A triple Newman-Kwart rearrangement at a fully substituted benzene ring

Article abstract of DOI:10.1515/znb-2009-11-1250

The Newman-Kwart rearrangement of O-thiocarbamates to S-thiocarbamates is a versatile method for the conversion of phenols into thiophenols. The triple rearrangement of the O-trithiocarbamate 2,4,6-triacetyl-1,3,5-tri- (dimethylthiocarbamoyloxy)benzene (2

Full text of DOI:10.1515/znb-2009-11-1250

A Triple Newman-Kwart Rearrangement
at a Fully Substituted Benzene Ring
Hubert Theila, Roland Fro¨hlichb, and Thorsten Glasera
a Fakulta¨t fu¨r Chemie, Universita¨t Bielefeld, Universita¨tsstr. 25, D-33615 Bielefeld, Germany
b Organisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t D-Mu¨nster, Corrensstr. 40,
48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. Thorsten Glaser. Fax: +49-(0)521-106-6003.
E-mail: thorsten.glaser@uni-bielefeld.de
Z. Naturforsch. 2009, 64b, 1633 – 1638; received September 16, 2009
Dedicated to Professor Hubert Schmidbaur on the occasion of his 75th birthday
The Newman-Kwart rearrangement of O-thiocarbamates to S-thiocarbamates is a versatile method
for the conversion of phenols into thiophenols. The triple rearrangement of the O-trithiocarbamate
2,4,6-triacetyl-1,3,5-tri-(dimethylthiocarbamoyloxy)benzene (2) to the S-trithiocarbamate 2,4,6-
triacetyl-1,3,5-tri-(dimethylthiocarbamoylthio)benzene (3) is performed, which is unprecendented
for fully substituted benzene rings. The synthesis and characterization of the O-trithiocarbamate 2
and the S-trithiocarbamate 3 including the single-crystal X-ray structure analysis of compound 3 are
presented.
Key words: Newman-Kwart Rearrangement, Thiocarbamates, Thiols
Introduction
rearrangement at one aromatic ring [8 – 13]. To the
best of our knowledge, only one triple rearrangement
at one benzene ring has been reported, which is the
conversion of the triple O-thiocarbamate of 1,3,5-
trihydroxybenzene into its triple S-thiocarbamate [9].
The rearrangement conditions vary tremendously in
dependence of the other substituents of the aromatic
ring. Some synthetic protocols are without a solvent
at high temperatures (170 ◦C to 335 ◦C) with different
reaction times (2 min to 20 h) [7 – 10, 12, 14– 21].
Other routines use high-boiling solvents like diphenyl
ether (b. p. 259 ◦C) or o-dichlorobenzene (b. p.
The synthesis of thiols has broad applications in
chemistry ranging from biochemistry to coordination
chemistry [1 – 5]. A convenient synthetic route to thio-
phenols is the Newman-Kwart rearrangement of phe-
nols (Scheme 1) [6, 7].
In the first step, the phenol is converted to a O-
thiocarbamate. A typical procedure is the reaction of
the phenol with dimethylthiocarbamoyl chloride under
basic conditions. This O-thiocarbamate is converted
into the S-thiocarbamate by a thermal rearrangement.
A four-membered cyclic transition state is proposed,
which results from the nucleophilic attack of the sulfur
atom to the ipso-carbon atom of the benzene ring. The
thermodynamic driving force is the conversion of the
formal C=S double bond into the energetically more
favorable C=O double bond. While this Newman-
Kwart rearrangement has been applied to many differ-
ent substrates, there are only few examples for a double
◦
180 C) [8, 14, 18]. It is well accepted that electron-
withdrawing substituents at the aromatic ring reduce
the temperature needed for the rearrangement. For
example, formyl or nitro substituents allow successful
rearrangement in toluene (b. p. 110 ◦C) [22, 23].
