Easy entry to terminal 1,5- and 1,6-enynes from (indenyl)ruthenium(II) allenylidenes and grignard reagents

Article abstract of DOI:10.1021/om020330n

An easy entry to terminal 1,5- and 1,6-enynes from (indenyl)ruthenium(II) and allenylidenes and Grignard reagents was discussed. Although the reactivity of cationic allenylidenes was governed by the electron-deficient character of both the C_α a

Full text of DOI:10.1021/om020330n

Organometallics 2002, 21, 3837-3840
3837
Ea sy En tr y to Ter m in a l 1,5- a n d 1,6-En yn es fr om
(In d en yl)r u th en iu m (II) Allen ylid en es a n d Gr ign a r d
Rea gen ts
Victorio Cadierno, Salvador Conejero, M. Pilar Gamasa, and J ose´ Gimeno*
Departamento de Quı´mica Orga´nica e Inorga´nica, Instituto Universitario de Quı´mica
Organometa´lica “Enrique Moles” (Unidad Asociada al CSIC), Facultad de Qu´ımica,
Universidad de Oviedo, E-33071 Oviedo, Spain
Received April 25, 2002
Summary: The allenylidene complexes [Ru(dCdCd
CR1R2)(η5-C9H7)(PPh3)2][PF6] (R1 ) R2 ) Ph (1a ); R1 )
H, R2 ) Ph (1b), C(Me)dCPh2 (1c); CR1R2 ) (1R)-1,3,3-
trimethylbicyclo[2.2.1]hept-2-ylidene (1d )) react with
Grignard reagents CH2dCH(CH2)nMgBr (n ) 1, 2) to
yield the σ-alkynyl derivatives [Ru{CtCCR1R2(CH2)n-
CHdCH2}(η5-C9H7)(PPh3)2] (2a -d , 3a -c). Protonation
of 2a -d and 3a -c with HBF4‚Et2O leads to the vin-
ylidene complexes [Ru{dCdC(H)CR1R2(CH2)nCHdCH2}-
(η5-C9H7)(PPh3)2][BF4] (4a -d and 5a -c), which can be
easily demetalated with acetonitrile to afford the termi-
nal enynes HCtCCR1R2(CH2)nCHdCH2 (6a -d and
7a -c) and the nitrile complex [Ru(NtCMe)(η5-C9H7)-
(PPh3)2][BF4] (8).
CR and Cγ atoms of the cumulenic chain,5 regioselective
nucleophilic additions at Cγ are usually observed with
electron-rich and/or bulky metallic fragments, leading
to a wide variety of the functionalized σ-alkynyl com-
plexes [M]-CtCC(Nu)R1R2.4 On the basis of these
nucleophilic attacks, we have recently developed an
alternative procedure for the propargylic substitution
of 2-propyn-1-ols mediated by the (indenyl)ruthenium-
(II) moiety [Ru(η5-C9H7)(PPh3)2]+ (Scheme 1; path B).6
Thus, in the first step allenylidene complexes B are
formed and subsequently transformed into the corre-
sponding σ-alkynyl derivatives C (Nu- ) HCtC-,
R3COCH2-, (CO)5WdC(OMe)CH2-), which undergo a
selective Câ protonation to afford the vinylidene com-
plexes D.7 Finally, demetalation of D with acetonitrile
leads to the functionalized terminal alkynes in excellent
yields.
In tr od u ction
The Nicholas reaction (Scheme 1; path A) has found
elegant applications in organic synthesis, especially in
the field of carbon-carbon bond formation.1 This type
of propargylic substitution proceeds via dicobalt hexa-
carbonyl-π-alkyne complexes containing coordinated
propargyl cations (A).2 These intermediates selectively
react at C-3 with a wide variety of heteroatom- and
carbon-centered nucleophiles to furnish functionalized
propargylic derivatives after oxidative demetalation. In
this regard, the cationic transition-metal allenylidene
complexes [M]+dCdCdCR1R2, readily available by
dehydration of 2-propyn-1-ols upon coordination to an
unsaturated metal center,3 can be considered as stabi-
lized propargyl cations, due to the extensive contribution
of the metal-alkynyl resonance form [M]-CtCC+R1R2.4
Although the reactivity of cationic allenylidenes is
governed by the electron-deficient character of both the
Pursuing our studies aimed at exploiting the utility
of this synthetic route, in this paper we report an easy
entry to terminal 1,5- and 1,6-enynes (see Chart 1)
based on the regioselective coupling of Grignard re-
agents CH2dCH(CH2)nMgBr (n ) 1, 2) with the allen-
ylidene chain in the complexes [Ru(dCdCdCR1R2)(η5-
C9H7)(PPh3)2][PF6] (R1 ) R2 ) Ph; R1 ) H, R2 ) Ph,
C(Me)dCPh2; CR1R2 )(1R)-1,3,3-trimethylbicyclo[2.2.1]-
hept-2-ylidene (C(C9H16)), followed by the corresponding
demetalation of the resulting σ-alkynyl complexes
(Scheme 1; path B).
