Steric variations between the synthesis of a stable chiral C 2-symmetric diimidazolidinylidene and an electron-rich tetraazafulvalene

Article abstract of DOI:10.1055/s-2006-926361

C₂-Symmetric electron-rich olefin dimers and imidazolidinylidene ligands with a 2,2-dimethyl-1,3-dioxolane backbone were synthesised and characterised. The steric protection of the carbene dictates which product is obtained. Georg Thieme Verlag

Full text of DOI:10.1055/s-2006-926361

1040
PAPER
Steric Variations between the Synthesis of a Stable Chiral C2-Symmetric
Diimidazolidinylidene and an Electron-Rich Tetraazafulvalene
A
Stable Chiral
C
o
-Symmetri
l
D
iimi
i
dazolid
n
inylidene
a
ndan
E
M
lectron-RichTetraazaf
a
ulvalene rshall,* Mark F. Ward, Janet M. S. Skakle
Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB24 3UE, Scotland, United Kingdom
Fax +44(1224)272921; E-mail: c.marshall@abdn.ac.uk
Received 14 October 2005
R1
Abstract: C2-Symmetric electron-rich olefin dimers and imidazo-
R
lidinylidene ligands with a 2,2-dimethyl-1,3-dioxolane backbone
were synthesised and characterised. The steric protection of the car-
bene dictates which product is obtained.
HBr
i
ii
Br
N
N
R1
H2N
R
R
NH
R
Key words: electron-rich olefin dimer, imidazolidinylidene, chiral,
C2-symmetric
H2N
1
2a,b
3a,b
R = R1= Me (a), R = i-Pr, R1 = H (b)
In recent years, synthetic efforts towards transition-metal
complexes containing N-heterocyclic carbene ligands
(NHC) have been the subject of intense interest.1 Major
stimuli for the renewed appeal of these complexes lie in
their use as catalysts, where they often display significant
advantages over the analogous phosphine-containing
compounds.2 In catalytic systems, NHCs were shown to
prevent the formation of elemental metal, a problem often
associated with weak ligand–metal interactions.3 Conse-
quent stability towards heat, oxygen and moisture led to
palladium, rhodium and ruthenium complexes of NHCs
featuring prominently in catalytic reactions, for which
these metals are already well known.4
Scheme 1 Synthesis of 1-arylimidazolidines 3; i) ArNH2, toluene;
ii) HC(OEt)3, TsOH.
in yields of 91–92% as colourless oils, which crystallised
over time (Scheme 1).
Manipulation of commercially available enantiopure L-
tartaric acid by conventional means9 gave access to
(4S,5S)-bis(bromomethyl)-2,2-dimethyl-1,3-dioxolane
(4), which, when heated with 1-arylimidazolidines 3a–b,
produced almost quantitative yields of the salts 5a,b as
light yellow glassy solids.
The literature abounds with examples of chiral monoden-
tate carbene complexes designed for asymmetric synthe-
sis,5 but, until recently, C2-symmetric bidentate
complexes were notable by their absence.6 Mindful of
this, we began the synthesis and isolation of stable free C2-
symmetric diimidazolidinylidenes bridged by a trans-2,2-
dimethyl-1,3-dioxalane backbone to use as chiral ligands
in catalytic asymmetric transformations.
