Sulfination of Alcohols with p-Toluenesulfonylmethyl Isocyanide under Metal-Free Conditions: A Mitsunobu Approach

Article abstract of DOI:10.1002/adsc.201600997

A Mitsunobu approach for the synthesis of sulfinate esters by direct nucleophilic substitution of alcohols is described. The salient features of this strategy include neutral and metal-free conditions for the rapid synthesis of sulfinates in high yields. The present protocol using p-toluenesulfonylmethyl isocyanide (TosMIC) and the triphenylphosphine (TPP)/diisopropyl azodicarboxylate (DIAD) reagent system represents the general synthetic route to this important class of compounds. (Figure presented.).

Full text of DOI:10.1002/adsc.201600997

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DOI: 10.1002/adsc.201600997
Sulfination of Alcohols with p-Toluenesulfonylmethyl Isocyanide
under Metal-Free Conditions: A Mitsunobu Approach
Lingaswamy Kadari,a Palakodety Radha Krishna,a and Y. Lakshmi Prapurnaa,*
a
D-207, Discovery Laboratory, Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad – 500007, India
Fax : (+91)-40-2716-0387; e-mail: ylprapurna.iict@gov.in
Received: July 11, 2016; Revised: September 22, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600997.
Abstract: A Mitsunobu approach for the synthesis
of sulfinate esters by direct nucleophilic substitution
of alcohols is described. The salient features of this
strategy include neutral and metal-free conditions
for the rapid synthesis of sulfinates in high yields.
The present protocol using p-toluenesulfonylmethyl
isocyanide (TosMIC) and the triphenylphosphine
(TPP)/diisopropyl azodicarboxylate (DIAD) re-
agent system represents the general synthetic route
to this important class of compounds.
varied functional groups: an isocyano group, an acidic
a-carbon atom and a sulfonyl group. Over the past
decade, our group has been involved in expanding the
synthetic horizon of TosMIC chemistry.[5] In this con-
text, we wished to probe the feasibility of TosMIC as
a nucleophile precursor in Mitsunobu reaction. This
study led to an unexpected sulfinylation of alcohols
using TosMIC under simple, metal-free and neutral
conditions.
Sulfinates are highly valuable precursors in organic
synthesis and can act as either electrophilic or nucleo-
philic agents depending on the reaction conditions.[6,7]
In the light of the emergence of sulfinates as potential
coupling partners and their biological importance,
a practical metal-free sulfinylation involving TosMIC
would be of great interest from the synthetic organic
chemistry standpoint. In general, sulfinates could be
produced from sulfinyl chlorides, sodium sulfinate
Keywords: alcohols; metal-free conditions.; Mitsu-
nobu reaction; p-toluenesulfonylmethyl isocyanide
(TosMIC); sulfinates
Alcohols are among the most versatile compounds in and sulfinic acid. The common sulfinyl halides are liq-
organic synthesis because of their wide availability, uids of limited stability and the preparation of pure
easy preparation and high tractability, and are excel- samples is tedious. In addition, the sulfinic acids and
lent electrophilic partners for substitution.[1] Because their salts are not powerful nucleophiles, and a simple
of the poor leaving-group potential of the hydroxide alkylation reaction of the sulfinate anion is not trivial
ion, however, an alcohol must typically be converted and often requires readjusting the reaction condi-
to a more activated functionality before nucleophilic tions.[8] An overview of recent relevant work on the
substitution is feasible. Thus, the direct nucleophilic sulfinylation of alcohols reveals that Li et al. have
substitution of an alcohol remains a long-standing reported the substitution of benzyl alcohols
challenge and has received sustained attention in or- with TosMIC catalyzed by bismuth(III) bromide
ganic synthesis.[2] The Mitsunobu reaction is one of (Scheme 1a).[9a] However, the major disadvantages of
the most popular methods for the direct substitution this method are the corrosive nature and moisture
of alcohols with nucleophiles mediated by a redox sensitivity of BiBr3, acidic conditions, long duration of
combination of trialkyl- or triarylphosphine and a di- reaction time (1 day) and it is limited to activated al-
