Platinum-catalyzed addition of dimethylsilylene to β-methyl α-β-unsaturated ketones: γ-silylation forming 1-Oxa-2-silacyclohex-5-enes

Article abstract of DOI:10.1021/ol702357n

The reaction of β-methyl α,β-unsaturated ketones with pentamethyldisilane in the presence of a platinum catalyst brought about silylation on the β-methyl group giving high yields of oxasilacyclohexenes.

Full text of DOI:10.1021/ol702357n

ORGANIC
LETTERS
2007
Vol. 9, No. 24
5067-5069
Platinum-Catalyzed Addition of
Dimethylsilylene to -Methyl
-Unsaturated Ketones: γ-Silylation
â
r
,
â
Forming 1-Oxa-2-silacyclohex-5-enes
Kazuhiro Okamoto and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Sakyo, Kyoto 606-8502, Japan
thayashi@kuchem.kyoto-u.ac.jp
Received September 27, 2007
ABSTRACT
The reaction of
â-methyl r,â-unsaturated ketones with pentamethyldisilane in the presence of a platinum catalyst brought about silylation on
the -methyl group giving high yields of oxasilacyclohexenes.
â
It is well-documented that silylenes (divalent silicon species,
R2Si:) are very active intermediates which react with various
types of compounds giving tetravalent organosilanes.1 How-
ever, the use of silylenes for organic synthesis had not been
extensively studied before Woerpel reported a silver-
catalyzed silylene transfer reaction to alkenes giving sila-
cyclopropanes with high selectivity.2 Among several reported
methods of generating silylenes,1 we became interested in
transition-metal-catalyzed reaction of hydrodisilanes generat-
ing silylenes together with hydrosilanes because of their mild
reaction conditions and the ready availability of the hydro-
disilanes. Here we wish to report that dimethylsilylene
generated from pentamethyldisilane under the catalysis by
a platinum complex3 is incorporated into R,â-unsaturated
ketones giving two types of addition products depending on
the substitution pattern at the â-position. These reactions
provide oxasilacycles which are potentially useful as versatile
intermediates for organic synthesis.
The reaction of (E)-1-phenyl-2-buten-1-one (1a) with
pentamethyldisilane (2, 1.2 equiv to 1a) in the presence of
PtCl2(cod) (3 mol %) in toluene at 40 °C for 12 h gave 67%
yield of oxasilacyclopentene 3a (eq 1). The formation of
oxasilacyclopentene is as expected because there have been
several examples of the [4 + 1] type cycloaddition of
silylenes4 including Woerpel’s report2b where di(tert-butyl)-
silylene adds to conjugate enones and enoates. Treatment
of the oxasilacyclopentene 3a with methyllithium at 0 °C
followed by hydrolysis gave â-trimethylsilyl ketone 4a.
(1) Ottosson, H.; Steel, P. G. Chem.sEur. J. 2006, 12, 1576.
(2) (a) Cirakovic, J.; Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc.
2002, 124, 9370. (b) Calad, S. A.; Woerpel, K. A. J. Am. Chem. Soc. 2005,
127, 2046.
(3) (a) Yamamoto, K.; Okinoshima, H.; Kumada, M. J. Organomet.
Chem. 1970, 23, C7. (b) Yamamoto, K.; Okinoshima, H.; Kumada, M. J.
Organomet. Chem. 1971, 27, C31.
On the other hand, the reaction of â,â-dimethyl-substituted
enones with the dimethylsilylene proceeded in a different
manner. Thus, the platinum-catalyzed reaction of 3-methyl-
1-phenyl-2-buten-1-one (1b) with hydrodisilane 2 (1.2 equiv
10.1021/ol702357n CCC: $37.00
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Published on Web 11/01/2007

