DOI: 10.1002/chem.201202749
Protonation-Triggered Generation of Acylcobalt Species from Alkyne–
[Co2(CO)6] Complexes and Their Reaction with Alkenes
Isao Ooi, Takafumi Sakurai, Jun Takaya, and Nobuharu Iwasawa*[a]
Acylcobalt carbonyl complexes of the type [RC(O)-
Table 1. Effect of additives.
ACHTUNGTRENNUNG
À
À
À
balt complexes undergo C H, C O, or C N bond forma-
tion. However, in spite of its possibility as an acyl anion
À
equivalent, C C bond-forming reactions of acylcobalt com-
plexes are rather limited to those with reactive substrates,
such as conjugated dienes,[3,4] and efficient reaction with al-
kenes has scarcely been realized in either catalytic or stoi-
chiometric reactions.[5–9] Moreover, generation of acylcobalt
complexes from alkyne–[Co2(CO)6] complexes, which are
recognized as stable, easy to handle metal complexes in or-
ganic chemistry,[10] has not been developed except for sever-
al specific reactions.[11] Herein, we report a general and effi-
cient method for the generation of acylcobalt species from
alkyne–[Co2(CO)6] complexes and their addition reaction to
various alkenes. This reaction affords another synthetic utili-
ty of alkyne–[Co2(CO)6] complexes for carbon–carbon
bond-formation reactions.
Additives
Yield [%]
acid
base
2
3
1[a]
2
AcOH
–
–
18[c]
1[c]
85
20
27
78
54
65
42
50[c]
50[c]
–
pyridine
pyridine
2,6-(tBu)2-pyridine
DABCO[d]
pyridine
pyridine
PPh3 (1.0 equiv)
PCy3 (1.0 equiv)
3
AcOH
AcOH
AcOH
CF3COOH
TfOH
AcOH
AcOH
4[b]
5
33[c]
2[c]
6
7
8
9
7[c]
15[c]
trace
17
[a] Heating for 3.5 h. [b] Heating for 4.0 h. [c] Based on 1H NMR analy-
sis. [d] 1,4-Diazabicyclo[2.2.2]octane.
AHCTUNGTRENNUNG
When a mixture of 5-fluorocycloheptyne-[Co2(CO)6] com-
plex 1 and ten equivalents of ethyl acrylate was heated at
808C in toluene in the presence of ten equivalents of AcOH
for 1.5 h, formation of ketone 2, which was thought to be
produced by addition of acylcobalt species generated from
the alkyne–[Co2(CO)6] complex to ethyl acrylate, was ob-
served in 18% yield along with a cyclic ketone 3, the
Pauson–Khand adduct, in 50% yield (Table 1, entry 1).[12]
When the same reaction was carried out in the presence of
ten equivalents of pyridine instead of AcOH, only the
Pauson–Khand reaction was promoted to give moderate
yield of 3 (Table 1, entry 2).[13] Interestingly, when the same
reaction was carried out in the presence of ten equivalents
each of AcOH and pyridine, the reaction pathway changed
dramatically, and ketone 2 was obtained in 85% yield
(Table 1, entry 3).[14] Use of 2,6-di-tBu-pyridine as a hin-
considerably (Table 1, entries 4 and 5). By using more acidic
CF3COOH or TfOH instead of AcOH somewhat lowered
the yield of ketone 2 (Table 1, entries 6 and 7). Ketone 2
was also obtained in reasonable yield by adding one equiva-
lent of PPh3 instead of pyridine (Table 1, entry 8). These re-
sults suggest that pyridine is actually acting not as a base,
À
but as a ligand to cobalt to facilitate protonation of C Co
bond or CO insertion. The bulky di-tBu-pyridine is unfavor-
able for coordination and DABCO, a stronger base, would
be protonated by AcOH. In a similar manner, stronger
acids, such as CF3COOH and TfOH, protonate pyridine to
retard its coordinating ability. Thus, the combination of
AcOH and pyridine is the most efficient promoter for gen-
erating acylcobalt species (see the proposed mechanism in
Scheme 1).
dered pyridine or 1,4-diazabicyclo
a more basic alkyl amine lowered the yield of the ketone 2
[2.2.2]octane (DABCO) as
Generality of alkenes employable in this reaction is sum-
marized in Table 2. A noteworthy feature of this reaction is
the wide applicability of the kind of alkenes. Thus, not only
electron-deficient alkenes, such as ethyl methacrylate and
acrylamide (Table 2, entries 1 and 2), but also an electron-
rich vinyl ether and enamide were employable (Table 2, en-
tries 3 and 4).[15] The reaction proceeded under nearly neu-
tral conditions to allow the use of such acid-sensitive al-
kenes. Moreover, simple alkenes, such as styrene (Table 2,
entry 5) and an allyl ether or ester (Table 2, entries 6 and 7)
[a] I. Ooi, T. Sakurai, Dr. J. Takaya, Prof. Dr. N. Iwasawa
Department of Chemistry
Tokyo Institute of Technology
2-12-1, O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
Fax : (+81) 3-5734-2931
Supporting information for this article is available on the WWW
14618
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 14618 – 14621