Metal-free, radical addition to alkenes via desulfitative chlorine atom transfer

Article abstract of DOI:10.1002/adsc.201100473

An efficient method for radical additions to unactivated alkenes via desulfitative chlorine-atom transfer is described. The reaction is based on the use of readily available sulfonyl chlorides as starting materials and cheap radical initiators such as azobisisobutyronitrile (AIBN), di-tert-butyldiazene (DTBD), and dilauroyl peroxide (DLP). No transition metal catalyst is required and the reaction takes place under mild conditions at temperatures °C. Copyright

Full text of DOI:10.1002/adsc.201100473

FULL PAPERS
DOI: 10.1002/adsc.201100473
Metal-Free, Radical Addition to Alkenes via Desulfitative
Chlorine Atom Transfer
Lidong Cao,a Karin Weidner,a and Philippe Renauda,*
a
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
E-mail: philippe.renaud@ioc.unibe.ch
Received: June 14, 2011; Revised: August 10, 2011; Published online: December 13, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100473.
Abstract: An efficient method for radical additions tion metal catalyst is required and the reaction takes
to unactivated alkenes via desulfitative chlorine- place under mild conditions at temperatures ꢀ858C.
atom transfer is described. The reaction is based on
the use of readily available sulfonyl chlorides as Keywords: amides; atom-transfer radical addition
starting materials and cheap radical initiators such as (ATRA) reaction; chlorine atom transfer; desulfita-
azobisisobutyronitrile (AIBN), di-tert-butyldiazene tion; esters; radical reaction; sulfonyl chloride; tri-
(DTBD), and dilauroyl peroxide (DLP). No transi- chloromethyl radical
Introduction
clearly be of high interest for preparative applications.
Interestingly, sulfonyl chlorides are excellent radical
The development of mild and selective methods for chorinating agents. For instance, the rate constant for
carbon-carbon bond formation via radical pathway the chlorination of primary alkyl radicals has been
has attracted the interest of many synthetic chemists measured to be k=1.3ꢀ106 MÀ1 sÀ1 at 258C.[10] This
during the last decades.[1] Among various types of rad- value is one order of magnitude smaller than the rate
ical processes, halogen atom-transfer radical addition constant reported for iodine atom transfer between
(ATRA) reactions, pioneered by Kharasch[2] and fur- primary alkyl radicals and methyl iodoacetate, one
ther developed by Curran and others,[3] have received order of magnitude higher than the rate constant re-
considerable attention because of their atom-econom- ported for chlorine atom transfer with CCl4 and two
ic nature and high efficiency.[4] Despite the instability order of magnitudes higher than the rate of bromine
of the starting iodides, iodine atom transfer processes atom transfer reported from methyl bromoacetate
represent a powerful method for C C bond forma- (Scheme 2).[11] As a consequence, sulfonyl chlorides
À
tion. Bromine atom transfers were among the earliest
examples of atom transfer processes,[2a] however, they
are restricted to a limited number of cases involving
highly reactive bromides such as bromotrichlorome-
thane and bromomalonitrile.[5–7] Due to the high C Cl
À
bond dissociation energy, the chlorine ATRA reaction
is less common. It is mainly limited to polychlorinated
substrates and it requires catalysis by transition metal
complexes.[8] The use of monochlorinated species is
still highly challenging despite the early report by
Julia about the addition of ethyl chloroacetate to 1-
decene in the presence of a CuCl2-bipyridine catalyst
system (Scheme 1).[9]
Due to their stability and long-term storage proper-
ties, chlorides are very attractive synthetic intermedi-
ates. An efficient method for chlorine ATRA reaction
Scheme 1. Copper-catalyzed chlorine atom transfer pro-
that does not rely on a transition metal catalyst would cess.[9]
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Scheme 4. General mechanism for the desulfitative chlorine
ATRA to alkenes (REl =radical with electrophilic charac-
ter).
