Nickel(0)-catalyzed unprecendented zipper annulation of certain conjugated enynes [5]

Full text of DOI:10.1021/ja993861k

1810
J. Am. Chem. Soc. 2000, 122, 1810-1811
Table 1. Nickel(0)-Catalyzed Zipper Annulation of Conjugated
Nickel(0)-Catalyzed Unprecendented Zipper
Annulation of Certain Conjugated Enynes
Enynesa
Shinichi Saito,*,† Tadashi Tanaka,‡ Toru Koizumi,‡,¶
Norie Tsuboya,| Hitoshi Itagaki,| Taishi Kawasaki,|
Shota Endo,| and Yoshinori Yamamoto*,|
Institute for Chemical Reaction Science
Tohoku UniVersity, Sendai 980-8578, Japan
Faculty of Pharmaceutical Sciences
Toyama Medical and Pharmaceutical UniVersity
2630 Sugitani, Toyama 930-0194, Japan
enyne (1)
R2
yield (%)
entry R1
R4
catalystb
time
2
3
c
Department of Chemistry, Graduate School of Science
Tohoku UniVersity, Sendai 980-8578, Japan
1
2
3
4
5
6
7
H
H
H
H
H
F
n-C6F13 (1a)
H
H
H
H
Ni(PPh3)4 20 min 89
0
0
d
n-C6F13 (1a)
n-C6F13 (1a)
PhCF2 (1b)
n-C6H13CF2 (1c) H
n-C9H19 (1d)
CN (1e)
Ni(PPh3)4 2 h
Pd(PPh3)4 5.5 h
79
33 30
ReceiVed NoVember 1, 1999
Ni(PPh3)4 20 min 55 0g
Ni(PPh3)4 50 mine 52 0g
The transition-metal catalyzed cycloaddition reaction between
unsaturated hydrocarbons is a powerful synthetic method, and
many reactions have been reported in the literature.1 Recently,
we discovered palladium-catalyzed [4+2] homo-benzannulation
of conjugated enynes2 (eq 1) and this reaction was further
H
Ni(PPh3)4 20 h
27 0h
29 39
f
H
n-C6H13 Pd(PPh3)4 16 h
a To a mixture of Ni(cod)2 (27.5 mg, 0.1 mmol) and PPh3 (105 mg,
0.4 mmol) in dry toluene (0.5 mL) was added a solution of 1a (1 mmol)
in toluene (0.5 mL) at room temperature. For details, see Supporting
Information. b The reaction was carried out in the presence of 10 mol
% of the catalyst, unless otherwise noted. c The reaction was carried
out in the presence of 5 mol % of the catalyst. d The catalyst was
prepared by the reaction of NiBr2(PPh3)2 and PPh3 in the presence of
Zn dust. See ref 5 e A solution of enyne was added dropwise to a
solution of the catalyst for 30 min and stirred for 20 min at room
temperature. f The reaction was carried out in the presence of 2 mol %
of the catalyst. g Other products were not isolated. h A mixture of
oligomerized compounds was isolated as a byproduct.
extended to the [4+2] cross-benzannulation of conjugated enynes
with diynes,3 which alleviated significantly the inherent problem
associated with the [2+2+2] approach. In contiunation of our
interest in understanding the reactivity profile of the [4+2]
approach, we undertook an investigation of the reaction of
conjugated enynes with electron-withdrawing groups. Quite
interestingly, certain enynes 1 having electron-withdrawing groups
underwent an unprecedented annulation reaction, which we call
“zipper annulation”, in the presence of nickel catalysts (eq 2).
Figure 1. ORTEP drawing of 2b.
produced. To make clear whether the formation of this unexpected
product is due either to the substrate or to the Ni catalyst, the
reaction of 1a was carried out in the presence of Pd(PPh3)4 (entry
3). The Pd-catalyzed reaction was slower than the Ni-catalyzed
reaction and a nearly 1:1 mixture of 2a and 3a was obtained in
a lower yield (63% combined yield). Furthermore, the reaction
of 2-hexyl-1-butene-3-yne in the presence of Ni(PPh3)4 (10
mol %) gave neither the corresponding zipper annulation product
nor the [4+2] benzannulation adduct. Accordingly, it is clear that
both the fluorinated substituent at C-2 and Ni(0) catalyst are
essential in obtaining the zipper annulation product in high yield.
2-Perfluorohexyl-1-butene-3-yne (1a) dimerized in a highly
regiospecific manner in the presence of a catalytic amount of
Ni(PPh3)4 (5 mol %), which was prepared in situ from Ni(cod)2
and PPh3, to give the strained bicyclo[4.2.0]octa-1,3,7-triene (2a)
in 89% yield (Table 1, entry 1). When Ni(PPh3)4 was generated
in situ from (PPh3)2NiBr2, PPh3, and Zn dust, a longer reaction
time was required and the yield of 2a decreased slightly. In entries
1 and 2, none of the usual [4+2] benzannulation product 3a was
† Institute for Chemical Reaction Science, Tohoku University.
‡ Toyama Medical and Pharmaceutical University.
Other catalysts such as CpCo(CO)2 and RhCl(PPh3)3,4 which
1c
¶ Deceased January 12, 1998.
| Department of Chemistry, Graduate School of Science, Tohoku University.
