Selective sulfenylative desulfonylation or decarbalkoxylation of α-sulfonyl malonates with DABCO or Bu3N: Reactivity and conformational analysis

Article abstract of DOI:10.1080/00397910902962852

The study on reactivity of severalαsubstituted αsulfonyl malonates toward 1,4-diazabicyclo[2.2.2]octane (DABCO) and Bu₃N is described. The reactivity with DABCO revealed the possible competition between decarbalkoxylation and unexpected desulfonylation, depending on the-substituent, because of sterical hindrance around the electrophilic centers (SO₂ and CO₂R). The derivatives with crowded α-substituents suffer selective desulfonylation, and a novel and efficient desulfonylation method can be proposed. The dependence of the reactivity ofα-sulfonyl malonates on the sterical hindrance around the electrophilic centers is confirmed by conformational analysis (Macromodel/MM2* and Mopac/MP3). The carbanionic mechanism is proved because the corresponding protonated, deuterated, and sulfenylated products were obtained by addition of the corresponding electrophilic agents. Bu3N showed itself to be a novel selective decarbalkoxylation agent for any-substituted-sulfonyl malonate.

Full text of DOI:10.1080/00397910902962852

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Selective Sulfenylative Desulfonylation
or Decarbalkoxylation of α-Sulfonyl
Malonates with DABCO or Bu3N:
Reactivity and Conformational Analysis
Claudio L. Donnici a , Elaine Henriques Teixeira Pereira a , Júlio C.
Dias Lopes a , Liliana Marzorati b & Blanka Wladislaw b
a Chemistry Department, Institute of Exact Sciences, Federal
University of Minas Gerais, Belo Horizonte, Brazil
b Institute of Chemistry, University of São Paulo, São Paulo, Brazil
Published online: 06 Jan 2010.
To cite this article: Claudio L. Donnici , Elaine Henriques Teixeira Pereira , Júlio C. Dias
Lopes , Liliana Marzorati & Blanka Wladislaw (2010): Selective Sulfenylative Desulfonylation or
Decarbalkoxylation of α-Sulfonyl Malonates with DABCO or Bu3N: Reactivity and Conformational
Analysis, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 40:3, 342-350
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Synthetic Communications1, 40: 342–350, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902962852
SELECTIVE SULFENYLATIVE DESULFONYLATION OR
DECARBALKOXYLATION OF a-SULFONYL MALONATES
WITH DABCO OR Bu3N: REACTIVITY AND
CONFORMATIONAL ANALYSIS
Claudio L. Donnici,1 Elaine Henriques Teixeira Pereira,1
1
Julio C. Dias Lopes, Liliana Marzorati,
2
´
and Blanka Wladislaw2
1Chemistry Department, Institute of Exact Sciences, Federal University of
Minas Gerais, Belo Horizonte, Brazil
2Institute of Chemistry, University of Sa˜o Paulo, Sa˜o Paulo, Brazil
The study on reactivity of several a-substituted a-sulfonyl malonates toward
1,4-diazabicyclo[2.2.2]octane (DABCO) and Bu3N is described. The reactivity with
DABCO revealed the possible competition between decarbalkoxylation and unexpected
desulfonylation, depending on the a-substituent, because of sterical hindrance around
the electrophilic centers (SO2 and CO2R). The derivatives with crowded a-substituents
suffer selective desulfonylation, and a novel and efficient desulfonylation method can be
proposed. The dependence of the reactivity of a-sulfonyl malonates on the sterical
hindrance around the electrophilic centers is confirmed by conformational analysis
(Macromodel/MM2Ã and Mopac/MP3). The carbanionic mechanism is proved because
the corresponding protonated, deuterated, and sulfenylated products were obtained by
addition of the corresponding electrophilic agents. Bu3N showed itself to be a novel
selective decarbalkoxylation agent for any a-substituted a-sulfonyl malonate.
Keywords: Conformational analysis; DABCO; decarbalkoxylation–desulfonylation; regioselectivity;
a-sulfonyl-malonates; tributylamine
INTRODUCTION
Activated compounds such as malonate esters, a-keto-esters, a-cyano esters,
and a-aryl(alkyl)sulfonyl esters find considerable utility in organic synthesis.[1,2] Tra-
ditionally, the malonic ester synthesis is one of the most important examples of the
synthesis of higher homologous esters, starting from malonic ester.[3] Nevertheless,
these conditions sometimes are not selective, and substrates containing either acid-
or base-sensitive groups cannot be employed in this hydrolysis–decarboxylation pro-
cess. Krapcho[3,4] developed an easier methodology for the dealkoxycarbonylation of
Received August 25, 2008.
´
ˆ
Address correspondence to Claudio L. Donnici, Departamento de Quımica, Instituto de Ciencias
Exatas, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, BH, MG, Brazil, 31270-901.
E-mail: cdonnici@terra.com.br
342