Scheme 2. Rate constants for halogen atom transfer involv-
ing primary alkyl radicals.[10,11]
Results and Discussion
Reactions with Trichloromethanesulfonyl Chloride
The desulfitative chlorine ATRA reaction with com-
mercially available Cl3CSO2Cl 1 was investigated first.
Under the neat conditions reported by Roques,[18] low
to moderate yields were obtained due to the forma-
tion of a polymeric material. In a rapid screening of
solvents, tert-butanol, a solvent of low toxicity, gave
the best results in terms of yield and cleanness of the
reaction (Scheme 5). Both AIBN and di-tert-butyldia-
zene (DTBD) can efficiently initiate the reaction. Re-
Scheme 3. Desulfitative chlorine[18] and azide transfer reac-
tions.[19]
are very attractive precursors for chlorine ATRA re- action of Cl3CSO2Cl with monosubstituted terminal
actions. However, so far these reactions are mostly re- alkenes 2a and 2b and methylenecyclohexane deriva-
stricted to the addition of arenesulfonyl radicals to al- tives 2c and 2d afforded the products of the desulfita-
kenes[12–14] and in most cases a transition metal cata- tive chlorine ATRA reaction 3a–3d in excellent
lyst is needed.
yields. Interestingly, para-methoxystyrene 2e reacted
The formation of carbon-centered radicals by a- with Cl3CSO2Cl in almost quantitative yield demon-
scission of sulfonyl radicals is a well-established pro- strating that the chlorination of benzylic radicals can
cess that has been used to generate alkyl radicals in-
volved in chain reactions.[15] Trifluoromethanesulfonyl
chloride (CF3SO2Cl) and trichloromethanesulfonyl
chloride (Cl3CSO2Cl) have been employed as radical
precursors in desulfitative chlorine ATRA reactions
by Kamigata under ruthenium(II) catalysis.[16] Cobal-
t(II) was also shown to catalyze reactions involving
tricholoromethyl radicals.[17] Roques discovered that
chlorine ATRA reactions involving perfluorinated
sulfonyl chlorides could be performed under solvent-
free conditions with azobisisobutyronitrile (AIBN) as
a radical initiator [Scheme 3, Eq. (1)].[18] Recently, we
reported an efficient metal-free desulfitative azide
transfer reaction involving various electrophilic C-
centered radicals [Scheme 3, Eq. (2)].[19]
Based on these results, we hypothesized that the
metal-free desulfitative chlorine ATRA reaction
could become a general approach applicable to vari-
ous types of electrophilic radicals (Scheme 4). We
report here our study of the scope and limitations of
this approach that demonstrates its generality.
Scheme 5. Chlorine ATRA reactions of trichloromethane-
sulfonyl chloride 1.
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Metal-Free, Radical Addition to Alkenes via Desulfitative Chlorine Atom Transfer
Scheme 7. Preparation of ethyl 2-(chlorosulfonyl)acetate
4.[20,21]
Table 1. Optimization of the reaction conditions for the de-
sulfitative chlorine ATRA reaction of 4 to 1-octene 2a.
Scheme 6. Ring-opening and ring-closing reactions involving
desulfitative chlorine ATRA reactions of trichloromethane-
sulfonyl chloride 1.
Entry 4
Initiator
Solvent
5a
(equiv.) (equiv.)
Yield[a]
1
2
3
4
5
1.2
2
2
2
2
DTBD (0.2)[b]
benzene
benzene
benzene
benzene
55%
72%
70%
91%
DTBD (0.2)[b]
AIBN (0.2)
DLP (0.2)
be achieved very efficiently with this reagent. Reac-
tion with an internal cyclic olefin such as norbornene
2f gave 3f in almost quantitative yield. As expected
for a radical reaction, the less reactive cyclohexene 2g
afforded 3g in only 50–53% yield as a mixture of ster-
eomers.
DLP (0.2)
ClCH2CH2Cl 90%
[a]
[b]
Isolated yield.
