Cyclo-propane, isomerization

The interaction of unsaturated molecules, for example olefins and acetylenes, with transition metals is of paramount importance for a variety of chemical processes. Included among such processes are stereospecific polymerization of olefin monomers, the production of alcohols and aldehydes in the hydroformylation reaction, hydrogenation reactions, cyclo-propanation, isomerizations, hydrocyanation, and many other reactions. 

Figure 2.10 Pressure dependence of the first-order rate constant for cyclo-propane isomerization. [After H.O. Pritchard, R.G. Sowden, and A.F. Trotman-Dickenson, Proc. Roy. Soc. (London), A217, 563, with permission of The Royal Society, (1953).]...

Methoxycyclopropanemcthanols, as potential precursors of cyclobutanones. are also obtained by addition of 1-methoxy-l-vinyllithium to carbonyl compounds followed by cyclo-propanation of the resulting allylic alcohol. Starting with cyclohexanone the final product was spiro[3.5]nonan-l-one (3).154 Cyclopropanation and rearrangement of an isomeric allylic alcohol 4 yielded spiro[3.5]nonan-2-one (5).15S... 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

The experimental and theoretical work published by the early 1970s viewed the stereomutations of cyclopropanes as kinetically competitive one-center and two-center stereomutations some details, especially regarding relative rate constants for one-center epimerizations which defined relative rotational propensities, remained unclear, but all agreed that neither the Smith mechanism (one-center only) nor any two-center-only formulation for stereomutations could be sufficient. Thus when kinetic studies275 278 on the isomerizations shown by chiral samples of l-phenyl-2-d-cyclopropane and 1,2-d2-cyclo-propane purported to show that, actually, two-center stereomutations were kinetically dominant, many were stimulated to fresh speculations and accommodations. Theoretical work at times hinted that the parent hydrocarbon might be an exceptional case and might... 

What are the properties of these compounds and what is their structure which can thus explain their isomerism with the olefines, and the fact that they are not unsaturated The simplest member of the group, viz., CaHe, is known as tri-methylene or cyclo propane. 

Tri-methylene, Cyclo Propane.—It is isomeric with propylene for which the structure has been shown to be CH3—CH = CH2. Now propylene is related to propane in that two hydrogen atoms in propane are lost from two adjacent carbon groups, the two carbons becoming doubly linked. 

The only way out is to express the transformation of the molecules in terms of the elementary steps of carbenium-ion chemistry protonation, H- and Me-shifts, protonated cyclo-propane-type isomerization, P-scission. .. [Froment, 1996]. In Fig. 3 the cracking of n-hexane is represented in terms of the single event approach introduced by Froment and co-workers [Baltanas et al 1989 Vynckier and Froment, 1991 Froment, 1991 Svoboda et al, 1995]. 

Several reagents can effect dehydrochlorination from an alkyl side chain attached to a cyclopropane ring. Thus, for example, short heating of equivalent amounts of (1,1-di-chloroethyl)cyclopropane (1) and quinoline gave an 80% yield of (l-chloroethenyl)cyclo-propane (2) no isomeric ethylidenecyclopropane was reported. 

The reluctance of phenyl-substituted methylenecyclopropanes to rearrange to products in which the phenyl group is located at the exocyclic methylene group was also demonstrated with l-methylene-2-phenylcyclopropane and l-methyl-2-methylene-l-phenylcyclopropane. When used as single enantiomers, both compounds underwent facile racemization at 100 C in chloroform without formation of benzylidenecyclopropane or (l-phenylethylidene)cyclo-propane. Similar results were obtained when a mixture of as- and tra i-l-methyl-2-methyl-ene-3-phenylcyclopropane (Table 1, R = R = R = R = H R = Me R = Ph) was isomerized under thermal or photochemical conditions. 

The protolytic transformation of homofuran derivative 15 into methyl ( , )-6-oxohexa-2,4-dienoate is one of the key steps in a synthesis of the arachidonic acid metabolite 5-HETE. Activated, low acidity zeolites can be used to induce the isomerization of cyclo-propanes. " ... 

