Samarium iodide-catalyzed pinacol coupling of carbonyl compounds

Full text of DOI:10.1021/ja962331a

11666
J. Am. Chem. Soc. 1996, 118, 11666-11667
Scheme 1
Samarium Iodide-Catalyzed Pinacol Coupling of
Carbonyl Compounds
Ryoji Nomura, Tatsuya Matsuno, and Takeshi Endo*
Research Laboratory of Resources Utilization
Tokyo Institute of Technology, Nagatsuta-cho
Midori-ku, Yokohama 226, Japan
ReceiVed July 8, 1996
samarium species.7 UV-vis spectroscopic analyses clearly
indicated the existence of the reduction pathway (Table 1). After
the suspension of SmI3 in THF was treated with Mg, the color
change from yellow to dark-blue occurred immediately, and the
characteristic absorptions attributed to SmI2 were observed at
621 and 560 nm (run 1), indicating the formation of SmI2.8 It
should be noted that the absorptions due to SmI2 were also
detected for reactions employing SmBrI2 and SmClI2 (runs 3
and 4). The decrease of the absorbance compared with the
parent SmI2 solution is probably due to the existence of an
equilibrium between SmI2 and SmI3, which might be supported
by the results that the addition of MgI2 to a solution of SmI2
led to the decrease in its absorbance (run 5). These results
indicate that the addition of Mg shifts the equilibrium between
Sm(II) and Sm(III) toward Sm(II) via Scheme 2.
The present SmI2/Mg system could be applied to the pinacol
coupling of carbonyl compounds.9,10 One of the significant
points for the successful pinacol coupling under this system is
to maintain the concentration of divalent samarium higher than
that of carbonyl compounds to avoid the competition between
the coupling and the Sm(III)-promoted reactions, such as
benzoin condensation and the Tishchenko reaction.9e In other
words, a fast regeneration of Sm(II) is required. The reduction
of initially formed samarium(III) pinacolate is expected to be
slower than that of SmClI2 due to the electron-donating alkoxide.
Therefore, the use of chlorotrimethylsilane (TMSCl)11 would
transform the samarium(III) pinacolate into its silyl ether along
with the formation of SmClI2 to afford the desired reaction
pathway. Indeed, the highest yield of the product (66%),
comparable with that of the stoichiometric reaction (run 5), was
attainable by slow addition of a mixture of benzaldehyde (1
equiv) and TMSCl (1 equiv) to a mixture of SmI2 (0.1 equiv),
TMSCl (0.5 equiv), and an excess of Mg in THF at room
temperature (run 1 in Table 2).12 It is notable that the color of
the reaction mixture of dark blue immediately changed into pale
One of the most remarkable developments in recent organic
synthesis is the application of divalent samarium compounds
as excellent reducing agents.1 Since the pioneering works by
Kagan and Inanaga in the chemistry of samarium(II) iodide
(SmI2), a wide variety of novel types of electron transfer
reactions which are far superior to the traditional ones in both
efficiency and selectivity have been reported.1 Unfortunately,
almost all of the electron transfer reactions required more than
a stoichiometric amount of samarium complexes, except for few
examples of hydride transfer reductions such as the Meerwein-
Ponndorf-Verley (MPV) reduction2 and the Tishchenko reac-
tion.3 This is simply because a catalytic cycle of them has not
been established.4 This limitation strongly decreases the
synthetic value of the reactions promoted by low-valent sa-
marium complexes. In this work, we report our preliminary
investigation of the establishment of a catalytic cycle of SmI2
for the reduction of carbonyl compounds.
In order to achieve the catalytic cycle of SmI2, a reduction
pathway of trivalent into divalent samarium species is required
(path A in Scheme 1). The previous studies on low-valent
samarium-mediated reductions using Sm as a reducing agent5
and the similar reduction potential between Sm and Mg (-2.41
and -2.37 V, respectively) would offer a possibility that Mg
serves as reducing agent of trivalent samarium.6 Indeed, it was
easily found that trivalent samarium salts such as SmI3, SmBrI2,
and SmClI2 underwent the reduction by Mg to provide divalent
(1) For recent reviews for the application of low-valent samarium
compounds into organic chemistry, see: (a) Molander, G. A.; Harris, C. R.