to 1b) under the same reaction conditions gave 72% yield
of oxasilacyclohexene 5b5 as a new type of addition product
together with a minor amount (7%) of oxasilacyclopentene
3b4 (eq 2). Treatment of 5b with methyllithium followed by
hydrolysis gave γ-trimethylsilyl ketone 6b, which confirms
the structure of six-membered ring product 5b.
Table 1. Platinum-Catalyzed Silylene Addition to â-Methyl
R,â-Unsaturated Ketonesa
The formation of oxasilacyclohexene 5 was observed only
for the â,â-disubstituted enones where at least one of the
â-substituents is a methyl group. Some of the results obtained
for this new type of six-membered ring formation are
summarized in Table 1. Phenyl ketones containing electron-
donating or -withdrawing substituents on the phenyl ring 1c-
1e are the substrates which gave oxasilacyclohexenes 5 as
major products (entries 3-5). It is remarkable that the
selectivity giving 5 over 3 is higher with a more electron-
donating group at the para position. In the reaction of
isopropylideneindanone 1f and -tetralone 1g, the selectivity
in giving oxasilacyclohexene 5 is very high (entries 6 and
7). It is interesting that both E and Z isomers of enone 1h
underwent the silylene addition giving oxasilacyclohexene
5 (entries 8 and 9).6
Because the oxasilacyclohexenes 5 formed by the present
silylation have a cyclic structure, the lithium enolates
generated by treatment with methyllithium should have
exclusive Z geometry.7 The (Z)-lithium enolates can be
trapped as (Z)-enol esters by O-acylation with an acyl
chloride. For example, the reaction mixture obtained by the
platinum-catalyzed silylene reaction of enone 1b was treated
subsequently with methyllithium and pivaloyl chloride to
give (Z)-enol pivalate 7 in 82% overall yield (eq 3). Synthetic
utility of the silylation reaction is also exemplified by the
oxidation of the carbon-silicon bond in oxasilacyclohexene
5b under Tamao conditions8 giving γ-hydroxy ketone 8 in
a high yield (eq 4).
a Reaction conditions: enone 1 (0.20 mmol), disilane 2 (0.24 mmol),
PtCl2(cod) (3 mol %), toluene (0.80 mL) at 40 °C for 12 h. b Method A:
Distillation under vacuum. Method B: MeLi in Et2O/THF (1.5 equiv),
0 °C, 20 min, then aq NH4Cl. c Isolated yield. d Ratio of 5/3 or 6/4. e A
1:1 mixture of diastereomeric isomers.
(4) (a) Ishikawa, M.; Nakagawa, K.; Kumada, M. J. Organomet. Chem.
1977, 135, C45. (b) Heinicke, J.; Gehrhus, B. J. Organomet. Chem. 1992,
423, 13. (c) Heinicke, J.; Gehrhus, B.; Meinel, S. J. Organomet. Chem.
1994, 474, 71. (d) Heinicke, J. W.; Gehrhus, B. Heteroat. Chem. 1995, 6,
461. (e) Heinicke, J.; Meinel, S. J. Organomet. Chem. 1998, 561, 121.
(5) Formation of 1-oxa-2-silacyclohex-5-enes by other reactions: (a)
Tanaka, Y.; Yamashita, H.; Tanaka, M. Organometallics 1996, 15, 1524.
(b) Hatanaka, Y.; Watanabe, M.; Onozawa, S.; Tanaka, M.; Sakurai, H. J.
Org. Chem. 1998, 63, 422. (c) Tanaka, Y.; Yamashita, M. Appl. Organomet.
Chem. 2002, 16, 51.
(6) In the reaction of (Z)-1h stopped in a shorter reaction time (30 min),
a minor amount of (E)-1h (Z/E ) 1/0.2) was observed, indicating the
possibility of isomerization of (Z)-1h to (E)-1h before the silylene addition.
(7) For an example of the selective formation of â-silyl enolates, see:
Crump, R. A. N. C.; Fleming, I.; Hill, J. H. M.; Parker, D.; Reddy, N. L.;
Waterson, D. J. Chem. Soc., Perkin Trans. 1992, 3277.
A catalytic cycle proposed for the present silylation giving
oxasilacyclohexenes 5 is shown in Scheme 1. Generation of
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Org. Lett., Vol. 9, No. 24, 2007