Resu lts
The allenylidene complexes [Ru(dCdCdCR1R2)(η5-
C9H7)(PPh3)2][PF6] (R1 ) R2 ) Ph (1a );5a R1 ) H, R2 )
(5) For theoretical calculations on cationic transition-metal allen-
ylidene complexes see: (a) Cadierno, V.; Gamasa, M. P.; Gimeno, J .;
Gonza´lez-Cueva, M.; Lastra, E.; Borge, J .; Garc´ıa-Granda, S.; Pe´rez-
Carren˜o, E. Organometallics 1996, 15, 2137. (b) Pe´rez-Carren˜o, E.
Ph.D. Thesis, University of Oviedo, 1996. (c) Esteruelas, M. A.; Go´mez,
A.; Lo´pez, A. M.; Modrego, J .; On˜ate, E. Organometallics 1997, 16,
5826. (d) Baya, M.; Crochet, P.; Esteruelas, M. A.; Gutie´rrez-Puebla,
E.; Lo´pez, A. M.; Modrego, J .; On˜ate, E.; Vela, N. Organometallics 2000,
19, 2585.
(6) (a) Cadierno, V.; Gamasa, M. P.; Gimeno, J .; Pe´rez-Carren˜o, E.;
Garc´ıa-Granda, S. Organometallics 1999, 18, 2821. (b) Cadierno, V.;
Conejero, S.; Gamasa, M. P.; Gimeno, J . J . Chem. Soc., Dalton Trans.
2000, 451. (c) Cadierno, V.; Conejero, S.; Gamasa, M. P.; Gimeno, J .;
Pe´rez-Carren˜o, E.; Garc´ıa-Granda, S. Organometallics 2001, 20, 3175.
(d) Cadierno, V.; Conejero, S.; Gamasa, M. P.; Gimeno, J .; Falvello, L.
R.; Llusar, R. M. Organometallics, in press.
* To whom correspondence should be addressed. E-mail: jgh@
sauron.quimica.uniovi.es.
(1) For reviews on the Nicholas reaction see: (a) Nicholas, K. M.
Acc. Chem. Res. 1987, 20, 207. (b) Melikyan, G. G.; Nicholas, K. M. In
Modern Acetylene Chemistry; Stang, P. J ., Diederich, F., Eds.; VCH:
New York, 1995; p 118. (c) Caffyn, A. J . M.; Nicholas, K. M. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon: New York, 1995; Vol. 12, p 685.
(d) Mu¨ller, T. J . J . Eur. J . Org. Chem. 2001, 2021. (e) Green, J . R.
Curr. Org. Chem. 2001, 5, 809. (f) Teobald, B. J . Tetrahedron 2002,
4133.
(2) For a review on transition-metal-stabilized propargyl cations
see: El Amouri, H.; Gruselle, M. Chem. Rev. 1996, 96, 1077.
(3) Selegue, J . P. Organometallics 1982, 1, 217.
(4) For comprehensive reviews on allenylidene complexes see: (a)
Werner, H. Chem. Commun. 1997, 903. (b) Bruce, M. I. Chem. Rev.
1998, 98, 2797. (c) Touchard, D.; Dixneuf, P. H. Coord. Chem. Rev.
1998, 178-180, 409. (d) Cadierno, V.; Gamasa, M. P.; Gimeno, J . Eur.
J . Inorg. Chem. 2001, 571.