300 W sun lamp irradiation was used.
The radical nature of the process is demonstrated
by the opening of the cyclobutane ring observed with
(À)-b-pinene 2h that afforded 3h (Scheme 6). Similar- of ethyl chloroacetate with sodium sulfite and oxalyl
ly, dienes 2i and 2j gave products 3i and 3j resulting chloride[22] (Scheme 7), it was of interest to test its re-
from a 5-exo-trig radical cyclization in 69% yield activity in desulfitative chlorine ATRA reactions.
(Scheme 6).
The reaction was optimized with 1-octene 2a and
The chlorine ATRA reaction of trichloromethane- the results are summarized in Table 1. A moderate
sulfonyl chloride and that of CCl4 afford identical yield was obtained with 1.2 equivalents of 4 and
products. The advantages of the sulfonyl chloride- DTBD/sun lamp irradiation to initiate the process. A
mediated process over the CCl4-mediated one are the better yield was obtained with 2 equivalents of the
absence of a metal catalyst [the reaction with CCl4 re- sulfonyl chloride under the same initiation conditions.
quires a metal catalyst (Cu, Ru, etc.) to achieve high Initiation with AIBN did not enhance the yield. Final-
efficiency] and the reduced dangerousness of tri- ly, dilauryl peroxide (DLP) as an initiator in benzene
ACHTUNGTRENNUNG
cinogenic). The CCl4 method is slightly superior in
The scope of the chlorine ATRA reaction of 4 was
terms of cost and atom efficiency despite the fact that then examined with different terminal alkenes
a substochiometric amount of a reducing species is (Scheme 8, compounds 5a–5d, 5k–5l). The products
necessary to regenerate the catalyst.[20]
were obtained in good to excellent yields. Under
these conditions, the tertiary chlorides 5c and 5d are
stable and were isolated in high yields, other tertiary
chlorides proved to be more prone to elimination
(vide infra). Reactions with non-terminal alkenes
Reaction with Ethyl 2-(Chlorosulfonyl)acetate (4)
The results obtained with Cl3CSO2Cl incited us to in- were also examined and as expected only strained sys-
vestigate some other sulfonyl chlorides. Despite the tems such as norbornene 2f gave a good yield of 5f.
high synthetic potential of ATRA reactions involving Internal alkenes such as cyclohexene 2g afforded the
chloroacetates, they remain extremely difficult to run chlorine ATRA product 5g in 22% yield.
even under transition metal catalysis.[9] Since ethyl 2-
Reaction of 4 with different dienes was attempted
(chlorosulfonyl)acetate 4 is easily prepared either by next (Scheme 9). When (S)-limonene 2m was used,
chlorination of ethyl thioglycolate[21] or by treatment the chlorine ATRA product 5m was formed in low
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DLP could also be at the origin of the low efficiency
of the ATRA reaction. Therefore, another radical ini-
tiator that allows running the reaction under strict
non-acidic conditions and at lower temperature was
tested. When DTBD (sun lamp irradiation, <808C)
was used, product 5m was isolated in 62% yield. As
expected, the reaction takes place exclusively at the
terminal alkene. Interestingly, when the iodine ATRA
reaction using ethyl iodoacetate was attempted, none
of the expected tertiary iodides were isolated. Reac-
tion of 4 with the 1,6-diene 2n was then examined.
Under the standard DLP procedure, the desired cy-
clized product 5n was obtained in 30% yield, together
with 26% of HCl elimination products 6n (mixture of
isomers). Using the DTBD/irradiation initiation pro-
cedure, the expected product 5n was obtained in 44%
yield and the diastereoselectivity was increased from
2:1 to 4:1.
Reaction with N-Methoxy-N-methyl-2-
(chloroACTHNUGRTENUNGsulfonyl)acetamide
Scheme 8. Chlorine ATRA reactions of ethyl 2-(chlorosulfo-
nyl)acetate 4.
