Mechanistic studies on the catalytic cycle of metal fluoride-catalyzed allylation using allyltrimethoxysilane in protic solvents

Article abstract of DOI:10.1021/jo0346196

The catalytic cycles of CdF₂·1-catalyzed and AgF·BINAP-catalyzed allylation using allyltrimethoxysilane in protic solvents were investigated. The experimental and ¹⁹F NMR studies strongly supported that metal fluorides were regenerat

Full text of DOI:10.1021/jo0346196

Mech a n istic Stu d ies on th e Ca ta lytic Cycle of Meta l
F lu or id e-Ca ta lyzed Allyla tion Usin g Allyltr im eth oxysila n e in
P r otic Solven ts
Naohiro Aoyama, Tomoaki Hamada, Kei Manabe, and Shuj Kobayashi*
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-0033, J apan
skobayas@mol.f.u-tokyo.ac.jp
Received May 11, 2003
The catalytic cycles of CdF2‚1-catalyzed and AgF‚BINAP-catalyzed allylation using allyltrimethox-
ysilane in protic solvents were investigated. The experimental and 19F NMR studies strongly
supported that metal fluorides were regenerated from silyl fluoride species, and that these reactions
were truly fluoride-catalyzed reactions. It was revealed that these catalytic cycles were much
different from that of TBAF-catalyzed allylation using allyltrimethylsilane in THF.
SCHEME 1. Cd F 2‚1-Ca ta lyzed Allyla tion Rea ction
in Aqu eou s Med ia
In tr od u ction
Allylation of carbonyl compounds to afford syntheti-
cally useful homoallylic alcohols has been a subject of
extensive investigations.1 While numerous allyl organo-
metallic reagents have been used so far,2 among them,
allylsilanes have been widely used and are more desirable
reagents than some other allylating agents such as
allyltins because of their lower toxicities.3 Not only Lewis
or Brønsted acids but also fluoride anion sources such
as tetrabutylammonium fluoride (TBAF) can be employed
as catalysts for allylation using allylsilanes, especially
allyltrimethylsilane.4 Recently, allyltrimethoxysilane5 has
also been used because it is activated more easily by
fluoride anion.6 For example, Yamamoto et al. reported
AgF‚BINAP-catalyzed enantioselective allylation of al-
dehydes,7 and Shibasaki et al. reported CuCl-tetrabu-
tylammonium difluorotriphenylsilicate-catalyzed allyla-
tion of aldehydes, ketones, and imines.8 Quite recently,
we have also reported CdF29‚terpyridine (1)-catalyzed
allylation using allyltrimethoxysilane in aqueous media
(Scheme 1).10 Interestingly, CdF2‚1-catalyzed reactions
in anhydrous THF or methanol gave only a trace amount
of the product. These results show that water is essential
for this catalytic system. Our next interest was the
mechanism of these fluoride-catalyzed reactions, and it
is indeed an important issue to clarify how fluoride anion
is involved in the reactions. Herein, we report studies
on the mechanism of catalytic cycle of the CdF2‚1-
catalyzed allylation in aqueous media. Furthermore,
based on the insight into the CdF2‚1 catalysis, we also
studied the mechanism of the AgF‚BINAP-catalyzed
allylation in methanol. These studies revealed that the
catalytic cycles of these reactions using allyltrimethox-
ysilane in protic solvents differ greatly from the cycle of
the TBAF-catalyzed reaction using allyltrimethylsilane.
For the TBAF-catalyzed allylation using allyltrimeth-
ylsilane in THF, two possible mechanisms of the catalytic
cycle have been proposed by Sakurai et al. The first
(1) For reviews, see: (a) Fleming, I. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Ed.; Pergamon Press: Oxford,
1991; Vol. 2, p 563. (b) Denmark, S. E.; Almstead, N. G. In Modern
Carbonyl Chemistry; Otera, J ., Ed.; Wiley-VCH: Weinheim, 2000; p
299.
(2) For a review, see: Yamamoto, Y.; Asao, N. Chem. Rev. 1993,
93, 2207.
(3) For reviews, see: (a) Sakurai, H. Pure Appl. Chem. 1982, 54, 1.
(b) Fleming, I. Org. React. 1989, 37, 57. (c) Hosomi, A. Acc. Chem. Res.