(7) For reviews on the synthesis and reactivity of vinylidene
complexes see: (a) Bruce, M. I. Chem. Rev. 1991, 91, 197. (b) Werner,
H. J . Organomet. Chem. 1994, 475, 45. (c) Bruneau, C.; Dixneuf, P. H.
Acc. Chem. Res. 1999, 32, 311. (d) Puerta, M. C.; Valerga, P. Coord.
Chem. Rev. 1999, 193-195, 977.
10.1021/om020330n CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/02/2002

3838 Organometallics, Vol. 21, No. 18, 2002
Sch em e 1. P r op a r gylic Su bstitu tion Rea ction s
Notes
Ch a r t 1
failed, and instead complicated mixtures of uncharac-
terized species were obtained.9
1
Analytical and spectroscopic data (IR and H, 31P{1H},
and 13C{1H} NMR) support the proposed formulations
for 2a -c and 3a -c.10 In particular, the formation of an
enynyl chain was identified on the basis of (i) the
presence of a typical ν(C≡C) absorption band in the IR
spectra (KBr) at 2076-2090 cm-1 and (ii) characteristic
resonances in the 13C{1H} NMR spectra of the Ru-Ct
C, Cγ, CH2, and CHdCH2 carbon nuclei in the ranges δ
90.26-95.29 (2J CP ) 22.6-24.9 Hz, CR), 112.22-115.02
(Câ), 40.10-52.05 (Cγ), 30.85-46.97 (CH2), 137.73-
140.35 (dCH), and 113.91-115.81 (dCH2).
Demetalation of complexes 2a -c and 3a -c using our
two-step method (path B; Scheme 1) proceeds cleanly,
yielding the terminal 1,5- and 1,6-enynes HCtCCR1R2-
(CH2)nCHdCH2 (R1 ) R2 ) Ph, n ) 1 (6a ), 2 (7a ); R1 )
H, R2 ) Ph, n ) 1 (6b), 2 (7b); R1 ) H, R2 ) C(Me)d
CPh2, n ) 1 (6c), 2 (7c)) in good overall yields (Scheme
2).11 Thus, the air-stable vinylidene derivatives
[Ru{dCdC(H)CR1R2(CH2)nCHdCH2}(η5-C9H7)(PPh3)2]-
[BF4] (4a -c and 5a -c) were initially prepared (85-95%
yield), via selective Câ protonation of 2a -c and 3a -c
Ph (1b),5a C(Me)dCPh2 (1c)8e) react with the Grignard
derivatives CH2dCH(CH2)nMgBr (n ) 1, 2), in tetrahy-
drofuran at -20 °C, to afford the neutral σ-alkynyl
derivatives [Ru(CtCCR1R2(CH2)nCHdCH2)(η5-C9H7)-
(PPh3)2] (R1 ) R2 ) Ph, n ) 1 (2a ), 2 (3a ); R1 ) H, R2 )
Ph, n ) 1 (2b), 2 (3b); R1 ) H, R2 ) C(Me)dCPh2, n )
1 (2c), 2 (3c)) (Scheme 2).8 Complexes 2a -c and 3a -c
have been isolated as air-stable orange solids in 76-
88% yield after chromatographic workup on neutral
Al2O3. Attempts to synthesize the σ-alkynyl derivatives
[Ru(CtCCR1R2CHdCH2)(η5-C9H7)(PPh3)2] by reaction
of the allenylidenes 1a -c with vinylmagnesium bromide
(9) We note that the coupling of the neutral diphenylallenylidenes
trans-[RhCl(dCdCdCPh2)(PiPr3)2], [RuCl(dCdCdCPh2)(η5-C5Me5)-
(κ1P-iPr2PCH2CO2Me)], and [OsCl(dCdCdCPh2)(η5-C5H5)(PiPr3)] with
CH2dCHMgBr has been reported to produce the pentatrienyl deriva-
tives [Rh{(3-5-η)-CH2CHCdCdCPh2}(PiPr3)2], [Ru{(3-5-η)-CH2CHCd
CdCPh2}(η5-C5Me5)(κ1P-iPr2PCH2CO2Me)], and [Os{(3-5-η)-CH2CHCd
CdCPh2}(η5-C5H5)(PiPr3)], respectively, via initial substitution of the
chloro ligand and subsequent migratory insertion of the allenylidene
unit into the M-CHdCH2 bond: (a) Wiedemann, R.; Steinert, P.;
Gevert, O.; Werner, H. J . Am. Chem. Soc. 1996, 118, 2495. (b) Braun,
T.; Meuer, P.; Werner, H. Organometallics 1996, 15, 4075. (c) Crochet,
P.; Esteruelas, M. A.; Lo´pez, A. M.; Ruiz, N.; Tolosa, J . I. Organo-
metallics 1998, 17, 3479.
