried out under conditions similar to those in the case
of Yb metal (Table 4). It is of interest to note that Yb
and Sm metals exhibit different reactivities in this
reaction. The yields of diimines by Sm in Table 4,
compared with those in the case of Yb metal (Table 3),
are low (entries 1-7 in Table 4). The different reactivities
of Yb and Sm metals have been observed in the reduction
of nitroarenes.16
When the substrates are nonsubstituted, or p-methyl-
and p-methoxy-substituted aromatic aldimines (1a -
c,h ,i,l,m ) (entries 1-3, 8, 9, 12, and 13), the reactions
proceed smoothly to give dark-brown or dark-red suspen-
sion in several minutes, and addition of 1-NpCHO affords
the corresponding diimines (2a -c,h ,i,l,m ) in good to high
yields. On the contrary, the yields of 2 are low in the
reactions of p-cyano- and p-chloro-substituted aldimines
In addition, when the samarium diiodide (SmI2) was
used in the reaction of 1c, the yield of diimine (2c) was
(16) Hou, Z.; Fujiwara, Y.; Taniguchi, H. J . Org. Chem. 1988, 53,
3118.

Dimerization/Oxidative Dehydrogenation of Aldimines
J . Org. Chem., Vol. 66, No. 2, 2001 517
Ta ble 4. Deh yd r ogen a tive Cou p lin g Rea ction of Va r iou s
Ald im in es Med ia ted by Sa m a r iu m Meta la
entry aldimine
R1
R2
product yieldb (%)
1
2
3
4
5
6
7
8
1a
1b
1b
1c
1h
1i
Ph
p-MeC6H4
p-MeC6H4
p-MeOC6H4 Ph
Ph
Ph
Ph
o-MeC6H4
Ph
Ph
Ph
2a
2b
2b
2c
2h
2i
38c
68
38c
37
p-MeC6H4
p-MeOC6H4
p-ClC6H4
Ph
56
23
1j
1q
2j
2q
40
36d
a
Reaction conditions: 1 (1.0 mmol), Sm (0.5 mmol), THF (2 mL),
b
HMPA (0.5 mL), MeI (2 µL), 1-NpCHO (1.0 mmol), rt, 2 h. GC
yield based on aldimine 1. c Sm (1.0 mmol) was used. At reflux
d
temperature.
corresponding deuterated 1-naphthalenemethanol 5′
(eq 7).
This result clearly indicates that transformation of
hydride from aldimine 1a ′ to 1-NpCHO takes place. In
addition, D2O treatment of the final reaction mixture did
not afford C-deuterated alcohol 5′ (1-NpCHDOH), indi-
cating that the deuterium atom of 1-NpCHDOH is not
derived from water.
On the basis of the above several control experiments,
we propose a plausible mechanism of dehydrogenative
coupling reaction of aldimines mediated by Yb metal as
depicted in Scheme 1.
very low, only 13% yield (eq 5). Metallic Yb or Sm plays
as a two-electron reductant in this reaction, while the
SmI2 generally plays as a one-electron reductant;1a,e,f,g
thus, SmI2 seems to be a poor regent for the dehydroge-
native coupling reaction.
