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

Abstract

Dedicated to Prof. Kurt Schaffner on the occasion of his 60th birthday (29.V.91)The stereo and face selectivities of the cycloaddition of 1,2,3-trichloro-3-fluorocyclopropene (la) with acyclic dienes and furans has been re-investigated by X-ray determination and correlation of I9F-NMR data. The isolated adducts of dienes exclusively have exo-configuration, and exo-configuration predominates with furans. The CI substituents of the resulting cyclopropane ring are cis-oriented. The face selectivity of the reaction with both types of substrates is attributed to electrostatic interactions between the F and the bridgehead CI substituents, which destabilize the F-cis-transition state (13 (F-cis)) over 13 (F-trans).

Structure, Synthesis, and Chemical Reactions of Fluorinated Cyclopropanes and Cyclopropenes

“…188,224,225 The cycloadducts of tetrachlorocyclopropene and 3,3-difluoro-1,2-dichlorocyclopropene with 1-substituted and 1,4-disubstituted 1,3butadienes have been similarly determined to have the exo configuration. 224,226,227 Almost simultaneously with the Law and Tobey report, 202 Sargeant reported the [4 + 2] cycloaddition reactions of perfluorocyclopropene (101) and perfluoro-1,2-dimethylcyclopropene (82) with cyclopentadiene and furan at room temperature. 182 In the case of 101, both tricyclic adducts 125 (X ) O or CH 2 ) were assigned the endo structure, but again the evidence (NMR chemical shifts) is unconvincing.…”

“…Whereas the stereochemistry of cycloaddition could not be established for the tetrachloro and tetrabromo adducts because of facile rearrangement to the bicyclic allylic adducts 123 , the stereochemistry of the 3,3-difluoro-1,2-dihalo adducts 124 was assigned the endo configuration 124a , based largely on analogy to previous lore in the cyclopropene series. , It should be mentioned that at the time these cycloadditions of perhalocyclopropenes, especially the perbromo and perchloro examples, were surprising in view of previously disclosed failures of 3,3-disubstituted (methyl or phenyl) cyclopropenes to undergo cycloaddition with cyclopentadiene or 1,3- butadienes, , presumably for steric reasons. Further examination of the structure of adducts 124 by several groups has unambiguously established the stereochemistry of the isolated adducts as exo, i.e., 124b , largely by X-ray crystallography. ,, The cycloadducts of tetrachlorocyclopropene and 3,3-difluoro-1,2-dichlorocyclopropene with 1- substituted and 1,4-disubstituted 1,3-butadienes have been similarly determined to have the exo configuration. ,,

…”

“…Since furan invariably yields exo adducts, even with cyclopropene, it is likely that this adduct has the exo structure. Other dienes that yield only, or predominantly, exo cycloadducts with perhalocyclopropenes include isobenzofurans, ,− 6,6-dimethyl- and 6,6-diphenylfulvene, [2.2]furanophane, and norbornadiene. , …”

Cycloproparenes

“…Initially, the configuration of such Diels−Alder adducts was presumed to result from compliance with the Alder endo rule . However, it was shown subsequently that cyclopropenes which carry a bulky flagpole (C-3) substituent add instead from the exo face − to provide a substrate that carries an antiperiplanar proton and halogen atom as depicted by 40 in Scheme (see below). In the early work, tetrahalocyclopropenes were used almost exclusively, ,, but the mid-1980s saw the development of a facile and straightforward synthesis of 1-bromo-2-chlorocyclopropene by Billups and co-workers …”

Synthesis of Dinuclear and Trinuclear Ruthenium Cyclopropenyl Complexes

“…This molecule has hence been under intense investigation and has played a crucial role in the development of the concept of aromaticity . Chemical reactivity of this molecule has also been addressed. − However, transition metal cyclopropenyl complexes are rare, even though participation of d orbitals in these complexes is expected to significantly stabilize the molecule. Previously we reported the facile synthesis of several mononuclear ruthenium cyclopropenyl complexes by deprotonation of (η 5 -C 5 H 5 )(PPh 3 ) 2 RuCC(Ph)CH 2 R + in which C α of the vinylidene ligand is known to be electron deficient.…”