(10) Analytical and spectroscopic data for compounds 2a -d through
7a -c have been provided as Supporting Information.
(11) To the best of our knowledge only terminal enynes 6b and 7b
have been previously reported: (a) O’Boyle, J . E.; Nicholas, K. M.
Tetrahedron Lett. 1980, 21, 1595. (b) Khun, O.; Rau, D.; Mayr, H. J .
Am. Chem. Soc. 1998, 120, 900. (c) Caporusso, A. M.; Filippi, S.;
Barontini, F.; Salvadori, P. Tetrahedron Lett. 2000, 41, 1227.
(8) Other regioselective nucleophilic additions at Cγ of (indenyl)-
ruthenium(II) allenylidenes [Ru(dCdCdCR1R2)(η5-C9H7)(PPh3)2]+ are
reported in: (a) Cadierno, V.; Gamasa, M. P.; Gimeno, J .; Borge, J .;
Garc´ıa-Granda, S. Organometallics 1997, 16, 3178. (b) Cadierno, V.;
Gamasa, M. P.; Gimeno, J .; Lo´pez-Gonza´lez, M. C.; Borge, J .; Garc´ıa-
Granda, S. Organometallics 1997, 16, 4453. (c) Cadierno, V.; Gamasa,
M. P.; Gimeno, J .; Lastra, E. J . Chem. Soc., Dalton Trans. 1999, 3235.
(d) Cadierno, V.; Conejero, S.; Gamasa, M. P.; Gimeno, J .; Rodr´ıguez,
M. A. Organometallics 2002, 21, 203. (e) Conejero, S. Ph.D. Thesis,
University of Oviedo, 2001.

Notes
Organometallics, Vol. 21, No. 18, 2002 3839
Sch em e 2
Sch em e 3
7
with HBF4 and fully characterized by means of stan-
dard spectroscopic techniques and elemental analysis.10
The most relevant spectroscopic features of 4a -c and
5a -c are (i) (1H NMR) the singlet (4a and 5a ) or doublet
(4b,c and 5b,c; J HH ) 6.9-9.1 Hz) signal for the acidic
vinylidene proton [Ru]dCdCH at δ 4.26-4.63 and (ii)
(13C{1H} NMR) the characteristic deshielded RudCR
resonance, which appears as a triplet (4a and 5a ) or a
doublet of doublets (4b,c and 5b,c) in the range 343.53-
354.55 ppm (2J CP ) 15.9-17.4 Hz). In a second step
acetonitrile solutions of vinylidenes 4a -c and 5a -c
were heated under reflux, affording the terminal enynes
6a -c and 7a -c and the cationic nitrile complex [Ru-
(NtCMe)(η5-C9H7)(PPh3)2][BF4] (8).6a Enynes 6a -c and
7a -c were purified from the reaction mixture by column
chromatography on silica gel (86-95% isolated yields)
after filtering off the unsoluble complex 8. Spectroscopic
data of 6a -c and 7a -c are in agreement with the
proposed structures.10 Thus, the IR spectra show the
expected ν(CtC) absorption in the range 2113-2132
The optically active 1,5-enyne HCtCC(C9H16)CH2-
CHdCH2 (6d ) has been also prepared by starting from
the chiral allenylidene complex [Ru{dCdCdC(C9H16)}-
(η5-C9H7)(PPh3)2][PF6] (1d )6c by reaction with allylmag-
nesium bromide (see Scheme 3). Compounds 2d , 4d , and
6d were obtained in a diastereoselective manner as the
result of the selective exo addition of the 2-propen-1-yl
fragment to the allenylidene chain in 1d . We have
recently reported analogous stereoselective nucleophilic
exo additions to 1d .6c,d,8e Since the analytical and
spectroscopic data of these species are comparable to
those observed for their achiral counterparts, 2a -c, 4a -
c, and 6a -c are not worth further discussion.
