Cyclopropanes energetics

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

These processes constitute a chain reaction. There are many possible chain-ending steps, which could account for some of the minor products observed in the pjorolysis. Certainly the pyrolyses of 3-ethylpent-l-ene and 3-ethylpent-2-ene do yield ethylbutadiene, but the rate of production of the diene is considerably slower than in the pyrolysis of the cyclopropane. Thus reaction (4) represents a sensitization of these latter processes. Further, unlike Chesick, we envisage the formation of the methyl radical from the cyclopropane to involve the rupture of the cyclopropane ring, and believe that this concerted process should be energetically favoured over the two-step mechanism. There are several simple experimental tests that could be used to decide between these mechanisms. 

For maximum stabilization this intermediate must be planar, and this means that some twisting must take place in the cyclopropane before the ring ruptures. This process is greatly helped energetically by the resulting overlap of the w electrons of the double bond with the vacant orbitals which result from the incipient carbon-carbon bond rupture. We will consider this process further in a later section. 

While primary aliphatic amines are converted to nitro compounds on reaction with ozone these reactions are accompanied by numerous by-products.Such side-reactions are largely suppressed by first dissolving the amine onto silica gel followed by passing a stream of 3 % ozone in oxygen through the solid at -78 °C under anhydrous conditions, where yields of between 60 and 70 % are reported. This route has been used to synthesize the energetic cyclopropane (65) from the diamine (64) (Table 1.7). ... 

The energetics consequences of the conjugation of double bonds has long been known and will not be reviewed here. What about cyclopropanation of conjugated olefins The... 

The author wishes to thank James S. Chickos, Eugene S. Domalski, Arthur Greenberg, Sharon G. Lias and Suzanne W. Slayden for numerous discussions of science and of style on the energetics of general organic compounds and, in particular, of cyclopropanes and other compounds with three-membered rings. The author also thanks those individuals, mentioned in footnotes, who shared with him unpublished data and/or insights. 

J. F. LiebmanandA. Greenberg, Chem. Rev., 89,1225(1989). This source alone cites some seventeen other reviews that relate directly to the energetics of cyclopropanes. In addition, pp. 1228-1231 provide four tables that represent our major archives for the enthalpies of formation of compounds of cyclopropanes and other compounds with three-membered rings. 

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