798
M. Rubin, V. Gevorgyan
PRACTICAL SYNTHETIC PROCEDURES
step dissolved in anhyd Et2O (800 mL). The mixture was stirred un-
der argon at r.t. and 3 N etheral solution of EtMgBr (1.3 equiv, 280
mL) was added dropwise over 1 h. The reaction mixture was addi-
tionally stirred for 40 min at r.t. until GC analysis indicated the ab-
sence of starting material. The mixture was quenched with H2O
(200 mL) and aq H2SO4 (1:5, 600 mL) and stirred until all the pre-
cipitate had dissolved. The etheral layer was separated and the aque-
ous layer was extracted with Et2O. The combined organic phases
were washed with H2O and brine, dried (MgSO4), filtered, and con-
centrated. The residue was filtered through a short column of silica
gel (hexane), the eluate was evaporated, and the residue was dis-
tilled in vacuum; bp 97–101 °C/10 mm Hg); yield: 115.13 g (85%).
5b
1H NMR (CDCl3, 500.13 MHz): d = 7.31–7.25 (m, 4 H), 7.16 (m, 1
H), 1.42 (s, 3 H), 1.32 (dd, J = 10.3, 3.9 Hz, 1 H), 0.79 (dd, J = 7.7,
3.9 Hz, 1 H), 0.39 (dd, J = 10.3, 7.7 Hz, 1 H), 0.17 (s, 9 H).
13C NMR (CDCl3, 125.76 MHz): d = 149.1, 128.6 (+, 2 C), 126.6
(+, 2 C), 125.6 (+), 26.7 (+), 23.9, 20.8 (–), 15.9 (+, 1JSn-C = 446.7,
467.9 Hz), –8.6 (+, 3 C).
1H-13C HMQC (CDCl3, 500.13 MHz, 125.76 MHz): dH/dC = 7.29/
128.6, 7.27/126.6, 7.16/125.6, 1.42/26.7, (1.32/20.8 and 0.79/20.8),
0.39/15.9, 0.17/–8.6.
119Sn NMR (CDCl3, 186.50 MHz): d = 0.5.
MS (GC): m/z = 296 (M+, <1%), 281 (M+ – Me, 3%), 165 (Me3Sn+,
3-Methyl-3-phenylcyclopropene (4)
100%), 131 (M+ – Me3Sn, 90%).
t-BuOK (73.4 g, 0.654 mol) was stirred in anhyd DMSO (400 mL)
at 60 °C under argon until the solution became homogenous, then
the temperature was brought to 20 °C and bromocyclopropane 3
(114.0 g, 0.54 mol) was added dropwise over 1 h. Occasional cool-
ing with an ice bath helped to keep the temperature at 20 °C. A deep
bluish-green color was developed quickly in the reaction mixture
and was retained through the course of the reaction. The mixture
was stirred for 10 h at r.t., then poured into ice-cold H2O (1 L) and
extracted with hexane (3 × 350 mL). The combined organic phases
were washed with H2O (3 × 500 mL) and brine (400 mL), dried
(MgSO4) and concentrated. Fractional distillation of the residue in
vacuum gave 4; yield: 55.42 g (79%); bp 61–62 °C/10 mm Hg.
5c
1H NMR (CDCl3, 500.13 MHz): d = 8.04 (m, 6 H), 7.72 (m, 10 H),
7.64 (m, 3 H), 7.50 (m, 1 H), 1.92 (m, 1 H), 1.82 (s, 3 H), 1.41 (m,
2 H).
13C NMR (CDCl3, 125.76 MHz): d = 148.7, 139.8 (3 C), 138.0 (+,
6 C), 129.8 (+, 2 C), 129.4 (+, 6 C), 129.2 (+, 3 C), 127.4 (+, 2 C),
126.5 (+), 28.0 (+), 24.5, 20.6 (–), 16.2 (+, 1JSn-C = 504.4, 527.8 Hz).
1H-13C HMQC (CDCl3, 500.13 MHz, 125.76 MHz): dH/dC = 8.04/
138.0, 7.72/129.8, 7.72/129.4, 7.72/127.4, 7.64/129.2, 7.50/126.5,
(1.92/20.6 and 1.41/20.6), 1.82/28.0, 1.41/16.2.
Pd-Catalyzed Hydrostannation;5a Tributyl(2-methyl-2-phenyl-
cyclopropyl)stannane (5a) ; Typical Procedure (Scheme 4)
An oven-dried 50 mL two-neck round-bottom flask was loaded with
Pd(PPh3)4 (116 mg, 0.1 mmol) under argon. Anhyd THF (20 mL)
was added and the mixture was stirred at r.t. until the catalyst had
dissolved. The solution was cooled down to –78 °C, 3-methyl-3-
phenylcyclopropene (4; 1.9 g, 14 mmol) was added and the mixture
was stirred at –78 °C for 5 min. Bu3SnH (4 mL) was added via a sy-
ringe pump over 1.5 h. Then, the mixture was concentrated and the
residue was purified by preparative column chromatography (hex-
ane) to obtain 4.70 g (80%) of 5a.