DESULFONYLATION OF SULFONYL MALONATES
343
Figure 1. Synthesized sulfonyl malonates.
malonate esters and other activated compounds: the use of anhydrous salts in
dipolar aprotic media such as dimethylsulfoxide (DMSO). a-Sulfenylated carboxylic
acids exhibit some noteworthy synthetic applications. These compounds are useful
for convenient two-step conversion of carboxylic acids to the corresponding
ketones;[5] a-sulfonyl carboxylic acids can also be used to generate the corresponding
a-keto esters and thioesters.[6,7] Interestingly, Corey, and Lowry reported an anionic
decarboxylation of a-sulfonyl carboxylic acids with retention of configuration.[8]
Triton-B showed itself to be a very efficient and mild agent for decarbalkoxylation
of a-sulfonylmalonates.[9] Miles and Chuang[10] reported that geminal esters treated
with 1,4-diazabicyclo[2.2.2]octane (DABCO) in refluxing xylene give decarbalkoxy-
lation products. However, preliminary studies showed that when diethyl a-sulfonyl-
malonates were treated with DABCO in refluxing toluene, there was competition
between decarboxylation and sulfenylation.[11] Despite that, just recently it became
possible to continue these investigations and to study this unusual reactivity, as
described herein. In fact, the removal of the sulfonyl group from carboxylic deriva-
tives is not so easy, even with the common desulfurative process using Raney
nickel,[12] as described by Shioiri and coworkers in the study of the reactivity of some
a-benzylsulfonyl acetates.[13] We report herein the results of our study on this dual
behavior of some derivatives of a-sulfonyl malonates (Fig. 1) toward DABCO and
tributylamine and the conformational analysis of these sulfonyl malonic derivatives.
The strong influence of the steric hindrance around the electrophilic center on the
reactivity is shown. Even though the desulfonylation could be effected using several
different methods (for example, thermal rearrangement,[14] catalyzed reaction with
Ru(II),[15,16] via radical-initiated cleavage,[17] by cathodic reduction,[18,19] by oxygen-
ation of an a-sulfonyl carbanion by molybdenum peroxide,[20] by coupling with
Grignard reagents in the presence of copper,[21] or by coupling with methyl=phenyl
lithium,[22] by treatment with TiCl4
or TiCl4-Sm[24]) in our oppinion, the novel
[23]
desulfonylation method with DABCO developed by us may be a good preparative
method for selective desulfonylation in crowded polyfunctionalized molecules.
RESULTS AND DISCUSSION
In the present work, several a-sulfonyl malonates (1–7) were synthesized
(Scheme 1) starting from the corresponding alkyl or phenyl malonates, which were
submitted as a suitable method for sulfenylation (generally NaH=DMSO
and MeSO2SMe or PhSO2SPh) and oxidation (H2O2=HOAc), generating the