1988, 21, 200. (d) Masse, C. E.; Panek, J . S. Chem. Rev. 1995, 95, 1293.
(4) For examples of fluoride-catalyzed allylation reactions using
allylsilanes, see: (a) Hosomi, A.; Shirahata, A.; Sakurai, H. Tetrahe-
dron Lett. 1978, 19, 3043. (b) Wang, D.-K.; Zhou, Y.-G.; Tang, Y.; Hou,
X.-L.; Dai, L.-X. J . Org. Chem. 1999, 64, 4233. (c) Asao, N.; Shibato,
A.; Itagaki, Y.; J ourdan, F.; Maruoka, K. Tetrahedron Lett. 1998, 39,
3177. (d) Shibato, A.; Itagaki, Y.; Tayama, E.; Hokke, Y.; Asao, N.;
Maruoka, K. Tetrahedron 2000, 56, 5373. (e) Nakamura, K.; Naka-
mura, H.; Yamamoto, Y. J . Org. Chem. 1999, 64, 2614.
(7) (a) Yanagisawa, A.; Kageyama, H.; Nakatsuka, Y.; Asakawa, K.;
Matsumoto, Y.; Yamamoto, H. Angew. Chem., Int. Ed. Engl. 1999, 38,
3701. (b) During the review of this paper, Yamamoto et al. have
reported that BINAP/AgOTf/KF/18-crown-6 was also effective for
enantioselective allylation reactions using allyltrimethoxysilane. Wad-
amoto, M.; Ozasa, N.; Yanagisawa, A.; Yamamoto, H. J . Org. Chem.
2003, 68, 5593.
(8) Yamasaki, S.; Fujii, K.; Wada, R.; Kanai, M. Shibasaki, M. J .
Am. Chem. Soc. 2002, 124, 6536.
(5) For allylation reactions using pentacoordinate silicates prepared
from allyltrimethoxysilane, see: (a) Sato, K.; Kira, M.; Sakurai, H. J .
Am. Chem. Soc. 1989, 111, 6429. (b) Hosomi, A.; Kohra, S.; Ogata, K.;
Yanagi, T.; Tominaga, Y. J . Org. Chem. 1990, 55, 2415.
(6) For reviews, see: (a) Chuit, C.; Corriu, R. J . P.; Reye, C.; Young,
J . C. Chem. Rev. 1993, 93, 1371. (b) Sakurai, H. Synlett 1989, 1.
(9) According to the literature, the hydrolysis constant (pKh) of Cd-
(II) is 10.08. Therefore, Cd(II) is cannot be readily hydrolyzed in water.
Baes, C. F., J r.; Mesmer, R. The Hydrolysis of Cations; Wiley: New
York, 1976.
(10) Aoyama, N.; Hamada, T.; Manabe, K.; Kobayashi, S. Chem.
Commun. 2003, 676.
10.1021/jo0346196 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/19/2003
J . Org. Chem. 2003, 68, 7329-7333
7329

Aoyama et al.
SCHEME 2. TBAF -Ca ta lyzed Allyla tion Rea ction 4a
TABLE 1. Effect of F lu or id e An ion
entry
catalyst (mol %)
yield (%)
1
2
TBAF (10)
Cd(ClO4)2 (5)
1 (5)
1
0
3
4
Cd(ClO4)2 (5)
1 (5)
TBAF (5)
Cd(ClO4)2 (5)
1 (5)
0
95
TBAF (10)
SCHEME 3. TBAF -In itia ted Allyla tion
Rea ction 4b,11
SCHEME 4. Cd F OH‚1-Ca ta lyzed Allyla tion
Rea ction in Aqu eou s Med ia
SCHEME 5. Ca ta lytic Cycle of Cd F 2‚1-Ca ta lyzed
Allyla tion Rea ction in Aqu eou s Med ia
mechanism is a truly fluoride anion-catalyzed reaction
(Scheme 2).4a The released trimethylsilyl fluoride reacts
with the tetrabutylammonium alkoxide to regenerate
TBAF in the catalytic cycle. The second mechanism is a
fluoride anion-initiated (-triggered) mechanism (Scheme
3). In this mechanism, the fluoride anion just serves as
an initiator of the reaction and is not involved in the
catalytic cycle. The tetrabutylammonium alkoxide reacts
with the allylsilane to regenerate the active species, and
the catalytic cycle is completed. Sakurai et al. later stated
that the latter mechanism was plausible,11 and Hou et
al. showed experimental evidence which supported the
fluoride anion-initiated autocatalytic mechanism.4b
However, CdFOH‚1 was found to be totally ineffective
for the allylation (Scheme 4). This result suggests that
the mechanism of the catalytic cycle is not a fluoride-
initiated one.
