As for the catalytic systems A and B, they comparison showed
that the latter was more efficient in terms of reaction rate and
selectivity (Table 1). Thus the cyclization of allylmalonates
1a–1c (entries 1–3) proceeded with good selectivity for methy-
lidene(methyl)cyclopentanes 2a–2c (75–92%) under condition
B. By using condition A a mixture of positional isomers 3 and
4 in rather low yields was usually formed. The cyclization of 1a
was also run with NiBr2(dppm), NiBr2(dppe), NiBr2(dppp),
and NiBr2(dppb) in a combination with Et2AlCl to assess
catalytic activity of other complexes. However, the yields of 2a
were rather poor (2%, 18%, 12%, and 1%, respectively).
Reactions with the substrates bearing a keto group 1d and
1e (entries 4 and 5) furnished medium to low yields of 2d
(60%) and 2e (28%) (condition B). Complex reaction mixtures
were obtained under condition A.
The cyclization of diallylphenylacetate 1f and diallylcou-
maranone 1g proceeded almost quantitatively under condition
B to 2f (94%) and 2g (93%). By using condition A the yields of
2f and 2g were 33% and 47%, respectively. The cyclization of
diallylcyanoacetate 1h to 2h proceeded in rather low yield
(30%) only under condition B and it was accompanied by
double bond migration giving allylpropenylcyanoacetate 5h in
16% yield.
Scheme 3 Proposed reaction mechanism for Ni hydride catalyzed
cyclization of dienes.
However, in the case of the reaction of diallylmalonates and
similar compounds the crucial moment for the further course
of the reaction depends on the amount of organoaluminium
present. In excess it also coordinates to the carbonyl group
and promotes deallylation.4,9 When organoaluminium is used
in an amount sufficient to generate just the cationic Ni
hydride, preferential intramolecular addition of the intermedi-
ate 8 to the second double bond takes place to give inter-
mediate 9.
The cyclization of ethers 1i and 1j proceeded in high yields
(condition B) to give mixtures of 2i (29%) and 3i (65%), and
of 2j (51%) and 4j (44%). Although the overall yields of
cyclized products were lower under condition A, higher selec-
tivity for methylidenemethylcyclopentanes 2i (37%) and 2j
(71%) was observed. Similarly the cyclization of diallylfluor-
ene 1k afforded 85% of 2k, whereas under condition B only
the cyclopentene derivatives 3k (72%) and 4k (26%) were
formed. High selectivity for the formation of the pyrrolidine
derivatives 2l (71%) was observed under condition B.
In conclusion, we have shown that it is possible to re-route
the course of the reaction from C–C bond cleavage (deal-
lylation) to C–C bond formation (cyclization) in Ni hydride
catalyzed reactions by a change in the amount of organoalu-
minium. It proceeded in high yields even in cases where
deallylation could compete with cyclization (e.g. diallylmalo-
nate, etc.) in high selectivity. In addition, generation of Ni
hydride species in situ provides an effective means for fast and
selective cyclization of variously substituted 1,6-heptadienes to
cyclopentanes. In addition by the appropriate choice of the
organoaluminium (Et3Al or Et2AlCl) certain levels for the
preferential formation of methylidenemethylcyclopentanes or
cyclopentenes could be achieved.
Results obtained with the catalytic system C with com-
pounds 1a, 1d, 1f, 1g, and 1h are presented for comparison.
They clearly demonstrate that when excess of triethylalumi-
nium (200 mol%) was used clean deallylation proceeded to
furnish compounds 6a, 6g and 6h. On the other hand, in the
case of 1d and 1f complex reaction mixtures were formed.
The proposed mechanistic rationale for the Ni-catalyzed
cyclization is presented in Scheme 3. It is initiated by alkyla-
tion of the Ni(II) complex and results probably in a dialkyl-
nickel compound, reaction of which with the present
organoaluminium followed by b-hydrogen elimination gives
the cationic Ni hydride 7. (The cationic Ni species are formed
upon the reaction with alkylaluminiums.10,11) Then 7 hydro-
metalates the double bond to give the sec-alkylnickel inter-
mediate 8. This intermediate is the result of anti-Markovnikov
addition to the terminal double bonds and is typical for late
transition metal hydrides.12 Then intramolecular addition to
the second double bond furnishes the alkylnickel compound 9.
b-Hydrogen elimination releases 7 back into the catalytic cycle
and yields the cyclopentane derivative 2. Repetitive addition
and elimination of the Ni hydride 7 accounts for the observed
migration of the double bond giving rise to 3 and 4. Migration
of the double bond from 1f to the propenyl derivative 5f can be
explained by the same reaction mechanism.
Experimental
Typical procedure for the cyclization reaction: to a solution of
a diene (1a–1l) (0.5 mmol) in dry toluene was added NiBr2
(PBu3)2 (15.6 mg, 0.025 mmol) (3 mL) and 1.8 M solution of
Et2AlCl in toluene (55 mL, 0.1 mmol) (cond. B) or
NiBr2(PPh3)2 (18.6 mg, 0.025 mmol) and 1.9 M solution of
Et3Al in toluene (53 mL, 0.1 mmol) (cond. A) under argon. The
reaction mixture was stirred at 20 1C for 1 or 3 h, respectively.
After that it was quenched with a portion of water (1 mL)
followed addition of a 3 M solution of HCl (3 mL). The
organic layer was separated and dried (MgSO4). The products
were isolated by HPLC (silica gel, hexane–EtOAc).
Comparison of the two processes, deallylation4 and cycliza-
tion, indicates that they share the common intermediate 8.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 671–674 | 673
Necas, David
