S. Hanada, A. Yuasa, H. Kuroiwa, Y. Motoyama, H. Nagashima
SHORT COMMUNICATION
If a similar ionic species is involved in the mechanism of described above. Application of this unique catalytic prop-
the present deprotection, Equations (5) and (6) shown in erty of 1 to other organic transformations is currently under
Scheme 4 would reasonably explain the reactions. The investigation.
Lewis acidic [R3Si]+ species coordinates to the carbonyl or
ether oxygen atom, leading to loosening of the C–O bond
Experimental Section
General Remarks: 1H and 13C NMR spectra were measured with
JEOL GSX-270 (270 MHz) and ECA 400 (396 MHz) spectrome-
ters. Chemical shifts for 1H NMR are described in parts per million
of the OtBu groups. Elimination of isobutene is ac-
companied by generation of H2 from a proton of the tBu
group and a hydride of the hydrosilane. In the reactions of
N-Boc and O-Boc derivatives, subsequent elimination of
CO2 from intermediate A proceeds by treatment with meth- downfield from tetramethylsilane as an internal standard (δ =
0 ppm) in CDCl3, unless otherwise noted. Chemical shifts for 13C
anol to afford the corresponding amine and alcohol.
NMR are expressed in parts per million in CDCl3 as an internal
standard (δ = 77.1 ppm), unless otherwise noted. IR spectra were
measured with a JASCO FT/IR-4200 spectrometer. Analytical
thin-layer chromatography (TLC) was performed on glass plates
precoated with silica gel (Merck, Kieselgel 60 F254, layer thickness
0.25 mm). Visualization was accomplished by UV light (254 nm),
iodine, and phosphomolybdic acid. (µ3,η2,η3,η5-acenaphthylene)-
Ru3(CO)7 (1)[3c] and 1,2-bis(dimethylsilyl)ethane[12] were prepared
by literature methods.
Typical Procedure for Deprotection: To a stirred solution of tert-
butyl 4-biphenyl ether (7b; 113 mg, 0.5 mmol) and (µ3,η2,η3,η5-
acenaphthylene)Ru3(CO)7 (1; 10 mg, 3 mol-%) in dimethoxyethane
(0.25 mL) was added dimethylphenylsilane (93 µL, 0.6 mmol). Af-
ter the mixture was stirred at 40 °C for 7 h, the reaction mixture
was quenched by the addition of methanol (100 µL). Following
stirring at room temperature for an additional 30 min, the resultant
mixture was concentrated under reduced pressure. Then, the resi-
due was treated with tetrabutylammonium fluoride (0.6 mmol) in
ether at room temperature for 1 h. After removal of the solvent,
purification of the residue by silica gel column chromatography
gave 4-phenylphenol (10b) in 95% yield (81 mg). White solid. IR
Scheme 4. Possible reaction mechanisms.
(KBr): ν = 3411, 3036, 1605, 1522, 1456, 1427, 1379, 1247, 1109,
˜
835, 752, 680 cm–1. 1H NMR (396 MHz, CDCl3): δ = 4.75 (br. s, 1
H, OH), 6.91 (d, J = 8.7 Hz, 2 H, 2-H), 7.31 (t, J = 7.5 Hz, 1 H,
Conclusions
As described above, cleavage of the C–O bond of OtBu p-Ph), 7.42 (dd, J = 7.5, 7.2 Hz, 2 H, m-Ph), 7.49 (d, J = 8.7 Hz, 2
H, 3-H), 7.55 (d, J = 7.2 Hz, 2 H, o-Ph) ppm. 13C NMR
(99.5 MHz, CDCl3): δ = 115.7, 126.8 (2 C), 128.5, 128.8, 134.2,
140.8, 155.1 ppm.
groups in carbamates, carbonates, esters, and ethers is fac-
ilely accomplished by PhMe2SiH activated by triruthenium
cluster 1, giving rise to a new deprotection method of OtBu
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and characterization data of
both the substrates and the products.
groups under neutral conditions. The proposed mechanism
indicates in situ generation of the Lewis acidic [R3Si]+ spe-
cies; however, it is noteworthy that the [R3Si]+ species is
formed from a stable and neutral hydrosilane and the ruthe-
nium catalyst. The typical Lewis acidic [R3Si]+ species is
produced from Me3SiI and Me3SiOTf, of which the Si–I or
Si–O bond is highly polarized and instantly formed acidic
HI or HOTf in contact with moisture. Hydrosilanes acti-
vated by transition-metal catalysts have been utilized for the
hydrosilylation of alkenes and alkynes, reduction of car-
bonyl compounds, dehydration of primary amides to ni-
triles, and polymerization of cyclic ethers and vinyl ethers.
The present reaction is the first example of a transition-
metal-catalyzed activation of Si–H bonds leading to depro-
tection of OtBu groups. It is noteworthy that plati-
num,[11a,11b] iridium,[11c] and iron catalysts[11d] are active
towards reduction of tertiary amides with Me2SiHOSiHMe2,
but do not induce deprotection of tert-butyl groups in carb-
amates, carbonates, esters, or ethers. This implies that 1 exhi-
bits the highest activity towards the generation of the Lewis
acidic [R3Si]+ species from Si–H groups among the catalysts
Acknowledgments
This work was partially supported by a Grant-in Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. S.H. is grateful to the Japan Society for the
Promotion of Science for Young Scientists for a Research Fellow-
ship.
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