Organic Letters
Letter
a
Table 1. Copper-Catalyzed 1,2-/1,4-hydrosilylation of 1-Substituted 1,3-Dienes
entry
catalyst
ligand (mol %)
[Si]H (equiv)
solvent (0.5 mL)
temp (°C)
time (h)
yield (%) (2/3)
1
2
3
4
5
6
7
8
9
Cu(OAc) (10)
PPh (10)
PhSiH (3.0)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
30
1.5
1.5
1.5
1.5
1.5
1.5
20
20
20
20
20
1.5
1.5
1.5
1.5
1.5
1.5
13
25 (68:32)
33 (70:30)
69 (84:16)
48 (67:33)
52 (62:38)
37 (89:11)
75 (93:7)
51 (94:6)
94 (90:10)
98 (93:7)
2
3
3
Cu(OAc) (10)
Ph PMe (10)
PhSiH (3.0)
2
2
3
Cu(OAc) (10)
XantPhos (10)
DPEphos (10)
dppbz (10)
dppp (10)
dppp (10)
dppp (10)
dppp (10)
dppp (10)
dppp (5)
dcype (10)
dcype (10)
dcype (10)
dcype (10)
dcype (10)
dcype (10)
dcype (1)
PhSiH (3.0)
2
3
Cu(OAc) (10)
PhSiH (3.0)
2
3
Cu(OAc) (10)
PhSiH (3.0)
2
3
Cu(OAc) (10)
PhSiH (3.0)
2
3
Cu(OAc) (10)
PhSiH (3.0)
2
3
Cu(OAc) (10)
PhMe SiH (3.0)
2
2
Cu(OAc) (10)
Ph SiH (3.0)
2
2
2
10
11
12
13
14
15
16
17
18
Cu(etacac) (10)
Ph SiH (3.0)
2
2
2
b
Cu(etacac) (5)
Ph SiH (2.0)
95 (93:7)
2
2
2
Cu(OAc) (10)
PhSiH (3.0)
57 (68:32)
41 (56:44)
75 (45:55)
65 (28:72)
62 (26:74)
75 (17:83)
2
3
Cu(OAc) (10)
PhSiH (3.0)
Et O
2
3
2
Cu(OAc) (10)
PhSiH (3.0)
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
2
3
Copper methacrylate (10)
PhSiH (3.0)
3
Cu(etacac) (10)
PhSiH (3.0)
2
3
Cu(acac) (10)
PhSiH (3.0)
2
3
b,c
Cu(acac) (1)
PhSiH (2.0)
1,4-dioxane
73 (10:90)
2
3
a
1
Conditions: 1a, [Si]H (0.6 mmol) in solvent (0.5 mL). The yields were determined by H NMR using Cl CHCHCl as an internal standard. The
2
2
1
b
c
ratio of 2/3 was determined by crude H NMR. Isolated yield. 1,4-Dioxane = 1 mL; XantPhos = 4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene; DPEphos = bis(2-diphenylphosphinophenyl)ether; dppbz = 1,2-bis(diphenylphosphanyl)benzene; dppp = bis(1,3-
diphenylphosphino)propane; Cu(etacac) = copper(II) ethylacetoacetate; dcype = 1,2-bis(dicyclohexylphosphino)ethane.
2
copper-catalyzed 1,2- and 1,4-hydrosilylations of 1-substituted
,3-dienes. Initially, PPh as ligand was applied to this reaction.
Compared with the bite angle of dppp with copper center, a
1
smaller bite angle of dcype with copper center may be beneficial
3
19
About 25% yield of mixed hydrosilylation products containing
the 1,2-hydrosilylation product 2u and the 1,4-hydrosilylation
product 3u was observed (Table 1, entry 1). The yields for the
formation of 2u increased when the hydrosilylation reaction was
catalyzed by Cu(OAc) bearing a ligand other than PPh , such as
for the regioselective 1,4-hydrosilylation.
Having established the conditions for the regioselective 1,2-
hydrosilylation of 1-substituted 1,3-dienes, the substrate scope
of 1,3-dienes was investigated (Figure 1). In general, a wide
range of 1,3-dienes reacted smoothly with Ph SiH under the
2
3
2
2
monophosphine and diphosphine ligands (Table 1, entries 2−
optimized conditions to afford the corresponding 1,2-hydro-
silylation products in high yields and with excellent
regioselectivities. The substrates bearing an alkyl substituent at
different positions of the phenyl ring were smoothly conducted
(2b−2e). Again, the (E)-4-(buta-1,3-dien-1-yl)-1,1′-biphenyl
and (E)-2-(buta-1,3-dien-1-yl)naphthalene could also afford the
corresponding products in good yields (2f, 2l). When different
halide-substituted substrates were subjected to the hydro-
silylation reaction, the desired products were obtained in good
results (2g−2j). Besides, installation of methoxy group at ortho-
position of phenyl ring led to a slightly lower regioselectivity
(2k). The result could be rationalized by the directing role of the
oxygen atom for the copper intermediate generated in situ.
Heterocycle-substituted 1,3-dienes like (E)-2-(buta-1,3-dien-1-
yl)thiophene and (E)-2-(buta-1,3-dien-1-yl)furan could also
afford the desired products in high yields and with excellent
regioselectivity (2n, 2o). Furthermore, we found that the
regioselective 1,2-hydrosilylation was not limited to 1-
monosubstituted 1,3-dienes. 1,1-Disubstituted 1,3-dienes were
also successfully transformed to the allylsilane products in high
yields and regioselectivities under the standard reaction
conditions (2p−2t).
6
). Finally, using dppp as a ligand resulted in notable
improvement which gave the 1,2-hydrosilylation product 2u as
the major product (Table 1, entry 6). When the reaction time
was prolonged to 20 h, the yield of 2u and 3u was enhanced to
7
5% with 93:7 regioselectivity (Table 1, entry 7). Further
optimization studies indicated that the choice of silane had a
significant impact on improving yield of the desired product 2
(
Table 1, entries 7−9). Using Ph SiH instead of PhSiH , the
2
2
3
yield of desired product was elevated to 94% with 90:10
regioselectivity (Table 1, entry 9). The yield was slightly
improved when Cu(etacac) was employed as a catalyst (Table
2
1
, entry 10). The reaction proceeded without a significant
decrease in yield when 5 mol % catalyst and 2.0 equiv of Ph SiH
2
2
were used (Table 1, entry 11). During ligand screening, it was
found that the electron-rich ligand dcype could improve the
ratio of 1,4-hydrosilylation product (Table 1, entry 12). With
this encouraging observation, we next examined a range of
reaction parameters including solvent, copper catalyst, temper-
ature, and concentration (Table 1, entries 13−18). These
experiments demonstrated that the 1,4-hydrosilylation product
formed selectively in 73% yield in the presence of Cu(acac) as a
2
catalyst and dcype as a ligand with PhSiH as the silylation
Subsequently, the regioselective 1,4-hydrosilylation of 1-
substituted 1,3-dienes were conducted, and the results are
summarized in Figure 2. The 1,4-hydrosilylation of 1-substituted
1,3-dienes bearing an alkyl group on the phenyl ring proceeded
3
reagent in 1,4-dioxane solution at 30 °C (Table 1, entry 18).
Using Ph SiH as the silyl source did not support the formation
2
2
of 1,4-hydrosilylation product due to its large steric hindrance.
4
738
Org. Lett. 2021, 23, 4736−4742