Lewis acids like BF3·Et2O reduce the rearrange-
ment temperatures strongly [7, 24]. Newman-Kwart
rearrangements with several substituents in the ortho
Scheme 1.
c
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1634
H. Theil et al. · Triple Newman-Kwart Rearrangement
Scheme 2.
position to the thiocarbamate have been described hexane/ethyl acetate 1 : 1). – IR (KBr): ν = 2934, 1699, 1546,
1395, 1276, 1194, 1109 cm−1. – 1H NMR (500.05 MHz,
in the literature [6, 7, 16, 22, 23, 25]. The above-
CDCl3): δ = 3.39, 3.30 (2 × s, 18H, NMe2), 2.47 (s, 9H,
COMe). – 13C NMR (125.75 MHz, CDCl3): δ = 196.2
(COMe), 184.4 (OCS), 148.8 (Car-O), 128.5 (Car-COMe),
43.4, 39.2 (2 × NMe2), 31.6 (COMe). – MS (MALDI,
matrix: DHB): m/z = 514 [M+H]+, 536 [M+Na]+. –
C21H29N3O7S3 (531.68): calcd. C 47.44, H 5.50, N 7.90;
found C 47.75, H 5.26, N 7.92.
mentioned multiple Newman-Kwart rearrangements at
one aromatic ring were performed on little-substituted
benzene rings with at least one hydrogen atom in or-
tho-position to the thiocarbamate.
Herein, we describe the synthesis of O-trithiocarb-
amate 2 starting from the triketone 1 which has been
frequently used in our group for the synthesis of
trinucleating ligands (Scheme 2) [26– 31]. The triple
Newman-Kwart rearrangement of the O-trithiocarb-
amate 2 to the S-trithiocarbamate 3 at a fully substi-
tuted benzene ring is unprecedented.
Synthesis of 2,4,6-triacetyl-1,3,5-tris(dimethylthiocarbamo-
ylthio)benzene (3)
O-Trithiocarbamate 2 (3.053 g, 5.94 mmol) was dis-
solved in o-dichlorobenzene (150 mL) and purged with ar-
gon for 30 min. The solution was heated at reflux for 8 h.
The solvent was removed under reduced pressure. The raw
material was dissolved in CHCl3 and purified via column
Experimental Section
Dimethylformamide (DMF), 1,4-diazabicyclooctane
(DABCO), dimethylthiocarbamoyl chloride, chloroform, chromatography (silica gel, 1. n-hexane/ethyl acetate 1 : 1,
2. EtOH). The solvents were removed under reduced pres-
n-hexane, ethyl acetate, and ethanol have been received
sure. S-Trithiocarbamate 3 was obtained as a pale-yellow
from commercial sources and were used without further
purification. Triketone 1 was synthesized as described in solid. Yield: 1.767 g (3.44 mmol, 57.9 %). – IR (KBr) ν =
the literature [32]. IR spectra were recorded on a Bruker
Vektor 22 spectrometer, NMR spectra on a Bruker AC200
2931, 1713, 1673, 1370, 1256, 1177, 1098, 903, 676 cm−1. –
1H NMR (200.13 MHz, CDCl3): δ = 2.98 (s, 18H, NMe2),
spectrometer, and MS-MALDI-ToF spectra on a Bruker 2.47 (s, 9H, COMe). – 13C NMR (50.33 MHz, CDCl3):
δ = 201.4 (COMe), 163.2 (SCO), 157.1 (Car-S), 120.8 (Car-
Reflex 4 instrument with the use of 2,5-dihydroxybenzoic
COMe), 37.2 (NMe2), 31.5 (COMe). – MS (MALDI, matrix:
DHB): m/z = 514 [M+H]+, 536 [M+Na]+. – C21H29N3O7S3
(531.68): calcd. C 47.44, H 5.50, N 7.90, S 18.09; found
C 47.69, H 5.25, N 8.02, S 17.87.
acid (DHB) as matrix. Elemental analysis were performed
on an Elementar Vario EL III instrument.
