A new approach to organomanganese compounds: The tellurium/manganese exchange reaction

Article abstract of DOI:10.1016/j.tetlet.2010.08.005

Diorganomanganese compounds react with aryl, vinyl, and alkynyl tellurides in a tellurium/manganese exchange reaction. The new mixed organomanganese reagents react selectively with electrophiles.

Full text of DOI:10.1016/j.tetlet.2010.08.005

Tetrahedron Letters 51 (2010) 5426–5429
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A new approach to organomanganese compounds: the tellurium/manganese
exchange reaction
⇑
⇑
Márcio S. Silva, João V. Comasseto , Alcindo A. Dos Santos
Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, CxP 26077, 05508-000 São Paulo, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Diorganomanganese compounds react with aryl, vinyl, and alkynyl tellurides in a tellurium/manganese
exchange reaction. The new mixed organomanganese reagents react selectively with electrophiles.
Ó 2010 Published by Elsevier Ltd.
Received 8 July 2010
Revised 23 July 2010
Accepted 2 August 2010
Available online 10 August 2010
Keywords:
Tellurium compounds
Organomanganese
Exchange reaction
The use of organomanganese compounds for synthetic purposes
is attractive because this element is readily available, cheap, and
more environmentally benign in comparison with some other met-
als commonly used in synthesis. The intensive study of organo-
manganese compounds started only in 1976 when Professor J. F.
Normant and G. Cahiez initiated a systematic study on their prep-
aration and reactivity. Currently there are many studies that
describe the preparation and the reactivity of this class of organo-
metallic compounds.1
On the other hand, organotellurides have been efficiently trans-
formed into a number of organometallics by tellurium/metal ex-
change. This methodology focused mainly on the Te/Li exchange,
because it is fast and clean, leading to useful organolithiums2
and other organometallics derived from them by transmetalla-
tion3, or even to other organoelemental compounds not easily ob-
tained by other methods, such as the preparation of Z-vinyl
stananes from Z-vinyl-lithiums, derived from Z-vinyl-tellurides.4
However, the direct Te/metal exchange is also well documented
for Te/Zn,5 Te/Cu,6 Te/Na,7 Te/Mg7, and Te/Ca7 exchanges. This easy
access to a range of organometallics from organotellurides allowed
the use of these organochalcogenides in the synthesis of biologi-
cally active compounds.8 In addition, there are many efficient
and well-established methodologies to prepare organotellurides
from elemental tellurium.2,3
rides as one of the most versatile precursors of reactive
organometallics.
Two general methods are available for the preparation of organo-
manganese species, namely, the transmetallation of organolithium
or organomagnesium reagents with manganese salts and the oxida-
tive addition of organic halides to manganese.8 A third method,
involvinga heteroatom/manganeseexchange is restrictedto organic
halides and even so, the method is scarcely studied.1,9 In view of the
easy Te/metal exchange reactions commented above and the avail-
ability of several classes of organic tellurides,2 we decided to inves-
tigate the Te/Mn exchange reaction as a straightforward method to
access structurally more complex representatives of this interesting
but still little-explored class of organometallics.
In this work we introduce the unprecedented tellurium/manga-
nese exchange and the capture of the new organometallics with
aldehydes and ketones.
This study was initiated by reacting telluride 5a with different
organomanganese reagents 4 in order to determine the reagent
of choice to perform the tellurium/manganese exchange reaction.
The formed organomanganese 6 was captured with benzaldehyde
(7a) or acetophenone (7b). The vinyl telluride 5a was prepared in
85% yield by hydrotelluration of phenyl acetylene, starting from
elemental tellurium and n-butyllithium as depicted in the scheme
of Table 1.13–16 All other organotellurides used in this study were
prepared according to the references indicated in Table 2.
The exchange reaction could be visualized by the change in the
color of the solution of 4 after the addition of 5a, from limpid
brownish to limpid purple. This change in the reaction mixture
appearance was almost instantaneous and identical for all the spe-
cies 4 shown in Table 1, except for the reaction with compound 4a,
Having in mind these features and the feasible Te/metal ex-
change reaction presented before, we can consider organotellu-
⇑
Corresponding authors. Tel.: +55 11 3091 1180.
E-mail addresses: jvcomass@iq.usp.br (J.V. Comasseto), alcindo@iq.usp.br (A.A.
Dos Santos).
0040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.08.005