First, Yb metal reacts with one molecule of aldimine 1
via single-electron transfer (SET) mechanism in a man-
ner similar to pinacol coupling of carbonyl compounds,17
to give univalent species A. The highly reactive species
A would immediately react with another molecule of
aldimine 1 to produce azametallacyclopentane C18 via
formation of a diradical intermediate B, which rapidly
forms C via intramolecular homocoupling. Then two
molecules of the aldehyde coordinate to the Yb of inter-
mediate C to give complex D, in which double-hydride
transfer to the carbon atom of the carbonyl group of the
coordinated aldehyde takes place as in a manner similar
to that of the Meerwin-Pondorf-Varley reduction/Op-
penauer oxidation,19 to give vicinal diimine 2 and ytter-
bium alkoxides E. The later species E gives an alcohol
R3CH2OH (5) and trivalent ytterbium(III) salts F when
quenched with water. In addition, if the reaction inter-
mediate C is treated with water, reductive dimerization
product, diamine 3 can be obtained.11 Thus, the present
reaction proceeds via a reductive dimerization/oxidative
dehydrogenation mechanism. As mentioned above, the
diimine yields of the reaction of Yb metal with aldimines
are higher than those of the reaction of Sm metal. It is
As mentioned above, the present reaction seems to
consists of two steps: (1) formation of a Yb-aldimine
complex by the reaction of aldimines with Yb metal in
the first step and (2) coordination and reduction of
1-NpCHO to the Yb-aldimine complex to give the
diimine product in the second step. We believe that the
reaction proceeds according to a pinacol-type mechanism
in a manner similar to that of carbonyl compounds in
the first step.3b,17 Deuterium oxide (D2O) treatment of the
reaction mixture of 1c and Yb metal was carried out
under the same reaction conditions. The reaction gave
diamine 3c (82%) and amine 4c (15%), but no C-
deuterated product 4c′ was formed (eq 6). This result
suggests that a metallacycle aldimine dianion interme-
diate3b is not the precursor of amine 4c. Thus, 4c would
be formed by hydrogen abstraction from solvent (THF
or HMPA).12c
In this reaction, hydrogen transfer from the aldimine
to 1-NpCHO must take place to give the diimine product
and 1-NpCH2OH (1-naphthalenemethanol). To comfirm
this point, a C-deuterated aldimine [N-benzylidene-
aniline-d (1a ′)] was allowed to react with Yb metal under
the same conditions. The reaction of 1a ′ with Yb metal
followed by treatment with 1-NpCHO gave 2a and the
(18) We attempted to prepare and isolate the intermediate C (Ar )
Ar′ ) Ph) and obtained dark-red needles. Since the complex is very
air- and moisture-sensitive the X-ray analysis was not successful.
However, this complex gave the diimine upon treatment with 1-N-
pCHO in THF.
(19) (a) Ponndorf, W. Angew. Chem. 1926, 39, 138. (b) Djerassi, C.
Org. React. 1951, 6, p 208.
(17) Wirth, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 61.

518 J . Org. Chem., Vol. 66, No. 2, 2001
J in et al.
Sch em e 1
spectra were recorded with a HORIBA FT-200 spectropho-
tometer. H NMR spectra were recorded on a J EOL J NM-AL
noted that the tendency to hydride transfer toward
aldehyde is strongly dependent on the Lewis acidity of
the center metal ion in this reaction. The carbonyl group
of the aldehyde in D is more easily polarized and more
easily accepts hydride in complex D having metal atom
which has the stronger Lewis acidity. Thus Yb metal
which has the higher stronger Lewis acidity than Sm,20
would give higher yield of diimines (2).
1
300 FT-NMR (300 MHz) spectrometer in CDCl3 solution and
are reported in parts per million (δ) downfield from internal
tetramethylsilane (TMS). 13C NMR spectra were recorded on
a J EOL J NM-AL 300 FT-NMR (75.5 MHz) spectrometer in
CDCl3 solution. The center peak of CDCl3 (77.0 ppm) was used
as the internal reference. GC analysis of the reaction mixture
was done on a Shimadzu GC-6A system. Elemental analyses
were performed by the Chemical Analysis Center of Kyushu
University. Ytterbium (40 mesh) and samarium (20 mesh)
metals were obtained from Shiga Rare Metallic Co. Ytterbium
metal was washed with anhydrous hexane under argon and
dried in vacuo. Tetrahydrofuran (THF) was distilled from
sodium benzophenone ketyl under argon prior to use. Hex-
amethylphosphoramide (HMPA) was dried over calcium hy-
dride, distilled, and then stored under argon. Caution: care
must be taken in the handing of HMPA because of toxicity.
Toluene was distilled from sodium metal under argon. All
reactions were carried out under argon atmosphere. The
aldimines were prepared from aldehydes and the correspond-
ing aniline derivatives by published procedures.22 All other
starting organic compounds were commercially available and
were used without further purification.