344
C. L. DONNICI ET AL.
Scheme 1. Reagents and conditions: (a) NaH=DMSO; rt, MeSO2SMe, 77–85%; (b) H2O2=HOAc, reflux,
1.5 h, 62–88%; (c) acetone, ZnCl2, Ac2O, reflux, 24 h, 58%; (d) MeMgl=ethyl ether, CuCl, 30 min. rt,
H2SO4=H2O, 60%; (e) LDA, THF, ꢁ78ꢀC, MeSO2SMe, rt, 2 h, 89%; (f) NaH=DMSO, rt; PhSO2SPh,
86%; and (g) phenol, POCl3, 100ꢀC, 30 min, 50%.
corresponding sulfonyl malonates. In fact, the synthesis of the tert-butyl analog 3
started from preparation of the diethyl isopropylidenemalonate (40%), followed by
the reaction with methyl magnesium bromide in anhydrous diethyl ether (65%)
and sulfenylation with lithium diisopropylamide (LDA), (89%). For the synthesis
of the diphenyl ester 7, the diethyl methylmalonate was first submitted to the trans-
esterification reaction with phenol in POCl3 (40%).
The data related to the studies on the reactivity of several a-sulfonyl malonates
to to DABCO and tributylamine (Bu3N) are given in Table 1. With Bu3N, all
a-sulfonyl-malonates gave the decarbalkoxylation products, but under quite vigor-
ous conditions (110ꢀC, 48 h). Therefore, the Krapcho[4] and Triton-B[7] methodolo-
gies are milder. The reactivity of a-sulfonyl-malonates to DABCO revealed the
possible competition between decarbalkoxylation and desulfonylation: the methyl
1 and ethyl 2 derivatives gave only the decarbethoxylated products 1.1 and 2.1
(69–81%), but the a-benzyl derivative 4 led to partial decarbalkoxylation and desul-
fonylation (4.1 and 4.2: 49–51%). On the other hand, for the a-tert-butyl 3 and a-
phenyl 5 analogs, only the desulfonylation occurs (3.4–5.4: 68–83%). The influence
of the sterical hindrance exerted by the a-substituents on the reactivity of the a-sul-
fonylmalonates toward DABCO is strongly noticed, and it can be proposed that the
crowd around the electrophilic centers is an important factor, as suggested by Chou
and Chang[22] for nucleophilic attack on the sulfonyl sulfur atom. This hypothesis

DESULFONYLATION OF SULFONYL MALONATES
345
Table 1. a-Sulfonyl malonates 1–7, synthesized as described in Scheme 1, and malonates, studied with
DABCO and tributylamine
a-Sulfonyl
malonate
Product with
DABCO
Yield
(%)
Product with
tris-butylamine
Yield
(%)
Entry
MeCH(CO2Et)SO2Mea
1.1
78
72
68
68
72
61
68
MeC(CO2Et)2SO2Me
1
MeCSMe(CO2Et)SO2Meb
1.2
MeCH(CO2Et)SO2Me
1.1
1
81
69
MeCSPh(CO2Et)SO2Mec
1.3
EtCH(CO2Et)SO2Mea
2.1
EtC(CO2Et)2SO2Me
2
EtCSMe(CO2Et)SO2Meb
EtCH(CO2Et)SO2Me
2.1
2
2.2
EtCSPh(CO2Et)SO2Mec
2.3
a
t-BuCH(CO2Et)2
3.4
t-BuC(CO2Et)2SO2Me
3
t-BuCH(CO2Et)SO2Me
3.1
b
3
4
t-BuCSMe(CO2Et)2
3.2
67
70
42
49
a
PhCH2CH(CO2Et)2
4.1
PhCH2C(CO2Et)2SO2Me
4
PhCH2CH(CO2Et)SO2Me
4.1
PhCH2CH(CO2Et)SO2Mea
4.2
51
83
62
72
68
62
60
a
PhCH(CO2Et)2
5.4
PhC(CO2Et)2SO2Me
5
PhCH(CO2Et)SO2Me
5.1
b
PhCSMe(CO2Et)2
5.2
68
65
5
6
7
PhCH(CO2Et)SO2Pha
6.1
PhCH(CO2Et)SO2Ph
6.1
PhC(CO2Et)2SO2Ph
6
PhCSMe(CO2Et)SO2Phb
6.2
MeCH(CO2Ph)SO2Meb
MeCH(CO2Ph)SO2Me
7.1
MeC(CO2Ph)2SO2Me
7
7.1
MeCH(CO2Ph)SO2Mec
7.2
60
a,b,cAfter reaction with DABCO and addition of water,a MeSO2SMe,b or PhSO2SPh.c
dAll products were characterized by IR, NMR, and mass spectroscopy; isolated yields of products after
column chromatography.
can be corroborated by the fact that the diethylmalonate 6, with steric crowd around
the sulfone group, suffered decarbalkoxylation, generating the corresponding
decarbalkoxylated products 6.1 (72% yield). The anionic mechanistic pathway can
be confirmed by addition of water, or deuterated water, after the decarbalkoxylation=
desulfonylation step. Moreover, when electrophilic sulfenylating agents, such as
MeSO2SMe or PhSO2SPh, were added to the reaction mixture, the sulfenylated
products were obtained in quite reasonable yields (62–72%).
Finally, a better comprehension of steric hindrance influence on the reactivity
could be achieved by a theoretical approach, for example, through the data obtained
from the mechanical molecular calculations (Macro Model=MM2ꢂ), which showed

