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(entry 10). The reaction of 1a with 3-hexyne (2b) proceeded
pentenes (3ae, 3af) with high regioselectivity. The reaction of
1-trimethylsilyl-2-phenylacetylene (2g; R4 = Ph, R5 = TMS)
with 1a gave a mixture of regioisomers, however, a reduced
regioselectivity was observed compared with the reactions of
either 2e or 2 f. These results suggest that the regioselectivity
might be controlled by the steric bulk of the substituents. The
molecular structure of the major regioisomer of 3ag was
determined by X-ray crystallography.[11] Although terminal
alkynes are prone to undergoing dimerization or trimeriza-
tion in the presence of nickel(0) species, the cycloaddition of
triisopropylsilyl acetylene (2h; R4 = TIPS, R5 = H) with 1a
occurred to give the expected cyclopentene product as a
single regioisomer (3ah).[12] Use of activated alkynes of
functional groups containing ester, ether, or carbonyl groups
led to the recovery of starting material presumably because of
the coordination to Me2AlCl, thus decreasing its reactivity.
The reaction of 1a with 2-methyl-1-hexen-3-yne (2i; R4 = Et,
R5 = 2-propenyl) gave the expected cyclopentene derivatives
3ai in moderate yield.
to give 3ab (entry 11). A decrease in the amount of Me2AlCl
from 100 mol% to 20 mol% lead to the low conversion and
yield of 3ab (entry 12). Thus, in the presence of 20 mol% of
Me2AlCl, the reaction was carried out at 508C and 3ab was
obtained quantitatively (entry 13).
We next examined the scope of this cycloaddition
reaction. The reaction of 1a with symmetric internal alkynes
gave the corresponding cyclopentene derivatives in high
yields (Scheme 1; 3aa–3ad). Unsymmetric alkynes were also
applicable for this reaction, and both 1-trimethylsilylpropyne
(2e; R4 = TMS, R5 = Me) and 1-phenylpropyne (2 f; R4 = Ph,
R5 = Me) reacted with 1a to give the corresponding cyclo-
A broad range of cyclopropyl aryl ketones were success-
fully employed for the cycloaddition reaction (3ba–3ga).
Electronic variation on the phenyl ring of the cyclopropyl
ketones had little effect on the reaction efficiency, except for
the methoxy group. The methoxy group is supposed to
decrease the reactivity of Me2AlCl by coordinating to
Me2AlCl. Moreover, cyclopropyl 3-methoxyphenyl ketone
(1d; R1 = 3-MeOC6H4, R2 = H, R3 = H) gave the product
(3da) in 21% yield even in the presence of a stoichiometric
amount of Me2AlCl. Cyclopropyl 4-fluorophenyl ketone (1e;
R1 = 4-FC6H4, R2 = H, R3 = H) was also converted into the
desired product. Cyclopropyl 2-thienyl ketone (1 f; R1 = 2-
thienyl, R2 = H, R3 = H) also underwent the reaction to give
3 fa in moderate yield. Although the oxidative addition of
À
À
Csp2 F and Csp2 S bonds in fluoroarenes and thiophenes,
respectively, to nickel(0) is known,[13,14] neither of them
underwent the oxidative addition under the reaction con-
ditions. Cyclopropyl methyl ketone (1h; R1 = Me, R2 = H,
R3 = H) reacted with 2c efficiently to give the expected
product (3hc) in 69% yield with a stoichiometric amount of
Me2AlCl. This observation could be rationalized by the
difference in coordination ability between 1a and 1h.[15] Ethyl
cyclopropanecarboxylate did not proceed the reaction at all.
Although the reaction of disubstituted cyclopropanes
required a stoichiometric amount of Me2AlCl or Me2AlOAc,
both 1i (R1 = Ph, R2 = Me, R3 = H) and 1j (R1 = Ph, R2 = Ph,
R3 = H) also underwent the cycloaddition reaction with 2a.
The reaction of trans-1-benzoyl-2-methylcyclopropane (1k;
R1 = Ph, R2 = H, R3 = Me) or trans-1-benzoyl-2-phenylcyclo-
propane (1l; R1 = Ph, R2 = H, R3 = Me) with 2a each gave a
mixture of three isomers: 1,4-trans (A), 1,4-cis (B), and 1,5-
trans (C; 3ka, 3la). In these reactions, Me2AlOAc was more
effective than Me2AlCl. By monitoring the progress of the
reaction of 1k with 2a using GC, it was observed that the
isomer ratio of 3ka remained unchanged during the reaction.
In addition, when two different mixtures of 3ka (A/B/C =
62:26:12 and 46:45:9) were heated at 508C for 3 hours in the
presence of 10 mol% of [Ni(cod)2], 100 mol% of Me2AlOAc,
and 2a, the isomer ratio of 3ka also remained unchanged.[11]
Scheme 1. [3+2] Cycloaddition reaction of 1 with 2. Reaction condi-
tions: cyclopropyl ketone (1.0 mmol), alkyne (3.0 mmol), [Ni(cod)2]
(0.1 mmol), and Me2AlCl (0.2 mmol) in THF (1.0 mL) at 508C for 3 h.
Yield of isolated product given; the regioisomer ratio is given in
parentheses. The major product is depicted. [a] Run at 808C. [b] Used
40 mol% of Me2AlCl. [c] Used 100 mol% of Me2AlCl. [d] Used 100
mol% of Me2AlOAc. [e] The isomer ratio of 1,4-trans (A)/1,4-cis (B)/
1,5-trans (C) is given in parentheses. TIPS=triisopropylsilyl, TMS=tri-
methylsilyl.
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 12067 –12070