Frans-1.2-Dimethylcyclopentane

PROBLEM 5.20 Show that frans-1,2-dimethylcyclopentane can exist in chiral, enantiomeric forms. 

Interestingly, zirconacyclopentane 246 formed by the reaction of 1,6-heptadiene with the Zr complex has the firms ring junction mainly [108]. It should be noted that the preparation of the trans ring junction in the bicyclo[3.3.0]octane system by other means is difficult. Carbonylation of 246 affords trans-fuzed bicyclo[3.3.0]octanone 247 [109,111]. The diacetoxy compound 248 is obtained by oxidative cleavage of 246. Protonation affords the frans-dimethylcyclopentane skeleton. Similar reactions occur with 1,6-enynes, and Pauson Khand-type cyclopentenone synthesis is possible by carbonylation. 

Mention should be made of the high activity of ruthenium deposited on alumina in the isomerization reaction both to methylcyclohexane and especially to the mixture of trans- and cis-1,2- and 1,3-dimethylcyclo-pentanes, the dimethylcyclopentane content in the cyclane-alkane portion of the catalysate obtained in the presence of this catalyst being 60%, of which 85-90% constituted frans-1,2Kiimethylcyclopentane. The cyclane-alkane portion of the catalysate obtained on contact with Pd-Si02 also contained 10 % dimethylcyclopentanes. 

This result confirmed the earlier work of Whitmore, F. C. Johnston, F. /. Am. Chem. Soc. 1933, 55,5020, who had found that the addition of HCl to 3-methyl-l-butene without solvent, in a sealed reaction tube for 7 weeks, gave both 2-chloro-3-methylbutane and 2-chloro-2-methylbutane. This result contradicted the suggestion of earlier investigators that the 3° alkyl halide was formed by rearrangement of the 2° alkyl halide formed from the addition reaction. Hammond, G. S. Collins, C. H. /. Am. Chem. Soc. 1960, 82, 4323 found that addition of HCl to 1,2-dimethylcyclopentene produced Tchloro-frans-l,2-dimethylcyclopentane as the major (perhaps only) addition product, but it isomerized to l-chloro-c/s-l,2-dimethylcyclopentane. 

Cis- and frans-l,2-dimethylcyclopentane can also be drawn as if the plane of the ring goes through the plane of the page. Each carbon in the ring then has one bond that points above the ring and one that points below. 

---