Ring opening of cyclopropanes

We began our calculations of the PES for the ring opening of cyclopropane in an attempt to resolve an apparent conflict between the results of experiments by Berson and co-workers and by Baldwin and co-workers. However, graduate students in our research group were among the experimentalists whose research ultimately benefited the most from the predictions made by our calculations. 

Figure 22.8. Conrotatory ring opening of cyclopropane (7) to what Hoffmann called the (0,0) conformation of propane-1,3-diyl (8). The in-phase combination of Ip-n AOs in the LUMO is destabilized by an antibonding interaction with the re combination of C—H bonding orbitals at C2. A lower energy MO, which is not shown, is stabihzed by h3fperconjugative electron donation that is, the bonding version of this interaction. The out-of-phase combination of 2p-n AOs in the HOMO has a node at C2 hence, it does not mix with the C—H orbitals at this carbon.

With the conrotatory path a reasonable model for optical isomerization, and the EF(ts) mediated path a serviceable rationale for geometrical isomerization, the possible contribution of a disrotatory ring opening of cyclopropane did not receive full consideration. Yet its potential to contribute, and to complicate the situation, did not pass entirely un-noticed268. 

Although C—H insertion reactions rarely occur in intermolecular reactions with diazoacetates, these are common side reactions with diazomalonates3132 (equation 10) and diazo ketones (with a-allyl vinyl ethers).33 Several mechanistic pathways are available to generate the products of an apparent direct C—H insertion reaction and these include dipolar intermediates, ir-allyl complexes and ring opening of cyclopropanes.1 Oxidative problems due to the presence of oxygen are common with copper catalysts, but these are rarely encountered with rhodium catalysts except in systems where the carbenoid is ineffectively captured.34... 

Strained cyclic systems such as cyclopropane derivatives proved sensitive to silver ions. Indeed, silver usually induced ring opening of cyclopropanes, leading to argento cationic intermediates, which evolved further depending on substituents. Most, if not all, of these ring rearrangements resulted from initial formation of a a complex of the cyclopropane with silver ion. 

Whereas methyl 2-siloxycyclopropanecarboxylates are thermally stable up to temperatures as high as 170 °C, they readily rearrange at low temperatures under the influence of appropriate Lewis acids. Catalytic amounts (0.05-0.4 equiv.) of iodo-trimethylsilane within minutes to days promote a quantitative ring opening of cyclopropanes 755 to the corresponding silyl enol ethers 156 (Eq. 68, Table 4)88). 

The reaction mechanism involved the cyclisation of ketyl radical anion 3 on to the methylenecyclopropane moiety in a 5-exo-trig manner. Ring opening of cyclopropane intermediate 4 gave rise to the cyclohexyl radical 5, which then cyclised in a 5-exo-dig fashion to form the second ring (Scheme 6.2).5,6... 

The potential ring-opening of cyclopropane radical cations, breaking the weakened bond of type A radical cations (21 +), has been a subject of both interest and controversy. The ESR spectra of cyclopropane radical cation and its methyl-substituted derivatives decayed at temperatures near 100 K. They were replaced by secondary spectra, in which the protons at one cyclopropane center do not interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species (22 +) in which one terminal carbon has rotated into an orthogonal orientation [105, 106, 140]. 

In a similar way an isopropenyl substituent can be transformed to a t-butyl group by cyclopropanation and hydrogenolysis . The regiochemistry in the ring-opening of cyclopropanes bearing other substituents (e.g. acceptor or donor groups) has also been studied ... 

TABLE 24. Ketones from ring-opening of cyclopropane monothio or dithioketals. 

The relief of steric strain plays a central role in the anodic oxidative ring opening of cyclopropanes. For instance, 1,1,2,2-tetraphenylcyclopropane is converted via the... 

Pro to nation of Cyclopropanes. Unassisted ionization does not preclude the subsequent formation of alkyl-bridged ions. Evidence for o-delocalization must then be derived from the product-forming step of the rearrangement. Useful information on this point may be obtained from the electrophilic ring opening of cyclopropanes. Several comprehensive reviews on protonated cyclopropanes are available66,4S8 460 Some observations pertinent to the present discussion will be summarized ... 

A related subject dealing with coordination chemistry of cyclopropanes is discussed in Section l.C. There, conversion with or without ring-opening of the three-membered ring is described. Included is the intermediate formation and ring opening of cyclopropanes with transition-metal complexes. 

The earlier reported palladium-catalyzed, regioselective nucleophilic substitution reaction of 1-halo-l-vinylcyclopropanes 10 to give alkylidenecyclopropanes can also be extended to stabilized carbanion nucleophiles. Use of a BINAP-modified catalyst leads to asymmetric induction with up to 47% ee. Again no ring opening of cyclopropanes was observed. 

Palladium(II) salts catalyze the rearrangement of 1-allyloxy-l-siloxycyclopropanes to provide a mixture of hexenoic acids. The rearrangement proceeds via a double-bond isomerization followed by a ring opening of cyclopropane rather than homo-Claisen rearrangement. 

Many other examples of transition metal mediated ring openings of cyclopropanes have been reported using complexes of rhodium and palladium or in the gas phase with transition-metal cations. ... 

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