The first oxidative addition of a hypervalent compound to metallic lanthanoid: Synthesis, characterization, and reaction of samarium(ii) bis(trifluoromethanesulfonate) derived from metallic samarium and 1, 5-dithioniabicyclo[3.3.0]octane bis(trifluorometh

Full text of DOI:10.1021/jo981436o

7114
J . Org. Chem. 1998, 63, 7114-7116
proved that the hypervalent compound 1,5-dithioniabi-
Th e F ir st Oxid a tive Ad d ition of a
Hyp er va len t Com p ou n d to Meta llic
La n th a n oid : Syn th esis, Ch a r a cter iza tion ,
a n d Rea ction of Sa m a r iu m (II)
Bis(tr iflu or om eth a n esu lfon a te) Der ived
fr om Meta llic Sa m a r iu m a n d
cyclo[3.3.0]octane (1), which has a transannular S-S
bond,19 was a highly reactive oxidant toward organo-
metallic compounds.20 Thus, these facts prompted us to
investigate the reaction of lanthanoid metal with 1.
Herein, we report the first synthesis and characterization
of salt-free samarium(II) bis(trifluoromethanesulfonate)
1,5-Dith ion ia bicyclo[3.3.0]octa n e
Bis(tr iflu or om eth a n esu lfon a te)
complexes of the formula [Sm(OTf)2(L)1.5
]
(2a , L )
n
MeCN; 2b, L ) t-BuCN; 3, L ) THF) and also report
that the complex 2a mediates intermolecular pinacol
coupling reactions of aromatic ketones with high dia-
stereoselectivity.
Kazushi Mashima,* Toshiyuki Oshiki, and
Kazuhide Tani*
Department of Chemistry, Graduate School of Engineering
Science, Osaka University,
Resu lts a n d Discu ssion
Toyonaka, Osaka 560-8531, J apan
Complex 2a can be prepared by the oxidative reaction
of metallic samarium with 1 in acetonitrile. When a
mixture of 1 and an excess of samarium metal in the
presence of a catalytic amount of iodine in acetonitrile
was stirred at 50 °C for 24 h, the complex 2a was isolated
as a purple powder in 66% yield (eq 1). A similar reaction
Received J uly 22, 1998
In tr od u ction
Diiodolanthanoid(II) compounds such as SmI2 and
YbI2, first introduced by Kagan, are versatile, homoge-
neous, and powerful reducing reagents1 which have been
utilized for chemo- and stereoselective C-C bond forma-
tion.2-9 Recently, a new lanthanoid(II) reagent, a sa-
marium(II) bistriflate “Sm(OTf)2”, which is generated in
situ by reducing Sm(OTf)3 with Grignard reagents10 or
alkyllithium,11 has been developed.12 Despite its versa-
tility, it has not been elucidated whether this Sm(II)
species includes the complexation of a triflate anion with
alkali and alkaline earth elements. Shibasaki et al. have
reported that a lithium ate complex of lanthanum bearing
chiral auxiliary binaphthoxy ligands is an excellent
asymmetric catalyst for nitro-aldol reactions, in which
the lithium plays an important role.13 We anticipated
that a salt-free, well-defined samarium(II) bistriflate
compound would give direct information about the reac-
tivity of Sm(II) supported with triflate ligands.
in pivalonitrile gave 2b in 60% yield. Both complexes
are highly air- and moisture-sensitive, but solid samples
of these complexes can be stored under argon. Aceto-
nitrile bound to the Sm center of 2a was labile; thus, on
dissolving the complex in THF at room temperature, the
acetonitrile was easily replaced by THF to form [Sm-
(OTf)2(thf)1.5]n (3) in quantitative yield. The addition of
excess acetonitrile to 3 regenerates 2a . The formulation
of 3 was further confirmed by X-ray analysis; the complex
3 has an infinite 2D sheet structure (Figure 1),21 though
a one-dimensional coordination polymer of [Eu(µ-SePh)2-
(thf)3]n has been reported.22
The triflate complex 2a was found to be an excellent
reagent for pinacol coupling reactions of carbonyl
compounds.23-25 Coupling reactions of aromatic ketones,
using stoichiometric amounts of 2a , yielded the corre-
sponding 1,2-diols. The results are summarized in Table
1. The pinacol coupling reaction of acetophenone using
2a in acetonitrile proceeded smoothly at room tempera-
ture to give 2,3-diphenyl-2,3-butanediol in 99% yield with
We have already reported that the reaction of oxidants
such as disulfide with a metallic lanthanoid gave rise to
salt-free lanthanoid thiolate complexes,14-18 and we
(1) Girard, P.; Namy, J . L.; Kagan, H. B. J . Am. Chem. Soc. 1980,
102, 2693.