Chem. ReV. 1996, 96, 307. (b) Imamoto, T. Lanthanides in Organic
Synthesis; Academic Press: London, Great Britain, 1994. (c) Molander,
G. A. Chem. ReV. 1992, 92, 29. (d) Molander, G. A. ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
Great Britain, 1991; Vol. 1, Chapter 9, p 251. (e) Inanaga, J. Yuki Gosei
Kagaku Kyokaishi 1989, 47, 200. (f) Kagan, H. B.; Namy, J. L.
Tetrahedron 1986, 42, 6573. (g) Kagan, H. B.; Namy, J. L. In Handbook
on the Physics and Chemistry of Rare Earths; Gschneidner, Jr., K. A.,
Eyring, L., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands,
1984; Vol. 6, Chapter 50, p 525.
(2) (a) Evans, D. A.; Nelson, S. G.; Gagne´, M. R.; Muci, A. R. J. Am.
Chem. Soc. 1993, 115, 9800. (b) Lebrun, A.; Namy, J. L.; Kagan, H. B.
Tetrahedron Lett. 1991, 32, 2355. (c) Okano, T.; Matsuoka, M.; Konishi,
H. Chem. Lett. 1987, 181. (d) Namy, J. L.; Souppe, J.; Collin, J.; Kagan,
H. B. J. Org. Chem. 1984, 49, 2045.
(7) Magnesium metal was activated by vigorous stirring as a usual
Grignard technique. SmI3 and SmBrI2 were prepared by the reaction of
SmI2 with iodine or bromine. SmClI2 was synthesized by the reaction of
SmI2 with 1 equiv of benzyl chloride. The concentration of SmClI2 was
not determined in this experiment.
(8) Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102,
2693.
(3) (a) Evans, D. A.; Hoveyda, A. H. J. Am. Chem. Soc. 1990, 112, 6447.
(b) Uenishi, J.; Masuda, S.; Wakabayashi, S. Tetrahedron Lett. 1991, 32,
5097. (c) Yokoo, K.; Mine, N.; Taniguchi, H.; Fujiwara, Y. J. Organomet.
Chem. 1985, 279, C19.
(4) SmCl3-associated electroreductions were reported although it appears
to be difficult to conclude that electrochemically formed Sm(II) species
work as reducing agents. See: (a) He´bri, H.; Dun˜ach, E.; Pe´richon, J. J.
Chem. Soc., Chem. Commun. 1993, 499. (b) Espanet, B.; Dun˜ach, E.;
Pe´richon, J. Tetrahedron Lett. 1992, 33, 2485. (c) He´bri, H.; Dun˜ach, E.;
Pe´richon, J. Synlett 1992, 293. (d) He´bri, H.; Dun˜ach, E.; Pe´richon, J. Syn.
Commun. 1991, 21, 2377. (e) He´bri, H.; Dun˜ach, E.; Heintz, M.; Troupel,
M.; Pe´richon, J. Synlett 1991, 901. (f) Le´onard, E.; Dun˜ach, E.; Pe´richon,
J. J. Chem. Soc., Chem. Commun. 1989, 276.
(9) For examples of SmI2-mediated pinacol coupling, see: (a) Namy, J.
L.; Souppe, J.; Kagan, H. B. Tetrahedron Lett. 1983, 24, 765. (b) Souppe,
J.; Danon, L.; Namy, J. L.; Kagan, H. B. J. Organomet. Chem. 1983, 250,
227. (c) Fu¨rstner, A.; Csuk, R.; Rohrer, C.; Weidmann, H. J. Chem. Soc.,
Perkin Trans. 1 1988, 1729. (d) Shiue, J. S.; Lin, C. C.; Fang, J. M.
Tetrahedron Lett. 1993, 34, 335. (e) Okaue, Y.; Isobe, T. Mem. Fac. Sci.,
Kyushu UniV., Ser. C 1992, 18, 179.
(10) For examples of Mg-promoted pinacol coupling reactions, see: (a)
Rausch, M. D.; McEwen, W. E.; Kleinberg, J. Chem. ReV. 1957, 57, 417.
(b) Fu¨rstner, A.; Csuk, R.; Rohrer, C.; Weidmann, H. J. Chem. Soc., Perkin
Trans. 1 1988, 1729. (c) Csuk, R.; Fu¨rstner, A.; Weidmann, H. J. Chem.