O
O
O
O
O
O
i
ii
N
N
N
2Br
N
Br
Br
S
N
S
N
N
N
Ar
Ar
Ar
Ar
4
5a,b
6a,b
Ar = 2,4,6-trimethylphenyl (a), 2,6-diisopropylphenyl (b)
It is well established that in order to isolate free imidazo-
lidinylidenes the nitrogen substituents have to bear suffi-
cient steric bulk to protect the carbene from dimerisation.7
The key to the synthesis of these stable chiral dicarbenes
involved the preparation of 1-arylimidazolidines with
large bulky substituents. The groups selected were 2,4,6-
trimethylphenyl and 2,6-diisopropylphenyl, as these sub-
stituents had been successfully used in order to isolate
simple symmetrical N-heterocyclic carbenes.8 Amination
of commercially available 1-bromoethylamine hydrobro-
mide (1) by refluxing excess arylamine in toluene gave
access to the diamines 2a,b in yields of 74–83%. Cyclisa-
tion of 2a,b with triethyl orthoformate in the presence of
p-toluenesulfonic acid gave the 1-arylimidazolidines 3a,b
Scheme 2 Synthesis of diimidazolidin-2-thiones 6a,b; i) 3a,b; ii)
DBU, S, MeOH, pyridine
Modifying the methods set out by Karkhanis et al.10 al-
lowed access to the thiones 6a,b in yields of 54–65% by
deprotonation of the crude diimidazolidinium salts 5a,b
with DBU in the presence of sulfur (Scheme 2). The
dithiones were characterised by HRMS and their typical
C=S resonance in the 13C NMR proved diagnostic. Suit-
able crystals for solid-state analysis of these colourless
thiones were obtained from methanol for 6a and diethyl
ether–petroleum spirit (40–60 °C) (1:1) for 6b (Figures 1
and 2).11–12
The structure determination of the solid dithione revealed
that both molecules contained puckered 2,2-dimethyl-1,3-
dioxolane and imidazolidin-2-thione rings. The carbon–
SYNTHESIS 2006, No. 6, pp 1040–1044
Advanced online publication: 27.02.2006
DOI: 10.1055/s-2006-926361; Art ID: P16705SS
x
x
.
x
x
.2
0
0
6
© Georg Thieme Verlag Stuttgart · New York

PAPER
A Stable Chiral C2-Symmetric Diimidazolidinylidene and an Electron-Rich Tetraazafulvalene
1041
O
O
O
O
O
O
i
i
N
N
N
N
N
N
N
N
S
S
N
N
N
N
Ar
Ar
6a,b
7a
7b
Scheme 3 Synthesis of bisimidazolidin-2-ylidene and electron-rich
olefin dimer 7a,b; i) Na/K, toluene
or tetrakis-carbene complexes was achieved by reaction
of the dimers with bridged dinuclear or mononuclear or-
ganometallic systems. Carbene complexes of a vast array
of metals, in various oxidation states, were obtained by
this method.14
Figure 1 ORTEP diagram of the molecular structure of 6a
Employing the same reduction methodology with the
bulkier 6b and potassium/sodium resulted in the isolation
of a viscous orange oil, which, when analysed by 13C
NMR, gave the typical downfield shift for the N-heterocy-
clic carbene at 242.2 ppm (Scheme 3). In view of this, the
need for the larger 2,6-diisopropylphenyl over 2,4,6-tri-
methylphenyl groups proved necessary for the isolation of
the free stable chiral dicarbene. The formation of NHC
complexes of a vast array of metals, in various oxidation
states, is well established by reacting free stable carbenes
with a metal precursor.15
The synthesis and isolation of stable free C2-symmetric
diimidazolidinylidenes bridged by a trans-2,2-dimethyl-
1,3-dioxalane backbone was achieved. Firstly, the synthe-
sis of bulky 1-arylimidazolidines was accomplished by
amination of 1-bromoethylamine hydrobromide followed
by cyclisation of the resultant diamine. Quaternisaton of
the imidazolidines with a C2-symmetric dibromo back-
bone gave the crude diimidazolidinium salts, which were
converted to their corresponding crystalline dithiones. Re-
duction of these chiral C2-symmetric dithiones with bulky
aryl groups on the imidazolidine was anticipated to allow
isolation of the free dicarbenes. Interestingly, employing
2,4,6-trimethylphenyl groups, which were successful in
the isolation of the simple symmetrical 1,3-(2,4,6-trimeth-
ylphenyl)imidazolidin-2-ylidene prepared by Arduengo
et al.,8 did not allow access to the free stable chiral dicar-
bene and resulted in the isolation of the electron-rich tet-
raazafulvalene. This unexpected result revealed that the
steric protection was insufficient to prevent dimerisation.
Conversely, when the steric bulk around the carbene was
increased to 2,6-diisopropylphenyl groups, the free dicar-
bene was isolated giving the first stable dicarbene bridged
by a chiral trans-2,2-dimethyl-1,3-dioxolane backbone.
