COMMUNICATION
DOI: 10.1002/chem.201100669
(1-Alkynyl)dicarbonylcyclopentadienyliron Complexes as Electron-Rich
Alkynes in Organic Synthesis: BF3-Mediated [2+2] Cycloaddition/Ring-
Opening Providing (2-Alkenyl-1-imino)iron Complexes
Ryotaro Nakaya,[a, b] Shigeo Yasuda,[b] Hideki Yorimitsu,*[a] and Koichiro Oshima*[b, c]
Formal stepwise [2+2] cycloadditions between alkynes
and alkenes are useful for the synthesis of functionalized cy-
clobutenes.[1] Among them, combinations of electron-rich al-
kynes and electron-deficient double bonds are rather rare
because of the limited repertoire of electron-rich alkynes.
One can employ heteroatom-substituted alkynes such as
ynamides[2] and 1-alkynyl ethers,[3] sulfides,[4] and selenides[5]
or their vinylogous and phenylogous analogues. To increase
the variety of the cycloaddition, new families of electron-
rich alkynes need to be uncovered.
Iron is an exceptionally ubiquitous and safe transition-
metal and has been attracting much attention. Because of
the inherent cost efficiency of iron, development of new re-
actions by using organoiron reagents is an interesting chal-
lenge.[6] We have been interested in the unusual reactivity of
kenes such as tetracyanoethylene. Organometallic chemists
have been interested only in the basic reactivity of the alky-
nylmetal complexes; however, no applications to organic
synthesis have been reported so far.
Attempted reactions of dicarbonylcyclopentadienyl(phe-
nylethynyl)iron (1a) with a variety of electron-deficient
carbon–carbon double bonds resulted in failure. No thermal
reactions took place in the absence of any Lewis acids and
complex mixtures were obtained in the presence of the
Lewis acids. However, the reaction of 1a with N-phenylben-
zaldehyde imine (2a) occurred with the aid of an equimolar
amount of BF3·OEt2 to yield the unexpected adduct 3a in
43% yield (Scheme 1). The reaction proceeds through a
BF3-mediated formal [2+2] cycloaddition reaction followed
by ring-opening of the resulting azacyclobutenes.[14,15]
aryldicarbonylcyclopentadienyliron
complexes
[CpFe(-
CO)2Ar] (CpFe(CO)2 is abbreviated as Fp hereafter) and
developed several useful reactions with FpAr in organic syn-
thesis.[7] Along this line, we have developed an efficient
ꢀ À
method for the synthesis of the alkynyl analogues FpACTHNUTRGNEUNG(C C
R) by Sonogashira-type cross-coupling reactions of FpI with
terminal alkynes.[8]
ꢀ À
With FpACHTUNGTRENNUNG(C C R) in hand, we report herein the use of the
alkynyliron complexes as electron-rich alkynes in stepwise
[2+2] cycloaddition reactions for organic synthesis. Such al-
kynyliron complexes[9] and their Ru,[10] W,[9b,c,11] Mn,[9b,c,12]
and Ni[9b,13] analogues are known to undergo thermal step-
wise [2+2] cycloaddition reactions with highly activated al-
[a] R. Nakaya, Prof. Dr. H. Yorimitsu
Department of Chemistry, Graduate School of Science,
Kyoto University, Kitashirakawa
Scheme 1. BF3-Mediated addition of N-phenylbenzaldehyde imine (2a)
to phenylethynyliron 1a followed by ring-opening.
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
The use of a larger amount of BF3·OEt2 (2.2 equiv) in-
creased the yield of 3a to 51% (Scheme 1). The reaction
did not proceed in a catalytic fashion; the reaction in the
presence of 10 mol% of BF3·OEt2 provided 3a in 7% yield.
BF3·OEt2 was the best Lewis acid among those tested; AlCl3
could promote the addition reaction much less efficiently to
give 3a in 21% yield and other Lewis acids such as TiCl4,
ZnCl2, MgBr2, FeCl3, and SnCl4 failed to promote the reac-
tion efficiently.
[b] R. Nakaya, Dr. S. Yasuda, Prof. Dr. K. Oshima
Department of Material Chemistry, Graduate School of Engineering
Kyoto University, Kyoto-daigaku Katsura
Nishikyo-ku, Kyoto 615-8510 (Japan)
[c] Prof. Dr. K. Oshima
Environment, Safety, and Health Organization
Kyoto University, Yoshida
Sakyo-ku, Kyoto 606-8501 (Japan)
Fax : (+81)75-753-7710
The substituent on the nitrogen proved to have a signifi-
cant influence on the efficiency of the reaction (Table 1).
Supporting information for this article is available on the WWW
Chem. Eur. J. 2011, 17, 8559 – 8561
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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