Direct synthesis of methylene-1, 2-dichalcogenolanes via radical [3 + 2] cycloaddition of methylenecyclopropanes with elemental chalcogens

Article abstract of DOI:10.1021/ol3031846

Direct [3 + 2] radical cycloaddition of methylenecyclopropanes and elemental chalcogens (S, Se, Te) can readily occur under simple thermal conditions, providing an efficient, practical method for preparation of useful but not easily accessed methylene-1,2

Full text of DOI:10.1021/ol3031846

ORGANIC
LETTERS
2013
Vol. 15, No. 1
144–147
Direct Synthesis of Methylene-1,
2-dichalcogenolanes via Radical [3 þ 2]
Cycloaddition of Methylenecyclopropanes
with Elemental Chalcogens
Lei Yu,*,†,‡,§ Yulan Wu,† Tian Chen,† Yi Pan,§ and Qing Xu*,‡
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,
Jiangsu 225002, China, College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou, Zhejiang 325035, China, and School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210089, China
yulei@yzu.edu.cn; qing-xu@wzu.edu.cn
Received November 19, 2012
ABSTRACT
Direct [3 þ 2] radical cycloaddition of methylenecyclopropanes and elemental chalcogens (S, Se, Te) can readily occur under simple thermal
conditions, providing an efficient, practical method for preparation of useful but not easily accessed methylene-1,2-dichalcogenolanes.
Methylenecyclopropanes (MCPs) are readily accessible
compounds of high intramolecular ring strain and high
reactivity.1,2 They have become a class of versatile building
blocks in organic synthesis because they readily undergo a
variety of interesting transformations under mild condi-
tions, giving various useful organic skeletons that are
otherwise difficult to achieve.1ꢀ14 Generally, MCPs can
undergo ring-breaking reactions to afford allylic and
homoallylic compounds,3,4 ring-expansion reactions to
afford cyclobutane derivatives,5,6 and cycloaddition reac-
tions.7ꢀ14 Cycloaddition is one of the most important trans-
formations of MCPs, because various unsaturated com-
pounds, such as alkenes,8 allenes,9 alkynes,10 aldehydes,11
imines,11a,b,12 nitrones,13 and 1,2-diazines,14 can be employed
† Yangzhou University.
‡ Wenzhou University.
§ Nanjing University.
(7) (a) Zhou, H.; Huang, X.; Chen, W. J. Org. Chem. 2004, 69, 5471.
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2006, 423.
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r
10.1021/ol3031846
Published on Web 12/18/2012
2012 American Chemical Society

to react with MCPs to prepare a wide range of useful ring
compounds.
related to sulfur’s solubility in the solvents, for DCE was
found to be a comparatively better solvent for S(0) at
80 °C.19,20 S(0) loading could be reduced to only 1.1 equiv
without affecting the product yield (run 4), but the reaction
of 1.0 equiv of S(0) gave a much lower yield (run 5).
Temperature screening also showed its key impact on the
reaction. Thus, only trace product was detected at 60 °C
(run 6), and a reaction at 100 °C using dioxane as the
solvent also failed to give a better result (run 7).
Yet, organochalcogenides are also important chemicals
recently drawing great interest due to their wide applica-
tions in many fields.4,6,15,16 Previously, we have investi-
gated reactions of MCPs and some organochalcogeno
reagents.6 With an ongoing interest in organochalcogen-
ides and MCPs,6,7,16 we envisioned a direct reaction of
elemental chalcogens and MCPs that may provide certain
organochalcogenide compounds in a more concise way,
which may also shorten the synthetic procedures by avoid-
ing multistep preparation of the conventional organo-
chalcogeno reagents. Herein we report that MCPs and
elemental chalcogens (S, Se, Te) can readily undergo a ther-
mally induced direct [3 þ 2] radical cycloaddition reaction
to afford the useful methylene-1,2-dichalcogenolanes.17
To our knowledge, cycloaddition of MCPs with elemental
chemicals was unknown.
Table 1. Optimization of the Reaction Conditionsa
Initially, MCP 1aand sulfurpowderweredirectlyheated
in toluene (Table 1, run 1). The product, isolated as pale
yellow crystals in low yield, was analyzed by NMR, IR,
MS, and X-ray diffraction. Interestingly, itunambiguously
proved to be 3-(diphenylmethylene)-1,2-dithiolane (2a).