As NHC complexes found application in catalysis, com-
Figure 2 ORTEP diagram of the molecular structure of 6b
sulfur bond lengths are consistent with literature imidazo-
lidin-2-thiones.13
Reduction of the dithione 6a by an alloy of potassium and
sodium in hot toluene was monitored by TLC and re-
vealed a clean reaction. After consumption of the thione,
the metal sulfide by-product was filtered. Concentration
of the solution deposited a light yellow solid, which was
identified as the dimeric product 7a by the characteristic
carbon–carbon double bond at 122.4 ppm in the 13C NMR
(Scheme 3). No resonance for the free carbene was detect-
ed.
This indicated that the 2,4,6-trimethylphenyl groups on
the imidazolidines did not carry enough steric bulk to pro-
tect the carbenes from dimerisation. Lappert devised a
method of cleaving electron-rich olefin dimers to form
NHC metal complexes. The formation of mono-, bis-, tris-
Synthesis 2006, No. 6, 1040–1044 © Thieme Stuttgart · New York

1042
C. Marshall et al.
PAPER
over MgSO4, filtered and concentrated to give a brown oil. Distilla-
tion under reduced pressure gave the imidazolidine (1.90 g, 91%) as
a colourless oil, which crystallised over time.
plexes containing these chiral ligands are anticipated to al-
low catalytic asymmetric transformations.
Starting materials were used as supplied by Lancaster synthesis and
Aldrich Chemical Company without further purification. Reactions
involving air-sensitive reagents were carried out in an atmosphere
of nitrogen or argon using standard Schlenk techniques. Toluene
was dried and distilled from sodium and benzophenone before use.
NMR spectra were recorded using a Bruker AC 250 spectrometer.
Bp 198 °C (3 mmHg); mp 66 °C.
IR (KBr): 2967, 2927, 2861, 1486, 1373 cm–1.
1H NMR (250 MHz, CDCl3): d = 1.98 (s, 6 H, CH3), 2.03 (s, 3 H,
CH3), 3.28 (t, J = 10.3 Hz, 2 H, NCH2), 3.79 (t, J = 10.3 Hz, 2 H,
NCH2), 6.52 (s, 1 H, N2CH), 6.64 (m, 2 H, ArH).
1
All H and 13C NMR spectra were obtained at 250 MHz and 62.9
13C NMR (62.9 MHz, CDCl3): d = 17.9, 20.7, 48.6, 55.1, 129.2,
MHz respectively. Chemical shifts are reported in ppm relative to
TMS and were determined by reference to the residual 1H or 13C sol-
vent peaks. Infrared spectra were recorded as KBr pellets or neat
samples on NaCl plates using an ATI Mattson Genesis series FTIR
instrument and are reported in cm–1. The Butterworth Contract An-
alytical Chemistry and Microbiology Company provided elemental
microanalysis. Mass spectrometry was recorded by the EPSRC Na-
tional Mass Spectrometry Service Centre, University of Wales,
Swansea. Melting point were determined using a Kofler hot-stage
microscope and are uncorrected. Polarimetry was carried out on a
Bellingham and Stanley P20 polarimeter and the values are given in
10–1 deg cm2 g–1.
134.7, 136.5, 136.9, 155.5.
ESI-MS: m/z (%) = 189.1 (100) [M + H]+.
1-(2,6-Diisopropylphenyl)imidazolidine (3b)
The procedure for 3a was followed using N-2,6-diisopropylphenyl-
1,2-diaminoethane (2b) (7.89 g, 36.0 mmol), triethyl orthoformate
(21.34 g, 144.0 mmol) and PTSA (0.30 g, 1.7 mmol). Distillation
under reduced pressure gave the imidazolidine (7.63 g, 92%) as a
light yellow oil, which crystallised over time and was recrystallised
from PE.
Bp 198 °C (3 mmHg); mp 74 °C.
IR (KBr) 3024, 2964, 2928, 2866, 1678, 1652, 1458, 1381 cm–1.
N-2,4,6-Trimethylphenyl-(1,2-diaminoethane) (2a)
1H NMR (250 MHz, CDCl3): d = 1.18 (dd, J = 6.8, 5.3 Hz, 12 H,
CH3), 3.08 (sept, J = 6.8 Hz, 2 H, CH3), 3.61 (t, J = 10.2 Hz, 2 H,
NCH2), 4.05 (t, J = 10.2 Hz, 2 H, NCH2), 6.82 (s, 1 H, N2CH), 7.17
(d, J = 8.0 Hz, 2 H, ArH), 7.29 (1 H, t, J = 8.3 Hz, ArH).