ACHTUNGTRENNUNG
alkyl azodicarboxylate.[3] However, the reaction has cohols, i.e., either benzyl or allyl alcohols. Xiong and
a serious limitation, i.e., the requirement of a relative- co-workers disclosed the BF3·Et2O-promoted sulfina-
ly acidic pro-nucleophile (H-Nu). p-Toluenesulfonyl- tion of alcohols, however, a stoichiometric amount of
methyl isocyanide (TosMIC), a unique isocyanide in- BF3·Et2O, a strong Lewis acid and high temperature
troduced by van Leusen has become well regarded in conditions
are
necessary
for
this
process
the literature for its diversity and has gained its place (Scheme 1b).[9b] A copper-catalyzed aerobic oxidative
in organic chemistꢀs tool box as a versatile synthon.[4] reaction of sulfonyl hydrazides and thiols with alco-
The multiple reactivity of TosMIC emerges due to its hols to afford sulfinates was reported by Du et al.[9c]
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in 92% yield. This result was in sharp contrast to the
traditional Mitsunobu reaction, which is known to be
inefficient for C–C bond formation because of the
weak acidity of C-nucleophiles. However, we realized
that the substitution of alcohol with a tosyl group, in-
stead of the expected alkylated TosMIC derivative
Ts-1a had occurred. Furthermore, the detailed prod-
1
uct analysis based on H NMR spectroscopy revealed
two peaks in a doublet for benzyl hydrogen atoms in-
dicating different chemical environments, i.e., due to
the influence of the chiral sulfur center of the sulfi-
nate but not the sulfone. The product was further con-
firmed by 13C NMR, mass spectral analysis and by
comparison with literature data. Given the impor-
tance of sulfinates in organic synthesis and recent ex-
ploration of new methods for the synthesis of these
compounds, we then turned to optimizing the reaction
conditions for this transformation to improve the
yield.
As can be seen from Table 1, a variety of solvents
as well as the required amounts of TosMIC and
DIAD-TPP reagents for an efficient formation of sul-
finate were investigated. Relatively high amounts of
the DIAD-TPP reaction system (2.5 equiv.) and
TosMIC (2 equiv.) were used when investigating the
scope of the reaction with solvents like THF, CH2Cl2,
CH3CN, Ether and DMF. However, we found that it
was possible to lower the DIAD-TPP reaction system
Scheme 1. Synthesis of sulfinate esters from alcohols.
and by Jang and co-workers[9d] independently, the to 1.5 equiv. and TosMIC to 1.2 equiv. and that this
drawbacks of these methods include the requirement had only a minor effect on the reaction yield over
of oxygen conditions, elevated temperature and long
reaction times (Scheme 1c and d). Notably, the major
Table 1. Optimization of the reaction conditions.[a]
disadvantage of these methods is the limited substrate
scope due to the effect of steric hindrance. The devel-
opment of new methods for sulfination of alcohols,
a fundamental transformation in organic synthesis,
persists as a worthwhile endeavor. Herein, we report
the synthesis of sulfinates from alcohols under Mitsu-
nobu conditions employing TosMIC as a sulfonyl
source (Scheme 1e).
To test our strategy, we started with the reaction of
benzyl alcohol 1a with TosMIC 2 under traditional
Mitsunobu conditions, i.,e., the DIAD-TPP reagent
(Scheme 2). To our surprise, the reaction proceeded
very quickly within 20 min to furnish the product 3a
[a]
Reaction conditions: 1a, TosMIC and TPP/DIAD reagent
system in solvent from 08C to room temperature.
Yields refer to isolated products purified by column chro-
[b]
Scheme 2. Sulfination of alcohols with TosMIC under Mitsu-
nobu conditions.
matography.
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a slightly increased reaction time of 5 min. The initial
To gain further insights into the mechanism and to
studies revealed the importance of the order of addi- determine the stereospecificity of the reaction, l-men-
tion of reagents. The following experimental proce- thol was subjected to our standard conditions to give
dure is illustrative: DIAD was added to a chilled solu- the corresponding sulfinate 3p in 84% yield [Eq. (3)].
tion (08C) of benzyl alcohol, TosMIC and PPh3 in 1H NMR spectral studies of 3p revealed the diastereo-
THF.
