Tin-free radical carbonylation: Thiol ester synthesis using alkyl allyl sulfone precursors, phenyl benzenethiosulfonate, and CO

Article abstract of DOI:10.1002/anie.200501606

(Chemical Equation Presented) Remove contents from tin ... Thiol esters have been successfully synthesized through tin-free radical carbonylation (see scheme; V-40 = initiator). This approach can be further extended to sequential radical reactions involving cyclization, carbonylation, and trapping of acyl radicals by phenyl benzenethiosulfonate.

Full text of DOI:10.1002/anie.200501606

Angewandte
Chemie
Synthetic Methods
DOI: 10.1002/anie.200501606
Tin-Free Radical Carbonylation: Thiol Ester
Synthesis Using Alkyl Allyl Sulfone Precursors,
Phenyl Benzenethiosulfonate, and CO**
Sangmo Kim, Sunggak Kim,* Noboru Otsuka, and
Ilhyong Ryu*
Free-radical carbonylation is synthetically very useful in
preparing various carbonyl compounds.[1] Synthetic methods
based on free-radical carbonylation utilize mainly highly toxic
organotin reagents as mediators.[2] In our efforts to address
the problems associated with toxic organotin reagents, we
reported that the use of alkyl allyl sulfone precursors is one of
the most useful and reliable methods for the generation of
alkyl radicals under tin-free conditions and are very effective
in radical carbon–carbon bond-formation reactions.[3,4] In our
continued efforts to achieve tin-free radical carbon–carbon
bond formations,[5] we have recently focused on tin-free
radical carbonylations that use alkyl allyl sulfone precursors
to prepare thiol esters [Eq. (1)].
Radical carboxylations were reported by Kharasch et al.
in the 1940s,[6] but no significant progress in this area was
made in the subsequent 50 years. Direct radical carboxylation
of alkyl radicals with carbon dioxide is an extremely difficult
process because decarboxylation is a greatly favored pro-
cess.[7] Thus, radical carboxylations using highly reactive
radical trapping agents such as oxalyl acid derivatives[8] and
S-phenyl chlorothioformate[9] have recently been reported
along with an indirect approach involving carbonylation and
iodine atom transfer.[10] For the synthesis of thiol esters,
radical reactions of aldehydes with disulfides are used.[11]
[*] S. Kim, Prof. Dr. S. Kim
Center for Molecular Design & Synthesis
and Department of Chemistry, School of Molecular Science (BK21)
Korea Advanced Institute of Science and Technology
Daejeon 305-701 (Korea)
Fax: (+82)42-869-8370
E-mail: skim@kaist.ac.kr
N. Otsuka, Prof. Dr. I. Ryu
Department of Chemistry, Graduate School of Science
Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan)
Fax: (+81)72-254-9695
E-mail: ryu@c.s.osakafu-u.ac.jp
[**] S.K. is grateful to the CMDS and the Korea Electric Power
Cooperation (KEPCO) for financial support. I.R. thanks JSPS and a
Grant-in-Aid for Scientific Research on Priority Areas (A) “Reaction
Control of Dynamic Complexes” from MEXT (Japan), for financial
support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
To uncover efficient radical-trapping agents of acyl
desulfonylation of the initially generated alkyl sulfonyl
radical along with formation of phenyl allyl sulfone (5). The
alkyl radical can react with CO and/or phenyl benzenethio-
sulfonate (2d) to yield the acyl radical and/or alkyl sulfide 4d.
Therefore, the success of this approach depends critically on
obviating the formation of 4d. To optimize the reaction
conditions, the effect of the pressure of CO and the concen-
tration of 1 were investigated (Table 2). As expected, the yield
radicals,[12,13] we screened several phenylsulfonyl derivatives
as shown in Table 1. When 4-phenoxybutyl allyl sulfone (1)
was treated with phenylsulfonyl bromide (2b) in the presence
of V-40 (1,1’-azobis(cyclohexane-1-carbonitrile)) as initiator
Table 1: Radical carbonylation of alkyl allyl sulfone 1 (R=PhO(CH2)4)
with arylsulfonyl derivatives 2.
Table 2: Effect of concentration of 1 (R=PhO(CH2)4) and pressure of
CO on the tin-free radical carbonylation with 2d.