(2) Molander, G. A. In The Chemistry of the Metal Carbon Bond;
Hartley, F. R., Ed.; J ohn Wiley: New York, 1989; Vol. 5.
(3) Soderquist, J . A. Aldrichimica Acta 1991, 24, 15.
(4) Molander, G. A. Chem. Rev. 1992, 92, 29.
(5) Imamoto, T. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Schreiber, S. L., Eds.; Pergamon: London, 1991; Vol. 1, p
231.
(6) Molander, G. A. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Schreiber, S. L., Eds.; Pergamon: London, 1991; Vol.
1, p 251.
(7) Kagan, H. B.; Sasaki, M.; Collin, J . Pure Appl. Chem. 1988, 60,
1725.
(8) Imamoto, T. Lanthanides in Organic Synthesis; Academic
Press: London, 1994.
(9) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96, 307.
(10) Hanamoto, T.; Sugimoto, Y.; Sugino, A.; Inanaga, J . Synlett
1994, 377.
(11) Fukuzawa, S.; Tsuchimoto, T.; Kanai, T. Chem. Lett. 1994, 1981.
(12) Fukuzawa, S.; Mutoh, K.; Tsuchimoto, T.; Hiyama, T. J . Org.
Chem. 1996, 61, 5400.
(13) Shibasaki, M.; Sasai, H.; Arai, T. Angew. Chem., Int. Ed. Engl.
1997, 36, 1236.
(14) Mashima, K.; Nakayama, Y.; Shibahara, T.; Fukumoto, H.;
Nakamura, A. Inorg. Chem. 1996, 35, 93.
(17) Mashima, K.; Nakayama, Y.; Fukumoto, H.; Kanehisa, N.; Kai,
Y.; Nakamura, A. J . Chem. Soc., Chem. Commun. 1994, 2523.
(18) Mashima, K.; Nakayama, Y.; Kai, Y.; Kanehisa, N.; Nakamura,
A. J . Chem. Soc., Chem. Commun. 1993, 1847.
(19) (a) Fujihara, H.; Akaishi, R.; Furukawa, N. J . Chem. Soc.,
Chem. Commun. 1987, 930. (b) Iwasaki, F.; Toyoda, N.; Akaishi, R.;
Fujihara, H.; Furukawa, N. Bull. Chem. Soc. J pn. 1988, 61, 2563.
(20) Mashima, K.; Oshiki, T.; Matsuo, Y.; Tani, K. Chem. Lett. 1997,
793.
(15) Mashima, K.; Shibahara, T.; Nakayama, Y.; Nakamura, A. J .
Organomet. Chem. 1995, 501, 263.
(16) Mashima, K.; Nakayama, Y.; Nakamura, A.; Kanehisa, N.; Kai,
Y.; Takaya, H. J . Organomet. Chem. 1994, 473, 85.
(21) Drawings of 3 and its structural feature are given in Supporting
Information.
(22) Berardini, M.; Emge, T.; Brennan, J . G. J . Am. Chem. Soc. 1993,
115, 8501.
S0022-3263(98)01436-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/11/1998

Notes
J . Org. Chem., Vol. 63, No. 20, 1998 7115
The choice of solvent is crucial in this reaction. The
reaction conducted in THF decreased both in the dias-
tereoselectivity and in the yield of the product (entry 4).