NOESY (CDCl3, 500.13 MHz): dH/dH (selected cross peaks) =
1.82/7.72, 1.82/8.04.
119Sn NMR (CDCl3, 186.50 MHz): d = –101.75.
HRMS (FAB): m/z Calcd for C28H26120SnNa: 505.0954; found:
505.0936.
Dimethyl Cycloprop-2-ene-1,1-dicarboxylate (11) (Scheme 5)
The titled cyclopropene was obtained previously by Wheeller in
28% overall yield via decomposition of dimethyl diazomalonate (6)
in the presence of Cu-catalyst and bis(trimethylsilyl)acetylene (7),
which was used as a solvent, followed by exhaustive desilylation of
the formed bis(trimethylsilyl)cyclopropene 9. The authors have also
reported that employment of Rh2(OAc)4 as a catalyst for cyclopro-
penation reaction, however, no desired compound 9 was formed.
Nonetheless, we found that Rh2(OAc)4 catalyzes smooth
cyclopropenation15 of terminal trimethylsilylacetylene (8), provid-
ing cyclopropene 10 as a major product in a 10:1 mixture with furan
12.16 When this mixture was submitted to deprotection, cyclopro-
pene 10 was desilylated smoothly to give the desired compound 11,
while silylfuran 12 stayed unchanged. Compounds 11 and 12 were
easily separable by short column chromatography. This modified
procedure allowed for significant improvement of the yield of 11.
Ph
Me
Ph
Pd(0)
R3SnH
-78 °C
Me
SnR3
4
5
5a: R = Bu, 92%
(80% in 14 mmol scale)
5b: R = Me, 91%
5c: R = Ph, 92%
Scheme 4
5a
A solution of dimethyl diazomalonate (6; 15.8 g, 100 mmol) in tri-
methylsilylacetylene (8; 15 mL) was added over 18 h to a refluxing
stirred suspension of Rh2(OAc)4 (110 mg, 0.25 mmol, 0.5 mol%) in
trimethylsilylacetylene (8; 250 mL) via a syringe pump. After the
addition was complete, the reaction mixture was stirred at reflux for
an additional 4 h, then the reflux condenser was changed for a dis-
tillation unit and most of the trimethylsilylacetylene (8) was dis-
tilled from reaction mixture under ambient pressure. Normally, it
was possible to recycle about 200 mL of trimethylsilylacetylene (8),
which was reused without additional purification. The residue was,
filtered through a short column of silica gel (EtOAc) and the eluate
was concentrated and dissolved in THF (500 mL). To a stirred so-
lution of this at 0 °C was added 10% aq K2CO3 (190 mL) dropwise.
The mixture was allowed to warm to r.t. and stirred for another 1 h,
until GC analysis showed the completion of the reaction. The aque-
ous layer was separated, the organic phase was concentrated in vac-
uum to a volume of 150 mL and combined with the aqueous phase.
1H NMR (CDCl3, 500.13 MHz): d = 7.27 (m, 4 H), 7.10 (m, 1 H),
1.55 (m, 6 H), 1.42 (s, 3 H), 1.35–1.30 (m, 7 H), 0.94–0.85 (m, 15
H), 0.77 (dd, J = 7.9, 4.7 Hz, 1 H), 0.35 (dd, J = 10.3, 7.9 Hz, 1 H).
13C NMR (CDCl3, 125.76 MHz): d = 149.0, 128.1 (+, 2 C), 126.4
(+, 2 C), 125.2 (+), 29.2 (–, 3 C), 27.4 (–, 3 C), 26.8 (+), 23.3, 20.1
(–), 14.2 (+, 1JSn-C = 373.7, 391.4 Hz), 13.7 (+, 3 C), 9.9 (–, 3 C).
1H-13C HMQC (CDCl3, 500.13 MHz, 125.76 MHz): dH/dC = 7.27/
128.1, 7.27/126.4, 7.10/125.2, 1.55/29.2, 1.42/26.8, 1.33/27.4,
(1.30/20.1 and 0.77/20.1), 0.92/13.7, 0.88/9.9, 0.35/14.2.
119Sn NMR (CDCl3, 186.50 MHz): d = –12.4.
HRMS (FAB): m/z calcd for C18H29120Sn (M – Bu)+: 365.1291;
found: 365.1285.
Synthesis 2004, No. 5, 796–800 © Thieme Stuttgart · New York