Reactions of acetoxy(methylthio)carbene and S-methyl thiopyruvate

Article abstract of DOI:10.1139/v05-151

Thermolysis of 2-acetoxy-5,5-dimethyl-2-methylthio-Δ³-1,3, 4-oxadiazoline in benzene at 110°C afforded S-methyl 2-oxopropanethioate (S-methyl thiopyruvate) from 1,2-acyl migration in acetoxy(methylthio)carbene, as well as S,S-dimethyl 2-acetoxy-2-methylpropanebis(thioate) and S-methyl 2-acetoxy-2-(methylthio)butanethioate. The latter two, both composed of two carbene units, are derived from reaction of the first product with the carbene.

Full text of DOI:10.1139/v05-151

1
360
Reactions of acetoxy(methylthio)carbene and
1
S-methyl thiopyruvate
Arkadiusz Klys, Wojciech Czardybon, John Warkentin, and Nick Henry Werstiuk
3
Abstract: Thermolysis of 2-acetoxy-5,5-dimethyl-2-methylthio-∆ -1,3,4-oxadiazoline in benzene at 110 °C afforded S-
methyl 2-oxopropanethioate (S-methyl thiopyruvate) from 1,2-acyl migration in acetoxy(methylthio)carbene, as well as
S,S-dimethyl 2-acetoxy-2-methylpropanebis(thioate) and S-methyl 2-acetoxy-2-(methylthio)butanethioate. The latter two,
both composed of two carbene units, are derived from reaction of the first product with the carbene.
Key words: acetoxy(methylthio)carbene, carbene, S-methyl thiopyruvate, rearrangement.
3
Résumé : La thermolyse de la 2-acétoxy-5,5-diméthyl-2-méthylthio-∆ -1,3,4-oxadiazoline, dans le benzène à 110 °C,
conduit à la formation du 2-oxopropanethioate de S-méthyle (thiopyruvate de S-méthyle) qui provient d’une migration
1
,2 d’un groupe acyle dans l’acétoxy(méthylthio)carbène, ainsi que du 2-acétoxy-2-méthylpropanebis(thioate) de S,S-di-
méthyle et du 2-acétoxy-2-(méthylthio)butanethioate de S-méthyle. Ces deux derniers produits, qui sont formés de deux
unités de carbène, dérivent de la réaction du premier produit avec le carbène.
Mots clés : acétoxy(méthylthio)carbène, carbène, thiopyruvate de S-méthyle, réarrangement.
Klys et al. 1364
Introduction
Thermolysis of 4 in benzene gave 6, 8, and 10. Formation
of 6 probably occurs by concerted acetyl transfer from oxy-
gen to the carbene carbon, as was found for other acyloxy
carbenes (2–8). Formation of compounds 6 and 8 is most
easily formulated in terms of zwitterionic intermediates (9)
formed by reaction of 6 with 5 (Scheme 4).
Recently, we reported the generation of acetoxy-
methoxy)carbene (2) by thermolysis of 2-acetoxy-5,5-
(
3
dimethyl-2-methoxy-∆ -1,3,4-oxadiazoline (1, Scheme 1)
(
1). Thermolysis of 1 is relatively complex, affording the
carbene as well as 2-diazopropane as shown in the abbrevi-
ated Scheme 1. The carbene did not fragment to radicals, but
it rearranged to methyl pyruvate (3) by acetyl migration
from oxygen to the carbene carbon, a known process for
other acetoxycarbenes (2–7).
We now report that thermolysis of 4 does not afford 2-
diazopropane, but gives acetoxy(methylthio)carbene (5) un-
der the same conditions. The latter also undergoes a 1,2-acyl
shift, but the final products contained not only the rearrange-
ment product (6), analogous to 3, but also two additional
compounds. In particular, 4 afforded 6 (the known
propanethioic acid, 2-oxo-, S-methyl ester, also known as S-
methyl thiopyruvate) and two additional compounds (8 and
The first formed S-methyl thiopyruvate (6) reacts with the
carbene preferentially at the ketonic carbon, which must be
more electrophilic than the thioester carbonyl carbon. Most
likely, transfer of an acyl group occurs through a five-
membered transition state to afford the solid 8. Attack at the
thioester carbonyl group, followed by an analogous transfer
affords the liquid 10.
The structure of 8 was confirmed by means of single crys-
4
tal X-ray diffraction and that of 10 was determined by
NMR and mass spectrometry, with reference to the spectra
of 8, chemical shift tables, (10) and chemical shift predic-
tions from the ChemDraw program. Good models for 8 were
not found. Attack of dialkoxycarbenes on carbonyl groups
has been observed (11), but we are not aware of analogous
reactions of acetoxy(methylthio)carbene. Transfer of an
acetyl group in zwitterionic intermediates such as 7 and 9
also appears to be unprecedented.
