Communication
doi.org/10.1002/chem.202100877
Chemistry—A European Journal
Table 1. Selected examples for optimization of main-group Lewis acid-
Table 2. Scope I: Variation of the hydrosilane.
promoted hydrosilylation of diphenylketene.
Entry[a]
Catalyst
[mol%]
Et3SiH
[equiv.]
Solvent
[0.5 M]
Temp.
Yield
[%][b]
Entry[a]
Ketene
Hydrosilane
Z/E ratio
of 3f[b]
Yield of 3a
or 3f [%][c]
°
[ C]
1[c]
2
–
1.2
1.5
1.2
1.2
1.2
1.2
4.0
1.2
1.0
1.2
1.2
C6H5F
RT
17
trace
17
15
17
45
75
90
76
73
78
1
2
3
4
5
6
7
8
9
1a
1f
1a
1f
1a
1f
1a
1f
Et3SiH (2a)
Et3SiH (2a)
–
3aa: 84 (94)
3fa: 51 (67)[d]
3ab: 82 (94)
3fb: 78 (74)[d]
3ac: -[e] (98)
3fc: -[e] (89)
3ad: 56 (67)
3fd: 70 (68)[d]
3ae: -(8)
[Ph3C][B(C6F5)4] (5.0)
[Ph3C][B(C6F5)4] (2.0)
[Ph3C][B(C6F5)4] (2.0)
[Ph3C][B(C6F5)4] (2.0)
[Ph3C][B(C6F5)4] (2.0)
B(C6F5)3 (2.0)
B(C6F5)3 (5.0)
B(C6F5)3 (5.0)
B(C6F5)3 (5.0)
B(C6F5)3 (5.0)
CH2Cl2
Toluene-d8
Toluene-d8
C6H5F
C6H6
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
À 78
À 78
RT
RT
RT
RT
RT
RT
70
86:14
–
84:16
–
62:38
–
31:69
–
3[d]
4
nBu3SiH (2b)
nBu3SiH (2b)
Me2PhSiH (2c)
Me2PhSiH (2c)
tBuMe2SiH (2d)
tBuMe2SiH (2d)
iPr3SiH (2e)
5
6
7
8
9
1a
10
11[e]
RT
[a] All reactions were performed on a 0.20 mmol scale. [b] Z/E-ratio was
determined by 1H NMR analysis of the crude reaction mixture. [c] Unless
otherwise noted, yields are isolated yield of silyl enol ethers (in parentheses
yields determined by 1H NMR spectroscopy with CH2Br2 as an internal
standard). [d] Combined isolated yield of 3f and the corresponding silyl
ether. [e] Silyl enol ether decomposed during column chromatography on
alumina.
[a] All reactions were performed on a 0.10–0.20 mmol scale. [b] Yield
determined by 1H NMR spectroscopy with mesitylene as an internal
standard. [c] For 24 h. [d] Performed in a J-Young tube under argon
°
atmosphere. After addition of all reactants at À 78 C, the mixture was
stirred at RT for 12 h. [e] Performed at 0.25 M.
entry 2). The yield of 3aa remained at the level of the
uncatalyzed reaction in arene solvents (entries 3–5; see Table S1
in the Supporting Information). A higher yield of 45% was
obtained only when benzene was used as solvent (entry 6). This
led us to conclude that the silylium-ion-promoted hydro-
silylation of ketenes is possible but not efficient. Conversely,
2.0 mol% of B(C6F5)3 and 4.0 equiv. of hydrosilane 2a in C6H5F
afforded the desired silyl enol ether 3aa in 75% yield (entry 7).
However, excess Et3SiH caused the formation of a large amount
of (Et3Si)2O, rendering isolation and purification of 3aa problem-
atic. To address this issue, the reaction was optimized with
lower amount of the hydrosilane (1.2 equiv.) and higher catalyst
loading (5.0 mol%). Under these reaction conditions, the yield
of 3aa did improve to 90% after maintaining the reaction at
room temperature for 12 h (entry 8). Lower yields were
obtained with less hydrosilane, decreased concentration of the
reactants, and at elevated reaction temperature (entries 9–11).
During the optimization of the reaction, trace amounts of
the corresponding silyl ether were detected (not shown). This is
believed to originate from the hydrosilylation of the acid
chloride introduced with ketene 1a. Ketene formation was
found to be generally slow, affording the ketene as a mixture
with unreacted acid chloride. Most of the distillable disubsti-
tuted ketenes could be purified with the exception of ketenes
1f, 1h, and 1i. As for silyl enol ether 3aa, the combined
isolated yields of the silyl enol ether and the silyl ether are
reported for these transformations. The purity of the ketene
was important, and attempts to start directly from the acid
chloride followed by the hydrosilylation in the same pot or after
simple filtration were unsuccessful (see the Supporting Informa-
tion for procedures).
monosubstituted ketenes such as phenylketene were not
included because of their strong tendency to undergo self-
reaction. With 1a, the reactions with tertiary hydrosilanes Et3SiH
(2a), nBu3SiH (2b), and Me2PhSiH (2c) proceeded smoothly to
provide products 3aa-ac in good yields (entries 1, 3, and 5).
Sterically more hindered tBuMe2SiH (2d) and iPr3SiH (2e) led to
lower yield (as for 3ad; entry 7) or little conversion (as for 3ae;
entry 9). Yields were generally lower for ketene 1f but the
influence of the steric demand of the hydrosilane was less
pronounced (entries 2, 4, 6, and 8). Silyl enol ethers 3fa–fc
formed with moderate Z selectivity while E configuration was
preferred in the case of 3fd.
We continued exploring the scope of diaryl- and alkylaryl-
substituted ketenes using Et3SiH (2a) under the standard
protocol (cf. Table 1, entry 8). Extension to other diarylketenes
was possible but limited due to the difficulty in their synthesis
and isolation in analytically pure form (Scheme 2). A gram-scale
synthesis of 3aa from 1a with lower catalyst loading
(2.0 mol%) brought about 84% isolated yield. Because of their
low polarity and lack of stability during column chromatog-
raphy on alumina, the yields of silyl enol ethers 3aa–ea were
1
determined by H NMR spectroscopy with an internal standard.
Sterically demanding aryl groups such as α-naphthyl were
detrimental; the yield of 39% was improved to 67% at
10 mol% catalyst loading. No conversion was achieved with
mesityl groups (not shown).
As previously seen for ketene 1f (see Table 2), silyl enol
ethers derived from other alkylaryl-substituted ketenes formed
with moderate to good Z selectivity (1f–q!3fa–qa; Scheme 3).
The best stereoselectivity of Z:E=93:7 was obtained for a
cyclopentyl group as the alkyl substituent (1i!3ia). Both the
alkyl group (top) and the substituent on the aryl group
(bottom) were modified, including Ibuprofen-derived 1j (gray
box). Electron-withdrawing and electron-donating groups at
With an optimized procedure in hand, we began to
investigate the scope for model substrate 1a and alkylaryl-
substituted ketene 1f with various hydrosilanes (Table 2);
Chem. Eur. J. 2021, 27, 1–5
2
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