The diastereoselectivity increased slightly to 88:12 at -78
°C in THF, though the yield decreased (entry 5). The
addition of 10 equiv of hexamethylphosphoramide (HMPA)
retarded the coupling reaction (entry 6), in sharp contrast
to a previous finding that the addition of excess HMPA
to Sm(OTf)2 generated in situ dramatically accelerated
the rate of the pinacol coupling reaction.11 Thus, aceto-
nitrile proved to be the best solvent, which corresponds
to Kagan’s finding that the pinacol coupling of acetophe-
none mediated by SmI2 in the presence of a catalytic
amount of NiBr2(PPh3)2 proceeded in pivalonitrile to give
the diols with diasteroselectivity up to 79:21 (dl:meso).23e
The pinacol coupling reaction of various aromatic ketones
using 2a gave the corresponding diols; the ester group
remains intact (entry 10). It is assumed that mechanis-
tically two adjacent Sm(II) centers binding a ketyl radical
dimerize to give a bimetallopinacolate intermediate in
which the two samarium atoms are connected by chelat-
ing triflate ligands and that minimization of steric
interference through anti-orientation of the aryl groups
explains the observed diastereoselectivity. The bimet-
allopinacolate complexes have been synthesized recently
by the reaction of acetone with UCl4/Hg(Li)26 and by the
treatment of fluorenone with a samarium(II) alkoxide,27
the latter a reversible conversion to the monomeric
radical anion of fluorenone bound to the samarium
center.
F igu r e 1. A drawing of the 2D sheet structure of 3. All
hydrogen atoms are omitted for clarity. Two samarium(II)
centers, Sm1 and Sm2, have seven-coordinated geometry, a
distorted capped octahedral geometry around Sm(1), and a
distorted pentagonal bipyramidal geometry around Sm(2).
Ta ble 1. P in a col Cou p lin g of Ca r bon yl Com p ou n d s
Med ia ted by 2a a
It is of interest to compare the reactivity of 2a with
that of in situ generated Sm(OTf)2 species. When a
tetrahydrofuran solution of Sm(OTf)2, which was derived
from the reaction of Sm(OTf)3 with ethylmagnesium
bromide, was used for the reductive coupling of acetophe-
none at room temperature, the corresponding diols were
obtained in 51% yield with 2 diastereoselectivity of 62:
38 (dl:meso).10 The coupling reaction of acetophenone by
the in situ Sm(OTf)2 species generated using s-BuLi in
THF afforded the diols (67% yield) with a diastereo-
selectivity of 85:15 (dl:meso),11 which is comparable to
that obtained with 2a in THF (the Sm(II) species in THF
was 3). These findings are consistent with the fact that
the absorption spectrum of 328 is almost the same as that
of the Sm(OTf)2 species generated using s-BuLi11 but is
much different from that of the Sm(OTf)2 species gener-
ated using ethylmagnesium bromide,10 suggesting that
the former species is similar to the salt-free 3, while the
latter species likely involves an interaction with magne-
sium.
temp yield
entry
substrate
PhCOMe
PhCOMe
PhCOMe
PhCOMe
PhCOMe
PhCOMe
PhCOEt
p-MeC6H4COMe
p-MeOC6H4COMe
p-MeOOCC6H4COMe CH3CN
p-FC6H4COMe CH3CN
solvent
CH3CN
CH3CN
EtCN
THF
THF
(°C) (%) b dl:meso c
1
2
3
4
5
6
7
8
9
rt
-40
-78
rt
-78
rt
99
99
60
59
20
0
90:10
94:6
94:6
82:18
88:12
THF/HMPAd
CH3CN
CH3CN
CH3CN
-40
-40
rt
-40
-40
92
92
74
73
73
95:5
93:7
78:22
88:12
84:16
10
11
a
b
Reaction conditions: ketone/2a ) 1/1, 1 h. Isolated yield of
a mixture of diastereomers. c Ratio was determined by 1H NMR.
d
Hexamethylphosphoramide (10 equiv) was added.
a high diastereoselectivity of dl:meso ) 90:10 (entry 1).
The highest diastereoselectivity of 94:6 was obtained
when the same reaction took place at -40 °C in aceto-
nitrile (entry 2) and at -78 °C in propionitrile (entry 3),
though the yield of the 1,2-diol of entry 3 decreased to
60%.
In summary, we demonstrated the new, salt-free
divalent samarium triflates 2a , 2b, and 3, which were
synthesized by the novel oxidative reaction of metallic
samarium with the hypervalent compound 1. Complex
2a was found to be an excellent reagent for the highly
diastereoselective intermolecular pinacol coupling of
aromatic ketones in acetonitrile.