M. S. Silva et al. / Tetrahedron Letters 51 (2010) 5426–5429
5427
Table 1
uct was observed, the telluride 5a being recovered unchanged (Ta-
ble 1, entry 1).
As it is shown in Table 1, the species 4b–e, all undergo the Te/
Mn exchange reaction and react with 7a and 7b leading to prod-
ucts in variable yields. Typically, the exchange reaction took place
within 30 minutes at À78 °C.
Tellurium/manganese exchange reaction10
Te0
1) nBuLi/THF, r.t.
2) EtOH
3) C6H5
As a rule, benzaldehyde (7a) behaved as a better electrophile,
allowing the isolation of the corresponding allylic secondary alco-
hol 8a in higher yields and lower reaction times than the tertiary
alcohol 8b (entries 2–9). By comparing the performance of the
organometallics, the superiority of the di-alkylmanganese over
all the others (Table 1) becomes evident. Dimethyl (4e) and di-
butylmanganese (4b) had similar reactivity toward benzaldehyde
(7a) (Table 1, entries 2 and 8), but in the reaction with acetophe-
none (7b), compound 4b gave a better product yield than 4e (Table
1, entries 3 and 9). Magnesium manganate 4d presented slightly
better results than the corresponding lithium analog 4c (Table 1,
entries 4–7). It is worthy of note that in no instance was n-butyl
transfer observed.
Te nBu
C6H5
5a
MnCl2
[R-MnX]
R
M
THF, N2
-78 °C
THF, N2
-78 °C
4
nBuTeR
M = Li, MgBr
MnX
C6H5
6a
C6H5CHO
7a
C6H5COCH3
7b
The chemoselectivity of the mixed organomanganese reagent
6a was also investigated toward a mixture of an aldehyde and a ke-
tone. An equimolar solution of benzaldehyde (7a) and acetophe-
none (7b) in THF was added to a solution of the preformed
butyl-vinylmanganese reagent 6a at -78 °C. While compound 8b,
resulting from the addition of 6a to 7b, was isolated in 10% yield,
the secondary alcohol 8a resulting from the reaction of 6a with
7a was isolated in 65% yield after flash chromatography purifica-
tion (Scheme 1).
C6H5
C6H5
C6H5
OH
8a
C6H5
OH
8b
Entry
RMnXa
Electrophile
Product/time (h)/yieldb (%)
1
2
3
4
5
6
7
8
9
nBuMnClc 4a
nBuMnnBu 4b
4b
C6H5CHO 7a
—
7a
8a/0.5/82
8b/1.0/70
8a/0.5/48
8b/1.0/30
8a/0.5/58
8b/1.0/36
8a/0.5/77
8b/1.0/52
C6H5COCH3 7b
nBu3MnLi 4c
4c
7a
7b
7a
7b
7a
7b
nBu3MnMgBr 4d
4d
Such selectivity is similar to the one observed for organomanga-
nese halides.11
MeMnMe 4e
4e
In view of these results, di-butylmanganese was chosen as the
ideal partner in the preparation of other mixed organomanganese
reagents by Te/Mn exchange reaction. In Table 2 the results of the
reaction of some aldehydes and ketones with vinyl-, aryl-, hetero-
aryl-, and alkynyl-manganese reagents derived from the corre-
sponding organotellurides are summarized. It is worthy of note
that all tellurides used in this study are not pungent compounds,
and can be manipulated in the presence of light and air.
As can be observed, vinyl, aryl, heteroaryl, and alkynyltellurides
undergo the tellurium/manganese exchange reaction within
30 min under the reaction with di-butylmanganese 4b at À78 °C,
a
The organomanganese reagents were prepared by reacting organolithium or
Grignard reagents with MnCl2.
b
Isolated yields.
c
The tellurium/manganese exchange reaction failed.
in which case no color change was observed. After the formation of
the vinyl manganese species 6, it was captured with benzaldehyde
(7a), leading to 8a. In the case of compound 4a, no addition prod-
Table 2
Preparation of organomanganese reagents by tellurium/manganese exchange and their reaction with aldehydes and ketones
THF, N 2
4b
E+
R
TenBu
5
R
MnnBu
R
E
-78 °C,
nBuTenBu
-78 °C
6
8
-
Entry
1
Organotelluride 5
nButyl organomanganese 6
Electrophile E+
Product 8
Time (h)/yielda (%)
1.0 h/75%
O
C6H5
C6H5
C6H5
HO
8c
MnnBu
TenBu
2
2
5a13
6a
7c
O
OH
C6H5
2
3
C6H5TenBu 5b14
C6H5MnnBu 6b
0.5 h/88%
6
6
7d
8d
OH
o-MeC6H4TenBu 5c14
o-MeC6H4MnnBu 6c
7c
1.0 h/72%
o-MeC6H4
2
8e
(continued on next page)