Obviously, it was generated via an unuaual [3 þ 2] cyclo-
addition reaction of 1a and S(0).
It is well-known that the 1,2-dithiolane structure is a key
moiety of lipoic acid, presents in many naturally occurring
chemicals, and has become a useful building block espe-
cially in materials and medicinal chemistry.17 Since directly
using S(0) as the substrate is more advantageous than the
known methods for 1,2-dithiolane construction,17a,18 the
interesting results intrigued us to further investigate the
reaction in more depth. First, solvents were screened. Non-
polar (xylene, 20%) and polar (CH3CN, 28%; DMF,
27%; DMSO, 37%) solvents and neat conditions (26%)
were all found not suitable for the reaction. Then, although
THF (22%) and CHCl3 (12%) were not effective, similar
etheric and chloric solvents 1,4-dioxane and 1,2-dichlo-
roethane (DCE) could cleanly afford good yields of 2a at
80 °C in only 6 h (runs 2, 3). The solvent effect is possibly
run
S (equiv)b
solvent
toluene
temp (°C)
t (h)
2a%c
1
2
3
4
5
6
7
1.5
1.5
1.5
1.1
1.0
1.1
1.1
80
80
80
80
80
60
100
24
6
23
1,4-dioxane
DCE
DCE
60
6
69
73
6
DCE
10
24
6
51
DCE
trace
61
1,4-dioxane
a The mixture of 1a (0.3 mmol) and sulfur powder was heated in a
solvent (1 mL) under N2 and monitored by TLC. Molecular structure of
2a confirmed by X-ray diffraction analysis (H-atoms omitted for clarity).
b Sulfur loading: 1.5 equiv, 0.90 mmol; 1.1 equiv, 0.66 mmol; 1.0 equiv,
0.60 mmol. c Isolated yields based on 1a.
The optimized conditions (Table 1, run 4) were then
applied to a series of MCPs to extend the scope of the
method (Table 2). For disubstituted symmetrical MCPs
(R1 = R2), such as 1a, only one stereomer of the products
was obtained in moderate to good yields (runs 1ꢀ4). Like
diaryl-substituted MCPs, dialkyl-substituted 1d also gave
the target 2d in good yield (run 4). For unsymmetrical
MCPs (R1 ¼ R2), both (Z)- and (E)-stereomers were
obtained (runs 5ꢀ17). Most disubstituted unsymmetrical
MCPs generally gave good yields of the products (runs
5ꢀ9), but the (Z)- and (E)-stereomers were not selective,
which may be attributed to the steric similarities of the two
aryl groups. As for 1j, only a low yield of the product 2j was
obtained with the observation of a byproduct, 1-(4-chlo-
rophenyl)ethanone (run 10). This is possibly because 1j
easily decomposes togivethe byproduct under the reaction
conditions. Besides, the selectivity of (Z)- and (E)-2j was
determined to be 60/40 by NOESY and 1H NMR spectro-
scopicanalysis,21 higherthanotherdisubstituted MCPs. In
the case of monosubstituted MCPs 1kꢀq (runs 11ꢀ17),
they generally gave moderate yields of the products but in
much higher Z/E selectivities than disubstituted MCPs.21
(15) (a) Ogawa, A. In Main Group Metals in Organic Synthesis;
Yamamoto, H., Oshima, K., Eds.; Wiley-VCH: Weinheim, 2004; Vol. 2,
p 813. (b) Wirth, T. Organoselenium Chemistry; Springer: Berlin, 2000;
Vol. 208. (c) Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004,
~
104, 6255. (d) Perin, G.; Lenardao, E. J.; Jacob, R. G.; Panatieri, R. B.
Chem. Rev. 2009, 109, 1277. (e) Beletskaya, I. P.; Ananikov, V. P. Chem.
Rev. 2011, 111, 1596.
(16) (a) Huang, X.; Liang, C. G.; Xu, Q.; He, Q. W. J. Org. Chem.
2001, 66, 74. (b) Xu, Q.; Huang, X.; Yuan, J. J. Org. Chem. 2005, 70,
6948. (c) Liu, C.; Zang, X.; Yu, B.; Yu, X.; Xu, Q. Synlett 2011, 1143.
