Cycloalkanes hydrogenative ring-opening

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

Reduction of cycloalkane-condensed 2-phenyl-5,6-dihydro-4//-l,3-benzoxazines 144 with lithium aluminium hydride (LAH) afforded A -benzyl-substituted 2-(aminomethyl)cycloalkanols 145 in a reductive ring opening via the ring-chain tautomeric tetrahydro-l,3-oxazine intermediates. Catalytic reduction of 1,3-oxazines 144 under mild conditions in the presence of palladium-on-carbon catalyst similarly resulted in formation of the A -benzyl-1,3-amino alcohols 145. When the catalytic reduction was performed at elevated temperature at hydrogen pressure of 7.1 MPa, the N-unsubstituted 2-(aminomethyl)cycloalkanols 146 were formed in good yields (Scheme 22) <1998SC2303>. 

A third example of a polymeric ligand with pH-sensitive solubility is 97. This ligand was prepared by ring-opening metathesis polymerization of the 1,4,7-triazacyclononane-containing monomer 96 by the chemistry shown in Eq. 40 [132]. This polymer was capable of forming Mn(IV) complexes that oxidize alkenes and cycloalkanes with hydrogen peroxide. This basic polymer s solubility is affected by pH, as is the case with the other polymers 93 and 95 described above. 

In solution, open-chain 1,3-dicarbonyl compounds enolize practically exclusively to the czls-enolic form (4b), which is stabilized by intramolecular hydrogen bonding. In contrast, cyclic 1,3-dicarbonyl compounds e.g. cycloalkane-1,3-diones [46]), can give either trans-Qnols (for small rings) or czk-enols (for large rings). As the diketo form is usually more dipolar than the chelated cu-enolic form, the keto/enol ratio often depends on solvent polarity. This will be discussed in more detail for the cases of ethyl acetoacetate and acetylacetone [47-50, 134, 135]. 

The relative rates of abstraction of a hydrogen atom from medium-ring cycloalkanes by various free radicals have been studied. The rates increased in the series Cg < Cj a C7 < Cg, Cl being fairly unselective whereas Br was fairly selective. Compared with open-chain hydrocarbons, hydrogen abstraction from the C,—C j rings has a lower activation energy, but this is offset by a decrease in the pre-exponential factor... 

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