13C NMR (62.9 MHz, CDCl3): d = 24.9, 24.9, 28.2, 51.6, 54.9,
124.2, 128.7, 134.2, 148.9, 156.2.
To a suspension of 2-bromoethylamine hydrobromide (1) (10.00 g,
48.80 mmol) and toluene (20 mL) was added 2,4,6-trimethylaniline
(13.20 g, 97.61 mmol) and the mixture was heated at reflux for 18
h. The cooled solution was opened to H2O (50 mL) and 2 M KOH
soln (50 mL) was added. The aqueous layer was extracted with Et2O
(3 × 30 mL) and the extracts were washed with aq sat. NaCl soln
(100 mL) and dried with Na2SO4, filtered and concentrated. The re-
sidual yellow oil was purified by column chromatography (EtOAc–
MeOH–Et3N, 17:2:1) to give the diamine (14.48 g, 83%) as a co-
lourless oil.
ESI-MS: m/z (%) = 231.1 (100) [M + H]+.
(4S,5S)-4,5-Bis[1-(2,4,6-trimethylphenyl)imidazolidinium-3-
methyl]-2,2-dimethyl-1,3-dioxolane Dibromide (5a)
A mixture of (4R,5R)-4,5-bis(bromomethyl)-2,2-dimethyl-1,3-di-
oxolane (4) (0.38 g, 1.3 mmol) and 1-(2,4,6-trimethylphenyl)imida-
zolidine (3a) (0.50 g, 2.7 mmol) was heated at 100 °C under an
atmosphere of Ar for 14 h to give the salt (0.88 g, 100%) as a light
yellow glass.
1H NMR (250 MHz, DMSO-d6): d = 1.18 (s, 6 H, CH3), 1.91–2.01
(3 s, 18 H, ArCH3), 3.25–3.42 (m, 2 H, OCH), 3.73–4.35 (m, 12 H,
NCH2), 6.59 (s, 4 H, ArH), 9.08 (s, 2 H, N2CH).
IR (film): 3362, 2914, 1485, 1449, 1373 cm–1.
1H NMR (250 MHz, CDCl3): d = 1.97 (br s, 3 H, NH), 2.24 (s, 3 H,
CH3), 2.29 (s, 6 H, CH3), 2.90 (m, 2 H, NCH2), 2.99 (m, 2 H, NCH2),
6.83 (s, 2 H, ArH).
13C NMR (62.9 MHz, CDCl3): d = 18.4, 20.6, 42.6, 51.3, 129.5,
129.8, 131.2, 143.6.
ESI-MS: m/z (%) = 179.1 (100) [M + H]+.
13C NMR (62.9 MHz, DMSO-d6): d = 17.8, 18.0, 20.8, 27.1, 49.1,
N-2,6-Diisopropylphenyl-(1,2-diaminoethane) (2b)
51.3, 52.8, 76.1, 110.7, 129.3, 129.7, 130.5, 135.1, 139.8, 159.4.
The procedure for the synthesis of 2a was followed using 2-bromo-
ethylamine hydrobromide (1) (10.00 g, 48.80 mmol), 2,6-diisopro-
pylaniline (17.31 g, 97.62 mmol) and toluene (20 mL). The crude
product was purified by column chromatography (MeOH–CH2Cl2,
3:7) to give a light yellow oil, which was distilled (144 °C/3 mmHg)
to give the diamine (7.91 g, 74%) as a colourless oil.
(4S,5S)-4,5-Bis[1-(2,6-diisopropylphenyl)imidazolidinium-3-
methyl]-2,2-dimethyl-1,3-dioxolane Dibromide (5b)
The procedure for 6a was followed using (4R,5R)-4,5-bis(bromo-
methyl)-2,2-dimethyl-1,3-dioxolane (4) (0.45 g, 1.6 mmol) and 1-
(2,6-diisopropylphenyl)imidazolidine (3b) (0.72 g, 3.1 mmol) to
give the salt (1.17 g, 100%) as a light yellow glass.
