Tetrahedron Letters
SnCl4–Zn: a novel reductive system for deoxygenative coupling
of aliphatic, aromatic, chalcone epoxide, and indanone carbonyl
compounds to olefins
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Gulab Khushalrao Pathe, Naseem Ahmed
Department of Chemistry, Indian Institute of Technology Roorke, Roorkee 247 667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
SnCl4–Zn complex provided
a novel reductive system in the deoxygenative cross-coupling of
Received 17 December 2014
Revised 30 January 2015
Accepted 31 January 2015
Available online 7 February 2015
aliphatic, aromatic, chalcone epoxide and indanone carbonyl compounds to olefins in high yield
(55–86%) at reflux temperature in THF. The advantage of this reagent is inexpensive, short reaction time,
and high yield compared to the reagents used in the McMurry cross-coupling reaction.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
SnCl4–Zn
McMurry cross-coupling
Chalcone epoxide
Indanone
Aldehydes and ketones
Tamoxifen analogs
Introduction
reducing agent in THF. Thus, a complex of TiCl4–Zn–(THF)2 gener-
ated in situ known as low valent titanium (LVT) has been used in
In the carbon–carbon bonds formation, the McMurry reaction
plays an important role to obtain homo- and cross-coupled alkenes
from aliphatic and aromatic aldehydes and ketones in the presence
of in situ generated low valent titanium (LVT) reagents at reflux
temperature.1 However, the reaction gave a moderate yield of
cross-coupled product due to homo-coupled product formation.
To enhance the yield of the cross-coupled products under mild
reaction conditions, the different reagents are explored for the
McMurry reaction. For example, magnesium–mercury couple,
the reductive deoxygenating coupling of aldehydes and/or ketones
to titanium pinacolates, followed by the removal of TiO2 gave the
olefins, but the exact reaction mechanism is still debatable.10,11
Likewise, we expect that it might be taking place in SnCl4–Zn (used
in 1:2 ratio) and THF to form a complex of SnCl4–Zn–(THF)2 in situ
low valent tin (LVT) as Sn(II) which converted carbonyl oxygen to
tin pinacolate, followed by the removal of SnO2 to give the olefins.
Therefore, in continuation of our interest to develop new meth-
ods in the organic synthesis and the acid catalysis reactions12 here-
in, we report a novel and efficient reagent, SnCl4–Zn system for the
McMurry cross-coupling reaction in the conversion of aliphatic and
aromatic ketones, aldehydes, chalcone epoxides, and indanones
into olefins and also useful in the synthesis of molecules like tamox-
ifen analogs in good yield within 4–4.5 h at reflux temperature.
NbCl5/NaAlH4, zinc–copper couple,3 LiAlH4, dicyclopentadienyl
titaniumdichloride,5 trimethyl aluminum.6 Although, these proce-
dures have drawbacks like costly reagents, low yield, the longer
reaction time, and/or less functional group tolerance. In recent
years, tin tetrahalides (SnX4, X = Cl, Br) have been widely used as
Lewis acids in numerous organic syntheses.7 In many cases, they
have been reported as efficient catalysts and easy to handle as
compared to other metal halides such as TiX4 AlX3, ZnX2, and ZrX4.8
Generally, the metal alloys are prepared by the Rieke’s method
and used as a reductive deoxygenating agent for the carbonyl com-
pounds in the organic synthesis.9 For example, in the McMurry
reaction Ti(IV) reagent reduced to Ti(II) in the presence of Zn as
2
4
Results and discussion
Initially, we optimized the cross-coupling reaction conditions in
the reaction of chalcone epoxide 1 and propiophenone 2 used in
1:1.5 ratio and varying the equivalents of SnCl4–Zn (used in 1:2
ratio). We obtained the cross-coupled product in 50% and 55%
yields in 4 h using 1 and 2 equiv of SnCl4–Zn, respectively (Table 1,
entries 1 & 2). Then, SnCl4–Zn was used in 3 equiv, the yield was
serendipitously improved up to 75% in 4 h (Table 1, entry 3).
⇑
Corresponding author. Fax: +91 1332 285745.
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.