From the results mentioned above, we propose the
catalytic cycle of the CdF2‚1-catalyzed allylation as shown
in Scheme 5. In the first step, complex 2, an aldehyde,
and allyltrimethoxysilane react to give adduct 313 and
fluorotrimethoxysilane 4. At this moment, it is unclear
whether an allylcadmium or allylsilicate species is in-
volved in this step.14 In the second step, adduct 3 is
hydrolyzed to give complex 5 and the product as the
alcohol form. Fluorotrimethoxysilane is also hydrolyzed
Resu lts a n d Discu ssion
To elucidate the mechanism of the CdF2‚1-catalyzed
allylation in aqueous media, at first, we investigated the
effect of fluoride anion in the reaction by adding TBAF
to Cd(ClO4)2‚1 complex (Table 1). As a result, only TBAF,
only Cd(ClO4)2‚1, or Cd(ClO4)2‚1:TBAF ) 1:1 did not
catalyze the allylation (entries 1-3). However, in the case
of Cd(ClO4)2‚1:TBAF ) 1:2, the reaction proceeded
smoothly (entry 4). This result suggests that 2 equiv of
fluoride anion to Cd(ClO4)2‚1 is essential for the reaction.
Next, we used CdFOH12‚1 as a catalyst. If the CdF2‚
1-catalyzed allylation is a fluoride-initiated autocatalytic
reaction, CdFOH, which is formed by hydrolysis of
CdFOCH(Ph)CH2CHdCH2, should catalyze the reaction.
(12) For preparation of CdFOH, see: Srivastava, O. K.; Secco, E.
A. Can. J . Chem. 1967, 45, 1375.
(11) Sakurai, H.; Hosomi, A.; Saito, M.; Sasaki, K.; Iguchi, H.;
Sasaki, J .; Araki, Y. Tetrahedron 1983, 39, 883.
(13) A possibility that allylcadmium (or allylsilver) species affords
the product without forming adduct 3 (or 9) may not be excluded.
7330 J . Org. Chem., Vol. 68, No. 19, 2003

Allylation Using Allyltrimethoxysilane in Protic Solvents
F IGURE 1. 19F NMR spectra (at 282.65 MHz in D2O at 30 °C) of (i) complex 5, (ii) (EtO)3SiF, (iii) a mixture of complex 5 and
(EtO)3SiF (1:1), (iv) a mixture of complex 5 and (EtO)3SiF (1:0.5), and (v) complex 2.
SCHEME 6. Cd F OH‚1 a n d F Si(OEt)3 a s a Ca ta lyst
to give 6 along with generation of methanol. These
hydrolyses should occur rapidly in the presence of water.
In the third step, complex 5 reacts with 6 to regenerate
CdF2‚1 complex (2) for the next catalytic cycle.
To elucidate this third step, a mixture of complex 5
and fluorotriethoxysilane was used as a catalyst. Fluo-
rotriethoxysilane was used as an analogue of 415 and
should have been hydrolyzed to 6 in situ. It is noteworthy
that the reaction proceeded smoothly in the presence of
complex 5 and fluorotriethoxysilane (Scheme 6), contrary
to the reaction without fluorotriethoxysilane (Scheme 4).
Regeneration of complex 2 was also confirmed by 19F
NMR studies. 19F NMR of complex 5 in D2O16 at 30 °C
showed a peak at -122.67 ppm (Figure 1 (i)). Fluorotri-
ethoxysilane was readily hydrolyzed by D2O and showed
broad peaks at -129.62 and -130.41 ppm (Figure 1 (ii)).
On the other hand, a mixture of complex 5 and fluorot-
riethoxysilane in D2O showed new peaks at -125.81 ppm
(peak c) and -130.26 ppm (peak d ) (Figure 1 (iii)). The
peak d was assigned to be 6 on the basis of the satellite
peak derived from coupling with 29Si (J ) 108 Hz).