1
0), each made up of two carbene units (Scheme 2).
The precursor of was 2-acetoxy-5,5-dimethyl-2-
5
3
methylthio-∆ -1,3,4-oxadiazoline (4) prepared from 9 ac-
cording to Scheme 3.
Received 27 April 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 22 October 2005.
2
3
A. Klys, W. Czardybon, J. Warkentin, and N.H. Werstiuk. Department of Chemistry, McMaster University, 1280 Main St. W.,
Hamilton, ON L8S 4M1, Canada.
1
This article is part of a Special Issue dedicated to organic reaction mechanisms.
Corresponding author (e-mail: warkent@mcmaster.ca).
Corresponding author (e-mail: werstiuk@mcmaster.ca).
Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data,
2
3
4
Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada. DUD 4036. For more information
on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 274864 contains the crystallographic data
for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
Can. J. Chem. 83: 1360–1364 (2005)
doi: 10.1139/V05-151
© 2005 NRC Canada

Klys et al.
1361
Scheme 1.
MeO OAc
110 °C
110 °C
MeO OAc
+
Me) C=N=N
-
N
O
N + Me CO +
2 2
(
..
2
N
Me
2
Me
1
MeO CCOCH3
2
3
Fig 1. Different views of the gauche and antigauche conforma-
tions of the transition state for formation of diazopropane from
Scheme 2.
O
4
, with hydrogen atoms omitted for clarity: (a) antigauche con-
MeS
O
MeS
O
MeS
Me
O
O
formation; (b) gauche conformation. (c) ORTEP rendition of 8.
110 °C
PhH
..
Me
O
(a)
(b)
N
N
O
Me
5
6
(14%)
4
O
Me
O
OCOMe
Me
MeS
MeS
+
+
MeS
Me
SMe
O
O
O
O
(25%)
8
10 (10%)
Although the zwitterions 7 and 9 are conjectural interme-
diates, there is some precedent for formation of persistent
charge-separated structures from reactions of a nucleophilic
carbene. Imidazol-2-ylidenes react with tetracoordinate sili-
con compounds (12), with stannylenes (13), and with a
plumbylene (14), for example, to produce species that are
zwitterions or resonance hybrids that have zwitterionic con-
tributors. An imidazol-2-ylidene also attacks CS to form an
2
isolable zwitterion (15) (Scheme 5).
The reasons for the different thermolysis behaviours of 1
and 4 are not well understood but some potential causes of
the differences were examined by means of computation.
First, the free energy barriers to fragmentation of the two
oxadiazolines might be different, either favouring fragmenta-
tion of 1 in the sense that affords 2-diazopropane (7) or fa-
vouring fragmentation of 4 in the sense that affords 5. Either
could account for the fact that 1 affords 2-diazopropane
while 4 does not. To account for the fact that 4 affords 8 and
1
0 whereas 1 does not produce analogous 2:1 products, one
could imagine that the reactivity of 5 toward 6 could be
higher than that of 2 toward 3.
(
c)
Optimized equilibrium geometries and transition states
were obtained at the Becke3PW91/6-31+G** (16a) level,
which includes the three-parameter Becke–Perdew–Wang
(
Becke3PW91) exchange-correlation potential (16b) using
either GAUSSIAN 98 (16a) or GAUSSIAN 03 (16c).
Computation of the free energy barriers for fragmentation
of the oxadiazolines at 110 °C was carried out for both the
gauche and antigauche conformations, where gauche and
antigauche are defined in Fig. 1.
For 1, the antigauche conformation is more favourable for
diazopropane formation (G‡ = 24.6 kcal mol , 1 cal =
–1
4
.184 J), while the gauche conformation is favoured for
‡
–1
ylide formation (G = 26.9 kcal mol ) (7). For 4, the
©
2005 NRC Canada

1
362
Can. J. Chem. Vol. 83, 2005
Scheme 3.
MeS OAc
Me
Me
Pb(OAc)4
MeS OAc
N
O
N2 + Me CO +
NNHCOSMe
2
.
.
N
Me
2
CH Cl2
5
Me
11
4
Scheme 4.
O
O
Me
O
O
Me
Me 5
MeS
MeS
MeS
O
SMe
+
SMe
4
..