Gen er a l P r oced u r es for th e Syn th esis of N,N′-Dia r yl-
1,2-d ia r yleth a n e-1,2-d iim in es (2) fr om th e Deh yd r oge-
n a tive Cou p lin g Rea ction of Ald im in es Med ia ted by
La n th a n oid Rea gen ts. THF (2.0 mL), HMPA (0.5 mL), and
methyl iodide (2 µL) were successively added to a mixture of
aldimines 1 (1.0 mmol) and lanthanoid metals (0.5 mmol; in
the case of SmI2 (0.1 mol/l), 1.0 mmol (10 mL) was used) under
argon atmosphere, and when the solution turned to brown-
black or blue-black, it was stirred for 2 h at room temperature.
Then, 1-naphthaldehyde (1.0 mmol, 156 mg) was added to the
mixture. The resulting mixture was stirred for an additional
2 h at room temperature. The reaction mixture was quenched
with water (0.1 mL) and diluted with ether (20 mL). After
usual workup followed by a silica gel column chromatography
(n-hexanes-ethyl acetate), the desired compounds 2 were
Con clu sion s
We have demonstrated a novel reductive dimerization/
oxidative dehydrogenation reaction of aldimines medi-
ated by lanthanoid metals, to give ethane-1,2-diimines
2 in good to high yields. The reaction proceeds in the
presence of aromatic aldehydes under midle conditions
such as room temperature in THF. Although there are
many reports on the reductive dimerization of aldimines
by low-velant metal reagents,21 this kind of two-step one-
pot dehydrogenative coupling of aldimines is not reported
to date. To the best of our knowledge, this is the first
example of metal mediated dehydrogenative coupling of
aldimines. It has been made clear that the reaction of
aldimines with lanthanoid metals followed by treating
with an aromatic aldehyde proceeds via a reductive
dimerization/oxidative dehydrogenation mechanism. This
is a quite new type reaction mediated by lanthanoid
metals. The finding of this reaction should contribute to
the developments of synthetic chemistry as well as
chemistry of lanthanoids.
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were recorded on a
YANACO micro melting appratus and are uncorrected. IR
(20) The Lewis acidity is affected by the charge density (Z/r; Z )
nuclear charge, r ) cation radius) of metal cations. Because of
lanthanoid contraction, the atomic and the ionic radii decrease
monotonically with increasing atomic number. See also ref 8a.
(21) (a) Talukdar, S.; Banerji, A. J . Org. Chem. 1998, 63, 3468. (b)
Rieke, R. D.; Kim, S.-H. J . Org. Chem. 1998, 63, 5235 and references
quoted therein.
(22) (a) Layer, R. W. Chem. Rev. 1963, 63, 489 and references
therein. (b) Colebourne, N.; Foster, R. G.; Robson, E. J . Chem. Soc. C
1967, 685. (c) Padawa, A.; Bergmark, W.; Pashayan, D. J . Am. Chem.
Soc. 1969, 91, 2653.

Dimerization/Oxidative Dehydrogenation of Aldimines
J . Org. Chem., Vol. 66, No. 2, 2001 519
obtained. GC yields of the diimines were obtained by using
triphenylphosphine oxide as an internal standard based on
aldimine.
N,N′-Di(p -m et h ylp h en yl)-1,2-d i(p -m et h oxyp h en yl)e-
th a n e-1,2-d iim in e (2m ): mp 126-128 °C (methanol); 1H
NMR (CDCl3) δ 2.23 (s, 6H), 3.82 (s, 6H), 6.47-6.49 (m, 4H),
6.86-6.89 (m, 8H), 7.80-7.83 (m, 4H); 13C NMR (CDCl3) δ
20.9, 55.4, 114.0, 120.5, 128.9, 130.0, 130.6, 134.1, 147.0, 161.7,
163.2; IR (KBr) 1618 cm-1. Anal. Calcd for C30H28N2O2: C,
80.36; H, 6.25; N, 6.25. Found: C, 80.25; H, 6.43; N, 6.18.
N,N′-Di(p-ch lor op h en yl)-1,2-d i(p-m eth oxyp h en yl)eth -
N,N′-Dip h en yl-1,2-d ip h en yleth a n e-1,2-d iim in e (2a ): mp
141-143 °C (methanol); 1H NMR (CDCl3) δ 6.50-6.53 (m, 4H),
6.96-7.09 (m, 6H), 7.35-7.44 (m, 6H), 7.88-7.90 (m, 4H); 13
C
NMR (CDCl3) δ 120.0, 124.9, 128.3, 128.4, 128.7, 131.1, 137.6,
149.3, 163.9; IR (KBr) 1622 cm-1. Anal. Calcd for C26H20N2:
C, 86.67; H, 5.56; N, 7.78. Found: C, 86.69; H, 5.64; N, 7.84.