Although peak d remained, the integration of peak d was
much smaller than that of peak c (the ratio of integration
was peak c/peak d ) 9/1), and peak d was consumed
completely when using 0.5 equiv of fluorotriethoxysilane
(Figure 1 (iv)). As a result of fast equilibrium between
(14) Crotylation reactions are known to give information on the
transition state structure. However, CdF2‚1-catalyzed crotylation
reaction using (E)-crotyltrimethoxysilane (E/Z ) 96/4, 2 equiv) resulted
in low yield and low selectivity (12% yield, syn/anti ) 46/54). Therefore,
we could not get a clue for the first step from crotylation.
(16) D2O was used as a solvent because complex 5 is hardly soluble
(15) Fluorotriethoxysilane is commercially available in high purity.
in organic and other aqueous solvents.
J . Org. Chem, Vol. 68, No. 19, 2003 7331

Aoyama et al.
F IGURE 2. 19F NMR spectra (at 282.65 MHz in methanol-d4 at -20 °C) of (i) a mixture of AgOMe‚BINAP and (EtO)3SiF (1:1)
and (ii) AgF‚BINAP.
SCHEME 7. Ca ta lytic Cycle of
AgF ‚BINAP -Ca ta lyzed Allyla tion Rea ction
tion of the trimethoxysilyl ether. Therefore, adduct 9 is
considered to react with methanol to give the alcohol
product and AgOMe‚BINAP. The next step was evaluated
experimentally. AgOMe‚BINAP was prepared by mixing
AgClO4‚BINAP and NaOMe in methanol at -20 °C. This
catalyst was found to be ineffective for the reaction of
benzaldehyde with allyltrimethoxysilane at -20 °C for
5 h (2% yield and 59% ee). However, when fluorotriethox-
ysilane was added to this catalyst solution and then
benzaldehyde and allyltrimethoxysilane were added suc-
cessively, the desired homoallylic alcohol was obtained
in 63% yield with 93% ee. Although the yield was slightly
lower, the result is close to that of AgF‚BINAP-catalyzed
reaction (82% yield and 93% ee, lit.7a 92% yield and 94%
ee). Furthermore, 19F NMR of AgOMe‚BINAP and fluo-
rotriethoxysilane in methanol-d4 at -20 °C showed a
peak at -151.26 ppm (Figure 2 (i)), which coincided with
the signal of AgF‚BINAP in methanol-d4 (-151.36 ppm
(Figure 2 (ii)). These results suggest the regeneration of
AgF‚BINAP from AgOMe‚BINAP and fluorotriethoxysi-
lane.
The experimental results stated above strongly support
the truly fluoride-catalyzed mechanism for both the CdF2‚
1-catalyzed allylation in H2O-THF and the AgF‚BINAP-
catalyzed allylation in methanol. This conclusion is
contrary to the catalytic cycle of TBAF-catalyzed allyla-
tion using allyltrimethylsilane in THF.4b,11 In the CuCl-
tetrabutylammonium difluorotriphenylsilicate-catalyzed
allylation using allyltrimethoxysilane in THF, Shibasaki
et al. also proposed another mechanism where the
initially formed CuF was not regenerated, and instead,
allylfluorodimethoxysilane was involved in the catalytic
cycle.8 We assume that in the former two reactions (the
CdF2‚1-catalyzed and the AgF‚BINAP-catalyzed ones) the
protic solvents play important roles and facilitate the
regeneration of the metal fluoride species probably due
to effective solvation of fluoride anion.18
CdF2‚1 and CdFOH‚1 on the NMR time scale, only one
peak was observed. The peak c coincided with the signal
of complex 2 (f) whose chemical shift was -125.89 ppm
(Figure 1 (v)). Therefore, the peak c is assigned to be
complex 2. These experimental and 19F NMR results
strongly support the regeneration of complex 2 from
complex 5 and fluorotriethoxysilane in the presence of
water and, as a consequence, the truly fluoride-catalyzed
mechanism. It should be also mentioned that, while a
strong silicon-fluoride bond is generally considered hard
to cleave, the bond was cleaved under the present
conditions.