meric ratio as 1:1, the characteristic O–CH proton
Subsequently, questions related to the reaction resonated at d=4.19 as a triplet of doublets (J=10.8,
mechanism arose with respect to the unexpected high 4.4 Hz) with an integration of 0.5H and at d=4.12 as
reactivity of TosMIC as a C–H based nucleophile as a triplet of doublets (J=10.8, 4.4 Hz) with an integra-
well as the formation of sulfinates. Of note, the mech- tion of 0.5H. However, the diastereomeric ratio of 3p
anistic details of the Mitsunobu reaction, a well-estab- observed was found to be due to the chiral sulfur
lished fundamental reaction in synthetic organic center by comparision with literature data.[11] In order
chemistry, renowned for its stereospecificity with to know the stereochemistry of the alcohol center, we
clean inversion of alcohols is still a subject of debate have chosen the chemical correlation method. Ac-
and intensive studies.[10] In order to clarify the mecha- cordingly, the sulfinate 3p was subjected to hydrolysis
nism, the following experimental investigations were followed by benzoylation to provide menthyl ben-
carried out (Scheme 3). The reaction of 1a under the zoate 5 [Eq. (4)], as the two epimers of 5 are well
1
optimal conditions without TosMIC failed, thus dem- known in the literature.[12] In the H NMR of 5, the
onstrating the necessity of TosMIC. Reactions involv- resonance of O–CH proton at d=4.93 as a triplet of
ing the most commonly used sulfonyl sources such as doublets (J=10.9, 4.4 Hz) and the absence of signals
sodium p-toluenesulfinate and sulfinic acid instead of at d=5.38, clearly demonstrate the stereochemistry of
TosMIC under otherwise identical conditions did not the alcohol as “R’’. Moreover, the specific rotation of
afford the desired product. When 1a was subjected to menthol, derived after hydrolysis of 3p exactly
react with mono- and disubstituted TosMIC, 3a was matched with the literature value for l-menthol.
not observed, which suggests that the acidic methyl- These results prove that the reactions proceed with
ene group was required for the formation of sulfinate a double-inversion mechanism resulting in the reten-
[Eq. (1)]. To capture the intermediate, a quenching of tion of configuration. As the first inversion has al-
the reaction mixture after 5 min afforded Ts-1a [Eq. ready been identified as the formation of Ts-1a, simi-
(2)]. However, the reaction of diols with TosMIC did lar to the Mitsunobu reaction, we turned our atten-
not provide any mono-/disulfinate product.
tion to the second inversion, i.e., the subsequent sulfi-
nation pathway. One of the possible sulfination path-
ways, the intramolecular rearrangement of Ts-1a,
similar to previous reports by Wu and co-workers,[9a]
could not be applicable under the present conditions
due to unfavorable 4-membered transition state. This
assumption was further supported by the reaction of
Ts-1a under standard conditions in which the sulfinate
formation was not observed [Eq. (5)]. In view of the
recent emergence of TosMIC as a sulfonyl source,[13]
we believe that the dual role of TosMIC as a pronu-
cleophile and sulfonyl source and attack of sulfinate
on Ts-1a, affords the second inversion to provide 3a.
Based on the above results a tentative reaction
mechanism was proposed as shown in Scheme 4. The
initial steps for the formation of oxyphosphonium salt
(i) are believed to proceed through the generally ac-
cepted Mitsunobu path. Subsequently, in the first SN2
reaction the anion of TosMIC attacks on (i) to form
Ts-1a. Next, the attack of sulfinate on Ts-1a takes
place to afford 3a as the final product with regenera-
tion of TosMIC.
Next, we turned our attention to explore the scope
of structurally various alcohols with TosMIC and the
results are summarized in Table 2. The first experi-
ments were conducted with a series of benzyl alcohols
and TosMIC under the optimal conditions. The results
showed that reactions of benzyl alcohols with differ-
Scheme 3. Mechanistic investigations.
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ing sulfinate 3e in a yield of 22%, whereas the present
protocol furnished 3e in 75% yield. For p-nitrobenzyl
alcohol and p-cyanobenzyl alcohol, substrates with
highly electron-withdrawing groups on the aromatic
ring, the present reaction failed to give the desired
product, which can be attributed to the destabilization
of the benzyl carbocation. The 2,4-dichloro benzyl al-
cohol 1f reacted smoothly to provide 3f in 80% yield.