N,N′-Dip h en yl-1,2-d i(p -m et h ylp h en yl)et h a n e-1,2-d i-
1
a n e-1,2-d iim in e (2n ): mp 150-152 °C (methanol); H NMR
(CDCl3) δ 3.85 (s, 6H), 6.42 (d, J ) 8.4 Hz, 4H), 6.91 (d, J )
8.7 Hz, 4H), 7.04 (d, J ) 8.4 Hz, 4H), 7.82 (d, J ) 8.7 Hz, 4H);
13C NMR (CDCl3) δ 55.4, 114.2, 121.6, 128.4, 130.0, 130.1,
130.2, 147.8, 162.2, 163.8; IR (KBr) 1601 cm-1. Anal. Calcd
for C28H22Cl2N2O2: C, 68.73; H, 4.50; N, 5.73. Found: C, 69.12;
H, 4.62; N, 5.70.
1
im in e (2b): mp 147-149 °C (methanol); H NMR (CDCl3) δ
2.38 (s, 6H), 6.47-6.50 (m, 4H), 6.97-7.07 (m, 6H), 7.19 (d,
J ) 8.7 Hz, 4H), 7.78 (d, J ) 8.7 Hz, 4H); 13C NMR (CDCl3) δ
21.5, 120.1, 124.6, 128.3, 128.3, 129.4, 135.2, 141.5, 149.5,
163.9; IR (KBr) 1622 cm-1. Anal. Calcd for C28H24N2: C, 86.60;
H, 6.19; N, 7.22. Found: C, 86.32; H, 6.37; N, 7.20.
N,N′-Dip h en yl-1,2-d i(m -m et h ylp h en yl)et h a n e-1,2-d i-
1
im in e (2o): mp 168-170 °C (methanol); H NMR (CDCl3) δ
N,N′-Dip h en yl-1,2-d i(p-m eth oxyp h en yl)eth a n e-1,2-d i-
1
2.37 (s, 6H), 6.49-6.53 (m, 2H), 6.96-7.09 (m, 6H), 7.25-7.27
(m, 6H), 7.57-7.60 (m, 2H), 7.79 (s, 2H); 13C NMR (CDCl3) δ
21.5, 120.1, 124.8, 126.1, 128.2, 128.3, 128.5, 131.9, 137.7,
138.5, 149.4, 164.2; IR (KBr) 1622 cm-1. Anal. Calcd for
im in e (2c): mp 151-153 °C (methanol); H NMR (CDCl3) δ
3.83 (s, 6H), 6.47-6.50 (m, 4H), 6.88-7.08 (m, 10H), 7.83-
7.86 (m, 4H); 13C NMR (CDCl3) δ 55.3, 114.0, 120.2, 124.5,
128.3, 130.1, 130.7, 149.6, 161.8, 163.3; IR (KBr) 1622 cm-1
.
C
28H24N2: C, 86.60; H, 6.19; N, 7.22. Found: C, 86.61; H, 6.29;
Anal. Calcd for C28H24N2O2: C, 80.00; H, 5.71; N, 6.67.
Found: C, 79.89; H, 5.80; N, 6.72.
N, 7.23.
