one of its synthetic equivalents.10 The present method permits
the use of unactivated olefins.
80 °C in the presence of 10 mol% of the radical initiator AIBN.
These results suggest a free-radical chain process in which
cleavage of the Se–CCl3 bond initiates the reaction, followed by
addition of the trichloromethyl radical to the less substituted
alkenic atom, and finally transfer of the PhSe group from 1 to
the resulting 2-alkyl radical (Scheme 4).
Selenide 1 was conveniently prepared by a new method from
the base-catalysed reaction of diphenyl diselenide with CHCl3
(Scheme 2). Thus, a mixture of the diselenide, a catalytic
amount of Adogen 464® (methyltrialkylammonium chloride),
50% aqueous NaOH and CHCl3 was stirred for 9 h at 15–20 °C,
while air was bubbled through the reaction mixture to recycle
the byproduct selenolate (PhSe2). The product was isolated in
77% yield by flash chromatography.
The free-radical 1,2-addition of 1 to various alkenes to give 2
was then effected as shown in Scheme 3 and Table 1.
Monosubstituted alkenes afforded adducts 2a–e with only
traces ( < 5%) of their corresponding regioisomers, as indicated
by NMR analysis. Both cis- and trans-dec-5-ene produced 2f as
the same ca. 4:1 mixture of diastereomers. Cyclohexene
furnished a mixture of cis and trans-adducts 2h in a ratio of
1:1.8. Thus, the additions are highly regioselective, but only
moderately stereoselective. b-Pinene underwent ring-opening
during the addition to afford the rearranged 1,6-adduct 2i. In
general, the reactions were performed by irradiation of the
reaction mixture, either neat or in benzene solution, containing
an excess (usually five-fold) of the alkene with a 300 W
incandescent lamp. The use of a smaller excess of the alkene
resulted in lower yields and required longer reaction times. The
reaction was also performed in the case of 2a in benzene at
Several of the adducts 2 were then transformed into allylic
selenides 3 by dehydrochlorination with KOBut in THF at
230 °C (except for 3f, which required 6 h at 10 °C), and finally
into a,b-unsaturated carboxylic acids 4 by oxidation and in situ
[2,3]sigmatropic rearrangement (Scheme 3) of the resulting
selenoxides.11 Products 4a, 4b and 4f were obtained as the
E-isomers with high stereoselectivity. When Et2NH was present
during the latter step, the corresponding diethylamides were
obtained instead. These results are shown in Table 2. Pre-
sumably, the sigmatropic rearrangement produces 6 initially,
which then hydrolyses to the carboxylic acid 4 (Scheme 5). On
the other hand, 6 reacts preferentially by aminolysis in the
presence of diethylamine to afford the amide 5 instead of 4 after
aqueous workup.
1
All new compounds reported here gave IR, H NMR, 13C
NMR and low and high resolution mass spectra consistent with
their structures.
These results demonstrate that the free-radical additions of
readily available selenide 1 to alkenes occur efficiently and with
high regioselectivity. Moreover, when the above process was
used in conjunction with base-promoted dehydrochlorination
and [2,3]sigmatropic rearrangement of the corresponding
selenoxides, a novel method for the overall regioselective
carboxylation of alkenes was achieved.
hn or
PhSeCCl3
PhSe•
+
•CCl3
CCl3
heat, AlBN
1
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support.
R2 R3
•
R2
R3
R1
+
•CCl3
Footnote and References
R1
* E-mail: tgback@acs.ucalgary.ca
R2 R3
•
R2 R3
CCl3 + 1
CCl3
1 T. G. Back, in Organoselenium Chemistry, ed. D. Liotta, Wiley, New
York, 1987, ch. 7; C. Paulmier, Selenium Reagents and Intermediates in
Organic Synthesis, Pergamon, Oxford, 1986, pp. 214–218.
2 T. G. Back and S. Collins, J. Org. Chem., 1981, 46, 3249; T. G. Back,
S. Collins and R. G. Kerr, J. Org. Chem., 1983, 48, 3077; R. A. Gancarz
and J. L. Kice, J. Org. Chem., 1981, 46, 4899; T. Miura and
M. Kobayashi, J. Chem. Soc., Chem. Commun., 1982, 438.
3 T. G. Back and M. V. Krishna, J. Org. Chem., 1988, 53, 2533;
A. Ogawa, H. Yokoyama, K.Yokoyama, T. Masawaki, N. Kambe and
N. Sonoda, J. Org. Chem., 1991, 56, 5721.
PhSe
R1
R1
•CCl3
+
Scheme 4
R3
R2 R3
R1
Cl
R1
Cl
Cl
Ph
R2
Se
4 D. L. Boger and R. J. Mathvink, J. Org. Chem., 1989, 54, 1777.
5 T. Toru, T. Seko, E. Maekawa and Y. Ueno, J. Chem. Soc., Perkin
Trans. 1, 1988, 575.
O
Cl
PhSeO
6 J. H. Byers and G. C. Lane, J. Org. Chem., 1993, 58, 3355; J. H. Byers,
J. G. Thissell and M. A. Thomas, Tetrahedron Lett., 1995, 36, 6403;
D. P. Curran, E. Eichenberger, M. Collis, M. G. Roepel and G. Thoma,
J. Am. Chem. Soc., 1994, 116, 4279; T. G. Back, P. L. Gladstone and
M. Parvez, J. Org. Chem., 1996, 61, 3806; P. Renaud and S. Abazi,
Synthesis, 1996, 253.
6
ii
i
5
4
Scheme 5 Reagents and conditions: i, H2O; ii, Et2NH, then H2O
7 C. Walling and E. S. Huyser, Org. React., 1963, 13, 91.
8 D. P. Curran, A. A. Martin-Esker, S.-B. Ko and M. Newcomb, J. Org.
Chem., 1993, 58, 4691.
Table 2 Preparation of allylic selenides 3, carboxylic acids 4 and amides 5
from 2
9 D. H. R. Barton, T. Okano and S. I. Parekh, Tetrahedron, 1991, 47,
1823; L. M. Yagupol’skii and N. V. Kondratenko, Russ. J. Gen. Chem.,
1967, 37, 1686.
Isolated
yields (%)
10 R. C. Larock, Comprehensive Organic Transformations, VCH, New
York, 1989, pp. 185–188; R. P. A. Sneeden, in The Chemistry of
Carboxylic Acids and Esters, ed. S. Patai, Wiley, London, 1969, ch.
4.
11 H. J. Reich, in Organoselenium Chemistry, ed. D. Liotta, Wiley, New
York, 1987, ch. 8.
Adduct
R1
R2
R3
3
4
5
2a
2b
2f
H
H
Bun
C6H13
But
Bun
H
H
H
H
92
90
92
86
91
84
81
60
73
50
59
50
2g
–[CH2]4–
Received in Corvallis, OR, USA, 13th June 1997; 7/04153E
1760
Chem. Commun., 1997