Trans-l, 2-Dimethylcyclopentane

Class (1) reactions were observed in all four cycloalkanes. The highest rate constants were observed for reactions of cyclohexane hole with low-IP aromatic solutes, (3-4.5) x 10" sec at 25°C [75]. In these irreversible reactions, a solute radical cation is generated. Class (2) reactions were observed for reactants 1,1-dimethylcyclo-pentane, trans-l, 2-dimethylcyclopentane, and 2,3-dimethyl-pentane in cyclohexane [74], trans-dtcaXm, bicyclohexyl, and Ao-propylcyclohexane in methylcyclohexane [69], and benzene in cis-... 

The solid wedges in 37 and 38 show the group projected out of the paper and the dashed lines show groups projected behind the paper. If two like groups are on the same side of a ring, it is a c s-cycloalkane and if the like groups are on opposite sides of the ring, it is a ra s-cyclic alkane. Therefore, cyclic alkane 37 is named cis-l,2-dimethylcyclopentane and 38 is trans-l,2-dimethylcyclopentane. 

Two types of substrate probe, cis- and trans-l,3-dimethylcyclopentane and ethyl- and 2-propylcyclopropane, were used to shed light on mechanishc details of the hydroxylahon step [30]. In the use of the first two probes, the participation of Ti—OOH species in a concerted mechanism would predict either the retention or the inversion of configurahon at the chiral center, while the stereochemistry of a homolytic mechanism would be determined by the compehhon between the epimerization of the transient terhary carbon radical and C—O formation (Scheme... 

There are two different substances named tran.s-l,2-dimethylcyclopentane. Make molecular models and see if you can find the relationship between them. (See Section 9.7.)... 

Very recently, Garst and coworkers 100) have provided a means to suppress the cyclization of (l-methyl-5-hexenyl)sodium with t-butyl amine101 . t-Butyl amine is a sufficiently reactive proton donor to compete successfully with the anion cyclization 138 - cis/trans-139 and the intramolecular 1,4-proton transfer 138 -> 142, leaving 1-heptene 140 as well as cis- and tra -l, 2-dimethylcyclopentane cis,trans-141 formation through radical cyclization only. For sodium metal reactions excess t-butyl amine-nearly eliminates cis,trans-141 suggesting that radical cyclization is negligible. For the Na + N" reaction, 2-heptenes 143 are eliminated by added t-butyl amine, but the yields of cis,trans-141 are merely diminished. As expected, the cis/trans... 

For example, Kunitake used the shifts on going from a trans-to a ciA-l,2-dimethylcyclopentane as representative of that which would occur in an analogous 2,3-dimethylsuccinic anhydride. Thus he estimated a shift of -5.1 ppm for C-2 (and C-3) and -2.1 ppm for the carbonyl carbon. We found that the actual shifts on going from trans- to cis-2,3-dimethylsuccinic anhydride are -4.1 ppm (C-2 and C-3) and +1.1 ppm (carbonyls). 

The situation is more complicated in the case of the pinacolic rearrangement of the isomeric cyclopentane-1,2-diols (Bunton and Carr, 1963b). In aqueous perchloric acid, cis-1, 2-dimethylcyclopentane-l,2-diol (19) is converted into a mixture of the polymeric cyclopentadiene). No O18 is found in the unrearranged cis-diol. The trans-1,2-dimethylcyclopentane-1,2-diol (20), on the other... 

Figure 3-13 compares the cis-trans isomers of 2-butene with those of 1,2-dimethylcyclopentane. Make models of these compounds to convince yourself that cis- and rmn5-l,2-dimethylcyclopentane cannot interconvert by simple rotations about the bonds. 

The trans compound exists as a pair of enantiomers 5 and 6. c -l,2-Dimethylcyclopentane (7) is a meso compound. It has a plane of symmetry that is perpendicular to the plane of the ring ... 

An electrochemical counterpart to this cyclization is provided by the anodic reduction of non-conjugated olefinic ketones cyclic tertiary alcohols are formed (Scheme 101) in a process initiated by electron addition to the carbonyl group. This method is limited to cases leading to formation of five-and six-membered rings it provides some interesting stereochemical results, e.g, the isolation of cw-l,2-dimethylcyclopentan-l-ol, whereas methyl-magnesium iodide on 2-methylcyclopentanone gives mainly the trans-di-methyl isomer. 

A facile intramolecular carbomagnesiation becomes possible by inserting one more carbon between the reactive magnesium center and the double bond. For example, when a 6-chloro-l-heptene was refluxed with magnesium, 1,2-dimethylcyclopentane (cis/trans = ca 1/4) was obtained in 88% yield after hydrolysis (Scheme 47) °. The cyclization shows a 5-exo-trig selectivity and the product derived from 6-endo-trig cyclization (methylcyclohexane) is not observed. 

The most frequent cyclopentane conformations show carbon atoms that carry two substituents equally disposed above and below the ring plane (Fig. 1.2.7). These are called isoclinal (i). Substituted cyclopentanes usually do not show significant conformational preferences. cw-l,3-Dimethylcyclopentane, for example, is only 0.5 kcal/mol more stable than the trans isomer, compared with an approximate 2 kcal/mol difference between the 1,3-dimethyl-cyclohexanes. 

Other unsaturated organometallics such as those derived from sodium, lithium, aluminum, " gallium, indium, " zinc, silver, and silyllithium readily give cyclized compounds. Although the cyclization is easier in the Cy5/Cy6 case and gives the (Cy5) compound, a concerted rather than an homolytic pathway is probably involved. Nevertheless, it must be noted that l-methyl-5-hexenylaluminum gives the 1,2-dimethylcyclopentane with a cis/trans ratio of 2.9 1, similar to the free radical cyclization ratio. 

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