Discu ssion
As expected, nucleophilic addition of the Grignard
reagents CH2dCH(CH2)nMgBr (n ) 1, 2) to [Ru(dCd
CdCR1R2)(η5-C9H7)(PPh3)2][PF6] (1a -d ) proceeds regio-
selectively at the Cγ atom of the allenylidene chain,
affording the σ-alkynyl complexes [Ru{CtCCR1R2(CH2)n-
CHdCH2}(η5-C9H7)(PPh3)2] (2a -d and 3a -c). As we
have extensively reported,6,8 this process is based on the
ability of the moiety [Ru(η5-C9H7)(PPh3)2] to sterically
protect the electrophilic CR atom of the allenylidene
chain, which allows the selective formation of σ-alkynyl
cm-1. The acetylenic CtCH proton appears, in the H
1
NMR spectra, as a singlet (6a and 7a ) or a doublet (6b,c
and 7b,c; J HH ) ca. 2 Hz) at 1.92-2.66 ppm, and the
HCtC carbons resonate, in the 13C{1H} NMR, in the
ranges δ 70.32-74.14 (HCt) and 85.29-87.77 (tC).

3840 Organometallics, Vol. 21, No. 18, 2002
Notes
(1.0 M in Et2O; 1.1 mL, 1.1 mmol) or CH2dCH(CH2)2MgBr (0.5
M in THF; 2.2 mL, 1.1 mmol) for 30 min. The mixture was
warmed to room temperature, and the solvent was then
removed in vacuo. The resulting solid residue was dissolved
in dichloromethane (ca. 5 mL) and transferred to an Al2O3
(neutral; activity grade I) chromatography column. Elution
with a mixture of hexane and diethyl ether (3/1) gave an
orange band from which complexes 2a -d and 3a -c were
isolated after solvent removal. 2a : yield 79% (0.768 g). 3a :
yield 81% (0.799 g). 2b: yield 82% (0.735 g). 3b: yield 88%
(0.801 g). 2c: yield 87% (0.881 g). 3c: yield 76% (0.780 g). 2d :
yield 84% (0.791 g).
complexes. Taking advantage of this behavior and the
well-established synthetic methodology of functionalized
alkynes mediated by the (indenyl)ruthenium moiety
[Ru(η5-C9H7)(PPh3)2] (path B; Scheme 1), 1,5- and 1,6-
enynes HCtCCR1R2(CH2)nCHdCH2 (n ) 1,2; 6a -d and
7a -c) have been synthesized in high yields. This
synthetic approach can be regarded as the result of the
formal coupling of 2-propyn-1-ols with 1-propene or
1-butene (see eq 1).
Syn th esis of th e Vin ylid en e Com p lexes [Ru {dCdC(H)-
CR1R2(CH2)n CHdCH2}(η5-C9H7)(P P h 3)2][BF 4] (R1 ) R2
)
P h , n ) 1 (4a ), 2 (5a ); R1 ) H, R2 ) P h , n ) 1 (4b), 2 (5b);
R1 ) H, R2 ) C(Me)dCP h 2, n ) 1 (4c), 2 (5c); CR1R2
)
C(C9H 16), n ) 1 (4d )). A diluted solution of HBF4‚Et2O in
diethyl ether was added dropwise at -20 °C to a stirred
solution of the corresponding σ-alkynyl complex [Ru{Ct
CCR1R2(CH2)nCHdCH2}(η5-C9H7)(PPh3)2] (2a -d and 3a -c; 1
mmol) in 100 mL of diethyl ether. Immediately, an insoluble
solid precipitated but the addition was continued until no
further solid was formed. The solution was then decanted and
the brown solid washed with diethyl ether (3 × 20 mL) and
dried in vacuo. 4a : yield 93% (0.986 g). 5a : yield 95% (1.020
g). 4b: yield 89% (0.876 g). 5b: yield 85% (0.848 g). 4c: yield
89% (0.979 g). 5c: yield 87% (0.969 g). 4d : yield 95% (0.978
g).