Next, we turned our attention to the mechanism of
AgF‚BINAP-catalyzed allylation in methanol. A proposed
catalytic cycle is shown in Scheme 7.17 Although the
reaction of adduct 913 with 10 to give the corresponding
trimethoxysilyl ether as the product was conceivable,
monitoring the reaction by TLC did not show the forma-
In conclusion, we have investigated the catalytic cycles
of CdF2‚1-catalyzed and AgF‚BINAP-catalyzed allylation
using allyltrimethoxysilane in protic solvents. The ex-
(18) Hefter reported in his review that fluoride anion solvation by
nonprotic solvents (DMSO, DMF, THF) requires much more Gibbs
energies compared to protic solvents (H2O, MeOH). Hefter, G. H. Pure
Appl. Chem. 1991, 63, 1749.
(17) In the crotylation reaction, Yamamoto et al. noted that the
occurrence of transmetalation gives crotylsilver species. Therefore, it
is likely that allylsilver species is involved in the first step. See ref 7a.
7332 J . Org. Chem., Vol. 68, No. 19, 2003

Allylation Using Allyltrimethoxysilane in Protic Solvents
CdFOH12 (11.9 mg) and 1 (18.8 mg) in D2O (0.8 mL) was added
fluorotriethoxysilane (14.6 µL, 1 equiv). The mixture was
transferred into a NMR sample tube. After 30 min, 19F NMR
was measured, and the spectrum is shown in Figure 1 (iii):
19F NMR (282.65 MHz, D2O) δ -125.81 (s), -130.26 (s, satellite
peak J Si-F ) 108 Hz).
perimental evidence described here shows that each
metal fluoride is regenerated from silyl fluoride species
and that these reactions are interesting examples in
which truly fluoride-catalyzed mechanisms are operative.
These catalytic cycles are much different from that of
TBAF-catalyzed allylation using allyltrimethylsilane in
THF. Thus, the studies in this paper will provide a new
aspect in fluoride-catalyzed allylation using allylsilanes.
19F NMR of complex 5 in D2O at 30 °C (Figure 1 (i)): 19F
NMR (282.65 MHz, D2O) δ -122.67 (s). 19F NMR of FSi(OEt)3
in D2O at 30 °C (Figure 1 (ii)): 19F NMR (282.65 MHz, D2O) δ
-129.62 (br), -130.41 (br). 19F NMR of a mixture of complex
5 and FSi(OEt)3 (0.5 equiv) in D2O at 30 °C (Figure 1 (iv)):
19F NMR (282.65 MHz, D2O) δ -123.45 (s). 19F NMR of
complex 2 in D2O at 30 °C (Figure 1 (v)): 19F NMR (282.65
MHz, D2O) δ -125.89 (s).
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were performed under an
argon atmosphere. Chemical shifts are reported in δ ppm
referenced to an internal standard (tetramethylsilane, 0 ppm)
for 1H NMR, an internal standard (CDCl3, 77.0 ppm) for 13C
NMR, and an external standard (CFCl3, 0 ppm) for 19F NMR.
Typ ica l P r oced u r es for Allyla tion Rea ction s. F or Cd -
(ClO4)2‚1 a n d TBAF -Ca ta lyzed Allyla tion Rea ction in
Aqu eou s Med ia (Ta ble 1, En tr y 4). To a solution of
Cd(ClO4)2 (6.5 mg) and 1 (3.6 mg) in H2O (145 µL) were added
TBAF in H2O (75 wt %, 10.5 µL) and THF (1.40 mL). The
mixture was stirred at 30 °C for 30 min, and then benzalde-
hyde (33.1 mg) and allyltrimethoxysilane (75.5 mg) were added
successively. The mixture was stirred at 30 °C for 9 h and
diluted with water (ca. 5 mL). The aqueous layer was extracted
with CH2Cl2 three times, and the organic layer was washed
with saturated NaCl and dried over Na2SO4. The solvents were
evaporated after filtration. The residual crude product was
purified by preparative TLC on silica gel to give 1-phenyl-3-
buten-1-ol19 as a colorless oil (44.0 mg, 95% yield).
