Li et al.
JOCArticle
addition to Pd(0) species to give intermediate B, selective
insertion of alkyne into Pd-H to give intermediate C, and
subsequent decarboxylation and reductive elimination of
the C-C bond to finally afford cis-alkenes and regenerate
the Pd(0) species. The oxidative addition of HCOO-H to
Pd(0)9b,c and the formation of vinyl-Pd by the insertion of
alkynes to the Pd-H bond9b had been proposed as steps of
the catalytic cycle in the palladium-catalyzed hydrocarboxy-
lation of alkynes and reduction of alkenes with formic acid.
It should be noted that although HCOOH/NEt3 can be
used as the hydrogen source for the hydrogen transfer of 1a
in the presence of Pd(OAc)2 as shown in eq 1, the use of
HCOOH (2.0 equiv) or HCOOH (2.0 equiv)/KOH (1.5
equiv) resulted in the formation of 2a in trace amounts only.
These results indicated that the decomposition of HCOOH
into H2 and CO2 was the predominant reaction under the
reaction conditions, which limited the hydrogenation of 1a.
Therefore, the formation of HCOOH in a proper concentra-
tion in the reaction system is the crucial factor in the transfer
semihydrogenation of internal alkynes.10
GCMS m/z (% rel intensity) 180 (Mþ, 100), 165 (52), 152 (14),
102 (10), 89 (29), 77 (22).
cis-1-Deuterio-1,2-diphenylethene, 2a-d1:12 1H NMR (300
2
MHz, CDCl3) δ 7.24-7.15 (m, 10H), 6.58 (s, 1H); D NMR
(600 MHz, CDCl3) δ 6.72 (s, 1D); GCMS m/z (% rel intensity)
181 (Mþ, 90), 180 (100), 179 (68), 178 (32), 166 (32), 153 (11), 90
(24), 77 (21).
cis-4,40-Dimethylstilbene, 2b:13 1H NMR (300 MHz, CDCl3) δ
7.15 (d, 4H, J = 7.9 Hz), 7.01 (d, 4H, J = 7.9 Hz), 6.50 (s, 2H),
2.29 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 136.8, 134.6, 129.6,
129.0, 128.9, 21.4; GCMS m/z (% rel intensity) 208 (Mþ, 100), 193
(76), 178 (70), 165 (11), 152 (6), 115 (18), 102 (14), 89 (13), 77 (5).
cis-4-Ethoxy-40-propylstilbene, 2c: 1H NMR (300 MHz,
CDCl3) δ 7.18 (d, 4H, J = 7.9 Hz), 7.01 (d, 2H, J = 8.3 Hz),
6.73 (d, 2H, J = 8.6 Hz), 6.45 (s, 2H), 3.96 (q, 2H, J = 6.9 Hz),
2.53 (t, 2H, J = 7.5 Hz), 1.61 (m, 2H), 1.37 (t, 3H, J = 6.9 Hz),
0.92 (t, 3H, J = 7.5 Hz); 13C NMR (75 MHz, CDCl3) δ 158.1,
141.5, 135.0, 130.2, 129.9, 129.3, 128.8, 128.4, 114.2, 63.4, 37.9,
24.5, 15.0, 14.0; GCMS m/z (% rel intensity) 266 (Mþ, 100), 237
(81), 209 (54), 194 (8), 178 (13), 165 (26), 115 (9), 104 (8), 91 (8);
HRMS m/z [Mþ þ H] 267.1758, calcd for C19H23O 267.1743.
cis-1-Phenyl-2-(4-benzyloxyphenyl)ethane, 2d:14 1H NMR
(300 MHz, CDCl3) δ 7.43-7.17 (m, 12H), 6.83 (d, 2H, J =
8.9 Hz), 6.52 (s, 2H), 5.03 (s, 2H); 13C NMR (75 MHz, CDCl3) δ
158.0, 137.7, 137.0, 130.3, 130.0, 129.8, 128.9, 128.7, 128.4,
128.1, 127.6, 127.0, 114.6, 70.1; GCMS m/z (% rel intensity)
286 (Mþ, 22), 195 (19), 165 (10), 152 (7), 91 (100).
Conclusions
In summary, we have developed a practical and efficient
Pd(OAc)2-catalyzed transfer semihydrogenation of internal
alkynes to afford cis-alkenes in good to high yields with
excellent chemo- and stereoselectivity by using DMF/KOH
as the hydrogen source system. The most significant advan-
tages of the present catalytic system include the use of stable
Pd(OAc)2 as catalyst and the wide generality for semihydro-
genation of diarylacetylenes bearing different functional
groups, as well as dialkylacetyelenes to afford cis-alkenes
without reduction to alkanes. The catalytic process was also
found to be applicable to the synthesis of the analogues of
combretastatin A-4.
