Angewandte
Chemie
DOI: 10.1002/anie.200801778
Synthetic Methods
Cobalt-Catalyzed [2+2] Cycloaddition**
Jonas Treutwein and Gerhard Hilt*
Dedicated to Professor Reinhard W. Hoffmann on the occasion of his 75th birthday
Carbocyclic four-membered rings can be synthesized by
cycloaddition reactions through the photochemical conver-
sion of alkenes into cyclobutanes or thermal [2+2] cyclo-
addition reactions of acceptor-substituted alkynes with
alkenes to give cyclobutenes.[1] Transition-metal-catalyzed
Scheme 2. Cobalt-catalyzed [2+2] cycloaddition reaction between nor-
bornene and 3-hexyne.
reactions of non-activated starting materials,which only
undergo thermal cycloaddition under harsh conditions,are
usually carried out with ruthenium[2] or rhodium[3] complexes,
or alternatively with cobalt or nickel complexes.[4]
substrates. Furthermore,the catalyst system presented
herein does not lead to the dimerization of norbornene,as
reported by Cheng and co-workers,and the addition of a
further equivalent of the phosphine ligand is not necessary.
A further advantage of the Co(dppp) catalyst system is
that,in contrast to most transition-metal-catalyzed [2 +2]-
cycloaddition reactions,no large excess of either starting
material is required: The substrates can usually be used in a
1:1 ratio,which greatly simplifies the purification of the
products.[7] Laborious column-chromatographic purification
can be avoided: Simple filtration through a silica-gel plug is
sufficient to remove the residual (in)organic catalyst compo-
nents and provide analytically pure reaction products of type
2. The cobalt catalyst system described herein is also
considerably less expensive than alternative rhodium- or
ruthenium-based compounds,and the substrate scope is
significantly wider than previously reported for cobalt-based
systems.
The results of the cycloaddition of norbornene derivatives
in the presence of the Co(dppp) catalyst (2 or 10 mol%) are
summarized in Table 1. The transformations involving sym-
metrical internal alkynes and norbornene (Table 1,entries 1–
3) gave the desired [2+2] cycloaddition products in quanti-
tative yield. Unsymmetrical 1-phenylalkynes also yielded the
desired products (Table 1,entries 4 and 5) quantitatively,
whereas the reaction of ethyl 2-butynoate with norbornene
gave the desired cycloaddition product in an acceptable yield
of 51% (Table 1,entry 6),along with a small quantity of a 1:2
(alkene/alkyne) adduct (18%,as determined by GC–MS).
The [2+2] cycloaddition products were also formed in good to
very good yields from 1-substituted bicyclo[2.2.0]hept-3-enes
(Table 1,entries 7–14). These substrates were sometimes used
as an endo/exo mixture,whereby the cycloaddition products
were formed as an endo/exo mixture in the same ratio.[8]
When terminal alkynes were used as substrates,only very
little of the [2+2] cycloaddition product was detected by GC–
MS. With these substrates,[2 +2+2] cyclotrimerization of the
alkyne appears to occur preferentially.
A while ago we reported the first cobalt-catalyzed Alder-
ene reaction between internal alkynes and terminal alkenes to
give 1,4-dienes 1 (Scheme 1).[5] It is assumed that the low-
valent cobalt complex coordinates to both unsaturated
starting materials,and that the reaction proceeds via a
À
cobaltacycle to form the new C C bond. Subsequent b-
hydride elimination and reductive elimination lead to the
final product 1.[6]
Scheme 1. Cobalt-catalyzed Alder-ene reaction. dppp=1,3-bis(diphe-
nylphosphanyl)propane.
We report herein that the reaction of internal,strained
alkenes,such as norbornene,with internal alkynes in the
presence of a cobalt–diphosphine complex leads to cyclo-
butene derivatives in quantitative yield with high chemo-
selectivity (Scheme 2). The only other reported synthetically
useful transition-metal-catalyzed [2+2] cycloaddition reac-
tions with norbornene involve rhodium or ruthenium com-
plexes.[2,3] The use of a very simple cobalt complex,[CoI -
2
(PPh3)2],in the presence of a further equivalent of the free
ligand PPh3 under reductive conditions (Zn powder) in [2+2]
cycloaddition reactions was pioneered by Cheng and co-
workers.[4] These early cobalt-catalyzed transformations are
somewhat limited in that they require harsh conditions
(toluene,90 8C) and a 10-fold excess of the alkyne,and
seem to be restricted to 7-oxa-bicyclo[2.2.0]heptenes as
[*] J. Treutwein, Prof. Dr. G. Hilt
Fachbereich Chemie, Philipps-Universität Marburg
Hans-Meerwein-Straße, 35043 Marburg (Germany)
Fax: (+49)6421-282-5677
E-mail: Hilt@chemie.uni-marburg.de
It seems plausible that the transformation proceeds
through the coordination of the substrates to the cobalt
center,followed by the formation of a cobaltacyclopentene
intermediate by oxidative addition. A b-hydride elimination
[**] This research was supported by the German Research Foundation
(Deutsche Forschungsgemeinschaft).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2008, 47, 6811 –6813
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6811
Treutwein, Jonas
