Intramolecular Homolytic Substitution Behavior of Acyl Radicals at Sulfur: New Carbonylative Access to γ-Thiolactones

Full text of DOI:10.1021/jo971682k

7550
J . Org. Chem. 1997, 62, 7550-7551
Sch em e 1. Ra te Con sta n t for In tr a m olecu la r SH2
a t Su lfu r by a n Acyl Ra d ica la
In tr a m olecu la r Hom olytic Su bstitu tion
Beh a vior of Acyl Ra d ica ls a t Su lfu r : New
Ca r bon yla tive Access to γ-Th iola cton es
Ilhyong Ryu,* Tohru Okuda, Kiyoto Nagahara,
Nobuaki Kambe, Mitsuo Komatsu, and
Noboru Sonoda*,†
Department of Applied Chemistry, Faculty of Engineering,
Osaka University, Suita, Osaka 565, J apan
Received September 9, 1997
Although the inter- and intramolecular addition be-
havior of acyl radicals has been largely elucidated in the
past decade,1 relatively little is known about homolytic
substitution reactions of acyl radicals.2,3 In this paper,
we focus on the intramolecular SH2-type behavior of acyl
radicals at sulfur. Carbonylative approaches to the
synthesis of γ-thiolactones are still rare, and we are
aware of only one such method reported by Alper and
co-workers,4 who utilized a ring-expansion carbonylation
of thiethanes by a Co/Rh mixed-catalyst system. The
work reported herein offers a new carbonylative synthesis
of γ-thiolactones based on free radical processes (eq 1)
and insight into kinetic and mechanistic issues of the first
examples of the intramolecular homolytic substitution of
acyl radicals at sulfur.
a
The value of k′ was taken from ref 5.
Ta ble 1. Syn th esis of γ-Th iola cton es by Ca r bon yla tion /
S
H2 Rea ction Sequ en ce
To know how efficient the intramolecular homolytic
substitution process is, we prepared acyl selenide B
having a tert-butylthio group and carried out a competi-
tive kinetic study between decarbonylation and SH2-type
cyclization (Scheme 1). The rate of decarbonylation was
obtained from the data reported by Chatgilialoglu and
co-workers.5 The SH2-type reaction, which was ac-
companied by extrusion of tert-butyl radical, was found
to be reasonably fast compared with decarbonylation.
From the product distribution, the rate of cyclization is
approximately 7.5 × 103 s-1 at 25 °C.6
Encouraged by these results, we tested the carbony-
lation/intramolecular SH2 reaction sequence, which starts
with tert-butyl 3-bromopropyl thioether (1a ). When 1a
(0.01 M) was treated with tributyltin hydride (1.2 equiv)
and a catalytic amount of AIBN in benzene (100 °C, 5 h,
80 atm of CO), it afforded the desired intramolecular SH2-
type product, γ-thiobutyrolactone (2a ), in 74% yield (eq
2). Uncyclized 4-(tert-butylthio)butanal (5%) and reduced
† Present address: Department of Applied Chemistry, Faculty of
Engineering, Kansai University, Suita, Osaka 564, J apan.
(1) For recent reviews on acyl radicals, see: (a) Brown, C. E.; Neville,
A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J . Aust. J . Chem. 1995,
48, 363. (b) Ryu, I.; Sonoda, N. Angew. Chem., Int. Ed. Engl. 1996, 35,
1050. (c) Crich, D.; Yuan, H. In Advances in Free Radical Chemistry;
Rawal, V. H., Ed.; J AI Press: Greenwich, CT, 1997; Vol. 2, in press.
Also see a review containing C1 radical synthons: (d) Ryu, I.; Sonoda,
N.; Curran, D. P. Chem. Rev. (Washington, D.C.) 1996, 96, 177.
(2) For examples of intermolecular SH2-type reactions of acyl
radicals at group 16 elements, see: (a) (PhSSPh) Walling, C.; Basedow,
O. H.; Savas, E. S. J . Am. Chem. Soc. 1960, 82, 2181. (b) (PhSeCH2E)
Ryu, I.; Muraoka, H.; Kambe, N.; Komatsu, M.; Sonoda, N. J . Org.
Chem. 1996, 61, 6396. (c) (PhTeTePh, RCOTePh) Crich, D.; Chen, C.;
Hwang, J .-T.; Yuan, H.; Papadatos, A.; Walter, R. I. J . Am. Chem. Soc.
1994, 116, 8937. Also see examples of group 17 element (iodine): (d)
Tsunoi, S.; Ryu, I.; Yamasaki, S.; Fukushima, H.; Tanaka, M.;
Komatsu, M.; Sonoda, N. J . Am. Chem. Soc. 1996, 118, 10670. (e)
Nagahara, K.; Ryu, I.; Komatsu, M.; Sonoda, N. J . Am. Chem. Soc.
1997, 119, 5465.
(3) For reviews on inter- and intramolecular SH2 reactions of alkyl
and aryl radicals at group 16 elements, see: (a) Beckwith, A. L. J .
