Cyclopropane, 1,1-dimethyl

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

Unlike the constitutional isomers butane and isobutane (Section 3.2), which have their atoms connected in a different order, the two 1,2-dimethyl-cyclopropanes have the same order of connections but differ in the spatial orientation of the atoms. Such compounds, which have their atoms connected in the same order but differ in three-dimensional orientation, are called stereochemical isomers, or stereoisomers. 

Dibrom-pentan kann bei -2,2 V in DMSO/Tetraathylammoniumbromid (Vorelek-trolyse ) in hoher Ausbeute zu 1,2-Dimethyl-cyclopropan reduziert werden (die Stereo-isomeren entstehen zu gleichen Teilen)1. In Methanol gelingt die quantitative Herstel-lung von 2-Hydroxy-2-methoxy-1,1 -dimethyl- bzw. 3-Hydroxy-3-methoxy-tetramethyl-cyclopropan aus l,3-Dibrom-2-oxo-3-methyl-butan bzw. 2,4-Dibrom-3-oxo-2,4-dime-thyl-pentan2. 

Fig. 1-11. Ball-and-stick models of cis and trans isomers of 1,2-dimethyl cyclopropane.

Make molecular models of CIS- and trans-1,2-dimethyl-cyclopropane, and compare their strain energies. 

Procymidone, N-(3,5-dichlorophenyl)-1,2-dimethyl-cyclopropane-1,2-dicarbox-imide (27), is a fungicide with moderate systemic action (Fujinami et a/., 1969). The active substance is a white crystalline material stable to the action of light, heat and humidity. It is slightly soluble in water. It is moderately toxic to mammals, the acute oral LDjo for rats being 6800 mg/kg. It is not toxic to fish or bees. 

In their textbook, M. Diaz Pina and A. Roig Muntaner (Quimica Fisica, Vol. n, pp. 1073-1074, Editorial Alhambra, Madrid, 1976) reported the data below for the reversihle cis-trans isomerization of 1,2-dimethyl cyclopropane. Test these data to ascertain if they are consistent with a rate expression that is first-order in both the forward and reverse directions. 

Mishra and Crawford reported the synthesis of 3R,5R)- +)-trans-7>,5-dimethyl-1-pyrazoline 35 by different approach starting from alcohol 327 The pyrazoline 35 undergoes thermolysis, producing 25.6% of /ran -1,2-dimethyl-cyclopropane 36, processing 23% optical purity, and having the S S configuration. 

Neither rw-l,2-dimethylcyclopropane nor c -l,2-dimethylcyclohexane can be resolved, because the cyclopropane is a meso compound (Chapter 4, p. 168), and the cyclohexane flips into its mirror image. However, both 1,2-dimethyl-cyclopropane and /rawi-l,2-dimethylcyclohexane can be resolved they are not superimposable on their mirror images. The point is that in a practical sense the planar cyclopropanes and nonplanar cyclohexanes behave in the same way. This finding has important consequences. In deciding questions of stereochemistry, we can treat the decidedly nonplanar 1,2-dimethylcyclohexanes as if they were planar. Indeed, all cyclohexanes can be treated as planar for the purposes of stereochemical analysis, because the planar forms represent the average positions of ring atoms in the rapid chair-chair interconversions. 

Electrophilic addition of bromine at — 60°C to cis- and trans-1,2-dimethyl-cyclopropanes has been shown to open the C-1—C-3 bond preferentially in a non-stereospecific manner. This rules out predominant reaction through a 1,3-bromonium ion. The principal difference between the cis- and transisomers is that the former gives 33 % of a 1,2-dibromide by 1,2-hydride shift within an intermediate carbonium ion. It has been reported that oxymercura-tion of optically active trans-1,2-dimethylcyclopropane occurs with at least 85 % inversion of configuration at the site of nucleophilic attack in a reaction which also cleaves the less substituted cyclopropane bond. Under the influ-... 

---