Synthesis of 2,4,6-triacetyl-1,3,5-tris(dimethylthiocarbamo-
yloxy)benzene (2)
Crystal structure determination
A suspension of triketone 1 (2.504 g, 9.93 mmol) in
DMF (25 mL) was purged with argon for 30 min. DABCO
X-Ray crystal structure analysis of 3: C21H27N3O6S3,
(6.683 g, 59.58 mmol) was added, and the resulting yel- Mr = 513.64, colorless crystals, 0.15 × 0.15 × 0.10 mm3, a =
◦
˚
low solution was stirred for 30 min. Dimethylthiocarbamoyl 17.915(1), b = 11.680(1), c = 11.717(1) A, β = 91.76(1) ,
chloride (7.366 g, 59.59 mmol) was added, and the reaction V = 2450.6(3) A , ρcalc = 1.39 g cm−3, µ = 3.1 mm−1
,
3
˚
◦
mixture was heated to 80 C for 4 h. The suspension was empirical absorption correction (0.651 ≤ T ≤ 0.745), mono-
˚
filtered and the filter cake washed with a small amount of clinic, space group P21/c (no. 14), Z = 4, λ = 1.54178 A,
DMF. The filtrate was slowly poured into 500 mL of water, T = 223(2) K; ω and ϕ scans, 27402 reflections collected
and the resulting solid was collected and dried in vacuo. O- ( h, k, l), [(sinθ)/λ] = 0.58 A−1, 3949 independent
˚
Trithiocarbamate 2 was obtained as a pale-brown solid and (Rint = 0.042) and 3710 observed reflections [I ≥ 2 σ(I)],
used without further purification. Yield: 4.415 g (8.60 mmol, 298 refined parameters, R = 0.051, wR2 = 0.137, max. / min.
86.6 %). A small amount was purified via column chro- residual electron density: 0.74 / −0.36 e A−3; hydrogen
˚
matography to get satisfying analytical data (silica gel, n- atoms calculated and refined as riding atoms. The data set
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H. Theil et al. · Triple Newman-Kwart Rearrangement
1635
˚
Table 1. Selected interatomic distances (A) and angles (deg)
was collected with a Nonius KappaCCD diffractometer.
Programs used: data collection COLLECT [33], data reduc-
tion DENZO-SMN [34], absorption correction DENZO [35],
structure solution SHELXS-97 [36], structure refinement
SHELXL-97 [37].
CCDC 745611 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
in 3.
C(1)–S(1)
1.781(3)
1.780(3)
1.778(3)
1.390(4)
1.399(4)
1.396(4)
1.519(4)
1.392(4)
1.399(4)
1.519(4)
1.398(4)
1.516(4)
1.210(5)
1.352(5)
1.805(4)
1.202(4)
C(23)–N(21)
C(23)–S(2)
C(33)–O(31)
C(33)–N(31)
C(33)–S(3)
1.332(4)
1.834(3)
1.217(4)
1.340(4)
1.816(4)
C(3)–S(2)
C(5)–S(3)
C(1)–C(2)
C(1)–C(6)
C(2)–C(3)
C(2)–C(11)
C(3)–C(4)
C(4)–C(5)
C(4)–C(21)
C(5)–C(6)
C(6)–C(31)
C(13)–O(11)
C(13)–N(11)
C(13)–S(1)
C(23)–O(21)
O(11)–C(13)–N(11)
O(11)–C(13)–S(1)
O(21)–C(23)–N(21)
O(21)–C(23)–S(2)
O(31)–C(33)–S(3)
N(31)–C(33)–S(3)
C(1)–S(1)–C(13)
C(3)–S(2)–C(23)
C(5)–S(3)–C(33)
124.8(4)
121.7(3)
125.7(3)
119.8(3)
121.3(3)
113.3(3)
101.1(2)
96.1(2)
Results and Discussion
The reaction of triketone 1 with dimethylthiocar-
bamoyl chloride is the first step of the synthesis de-
scribed in Scheme 2. The reaction using NaH as a
base, which is frequently used for the synthesis of O-
thiocarbamates [7, 8], in DMF does not provide a ho-
mogeneous sample, as evidenced by FTIR, NMR, and
MS analyses. A possible explanation is a deprotonation
of the acetyl group and the reaction of the resulting car-
banion with dimethylthiocarbamoyl chloride, with an-
other molecule of 1, or even with the solvent DMF. Us-
ing the weaker base DABCO in DMF results in good
yields of the triple O-trithiocarbamate 2. A band at
1699 cm−1 in the FT-IR spectrum can be assigned to
the C=O stretching mode of the acetyl function which
is shifted to higher energy in comparison to the trike-
tone 1 due to the absence of hydrogen bonding with
the neighboring phenol function [38]. Another charac-
teristic vibration at 1546 cm−1 may be assigned to the
C=S stretching vibration [6, 8], although also other as-
101.2(2)
Fig. 1. Molecular structure of 3 and crystallographic labeling
scheme used (displacement ellipsoids at the 50 % probability
level, hydrogen atoms omitted for clarity).
