Copper-catalyzed extended pummerer reactions of ketene dithioacetal monoxides with alkynyl sulfides and ynamides with an accompanying oxygen rearrangement

Article abstract of DOI:10.1002/chem.201204072

The first examples of metal-catalyzed extended Pummerer reactions through the activation of sulfoxides are described. The copper-catalyzed reactions of ketene dithioacetal monoxides with alkynyl sulfides and ynamides provided a wide variety of γ,γ-disulfanyl-β,γ-unsaturated carbonyl compounds with an accompanying oxygen rearrangement. The products can be easily converted into 1,4-dicarbonyl compounds and substituted heteroaromatics. DFT calculations and mechanistic experiments revealed a new interesting stepwise addition/oxygen rearrangement mechanism. Beyond Pummerer: The first examples of copper-catalyzed extended Pummerer reactions of ketene dithioacetal monoxides with alkynyl sulfides and ynamides provided a wide variety of γ,γ-disulfanyl-β,γ-unsaturated carbonyl compounds with an accompanying oxygen rearrangement (see scheme). The products can be easily converted into 1,4-dicarbonyl compounds and substituted heteroaromatics. DFT calculations and mechanistic experiments revealed a new interesting stepwise addition/oxygen-rearrangement mechanism.

Full text of DOI:10.1002/chem.201204072

FULL PAPER
DOI: 10.1002/chem.201204072
Copper-Catalyzed Extended Pummerer Reactions of Ketene Dithioacetal
Monoxides with Alkynyl Sulfides and Ynamides with an Accompanying
Oxygen ACTHNURTGNEURGN earrangement
Kei Murakami,[a, b] Junichi Imoto,[a] Hiroshi Matsubara,*[c] Suguru Yoshida,[a]
Hideki Yorimitsu,*[b, d] and Koichiro Oshima*[a, e]
Abstract: The first examples of metal-catalyzed extended Pummerer reactions
through the activation of sulfoxides are described. The copper-catalyzed reactions
of ketene dithioacetal monoxides with alkynyl sulfides and ynamides provided a
Keywords: alkynes · copper · Pum-
wide variety of g,g-disulfanyl-b,g-unsaturated carbonyl compounds with an accom-
merer reactions · rearrangement ·
panying oxygen rearrangement. The products can be easily converted into 1,4-di-
sulfur heterocycles
carbonyl compounds and substituted heteroaromatics. DFT calculations and mech-
anistic experiments revealed a new interesting stepwise addition/oxygen rearrange-
ment mechanism.
Introduction
heterocycles. However, the Pummerer reactions of 1-alkenyl
sulfoxides have been much less studied.[4]
The Pummerer reaction has been extensively studied since
its discovery in 1909[1] and widely applied to the synthesis of
natural products and biologically active compounds.[2] Mech-
anistically, the reactions are initiated by the activation of
sulfoxides with acids (LA+) and then nucleophiles (NuÀ)
attack the resulting thionium ions to give the products
[Eq. (1)]. Recently, an extended version of the Pummerer
chemistry of aryl sulfoxides[3] and 1-alkenyl sulfoxides has
attracted increasing attention for the synthesis of complex
Currently, our group is interested in ketene dithioacetal
monoxides, members of the 1-alkenyl sulfoxide family, as
useful two-carbon building blocks under Pummerer condi-
tions.[5,6] During our study, we have developed a concise
[a] Dr. K. Murakami, J. Imoto, Dr. S. Yoshida, Prof. Dr. K. Oshima
Department of Material Chemistry, Graduate School of Engineering
Kyoto University, Kyoto-daigaku Katsura, Nishikyo-ku
Kyoto 615-8510 (Japan)
access to 2-trifluoromethyl-1,4-dicarbonyl compounds
F
through the reaction of 2-(2,2,2-trifluoroethylidene)-1,3-di-
thiane 1-oxide (A) with ketones (Scheme 1).[5e] The reaction
proceeded under acidic Pummerer conditions through a
[3,3] sigmatropic rearrangement (C into D). Although the
reactions are interesting and useful, the substituent of A is
Fax : (+81)75-753-7710
E-mail: oshima@orgrxn.mbox.media.kyoto-u.ac.jp
[b] Dr. K. Murakami, Prof. Dr. H. Yorimitsu
Department of Chemistry, Graduate School of Science
Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: yori@kuchem.kyoto-u.ac.jp
[c] Prof. Dr. H. Matsubara
Department of Chemistry, Graduate School of Science
Osaka Prefecture University
Gakuen-cho 1-1, Naka-ku, Sakai 599-8531 (Japan)
Fax : (+81)72-252-1161
E-mail: matsu@c.s.osakafu-u.ac.jp
[d] Prof. Dr. H. Yorimitsu
ACT-C, Japan Science and Technology Agency (Japan)
[e] Prof. Dr. K. Oshima
Environment, Safety and Health Organization, Kyoto University,
Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204072.
