Li and Shi
SCHEME 4. A Control Experiment on the Transformation
of 22a to 23a in the Presence of PdCl2
mohydrin derivatives 5 can be easily transformed to R-allenic
alcohols 19 in the presence of Et3N. In addition, the cyclocar-
bonylation of R-allenic alcohols 19 with CO catalyzed by
palladium chloride to give lactones 22 or 23 under different
reaction conditions has been described. Efforts are underway
to elucidate the mechanistic details and the scope and limitations
of these reactions in the laboratory.
Experimental Section
1
General Remarks. H NMR spectra were recorded on a 300
allylpalladium complexes has been proposed by Trost and
Verhoeven,23 and the same type of intermediate like D, where
the π-allyl group is directly attached to the double bond, has
been suggested for the reaction of allenyl esters and carbonates
using Pd(0) catalysts.24 The formation of the conjugate diene
would be the driving force for the conversion of the π-bonded
complex D to the σ-bonded complex E.21a The insertion of CO
into the Pd-OH bond produces intermediate F, which generates
a cyclopalladium complex G by an intramolecular oxidative
addition of OH to the double bond. This may explain why the
cyclocarbonylation cannot occur in the presence of base because
NEt3 or K2CO3 can inhibit the intramolecular oxidative addition
of OH to the double bond in intermediate F by forming a salt.
Moreover, the existence of intermediate F can be proved by
the isolation of trace amount of acid 24a in a large scale
cyclocarbonylation of 19a,25 which should be derived from the
reductive elimination of intermediate F. The double bond at
the γ-position of Pd is essential for the cyclocarbonylation of
19a to be able to take place since many other types of R-allenic
alcohols cannot undergo such a cyclocarbonylation in the
presence of palladium complexes under similar conditions.18,23
The reductive elimination of intermediate G gives the corre-
sponding lactone product 2(3H)-dihydrofuranone 22a and
regenerates the Pd(0) species. 2(3H)-Dihydrofuranone 22a can
be transformed to the corresponding 2(5H)-furanone 23a in the
presence of palladium complex at higher temperatures (100-150
°C) via a 1,5-H transfer (Scheme 3). This transformation is
proved in a control experiment by employing 2(3H)-dihydro-
furanone 22a and PdCl2 (8 mol %) in DMF at 150 °C under
CO atmosphere or ambient atmosphere, affording the corre-
sponding 2(5H)-furanone 23a in 89% or 82% yield, respectively
(Scheme 4). It also should be noted that upon heating at 150
°C, 22a could not be transformed to 23a, suggesting that PdCl2
is essential in this transformation.
MHz spectrometer in CDCl3 with tetramethylsilane as the internal
standard. Infrared spectra were measured on a spectrometer. Mass
spectra were recorded by the EI method, and HRMS was measured
on a Kratos Analytical Concept mass spectrometer (EI or MALDI).
Satisfactory CHN microanalyses were obtained with an analyzer.
Melting points are uncorrected. All reactions were monitored by
TLC with silica gel coated plates. Flash Column Chromatography
was carried out with 300-400 mesh silica gel at increased pressure.
General Procedure for the Synthesis of Vinylbromohydrins.
To a solution of vinylidenecyclopropanes (0.18 mmol) in MeCN
(1.8 mL) was added 0.6 mL of water with a syringe. Then, 40 mg
of NBS (1.2 equiv, 0.22 mmol) was added in one portion and the
mixture was stirred for another 10 min at room temperature. The
reaction mixture was quenched by addition of 10 mL of water then
extracted with ether (3 × 10 mL), and the combined organic phases
were washed with brine (1 × 20 mL) and dried over anhydrous
MgSO4. The products were purified by flash column chromatog-
raphy with petroleum ether-EtOAc (5%).