acetyl functions in ortho positions [7].
signments have been given for this vibration [7, 17].
Attempts to carry out the rearrangement of O-
1
The H NMR spectrum shows the methyl group of trithiocarbamate 2 without a solvent resulted in a mix-
the acetyl function at δ = 2.47 ppm in comparison to ture of products without showing spectral signatures of
2.71 ppm in 1. The two methyl groups of the three O- the exp◦ected S-trithiocarbamate 3. The rearrangements
◦
thiocarbamate functions give two signals at 3.39 ppm at 180 C and 250 C resulted in a mass reduction of
and 3.30 ppm. This behavior is due to a reduced rota- 9 % and 47 %, respectively.
tion along the C–N bond by partial double-bond char-
acter [9, 14– 17, 20, 21, 24, 39, 40].
The successful rearrangement of the O-trithio-
carbamate 2 into the S-trithiocarbamate 3 has been
The second step, the Newman-Kwart rearrangement performed in ortho-dichlorobenzene within 8 h at
of the O-trithiocarbamate 2 into the S-trithiocarbamate reflux. The FT-IR spectra showed two C=O double
3, has been thought to be the problematic step. bond vibrations at 1713 cm−1 (acetyl) and 1673 cm−1
Brooker et al. described the attempted Newman-Kwart (S-thiocarbamate) [8, 14]. Additional evidence for the
rearrangement of 2,6-diacetyl-4-methylphenyl-O-thio- successful triple rearrangement is the appearance of
1
carbamate. This reaction was not successful in boil- two singlets in the H NMR spectrum at 2.98 ppm
ing toluene even on addition of BF3·Et2O [24], which (S-thiocarbamate) and 2.47 ppm (acetyl), with an in-
is surprising due to the presence of the electron- tensity ratio of 2 : 1. While the acetyl methyl function
withdrawing acetyl substituents. Already the parent exhibits only a minor shift, the two methyl group
report of Newman and Karnes mentioned problems signals of the O-trithiocarbamate 2 merge to one
for the rearrangement of O-thiocarbamates that carry signal in the S-trithiocarbamate 3, bcause the C–N
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1636
H. Theil et al. · Triple Newman-Kwart Rearrangement
˚
Table 2. Structural parameters calculated for 3. d (A) is the
shortest distance of the respective atom to the best plane
formed by the six carbon atoms of the benzene ring. ψ (deg)
is the angle between the best planes of the six carbon atoms
of the benzene ring and the best plane of the atoms of the
respective substituent.
d
ψ
d
ψ
S1
S2
S3
−0.262
−0.049
0.292
90.5
83.7
95.5
C11
C21
C31
0.241
0.019
−0.032
92.8
90.8
87.2
bond of the S-thiocarbamate has less double-bond
character and therefore a lower rotation barrier [9, 14 –
17, 20, 21, 24]. Slow evaporation of a solution of the
S-trithiocarbamate 3 in CHCl3 led to the formation
of colorless crystals. Single-crystal X-ray diffraction
analysis confirmed the formation of 3. The molecular
structure is shown in Fig. 1, and selected interatomic
distances and angles are given in Table 1. The mean
˚
C–S distances of 1.78 A are in the range expected for
aromatic carbon-sulfur bonds [41]. All bond lengths
and angles are in the typical ranges and in good
agreement with related compounds reported in the
literature [42].
Fig. 2. Molecular structure of 3: a) View perpendicular to the
central benzene ring; b) view nearly parallel to the central
benzene ring.
The sterical demand of the six bulky substituents on
the carbon ring results in a nearly perpendicular orien-
tation of the planes of the substituents with respect to
the central benzene ring plane (Fig. 2a and Table 2).
