Cascade Reactions of Arylvinylcyclopropenes
COMMUNICATION
intramolecular Friedel–Crafts reaction to give intermediate
C. In the presence of BF3·OEt2, intermediate C undergoes
the release of an ethoxy group and aromatization to give
the corresponding cationic intermediate D. Intramolecular
cyclization to the exo-vinyl group produces intermediate E
through deprotonation to furnish the product 4.
Crafts reaction of intermediate J produces adducts 5.[12] On
the other hand, aromatization of intermediate I produces
naphthalene derivative 7 as the by-product.[5b] It should be
noted that the p attack and the s attack of H+ to cyclopro-
pene could be both possible in this reaction. However, since
the structures of arylvinylcyclopropenes 1 are planar and
highly hindered,[5b] the direct p attack could be favorable
and the sigma-attack could be blocked out.
In conclusion, we have succeeded in the construction of a
variety of aromatic systems via different regioselective addi-
tion of arylvinylcyclopropenes to acetals and aldehydes in
the presence of Lewis or Brønsted acid under mild condi-
tions. Efforts are in progress to elucidate further mechanistic
details of these reactions and to understand their scope and
limitations.
Experimental Section
General procedure for BF3·OEt2-catalyzed reaction of arylvinylcyclopro-
penes with acetals: Under an argon atmosphere, arylvinylcyclopropenes 1
(0.2 mmol), acetal
2 (0.4 mmol), BF3·OEt2 (0.02 mmol) and DCE
(1.0 mL) were added into a Schlenk tube. The reaction mixture was
stirred at 508C until the reaction completed. Then, the solvent was re-
moved under reduced pressure and the residue was purified by a flash
column chromatography (SiO2).
General procedure for TfOH-catalyzed reaction of arylvinylcyclopro-
penes with aldehyde: Under an argon atmosphere, arylvinylcyclopro-
penes 1 (0.2 mmol), aldehyde 3 (0.4 or 0.6 mmol), TfOH (0.02 mmol) and
DCE (1.0 mL) were added into a Schlenk tube. The reaction mixture was
stirred at 108C until the reaction completed. Then, the solvent was re-
moved under reduced pressure and the residue was purified by a flash
column chromatography (SiO2).
Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(06XD14005, 08dj1400100-2), National Basic Research Program of China
AHCTUNGERTG(NNUN 973)-2009CB825300, and the National Natural Science Foundation of
China for financial support (20872162, 20672127, 20821002 and
20732008).
Scheme 2. A plausible reaction mechanism of arylvinylcyclopropene 1
with acetal 2 and aldehyde 3 in the presence of acids.
Keywords: acid
catalysis
·
cascade
reactions
·
cyclopropenes · Friedel–Crafts reaction · Prins reaction
When aldehyde 3 (R=aromatic ring) is employed as the
substrate, the carbocation F and protonated aldehyde 3
could be formed in the presence of Lewis acid initially.
Their relative stabilities are also investigated theoretically.
The carbocation F is more stable than protonated aldehyde
3 in the gas phase and in solution (for computational details,
see Supporting Information). Based on theoretical investiga-
tion results, one possible mechanism starting from the car-
bocation F is proposed here. When aldehyde 3 is used in the
reaction, 1 undergoes protonation, ring-opening, allylic mi-
gration, and intramolecular Friedel–Crafts reaction via inter-
mediates F, G, and H to generate intermediate I.[5b] The
Prins-type reaction of intermediate I with protonated alde-
hyde 3 produces final products 8 (R4 =alkyl group) or inter-
mediate J (R4 =aryl group). Further intermolecular Friedel–
[1] For a review, see: M. S. Baird in Cyclopropenes: Synthesis by Con-
struction of the System, Vol. E17d/2 (Ed.: A. de Meijere), Houben-
Weyl, Stuttgart, 1996, pp. 2695–2744.
8103–8112; c) A. Fattahi, R. E. McCarthy, M. R. Ahmad, S. R. Kass,
[3] For reviews, see: a) P. Binger, H. M. Bꢁch, Top. Curr. Chem. 1987,
2241–2290; c) M. S. Baird in Cyclopropenes: Transformations,
Vol. E17d/2, (Ed.: A. de Meijere), Houben-Weyl, Stuttgart, 1996,
pp. 2781–2860; d) M. Nakamura, H. Isobe, E. Nakamura, Chem.
[4] For recent reviews, see: a) M. Rubin, M. Rubina, V. Gevorgyan,
Chem. Eur. J. 2009, 15, 7543 – 7548
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