5428
M. S. Silva et al. / Tetrahedron Letters 51 (2010) 5426–5429
Table 2 (continued)
Entry
Organotelluride 5
nButyl organomanganese 6
Electrophile E+
Product 8
OH
Time (h)/yielda (%)
0.5 h/80%
Te nBu
MnnBu
S
S
S
6
4
7d
5d15
6d
8f
O
THPO
TenBu
THPO
MnnBu
THPO
p-MeC6H4
5
0.5 h/75%
p-MeC6H4
OH
5e13
6e
7e
8g
THPO
C6H5
6
7
8
5e
6e
7a
7d
7b
0.5 h/68%
0.5 h/56%b
1.0 h/48%b
C6H5
OH
8h
C6H5
Te nBu
C6H5
MnnBu
OH
5f16
6f
6
OH
8i
Te nBu
MnnBu
3
3
3
C6H5
5g16
6g
8j
a
Isolated yields after column chromatography.
The low yield is a consequence of the competitive n-butyl transfer to the electrophile.
b
Table 3
Reactivity of (Z)-butyl-vinylmanganese compound 6b with enones
C6H5
C6H5
7a and 7b
C6H5
+
MnnBu
6a
O
THF/-78 °C
HO
8a
65%
C6H5
HO
C6H5
8b
10%
O
O
O
C6H5
Scheme 1. Chemoselectivity of the mixed organomanganese reagent (6a) toward
benzaldehyde (7a) and acetophenone (7b).
9
C6H5
+
MnnBu
6a
THF, - 78 °C
10
11
yielding the new mixed di-organomanganese 6, along with the inert
di-butyltelluride (Table 2). As commented before, for the reaction of
6a with benzaldehyde (7a) and acetophenone (7b), the organoman-
ganese reagents 6a–g prepared from tellurides 5a–g reacted faster,
leading to products in better yields, in the use of aldehydes as com-
pared to ketones. Contrary to vinyl, aryl, and heteroaryl organoman-
ganese reagents 6a–e, the alkynylbutylmanganese 6f and 6g
presented lower yield on reaction with both an aldehyde and a
ketone, due to the competitive butyl transfer (Table 2, entries 7
and 8). In all these reactions, di-butyltelluride was produced as the
only by-product in the exchange reaction step.
Entry
1
Enone
Time (h)
Yielda (%)
O
55:25
10a 11a
0.5
O
O
42:28
10b 11b
2
3
0.5
1.0
5:74
10c 11c
When Z-vinylic tellurides 5a and 5e were used, the retention of
the olefin configuration was observed. An interesting experimental
feature of the exchange step is that the addition of the tellurides 5b
through 5g to a solution of 4b did not cause any change in the color
of the reaction mixture, as did the addition of telluride 5a. How-
ever, TLC analysis showed the immediate disappearance of the
starting telluride, indicating that the exchange reaction in these
cases is also fast.
a
Isolated yield.
structurally complex reactive organometallics by element/metal
exchange reactions.
Another interesting experiment was performed by capturing
the butyl-vinylmanganese compound 6a with enones 9. In this
case, besides the 1,4-addition product 10, the formation of 11,
the product of the oxidative dimerization of the enone was ob-
served, as the result of a long time known reaction of organoman-
ganese compounds with enones.12 The results are shown in Table 3
In conclusion, the Te/Mn exchange reaction is easily performed
leading to mixed diorganomanganese reagents, which react with
organic substrates in the expected way. The present study and all
other previously reported Te/metal exchange reactions show that
the organotellurides constitute one of the best options to access
Acknowledgments
The authors acknowledge FAPESP, CNPq and CAPES for financial
support.
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