(1) For reviews, see, for example: (a) Lautens, M.; Klute, W.; Tam, W.
Chem. ReV. 1996, 96, 49-92. (b) Schore, N. E. Chem. ReV. 1988, 88, 1081-
1119. (c) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23 (8), 539-
644.
are effective catalysts for the cyclotrimerization of alkynes, were
tested, but products such as 2 or 3 were not obtained. The reaction
of R,R-difluorobenzyl- and 1,1-difluoroheptylenynes (1b and 1c)
proceeded smoothly to give the corresponding bicyclic products
2b and 2c, respectively, in fair yields (entries 4 and 5). Therefore,
the zipper annulation takes place in general for enynes bearing
perfluorinated or partially fluorinated C-2 substituents. Even a
(2) Saito, S.; Salter, M. M.; Gevorgyan, V.; Tsuboya, N.; Tando, K.;
Yamamoto, Y. J. Am. Chem. Soc. 1996, 118, 3970-3971.
(3) (a) Gevorgyan, V.; Takeda, A.; Yamamoto, Y. J. Am. Chem. Soc. 1997,
119, 11313-11314. (b) Gevorgyan, V.; Takeda, A.; Homma, M.; Sadayori,
N.; Radhakrishnan, U.; Yamamoto, Y. J. Am. Chem. Soc. 1999, 121, 6391-
6402. (c) Review: Gevorgyan, V.; Yamamoto, Y. J. Organomet. Chem. 1999,
576, 232-247.
(4) Neeson, S. J.; Steverson, P. J. Tetrahedron 1989, 45, 6239-6348 and
references therein.
10.1021/ja993861k CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/09/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1811
Scheme 1
The two rings (four- and six-membered rings) were coplanar, and
the distances of the carbon-carbon atoms are within the values
expected for the structure of 2b.7
Currently we assume the mechanism of this reaction as shown
in Scheme 1. The Ni(0) species would coordinate with the alkyne
moiety of the enynes 1 and the nickelacyclopentadiene 4 or bis
π-allylnickel 5 would be formed.8 The reductive elimination of
the Ni(0) species from these complexes might proceed through
the nickel cyclobutadiene complex 6 or directly give the unstable
cyclobutadiene intermediate 7. The existence of an electron-
withdrawing group may be essential for facilitating the reductive
elimination to 6 or 7. A pericyclic reaction of 7 (or 6) in either
the presence or the absence of Ni(0) catalyst would give the
bicyclo[4.2.0]triene 2.
It is not yet clear why an electron-withdrawing group, especially
a perfluoroalkyl group, is essential and why the Ni(0) catalyst is
more efficient than the Pd(0) catalyst for this unprecedented zipper
annulation reaction. Although it is premature to settle the
mechanism, the strained bicyclo[4.2.0]trienes are formed from
enynes in one step: especially the fluorinated trienes may be
useful as a monomer for the synthesis of fluorinated polymers.
We are currently investigating such a possibility in addition to
the detailed mechanistic study of the zipper reaction.
Acknowledgment. This work was supported in part by the Japan
Securities Scholarship Foundation.
Supporting Information Available: Synthetic procedures and char-
acterization data for the compounds 1-3, and the crystallographic data
of 2b (PDF). This material is available free of charge via the Internat at
http://pubs.acs.org. See any current masthead page for ordering informa-
tion and Web access instructions.
1,1-difluoroenyne 1d having an n-nonyl substituent at C-2
underwent zipper annulation to give 2d although the yield was
low (entry 6). 2-Cyanoenyne 1e reacted in the presence of a
Pd(0) catalyst to give a mixture of 2e and 3e in 68% combined
yield (entry 7), although the nickel-catalyzed reaction afforded a
complex mixture of the products.
The structures of the bicyclic products 2a-e were determined
unambiguously by spectroscopic methods. Furthermore, the
structure of 2b was confirmed by an X-ray crystallographic
analysis, and an ORTEP structure of 2b is shown in Figure 1.6
JA993861K
(6) Crystalographic data for 2b: MF ) C22H16F4, MW ) 356.36,
orthoronbic; a ) 10.123(3) Å, b ) 30.654(4) Å, c ) 5.709(3) Å, V ) 1771-
(6) Å3, space group P212121, Z ) 4, Dc ) 1.336 g/cm3. Data collection was
done using Mo KR radiation (µ ) 1.06 cm-1) with a rotating anode
diffractometer (50 kV, 120 mA). A total of 2316 reflections was collected, of
which 1383 reflections with Io > 1σ(Io) were used in the refinement. The
final R factor was 0.065 (Rw ) 0.059). The final difference peaks were Fmax
) 0.20 and Fmin ) -0.20 e/Å3. For details, see Supporting Information.
(7) For details, see Supporting Information.
(8) It is widely accepted that the matallacyclopentadiene is an important
intermediate in the transition metal catalyzed cyclotrimerization of alkynes.
See: Principles and Applications of Organotransition Metal Chemistry;
Collman, J. P., Hegedus, L. S., Norton, J. R., Finke, R. G., Eds.; University
Science Books: Mill Vally, 1987: Section 18.3c, pp 870-879, and ref 1.
(5) Iyoda, M.; Tanaka, S.; Otani, H.; Nose, M.; Oda, M. J. Am. Chem.
Soc. 1988, 110, 8494-8500.