The overall processes of Schemes 2 and 3 constitute
an efficient alternative to the known coupling of prop-
argylic alcohols with allylsilanes or -stannanes via [Co2-
(CO)6]-stabilized propargyl cations (Nicholas reac-
tion).1,11a,b Although the same number of steps are
required in both propargylic substitution reactions (see
Scheme 1), the quantitative recovery of the metal
fragment as the acetonitrile complex [Ru(NtCMe)(η5-
C9H7)(PPh3)2][BF4] (8) represents an advantage com-
pared with the classical Nicholas reaction in which the
metal auxiliary cannot be recovered after the oxidative
decomplexation step.12 Further studies directed to the
preparation of novel functionalized terminal alkynes
using the allenylidene complexes [Ru(dCdCdCR1R2)-
(η5-C9H7)(PPh3)2][PF6] as synthons of propargyl cations
HCtCC+R1R2 are in progress.
Syn th esis of Ter m in a l En yn es HCtCCR1R2(CH2)n CHd
CH2 (R1 ) R2 ) P h , n ) 1 (6a ), 2 (7a ); R1 ) H, R2 ) P h , n
) 1 (6b), 2 (7b); R1 ) H, R2 ) C(Me)dCP h 2, n ) 1 (6c), 2
(7c); CR1R2 ) C(C9H16), n ) 1 (6d )). A solution of the
corresponding vinylidene complex [Ru{dCdC(H)CR1R2(CH2)n-
CHdCH2}(η5-C9H7)(PPh3)2][BF4] (4a -d and 5a -c; 1 mmol) in
acetonitrile (30 mL) was heated under reflux for 30 min. The
solution was then evaporated to dryness and the resulting solid
residue extracted with diethyl ether (ca. 50 mL) and filtered.
A yellow solid containing the nitrile complex [Ru(NtCMe)-
(η5-C9H7)(PPh3)2][BF4] (8) remains insoluble. The extract was
evaporated to dryness and the crude product purified by
column chromatography on silica gel with hexane as eluent.
Evaporation of the solvent gave terminal enynes 6a -d and
7a -c as colorless oils. 6a : yield 88% (0.204 g). 7a : yield 90%
(0.222 g). 6b: yield 86% (0.134 g). 7b: yield 89% (0.151 g).
6c: yield 95% (0.259 g). 7c: yield 93% (0.266 g). 6d : yield
98% (0.198 g).
Exp er im en ta l Section
The manipulations were performed under an atmosphere
of dry nitrogen using vacuum-line and standard Schlenk
techniques. All reagents were obtained from commercial
suppliers and used without further purification. Solvents were
dried by standard methods and distilled under nitrogen before
use. Allenylidene complexes [Ru(dCdCdCR1R2)(η5-C9H7)-
(PPh3)2][PF6] (R1 ) R2 ) Ph (1a );5a R1 ) H, R2 ) Ph (1b),5a
C(Me)dCPh2 (1c);8e CR1R2 ) C(C9H16) (1d )6c) were prepared
by following the methods reported in the literature.
Syn th esis of th e σ-Alk yn yl Com p lexes [Ru {CtCCR1R2-
(CH2)n CHdCH2}(η5-C9H7)(P P h 3)2] (R1 ) R2 ) P h , n ) 1
(2a ), 2 (3a ); R1 ) H, R2 ) P h , n ) 1 (2b), 2 (3b); R1 ) H, R2
) C(Me)dCP h 2, n ) 1 (2c), 2 (3c); CR1R2 ) C(C9H16), n )
1 (2d )). A solution of the corresponding allenylidene complex
[Ru(dCdCdCR1R2)(η5-C9H7)(PPh3)2][PF6] (1a -d ; 1 mmol) in
30 mL of THF was treated at -20 °C with CH2dCHCH2MgBr
Ack n ow led gm en t. This work was supported by the
Ministerio de Ciencia y Tecnolog´ıa of Spain (Project
BQU2000-0227) and the FICYT of Asturias (Project PR-
01-GE-4). S.C. thanks the Ministerio de Educacio´n y
Cultura of Spain for the award of a Ph.D. grant.
Su p p or tin g In for m a tion Ava ila ble: Text giving analyti-
cal and spectroscopic data for compounds 2a -d through 7a -
c. This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) Related ruthenium-catalyzed propargylic substitutions of mono-
substituted propargylic alcohols, via allenylidene intermediates, have
been recently reported: (a) Nishibayashi, Y.; Wakiji, I.; Hidai, M. J .
Am. Chem. Soc. 2000, 122, 11019. (b) Nishibayashi, Y.; Wakiji, I.; Ishii,
Y.; Uemura, S.; Hidai, M. J . Am. Chem. Soc. 2001, 123, 3393.
OM020330N