Isomerizations cyclopropane rings

The increasingly common theme of developing new pyrrole syntheses that involve cyclopropane fragmentations appeared in a type Ilae pyrrole synthesis <06OL835>. Treatment of doubly activated cyclopropane 43 with benzylamine in the presence of magnesium sulfate led to complex pyrrole 44 via a nucleophilic cleavage of the cyclopropane ring, intramolecular condensation, and isomerization of the exocyclic JC-bond. 

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

No complete kinetic experiments have yet been described for the isomerization reactions of tricyclic systems containing two cyclopropane rings. A number of preliminary observations have been reported and some of the isomerizations noted are given below. 

While activation energies for these reactions have not yet been obtained, it is clear from the temperatures at which the isomerizations have been found to occur that they must have significantly smaller energies of activations (perhaps by as much as 10 to 15 kcal mole than those for the cis-trans isomerizations of dialkylcyclopropanes. This suggests that the two cyclopropane rings can co-operate, especially if the geometric arrangement is favourable. 

Sarel and co-workers have examined some reactions of alkynylcyclopropanes with iron carbonyl compounds [1]. Treatment of cyclopropylacetylene (5) with iron pentacarbonyl under photolytic conditions gives, after cerium(IV) oxidation, isomeric quinones 6 and 7, derived from two molecules of 5 and two carbonyls with both cyclopropane rings intact [6]. Furthermore, the photoreaction of dicyclopropylacetylene (8) with iron carbonyl gives some ten different products depending on the reagents and the reaction conditions, and some of them have the cyclopentenone skeleton formed by the opening of cyclopropane ring coupled with carbonyl insertion [7] (Scheme 2). 

Compounds of type 94 with substituted and spirocyclopropane-annelated cyclopropane rings showed an increased rate of rearrangement, and the corresponding dimethyl-substituted starting material 94 k was never isolated, its transformation into 109k proceeded even in the absence of Lil upon attempted preparation (36% yield). This isomerization apparently occurs regioselectively and ste-... 

Substitution of the VCP is tolerated both on and adjacent to the cyclopropane ring. Diester-substituted and heteroatom (O, NTs) tethers are well tolerated. Reactions were conducted with 2-10 mol% catalyst at up to 0.20 M, as illustrated. Most importantly, reactions with the naphthalene catalyst were found to be more rapid than those with other catalysts. For example substrate 54 is readily converted in >99% yield to cycloadduct 55 in only 15 min at room temperature (entry 1). Complex 93 efficiently catalyzes the reactions of both alkynes and alkenes with VCPs, offering greater generahty than thus far observed with non-rhodium catalysts. This catalyst is particularly advantageous in the cases of substrates 100 and 102, for which the desired product is not formed cleanly with Wilkinson s catalyst due to product isomerization. 

Fig. 6. 3C complexes as intermediates of isomerization. A two-site (large ensemble) mechanism via a bond shift (left) or cyclopropane ring (right), as suggested by various authors (see text). Except for the number of C atoms involved, all other aspects of the mechanisms are speculative. The same remark holds for Figs. 7-9. 

The latter proposal would lead one to conclude that radical 56, having a cyclopropane ring and a resonance-stabilized secondary radical, is more stable than is the isomeric form 55, which has a conjugated ketone and a primary radical. The product, therefore, is one derived from the intermediate 56. In the case where the 19-oxime is formed without any rearrangement, the initially formed intermediate 57, having a double bond and a primary radical, would be more stable than is the isomeric form 58, which contains a cyclopropane ring and a secondary radical. 

The mechanism proposed for the isomerization of perfluoro(alkylcyclopropanes)24 involves abstraction of fluoride ion from the a-position to form a cation that is partially stabilized by delocalization into the cyclopropane ring. Ring opening, addition of fluoride, and bond migration give the observed products, as shown for pcrfluoro(methylcyclopropane) (27a). 

A new and readily available catalytic system has been developed to open the cyclopropane ring in 4 + 2-1- 2-homo-Diels-Alder cycloadducts formed by reaction of norbornadienes and buta-1,3-diene.47 The cobalt-mediated homo-Diels-Alder reaction followed by this PtCB-promolcd isomerization is a key step in the efficient route to bicyclo[5.3.0]decanes. 

The structural isomerization of cyclopropane to propene does not involve radical chain processes if a trimethylene diradical intermediate is involved, it must be of such a short lifetime that it may not be easily trapped through gas-phase intermolecular reactions Yet the trimethylene diradical hypothesis does account for the thermal interconversion of cis and trans 1,2-d2-cyclopropanes in a most plausible manner. Homolytic cleavage of one carbon-carbon bond would form a 1,3-diradical intermediate rotations of terminal methylene groups in this trimethylene diradical followed by reformation of the cyclopropane ring would rationalize the isomerization According to this model, the net outcome of a stereomutation process would be dictated by the relative magnitudes of rate... 

Record the refractive indexes of the last two fractions ( d = 1-4410), and analyze them by vapor-phase chromatography. Only minor impurity peaks should show on the VPC trace at low attenuation, with the ester peak off-scale. Obtain an infrared spectram of a 5 percent solution of the final fraction in carbon tetrachloride. Isomerization of the ring to a terminally unsaturated straight chain is much less likely than with the cyclopropane ring. Check for absorption in the medium-strength 3085 cm and 3030 cm vinyl C—H stretching bands vinyl C—bending bands tend to be obscured by other features. 

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