1H NMR (250 MHz, DMSO-d6): d = 1.23 (4 d, J = 8.5, 6.7 Hz, 24
H, CH3), 1.38 (s, 6 H,CH3), 2.91–2.99 (m, 4 H, CH), 3.29–3.47 (m,
2 H, OCH), 3.78–4.27 (m, 12 H, NCH2), 7.38 (m, 4 H, ArH), 7.50
(t, J = 7.9 Hz, 2 H, ArH), 9.07 (s, 2 H, N2CH).
13C NMR (62.9 MHz, DMSO-d6): d = 24.0, 24.9, 27.2, 28.5, 49.1,
51.2, 53.7, 76.1, 111.0, 124.4, 124.6, 124.8, 129.7, 130.9, 146.0,
146.5, 159.1.
IR (film): 3365, 2960, 2867, 1589, 1458, 1382, 1362, 939, 802, 754
cm–1.
1H NMR (250 MHz, CDCl3): d = 1.20 (d, J = 6.8 Hz, 12 H, CH3),
2.13 (br s, 3 H, NH), 2.96 (m, 4 H, NCH2), 3.35 (sept, J = 6.8 Hz, 2
H, CH), 7.11 (m, 3 H, ArH).
13C NMR (62.9 MHz, CDCl3): d = 24.4, 27.6, 42.6, 54.4, 123.6,
123.8, 142.6, 143.4.
ESI-MS: m/z (%) = 221.3 (100) [M + H]+.
1-(2,4,6-Trimethylphenyl)imidazolidine (3a)
(4S,5S)-4,5-Bis[1-(2,4,6-trimethylphenyl)imidazolidin-2-thio-3-
methyl]-2,2-dimethyl-1,3-dioxolane (6a)
A mixture of (4S,5S)-4,5-bis[1-(2,4,6-trimethylphenyl)imidazol-
idinium-3-methyl]-2,2-dimethyl-1,3-dioxolane dibromide (5a)
(0.88 g, 1.3 mmol), MeOH (20 mL), pyridine (1.8 mL), 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (0.89 g, 5.9 mmol) and sulfur (0.13 g,
A mixture of N-2,4,6-trimethylphenyl-1,2-diaminoethane (2a)
(2.00 g, 11.2 mmol), triethyl orthoformate (6.65 g, 44.9 mmol) and
PTSA (0.10 g, 0.58 mmol) was heated at reflux for 18 h. Once
cooled to r.t., the suspension was dissolved in aq 5% NaOH soln (5
mL) and extracted with CHCl3 (3 × 50 mL). The extracts were dried
Synthesis 2006, No. 6, 1040–1044 © Thieme Stuttgart · New York

PAPER
A Stable Chiral C2-Symmetric Diimidazolidinylidene and an Electron-Rich Tetraazafulvalene
1043
4.1 mmol) was heated at 65 °C for 18 h. Once cooled to r.t., the mix-
ture was opened to H2O (50 mL) and extracted with CHCl3 (3 × 20
mL). The combined extracts were dried over MgSO4, filtered and
concentrated under reduced pressure to leave a brown residue. The
residue was purified by column chromatography (EtOAc–PE, 1:1,
loaded as a solution in CH2Cl2) to give the dithione (0.39 g, 54%) as
a colourless solid, which can be recrystallised from MeOH to give
colourless needles.
Mp 228–226 °C; [a]D25 +42.3 (c 1.87, CH2Cl2).
IR (KBr) 2983, 2909, 1610, 1486, 1448, 1411, 1375, 1081 cm–1.
1H NMR (250 MHz, CDCl3): d = 1.47 (s, 6 H,CH3), 2.19 (s, 6 H,
ArCH3), 2.22 (s, 6 H, ArCH3), 2.27 (s, 6 H, ArCH3), 3.71–4.13 (m,
12 H, NCH2), 4.45 (m, 2 H, OCH), 6.92 (s, 4 H, ArH).
13C NMR (62.9 MHz, C6D6): d = 18.6, 20.9, 21.3, 28.2, 52.0, 54.5,
55.5, 79.2, 109.2, 122.4, 129.7, 130.2, 133.2, 133.3, 136.4, 143.5.