Soc., Chem. Commun. 1986, 1802. (d) Adams, R.; Adams, E. W. Organic
Syntheses; Wiley: New York, 1941; Collect. Vol. I, p 459.
(5) (a) Ogawa, A.; Nanke, T.; Sumino, Y.; Ryu, I.; Sonoda, N. Chem.
Lett. 1994, 379. (b) Yanada, R.; Bessho, K.; Yanada, K. Chem. Lett. 1994,
1279. (c) Murakami, M.; Hayashi, M.; Ito, Y. Synlett. 1994, 179. (d)
Yoshizawa, T.; Hatajima, T.; Amino, H.; Imamoto, T. Nihonkagakukaishi
1993, 482. (e) Ogawa, A.; Takami, N.; Sekiguchi, M.; Ryu, I.; Kambe,
N.; Sonoda, N. J. Am. Chem. Soc. 1992, 114, 8729.
(6) Sonoda et al. reported that the Sm/SmI2 system promotes the
deoxygenative coupling of amides. They have also shown that this reaction
proceeds by using both a catalytic amount of SmI2 and excess of Mg
although any further investigation was not performed. See, ref 5e.
(11) (a) Fu¨rstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 2533. (b)
Fu¨rstner, A.; Hupperts, A. J. Am. Chem. Soc. 1995, 117, 4468. (c) Hirao,
T.; Hasegawa, T.; Muguruma, Y.; Ikeda, T. J. Org. Chem. 1996, 61, 366.
(12) A typical procedure was as follows: To a 0.1 M solution of SmI2
in dry THF (2 mL) containing 1 mmol of TMSCl and 400 mg of magnesium
turnings (Nacalai Tesque) was added dropwise the mixture of carbonyl
compound (2 mmol) and TMSCl (2 mmol) over 2 h. After the complete
consumption of carbonyl compound, an extractive workup with ether
followed by purification (SiO2 column chromatography) gave the mixture
(dl and meso) of pinacols.
S0002-7863(96)02331-1 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11667
Table 1. UV-Vis Absorption of the Reaction Mixture of
Scheme 3
Samarium Salts and Mg in THFa
run
conditions
SmI2
SmI3 + Mg
SmI2Br + Mg
SmI2Cl + Mg
SmI2 + MgI2
UV-visb λmax (absorbance)
1
2
3
4
5
620 (0.90), 558 (0.84)
621 (0.43), 560 (0.40)
620 (0.45), 560 (0.42)
621 (0.38), 556 (0.31)
620 (0.21), 555 (0.20)
a Conditions: samarium salts (0.01 mmol), dry THF (3.5 mL), room
b
temperature. λmax valves are in nanometers (nm).
Scheme 2
Table 3. Pinacol Coupling of Carbonyl Compounds under the
SmI2/Mg Systema
Table 2. Pinacol Coupling of Benzaldehyde (1) under the
SmI2/Mg System
run
substrates
yieldb (%)
1
2
3
4
5
6
7
8
9
PhCH2CH2CHO
p-CH3OC6H4CHO
p-(CH3)2NC6H4CHO
p-CH3O2CC6H4CHO
p-CNC6H4CHO
57c
81
76
45
23
run
SmI2/TMSCl/1/Mg (mmol)
yielda (%)
1
2
3
4
5
0.2/3.0/2.0/16
0.2/6.0/4.0/16
0.2/0/2.0/16
0/3.0/2.0/16
2.0/3.0/2.0/0
66
54
14
0
PhCOCH3
68
78
p-CH3OC6H4COCH3
PhCO(CH2)3COPh
PhCH2CH2COCH3
76
82d
58e
a Isolated yield.
a Conditions: SmI2 (0.2 mmol), TMSCl (3 mmol), substrate (2
mmol), THF (2 mL), room temperature. b Isolated yield. c Cyclic trimer
of hydrocinnamaldehyde was obtained in 36% yield. d Only a single
isomer (meso) was obtained. e Reaction time was 48 h; 42% of starting
material was recovered.
blue by the addition of a few drops of benzaldehyde/TMSCl
and again to dark blue within a few minutes. The formation of
benzyl alcohol, benzyl benzoate, and benzoin was not detected,
and a bond formation between para and carbonyl carbons9d did
not occur. There was no apparent difference in the diastereo-
selectivity between the present and the stoichiometric reactions
(almost 1:1).13 A reaction with 5 mol % of SmI2 also produced
the homocoupled product without evident decrease of the yield
(run 2). Reaction run in the absence of TMSCl resulted in a
complex mixture containing the pinacol, benzyl alcohol, ben-
zoin, benzyl benzoate, and (4-formylphenyl)phenylmethanol9d
(run 3).