2: X=
Yield [%]
3
4
1
a: Cl
b: Br
c: SePh
d: SPh
10[a]
0
0
0
75
84
12
76
20
0
[1] [m]
pCO [atm]
Yield [%]
3d
4d
1
75
7
0.05
0.02
0.02
0.01
0.01
0.01
95
95
50
50
82
77
81
61
70
92
11
3
8
3
0
5
18
9
[a] Isolated as the methyl ester.
34
19
6
95
95[a]
0
under pressurized CO (50 atm, 0.03m, autoclave) in heptane
at 1008C for 12 h, 4-phenoxybutyl bromide (4b) was obtained
in 75% yield along with recovery of the starting material 1
(20%) while no acid bromide was obtained. The use of phenyl
benzeneselenosulfonate (2c) gave 4-phenoxybutyl phenyl
selenide (4c) in 84% yield, whereas the use of phenylsulfonyl
chloride (2a) yielded a small amount of the acid chloride
(10%). Apparently, phenylsulfonyl bromide and phenyl
benzeneselenosulfonate react with the alkyl radical prior to
the carbonylation of the alkyl radical, whereas phenylsulfonyl
chloride is too unreactive toward the alkyl radical. When the
reaction was attempted using phenyl benzenethiosulfonate
(2d) under the same conditions, a mixture of thiol ester 3d
(75%) and alkyl sulfide 4d (12%) was isolated along with
some starting material (7%). Furthermore, the use of
diphenyl disulfide as a trapping agent under the same
conditions was not effective and 3d was obtained in 15%
yield along with 80% recovery of 1.
[a] Reaction time: 18 h.
of thiol ester 3d was increased at the higher pressure of CO
while a lower concentration of 1 led to a reduced yield of alkyl
sulfide 4d. The best result was obtained when the reaction
was carried out with 2d (1.5 equiv) and V-40 (0.2 equiv) as
initiator in a pressurized autoclave (95 atm of CO) in heptane
(0.01m) at 1008C for 18 hours. Furthermore, when the
effectiveness of alkyl benzenethiosulfonates relative to 2d
was briefly studied, methyl benzenethiosulfonate was found
to be equally effective and slightly more reactive than 2d
[Eq. (2)]. Additionally, we explored the application of the
present method to synthetically useful pentafluorophenyl
thiol esters [Eq. (3)].[14] Treatment of 1 with pentafluoro-
phenyl benzenethiosulfonate (7)[15] under the same conditions
afforded pentafluorophenyl thiol ester 8 in 82% yield along
with pentafluorophenyl sulfide 9 (16%). A similar result was
also obtained with 6.
As shown in Scheme 1, the addition of a phenylsulfonyl
radical to 1 produces an alkyl radical through the thermal
Table 3 illustrates the efficiency and the scope of the
present method. Primary alkyl radicals worked well, yielding
the corresponding thiol esters in high yields under the present
Scheme 1. Tin-free radical carbonylation of alkyl allyl sulfone 1
(R=PhO(CH2)4) with phenyl benzenethiosulfonate (2d).
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Table 3: Synthesis of thiol esters through tin-free radical carbonylation.
alkyl phenyl sulfide was reduced to
some extent to yield an 80:11 mix-
ture of 3d and 4d (entry 8). As we
anticipated, tertiary alkyl radicals
gave more direct addition products
(entries 11 and 12). The benzylic
radical did not undergo carbonyla-
tion and reacted with 2d to give a
benzyl phenyl sulfide in 51% yield
together with the recovery of some
starting material (46%; entry 13).
Sequential radical reaction involv-
ing cyclization and phenylthio car-
bonylation afforded the desired
product in 90% yield (entry 14).
However, in the case of 6-exo ring
closure, a 24:59 mixture of two
products was obtained in favor of
the direct carbonylation product,
apparently as a result of the com-
petition between 6-exo ring closure
and the direct carbonylation
(entry 15).[16] When a four-compo-
nent coupling reaction using 12,
allyl trimethylsilane, CO, and 2d
was carried out under the same
conditions, a 4.7:1 diastereomeric
mixture of the desired product 14
was isolated in 83% yield
(Scheme 2).[17] Evidently, the elec-
trophilic alkyl radical generated
from 12 failed to undergo carbon-
ylation and reacted with allyl trime-
thylsilane to yield intermediate 13.