Since radicals derived from Weinreb amides are know
to react efficiently with terminal alkenes,[23] sulfonyl
chloride 7 was prepared and tested. The chlorine
ATRA reaction of 7 with 1-octene 2a afforded 8a in
72% yield (Scheme 10) demonstrating further that
Weinreb amides are excellent reagents for intermolec-
ular radical addition to terminal alkenes.
To test its limitations, the chlorine ATRA reaction
involving an N,N-dialkylamide was attempted next.
This system is of interest since iodine ATRA reac-
tions of N,N-dimethyliodoacetamide do not afford the
expected iodine atom transfer products but rather lac-
tones resulting from nucleophilic substitution of the
iodide by the ester group.[24] These reactions proceed
inefficiently under standard atom-transfer conditions
and the use of 0.5 equiv. (Bu3Sn)2 and 3.5 equiv. EtI is
required to reach acceptable yields. To test the desul-
fitative chlorine ATRA reaction, N,N-diethyl-2-
(chlorosulfonyl)acetamide 9 was prepared and its re-
action with 1-octene was examined (Scheme 11). The
Scheme 10. Chlorine ATRA reaction of Weinreb amide 7.
Scheme 9. Desulfitative chlorine ATRA reactions of ethyl
(2-chlorosulfonyl)acetate 4 with dienes.
yield under the optimized DLP-initiated reaction con-
ditions. Decomposition products were obtained result-
ing most probably from processes involving cationic
intermediates. Traces of lauric acid generated from Scheme 11. Chlorine ATRA reaction of amide 9.
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Metal-Free, Radical Addition to Alkenes via Desulfitative Chlorine Atom Transfer
crude product was purified by flash chromatography using
silica gel.
The preparation of sulfonyl chlorides and detailed charac-
terization of all compounds can be found in the Supporting
Information.
desired chlorine atom transfer product 10a was ob-
tained but the yield was low. Optimization of the re-
action conditions did not allow us to improve signifi-
cantly the yield. This result demonstrates that the de-
sulfitative chlorine ATRA reaction offers an advant-
age over the iodine ATRA reaction in terms of prod-
uct stability (the chloride product does not undergo
an intramolecular nucleophilic substitution) but, as
anticipated, the limitation due to the lack of reactivity
(electrophilicity) of the intermediate radical is not al-
leviated.
Acknowledgements
We thank the Swiss National Science Foundation (grants 20-
135087) for financial support.
Conclusions
References
The desulfitative chlorine atom transfer radical addi-
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the formation of carbon-carbon bonds under metal-
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decomposition. The reaction can be performed in a
variety of solvents such as tert-butanol, 1,2-dichloro-
ethane, and benzene. Commercially and easily avail-
able radical initiators have been used with success to
trigger the reaction.
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Experimental Section
ATRA Reactions of Cl3CSO2Cl (AIBN Initiation)
In a two-necked, flame-dried flask equipped with a reflux
condenser, the alkene (1.0 equiv.), Cl3CSO2Cl (1.1 equiv.),
and AIBN (0.05 equiv.) were weighed. After purging with
N2, the solvent was added, and the reaction mixture was
heated to reflux and monitored by TLC. At the end of the
reaction time (typically 0.5–2 h), the mixture was concen-
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ATRA Reactions of Cl3CSO2Cl (DTBD Initiation)
In a quartz round-bottomed flask equipped with a reflux
condenser, the alkene (1.0 equiv.) and DTBD (0.5 equiv.)
were added to a stirred solution of sulfonyl chloride
(2.0 equiv.) in tert-butanol. The reaction mixture was irradi-
ated by a sunlamp (300 W) and heated to reflux. At the end
of the reaction time (typically 2 h), the crude product was
purified by flash chromatography using silica gel.
ATRA Reactions of Ethyl 2-(Chlorosulfonyl)acetate
In a two-necked flask, equipped with a reflux condenser and
charged with ethyl 2-(chlorosulfonyl)acetate 4 (2.0 equiv.),
alkene (1.0 equiv.) and benzene was added DLP (0.2 equiv.)
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