(23) For some 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) Hamann, B.; Namy,
J .-L.; Kagan, H. B. Tetrahedron 1996, 52, 14225. (f) Honda, T.; Katoh,
M. Chem. Commun. 1997, 369. (g) Kawatsura, M.; Kishi, E.; Kito, M.;
Sakai, T.; Shirahama, H.; Matsuda, F. Synlett 1997, 479. (h) Nomura,
R.; Matsuno, T.; Endo, T. J . Am. Chem. Soc. 1996, 118, 11666. (i)
Taniguchi, N.; Kaneta, N.; Uemura, M. J . Org. Chem. 1996, 61, 6088.
(24) Wirth, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 61.
(26) Maury, O.; Villiers, C.; Ephritikhine, M. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1129.
(27) Hou, Z.; Fujita, A.; Zhang, Y.; Miyano, T.; Yamazaki, H.;
Wakatsuki, Y. J . Am. Chem. Soc. 1998, 120, 754 and references
therein.
(25) SmBr2 was found to be an efficient reagent for pinacol coupling
reactions: Lebrun, A.; Namy, J .-L.; Kagan, H. B. Tetrahedron Lett.
1993, 34, 2311.
(28) UV spectra of Sm(II) species. Sm(OTf)3/EtMgBr system λmax
(THF) (ꢀ) 595 (240), 408 (390), 363 (400), 408 nm (460); Sm(OTf)3/s-
BuLi system λmax (THF) (ꢀ) 563 (110), 480 (85), 347 (270), 313 nm (320).

7116 J . Org. Chem., Vol. 63, No. 20, 1998
Notes
synthesis of 2a , except for the use of pivalonitrile as a solvent,
were conducted to give complex 2b in 60% yield: mp >200 °C
Exp er im en ta l Section
(dec); IR (Nujol/KBr) 2269 (w), 1271 (s), 1065 (s), 647 (s) cm-1
.
Gen er a l. All manipulations involving air- and moisture-
sensitive compounds were carried out using standard Schlenk
techniques under argon. THF, toluene, diethyl ether, and
hexane were dried over sodium-benzophenone ketyl and then
distilled before use. Acetonitrile, propionitrile and pivalonitrile
were distilled from CaH2. The compounds 1,5-dithioniabicyclo-
The composition of 2b was also determined by iodometric
titration and by the 1H NMR spectrum of the decomposition
product.
P r ep a r a tion of 3. Complex 2a (50.6 mg, 9.96 × 10-2 mmol)
was dissolved in 2 mL of THF. The resulting purple solution
was stirred at room temperature for 5 min and then all volatiles
were removed in vacuo. Complex 3 was obtained as a reddish
purple powder in 97% yield (54.0 mg): mp >255 °C (dec); IR
30
[3.3.0]octane bis(trifluoromethanesulfonate) (1)29 and Sm(OTf)3
were prepared according to the literature procedures. Metallic
samarium was purchased from Nihon Yttrium Co. Ltd. Ethyl-
magnesium bromide (1.0 M diethyl ether solution) and sec-
butyllithium were purchased from Kanto Chemical Co. Ltd.
P r ep a r a tion of 2a . A mixture of 1 (1.392 g, 3.12 mmol),
finely divided samarium metal (938 mg, 6.24 mmol), and iodine
(39.5 mg, 0.156 mmol) in acetonitrile (40 mL) was stirred at 50
°C for 24 h. After excess samarium was removed by centrifuga-
tion, all volatiles were removed under reduced pressure. The
crude product was dissolved in acetonitrile (5 mL), followed by
precipitation by adding toluene (25 mL). The supernatant
solution was removed by a syringe, and a purple powder 2a
(2.179 g) was obtained in 66% yield after being dried in vacuo:
mp >230 °C (dec); IR (Nujol/KBr) 2267 (w), 1262 (s), 1045 (s),
and 633 (s) cm-1. The composition of 2a was determined both
by iodometric titration and by the amount of the nitrile molecules
estimated by the 1H NMR of the decomposition product of 2a
with air. UV spectrum of 2a : λmax (CH3CN) (ꢀ) 659 nm (160).
