Organic Letters
Letter
The hydrosilylation proceeds selectively via syn-addition of Si−
H bond to the alkyne group incorporating the silyl group at the
site proximal to the alkene.
incorporating the silyl group at the site proximal to the alkene.
The high regioselectivity achieved by 1d inspired us to do
further structural modification on the IPO ligand. To our
delight, the sterically more hindered (IPO)Fe complex 1e
bearing a tert-Bu group on the oxazoline arm and diphenyl
methyl groups at the ortho positions of the N-aryl group
showed excellent activity and regioselectivity, affording the 1,2-
hydrosilylation product 3a in satisfying yield (77%) with the
ratio 3a:3a′ being more than 38:1 (entry 6). The influence of
solvent, base, and temperature was investigated. Solvent
screening revealed the use of nonpolar solvent (hexane)
improved both the yield of 3a (87%) and the regioselectivity
(3a′: 1%, entry 7). Excellent yield (94%) and selectivity were
obtained with NaOtBu as the base (entry 8). The reaction
proceeded smoothly even at 0 °C furnishing 3a in 96% yield
with excellent regioselectivity (3a′: 2%, entry 9).
In an initial investigation, we employed 2a as the substrate
and attempted the hydrosilylation in the presence of an iron
precatalyst with NaBEt3H as the activator in THF at rt. We
focused on the evaluation of tridentate iron complexes, which
have proven to be efficient precatalysts for alkene or alkyne
hydrosilylation.14 High conversions but low selectivities were
observed for iron complexes supported by pyridinediimine
(L1), phosphine-bipyridine (1a), and phosphinite-iminopyr-
idine (1b) ligands (Table 1, entries 1−3). In these cases, 2a
Table 1. Identification of Iron Catalyst for Selective
a
Monohydrosilylation of 1,3-Enyne 2a
With optimal reaction conditions established, the scope and
limitations of this iron catalyzed selective hydrosilylation were
explored using 1,3-enynes with various substitution patterns
(Scheme 1). Linear 1,3-enynes (3a−3c) bearing a terminal
double bond and an internal triple bond were selectively
hydrosilylated at alkyne positions, affording 1,3-dienylsilanes in
good yields and excellent regioselectivity. In addition to
primary hydrosilane, the use of secondary hydrosilane15 also
produced the corresponding product (3b) in comparable yield
(85%) and regioselectivity (3b/3b′ = 98:2). The selective
hydrosilylation could be applicable to enynes with a branched
alkyl substitution, as demonstrated by the formation of 3e and
3f in moderate yields and excellent regioselectivities. The
reaction of enynes containing silyl and benzyl protected
hydroxyl groups proceeded smoothly to furnish the desired
products (3g−3k) selectively. Enynes with heteroatoms at the
progargylic and homopropargylic position (3l and 3m) were
readily transformed to the desired products with excellent
regioselectivities, but the yields were relatively low. These
results also demonstrated that the reducible functionalities
such as amide and ester could be tolerated under the
hydrosilylaltion conditions. The synthetically useful C(sp3)−
Cl bond was also tolerated, giving chloro-substituted 1,3-
dienylsilane 3n in good yield (70%) with excellent
regioselectivity (96:4). In contrast, poor selectivity was
observed for 4-phenyl 1-buten-3-yne 4a, leading to the
formation of complicated mixtures.
The reaction is not only suitable for enynes with terminal
alkenes but also feasible for 1,3-enynes bearing 1-monosub-
stitution and 1,1-disubstitution around the double bond.
Selective hydrosilylation was achieved for 1,1-dimethyl 1-
buten-3-yne to furnish 3o in 43% yield with 99:1 (3o:3o′)
regioselectivity. Besides aliphatic substitutions, aromatic
substituitions were successfully employed at the 1-position of
enynes, affording aryl 1,3-dienylsilanes (3p−3s) in good yields
with excellent regioselectivities. However, the introduction of
substitutions at the 2-position of enynes (such as 4b) resulted
in low conversion, probably as a result of the large steric
hindrance around the alkyne group. In addition, the catalytic
system is not compatible with enynes bearing termianl alkynes,
as demonstrated by the low conversion of substrate 4c.
To further showcase the synthetic potential of this method,
the selective hydrosilylation of 1,3-enyes was carried out on
gram scale. With reduced catalyst loading (1.0 mol %), the
gram scale synthesis of 3a and 3r was realized in high yields
with excellent regioselectivities (Scheme 2a). The utility of 1,3-
dienylsilanes was demonstrated by further derivatization.
b
yield (%)
b
entry
cat.
activator
conv (%)
3a
3a′
<1
3
<1
16
5
2
1
1
2
1
2
3
4
5
6
7
8
9
FeBr2/L1
NaBEt3H
NaBEt3H
NaBEt3H
NaBEt3H
NaBEt3H
NaBEt3H
NaBEt3H
NaOtBu
82
100
63
85
98
100
100
100
100
<1
4
1a
1b
1c
1d
1e
1e
1e
1e
<1
15
57
77
87
94
96
c
c
cd
,
NaOtBu
a
Conditions: 2a (0.3 mmol), PhSiH3 (0.33 mmol), cat. (2 mol %),
and NaBEt3H (6 mol %) in THF (2 mL) at room temperature for 12
h. Determined by GC using mesitylene as the internal standard.
b
c
d
Hexane as the solvent. 0 °C.
was consumed in varying degrees. However, vinyl silanes 3a
and 3a′ originated from hydrosilylation of the alkyne group of
enyne were barely detected, and the poor chemoselectivity
made the identification of the products difficult. The use of
iron complex 1c bearing a pyridinebisoxazoline ligand led to
the formation of 3a and its regioisomer 3a′ in yields of 15%
and 16%, respectively (entry 4). With one oxazoline arm
replaced by an imine, the iminopyridine-oxazoline (IPO)
ligated iron complex 1d enhanced the selectivity significantly,
affording 3a in 57% yield with a minor amount (5%) of the
regioisomer 3a′ (entry 5). The structure of 3a confirmed by
NMR analysis indicates hydrosilylaton of 1,3-enynes proceeds
selectively via syn-addition of a Si−H bond to the alkyne group
2376
Org. Lett. 2021, 23, 2375−2379
Guo, Zhihao