(4S,5S)-4,5-Bis[1-(2,6-diisopropylphenyl)imidazolidin-2-
ylidene-3-methyl]-2,2-dimethyl-1,3-dioxolane (7b)
The procedure for 8a was followed using Na (0.05 g, 2.2 mmol), K
(0.16 g, 4.2 mmol), toluene (20 mL) and (4S,5S)-4,5-bis[1-(2,6-di-
isopropylphenyl)imidazolidin-2-thio-3-methyl]-2,2-dimethyl-1,3-
dioxolane (6b) (0.20 g, 0.31 mmol), which gave the diylidene as a
viscous orange oil.
1H NMR (250 MHz, C6D6): d = 1.37 (m, 24 H, CH3), 1.55 (s, 6 H,
CH3), 2.82 (m, 2 H, CH), 2.98 (m, 2 H, CH), 3.20–3.80 (m, 12 H,
NCH2), 4.78 (app br t, 2 H, OCH), 7.30 (m, 6 H, ArH).
13C NMR (62.9 MHz, C6D6): d = 24.5, 25.0, 25.8, 27.8, 28.5, 29.1,
51.4, 52.3, 54.8, 78.3, 108.7, 129.7, 139.9, 143.4, 147.7, 147.9,
242.2.
13C NMR (62.9 MHz, CDCl3): d = 17.9, 18.0, 21.1, 27.0, 47.3, 49.2,
49.6, 77.3, 108.9, 129.5, 134.9, 136.4, 136.7, 138.2, 183.0.
ESI-MS: m/z (%) = 589.2 (95) [M + Na]+, 567.2 (100) [M + H]+.
HRMS–ESI: m/z [M + H]+ calcd for C31H43N4O2S2: 567.2827;
found 567.2824.
References
(1) (a) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003,
14, 951. (b) Zang, T. Y.; Zang, H. Tetrahedron Lett. 2002,
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Int. Ed. Engl. 1997, 36, 2162.
(2) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.;
Herrmann, W. A. Angew. Chem. Int. Ed. 1998, 37, 2490.
(b) Herrmann, W. A.; Reisinger, C. P.; Spiegler, M. J.
Organomet. Chem. 1998, 557, 93. (c) Haung, J.; Nolan, S.
P. J. Am. Chem. Soc. 1999, 121, 9889.
(3) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Valley, 1987.
(4) (a) Herrmann, W. A.; Goossen, L. J.; Artus, G. R. J. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2805. (b) Haung, J.;
Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem.
Soc. 1999, 121, 2674. (c) Weskamp, T.; Kohl, F. J.;
Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem.
Int. Ed. 1999, 38, 2416. (d) Grasa, G. A.; Moore, Z.; Martin,
K. L. J. Organomet. Chem. 2002, 658, 126.
(5) (a) Enders, D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J.
H. Chem. Ber. 1996, 129, 1483. (b) Herrmann, W. A.;
Goossen, L. J.; Artus, G. R. J.; Köcher, C. Organometallics
1997, 16, 2472. (c) Seiders, T. J.; Ward, D. W.; Grubbs, R.
H. Org. Lett. 2001, 3, 3325.
Anal. Calcd for C31H42N4S2O2: C, 65.69; H, 7.47; N, 9.88; S, 11.31.
Found: C, 65.42; H, 7.11; N, 9.62; S, 11.25.
(4S,5S)-4,5-Bis[1-(2,6-diisopropylphenyl)imidazolidin-2-thio-3-
methyl]-2,2-dimethyl-1,3-dioxolane (6b)
The procedure for 7a was followed using (4S,5S)-4,5-bis[1-(2,6-di-
isopropylphenyl)imidazolidinium-2-methyl]-2,2-dimethyl-1,3-di-
oxolane dibromide (5b) (1.15 g, 1.54 mmol), MeOH (20 mL),
pyridine (1.8 mL), 1,8-diazabicyclo[5.4.0]undec-7-ene (1.03 g,
6.76 mmol) and sulfur (0.15 g, 4.6 mmol). The crude product was
purified by column chromatography (EtOAc–PE, 1: 2, loaded as a
solution in CH2Cl2) to give the dithione (0.63 g, 63%).
Mp 188 °C (from Et2O–PE, 1:2); [a]D25 +50.3 (c 2.04, CH2Cl2).
