Angewandte
Chemie
However, without irradiation (Table 2, entries 3 and 4), 1a led
to a much better yield of 3a than [CpCo(CO)2] (85%), the
best yield of the series. Furthermore, some analytically pure
catalyst could be recovered by flash chromatography,[14] which
is not possible with [CpCo(CO)2] owing to fast decomposi-
tion. Catalysts 1a–d were also effective under microwave
(MW) conditions (Table 2, entries 5–8, DMF, 2008C,
10 min).[15,16] Complex 1a (Table 2, entry 5) gave rise to a
significantly better yield[17] than 1b–d (Table 2, entries 6–8)
and [CpCo(CO)2] (Table 2, entry 9)., whereas [CpCo(C2H4)2]
proved as efficient as 1a (Table 2, entry 10). Decreasing the
loading of 1a from 5 mol% to 1 mol% led to a decrease in the
yield (Table 2, entry 5, 53% vs. 75%). In addition to the fact
that complexes 1a–d are much easier to handle than
[CpCo(CO)2] and [CpCo(C2H4)2], it is especially notable
that the transformations involving these new catalysts were
achieved in solvents taken “straight from the bottle”. Neither
toluene nor DMF were distilled or degassed prior to use.
Even in a 1:3 mixture of H2O and DMF, formation of 3a using
1a still occurred, albeit in lower yield (50%).
Scheme 2. Intermolecular [2+2+2] cycloaddition reactions: [a] MW,
DMF, 2008C, 10 min; [b] toluene, reflux, 3 h.
ing toluene, without visible-light irradiation. The cyclotrime-
rization of dimethyl acetylenedicarboxylate (DMAD) gave
rise to hexamethyl benzenehexacarboxylate in 44% (MW/
DMF) and 79% (refluxing toluene) yields of isolated product.
Phenylacetylene could be transformed into the expected
regioisomeric mixture of 1,3,5- and 1,2,4-triphenylbenzene in
98% (MW/DMF) and 92% (refluxing toluene) total yields
(ratio 3:7, in both cases).[18]
To further evaluate the potential of 1a as a catalyst for
[2+2+2] cycloaddition, we carried out bimolecular reactions
between diynes and nitriles, to construct bicyclic fused
pyridines (Table 3). The best yields were obtained in refluxing
To explore the scope of this intramolecular transforma-
tion, triynes displaying various tethers and alkyne termini (H,
Me, Ph, CO2Me, SiMe3, Br) were heated by microwave in
unpurified DMF in the presence of 1a, giving 3b–l in
moderate to excellent yields (Scheme 1). Only the bis-
Table 3: Optimization of the reaction conditions for construction of
fused bicyclic pyridines.
X
R
Product
Yield [%]
(CH2)2
(CH2)2
NTs
Et
Ph
Et
4a
4b
4c
4d
65[a]
70[b]
63
NTs
Ph
66
Without hn: [a] toluene, reflux, 7 h (44%); xylenes, reflux, 3 h (53%);
MW, DMF, 2008C, 10 min (45%). [b] Toluene, reflux, 7 h (51%).
Scheme 1. Tricyclic compounds prepared by intramolecular [2+2+2]
cycloadditions of triynes (1a (5 mol%), MW, DMF, 2008C, 10 min).
E=CO2Me; TMS=trimethylsilyl. Values in parentheses correspond to
yields of reaction for 4 h in refluxing toluene, catalyzed by 1a
(5 mol%). [a] 1a (10 mol%), toluene, reflux, 10 h.
toluene.[19] Visible-light irradiation improved the yields sig-
nificantly.[20] Microwave conditions were still effective,
although the yields were markedly lower.
Stoichiometric reactions were also briefly examined. The
fused 1,3-cyclohexadiene 6 could be formed diastereoselec-
tively in 50% yield from enediyne 5 in refluxing toluene
[Scheme 3, Eq. (1)]. The synthesis of the {CpCo}-complexed
cyclobutadiene 7 was also accomplished under microwave
irradiation, following a procedure described for diphenylace-
tylene and [CpCo(CO)2] [Scheme 3, Eq. (2)].[21] Interestingly,
the yield (83%) was much better than that reported with
[CpCo(CO)2] (52%), owing to a far lower contamination by
the [2+2+1] adduct 8 (9% vs. 40% with [CpCo(CO)2]).
In summary, complexes 1a–d are versatile catalysts for
various [2+2+2] cycloadditions, and related reactions, for
forming benzenes, pyridines, and 1,3-cyclohexadienes.
Although, in some cases, they exhibit similar activity, they
present major advantages compared to common catalysts
such as [CpCo(CO)2] and [CpCo(C2H4)2]. They are air-stable
and retain their activity after several months of storage in
brominated triyne proved poorly compatible with these
experimental conditions, 3l being isolated in only 20%
yield, accompanied by the corresponding monodebrominated
product. This problem could be circumvented in refluxing
toluene, 3l being obtained in 91% yield. All other triynes
could be transformed into the corresponding tricyclic deriv-
atives in at least 48% (3j), and up to 92% yield (3b and 3h).
With the exception of 3c, refluxing in toluene for four hours
instead of microwave heating in DMF for ten minutes gave
rise to lower yields. Application of visible-light irradiation in
addition to heating did not improve the amount of isolated
product. Interestingly, 3j and 3k could be obtained only
under microwave conditions.
Intermolecular cyclizations could also be achieved
(Scheme 2), either under microwave conditions or in reflux-
Angew. Chem. Int. Ed. 2009, 48, 1810 –1813
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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