dialkylsilylenes from hydrodisilanes has been reported to take
place on transition metal complexes including platinum.9 It
is most probable that the reaction proceeds through nucleo-
philic attack of the carbonyl oxygen of enone 1 onto an
dimethylsilylene B stabilized by coordination to platinum,
which is generated from pentamethyldisilane (2) and a low-
valent platinum A. One of the methyl hydrogens on zwit-
terionic species C10 is abstracted by platinum to form
deuterium in 70%. The deuterium shift from γ-carbon to
â-carbon is consistent with the proposed catalytic cycle which
involves this hydrogen transfer from γ to â during the
transformation of the intermediates from C to E. Another
reaction is a platinum-catalyzed intramolecular hydrosilyla-
tion11 of dienyloxy(hydro)silane 9, which gave a high yield
of cyclization product, oxasilacyclohexene 5b, in the pres-
ence of platinum catalyst under similar reaction conditions
to the silylene addition. This intramolecular hydrosilylation
is generally accepted11 to proceed by way of platinum hydride
intermediate D which should be generated by oxidative
addition of the hydrosilane moiety of 9, demonstrating that
the last two steps of the catalytic cycle after D (Scheme 1)
take place once intermediate D is formed from C in the
silylene addition.
Scheme 1. A Catalytic Cycle Proposed for the Silylene
Addition
In summary, we found a new type of platinum-catalyzed
addition reaction of dimethylsilylene to â,â-dimethyl-
substituted R,â-unsaturated ketones. A carbon-silicon bond
is formed on the γ-methyl carbon to give 1-oxa-2-silacy-
clohex-5-enes with high selectivity. This catalytic silylene
addition reaction has been studied for its synthetic utility
and the catalytic cycle.
platinum hydride D. Insertion of the exo-methylene into the
platinum hydride bond followed by reductive elimination of
the carbon-silicon bond on E produces oxasilacyclohexene
5.
This catalytic cycle is supported by the results obtained
for the following two reactions (eqs 5 and 6). One is the
deuterium-labeling experiment using hexadeuterated enone
1b-d6. The platinum-catalyzed reaction of 1b-d6 with hy-
drodisilane 2 under the standard conditions gave the silylation
product 6b where the â-position is incorporated with
Acknowledgment. Support has been provided in part by
a Grant-in-Aid for Scientific Research, the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
K.O. thanks the Japan Society for the Promotion of Science
for the award of a fellowship for graduate students.
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(8) (a) Tamao, K.; Kakui, T.; Akita, M.; Iwahara, T.; Kanatani, R.;
Yoshida, J.; Kumada, M. Tetrahedron 1983, 39, 983. (b) Tamao, K.; Ishida,
N.; Tanaka, T.; Kumada, M. Organometallics 1983, 2, 1694. (c) Tamao,
K.; Ishida, N. J. Organomet. Chem. 1984, 269, C37.
(9) (a) Ogino, H. Chem. Rec. 2002, 2, 291 and references therein. (b)
Feldman, J. D.; Mitchell, G. P.; Nolte, J.-O.; Tilley, T. D. Can. J. Chem.
2003, 81, 1127.
OL702357N
(11) (a) Lane, T. H.; Frye, C. L. J. Organomet. Chem. 1979, 172, 213.
(b) Tamao, K.; Tanaka, T.; Nakajima, T.; Sumiya, R.; Arai, H.; Ito, Y.
Tetrahedron Lett. 1986, 27, 3377. (c) Tamao, K.; Nakagawa, Y.; Arai, H.;
Higuchi, N.; Ito, Y. J. Am. Chem. Soc. 1988, 110, 3712. (d) Tamao, K.;
Nakagawa, Y.; Ito, Y. J. Org. Chem. 1990, 55, 3438. (e) Tamao, K.;
Nakagawa, Y.; Ito, Y. Organometallics 1993, 12, 2297.
(10) Ando, W.; Hagiwara, K.; Sekiguchi, A. Organometallics 1987, 6,
2270.
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