Next, the possibility of a double
Entry
Alkyl allyl sulfone
Conditions[a]
Thiol ester
Yield [%][b]
=
Y=SO2CH2CH CH2
1
A
97
2
3
A
A
98
94
4
A
83
5
6
A
A
95
84[c]
7
8
A
B
64 (26)
80 (11)
9
10
11
A
A
A
87 (10)
83 (13)
33 (57)
12
13
14
A
A
A
72 (24)
– (51)[d]
90
carbonylation was explored.[18]
Reaction of 15 with 2d and CO
yielded acyl radical 17 through
carbonylation of the radical inter-
mediate 16. The subsequent 5-exo
ring closure of 17 and CO trapping
followed by quenching with 2d
afforded thiol ester 18 according to
the scheme proposed (Scheme 3).
When 15 was subjected to the
15
A
24
59
[a] A: 2d (1.5 equiv), CO (95 atm), heptane, 1008C, 18 h; B: 2d (1.5 equiv), CO (130 atm), heptane,
1008C, 18 h. TBDPS=tert-butyldiphenylsilyl. [b] The numbers in parentheses indicate isolated yields of
alkyl phenyl sulfides. [c] Starting material (12%) was recovered. [d] Some starting material (46%) was
also recovered.
conditions (95 atm of CO, 0.01m solution of 1). There was no
indication of the formation of alkyl sulfide 4d. In one case, a
small amount of the starting material was recovered (entry 6).
However, secondary alkyl radicals led to a significant amount
of formation of 4d. As the radical carbonylation of secondary
alkyl radicals is less efficient than that of primary alkyl
radicals, secondary alkyl radicals would have more chance to
react with 5 prior to carbonylation. At 95 atm of CO, a 64:26
mixture of the thiol ester and the alkyl phenyl sulfide was
isolated (entry 7). When the same reaction was repeated at a
higher pressure of CO (130 atm) for 18 h, the formation of the
Scheme 2. E=CO2Et.
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Communications
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Scheme 3. E=CO2Et.
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Free Radicals in Organic
standard carbonylation conditions, 18 was isolated in 66%
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In conclusion, we have reported that tin-free radical
carbonylation is successfully achieved using alkyl allyl sulfone
precursors and have developed a highly efficient method for
the synthesis of thiol esters using phenyl benzenethiosulfo-
nate as a trapping agent. This approach provides ready access
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Experimental Section
Typical procedure: Heptane (12 mL), 4-(prop-2-ene-1-sulfonyl)buty-
ric acid ethyl ester (26 mg, 0.12 mmol), phenyl benzenethiosulfonate
(2d; 45 mg, 0.18 mmol), and V-40 (8 mg, 0.03 mmol) were placed in a
50-mL stainless steel autoclave. The autoclave was sealed and purged
with CO (3 ” 10 atm). The autoclave was then pressurized with CO
(95 atm) and heated, with stirring, at 1008C for 18 h. After excess CO
was discharged at room temperature, the solvent was evaporated, and
the residue was purified by column chromatography on silica gel using
ethyl acetate and n-hexane (1:20) as eluant to give 4-phenylsulfanyl-
carbonylbutyric acid ethyl ester (28 mg, 94%). 1H NMR (CDCl3,
400 MHz): d = 1.24 (t, J = 7.1 Hz, 3H), 2.01 (quin, J = 7.3 Hz, 2H),
2.38 (t, J = 7.3 Hz, 2H), 2.72 (t, J = 7.3 Hz, 2H), 4.12 (q, J = 7.1 Hz,
2H), 7.39 ppm (s, 5H); 13C NMR (CDCl3, 100 MHz): d = 14.2, 20.6,
33.0, 42.5, 60.5, 127.6, 129.2, 129.4, 134.5, 172.7, 196.8 ppm; IR
(polymer): n˜ = 749, 1026, 1187, 1442, 1479, 1708, 1735, 1963,
2983 cmÀ1; HRMS [M+] calcd for C13H16O3S: 252.0820; found:
252.0815
Received: May 11, 2005
Published online: August 31, 2005
Keywords: carbonylation · radical reactions ·
.
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radicals · synthetic methods
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6186
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