N,N′-Dip h en yl-1,2-d i(m -m eth oxyp h en yl)eth a n e-1,2-d i-
im in e (2p ): mp 144-146 °C (methanol); H NMR (CDCl3) δ
N,N′-Dip h en yl-1,2-d i(p-d im eth yla m in op h en yl)eth a n e-
1,2-d iim in e (2d ): mp 245-247 °C (methanol); 1H NMR
(CDCl3) δ 3.02 (s, 12H), 6.47 (d, J ) 7.5 Hz, 4H), 6.65 (d, J )
9.0 Hz, 4H), 6.93-7.06 (m, 6H), 7.80 (d, J ) 9.0 Hz, 4H); 13C
NMR (CDCl3) δ 40.1, 111.3, 120.5, 123.8, 126.2, 128.1, 130.0,
150.2, 151.9, 164.0; IR (KBr) 1618 cm-1. Anal. Cacd for
1
3.82 (s, 6H), 6.51 (d, J ) 7.2 Hz, 4H), 6.99-7.10 (m, 8H), 7.26-
7.38 (m, 4H), 7.54 (s, 2H); 13C NMR (CDCl3) δ 55.4, 112.1,
117.6, 120.1, 121.6, 124.9, 128.4, 129.6, 139.1, 149.2, 159.8,
163.6; IR (KBr) 1622 cm-1. Anal. Calcd for C28H24N2O2: C,
80.00; H, 5.71; N, 6.67. Found: C, 80.00; H, 5.77; N, 6.71.
N,N′-Di(m -m et h ylp h en yl)-1,2-d ip h en ylet h a n e-1,2-d i-
C
30H30N4: C, 80.72; H, 6.73; N, 12.56. Found: C, 80.63; H,
6.82; N, 12.56.
1
N,N ′-Dip h en yl-1,2-d i(p -ch lor op h en yl)et h a n e-1,2-d i-
im in e (2s): mp 105-106 °C (methanol); H NMR (CDCl3) δ
1
im in e (2e): mp 145-147 °C (methanol); H NMR (CDCl3) δ
2.13 (d, 6H), 6.25 (d, 2H), 6.38 (d, J ) 7.8 Hz, 2H), 6.81 (d,
J ) 7.8 Hz, 2H), 6.96 (t, J ) 7.8 Hz, 2H), 7.36-7.47 (m, 6H),
7.86 (d, J ) 7.8 Hz, 4H); 13C NMR (CDCl3) δ 21.3, 116.3, 121.3,
125.4, 128.1, 128.3, 128.7, 131.0, 137.8, 138.0, 149.4, 163.9;
IR (KBr) 1626 cm-1. Anal. Calcd for C28H24N2: C, 86.60; H,
6.19; N, 7.22. Found: C, 86.45; H, 6.23; N, 7.25.
6.49-6.52 (m, 4H), 7.02-7.12 (m, 6H), 7.36-7.40 (m, 4H),
7.77-7.81 (m, 4H); 13C NMR (CDCl3) δ 120.0, 125.3, 128.5,
129.1, 129.5, 135.7, 137.5, 148.9, 162.3; IR (KBr) 1610 cm-1
.
Anal. Calcd for C26H18N2Cl2: C, 72.74; H, 4.20; N, 6.53.
Found: C, 72.91; H, 4.34; N, 6.43.
N ,N ′-Dip h e n yl-1,2-d i(p -cya n op h e n yl)e t h a n e -1,2-d i-
im in e (2f): mp 222-224 °C (methanol); 1H NMR (CDCl3) δ
6.59-6.63 (m, 4H), 7.06-7.15 (m, 6H), 7.66-7.69 (m, 4H),
7.89-7.92 (m, 4H); 13C NMR (CDCl3) δ 114.8, 118.1, 120.1,
126.3, 128.5, 128.8, 132.7, 140.4, 148.3, 161.4; IR (KBr) 2229,-
1614 cm-1. Anal. Calcd for C28H18N4: C, 81.95; H, 4.39; N,
13.66. Found: C, 81.66; H, 4.47; N, 13.57.
N,N′-Di(m -m et h oxyp h en yl)-1,2-d ip h en yl-1,2-d iim in e
(2t): mp 140-141 °C (methanol); H NMR (CDCl3) δ 3.50 (s,
1
6H), 6.00 (s, 2H), 6.18 (d, J ) 7.8 Hz, 2H), 6.57 (dd, J ) 2.1
Hz, 2H), 7.98 (t, J ) 7.8 Hz, J ) 8.4 Hz, 2H), 7.38-7.46 (m,
6H), 7.89 (d, J ) 8.4 Hz, 4H); 13C NMR (CDCl3) δ 54.8, 105.2,
111.6, 111.7, 128.4, 128.8, 129.1, 131.2, 137.7, 150.7, 159.6,
164.1; IR (KBr) 1610 cm-1. Anal. Calcd for C28H24N2O2: C,
80.00; H, 5.71; N, 6.67. Found: C, 80.01; H, 5.79; N, 6.69.