These results cannot prove precisely that SmI2 serves as a
mediator in the reduction system. However, the presence of
the catalytic cycle of SmI2 is suggested by the results as follows.
First, the use of other lanthanide salts such as CeCl3, LaCl3,
NbCl3, Yb(OTf)3, YbI3, and SmCl3 gave homocoupled product
in low yields (17-19%), whereas these salts and SmI2 are
known to work as Lewis acids. Therefore, the possibility that
SmI2 or trivalent samarium salts only act as a Lewis acid is
negligible. Second, no reactions took place in the absence of
SmI2 (run 4), although the TMSCl/Mg system in hexameth-
ylphosphoramide is known to be effective for the pinacol
coupling.14 It rules out the promotion of the pathway by the
TMSCl/Mg system. Furthermore, the reduction of Sm(III) into
Sm(II) by the Grignard reagent15 of 4-iodobutyl trimethylsilyl
ether can be neglected since no oxidized products of the
Grignard reagent (i.e., 1-butanol, 1,8-octandiol) were formed.
Third, the reaction without SmI2 using Mg pretreated with SmI2
resulted in the recovery of benzaldehyde. This result excludes
the possibility that SmI2 simply activates the Mg surface.
Finally, as mentioned above, the recovery of the initial dark
blude color proves the regeneration of Sm(II). These observa-
tions suggest that Sm(II) serves as the true reducing species.
The possible reaction pathway involves the reduction of carbonyl
compounds by SmI2 to give Sm(III) alcoholates followed by
silylation of the alcoholates by TMSCl providing the corre-
sponding silyl ether and SmI2Cl. SmI2 or SmClI would be
regenerated by the electron transfer from Mg (Scheme 3).
Table 3 lists examples of the reductive coupling of carbonyl
compounds including aliphatic and aromatic ketones and
aldehydes. The aliphatic aldehyde (run 1) as well as aromatic
aldehydes (runs 2-5) and ketones (runs 6-8) underwent the
pinacol coupling by the SmI2/Mg system. Although reducible
functional groups (ester, cyano) tended to decrease the yield,
the present reaction includes a tolerance to methoxy and
dimethylamino groups remaining intact under the conditions.
Aliphatic ketone was also available in this reduction system to
give the corresponding pinacol (run 9). The reaction of 1,5-
diphenyl-1,5-pentanedione gave a single isomer (meso) in high
yield. The formation of iodotrimethylsilane (TMSI) in situ
seems to be negligible since the cleavage of methoxy group by
TMSI16 did not occur.
In summary, we have demonstrated a novel SmI2-catalyzed
reduction system of carbonyl compounds. SmI2 was found to
act as a catalyst in the reduction where trivalent samarium
formed is smoothly reduced by Mg to regenerate divalent
samarium salts. This method would provide a new route in
the field of lanthanide chemistry.
Acknowledgment. This research was supported by grants from the
Nissan Science Foundation. We also thank the reviewers for their
valuable suggestion for revision of the paper.
Supporting Information Available: Detail experimental procedures
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JA962331A
(13) The stoichiometric reaction gave 4-iodobutyl trimethylsilyl ether
as a byproduct, whereas it was not detected in the catalytic reaction.
(14) Chan, T. H.; Vinokur, E. Tetrahedron Lett. 1972, 13, 75.
(15) Fukuzawa, S.; Mutoh, K.; Tsuchimoti, T.; Hiyama, T. J. Org. Chem.
1996, 61, 5400 and references therein.
(16) Olah, G. A.; Surya Prakash, G. K. In AdVances in Silicon Chemistry,
Larson, G. L., Ed.; JAI Press Inc.: London, Great Britain, 1991; Vol. 1,
Chapter 1, p 1.