Chem. Soc. Rev. 1993, 143. (b) Schiesser, C. H.; Wild, L. M. Tetrahedron
1996, 52, 13265. (c) Schiesser, C. H. Main Group Chem. News 1993,
1, 8. (d) Crich, D. In Organosulfur Chemistry; Page, P., Ed.; Academic
Press: London, 1995; p 49. Also see an early review: (e) Kampmeier,
J . A.; J ordan, R. B.; Liu, M. S.; Yamanaka, H.; Bishop, D. J . In Organic
Free Radicals; Pryor, W. A., Ed.; ACS Symposium Series 69; American
Chemical Society: Washington, DC, 1978; p 275. (f) Ingold, K. U.;
Roberts, B. P. Free Radical Substitution Reactions; Wiley-Inter-
science: New York, 1971.
propyl tert-butyl thioether (12%) were the only detectable
byproducts. Formation of the anticipated γ-thiobutyro-
lactone (2a ) may involve the addition of tert-butylthio-
propyl radical to carbon monoxide to give an acyl radical
and subsequent intramolecular SH2 reaction of the acyl
radical at the sulfur atom as outlined in eq 1. In this
step, tert-butyl radical is extruded and is quenched by
tributyltin hydride to produce isobutane.
(5) Chatgilialoglu, C.; Ferreri, C.; Lucarini, M.; Pedrielli, P.; Pedulli,
G. F. Organometallics 1995, 14, 2672.
(4) Wang, M.-D.; Calet, S.; Alper, H. J . Org. Chem. 1989, 54, 20.
Also see a review: Khumtaveeporn, K.; Alper, H. Acc. Chem. Res. 1995,
28, 414.
(6) For rate constants of the corresponding SH2 by a primary alkyl
radical, see: Franz, J . A.; Roberts, D. H.; Ferris, K. F. J . Org. Chem.
1987, 52, 2256.
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Communications
J . Org. Chem., Vol. 62, No. 22, 1997 7551
The generality of the carbonylative approach to γ-thio-
lactones was examined with a variety of substrates.
Some of the results are shown in Table 1. In general,
the benzyl leaving group is considered to be superior to
tert-butyl for intramolecular SH2 reactions, but in our
carbonylation/SH2 system tert-butyl is superior. This is
due to an undesirable 1,5-H abstraction at the benzylic
position that precludes carbonylation (Table 1, entries 2
and 6).7
The present reaction need not be restricted only to thio-
substituted alkyl halides. Equations 3 and 4 illustrate
the reactivities of vinyl iodide 1e and aromatic iodide 1f,
respectively. The carbonylation of 1e gave R,â-unsatur-
and E via ketene R radical D satisfactorily accounts for
this result.8 The reaction of o-iodobenzyl tert-butyl
sulfide (1f) gave benzo thiolactone 2f in good yield. The
bond cleavage was selective at a in F , and no product
derived from b, which would afford a more stable benzyl
radical, was detected. This selectivity strongly suggested
the importance of collinear arrangement of both attacking
and leaving radicals for the SH2 process. This led us to
undertake an ab initio MO calculation study. Prelimi-
nary calculations at the levels of UMP2/3-21G* and UHF/
3-21G* predict that homolytic substitution by an acyl
radical at sulfur atom proceeds via a T-shaped transition
structure rather than a hypervalent species (for geom-
etries, see the Supporting Information). This transition
state is similar to those predicted for intramolecular SH2
attack by alkyl radicals.9-11 However, further work based
on higher levels of calculations is needed to clarify this
mechanistically intriguing point.
In summary, we have shown that intramolecular
homolytic substitution by acyl radicals at sulfur occurs
efficiently with extrusion of tert-butyl radical. Coupled
with this SH2-type reaction, free radical carbonylation
methods can be useful alternatives to transition metal
methods for γ-thiolactone synthesis with incorporation
of carbon monoxide.
Ack n ow led gm en t. We are grateful to Dr. Cathleen
M. Crudden for useful discussions.
Su p p or tin g In for m a tion Ava ila ble: Detailed kinetic
data of Scheme 1, geometries of transition state A (UHF/3-
21G(*) and UMP2/3-21G(*)), and characterization data and a
general procedure for compounds 2b-f (15 pages).
J O971682K
(8) (a) Hayes, C. J .; Pattenden, G. Tetrahedron Lett. 1996, 37, 271.
(b) Herbert, N.; Pattenden, G. Synlett 1997, 69. (c) Balogs, A. Ph.D.
Thesis, University of Pittsburgh, Pittsburgh, PA, 1996, pp 128-129.
(9) In general, linear transition structure is important in alkyl
radical-induced intramolecular SH2 reactions. For recent theoretical
studies, see: (a) Smart, B. A.; Schiesser, C. H. J . Chem. Soc., Perkin
Trans. 2 1994, 2269. (b) Schiesser, C. H.; Smart, B. A. Tetrahedron
1995, 51, 6051.
(10) For synthesis of γ-thiolactones by an intramolecular SH2 process
by attacking of alkyl radical to sulfur of thioester, see: (a) Tada, M.;
Nakamura, T.; Matsumoto, M. J . Am. Chem. Soc. 1988, 110, 4647. (b)
Tada, M.; Matsumoto, M.; Nakamura, T. Chem. Lett. 1988, 199.
(11) (a) Beckwith, A. L. J .; Duggan, S. A. M. J . Chem. Soc., Perkin
Trans. 2 1994, 1509. Crich and Yao recently reported an example of
intramolecular SH2 in which aryl radical attacks at sulfur with
extrusion of acyl radicals; see: (b) Crich, D.; Yao, Q. J . Org. Chem.
1996, 61, 3566.
ated γ-thiolactone 2e in 60% yield after isolation by flash
chromatography on silica gel. The rapid isomerization
between two isomeric R,â-unsaturated acyl radicals C
(7) Indeed, the reaction of 1a ′ with Bu3SnD and AIBN (0.02 M, 80
°C) gave a 1:1 mixture of PhCH2SCH2CH2CH2D and PhCHDSCH2-
CH2CH3.