Deh yd r ogen a t ive Cou p lin g of o-Su b st it u t ed Ald i-
m in es (1q,u ) Med ia ted by Ytter biu m Meta l. THF (2.0 mL),
HMPA (0.5 mL), and methyl iodide (2 µL) were successively
added to a mixture of aldimines 1q (or 1u ) (1.0 mmol, 195 mg)
and ytterbium metal (0.5 mmol, 87 mg) under argon atmo-
sphere, and when the solution turned brown-black, it was
stirred for 2 h at room temperature. Then, the reaction mixture
was heated to refluxing temperature, and 1-NpCHO (1.0 mmol,
156 mg) was added to the mixture. The resulting mixture was
stirred for an additional 2 h at reflux temperature. The
reaction mixture was quenched with water (0.1 mL) and
diluted with ether (20 mL). After usual workup followed by a
silica gel column chromatography (n-hexanes-ethyl acetate)
the desired compound 2q (or 2u ) was obtained.
N,N′-Di(p -m et h ylp h en yl)-1,2-d ip h en ylet h a n e-1,2-d i-
1
im in e (2h ): mp 160-162 °C (methanol); H NMR (CDCl3) δ
2.24 (s, 6H), 6.50-6.53 (m, 4H), 6.88 (d, J ) 7.8 Hz, 4H), 7.34-
7.42 (m, 6H), 7.85-7.88 (m, 4H); 13C NMR (CDCl3) δ 20.9,
120.3, 128.2, 128.7, 129.0, 130.9, 134.6, 137.6, 146.8, 163.8;
IR (KBr) 1622 cm-1. Anal. Calcd for C28H24N2: C, 86.60; H,
6.19; N, 7.22. Found: C, 86.37; H, 6.30; N, 7.12.
N,N′-Di(p-m eth oxyp h en yl)-1,2-d ip h en yleth a n e-1,2-d i-
im in e (2i): mp 171-172 °C (methanol); 1H NMR (CDCl3) δ
3.72 (s, 6H), 6.65-6.66 (m, 8H), 7.36-7.41 (m, 6H), 7.84-7.87
(m, 4H); 13C NMR (CDCl3) δ 55.4, 113.7, 122.1, 128.1, 128.7,
130.8, 137.4, 142.5, 157.3, 163.3; IR (KBr) 1622 cm-1. Anal.
Calcd for C28H24N2O2: C, 80.00; H, 5.71; N, 6.67. Found: C,
80.08; H, 5.76; N, 6.74.
N,N ′-Di(p -ch lor op h en yl)-1,2-d ip h en ylet h a n e-1,2-d i-
im in e (2j): mp 168-170 °C (methanol); 1H NMR (CDCl3) δ
6.42-6.47 (m, 4H), 7.04-7.07 (m, 4H), 7.38-7.48 (m, 6H),
7.84-7.88 (m, 4H); 13C NMR (CDCl3) δ 121.4, 128.3, 128.5,
N,N′-Dip h en yl-1,2-d i(o-m et h ylp h en yl)et h a n e-1,2-d i-
1
im in e (2q): mp 181-182 °C (methanol); H NMR (CDCl3) δ
2.31 (s, 6H), 6.60-6.61 (m, 4H), 6.63-6.64 (m, 2H), 6.86-7.19
(m, 12H); 13C NMR (CDCl3) δ 20.49, 119.0, 120.1, 124.2, 125.0,
128.1, 128.2, 128.7, 129.7, 135.8, 149.9, 169.8; IR (KBr) 1612
cm-1. Anal. Calcd for C28H24N2: C, 86.60; H, 6.19; N, 7.22.
Found: C, 86.26; H, 6.26; N, 7.21.
128.9, 130.4, 131.6, 137.2, 147.7, 164.3; IR (KBr) 1622 cm-1
.
Anal. Calcd for C26H18N2Cl2: C, 72.74; H, 4.20; N, 6.53.
