ArS-mediated AdE reaction of the cobaltcarbonyl-complexed conjugated enynes

Article abstract of DOI:10.1080/10426500590911584

A novel approach to the synthesis of functionalized enynes is elaborated based on the protocol of the three-component coupling of arenesulfenyl chlorides, dicobalthexacarbonyl complexes of the conjugated enynes, and nucleophiles of the π-donor type and a

Full text of DOI:10.1080/10426500590911584

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Phosphorus, Sulfur, and Silicon
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ArS-Mediated Ad Reaction of
E
the Cobaltcarbonyl-Complexed
Conjugated Enynes
Vasilii V. Tumanov a & William A. Smit a
a N.D. Zelinsky Institute of Organic Chemistry,
Russian Academy of Sciences , Moscow, Russia
Published online: 16 Aug 2006.
To cite this article: Vasilii V. Tumanov & William A. Smit (2005) ArS-Mediated Ad
Reaction of the Cobaltcarbonyl-Complexed Conjugated Enynes, Phosphorus,
E
Sulfur, and Silicon and the Related Elements, 180:5-6, 1279-1283, DOI:
10.1080/10426500590911584
To link to this article: http://dx.doi.org/10.1080/10426500590911584
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Phosphorus, Sulfur, and Silicon, 180:1279–1283, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590911584
ArS-Mediated AdE Reaction of the
Cobaltcarbonyl-Complexed Conjugated Enynes
Vasilii V. Tumanov
William A. Smit
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of
Sciences, Moscow, Russia
A novel approach to the synthesis of functionalized enynes is elaborated based on
the protocol of the three-component coupling of arenesulfenyl chlorides, dicobalthex-
acarbonyl complexes of the conjugated enynes, and nucleophiles of the π-donor type
and a possibility to employ the 1,6-enyne derivatives thus prepared as the substrates
for the intramolecular Pauson–Khand cyclization is demonstrated.
INTRODUCTION
Nicolas reaction1 is one of the most effective and convenient methods for
the preparation of various functionalized alkynes. A useful modification
of this reaction involves the controlled two-step sequence of AdE reac-
tions across the double bond of dicobalthexacarbonyl (DCHC) complexes
of the conjugated enynes via an intermediacy of the cobalt-complexed
propargylic cation.2−4 However, the latter option is of a limited prepar-
ative usefulness for the substrates bearing an additional substituent at
the propargylic center and in general its diastereoselectivity is rather
low.
Here we present a new scheme of the modified Nicolas reaction based
upon the use of arylsulfenium chlorides as the starting electrophiles in
the aforementioned sequence. The presence of ArS moiety secures the
formation of extra stabilized episulfonium-like intermediate. As a result
a significant extention of the scope and improvement of the diastereos-
electivity of the coupling is achieved.
Received July 9, 2004; accepted October 5, 2004.
We are indebted to Deutsche Forschungsgemeinschaft (Grant No. MA673/19) and to
RFBR (Grants Nos. 03-03-32884 and 03-03-04001) for financial support of this work.
Address correspondence to Vasilii V. Tumanov, N.D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Russian Federation, 119991 Moscow, Leninsky
prosp. 47, Russia. E-mail: Vasilii Tumanov@mail.ru
1279

1280
V. V. Tumanov and W. A. Smit
RESULTS
DCHC complexes of vinylacetylene 1 and isopropenylacetylene 2 were
chosen as the model substrates in the reaction sequence shown in
Scheme 1. p-Chlorobenzene and p-methylbenzene sulfenyl chlorides
were used as a starting electrophiles at step (i). Various Lewis acids
were shown to be efficient at step (ii). At the final step (iii), we used a
set of the standard silyl-capped π-donors as carbon nucleophiles (NuC).
SCHEME 1
A list of the represented examples shown in Scheme 2 demonstrates
a rather wide scope of the investigated reaction, which was found to be
applicable to virtually all combinations of the substrates involved (in
all these cases EtAlCl2 was employed as Lewis acid).
SCHEME 2
Classic Nicolas reaction and its analogues,1−4 except for some
particular cases,5 exhibits a rather low diastereoselectivity of the
overall coupling. We disclosed that β-arylthiosubstituted cobalt-
complexed propargylic cation prepared from 1 reacts with prochiral
1-trimethylsilyloxycyclopentene 4 and 1-trimethylsilyloxycyclohexene
5 to give the corresponding adducts with good to excellent diastereos-
electivity depending on the Lewis acid used (Scheme 3, Table I). As
might have been expected, a substantially lower diastereoselectivity
was observed for the coupling using enyne complex 2.
The usefulness of the aforementioned sequence for the synthesis of
the substrates directly amenable for the intramolecular Pauson–Khand
(IMPK) reaction6 cyclization is shown in Scheme 4. This option is based
on the use of 2-(trimethylsilylmethyl)buta-1,3-diene 6, as the final NuC

