TETRAHEDRON
LETTERS
Tetrahedron Letters 42 (2001) 9093–9095 Pergamon
Highly active salted low-valent titanium reagents for various
SET induced reactions
Shyam Rele, Subrata Chattopadhyay and Sandip K. Nayak*
Bio -Organic Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Received 24 August 2001; revised 26 September 2001; accepted 19 October 2001
Abstract—External addition of inorganic salts (Group I and II metal halides) to the preformed low-valent titanium reagent A
(TiCl
3
–Li–THF) dramatically enhanced its activity. The new reagents were used to carry out various SET reactions including
McMurry’s olefination at a faster rate even at ambient temperature. © 2001 Published by Elsevier Science Ltd.
Low-valent titanium (LVT) reagent-mediated McMurry
coupling is a key reaction in organic synthesis.
1–3
How-
ever, olefin synthesis by this reaction requires higher
temperatures
1,2
especially with aliphatic substrates,
4
which might be detrimental for compounds possessing
thermolabile functionalities. This warrants activation of
the LVT reagents, which we have earlier accomplished
by changing the ligand environment or addition of
chemical redox agents leading to some new LVT prepa-
rations.
5
Considering the formation of intermetallic
Timetal bonds between Ti and inorganic salts,
6
we
envisaged that external addition of an inorganic salt
might be an alternative for LVT activation. This com-
munication constitutes the first report on the activation
of LVT reagents by addition of inorganic salts and their
use in various single electron transfer (SET) reactions
including the McMurry coupling.
The ‘salted’ LVT reagents used for the study were
prepared by refluxing a mixture of the preformed LVT
reagent A [TiCl
3
–Li–THF] and various metal salts for 1
h. The new reagents, obtained as viscous and uniform
slurries (unlike conventional LVT reagents), were used
for various organic transformations as discussed below.
McMurry olefination : Reductive dimerization of ace-
tophenone (1) to 2,3-diphenyl-2-butene (3) (Scheme 1)
was chosen as the model reaction to optimize the
influence of salted LVT reagent and the results are
summarized in Table 1. The reagent A as such, afforded
the pinacol 2 (89%) (Table 1, entry 1) at ambient
temperature, while the stilbene 3 (87%) could be
obtained only after prolonged (16 h) refluxing (Table 1,
entry 2). In contrast, reaction of 1 with reagent A/
MgCl
2
(2 equiv.) furnished the stilbene 3 (25%) along
with the pinacol 2 (62%) even at ambient temperature
in 16 h (Table 1, entry 3), clearly indicating the positive
effect of MgCl
2
on the activity of the reagent A. As
Scheme 1.
Table 1. Effect of salts on the LVT reagent A (TiCl
3
–Li–
THF) in the reductive coupling of acetophenone (1)
Entry Salt
a
Product yields (%)
b
Pinacol 2 (dl :meso ) Stilbene 3 (E:Z)
Nil 1 89 (75:25) Trace
Nil
c
2 – 87 (75:25)
62 (70:30) 25 (65:35) 3 MgCl
2
MgCl
2
d
4 64 (62:38) 22 (62:38)
LiI 5 75 (80:20) Trace
65 (70:30) – 6 CsCl
e
– 82 (85:15) 7 KCl
e
45 (74:26) 8 32 (32:68) ZnCl
2
a
2 equiv. of salt were used.
b
Isolated yields.
c
The reaction was carried out at reflux temp., in other cases the
reaction temp. was 25°C.
d
8 equiv. of salt were used.
e
The reaction was over in 2 h, in other cases the reaction period was
16 h. * Corresponding author. E-mail: sknayak@magnum.barc.ernet.in
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII:S0040-4039(01)02009-3