Synthesis of 1,2-divinylcyclopropanes by metal-catalyzed cyclopropanation of 1,3-dienes with cyclopropenes as vinyl carbene precursors

Abstract

The synthesis of 1,2-divinylcyclopropanes by the reaction of cyclopropenes with 1,3-dienes is reported. The process relies on the ability of ZnCl₂ or [Rh₂(OAc)₄] to generate metal-vinyl carbene intermediates from cyclopropenes, which effect cyclopropanation of 1,3-dienes. Most of the reactions proceeded in reasonable yields while the diastereoselectivity strongly depends on the structure of the diene. An example of an intramolecular process as well as the use of furan and 1,4-cyclohexadiene as dienes are also reported.

Keywords: cyclopropanation; cyclopropenes; dienes; divinylcyclopropanes; transition metal catalysis.

Scheme 1

Typical syntheses of 1,2-divinylcyclopropanes and rationale hypothesis for their syntheses from cyclopropenes and 1,3-dienes.

Scheme 2

Synthesis of 1,2-divinylcyclopropane 3a: Optimization studies. ^aIsolated yield. ^bDetermined by ¹H NMR on the reaction crude mixture (unaltered after purification). ^cEstimated by ¹H NMR. (IPr = 1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene).

Scheme 3

Synthesis of 1,2-divinylcyclopropanes 3 from cyclopropenes 1 and unbiased 1,3-dienes 2: Scope. (Yields of isolated products, diastereoisomeric ratios were determined by ¹H NMR). ^aAt 50 °C. ^b1 was completely consumed. ^c1,3-Butadiene was used in large excess (ca. 0.5 mL condensed prior the reaction). ^dcis relationship is referred to substituents labelled with an asterisk.

Scheme 4

Rh-catalyzed intramolecular cyclopropanation with dienylcyclopropene 4 (the trans/cis ratio is related to the C-atom labelled with an asterisk).

Scheme 5

Zn- or Rh-catalyzed reactions of cyclopropenes 1 with furan (6) and 1,4-cyclohexadiene (8) and comparison with related processes (1b, R¹ = n-Hex).