Scheme 1. Our previous approach to 1,4-dicarbonyl compounds from A
under Pummerer conditions.[5e] Tf: trifluoromethanesulfonyl.
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limited to only the trifluoromethyl group because the elec-
tron-withdrawing group plays an important role to suppress
the formation of the too-reactive dicationic intermediate B’.
We hence envisioned that the use of a mild Lewis acid
would be the key to extend our Pummerer reactions by mild
activation of the ketene dithioacetal monoxides without
breaking the oxygen–sulfur bond of the substrates prior to
the reaction.
Herein, we wish to report copper-catalyzed extended
Pummerer reactions of ketene dithioacetal monoxides with
alkynyl sulfides and ynamides, with the aim of versatile syn-
theses of 1,4-dicarbonyl compounds (Scheme 2).[7] To the
stituted alkynyl sulfides afforded the corresponding products
3c and 3d in excellent yields. Phenylethynyl sulfide was con-
verted into 3e in 75% yield. The bulky tert-butyldimethylsil-
yl-substituted alkynyl sulfide reacted smoothly to provide 3 f
in 51% yield.[12] The reaction with the less nucleophilic S-
phenyl alkynyl sulfide gave 3g in low yield. The scope of
the ketene dithioacetal monoxides was then studied. Methyl
substitution was tolerated to yield 3h in 95% yield, whereas
a bulky cyclohexyl substituent completely inhibited the for-
mation of 3i. The reaction was applicable with an acyclic
ketene dithioacetal monoxide to provide 3j, albeit in lower
yield. With a higher catalyst loading, aryl-substituted 2[13] re-
acted efficiently, regardless of the electronics of the aryl
groups (3k–3p). It is noteworthy that the reaction can be
performed on a large scale to provide 3n in high yield.
Ynamides[14] were also applied to the copper-catalyzed re-
action (Table 1, lower section, X: NR’). The reactions with
the more nucleophilic ynamides were faster than those with
alkynyl sulfides and reached full conversion in 2 h. It is
noteworthy that the reactions proceeded without the addi-
tion of H2O. The reaction with the less nucleophilic nosylat-
ed ynamide required 6 h for completion to afford 4e in
78% yield. The scope was wide enough to apply various
ketene dithioacetal monoxides and ynamides. The structure
of 4d was unambiguously determined by X-ray crystallo-
graphic analysis.[15]
The products were utilized in divergent and short synthe-
sis of unsymmetrical 1,4-dicarbonyl compounds (Scheme 3).
The ketene dithioacetal moieties of products 3n and 4h
were easily converted into thioesters[16] by acidic hydrolysis
and methylation to provide 5a and 5b in 55% and 92%
yields, respectively (paths a and b). Concise and designed
synthesis of 1,4-diketones, good precursors of heteroaromat-
ics, were accomplished by stepwise Fukuyama coupling reac-
tions[16b] of the g,g-disulfanyl-b,g-unsaturated thioesters. The
thioester moiety of 3a was first converted into a benzoyl
group by palladium-catalyzed Fukuyama coupling with the
phenylzinc iodide–lithium chloride complex[17] (path c).
After transformation of the remaining ketene dithioacetal
moiety into a thioester, a subsequent coupling reaction with
a p-tolylzinc reagent provided the corresponding 1,4-dicar-
bonyl compound 5c in 59% yield over four steps. Similarly,
tetraaryl-substituted unsymmetrical 1,4-diketone 5d was
synthesized from 3n over three steps in 42% yield (path f).
Paal–Knorr reactions of 5c and 5d yielded multisubstituted
heterocycles 6a–6d in high yields (paths d, e, g, and h);
these compounds exhibit bright blue fluorescence (fluores-
cence quantum yields up to 97%).[18] The present route has
proved to be useful to synthesize not only various 1,4-dicar-
bonyl compounds but also substituted heteroaromatics.