346
C. L. DONNICI ET AL.
different conformational equilibria for the different a-sulfonyl-malonates. The
analysis of the preferential conformations for the derivatives 1 (a-methyl), 2
(a-ethyl), 6 (a-phenyl-phenyl sulfonyl), and 7 (a-methyl diphenylester) revealed that
those conformers do not have any steric hindrance around the carboxyl groups,
and the decarbalkoxylation is favored. For the derivatives 3 (a-tert-butyl) and 5
(a-phenyl), the most stable conformations show that the bulky a-tert-butyl or a-phe-
nyl groups are closer to the carbalkoxy groups. Additionally, the methylsulfonyl
group is very close to one of these carbalkoxy groups, which creates steric hindrance
around it, leading preferably to the desulfonylation. For the benzyl derivative 4,
there are two major conformers 4a and 4b: the most stable conformer 4a shows that
only one of the carbalkoxy groups is accessible and the decarbalkoxylation is
favored, but the conformer 4b shows steric hindrance around both carbomethoxy
groups favors the desulfonylation. In fact, for 4, both partial decarbalkoxylation
and desulfonylation occur. It is remarkable that the analysis of dihedral angles
(SꢁCꢁCꢁO: a or b angles) of the studied sulfonyl-malonates generally demonstrate
the presence of gauche configuration (ca. 60–90ꢀ), which can lead to a hiperconjuga-
tive interaction pco=rꢂcꢁs, which makes the CꢁO bond longer, more polar, and
maybe more reactive toward nucleophilic attack. The results obtained from a
semi-empirical method (Mopac=PM3) allowed us to calculate the partial atomic
charges from the functional groups CO2R and SO2R. These data could generate
the building of electrostatic potential surfaces, which revealed the existence of a
charge-transfer interaction between the molecular orbitals of the sulfonyl and car-
bonyl groups, OSO ! pco. Besides the analysis of some dihedral angles from selected
2
malonates, 1 and 3 present angles around 60ꢀ between sulfonyl oxygen. The carbonyl
carbon also suggests the occurrence of an OSO ! pco interaction, as already
2
described.[25] Moreover these semi-empirical data about the sum of the partial
atomic charges of the functional groups CO2R and SO2R showed that the sulfonyl
group must present the higher positive-density charge. Consequently, the sulfonyl
group should be the preferred electrophilic center for the DABCO’s nucleophilic
attack, even though the preferential attack of DABCO is over the carbalkoxyl
groups. This contradiction might be explained by the fact that carbonyl carbon
has sp2 geometry, which is more sterically acessible, whereas the sulfur atom of
the sulfone group has sp3 geometry, less sterically acessible to DABCO’s attack.
Hence, the desulfonylation only occurs when there is a great steric hindrance around
the carbalkoxy groups and the a-sulfone group is available, such as for 3 and 5. A
global analysis of the results suggests that in the mechanism of decarbalkoxylation
by DABCO, the nucleophilic attack should be on the carbonyl carbon atom, because
the dicarbophenoxy derivative 6 suffers decarbalkoxylation, even with the phenyl
group being attached to the carboxylic group. It is difficult to propose a nucleophilic
mechanism for DABCO on a high eletronic density Csp2 from an aromatic ring. For
the mechanism of desulfonylation, it can be proposed that the nucleophilic attack by
DABCO occurs on the alkyl group attached to the sulfone and not on the sulfonyl
sulfur atom. All these results reveal that in the reactivity of a-sulfonyl-malonates
with DABCO, leading to decarbalkoxylation or desulfonylation, the steric factor
is most important for the regioselectivity of DABCO’s nucleophilic attack. If there
is a very bulky group, only the desulfonylation occurs. Finally, it is noteworthy to
point out that because it is rather easy to functionalize any molecule with a