MeS
O_
O
O
_
H C
O
O
3
Me
Me
O
5
6
7
8
O
O
O
O
MeS
MeS
O
5
Me
O
6
Me
+
MeS
MeS
O
Me
Me
9
O
O
10
1
10 °C by 1,2-acyl migration to S-methyl thiopyruvate. The
Scheme 5.
latter reacts with the carbene at both carbonyl groups to
afford S,S-dimethyl 2-acetoxy-2-methylpropanebis(thioate)
and S-methyl 2-acetoxy-2-(methylthio)butanethioate. Those
products probably arise by acetyl group transfer, through
five-membered transition states, in zwitterionic intermedi-
ates. Support for the formation of the carbene via cyclo-
reversion of an oxadiazoline precursor to a carbonyl ylide,
and rearrangement of the carbene by concerted acyl group
transfer, was sought by means of computation.
Me
Me
Me
Me
Me
N
N
-
CS
:
+
CS2
+
2
N
N
Me
Me
Me
antigauche conformation is again favoured for diazopropane
formation (G
‡
–1
= 20.2 kcal mol ), while the gauche
diazopropane
conformation is slightly favoured for ylide formation
–
1
(
22.5 kcal mol ).
It is clear that the computations do not predict all of the
Experimental
experimental results, which indicate that 1 should fragment
to 2-diazopropane at 110 °C (which it does) and that 4
should also do so (which it does not). However, the free en-
ergy differences for the two possible thermolysis paths open
General
1
H NMR spectra were obtained from solutions in C D or
6
6
–
1
CDCl , with Bruker spectrometers operating at 200 or
to 1 and 4 are only about 2 kcal mol , which could easily be
within the combined errors in the computations (Table 1).
Calculated free energies of activation for rearrangement
3
6
00 MHz, unless otherwise indicated, and are referenced to
the signal at 7.16 attributed to C HD or to CHCl3 at
7
6
5
1
3
.26 ppm. C NMR spectra, run at 50.9 or 150.9 MHz, in
(
1
1,2-acyl migration) of the carbenes gave 16.6 and
–
1
C D or CDCl , are referenced to the center line of the sol-
3.1 kcal mol for 2 and 5, respectively. This result sug-
6
6
3
vent triplet at 128.1 or 77.2 ppm, respectively. Mass spectra
were run with a Micromass/Waters GCT time-of-flight in-
strument. GC separations were performed with a 6 ft ×
gests that 5 probably rearranges more rapidly than 2. As a
reasonable explanation for the fact that 5 affords not only 6,
but also 8 and 10, we suggest that 5 is significantly more re-
active toward 6 than 2 is toward 3. The methylthio group is
poorer (compared to the methoxy group) at conjugative sta-
bilization of both a carbene and a carbonyl group and that
feature could make both 5 and 6 more reactive than their oxy
counterparts (2 and 3). Ketoester 6 can be regarded as some-
what like an α-diketone rather than a simple α-ketoester,
with a consequent increase in reactivity of both carbonyl
groups toward nucleophiles.
4
.0 mm (1 ft = 0.0348 m) (internal) column containing 10%
–
1
OV 101. The helium flow rate was 40 mL min and the
temperature program was 60 °C for 3 min, then 5 °C / min
to 200 °C. IR spectra were taken with neat samples on a
Bruker Tensor 27 FT IR instrument equipped with a Harrick
ATR accessory and a ZnSe crystal.
Yields are calculated in the usual way, as 100 (mol product)/
mol 4, with the different stoichiometries involved in the for-
mation of 6, 8, and 10 being taken into account. For exam-
ple, 2 mol of 4 are required to make 1 mol of 8. About 80%
of the potential carbene units from 4 were accounted for by
products 6, 8, and 10.
Summary
Acetoxy(methylthio)carbene rearranges in benzene at
©
2005 NRC Canada

Klys et al.
1363
Table 1. Energies expressed as total energies, zero point corrected total energies, free energies, and
enthalpies calculated at the B3PW91/6-31+G** level.
(
a) MeSOAc system
gauche
antigauche
Thermochemical
parametera
TS to
ylide
TS to
diazopropane
TS to
ylide
TS to
diazopropane
TS carbene (W)
to pyruvate
∆
∆
∆
∆
∆
∆
∆
Eelecb
Eoc
E110 °Cd
H25 °Ce
H110 °C
G25 °Cf
G110 °C
27.10
24.31
25.19
25.06
25.19
23.10
22.52
25.91
23.43
24.28
24.19
24.28
21.85
21.16
27.63
24.75
25.65
25.51
25.65
23.64
23.09
24.88
22.41
23.28
23.19
23.28
20.86
20.18
13.15
12.41
12.05
12.14
12.05
12.86
13.08
(
b) MeOOAc system
gauche
antigauche
Thermochemical
parametera
TS to
ylide
TS to
diazopropane
TS to
ylide
TS to
diazopropane
TS carbene (W)
to pyruvate
∆
∆
∆
∆
∆
∆
∆
Eelecb
Eoc
E110 °Cd
H25 °Ce
H110 °C
G25 °Cf
34.53
31.43
37.22
29.84
16.71
30.64
32.24
28.78
28.06
32.17
32.78
27.18
28.07
15.96
15.57
31.98
32.24
28.33
26.93
29.53
28.01
27.45
34.05
32.74
28.24
29.86
27.97
28.07
25.51
24.58
15.70
15.57
16.34
16.60
G110 °C
30.16
a
–1
At the B3PW91/6-31+G** level in kcal mol .