ꢀ
ꢁ
ꢀ
(17) Selected examples: (a) Markovic, R.; Rasovic, A. In Compre-
hensive Heterocyclic Chemistry III; Katritzky, A. R., Ramsden, C. A.,
Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsevier: Netherland, 2008; Vol. 4, p
893. (b) Palui, G.; Avellini, T.; Zhan, N.; Pan, F.; Gray, D.; Alabugin, I.;
Mattoussi, H. J. Am. Chem. Soc. 2012, 134, 16370. (c) Li, M.-J.; Liu, X.;
Nie, M.-J.; Wu, Z.-Z.; Yi, C.-Q.; Chen, G.-N.; Yam, V. W.-W. Orga-
nometallics 2012, 31, 4459.
(18) (a) Russell, G. A.; Law, W. C.; Zaleta, M. J. Am. Chem. Soc.
1985, 107, 4175. (b) Morera, E.; Pinnen, F.; Lucente, G. Org. Lett. 2002,
4, 1139. (c) Joly, K. M.; Mirri, G.; Willener, Y.; Horswell, S. L.; Moody,
C. J.; Tucker, J. H. R. J. Org. Chem. 2010, 75, 2395.
(19) We found S(0) alone (0.66 mmol) could not be totally dissolved
in toluene or DMF (1 mL) at 80 °C, while total dissolution of S(0) was
observed in DCE, giving a homogeneous-like emulsion.20
(20) We thankful the reviewers for their kind, proper suggestions.
(21) See Supporting Information for details.
Org. Lett., Vol. 15, No. 1, 2013
145

For example, the highest Z/E selectivity of the products
(2o, 96/4) was obtained from 1o bearing the most bulky
1-naphthyl group (run 15).
the target 1,2-diselenolane (3a) and 1,2-ditellurolane (4a),
respectively. The reaction of Se(0) was then investigated at
high temperatures under neat conditions and was found
to proceed best at 220 °C for 3 h (Table 3, run 1), giving a
63% yield of 3a.22
Table 2. [3 þ 2] Cycloaddition of MCPs with S(0)a
Table 3. [3 þ 2] Cycloaddition of MCPs with Se(0) and Te(0)a
run
1: R1, R2
Y
yield% (Z/E)b
1
2
3
4
5
6
7c
8
1a: Ph, Ph
Se
Se
Se
Se
Se
Se
Se
Te
3a: 63
run
1: R1, R2
t (h)
2: yield% (Z/E)b
1c: 4-ClC6H4, 4-ClC6H4
1d: -CH2CH2CH(Ph)CH2CH2-
1e: 4-MeC6H4, Ph
1f: 4-MeOC6H4, Ph
1g: 4-FC6H4, Ph
3c: 43
3d: 56
1
1a: Ph, Ph
6
2a: 73
3e: 67 (50/50)
3f: 51 (50/50)
3g: 51 (50/50)
3n: 54 (68/32)
4a: 22
2
1b: 4-FC6H4, 4-FC6H4
1c: 4-ClC6H4, 4-ClC6H4
1d: -CH2CH2CH(Ph)CH2CH2-
1e: 4-MeC6H4, Ph
1f: 4-MeOC6H4, Ph
1g: 4-FC6H4, Ph
20
28
10
7
2b: 43
3
2c: 54
4
2d: 73
1n: 2,4,6-Me3C6H2, H
1a
5
2e: 87 (50/50)
2f: 79 (50/50)
2g: 52 (50/50)
2h: 72 (50/50)
2i: 58 (50/50)
2j: 28 (60/40)
2k: 65 (70/30)
2l: 69 (68/32)
2m: 60 (76/24)
2n: 50 (85/15)
2o: 65 (96/4)
2p: 42 (82/18)
2q: 57 (92/8)
6
7
a Unless otherwise noted, MCPs 1 (0.3 mmol) and Se or Te powder
(0.66 mmol, 1.1 equiv) were heated under N2 at 220 °C for 3 h. b See note
b in Table 2 for reference. c N2, DCE (1 mL), 80 °C, 3 d.