DESULFONYLATION OF SULFONYL MALONATES
347
sulfonyl-malonate group, if this group was a crowded one, this novel desulfonylation
method with DABCO might be a good preparative method for selective removal of
sulfonyl group and possible subsequent functionalization by electrophiles in crowded
polyfunctionalized molecules.
EXPERIMENTAL
General
Melting points were determined on a Mettler apparatus and were uncorrected.
Starting materials were of the highest commercial quality and were used without
further purification. 1H and 13C NMR spectra were recorded on a Bruker instrument
(AC-200 MHz) in CDCl3 using tetramethylsilane (TMS) as an internal standard.
Fourier transform–infrared FT-IR spectra were recorded on a Mattson instrument,
model Galaxy 3000. Microanalyses were carried out on a Perkin-Elmer CHNS 2400
elemental analyzer.
Compounds 1–7 were prepared from the corresponding malonic diacid or ester
derivatives as starting materials according to the synthetic methodology described in
Scheme 1. These sulfonylmalonates were obtained, after purification and isolation, in
good global yields, and the analysis of the spectroscopic data of the pure compounds
are given.
General Procedure for the Reaction of a-Sulfonylmalonates 1–7
with DABCO
A mixture of sulfonylmalonate (10 mmol) and DABCO (50 mmol) in dry tolu-
ene (15 mL) was stirred under reflux for 8 h and cooled to room temperature. Water
(2 mL) was added, and the mixture was filtered and concentrated under reduced
pressure. Silica-gel chromatography (hexane–ethylacetate 8:2 vol=vol) gave the pur-
ified products. The reactions underwent subsequent sulfenylation after reaction with
DABCO, MeSO2SMe, or PhSO2SPh (10.5 mmol). The mixture was stirred for 6 h at
rt; and the following workup is described.
General Procedure for the Reaction of a-Sulfonylmalonates 1–7
with Tributylamine
A mixture of sulfonyl-malonate (1.0 mmol), tributylamine (18 mmol, 4.0 mL),
and p-xylene (3 mL) was refluxed for 48 h, cooled to rt, poured into water
(10 mL), extracted with CH2Cl2 (3 ꢃ 10 mL), dried over anhydrous sodium sulfate,
and concentrated under reduced pressure to give a crude product, which was purified
by silica-gel column chromatography to afford the decarbalkoxylated product.
Selected Data
Diethyl methyl(methylsulfonyl)malonate (1). Yellowish oil; IR nmax
1
(KBr)=cmꢁ1 1740, 1322, 1152. H NMR (CDCl3) d 4.34 (4H, q, J 7.2, OCH2),
3.29 (3H, s, CH3SO2), 1.90 (3H, s, CH3C), 1.35 (6H, t, J 7, CH3diester), 1.16 (3H,