The titration method31 was as follows. An acetonitrile solution
(1.0 mL) of compound 2a (42.4 mg) in a Schlenk tube was
titrated with a standard toluene solution of iodine (8.38 × 10-3
mmol/mL) using a 1.0 mL syringe until the initial green solution
of 2a changed to pale yellow. This titration was repeated three
times, and the averaged concentration of the solution of 2a was
determined to be 8.25 × 10-2 mmol/mL.
(Nujol/KBr) 1262 (s), 1039 (s), 887 (w), 623 (s) cm-1
. The
composition of 3 was also determined by similar methods to
those employed for 2a . UV spectrum of 3: λmax (THF) (ꢀ) 563
(340), 478 (230), 303 nm (180).
Liga n d Exch a n ge Rea ction of 3 w ith Aceton itr ile. An
acetonitrile solution (2 mL) of 3 (45.0 mg, 8.09 × 10-2 mmol)
was stirred at room temperature for 5 min. All volatiles were
removed, and the resulting purple powder was dried under
vacuum. Complex 2a was obtained in 95% yield (39.1 mg).
P in a col Cou p lin g Rea ction s of Ca r bon yl Com p ou n d s
Med ia ted by Com p lex 2a . A typical procedure is described
for the reaction of 2a with acetophenone. A solution of 2a (82.5
mg, 8.71 × 10-2 mmol) in acetonitrile (1.0 mL) was cooled to
-40 °C. Acetophenone (10.0 µL, 8.71 × 10-2 mmol) was added
via a microsyringe. After the mixture was stirred for 1 h at -40
°C, the reaction was quenched by adding silica gel and then
hexane (3 mL). The organic products were passed through a
short column of silica gel using ether as an eluent. The crude
products obtained were subjected to preparative TLC (ethyl
acetate/hexane 1:3) to give the 1,2-diols (10.4 mg, 99% yield).
The diastereomeric ratio of the product was determined to be
94:6 (dl:meso) by 1H NMR spectroscopy; a resonance (δ 1.52) due
to the methyl group in the dl-isomer was observed further
upfield than that (δ 1.59) in the meso-isomer.22
Air oxidation of 2a was accomplished as follows. Complex
2a (7.2 mg) was dissolved into CDCl3 (0.5 mL) containing
dichloromethane (2.00 µL, 3.12 × 10-2 mmol) as an internal
standard. The mixture was exposed to air and then was passed
through a short column of Na2SO4 using CDCl3 as an eluent.
The 1H NMR spectrum of the eluate showed a singlet signal due
to the free acetonitrile and a singlet signal due to dichlo-
romethane of the internal standard. Peak areas of the aceto-
nitrile and the dichloromethane were given as follows: 213 for
the acetonitrile (3 H) and 200 for dichloromethane (2 H).
Thereby, the weight of acetonitrile was calculated to be 9.10 ×
10-1 mg (2.22 × 10-2 mmol). This corresponds to the presence
of 1.5 acetonitrile molecules per Sm(OTf)2 fragment. The
composition of 2a obtained by this method agreed with that
obtained by the iodometric titration.
Ack n ow led gm en t. We thank Dr. T. Yamagata
(Osaka University) for his contribution to the structure
determination of 3. This work was financially supported
by a Grant-in-Aid for Scientific Research (Priority Area
283, Innovative Synthetic Reactions, and 09309006)
from the Ministry of Education, Science and Culture,
J apan. K.M. appreciates the partial financial support
from the Asahi Glass Foundation. T.O. is supported by
a J SPS research fellowship, 1996-1998.
Su p p or tin g In for m a tion Ava ila ble: Structure, final
positional parameters, final thermal parameters, bond dis-
tances, angles, and torsion angles for the complex 3 (15 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
P r ep a r a tion of 2b. Similar procedures described for the
(29) Fujihara, H.; Akaishi, R.; Furukawa, N. J . Chem. Soc., Chem.
Commun. 1987, 930.
(30) Forsberg, J . H.; Spaziano, V. T.; Balasubramanian, T. M.; Liu,
G. K.; Kinsley, S. A.; Duckworth, C. A.; Poteruca, J . J .; Brown, P. S.;
Miller, J . L. J . Org. Chem. 1987, 52, 1017.
(31) Imamoto, T.; Ono, M. Chem. Lett. 1987, 501.
J O981436O