Found: C, 73.00; H, 4.22; N, 6.59.
N,N′-Di(p-m eth ylph en yl)-1,2-di(p-m eth ylph en yl)eth an e-
1,2-d iim in e (2l): mp 164-166 °C (methanol); 1H NMR (CDCl3)
δ 2.23 (s, 6H), 2.37 (s, 6H), 6.47 (d, J ) 8.4 Hz, 4H), 6.86 (d,
J ) 8.4 Hz, 4H), 7.17 (d, J ) 7.8 Hz, 4H), 7.76 (d, J ) 7.8 Hz,
4H); 13C NMR (CDCl3) δ 20.9, 21.5, 120.3, 128.2, 128.9, 129.4,
134.2, 135.1, 141.2, 146.9, 163.8; IR (KBr) 1618 cm-1. Anal.
Calcd for C30H28N2: C, 86.54; H, 6.73; N, 6.73. Found: C, 86.33;
H, 6.86; N, 6.77.
N,N′-Di(o-m eth ylp h en yl)-1,2-d iph en yl-1,2-diim in e (2u ):
1
mp 145-147 °C (methanol); H NMR (CDCl3) δ 1.31 (s, 6H),
6.52 (d, J ) 7.8 Hz, 2H), 6.77-6.95 (m, 6H), 7.40-7.47 (m,
6H), 7.95-7.99 (m, 4H); 13C NMR (CDCl3) δ 16.6, 116.7, 125.0,
125.5, 128.4, 128.8, 129.9, 130.9, 132.3, 138.3, 147.9, 162.5;
IR (KBr) 1614 cm-1. Anal. Calcd for C28H24N2: C, 86.60; H,
6.19; N, 7.22. Found: C, 86.33; H, 6.27; N, 7.25.

520 J . Org. Chem., Vol. 66, No. 2, 2001
J in et al.
Cr oss-Cou p lin g Rea ction of Ald im in es 1b a n d 1c
Med ia ted by Ytter biu m Meta l. THF (2.0 mL), HMPA (0.5
mL), and methyl iodide (2 µL) were successively added to a
mixture of aldimines 1b (0.5 mmol, 98 mg) and 1c (0.5 mmol,
106 mg) and ytterbium metal (0.5 mmol, 87 mg) under argon
atmosphere, and when the color of the mixture turned brown-
black, it was stirred for 2 h at room temperature. Then,
1-naphthaldehyde (1.0 mmol, 156 mg) was added to the
mixture. The resulting mixture was stirred for an additional
2 h at room temperature. The reaction mixture was quenched
with water (0.1 mL) and diluted with ether (20 mL). Usual
workup followed by a silica gel column chromatography (n-
hexanes-ethyl acetate) gave the cross-coupling product 2bc.
N,N′-Dip h en yl-1-p-m eth oxyp h en yl-2-p-m eth ylp h en yle-
th a n e-1,2-d iim in e (2bc): mp 154-156 °C (methanol); 1H
NMR (CDCl3) δ 2.39 (s, 3H), 3.84 (s, 3H), 6.47 (d, J ) 7.8 Hz,
4H), 6.89 (d, J ) 8.7 Hz, 2H), 6.97-7.08 (m, 6H), 7.20 (d, J )
8.4 Hz, 2H), 7.80 (dd, J ) 8.4 Hz, J ) 8.7 Hz, 4H); 13C NMR
(CDCl3) δ 21.5, 55.4, 114.0, 120.1, 120.2, 124.5, 124.6, 128.3,
128.3, 129.5, 130.1, 130.6, 135.2, 141.5, 149.5, 149.5, 161.8,
163.2, 163.9; IR (KBr) 1620 cm-1. Anal. Calcd for C28H24N2O:
C, 83.17; H, 5.94; N, 6.93. Found: C, 83.09; H, 6.06; N, 6.88.
Ack n ow led gm en t. The present work was supported
in part by a Grant-in-Aid for Scientific Research on
Priority Areas No. 283, “Innovative Synthetic Reac-
tions”. from Monbusho. Y.M. gratefully acknowledges
the financial support of this research by the J apan
Society for the Promotion of Science in the form of
Research Fellowships for Young Scientists.
J O001328D