Scheme 2. Proposed mechanism for the copper-catalyzed Pummerer reac-
tions with alkynyl sulfides and anamides.
best of our knowledge, this is the first example of metal-cat-
alyzed Pummerer reactions in which the sulfoxides are di-
rectly activated by the catalyst.[8,9] Alkynyl sulfides and yn-
ACHTUNGTRENNUNGamides were selected as enolate equivalents that would be
good nucleophiles toward ketene dithioacetal monoxides ac-
tivated by copper (G into H). Importantly, the heteroatom-
substituted alkynes could accommodate the oxygen atom
after the nucleophilic attack on the ketene dithioacetal mon-
À
oxide (H into I). The following S O bond cleavage would
give zwitterionic intermediate J, which contains a highly
stable carbocation delocalized over the adjacent two sulfur
atoms.[10] Subsequent proton transfer should afford the cor-
responding g,g-disulfanyl-b,g-unsaturated carbonyl com-
pound K, an equivalent of 1,4-dicarbonyl compound L.
Results and Discussion
Treatment of ketene dithioacetal monoxide 2a with alkynyl
sulfide 1a in the presence of a catalytic amount of copper
bromide afforded 3a in 86% yield [Eq. (2)]. It is notewor-
thy that the addition of one equivalent of H2O was in
ACHTUNGTNERdNUNG is-
A
ACHTUNGTRENNUNG
cy. The role of the water will be discussed below. The reac-
tion failed to give any product without activation by copper
bromide.[11]
The scope of the reaction with various alkynyl sulfides
was then studied (Table 1, upper section, X: S). The reac-
tions with sterically hindered isopropyl- and tert-butyl-sub-
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Extended Pummerer Reactions
FULL PAPER
Table 1. Scope of the reactions with alkynyl sulfides and ynamides.
Products with alkynyl sulfides (X: S)
Scheme 3. Syntheses of 1,4-dicarbonyl compounds and multisubstituted
heterocycles. Procedures: a) 1) 3n, HCl, MeCN; 2) MeI, EtiPr2N, ace-
tone. b) 4h, HCl, MeCN, then MeI, K2CO3. c) 1) 3a, PdCl
PhZnI·LiCl/THF, THF; 2) HCl, MeCN; 3) MeI, EtiPr2N, acetone;
4) PdCl2A(dppf), p-tolZnI·LiCl/THF, toluene. d) 5c, p-TsOH, toluene.
e) 5c, Ti(OiPr)4, BnNH2, toluene. f) 1) 3n, PdCl2A(dppf), 2-thienylZnI·
LiCl/THF, THF; 2) HCl, MeCN, then MeI, K2CO3; 3) PdCl2A(dppf),
4-NCC6H4ZnI·LiCl/THF, toluene. g) 5d, p-TsOH, toluene. h) 5d, Ti-
AHCTUNGTERG(NNNU OiPr)4, BuNH2, toluene. Bn: benzyl; dppf: 1,1’-bis(diphenylphosphanyl)-
2ACHTUNGTRENNUNG(dppf),
CTHUNGTRENNUNG
A
CHTUNGTRENNUNG
CHTUNGTRENNUNG
ferrocene; p-tol: para-tolyl; p-Ts:toluene-4-sulfonyl; THF: tetrahydrofur-
an.
To clarify the role of water in the reactions with alkynyl
sulfides, we performed the reaction in the presence of
[18O]H2O or D2O. When [18O]H2O was used as an additive,
18O was not incorporated into the product [Eq. (3)]. This
result suggests that the oxygen-transfer step proceeds in an
intramolecular fashion, not an intermolecular fashion in
which compound H reacts with water.[19] The reaction with
D2O afforded a corresponding product that contained 34%
D at the a position of the thioester moiety [Eq. (4)].[20]
Judging from these results, we assume that the addition of
water accelerates the proton-transfer step from J to K
(Scheme 2).
Products with ynamides (X: NR’)
[a] Conditions: A) 1 (1.2 equiv), 2 (0.40 mmol), CuBr (10 mol%), H2O
(1 equiv), MeNO2 (0.25m), 1008C, 21 h; B) 1 (1.2 equiv), 2 (0.40 mmol),
CuBr (10 mol%), H2O (1 equiv), MeNO2 (1.0m), 808C, 6 h; C) 1
(1.44 equiv), 2 (0.18 mmol), CuBr (20 mol%), H2O (1 equiv), EtNO2
(0.25m), 1008C, 24 h; D) 1 (1.2 equiv), 2 (0.20 mmol), CuBr (10 mol%),
MeNO2 (0.25m), 1008C, 2 h. Bn: benzyl; Cy: cyclohexyl; p-tol: para-
tolyl; Ts: toluene-4-sulfonyl; Ns: 4-nitrobenzenesulfonyl. [b] Yield of iso-
lated product. [c] Performed on the 5.0 mmol scale. [d] Reaction was con-
tinued for 6 h.