348
C. L. DONNICI ET AL.
t, J 7.2, CH3 alkyl); 13C NMR (CDCl3) d 164.49, 75.52, 62.86, 39.56, 14.10, 13.75.
Anal. calcd. for C9H16O6S: C, 42.85; H, 6.40. Found: C, 42.79; H, 6.43.
Diethyl ethyl(methylsulfonyl)malonate (2). Yellowish oil; IR nmax (KBr)=
1
cmꢁ1 1739, 1323, 1153. H NMR (CDCl3) d 4.38 (4H, q, J 7.2, OCH2), 3.24 (3H, s,
MeSO2), 2.42 (2H, q, J 7.2, CCH2), 1.33 (6H, t, J 7.2, CH3 diester), 1.16 (3H, t, J 7.2,
CH3 alkyl); 13C NMR (CDCl3) d 164.26, 79.64, 62.60, 40.33, 22.75, 13.45, 9.20. Anal.
calcd. for C10H18O6S: C, 45.10; H, 6.82. Found: C, 45.13; H, 6.85.
Diethyl 1,1-dimethylethyl-(methylsulfonyl)malonate (3). Yellowish oil;
1
IR nmax (KBr)=cmꢁ1 1750, 1370, 1310, 1190 cmꢁ1; H NMR (CDCl3) d 4.20 (4H,
q, J 7.0, OCH2), 3.49 (3H, s, MeSO2), 1.43 (6H, t, J 7.0, CH3 diester), 1.12 (9H, t,
tert-Bu); 13C NMR (CDCl3) d 168.43, 107.45, 63.50, 60.80, 43.43, 14.10, 13.83. Anal.
calcd. for C12H22O6S: C, 48.96; H, 7.54. Found: C, 49.01; H, 7.58.
Diethyl benzyl(methylsulfonyl)malonate (4). Pale yellow crystals, mp
1
55–56ꢀC; IR nmax (KBr)=cmꢁ1 1727, 1310, 1146. H NMR (CDCl3) d 7.4–7.2 (5H,
m, HAr), 4.3–4.0 (4H, m, OCH2), 3.74 (2H, s, CH3SO2), 3.19 (3H, s, CH2Ph), 1.16
(3H, t, J 7.2, CH3 alkyl); 13C NMR (CDCl3) d 164.02, 133.64, 130.63, 127.94,
127.33, 81.00, 62.78, 40.27, 33.49, 13.36. Anal. calcd. for C15H20O6S: C, 54.86; H,
6.14. Found: C, 54.89; H, 6.18.
Diethyl phenyl(methylsulfonyl)malonate (5). Colorless crystals, mp 87–
1
88ꢀC; IR nmax (KBr)=cmꢁ1 1760, 1310, 1130. H NMR (CDCl3) d 7.6–7.5 (2H, m,
H
Arꢁortho), 7.5–7.4 (3H, m, HArꢁpara), 4.43 (4H, q, J 7.0, OCH2), 3.03 (3H, s,
CH3SO2), 1.35 (6H, t, J 7.0, CH3 alkyl); 13C NMR (CDCl3) d 164.15, 129.97,
128.86, 128.80, 128.58, 85.05, 63.52, 40.71, 13.84. Anal. calcd. for C14H18O6S: C,
53.49; H, 5.78. Found: C, 53.54; H, 5.75.
Diethyl phenyl(phenylsulfonyl)malonate (6). Colorless crystals, mp 57–
59ꢀC; IR nmax (KBr)=cmꢁ1 1750, 1325, 1140;1H NMR (CDCl3) d 7.6–7.2 (10H, m,
H
Ar), 4.37 (4H, q, J 7.2, OCH2), 1.30 (6H, t, J 7.2, CH3 alkyl); 13C NMR (CDCl3)
d 163.74, 136.68, 133.66, 131.20, 130.16, 129.47, 129.23, 127.99, 127.62, 86.07, 63.08,
13.69. Anal. calcd. for C19H20O6S: C, 60.62; H, 3.59. Found: C, 60.67; H, 3.61.
Diphenyl methyl(methylsulfonyl)malonate (7). Colorless crystals, mp
1
100–101ꢀC; IR nmax (KBr)=cmꢁ1 1750, 1320, 1140. H NMR (CDCl3) d 7.5–6.7
(10H, m, HAr), 3.43 (3H, s, CH3SO2), 2.20 (3H, s, CH3 alkyl); 13C NMR (CDCl3)
d 163.70, 149.90, 129.83, 127.01, 120.86, 76.76, 40.05, 14.88. Anal. calcd.
C17H16O6S: C, 58.61; H, 4.63. Found: C, 58.66; H, 4.67.
ACKNOWLEDGMENTS
`
The authors thank FAPEMIG (Fundac¸a˜o de Amparo a Pesquisa de Minas
Gerais: EDT 479=07, CEX PPM III 0207=09) and CNPq (Conselho Nacional de
´
´
Desenvolvimento Cientıfico e Tecnologico) for financial support and fellowships.

DESULFONYLATION OF SULFONYL MALONATES
349
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