The alkyl substituents in 1-alkyl-l-vinyl and /rfl/7 -l-alkyl-2-vinyl-cyclo-propanes have little effect on reactivity. A cis alkyl substituent may have a profound effect for example, the energy of activation is nearly 20 kcal.mole smaller and the entropy of activation is 10 eu more negative, for isomerization of cis-1 -methyl-2-vinylcyclopropane than for typical vinylcyclopropane rearrangements. Further, the sole product of this reaction is cis-1,4-hexadiene. This and related reactions, which occur by a concerted mechanism involving 1,5-hydrogen migration, are discussed in the next section. 

From Diazo-compounds. The routine cuprous-catalysed diazomethane cyclo-propanation method has been used on bicyclic and bridged olefins. With fneso-tetraphenylporphin, this procedure gave a single cyclopropane product, unlike the corresponding reaction with diazo-esters. Trishomobenzene has been prepared by multiple use of this diazomethane method. It was shown to be the trans-isomer (78) by n.m.r. spectroscopy in (78) there are seven types of H whereas in its cis-isomer there would be only three. Twelve isomeric products have been isolated by preparative g.l.c. from the reaction of diazomethane-Cu2Cl2 with biallenyl (79). Some of the products could be synthesized independently, otherwise structural assignments were based upon spectroscopic evidence. 

Unlike the isomerization of l-chloro[l- C]butane with aluminium trichloride, which proceeds without major participation of protonated cyclo-propanes, the isotopic scrambling from the reaction of [l- C]-l-propyl-mercuric perchlorate in trifluoroacetic acid does require intermediate protonated cyclopropanes. The isolated 1-propyl trifluoroacetate (565) was shown by degradation to have the label distributed as shown in Scheme 76. The greater scrambling from C-1 to C-3 than from C-1 to C-2 requires an edge-rather than a comer-protonated ion. Full details have appeared of the liquid-phase thermolyses of cycloalkyl and cycloalkylmethyl chloroformates which take place via carbonium ions. Protonated cyclopropanes are believed to be intermediates for 5-10% of the products. The alkylation of benzene and toluene by cyclopropane with acidic catalysts also involves initial formation of a protonated cyclopropane. ... 

Distribution measurements show that the silver complex of dr-cyclo-octene is less stable than that of cycloheptene, presumably owing to more ring strain in the latter 129,130). tronr-Cyclooctene is considerably more strained than the cis isomer, and can be separated from it by extraction with 20% aqueous silver nitrate 32), but there are no quantitative measurements of the stability of the silver complex of the tram isomer. The interesting possibility of isomerizing cis to trom-cyclooctene via metal complexes has not yet been achieved. tronr-Cyclooctene has been resolved via its platinum(II) complex with the optically active amine l-phenyl-2-amino-propane (am), 7r-CgH]4PtCl2am. 29). 

Perfluoropropene oxide is a convenient, volatile, thermal source of difluoro-carbene, and its use in the preparation of fluorocyclopropanes has been further exemplified, perfluorinated, polyfluorinated, and hydrocarbon olefins being employed as substrates (see also p. 17) it has also been employed to convert perfluorobut-2-yne into 3,3-difluoro-l,2-bis(trifluoromethyl)cyclo-propene. Qose examination of the reaction between the epoxide and a mixture of cis- and rra .r-l-chloro-l,2-difluoroethylene at ca. 200°C has revealed that stereospecific addition of difluorocarbene takes place, but that loss of configuration can subsequently result from slow thermal isomerization of the cyclopropane product. Thermal decomposition of perfluoropropene oxide at 200 "C in the absence of a trap yields mainly perfiuorocyclo-propane and trifluoroacetyl fluoride together with tetrafluoroethylene, perfluoroisobutene oxide, perfluorobut-l-ene, and poly(difluoromethylene). 

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