4-(cis-Styryl)biphenyl, 2e:15 1H NMR (300 MHz, CDCl3) δ
7.66-7.29 (m, 14H), 6.69 (s, 2H); 13C NMR (75 MHz, CDCl3) δ
140.8, 139.9, 137.4, 136.3, 130.5, 129.9, 129.5, 129.0, 128.9,
128.4, 127.4, 127.3, 127.0, 126.9; GCMS m/z (% rel intensity)
256 (Mþ, 100), 239 (22), 178 (21), 165 (18), 152 (9), 91 (10), 77 (5).
cis-1-(4-Methoxyphenyl)-2-phenylethene, 2f:16 1H NMR (300
MHz, CDCl3) δ 7.28-7.16 (m, 7H), 6.74 (d, 2H, J = 8.6 Hz),
6.51 (s, 2H), 3.75 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 158.8,
137.7, 130.3, 129.9, 129.7, 128.9, 128.3, 127.0, 113.7, 55.3;
GCMS m/z (% rel intensity) 210 (Mþ, 100), 195 (21), 179 (18),
165 (39), 152 (30), 128 (4), 115 (8), 89 (12).
cis-1-(2-Methoxyphenyl)-2-phenylethene, 2g:17 1H NMR (300
MHz, CDCl3) δ 7.27-7.16 (m, 7H), 6.90 (d, 1H, J = 8.2 Hz),
6.79-6.62 (m, 3H), 3.84 (s, 3H); 13C NMR (75 MHz, CDCl3) δ
157.3, 137.4, 130.4, 130.2, 129.0, 128.7, 128.2, 127.0, 126.3, 125.9,
120.3, 110.8, 55.6; GCMS m/z (% rel intensity) 210 (Mþ, 100), 195
(7), 179 (17), 165 (54), 152 (31), 139 (6), 119 (36), 104 (32), 91 (37).
cis-1-(2-Fluorophenyl)-2-phenylethene, 2h:18 1H NMR (300
MHz, CDCl3) δ 7.28-7.23 (m, 7H), 7.13-6.95 (m, 2H), 6.78
(d, 1H, J = 12.1 Hz), 6.68 (d, 1H, J = 12.1 Hz); 13C NMR (75
MHz, CDCl3) δ 160.5 (d, J = 247.3 Hz), 136.9, 132.3, 130.6 (d,
J = 28.8 Hz), 129.0 (d, J = 86.6 Hz), 128.9, 128.3, 127.5, 125.1
(d, J = 151.3 Hz), 127.7 (d, J = 36.0 Hz), 122.7 (d, J = 28.8 Hz),
115.7 (d, J = 216.3 Hz); GCMS m/z (% rel intensity) 198 (Mþ,
100), 183 (34), 177 (29), 165 (5), 120 (7), 98 (15), 77 (9).
Experimental Section
Typical Experimental Procedure for Transfer Semihydrogena-
tion of Diphenylacetylene (1a) Affording cis-Stilbene (2a) (Table 1,
entry 5). Diphenylacetylene (1a) (89.0 mg, 0.5 mmol), KOH (42.0
mg, 0.75 mmol), Pd(OAc)2 (2.3 mg, 0.01 mmol), and DMF (1.0
mL) were placed in a thick-walled Pyrex screw-cap tube (25 mL)
under a nitrogen atmosphere, and the tube was capped and the
mixture heated in an oil bath at 145 °C with stirring for 6 h. After
the reaction mixture was cooled to room temperature, the crude
reaction mixture was diluted with CH2Cl2 (2.0 mL) and cyclohex-
ane (2.0 mL), and n-octadecane (101.7 mg, 0.4 mmol) was then
added as an internal standard for GC analysis. After GC and GC-
MS analyses of the reaction mixture, volatiles were removed under
reduced pressure, and the residue was subjected to silica gel
column chromatography [eluting with cyclohexane] to afford 2a
as a colorless viscous oil (86.4 mg, 0.48 mmol, 96%). The GC
analysis of the reaction mixture showed the formation of 2a and 3a
in 99% GC yield, and the ratio of 2a:3a was 97:3.
cis-1-(4-Acetylphenyl)-2-phenylethene, 2i:19 1H NMR (300 MHz,
CDCl3) δ 7.80 (d, 2H, J = 8.2 Hz), 7.31 (d, 2H, J = 8.2 Hz),
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Characterization Data of Products. cis-Stilbene, 2a:11 1H
NMR (300 MHz, CDCl3) δ 7.25-7.17 (m, 10H), 6.59 (s, 2H);
13C NMR (75 MHz, CDCl3) δ 137.4, 130.4, 129.0, 128.3, 127.2;
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(10) One reviewer considered that the true reductant might be Me2NH or
Et3N, and suggested authors to perform the semihydrogenation using amine/
KOH as a possible reductant. However, under similar reaction conditions,
the use of Et3N or Bu2NH instead of DMF resulted in only a trace amount of
stilbenes formation in the reaction of 1a.
€
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