131. P. Müller, C. Gränicher, F.-G. Klärner, V. Breitkopf,
     “Metallocarbene Intermediates in Rhodium(II)-catalyzed Rearrangements of Ethyl 2-butyl cycloprop-2-ene-1-carboxylate”
     Gazz. Chim. Ital. 1995, 125, 459-463.

Ethyl 2-butylcycloprop-2-ene-1-carboxylate, 1 , was rearranged thermally, photochemically and in the presence of Cu(l) and Rh(Il) catalysts. The rearrangement leads to vinylcarbenes via ring opening. The products of the thermal and photochemical reactions derive from the more stable vinylcarbene 4 which, in tum, originates from cleavage of the more highly substituted cyclopropene single bond, while those of the transition metal-catalyzed rearrangement derive from cleavage of the less substituted bond through the vinylcarbenes (E)-2, (Z)-2 and (Z)- 10, respectively. When enantio-enriched 1 was rearranged with achiral [Rh₂(pfb)₄] the unreacted starting material retained the enantiomeric excess in the course of the reaction, while the product 3 was racemic. Rearrangement of 1 with the chiral Rh(Il) catalysts [Rh₂{(2S)-mepy}₄] or [Rh₂(R-bnp)₄ ) produced 3 in modest yield and modest enantiomeric excess (<52%).

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