7
20
25
27
23
10
14
10
24
10
24
36
8
1h: 4-ClC6H4, Ph
1i: 4-BrC6H4, Ph
1j: 4-ClC6H4, Me
1k: Ph, H
9
10
11
12
13
14
15
16
17
As shown in Table 3, other disubstituted MCPs also
reacted effectively with Se(0) to afford 3-methylene-1,2-
diselenolanes 3 in satisfactory yields under the same con-
ditions (runs 1ꢀ6). In contrast, only trace product was
detected in the reaction of monosubstituted MCP 1n,
probably because 1n easily decomposes under the harsh
reaction conditions. Fortunately, when 1n and Se(0) were
heated in DCE at 80 °C, the original conditions for S(0),
the desired 3n could be isolated in 54% yield after 3 days
(run 7). In comparison, the Z/E selectivity of 3n was lower
than that for the sulfur analogue 2n, but higher than that of
other diselenolanes 3eꢀg. In addition, we also attempted
many conditions for the reactions of MCPs and Te(0), but
the results were not satisfactory, which is possibly due to
the even lower reactivity of Te(0) vs Se(0). Thus, the best
result was still obtained from the neat reaction at 220 °C,
giving a 22% isolatd yield of 4a (run 8).
During substrate extension, we also investigated the
reactions of other reactivemolecules such ascyclopropane,
vinylcyclopropane, methylenecyclobutane, and allene de-
rivatives 5ꢀ10 with S(0) to broaden the scope of the
method (Scheme 1, eq 1). However, no cycloaddition
reaction occurred under the same conditions with recovery
of the reactants only. This indicated that the cycloaddition
reaction is most possibly dependent on the MCP structure,
further implying that the reaction should proceed via an
interesting mechanism correlated with the MCP structure.
Yet, since all reactions were simply conducted under
thermal conditions without using any catalysts or addi-
tives, and MCPs can readily react with chalcogeno and
1l: 4-MeC6H4, H
1m: 4-MeOC6H4, H
1n: 2,4,6-Me3C6H2, H
1o: 1-C10H7, H
1p: 4-CF3C6H4, H
1q: 4-BrC6H4, H
a MCPs 1 (0.3 mmol) and sulfur powder (0.66 mmol, 1.1 equiv) in
DCE (1 mL) were heated at 80 °C and then monitored by TLC. b Isolated
yields based on 1. Stereochemistry of the Z/E isomers was determined by
NOESY spectroscopic analysis. E/Z ratios of the products were deter-
mined by 1H NMR spectroscopic analysis.21
The above-mentioned results also showed that substi-
tuents on MCPs can affect the reaction significantly. Thus,
reactions of disubstituted MCPs were usually faster and
gave higher yields of products than analogous monosub-
stituted MCPs. Likewise, reactions of electron-rich MCPs
were also faster and gave higher yields than electron-
deficient ones (runs 1, 4 vs 2, 3; runs 5, 6 vs 7ꢀ9; runs
11ꢀ15 vs runs 16, 17). For unsymmetrical MCPs, the Z/E
selectivity of the stereomers seemed to correlate closely
with the steric and electronic properties of the substituents.
Thus, monosubstituted MCPs (runs11ꢀ17) generally gave
higher Z/E selectivities of the products than disubstituted
ones (runs 5ꢀ10), and MCPs (1n, 1o) with higher steric
asymmetry for bearing bulky groups (runs 14, 15) and
those (1j, 1p, 1q) bearing electron-withdrawing groups
(runs 10, 16, 17) usually afforded products in higher Z/E
selectivities than analogous MCPs.
The successful reactions of S(0) encouraged us to further
investigate the rections of Se(0) and Te(0) to prepare the
analogous 1,2-dichalcogenolanes. However, the reactions
were not effective under the standard conditions for S(0),
giving only 17% (in 3 d) and 13% (in 5 d) isolated yields of
(22) For comparison, a reaction at 140 °C for 24 h gave a 26%
isolated yield of 3a, a reaction at 220 °C for 1 h gave 54% 3a, and a
reaction at 220 °C for 5 h gave 64% 3a.
146
Org. Lett., Vol. 15, No. 1, 2013

other radicals,2,4,7 this cycloaddition reaction might also
proceed via radical pathways. Thus, 1,4-hydroquinone, a
well-known radical inhibitor, was first added to the reac-
tionof 1aand S(0), whichresultedin completeinhibition of
the cycloaddition reaction with revovery of 1a (Scheme 1,
eq 2). Besides, addition of a radical initiator AIBN did
accelerate the reactions and can reduce the reaction tem-
perature of Se(0) and Te(0) significantly (eq 3), but it failed
to enhance the product yield. This is possibly due to the
high reactivity of 1a under radical initiator-induced con-
ditions that may also lead to formation of unknown
byproducts. These results confirmed that the reaction is
indeed a radical reaction and should proceed via radical
pathways.