IR (KBr): 2965, 2868, 1492, 1458, 1404, 1347, 1083 cm–1.
1H NMR (250 MHz, CDCl3): d = 1.20 (4 d, J = 6.5, 5.0 Hz, 24 H,
CH3), 1.49 (s, 6 H, CH3), 2.88 (sept, J = 7.5 Hz, 2 H, CH), 3.02
(sept, J = 7.5 Hz, 2 H, CH), 3.99 (m, 12 H, NCH2), 4.52 (app br t, 2
H, OCH), 7.21 (m, 4 H, ArH), 7.34 (t, J = 7.5 Hz, 2 H, ArH).
13C NMR (62.9 MHz, CDCl3): d = 24.2, 24.4, 24.6, 24.7, 28.6, 28.7,
49.1, 49.3, 49.8, 77.4, 108.6, 124.4, 129.2, 135.1, 147.3, 147.7,
184.5.
ESI-MS: m/z (%) = 651.4 (100) [M + H]+, 619.3 (15) [M – S]+.
HRMS–ESI: m/z [M + H]+ calcd for C37H55N4O2S2: 651.3784;
(6) (a) Marshall, C.; Ward, M. F.; Harrison, W. T. A.
Tetrahedron Lett. 2004, 45, 5703. (b) Duan, W. L.; Shi, M.;
Rong, G. B. Chem. Commun. (Cambridge) 2003, 2976.
(c) Clyne, D. S.; Jin, J.; Genest, E.; Gallucci, J. C.; Rajan
Babu, T. V. Org. Lett. 2000, 2, 1125. (d) Perry, M. C.; Cui,
X.; Burgess, K. Tetrahedron: Asymmetry 2002, 13, 1969.
(e) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R.
Organometallics 2003, 22, 4384.
found: 651.3766.
Anal. Calcd for C37H54N4O2S2: C, 68.2; H, 8.5; N, 8.4; S, 9.5.
Found: C, 68.3; H, 8.4; N, 8.6; S, 9.9.
(4S,5S)-4,5-Bis[1-(2,4,6-trimethylphenyl)tetraazafulvalene]-
2,2-dimethyl-1,3-dioxolane (7a)
(7) (a) Hitchcock, P. B.; Lappert, M. F.; Terrenos, P.;
Wainwright, K. P. J. Chem. Soc., Chem. Commun. 1980,
1180. (b) Coleman, A. W.; Hitchcock, P. B.; Lappert, M. F.;
Maskell, R. K.; Müller, J. H. J. Organomet. Chem. 1983,
250, C9.
To a mixture of sodium (0.05 g, 2.3 mmol) and potassium (0.18 g,
4.7 mmol) in toluene (20 mL) was added (4S,5S)-4,5-bis[1-(2,4,6-
trimethylphenyl)imidazolidin-2-thio-3-methyl]-2,2-dimethyl-1,3-
dioxolane (6a) (0.20 g, 0.35 mmol), and the mixture was heated to
50 °C for 18 h. After cooling to r.t., the toluene was separated from
the suspension via cannular filtration and removed under reduced
pressure to give the dimer of the desired product as a light yellow
solid.
1H NMR (250 MHz, C6D6): d = 1.56 (s, 6 H, CH3), 1.73 (s, 6 H,
ArCH3), 2.35 (s, 6 H, ArCH3), 2.48 (s, 6 H, ArCH3), 2.64–2.70 (m,
4 H, NCH2), 3.02–3.11 (m, 4 H, NCH2), 3.21–3.27 (dd, J = 7.0, 5.0
Hz, 2 H, NCH2), 3.92–3.97 (dd, J = 7.0, 5.0 Hz, 2 H, NCH2), 4.84
(app br t, 2 H, OCH), 6.77–6.80 (2 s, 4 H, ArH).
(8) Arduengo, A. J. III; Krafczyk, R.; Schmutzler, R.
Tetrahedron 1999, 55, 14523.
(9) (a) Mash, E. A.; Neilson, K. A.; Van Deusen, S.; Hemperly,
S. B. Org. Synth. 1990, 68, 92. (b) Fujimara, O.; de la Mata,
F. J.; Grubbs, R. H. Organometallics 1996, 15, 1865.
(10) Karkhanis, D. W.; Field, L. Phosphorus Sulphur Relat.
