exo‐Stereoselectivity in Diels‐Alder Addition of Halogenocyclopropenes to Butadienes and Furans

Abstract

The stereochemistry of the Diels-Alder reaction of 1,2-dichloro-3,3-ditluorocyclopropene (Sa) to 1,4-diphenyl-1,3-butadiene (6) and 1,3-diphenylisobenzofuran (7) was unambiguously established by X-ray structure determination. In all cases known so far, tetrahalogenocyclopropenes add exo to open-chain dienes and furans. The previously reported exo-addition product (2a) of 1-bromo-2-chlorocyclopropene (5b) to 7 allows assignments of the stereochemistry of other additions of 5b to furans. exo-Addition usually predominates, but in some cases endo-adducts are also formed. This contrasts with reports in the literature that 5b adds preferentially endo to open-chain dienes.

Macrocyclic Cyclopropenes by Highly Enantioselective Intramolecular Addition of Metal Carbenes to Alkynes

“…By injection of carbon monoxide at room temperature the red solution turned orange and the monoacetonitrile complex 2 a was obtained in 61 % yield based on 1 a. Related ligand exchange reactions at the cationic complex [h 5 -CpFe(CO) 3 ] PF 6 À were previously described by Astruc et al [11] However, the cationic CpFe complexes with acetonitrile ligands reported therein are fairly stable compared to those of cyclopentadienones.…”

“…All three products were obtained with CuPF 6 /1 as the catalyst, but the formation of 13 was virtually the exclusive outcome of reactions with chiral dirhodium carboxamidates, although with diminished enantiocontrol from that obtained for the formation of 4, 8, or 11. Cyclopropene 13 was relatively unstable and, therefore, was treated with 1,3-diphenylisobenzofuran to form its stable exo-Diels ± Alder adduct; [11] the % ee of 13 was obtained following selective hydrogenation of the cyclopropene with diimide. [12] The ylide derived product 15 that was a major outcome of the CuPF 6 /1 catalyzed reaction was formed in only 18 % ee.…”

Stereo and Face Selectivity in Cycloadditions of 1,2,3‐Trichloro‐3‐fluorocyclopropenes to Acyclic Dienes and Furans

“…Part (cf Table 2 ) . The cyclohexene moiety of 3d is nearly planar as in the adduct of phenylbutadiene to l c [4] and not puckered as previously assumed [6].…”

“…This is the result of an ex0 -addition of the diene to the cyclopropene. Accordingly, the stereoselectivity of the cycloaddition of l a to open-chain dienes is identical to that of tetrachlorocyclopropene (lc) [4], which contradicts the view of Law and Tobey [6], that the cycloadditions of tetrahalogenocyclopropenes to all dienes should be endo.…”

“…Until recently, the information about the stereochemistry of the Diels-Alder reaction of substituted cyclopropenes was incomplete and, in part, contradictory. Several examples of this reactions with 1,2-di-and tetrahalogenocyclopropenes have recently been re-examined by X-ray crystallography and modern NMR techniques [4] [5], and as a consequence of these investigations, some of the reported configurations of the adducts had to be revised. The following empirical guidelines for predicting the configuration of the (kinetic) products in cycloadditions of cyclopropenes have been formulated by Apeloig et al [5]: 'The parent cyclopropene and 1,2-disubstituted cyclopropenes are expected to yield endo -adducts, exclusively or predominantly.…”