ArS-Mediated AdE Reaction
1281
SCHEME 3
SCHEME 4
bearing an additional vinyl group. It was shown that the presence of
the conjugated diene moiety in the silane 6 does not affect essentially
the efficiency of the coupling and the respective product 7 was prepared
in high yield.
Ample literature data on the synthetic application of the IMPK
cycloaddition6 do not contain any reference to its applicability for the
1,6-enynes that contain alkene unit as the part of the conjugated diene
TABLE I
Ratio of diastereomers
Substrate
NuC
Lewis acid
(syn:anti)
Yield (%)
1
1
1
1
1
1
1
2
2
2
2
2
2
2
4
4
4
4
4
5
5
4
4
4
4
4
5
5
AgSbF6
TiCl4
TMSOTf
Bu2BOTf
EtAlCl2
TiCl4
EtAlCl2
AgSbF6
TiCl4
TMSOTf
Bu2BOTf
EtAlCl2
TiCl4
20 : 1
17 : 1
12 : 1
10 : 1
3 : 1
52
90
56
61
92
87
86
50
91
54
59
91
85
82
12 : 1
7 : 1
3.8 : 1
3.5 : 1
3.5 : 1
3.0 : 1
2.5 : 1
2.5 : 1
2.2 : 1
EtAlCl2

1282
V. V. Tumanov and W. A. Smit
system. We have found that this type of compound can serve as the suit-
able substrates for this reaction and the adduct 7 readily underwent
the conversion into the respective bicyclo[3.3.0]octadienone 8 under the
standard cyclization conditions.7
An alternative approach for the synthesis of the 1,6-enynes takes
an advantage of an easy preparation of the DCHC complexes of 5-
oxoalkynes as a result of the above described three components cou-
pling with the use of silyl enol ethers as the C-nucleophiles. It was
anticipated that these adducts could be further transformed into the
required 1,6-enynes by the Grignard addition of vinylmagnesium bro-
mide. To check the viability of this approach we have chosen DCHC com-
plexes of oxoalkynes 10 and 11 prepared in virtually quantitative yields
with the use of 2-trimethylsilyloxypropene (9) as NuC (see Scheme 5).
SCHEME 5
Unfortunately, the attempts to carry out the Grignard reaction of
vinylmagnesium bromide directly with DCHC complexes 10 and 11
turned out to be rather unsuccessful. At the same time, we faced no
problems when performing the reaction of vinyl magnesium bromide
with the decomplexed adducts 10a and 11a and the respective 1,6-
enynes 12a,b and 13a,b were prepared in good yields as mixtures of
diastereomers.
The separation of diastereomers failed. Therefore, further transfor-
mations were performed with the mixtures of diastereomers 12a,b and
13a,b. The reaction of these adducts with Co2(CO)8 resulted in the for-
mation of the respective DCHC complexes. The latter were not isolated
but used directly as the substrates for the IMPK reaction under the
conventional conditions of the oxidative initiation under the action of
Me3NO.
It was found that the cyclization of the mixture 12a,b produced a
mixture of only two (out of four possible) diastereomers 14a,b in the
ratio 2.5:1, which roughly corresponded to that of the diastereomers in
the starting material (Scheme 6). Individual products 14a and 14b were
isolated by preparative TLC and their structures were unambiguosly
established by 1H and 13C NMR data.

ArS-Mediated AdE Reaction
1283
SCHEME 6
Because the cycloadducts 14a and 14b differ only in a configuration
of the ArSCH2-substituents at C-8, one may assume that the IMPK
cyclization of both diastereomers 12a and 12b proceeded with high
diastereoselectivity. A rather different diastereoselectivity pattern was
observed for the cyclization of the mixture 13a,b, which furnished a
mixture of three isomers 15a,b,c in a ratio 4:2:1. In this case, the IMPK
reaction of 13a proceeded with a nearly complete diastereoselectivity
to give the adduct 15a, while the conversion of 13b into the mixture
15b,c exhibited a rather modest diastereoselectivity.
In summary, we elaborated a preparatively convenient protocol for
the ready preparation of alkyne derivatives with the variable pattern
of substitution secured by the use arylsulfenyl moiety first as an ini-
tiator and then as a group stabilizing a carbocationic intermediate. An
opportunity to employ this approach for the concise synthesis of the
substrates for IMPK reaction was also demonstrated.
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[4] S. L. Schreiber, T. Sammakia, and W. E. Crowe, JACS, 108, 3128 (1986).
[5] T. J. J. Mu¨ller, Eur. J. Org. Chem., 2021 (2001).
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