Elem. 1985, 22, 49.
(11) Crystallographic data (excluding structure factors) for the
structure in this paper were deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
Synthesis 2006, No. 6, 1040–1044 © Thieme Stuttgart · New York

1044
C. Marshall et al.
PAPER
have very high Ueq values. These could be split to give
disorder over the two sites. Whilst this significantly
improved wR2, one of the methyl groups (C-16) still had
high Ueq values and the data/parameter ratio was worsened.
Both models, therefore, have their merits and data for both
were deposited.
numbers CCDC 273128, 273129 and 273130. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif or on application to CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK. Crystal Data for 6a (CCDC
273128 and 273129): Empirical formula C31H42N4O2S2,
formula weight 566.81, temperature 296 (2) K, wavelength
0.71073 Å, monoclinic, space group P21. Unit cell
dimensions a = 7.7147 (4) Å, b = 18.6723 (8) Å, c = 11.1002
(5) Å, b = 104.461 (2)°, V = 1548.34 (13) Å3, Z = 2,
Dc = 1.216 Mg m–3, absorption coefficient 0.205 mm–1,
F(000) = 608, crystal size 0.30 × 0.08 × 0.05 mm. Theta
range for data collection 1.89–24.99°, index ranges –9 £ h £
8, –18 £ k £ 22, –13 £ l £ 13, reflections collected 10775,
unique reflections 2812 [Rint = 0.0940], completeness to
q max (32.63°) = 99.9%, max. and min. transmission 0.9280
and 0.8000, refinement method full-matrix least-squares on
F2. Ordered Model: Data/restraints/parameters 2812/1/360,
goodness-of-fit on F2 = 0.811, final R indices [I > 2s(I)],
R1 = 0.0412, wR2 = 0.0624, R indices (all data), R1 =
0.0993, wR2 = 0.0727, absolute structure parameter 0.00(9),
largest diff. peak and hole 0.207 and –0.169 e Å–3.
(12) Crystal data of compound 6b (CCDC 273130): Empirical
formula C37H54N4O2S2, formula weight 650.96, temperature
295 (2) K, wavelength 0.71073 Å, monoclinic, space group
P21. Unit cell dimensions a = 13.5162 (13) Å, b = 6.9703 (6)
Å, c = 21.051 (2) Å, b = 102.897 (2)°, V = 1933.2 (3) Å3,
Z = 2, Dc = 1.118 Mg m–3, absorption coefficient 0.172
mm–1, F(000) = 704, crystal size 0.80 × 0.10 × 0.10 mm.
Theta range for data collection 1.98–32.63°, index ranges –
10 £ h £ 20, –10 £ k £ 10, –31 £ l £ 29, reflections collected
18951, unique reflections 7507 [Rint = 0.1700], completeness
to q max (32.63°) = 99.1%, max. and min. transmission
0.9830 and 0.8721, refinement method full-matrix least-
squares on F2, data/restraints/parameters 7507/1/416,
goodness-of-fit on F2 = 0.653, final R indices [I > 2s(I)],
R1 = 0.0487, wR2 = 0.0758, R indices (all data),
R1 = 0.3135, wR2 = 0.1151, absolute structure parameter
0.12 (9), largest diff. peak and hole 0.155 and –0.163 e Å–3.
(13) Bildstein, B.; Malaun, M.; Kopacka, H.; Wurst, K.;
Mitterböck, M.; Onganai, K. H.; Opromolla, G.; Zanello, P.
Organometallics 1999, 18, 4325.
Disordered Model: Data/restraints/parameters 2812/1/381,
goodness-of-fit on F2 = 0.791, final R indices [I > 2s(I)],
R1 = 0.0372, wR2 = 0.0464, R indices (all data), R1 =
0.0956, wR2 = 0.0547, absolute structure parameter –0.02
(7), largest diff. peak and hole 0.132 and –0.137 e Å–3. Two
of the methyl groups (numbered C-16, C-17) were found to
(14) Lappert, M. F. J. Organomet. Chem. 1988, 185.
(15) Herrmann, W. A. Angew. Chem. Int. Ed. 2002, 41, 1290.
Synthesis 2006, No. 6, 1040–1044 © Thieme Stuttgart · New York