Eelec is the uncorrected total energy.
b
c
E = E + ZPE.
o
elec
d
E = E + E + Erot + Etrans
.
o
vib
e
H = E + RT.
G = H – TS.
f
3
rated 8 from 10. The helium flow rate was 40 mL min–1 and
the temperature program was 60 °C for 3 min, then 5 °C / min
to 200 °C. The total yield of products was determined by
analysis of the H NMR spectrum of the crude product con-
taining an internal standard. The same yields were calculated
from GC data.
2
-Acetoxy-5,5 dimethyl-2-methylthio-⌬ -1,3,4-
oxadiazoline (4)
To a stirred solution of Pb(OAc) (8.4 g) in CH Cl at
4
2
2
1
0
3
°C was added hydrazone 11 (17) (2 g, 13.7 mmol). After
h the solution was filtered through Celite to remove
Pb(OAc) and the organic filtrate was washed with 100 mL
2
of aq. (5%) NaHCO before it was dried. Evaporation of the
3
solvent left a yellow oil (1.9 g, 68%) that was a mixture of 4
and an acyclic isomer. Very fast chomatography on “old”
neutral alumina (hydrated in an open beaker for 3 days) in a
Propanethioic acid, 2-oxo-, S-methyl ester (6) (18)
Pale yellow oil. Yield: 14%. IR (cm ): 1722, 1670. H
–1
1
13
NMR (200 MHz, CDCl ) δ : 2.35 (s, 3H), 2.43 (s, 3H).
C
3
1
5 cm × 3 cm column with hexane–EtOAc (95:5), collection
of the first fraction, evaporation of the solvent immediately,
NMR (50.3 MHz, CDCl ) δ : 11.42, 24.06, 191.86, 193.20.
3
and recrystallization from pentane at –20 °C gave 1.3 g
1
S,S-Dimethyl 2-acetoxy-2-methylpropanebis(thioate) (8)
(
46%) of colorless crystalline 4. H NMR (200 MHz,
–
1
White solid. Yield: 25%. IR (cm ): 1753, 1697, 1663.
The C=O stretching frequencies of esters, in nonpolar sol-
CDCl ) δ : 1.56 (s, 3H), 1.65 (s, 3H), 2.13 (s, 3H), 2.41 (s,
3
1
3
3
2
H). C NMR (50.3 MHz, CDCl ) δ : 13.55, 21.62, 23.35,
3
vents, are in the range 1735–1750 cm (19). 1H NMR
–
1
3.61, 125.72, 133.16, 166.98. MS (CI, NH ) m/z calcd. for
3
+
(200 MHz, CDCl ) δ : 1.88 (s, 3H), 2.22 (s, 3H), 2.32 (s,
C H N O S: 205.0642 (M + H) ; found: 205.0647.
3
7
13
2
3
1
6
6
H). H NMR (200 MHz, C D ) δ : 1.72 (s, 3H), 1.84 (s,
6 6
1
H), 1.96 (s, 3H). H NMR chemical shifts (CDCl ) pre-
3
1
3
Thermolysis of 4
dicted with the use of ChemDraw: 1.86, 2.01, 2.27.
C
Oxadiazoline 4 (204 mg, 1 mmol) in dry benzene (5 mL)
was heated for 24 h at 110 °C. The solvent was then evapo-
rated in vacuo and products 7 and 8 were separated by radial
chromatography on a 2 mm silica plate with hexane–EtOAc
NMR (50.3 MHz, CDCl ) δ : 11.4 (SCH ), 21.2 (CH or
3 3 3
CH CO ), 21.4 (CH or CH CO ), 91.5 (quat. carbon), 168.6
3
2
3
3
2
(O CCH ), 195.4 (O CSCH ) (assignments based on
2
3
2
3
1
3
ref. 10). C NMR chemical shifts (CDCl ) predicted with
3
(
(
95:5). Product 6 was separated by GC with a 6 ft × 4.0 mm
internal) column containing 10% OV 101, which also sepa-
the use of ChemDraw: 15.5, 17.2, 17.3, 117.1, 171.0, 197.6.
