Ring breaking

Cycloalkane properties depend on ring size. Strained molecules, i.e., moleeules with distorted geometries, tend to be more reaetive in ring-breaking ehemieal reaetions. For example, eombustion of a strained eyeloalkane should release more energy per CH2 group than eombustion of an unstrained moleeule. 

Phenyl radical reactions with 02, O, or H02 seem to be the most likely candidates for the first steps in the aromatic ring-breaking sequence [54, 61]. A surprising metathesis reaction that is driven by the resonance stability of the phenoxy product has been suggested from flow-reactor studies [54] as a key step in the oxidation of the phenyl radical ... 

General acid catalysis by H2PO4 and imidazolium cation has recently been detected in hydrolysis of 2-methoxy-3,3-dimethyl-oxetane [69] (Atkinson and Bruice 1974). Such catalysis must be due to relief of ground state strain as the ring breaks to give an open-... 

Later Julia and coworkers developed another important mode of ring breaking addition of Grignard compounds or reduction converts the ester 22 to the corresponding alcohols which are startingpoints for a cyclopropylcarbinyl/homoallyl cation rearrangement. After acid treatment p/y-unsaturated carbonyl compounds (e.g. 23) can be isolated, which sometimes isomerize to the oc,(3-unsaturated systems 10). 

Chauvin proposed that the metal-alkylidene catalyst forms an intermediate four-membered ring with an alkene, as shown in Mechanism 8-11. Then the ring breaks apart, either to give the starting alkene and catalyst or to give a new alkene that has traded one alkylidene group with the catalyst. 

The first reaction is th3 only known exception to the rule previously stated,that in the case of cyclopropane hydrocarbons the ring breaks between the carbon atoms having the most and the fewest substituents. 

The higher stability of addition products on olefinic carbon atoms C8 and C7 was explained based on loss of aromaticity because addition to the ring breaks the cyclic delocalization leading to the loss of accounting to around 30 kcal/mol of destabilization of the system. Further excellent linear correlation between the relative stabilities of the OH adducts (after accounting for the aromatic stabilization in olefinic adducts) and the maximum spin density values was also obtained. 

Figure 10 Model systems used for various ring-breaking sugar radicals radicals observed experimentally (I and II), model ring-breaking radical (M), C5 centered radical proposed experimentally (IV) and the model ring-breaking radical with a phosphate group (V).

So how are these products formed At such a high temperature, cr-bonds break and the weas -bonds in the molecule are the C-C and C-O bonds in the four-membered ring next to the ben ring. Breaking these bonds releases strain and allows one of the radical products to be secondar delocalized. 

Reaction (b) is a normal Favorskii reaction going to the right in the diagram. The only point of interest is the way in which the three-membered ring breaks up. The more stable carbanion is the doubly benzylic one so the right product is formed. 

The classic carbonium theory of this reaction (/3-rule) postulates the formation of a carbonium ion at one of the carbon atoms close to the bridgehead carbon. Among the possible Wagner-Meerwein transpositions, the ones leading to four-membered rings are not observed, and may, instead, produce ring breaking (Fig. 16). 

Fio. 15. Ring-breaking and ring-forming reactions of the C9H14 series. 

A combination of ring-breaking and ring-forming reactions of this kind could also explain the passage from the bicyclo[2.2.1]heptane series to the bicyclo[3.2.1]- and bicyclo[3.3.0]octane series, by the sequence ... 

This argument cannot be applied to our examples when large amounts of polymers are formed. Besides, Deno and co-workers 41) have detected in concentrated sulfuric acid solutions of camphene and related compounds (borneol, fenchol) high concentrations of a mono-cyclic cyclohexenic carbonium ion, produced also by proton addition to l-methyl-3-isopropylidene-l-cyclohexene, a clear case of ring breaking in acid media. More recently, Olah and co-workers reported that in highly acidic systems such as HF or HSO3F and SbFs, cyclohexane produces, besides the expected cyclohexyl- and methylcyclopentyl carbonium ions, hexyl and isohexyl carbonium ions 42). 

Right) When molten sulfur is heated above 150°C, an amorphous form of sulfur called plastic sulfur is formed as the Sg rings break and the fragments link to form long chains. 

---