Th e P r oced u r e for AgOMe‚(R)-BINAP - a n d F Si(OEt)3-
Ca ta lyzed Allyla tion Rea ction in Meth a n ol. To a solution
of AgClO4 (7.2 mg) and (R)-BINAP (13.3 mg) in methanol (2.17
mL) was added NaOMe (1.9 mg) in methanol (1.0 mL), and
the mixture was stirred at -20 °C for 10 min with exclusion
of direct light. To the mixture were added FSi(OEt)3 (6.4 mg)
in methanol (1.0 mL), benzaldehyde (36.7 mg), and allyltri-
methoxysilane (84.5 mg) successively at this temperature. The
mixture was stirred at -20 °C for 5 h and quenched with 0.2
N HCl (ca. 5 mL). The aqueous layer was extracted with CH2-
Cl2 three times, and the organic layer was washed with
saturated NaCl and dried over Na2SO4. The solvents were
evaporated after filtration. The residual crude product was
purified by preparative TLC on silica gel to give 1-phenyl-3-
buten-1-ol19 as a colorless oil (32.5 mg, 63% yield). The
enantiomeric excess of the product was determined to be 93%
ee by a chiral HPLC analysis (CHIRALCEL OD, hexane/iPrOH
) 20/1, flow rate 0.5 mL/min, tR 18.1 min (major, (R)-isomer)
and tR 19.6 min (minor, (S)-isomer).19
1-P h en yl-3-bu ten -1-ol: 1H NMR (300.4 MHz, CDCl3) δ 2.05
(br, 1H), 2.43-2.56 (m, 2H), 4.72 (dd, 1H, J ) 5.7, 7.3 Hz),
5.11-5.19 (m, 2H), 5.79 (ddt, 1H, J ) 7.1, 10.2, 17.1 Hz), 7.24-
7.36 (m, 5H); 13C NMR (75.45 MHz, CDCl3) δ 43.8, 73.3, 118.3,
125.8, 127.5, 128.4, 134.4, 143.8.
F or 19F NMR Mea su r em en t of a Mixtu r e of AgOMe‚
BINAP a n d F Si(OEt)3 (1 equ iv) in Meth a n ol-d 4 a t -20
°C. To a mixture of AgClO4 (8.8 mg) and (R)-BINAP (26.8 mg)
in methanol-d4 (1.1 mL) was added NaOMe (2.4 mg) in
methanol-d4 (0.53 mL) at -20 °C. After 10 min, to the mixture
was added FSi(OEt)3 (7.7 mg) in methanol-d4 (0.53 mL) at this
temperature. After 10 min, the resulting solution was trans-
ferred into an NMR sample tube at -20 °C. After 30 min, 19F
NMR was measured at -20 °C, and the spectrum is shown in
Figure 2 (i): 19F NMR (282.65 MHz, D2O) δ -151.26 (s).
19F NMR of AgF‚(R)-BINAP in methanol-d4 at -20 °C
(Figure 2 (ii)): 19F NMR (282.65 MHz, D2O) δ -151.36 (s).
F or Cd F OH‚1- a n d F Si(OEt)3-Ca ta lyzed Allyla tion
Rea ction in Aqu eou s Med ia (Sch em e 6). To a suspension
of CdFOH12 (2.3 mg) and 1 (3.6 mg) in H2O (155 µL) was added
THF (1.40 mL). The mixture was stirred at 30 °C for 30 min,
and then benzaldehyde (32.8 mg), allyltrimethoxysilane (75.5
mg), and fluorotriethoxysilane (2.8 mg) were added succes-
sively. The mixture was stirred at 30 °C for 9 h and diluted
with water (ca. 5 mL). The aqueous layer was extracted with
CH2Cl2 three times, and the organic layer was washed with
saturated NaCl and dried over Na2SO4. The solvent was
evaporated after filtration. The residual crude product was
purified by preparative TLC on silica gel to give 1-phenyl-3-
buten-1-ol19 as a colorless oil (41.2 mg, 90% yield).
Ack n ow led gm en t. This work was partially sup-
ported by CREST and SORST, J apan Science and
Technology Corporation (J ST), and a Grant-in-Aid for
Scientific Research from J apan Society of the Promotion
of Science. N.A. and T.H. thank the J SPS fellowship for
J apanese J unior Scientists.
F or 19F NMR Mea su r em en t of a Mixtu r e of Com p lex
5 a n d F Si(OEt)3 (1 equ iv) in D2O a t 30 °C. To a mixture of
(19) Ishihara, K.; Mouri, M.; Gao, Q.; Maruyama. T.; Furuta, K.;
Yamamoto, H. J . Am. Chem. Soc. 1993, 115, 11490.
J O0346196
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