To clarify the reaction mechanism of the oxygen-transfer
step, we then performed DFT calculations on the simplified
reaction of 2a with ethynyl methyl sulfide in the presence of
copper bromide (Scheme 4). All of the structures were opti-
mized and the energies were obtained at the M05/6-
311G**+ECP (S, Cu, Br) level by using the Gaussian 03
and 09 programs.[21–24] The energies in nitromethane were
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H. Matsubara, H. Yorimitsu, K. Oshima et al.
respectively) than those in the reaction with alkynyl sulfides.
The lower barriers properly reflect the results that ynamides
reacted with 2a much faster than alkynyl sulfides. These cal-
culations support the suggestion that the reaction is step-
wise, as shown in Scheme 2, rather than a Diels–Alder type
cycloaddition pathway.
Conclusion
We have developed novel copper-catalyzed extended Pum-
merer reactions to efficiently provide a wide range of g,g-di-
sulfanyl-b,g-unsaturated carbonyl compounds. DFT calcula-
tions have rationalized the intriguing stepwise reaction
mechanism, which involves copper-assisted addition of an al-
kynyl sulfide or ynamide to a ketene dithioacetal monoxide
and a subsequent intramolecular oxygen rearrangement.
The present chemistry will open up new possibilities of cata-
lyst-controlled Pummerer reactions for more selective and
efficient transformations.
Scheme 4. Reaction pathway of the reaction with alkynyl sulfide obtained
by DFT calculations. Energies in nitromethane were obtained by PCM
calculations. Atom colors: C: gray; H: white; S: yellow; O: red; Cu:
pink; Br: reddish brown.
Experimental Section
Typical procedure for the reaction of a ketene dithioacetal monoxide
with an alkynyl sulfide: The reaction of 2-methylene-1,3-dithiane 1-oxide
(2a) with butyl 1-octynyl sulfide (1a) is representative. Copper bromide
(5.7 mg, 0.04 mmol) was placed in a 20 mL reaction flask and was dried
evaluated by polarized continuum model (PCM) calcula-
tions.[25]
The initial complexation of copper bromide with 2a was
calculated to prove that O,S-bidentate coordination of 2a
leads to the second-most-stable complex M among the other
possible complexes tested.[26] Complex M then reacts with
ethynyl methyl sulfide with an activation barrier of
75.6 kJmolÀ1 to yield zwitterionic intermediate Oax, which
bears the oxygen atom in the axial position. Prior to oxygen
transfer, isomerization of Oax
in vacuo.
A dry nitromethane (1.6 mL) solution of 2a (95.2 mg,
0.48 mmol) and 1a (59.3 mg, 0.40 mmol) was added under an atmosphere
of argon. Water (7 mL) was then added to the mixture. The whole mix-
ture was heated at 1008C for 21 h. After the mixture was cooled to room
temperature, water (10 mL) was added. The product was extracted with
ethyl acetate (10 mLꢁ3). The combined organic layers were dried and
concentrated. Purification by chromatography on silica gel (hexane/ethyl
takes place to furnish Oeq,
which bears an equatorial
oxygen atom. Highly exother-
mic irreversible intramolecular
oxygen transfer occurs with an
activation
barrier
of
76.5 kJmolÀ1 to finally afford
copper enolate Q. Both the ad-
dition of ethynyl methyl sulfide
and the oxygen transfer require
high activation energies, a fact
that is consistent with the high
reaction temperature (1008C).
DFT calculations on the reac-
tion with ynamides revealed
that the reaction pathways of
the two reactions are quite sim-
ilar (Scheme 4 compared with
Scheme 5). However, the acti-
vation energies in the reaction
with ynamides are apparently
Scheme 5. Reaction pathway of the reaction with ynamide obtained by DFT calculations. Energies in nitro-
A
ACHTUNGTRENNaUNG ne were obtained by PCM calculations. Atom colors: C: gray; H: white; N: yellow: S: blue; O: red; Cu:
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Extended Pummerer Reactions
FULL PAPER
acetate, 30:1) provided S-butyl 2-((1’,3’-dithian-2’-ylidene)methyl)octane-
thioate (3a, 118 mg, 0.34 mmol, 86%).
esters; see: c) K. Ogura, Y. Ito, G. Tsuchihashi, Bull. Chem. Soc.
Jpn. 1979, 52, 2013.
[7] For recent advances in the synthesis of 2,3-disubstituted 1,4-dicar-
bonyl compounds, see: a) C. Liu, Y. Deng, J. Wang, Y. Yang, S.
