α-Allylation of Carbonyl Compounds via Silyl Enol Ethers and Silyl Ketene Acetals using a Hypervalent Iodine Reagent

Article abstract of DOI:10.1039/a701224a

Oxidation of allyltrimethylsilane with iodosobenzene and trimethylsilyl trifluoromethanesulfonate followed by treatment with trimethylsilyl enol ethers of ketones or trimethylsilyl ketene acetals of esters/lactones provides a new method for the α-allylation of the corresponding carbonyl compounds.

Full text of DOI:10.1039/a701224a

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262 J. CHEM. RESEARCH (S), 1997
J. Chem. Research (S),
1997, 262–263†
a-Allylation of Carbonyl Compounds via Silyl Enol Ethers
and Silyl Ketene Acetals using a Hypervalent Iodine
Reagent†
Robert M. Moriarty,* Widanagamage R. Epa and Om Prakash*‡
Chemistry Department, University of Illinois at Chicago, 845-West Taylor Street, Chicago, IL,
60607, USA
Oxidation of allyltrimethylsilane with iodosobenzene and trimethylsilyl trifluoromethanesulfonate followed by treatment
with trimethylsilyl enol ethers of ketones or trimethylsilyl ketene acetals of esters/lactones provides a new method for the
a-allylation of the corresponding carbonyl compounds.
There is considerable interest in the hypervalent iodine oxi-
dation of a wide variety of organic compounds.1 One import-
ant goal achieved using this approach is the formation of
carbon–carbon bonds.2 Oxidation of silyl enol ethers
reported from our laboratory3 and other research groups4,5
has offered a useful way of accomplishing such a goal.
We have reported that oxidation of silyl enol ethers with
iodosobenzene in the presence of boron trifluoride–diethyl
ether produces symmetrical 1,4-diones [e.g. 4, R = Ph in eqn.
(1)].3 The method could not be used to synthesise unsymme-
trical products. Zhdankin et al. modified this approach by
generating in situ the hypervalent iodine intermediate,
phenacyliodonium tetrafluoroborate (2, X = BF4), to synthe-
sise cross-coupled products.4 Only the a-(trimethylsilyloxy)
styrene was employed as a substrate. As an effort to intro-
duce a new method for carbon–carbon cross coupling, we
now report the a-allylation of carbonyl compounds via silyl
enol ethers and silyl ketene acetals.
Based on previous results that the iodine(III) intermediate
2, generated from the oxidation of a silyl enol ether, is equiva-
lent to carbocation 3 and capable of forming a carbon–
carbon bond with silyl enol ether itself or with external car-
bon nucleophiles thereby producing 4 [eqn. (1)], we first
investigated the a-allylation by treating silyl enol ether 1 with
iodosobenzene and trimethylsilyl trifluoromethanesulfonate
followed by addition of allyltrimethylsilane. The reaction,
however, gave the self coupling product butane-1,4-dione 4
(R = Ph) and the expected allylated ketone 5 was not
obtained in any detectable amount (Scheme 1). Use of
iodosobenzene and boron trifluoride–diethyl ether also gave
4.
The above observations led us to change the strategy with
the addition of allyltrimethylsilane followed by the addition
of the trimethylsilyl enol ether. Thus, allyltrimethylsilane was
added to a solution of (PhIO)n–Me3SiOTf in dichloro-
1
methane. The reaction was monitored by H NMR and the
formation of allyliodine(III) intermediate 6 was indicated by
the observation of a downfield shift of the methylene protons
from d 1.8 to 5.0. Addition of silyl enol ether 1 afforded the
a-allylated product 56 in 69% yield (Scheme 2). A similar
approach was successfully used for the a-allylation of other
silyl enol ethers (Table 1, entries 1–3). The method is regio-
selective as 8 gave only 9 in 76% yield (Table 1, entry 2).
Furthermore, trimethylsilyl ketene acetals of esters and lac-
tones can also be a-allylated using this approach (Table 1,
entries 4–6).
A plausible mechanism for this reaction involves the
generation of the reactive IIII species PhI(OSiMe3)OTf7 from
the reaction of iodosobenzene and trimethylsilyl trifluoro-
methanesulfonate. Allyltrimethylsilane then forms the initial
intermediate 6 which then reacts with the silyl enol ether (e.g.
1) to give another intermediate 7. Finally, ligand coupling
and reductive elimination of iodobenzene gives the a-allyl-
ated product (Scheme 2).
Experimental
Typical Procedure.sTo an ice cooled suspension of iodoso-
benzene (528 mg, 2.4 mmol) in dry dichloromethane (75 ml) under
nitrogen was added trimethylsilyl trifluoromethanesulfonate (0.48
ml, 566 mg, 2.4 mmol). To the resulting yellow solution was added
allyltrimethylsilane (0.38 ml, 274 mg, 2.4 mmol) which turned the
solution colourless. The cooling bath was then removed and the
X–
I+Ph
OSiMe3
O
O
O
OSiMe3
(PhIO)n–
R
R
+
CH2
(1)
Ph
Ph
Ph
Ph
BF3•Et2O
or
HBF4
O
2 X = OBF2 or BF4
3
1
4
R = Ph, substituted phenyl, etc
4 (R = Ph)
(PhIO)n–Me3SiOTf
CH2 CHCH2SiMe3
1
OSiMe3
(PhIO)n–Me3SiOTf
O
SiMe3
I
Ph
Ph
6
5
O
Scheme 1
Ph
1
5
–(Me3Si)2O
–PhI
*To receive any correspondence.
I
†This is a Short Paper as defined in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
‡On leave of absence from Kurukshetra University, Kurukshetra,
132119, India.
Ph
7
Scheme 2

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J. CHEM. RESEARCH (S), 1997 263
Table 1 a-Allylation of carbonyl compounds
silyl ketene crystals were converted into the corresponding a-allyl-
carbonyl compounds (Table 1).
Entry
Substrate
Producta
Yield (%)
(Lit.)
(8)
OSiMe3
O
We thank the National Science Foundation (Grant No.
9520157) for financial support.
52
1
Received, 21st February 1997; Accepted, 1st April 1997
Paper E/7/01224A
OSiMe3
O
65
61
70
40
75
(9)
(8)
2
3
4
5
6
8
9
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OSiMe3
O
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