4-Bromo-2-methyl-5,5-diphenyl-3-(propan-2-ylidene)pent-4-
1
en-2-ol, 5a. A white solid, mp 108-110 °C. H NMR (CDCl3,
300 MHz, TMS) δ 1.23 (s, 3H, CH3), 1.46 (s, 3H, CH3), 1.59 (s,
1H, OH), 1.94 (s, 3H, CH3), 1.97 (s, 3H, CH3), 7.18-7.38 (m,
10H, Ar). 13C NMR (CDCl3, 75 MHz, TMS) δ 21.7, 25.0, 30.3,
30.4, 72.9, 124.0, 127.3, 127.4, 127.9, 128.1, 128.4, 129.1, 137.4,
140.0, 140.6, 141.2, 143.2. IR (CH2Cl2) ν 3080, 3027, 2928, 2853,
1730, 1628, 1596, 1492, 1443, 1371, 1277, 1120, 1074, 1031, 965,
900, 790, 771, 758, 745, 697, 597, 567 cm-1. MS (%) m/e 353
(M+ - H2O, 1), 290 (M+ - Br, 6), 273 (M+ - Br - H2O), 233
(29), 232 (100), 217 (36), 215 (26), 202 (18), 91 (17), 59 (26).
HRMS (EI) for C21H21Br (M - H2O) 352.0827, found 352.0825.
Anal. Calcd for C21H23OBr: C, 67.93; H, 6.24. Found: C, 67.99;
H, 6.20.
General Procedure for the Synthesis of r-Allenic Alcohols.
To a solution of 5 (0.10 mmol) in DMF (3.0 mL) was added 3.0
equiv of Et3N with a syringe, and the resulting mixture was stirred
for 3-4 h at 100 °C, followed by addition of water (5 mL), and
extracted with ether (3 × 10 mL). The combined organic phases
were washed with brine (1 × 20 mL) and dried over anhydrous
MgSO4. The products were purified by a flash column chromatog-
raphy with petroleum ether-EtOAc (5%).
Conclusion
We have disclosed in this paper that the hydrobromination
or alkoxybromination of vinylidenecyclopropanes 1 could be
achieved in the presence of NBS and water or alcohols under
mild conditions. By this synthetic approach, the corresponding
vinylbromohydrin derivatives 5 or vinylbromoalkoxy derivatives
6-12 can be obtained in moderate to excellent yields. A
considerable range of aromatic groups-substituted VDCPs 1 has
been examined in the reaction, providing an efficient route to
the synthesis of various vinylbromohydrin or vinylbromoalkoxy
derivatives according to different substrates. These vinylbro-
2-Methyl-5,5-diphenyl-3-(prop-1-en-2-yl)penta-3,4-dien-2-
1
ol, 19a. A white solid, mp 124-126 °C. H NMR (CDCl3, 300
MHz, TMS) δ 1.56 (s, 6H, 2CH3), 1.95 (s, 3H, CH3), 5.15 (dd, J
) 1.2 Hz, J ) 1.5 Hz, 1H), 5.46 (s, 1H), 7.25-7.35 (m, 10H, Ar).
13C NMR (CDCl3, 75 MHz, TMS) δ 24.6, 30.5, 72.4, 111.9, 115.4,
118.3, 127.2, 128.1, 128.5, 136.6, 137.8, 204.3. IR (CH2Cl2) ν 3568,
3448, 2974, 2928, 1919, 1616, 1597, 1492, 1449, 1375, 1173, 1106,
1030, 900, 827, 767, 695, 636, 577, 550 cm-1. MS (%) m/e 290
(M+, 8), 257 (11), 232 (100), 217 (69), 202 (30), 189 (12), 165
(17), 59 (58), 43 (54). Anal. Calcd for C21H22O: C, 86.85; H, 7.64.
Found: C, 86.55; H, 7.88.
General Procedure for the Cyclocarbonylation of r-Allenic
Alcohols Catalyzed by PdCl2 to Give 2(3H)-Furanones. To a
solution of 19 (0.10 mmol) in DMF (3.0 mL) were added PdCl2
(10 mol%) and dppp (20 mol%), and the resulting mixture was
stirred for 28 h at 70-80 °C under 20 atm of carbon monoxide.
The reaction was quenched by addition of water (5 mL) and
(23) Trost, B. M.; Verhoeven, T. R. J. Am. Chem. Soc. 1980, 102, 4730–
4743, and references cited therein.
(24) Nokami, J.; Maihara, A.; Tsuji, J. Tetrahedron Lett. 1990, 31, 5629–
5630.
(25) A trace of acid 22a can be obtained in a large scale cyclocarbonylation
with use of 0.3 mmol of 18a under the standard conditions.
6704 J. Org. Chem. Vol. 73, No. 17, 2008