Tang, A. Lei, Angew. Chem. 2011, 123, 7475; Angew. Chem. Int. Ed.
2011, 50, 7337; b) M. P. DeMartino, K. Chen, P. S. Baran, J. Am.
Chem. Soc. 2008, 130, 11546, and references for classical routes to
1,4-dicarbonyl compounds therein; c) P. S. Baran, B. D. Hafenstein-
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Soc. 2006, 128, 8678; d) M. D. Clift, C. N. Taylor, R. J. Thomson,
Org. Lett. 2007, 9, 4667.
[8] Kita et al. showed efficient zinc-catalyzed Pummerer reactions, in
which zinc salts would work to accelerate silylation of the oxygen
atom of sulfoxides; see: a) Y. Kita, N. Shibata, N. Yoshida, S. Fujita,
J. Chem. Soc. Perkin Trans. 1 1994, 3335; b) Y. Kita, N. Shibata, Syn-
lett 1996, 289.
Typical procedure for the reaction of a ketene dithioacetal monoxide
with an ynamide: The reaction of 2-methylene-1,3-dithiane 1-oxide (2a)
with N-methyl-N-(phenylethynyl)-4-methylbenzenesulfonamide is repre-
sentative. Copper bromide (2.9 mg, 0.02 mmol) was placed in a 20 mL re-
action flask and was dried in vacuo. A dry nitromethane (0.8 mL) solu-
tion of 2a (29.7 mg, 0.20 mmol) and N-methyl-N-(phenylethynyl)-4-meth-
ylbenzenesulfonamide (68.5 mg, 0.24 mmol) was added under an at
ACHTUNGERTNmNUNG o-
ACHTUNGTRENNUNGsphere of argon. The mixture was heated at 1008C for 2 h. After the mix-
ture was cooled to room temperature, water (10 mL) was added. The
product was extracted with ethyl acetate (10 mLꢁ3). The combined or-
ganic layers were dried and concentrated. Purification by chromatogra-
phy on silica gel (hexane/ethyl acetate, 30:1) provided 3-(1’,3’-dithian-2’-
ylidene)-N-methyl-2-phenyl-N-tosylpropanamide (4a, 68 mg, 0.16 mmol,
78%).
[9] Interesting gold-catalyzed reactions of sulfoxides with alkynes were
reported. In these reactions, highly p-acidic gold activated alkynes
to provide gold–carbene complexes as active species; see: J. Xiao,
X. Li, Angew. Chem. 2011, 123, 7364; Angew. Chem. Int. Ed. 2011,
50, 7226, and references therein.
[10] Carbocations stabilized by two adjacent sulfanyl groups were utiliz-
ed in organic synthesis; see: a) K. Narasaka, N. Arai, T. Okauchi,
Bull. Chem. Soc. Jpn. 1993, 66, 2995; b) T. Mukaiyama, H. Sugumi,
H. Uchiro, S. Kobayashi, Chem. Lett. 1988, 1291; c) J. Ichikawa, T.
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[11] For optimization of the reaction conditions, see the Supporting In-
formation.
[12] Proto-desilylation product 3 f’ (S-butyl 3-(1,3-dithian-2-ylidene)pro-
panethioate) was also obtained in 17% yield.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research (nos.
24685007, 23655037, 22106523, and 22406721, “Reaction Integration”)
from MEXT. Prof. Jun-ichi Yoshida and Dr. Toshiki Nokami (Kyoto Uni-
versity) are acknowledged for their help in evaluating the amount of
water in nitromethane. H.Y. is thankful for financial support from The
Uehara Memorial Foundation, the NOVARTIS Foundation for the Pro-
motion of Science, and the Takeda Science Foundation. K.M. acknowl-
edges JSPS for financial support.
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[19] The reaction with the ynamide proceeded smoothly without the ad-
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[20] The vinylic deuterium atom of 3a’ would be introduced according to
the mechanism shown, which can be promoted with the aid of
copper bromide. When the reaction was quenched earlier at 6 h,
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H. Matsubara, H. Yorimitsu, K. Oshima et al.
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cates that a hydrogen/deuterium exchange occurs under the reaction
conditions.
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[26] The most stable complex is lower in energy by only 0.7 kJmolÀ1
than M, and no acceptable transition states were obtained from the
most stable complex. Details of the most stable and other complexes
are provided in the Supporting Information.
Received: November 14, 2012
Revised: December 29, 2012
Published online: March 5, 2013
[21] GAUSSIAN 03, Revision E.01, Gaussian, Inc., Wallingford, CT,
2004. Full citation is provided in the Supporting Information.
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