Scheme 2. Simplified Plausible Mechanism.20,23
is adequate for the whole reaction. 13 can account for the
high (Z)-selectivities of monosubstituted MCPs, since its
exocyclic CꢀC bond may rotate to reach a favored con-
figuration 130, in which the more bulky Ar group is at the
anti-position to the more bulky CP moiety, leaving the
least bulky Hcis toCP. Thesubsequent radicalfragment of
the CP ring with coinstantaneous reformation of the
exocyclic CdC bond may afford 14, which can selectively
produce (Z)-2 via direct radical ring closure.2,4,7,20
Scheme 1. Control Reactions
13 and 130 may explain the nonselectivity of disubsti-
tuted MCPs, for 13 are very stable and reactive and the two
Ar groups are sterically similar in these cases. Thus,
exocyclic CꢀC rotation may not occur prior to the fast
ring fragment and closure, which will directly give non-
selective products. 13 and 130 may also explain the higher
(Z)-selectivities of electron-deficient MCPs, for radicals 13
of electron-rich MCPs are more stable, easier to be gener-
ated, and more reactive than those of electron-deficient
MCPs, which can lead to faster ring fragment and closure
reactions thanexocyclic CꢀC rotation to form favored 130,
and consequently lower stereoselectivities of the products;
vice versa. For example, 1q is the slowest in reaction rate
(Table 1, run 17), but it may provide enough time for 13 to
rotate to 130 and finally give the highest Z/E (92/8)
selectivity among the electron-deficient MCPs.
In conclusion, we developed an interesting thermally
induced direct [3 þ 2] radical cyloaddition reaction of
MCPs and elemental chalcogens, providing a simple synthe-
sis for the useful methylene-1,2-dichalcogenolanes. This
method is advantageous and potentially useful in synthesis
because it can directly use elemental chalcogens and avoid
the conventional reagents. Further extension of the meth-
od and deeper mechanistic insights are underway.
Based on all the above results and MCPs’ radical reac-
tions,2,4,7 a simplified plausible mechanism was proposed
(Scheme 2).20,23 Thus, sulfur powder (S8) may first gen-
erate a sulfur biradical 11 under thermal conditions,23
which should then selectively attack MCPs’ least hindered
central carbon to give a stable radical intermediate 12 via
homoscission of the CdC bond (path a).2,4,7 Clearly, 12’s
stability depends on its substituents, which can account for
the faster reactions and higher product yields of disubsti-
tuted and electron-rich MCPs than monosubstituted and
electron-deficient ones. In contrast, homoscission of the
proximal bonds of the cyclopropane (CP) ring (path b) and
attack of 11 at the sterically more hindered exocyclic
carbon of MCPs (path c) should not be preferable paths
since they will give unstable radical intermediates.2,4,7
Then, by homoscission of the SꢀS bond, 12 may convert
to 13 and S6, which may be recycled to afford new sulfur
biradicals such as 11. Consequently, nearly 1 equiv of S(0)
Acknowledgment. We thank NNSFC (21202141, 2090-
2070), Priority Academic Program Development of Jiangsu
Higher Education Institutions, and Opening Foun-
dation of Zhejiang Provincial Top Key Disciplines
(100061200138) for financial support. We also thank the
Analysis Center of Yangzhou University.
(23) As suggested by a reviewer, the reactions of Se(0) and Te(0) at
220 °C may also proceed via a mechanism initiated by MCPs-derived
carbon biradicals. Indeed, we found heating 1a alone at 220 °C afforded
a complex mixture of unknown byproducts with recovery of 35% 1a,
and heating 1a and S(0) at 220 °C afforded 9% 2a and unknown
byproducts with recovery of 10% 1a.20 However, Se or Te biradical-
initiated mechanisms cannot be excluded completely at present, since 1a
is still very stable even at 220 °C (see above) and AIBN-induced reactions
could afford target 3a and 4a even at 80 °C in DCE (Scheme 1, eq 3),
implying that Se or Te biradicals were still possibly generated at 220 °C
or under AIBN-induced radical conditions.
Supporting Information Available. Experimental pro-
1
cedures, product characterization, H, 13C NMR and
NOESY spectra of the products. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 1, 2013
147