HR-MS (CI, NH ) m/z calcd. for C H O S : 237.0255 (M +
3
8
13
4
2
©
2005 NRC Canada

1
364
Can. J. Chem. Vol. 83, 2005
+
H) ; found: 237.0255. The structure of 8 was confirmed by
9. H. Zhou, G. Mloston, and J. Warkentin. Org. Lett. 7, 487
(2005).
0. H.-D. Kalinowski, S. Berger, and S. Braun. Carbon-13 NMR
spectroscopy. Wiley, New York. 1988. pp. 193–198.
means of single crystal X-ray diffraction.
1
S-Methyl 2-acetoxy-2-(methylthio)butanethioate (10)
Pale yellow oil. Yield: 10%. IR (cm ): 1754. H NMR
200 MHz, CDCl ) δ : 2.12 (s, 3H), 2.24 (s, 3H), 2.36 (s,
H), 2.42 (s, 3H). H NMR chemical shifts (CDCl ) pre-
dicted with the use of ChemDraw: 2.01, 2.09, 2.09, 2.27.
NMR (50.3 MHz, CDCl ) δ : 12.4 (CH S or CH SC=O),
2.6 (CH S or CH SC=O), 21.0 (CH CO ), 26.3 (CH C=O),
8.6 (quat. carbon), 169.1 (O CCH ), 193.9 (O=CSCH or
O=CCH ), 194.7 (O=CSCH or O=CCH ) (assignments
based on ref. 10). C NMR chemical shifts (CDCl ) pre-
dicted with the use of ChemDraw: 7.6, 14.8, 15.0, 16.6,
–
1
1
11. (a) M. Dawid and J. Warkentin. Can. J. Chem. 81, 598 (2003);
b) M. Dawid, G. Mloston, and J. Warkentin. Chem. Eur. J. 8,
2184 (2002); (c) P.C. Venneri and J. Warkentin. Can. J. Chem.
8, 1194 (2000).
2. N. Kuhn, T. Kratz, D. Bläser, and R. Boese. Chem. Ber. 128,
45 (1995).
(
(
3
3
1
3
7
1
3
C
1
1
3
3
3
2
1
9
3 3 3 2 3
3. A. Schäfer, M. Weidenbruch, W. Saak, and S. Pohl. J. Chem.
Soc. Chem. Commun. 1157 (1995).
14. F. Stabenow, W. Saak, and M. Weidenbruch. Chem. Commun.
(Cambridge), 1131 (1999).
2
3
3
3
3
3
1
3
3
1
5. N. Kuhn, H. Bohnen, and G. Henkel. Z. Naturforsch. B, 49b,
1
30.4, 171.0, 196.5, 206.0. MS (CI, NH ) m/z: 254 (M +
1473 (1994).
3
+
+
NH ) , 100%, 237 (M + H) , 12%. HR-MS (CI, NH )
16. (a) M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria,
M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgom-
ery, Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M.
Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J.
Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson,
P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck,
K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz,
A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe,
P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres,
C. Gonzalez, M. Head-Gordon, E.S. Replogle, and J.A. Pople.
GAUSSIAN 98 [computer program]. Gaussian, Inc., Pitts-
burgh, Penn. 1998; (b) J.P. Perdew and Y. Wang. Phys. Rev. B,
4
3
+
m/z calcd. for C H NO S : 254.0521 (M + NH ) ; found:
54.0524.
8
16
4
2
4
2
Search for 2-diazopropane
Phenol generates the stable isopropyl phenyl ether (1)
from 2-diazopropane. Thermolysis of oxadiazoline 4 (0.20 g,
.98 mmol) in benzene (1 mL) in the presence of phenol
0
(
0.184 g, 1.96 mmol) and analysis of the crude reaction mix-
ture by gas chromatography, with authentic ether as the ref-
erence in hand, showed at most a trace of the ether. Mass
spectrometry of the same crude product and a careful search
for m/z = 136.19 (C H O) failed to show any signal above
noise. The detection limit by this method was estimated to
be well below 1%.
9
12
4
5, 13244 (1992); (c) M.J. Frisch, G.W. Trucks, H.B. Schlegel,
G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery,
Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G.
Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E.
Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R.
Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi,
C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A.
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Acknowledgements
The authors are grateful to Professor Jim Britten, who car-
ried out the crystallography, and to the Natural Sciences and
Engineering Research Council of Canada (NSERC) for fi-
nancial support.
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2005 NRC Canada