The benzylic secondary alcohols, such as a-phenethyl
alcohol 1g and benzhydrol 1h provided the corre-
sponding sulfinates in 3g and 3h 82 and 86% yields,
respectively. The compound 3g was found to be a 1:1
diastereomeric mixture and the ratio was determined
from the isolated yields. Compared with the benzyl
carbocation, a lower stability exists for an aliphatic
carbocation, especially for primary and secondary
ones. To extend the utility of the reaction further, we
set out to explore various aliphatic alcohols, the less
reactive alcohols. Pleasingly, heptanol 1i reacted
smoothly, affording the desired product 3i in high
Scheme 4. Probable reaction mechanism.
ent substitutions like methyl, fluoro and methoxy (1a– yield. The phenethyl alcohol 1j, an aliphatic alcohol
1d) proceeded smoothly resulting in high yields of sul- possessing a phenyl group, provided 3j in 77% yield.
finates (3a–3d). It is pleasing to note that the sulfina- The sulfination of cyclohexanol 1k with TosMIC also
tion of 1d afforded 3d in 85% yield. The same conver- afforded the corresponding sulfinate 3k effectively.
sion was unsuccessful under the BF3·Et2O reaction Allyl alcohol 1l and propargyl alcohol 1m performed
conditions reported by Xiong and co-workers. In an- well in the reaction to give 3l and 3m in 78 and 80%
other example from the same group, the reaction of yields, respectively. The tertiary alcohol t-BuOH 1n,
1-naphthalenemethanol 1e afforded the correspond- underwent the reaction smoothly giving the desired
Table 2. Synthesis of sulfinate esters from various alcohols under Mistunobu conditions.
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product 3n in 72% yield. Next, the reaction of a-allyl- cohols under simple, metal-free and neutral condi-
benzyl alcohol 1o under the standard conditions fur- tions. Further studies on this process, the development
nished the sulfinate 3o in 81% yield as a diastereo- of a catalytic concept of the present reaction with
meric mixture in a 43:57 ratio. The CH3 protons res- new Mitsunobu reagents, mechanistic investigations,
onated at d=2.41 as a singlet with integration of and the exploration of TosMIC chemistry are current-
1.3H and at d=2.35 as a singlet with an integration ly underway and will be reported in due course.
of 1.7H. As mentioned above, the reaction of l-men-
thol 1p afforded the 3p in racemic form in 84% yield.
In further experiments, with regard to the importance Experimental Section
of allylic sulfinates, we investigated the sulfination of
cinnamyl alcohol 1q and p-fluorocinnamyl alcohol 1r Typical Procedure for the Synthesis of Benzyl 4-
which showed similar activity as benzyl alcohols and Methylbenzenesulfinate (3a)
the desired products 3q and 3r were obtained in good
yields. Unlike the previous reports for the sulfination
of alcohols with TosMIC under acidic conditions, the
To a solution of benzyl alcohol (0.10 g, 0.93 mmol), TosMIC
(0.22 g, 1.10 mmol) and PPh3 (0.36 g, 1.40 mmol) in THF
(5 mL), DIAD (0.28 g, 1.40 mmol) was added at 08C. The
compatibility of acid-sensitive groups under the pres-
ent conditions was probed with mono-TBS protected
pentanediol. Gratifyingly, the reaction provided the
corresponding sulfinate 3s in 79% yield.
It is noteworthy that when a-vinylbenzyl alcohol 1s
was subjected to the optimized reaction conditions,
the reaction proceeded with allylic rearrangement to
give 3q in 82% yield (Table 3, entry 1). The exclusive
formation of the g adduct indicates attack of the
anion on the g carbon of oxyphosphonium salt rather
than the normally expected a-carbon. In entry 2, the
sulfination of a-vinyl-p-fluorobenzyl alcohol 1t fur-
ther demonstrates the predominant formation of the
g adduct, i.e., 3r.
mixture was allowed to warm to room temperature and the
reaction was monitored by TLC. Upon complete consump-
tion of alcohol, the solvents were removed under vacuum
and the crude product was further purified by column chro-
matography in silica using ethyl acetate/hexane as eluent to
give sulfinate 3a; yield: 0.20 g (89%).
Acknowledgements
Financial support from DST, New Delhi (GAP 0485) is
greatly appreciated. The author K.L.S acknowledges UGC,
New Delhi for fellowship support.
In conclusion, we have demonstrated a Mitsunobu
approach employing TosMIC as an efficient pronu-
cleophile for the sulfination of alcohols. The prelimi-
nary mechanistic studies suggest that the reaction pro-
ceeds via an SN2–SN2 mechanism with retention of al-
cohol configuration in the case of chiral alcohol sub-
strates. The present reaction should greatly facilitate
the TosMIC-mediated synthesis of sulfinates from al-
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Sulfination of Alcohols with p-Toluenesulfonylmethyl
Isocyanide under Metal-Free Conditions: A Mitsunobu
Approach
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Adv. Synth. Catal. 2016, 358, 1 – 7
Lingaswamy Kadari, Palakodety Radha Krishna,
Y. Lakshmi Prapurna*
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