Interestingly, the structure does not show an alternat-
ing up-and-down arrangement of the substituents with
all thiocarbamates pointing to one side of the central
benzene ring and the three COCH3 carbonyl functions
to the other side (Fig. 2b). The equivalence of the in-
dividual substituents in the NMR spectrum indicates a
low barrier for the rotation along the Car–C and Car–S
bonds in solution.
The cleavage of the S-thiocarbamate groups of 3
should result in 2,4,6-triacetyl-1,3,5benzenetrithiol. S-
Thiocarbamates are usually cleaved under basic condi-
tions in protic solvents with at least two equivalents
of base per thiocarbamate. Temperatures ranging from
r. t. to reflux conditions and reaction times ranging
from several minutes to one day have been applied [7 –
9, 20, 22, 23, 25].
ation of the reaction times and of the metal hydroxide
(LiOH, NaOH) did not lead to any measurable reac-
tions. At even harsher reaction conditions using iso-
propanol, ethylene glycol, or 2-methoxyethanol reac-
tions occured, but the expected trithiol could not be
obtained. NMR spectroscopic inspection of the prod-
ucts showed the presence of a mixture of several as yet
unidentified compounds.
These results are consistent with a report by Lau et
al. [18]. They showed that the treatment of an ortho-
acetyl S-thiocarbamate unit with base under harsh con-
ditions led to the deprotonation of the acetyl function
which then attacks the neighboring S-thiocarbamate
group, resulting in 5-membered and 6-membered het-
erocycles. The presence of three acetyl and three thio-
carbamate functions in 3 may therefore result in a mix-
ture of various heterocyclic compounds.
The acidic cleavage of S-thiocarbamates has also
been reported [16], but treatment of the S-trithiocarb-
amate 3 in mixtures of concentrated HCl in methanol
or isopropanol only resulted in the recovery of the
starting material.
The reaction of S-trithiocarbamate 3 with KOH in
MeOH at reflux resulted in the quantitative isolation
of the starting material. This indicates some sterical
protection of the thiocarbamate so that more severe re-
The third kind of cleavage reaction is the reductive
action conditions had to be chosen. Changing the sol- cleavage of S-thiocarbamates with LiAlH4 in THF at
vent to ethanol or mixtures with chloroform and vari- reflux [10, 15, 19, 21]. These conditions lead to the re-
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H. Theil et al. · Triple Newman-Kwart Rearrangement
1637
duction of the acetyl functions, producing three stereo- Newman-Kwart rearrangement of 2 in boiling ortho-
centers and thus to diastereomeric mixtures. Therefore, dichlorbenzene afforded the S-trithiocarbamate 3. The
the reduction of the S-trithiocarbamate 3 has been per- latter reaction performed smoothly contrary to reports
formed by using only a minimum amount of LiAlH4 at that Newman-Kwart rearrangements with acetyl func-
r. t. However, NMR spectroscopic analysis of the prod- tions in ortho position cause severe problems [7, 24].
ucts showed the reduction of both the thiocarbamate The molecular structure of 3 obtained by single-crystal
and the acetyl functions. Performing the reaction at X-ray diffraction shows the bulkiness of the six sub-
lower temperatures did not result in a kinetically con- stituents on the central benzene ring resulting in an al-
trolled reduction of the thiocarbamates only, and the most perpendicular orientation of the planes of the six
reduction with sodium or sodium/naphthaline did also substituents relative to the central benzene plane. The
not lead to the chemoselective reduction of the thiocar- basic cleavage of S-trithiocarbamate 3 provides com-
bamates.
plex mixtures of heterocyclic rings initiated by the de-
protonation of the acetyl function. Attempts of acidic
cleavage of S-trithiocarbamate 3 were unsuccessful.
The reductive cleavage of 3 leads also to the reduction
of the acetyl functions and therefore to diastereomeric
mixtures.
Conclusions
The reaction of triketone 1 with dimethylthio-
carbamoyl chloride in the presence of DABCO re-
sulted in the formation of O-trithiocarbamate 2. The
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