SCHEME 4
under the above optimized reaction conditions. Further raising
the reaction temperature, loading the amount of catalyst to 10%
Ni3(PO4)2 ·8H2O and using stronger bases exhibit no effect
(Scheme 3).
Initially, we started out the reaction of 3-(2-(2,2-di(ethoxy-
carbonyl)ethyl)phenyl)prop-2-ynyl ethyl carbonate (5a) under
the optimized reaction conditions above. Satisfactorily, the
allenyl indene product 6a was isolated in 83% yield as a single
product (Table 2, entry 1).
On the basis of our knowledge of the transition metal-
catalyzed carboannulation of alkynes,7,9 combined with our
observed experiment features, a plausible mechanism is pro-
posed in Scheme 4; it consists of the following key steps: (a)
generation of a carbanion A by the base; (b) coordination of
the Ni(II) specie to the alkyne triple bond affords the alky-
nylnickel complex B; and (c) intramolecular nucleophilic attack
of the carbanion on the activated triple bond to afford a Ni(II)
vinyl specie C and then protonation of the C-Ni bond to give
the cyclization product 2.
Synthesis of Allenyl Indenes by the Nickel(II)-Cataly-
zed Cyclization Reaction of Propargylic Compounds. Allenes
are regarded as useful building blocks for organic synthesis10
and the development of versatile methods for the preparation
of allenes has been of great importance.11 Allenes can be
generally prepared from propargylic drivatives by SN2′-type
displacement with organocopper species.12 Methodologies to
the synthesis of allenes via nickel-catalyzed, such as carbon-
ylation of propargylic and allenyl halides,13 reactions of
propargylic compounds with Grignard reagents or other orga-
nometallic reagents,14 are generally driven by Ni(0) species. Up
to now, there is no report about carboannulation of propargylic
compounds driven by nickel(II) species for the preparation of
allene derivatives.
The general applicability of the reaction of primary, substi-
tuted secondary and tertiary propargylic esters was investigated
next. The reactions of propargylic acetate 5b and propargylic
phosphate 5c gave the desired product 6a in good yields under
the optimized reaction conditions above (Table 3, entries 2–3).
Secondary carbonates possessing various substituents (alkyl-,
heterocyclic-, and aryl- substituted) at the propargylic position
also worked well, and afforded the corresponding products in
moderate yields (entries 4–13). Functional groups, such as
methyl, chloro, bromo, and methoxyl, etc., were well tolerated
in the reactions. Meanwhile, the reactions of tertiary propargylic
acetates have also been studied. Moderate yields of the expected
products have been obtained from 5n, 5o, and 5p (entries
14–16).
Propargylic alcohols were also investigated under the
optimized reaction conditions. Propargylic alcohol 5q fur-
nished a 56% yield of the desired allenyl indene 6a in 5 h
(Table 3, entry 17). Propargylic alcohols with different
electron-withdrawing groups were subjected to the reaction,
such as ꢀ-ketoester 5r led to a 49% yield of product 6o and
shorter reaction time was required (entry 18). The reactivity
of substituted secondary and tertiary propargylic alcohols
have also been examined. Expectedly, longer reaction times
were needed and lower yields were obtained (entries 19–20).
Compared with the reaction of propargylic esters, the
reactivity of propargylic alcohols were comparatively a little
lower, because the hydroxy is not a good leaving group and
products 6 are very sensitive and could not be stored for
extended periods. Furthermore, additional attempts to extend
substrates to propargylic ethers were unsuccessful.
SCHEME 5
SCHEME 6
Recently, we have developed an efficient route to allenyl
indenes by the Pd/C-catalyzed carboannulation of propargylic
compounds8e,f (Scheme 5). These successful results encouraged
us to extend this nickel(II)-mediated methodology to the
synthesis of allenyl indenes. Herein, we report the full details
of this synthesis of allenyl indenes.
To further probe our Ni(II)-catalyzed carboannulation reac-
tions, the propargylic compounds with different leaving groups
has also been examined (Scheme 6). Gratifyingly, the desired
product 6a was isolated in 66 and 73% yields, and it was
observed that the yields of the reactions employing propargyl
bromide 7a and propargyl azide 7b are generally higher than
those utilizing propargylic alcohols.
The formation of these allenyl indenes can be explained by
the following processes (Scheme 7). It consists of the following
key steps: (a) generation of a carbanion A by the base; (b)
coordination of the Ni(II) specie to the alkyne triple bond affords
the alkynylnickel complex B, which activates the triple bond
(10) (a) Landor, S. R., Ed. The Chemistry of Allenes; Academic Press:
London, 1982. (b) Hoffmann-Röder, A.; Krause, N. Angew. Chem., Int. Ed. 2002,
41, 2933. (c) Krause, N., Hashmi, A. S. K., Eds. Modern Allene Chemistry;
Wiley-VCH: Weinheim, 2004.
(11) (a) Jarvi, E. T.; McCarthy, J. R. Nucleoside Nucleotide 1994, 13, 585.
(b) Dauvergne, J.; Burger, A.; Biellmann, J.-F. Nucleoside Nucleotide 2001, 20,
1775.
(12) (a) Schuster, H. F., Coppola, G. M., Eds. Allenes in Organic Synthesis;
Wiley: New York, 1984. (b) Alexakis, A.; Marek, I.; Mangeney, P.; Normant,
J. F. J. Am. Chem. Soc. 1990, 112, 8042.
(13) (a) Arzoumanian, H.; Cochini, F.; Nuel, D.; Petrignani, J. F.; Rosas, N.
Organometallics 1992, 11, 493. (b) Arzoumanian, H.; Cochini, F.; Nuel, D.;
Petrignani, J. F.; Rosas, N. Organometallics 1993, 12, 1871.
(14) (a) Pasto, D. J.; Chou, S.-K.; Waterhouse, A.; Shults, R. H.; Hennion,
G. F. J. Org. Chem. 1978, 43, 1385. (b) Pasto, D. J.; Chou, S.-K.; Fritzen, E.;
Shults, R. H.; Waterhouse, A.; Hennion, G. F. J. Org. Chem. 1978, 43, 1389.
J. Org. Chem. Vol. 73, No. 10, 2008 3839