β-Effects of silicon in directing fragmentation of β- silylcycloalkanone radical cations [12]

Full text of DOI:10.1021/ja992720f

J. Am. Chem. Soc. 2000, 122, 5899-5900
Scheme 2
5899
â-Effects of Silicon in Directing Fragmentation of
â-Silylcycloalkanone Radical Cations
Jih Ru Hwu,*,†,‡ Shui-Sheng Shiao, and Shwu-Chen Tsay‡
†
Organosilicon and Synthesis Laboratory, Department of
Chemistry, National Tsing Hua UniVersity, Hsinchu,
Taiwan 30043, Republic of China, and Institute of
Chemistry, Academia Sinica, Nankang, Taipei,
Taiwan 11529, Republic of China
ReceiVed August 2, 1999
Silicon can direct the Norrish type I cleavage of â-(trimeth-
ylsilyl)cycloalkanones at the C
1
2
-C bond in a highly regiose-
1
lective manner. This photochemical reaction has been applied
in organic synthesis.2 Carboradical intermediates generated
-4
therein are stabilized by a â-silyl group predominantly through
σ-π hyperconjugation”.5
-8
“
We planned to explore the possibility of using silicon to direct
chemical processes involving radical cationic intermediates gener-
ated from cycloalkanones. An ideal outcome would lead us to
obtain a single product bearing synthetically valuable function-
alities, instead of a mixture containing an ester and an alkenyl
1
aldehyde generated through the Norrish type I cleavage. Herein
we report a new silicon-directed fragmentation as shown in
Scheme 1, in which the highly regioselective cleavage of the C -
1
Scheme 1
One-electron oxidation of a carbonyl group to give the
corresponding radical cation can be accomplished by use of
CAN.9 Thus, we treated a 50% aqueous acetonitrile solution
,10
(20 mL) containing a â-(trimethylsilyl)cycloalkanone (2.0 mmol,
0
2
2
.10 M) and CAN (2.4 equiv) at various temperatures between
5 and 82 °C. The optimal conditions were found at 60 °C. Within
.0 h, the solution turned from dark brown to pale yellow or
C bond in â-(trimethylsilyl)cycloalkanones occurred by use of
2
colorless at 60 °C. Change of the color indicated completion of
the reactions. Workup followed by chromatographic purification
afforded ω-alkenylcarboxylic acids in 62-98% yields (Scheme
1). There was no byproduct detected through the C
cleavage.
ceric ammonium nitrate (CAN) to give the desilylated ω-alk-
enylcarboxylic acids.
†
National Tsing Hua University.
1 ω
-C bond
‡
Academia Sinica.
(
1) Hwu, J. R.; Gilbert, B. A.; Lin, L. C.; Liaw, B. R. J. Chem. Soc., Chem.
Commun. 1990, 161-163.
This new reaction was applicable to five- and six-membered
cycloalkanones bearing a â silyl group at an endo (i.e., 1 and 2)
or an exo (i.e., 3 and 4) position. Furthermore, the silicon-directed
fragmentation was successfully extended to bicyclic ketones 8
(
(
(
2) Tietze, L. F.; W u¨ nsch, J. R. Synthesis 1990, 985-990.
3) Sarkar, T. K. Synthesis 1990, 1101-1111.
4) Tietze, L. F.; Beifuss, U. Angew. Chem., Int. Ed. Engl. 1993, 32, 131-
1
63.
1
0.1021/ja992720f CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/31/2000

5900 J. Am. Chem. Soc., Vol. 122, No. 24, 2000
Communications to the Editor
and 10. The desired products 9 and 11 were obtained in 62 and
product 11. As a result, silicon plays a vital role on the
regioselective fragmentation of â-silyl ketones.
6
6% yields, respectively.
Deslongchamps et al.11 reported results from the oxidation of
In addition to silicon, the â-effect of tin has also been applied
to direct organic reactions;1 examples include the C-C bond
fragmentations in the Baeyer-Villiger reaction and the Beckman
reaction.14 We planned to realize the diversity of the electronic
effects of these two elements in group IV. Subsequently, 3-(tri-
n-butyl)stannylcyclopentan-1-one was treated with CAN in wet
acetonitrile at 60 °C. We found that destannylation took place to
afford 2-cyclopenten-1-one in 81% yield. Furthermore, we
performed the destannylation on 3-(tri-n-butyl)stannylcyclohexan-
2,13
nonsilylated bicyclic ketone 12 by CAN in wet acetonitrile. A
mixture of cis- and trans-cyclonitratocarboxylic acids 16 and 17
was generated in a 46% overall yield after 1.5 h (Scheme 2).
The oxidative cleavage occurs at the C
marked in the nonsilylated ketone 12. The regioselective cleavage
of the C -C bond likely comes from the generation of the
thermodynamically more stable secondary carboradicals 13 and
4 as well as the carbocation 15 instead of the less stable primary
species resulting from cleavage of the C -C bond. Under the
same conditions, we converted â-silyl bicyclic ketone 10, through
cleavage of the C -C bond therein, to alkenylcarboxylic acid
1 in 66% yield. Cleavage of either the C -C or the C -C
1 ω
-C bond, which is
1
ω
1
1-one with CAN at 60 °C to give 2-cyclohexen-1-one in 85%
1
2
yield. These successful conversions offer a new method by use
of a stannyl moiety as a “protective group”. Thus, cycloalkanones
bearing a â-silyl or -stannyl group gave an entirely different class
of products upon exposure to CAN under the same conditions.
In conclusion, silicon can direct the C-C bond cleavage in
â-silylcycloalkanones in a highly regioselective manner by use
of CAN. A single product, ω-alkenylcarboxylic acid, was
produced in good to excellent yields under mild conditions. In
comparison with the Norrish type I cleavage, this reaction may
have a greater potential in organic synthesis.
1
2
1
1
2
1
ω
bond in 10 would lead to a secondary carboradical. Nevertheless,
the silyl group at the â-position toward the carboradical centers
in 18 and 19 as well as the carbocationic center in 20 exerts an
additional stabilizing effect. The positive charge and the radical,
however, may be switched in the oxygen and carbon centers in
18; such a radical cationic intermediate would lead to the same
(
5) Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In The Chemistry of
Acknowledgment. This work was supported by the National Science
Council of Republic of China and Academia Sinica.
Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: New
York, 1998; Part 1, Vol. 2, pp 356-359.
(
6) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London, 1981;
Supporting Information Available: Synthetic procedures and spec-
troscopic data for 5-11, 23, and 24. This material is available free of
charge via the Internet at http://pubs.acs.org.
Chapter 3, p 15.
(
7) Pross, A.; Radom, L.; Riggs, N. V. J. Am. Chem. Soc. 1980, 102, 2253-
2
259.
(
8) Lyons, A. R.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 2 1972,
JA992720F
6
8, 622-630.
(
9) Ho, T.-L. Organic Syntheses by Oxidation with Metal Compound; Mijs,
(12) Pereyre, M.; Quintard, J.-P.; Rahm, A. Tin in Organic Synthesis;
Butterworth: London, 1987.
W. J.; de Jonge, C. R. H. I., Eds.; Plenum: New York, 1986; pp 569-631.
(
10) Ho, T.-L. Synthesis 1973, 347-354.
11) Soucy, P.; Ho, T.-L.; Deslongchamps, P. Can. J. Chem. 1972, 50,
(13) Sato, T. Synthesis 1990, 259.
(
(14) Bakale, R. P.; Scialdone, M. A.; Johnson, C. R. J. Am. Chem. Soc.
1990, 112, 6729.
2
047-2052.