Intramolecular hydrogen bonding hydroperoxides

The second largest number of hydrogen bonds in crystal structures of alkyl hydroperoxides refers to interactions of the type OO—H OR R, where R is an alkyl group and R denotes H, alkyl or R O. The OO OR R distances vary between 2.67-2.91 A and the associated O—H O angles range from 152 to 177°. In some compounds, formation of intramolecular hydrogen bonds of the type OO—H 0=X would in principle have been feasible. The number of examples documented in the literature so far is clearly in favor of the intermolecular type of H bonding. 

FIGURE 14. Intramolecular hydrogen bonds the carbonyl, sulfoxide and phosphine oxide group as acceptor for the hydroperoxide proton ... 

FIGURE 16. The formation of intramolecular hydrogen bonds nitrogen functionalities as acceptor for the hydroperoxide proton69-91... 

Since singlet oxygen is a dienophile, such oxidations are believed to involve cycloaddition of oxygen to an endoperoxide (46, Scheme 14), followed by rearrangement to the hydroperoxide. Reduction in steric strain and the formation of a strong intramolecular hydrogen bond (vmax 2800 cm 1) could provide the driving force in the second step.48... 

The enol tautomers of 1-(2, 4, 6 -trialkylphenyl)-2-methyl-1,3-diketones form a range of alkene, epoxide, ether, and hydroperoxide products on reaction with singlet oxygen. The product distribution is substantially affected by the solvent, apparently owing to the disruption of intramolecular hydrogen bonding of the enols in polar solvents. 

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

At high hydroperoxide concentrations, a bimolecular dissociation mechanism [Eq. (13)] between hydrogen-bonded molecules (for instance, between adjacent peroxide groups formed by an intramolecular abstraction reaction in PP) can also occur. 

Abstraction reaction in polymers can occur intramolecularly or intermolecularly. The former possibility is important in polymers which possess lateral groups (PP and PS), whereas it is nonexistent in linear polymers (HOPE). The possibility of intramolecular H abstraction has been advanced as one of the reasons for the high sensitivity of PP (in comparison to HOPE) toward oxidative degradation. Infrared studies have revealed that more than 90% of hydroperoxides in PP are hydrogen-bonded in sequences of two or more. This result supports an intramolecular hydrogen abstraction reaction, facilitated by a six-membered ring stereochemical arrangement [Eq. (40)]. 

Peroxyl radical reaction with the polymer. This is a generally much slower process, which is structure dependent. In saturated hydrocarbon polymers, e g. polyethylene (PE) and polypropylene (PP), it is exclusively a hydrogen atom abstraction. In this case. Per is a hydroperoxide group (POOH). The corresponding rate constant is very low = 10 -10 l.mor. s at ambient temperature (see Table 12.7). In polyenic elastomers, e g. polybutadiene (PBD) and polyisoprene (PIP), step 3 can also be an addition to double bonds. In this case. Per is a peroxide bridge (POOP). The corresponding rate constant is also very low typically = 10 -10 l.mor. s at ambient temperature for an intramolecular addition (see Table 12.7). 

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