Alkyl hydroperoxides, structure

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

The largest number of hydrogen bonds in crystal structures of alkyl hydroperoxides refer to intermolecular bonds between the hydroperoxide proton and functionalities of the type 0=X, where X denotes a sulfur (e.g. 27), carbon (e.g. 30) or a phosphorous atom (e.g. 32, Figure 14, Table 7)93,108,115 geometry of [l,2-bis(diphenylphosphinoyl)ethane] bis(2,2-dihydroperoxypropane) (32) in the solid state is a rare example of a bifurcated hydrogen bond between an OOH donor and an 0=X proton acceptor. 

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. 

A survey of crystal structures of 29 compounds (Table 8), in which the alkyl hydroperoxide anions serves as ligand to metal ions, transition metal ions or group 13-17 elements, provides a mean 0—0 bond length of 1.46 0.03 A, an O—O—C angle of 109 2.1° and a M—O—O angle of 112 6.9°. More specialized aspects that deserve to be addressed separately refer to the nature of the M—O bond, the magnitude of the dihedral angle M—O—O—C and the tetrahedral distortion of the peroxide bound C atom. 

The enthalpy of formation data appear in Table 1. Where there are numerous data for structurally related compounds, plotting the enthalpy of formation versus the number of carbons can reveal at a glance the quality of the data. For the alkyl hydroperoxides especially, the data are seen to be of mixed quality as evidenced by the lack of linearity in the plot (Figure 1). The error bars for the ethyl and n-propyl hydroperoxides are so large as to render the data meaningless. Also, the enthalpy of formation of n-propyl hydroperoxide is more negative than that of 1-hexyl hydroperoxide, another indication of its gross inaccuracy. The C2 and C3 hydroperoxide data will not be considered any further. 

Three peroxides with aromatic substituents have reported enthalpy of vaporization data, all from the same source". The enthalpies of vaporization of cumyl hydroperoxide and ferf-butyl cumyl peroxide are the same, which makes us skeptical of at least one of these values. The calculated b value for cumyl hydroperoxide is 31.5, consistent with the alkyl hydroperoxides. The calculated b value for tert-butyl cumyl peroxide is 15.4 and more than twice that for the mean of the dialkyl peroxides. The structurally related tert-butyl p-isopropylcumyl peroxide has a b value of 8.8 and so is consistent with the other disubstituted peroxides. 

Alkyl halides, hydroperoxide synthesis, 327-8 Alkyl hydroperoxides anion ligands, 114-19 covalent radii, 114, 118-19 dihedral angles, 119 geometric parameters, 115-8 tetrahedral distortion, 119 artemisinin formation, 133-4 chlorotriorganosilane reactions, 779-83 crystal structure, 105-14 anomeric effect, 110-11 geometric parameters, 106-9 hydrogen bonding, 103-5, 111-14 tetrahedral distortion, 110 determination, 674... 

Alkyl hydroperoxysilanes, preparation, 783 Alkyl hydrotrioxides, structural chemistry, 132 Alkyl iodides, dioxirane oxidation, 1158 Alkyl methyl sulfonates, alkyl hydroperoxide synthesis, 673... 

When the alkyl hydroperoxide has been fully formed, only one half of the oxidizing power of the oxygen has been utilized. Alkyl hydroperoxides are therefore unstable, the stability being dependent upon the structure. Tertiary hydroperoxides are the most, and primary hydroperoxides the least, stable. The degradation reaction, which is essentially a second stage in the oxidation, may be either inter- or intramolecular the degradation may be either bi- or monomolecular. The rate of degradation is a function of the temperature and is easily subject to catalysis. 

Organic peroxides can be classified according to peroxide structure. There are seven principal classes hydroperoxides dialkyl peroxides a-oxygen substituted alkyl hydroperoxides and dialkyl peroxides primary and secondary ozonides peroxyacids diacyl peroxides (acyl and organosul-fonyl peroxides) and alkyl peroxyesters (peroxyearboxylales. peroxysul-fonates, and peroxyphosphates). 

Acids react with alkyl hydroperoxides in two different ways, depending on the hydroperoxide structure and the acid strength. 

The susceptibility of dialkyl peroxides to acids and bases depends on peroxide structure and the type and strength of the acid or base. In acidic environments, unsyinmetrical acyclic alkyl aralkyl peroxides undergo carbon-oxygen fission, forming acyclic alkyl hydroperoxides and aralkyl carbonium ions. The latter react with nucleophiles, X. ... 

The first step consists of the formation of the dioxygen adduct which can have either a superoxo structure (1) if the metal is a potential one-electron donor, or a peroxo structure (2) if the metal is a potential two-electron donor. These superoxo or peroxo complexes can be considered as the formal, but not chemical, analogs of the superoxide 02 and peroxide 022- anions. The superoxo complex (1) can further react with a second reduced metal atom to give the /x-peroxo species (3), which can cleave itself into the oxo species (4), which may be hydrolyzed to give the hydroxo species (6) or react with a second metal atom to give the p.-oxo species (5). The alkylperoxo (7) and hydroperoxo (8) species can result from the alkylation or protonation of the peroxo species (2), or from anion exchange from metal salts by alkyl hydroperoxides or hydrogen peroxide. 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

Cobalt(III)-alkyl peroxide complexes with the formula Co(BPI)(OOR)(OCOR ) (205 BPI = l,3-bis(2 -pyridylimino)isoindoline R= Bu , CMe2Ph R = Me, Ph, Bu ) have been prepared from the oxidation of Con(BPI)(OCOR ) complexes by alkyl hydroperoxides. The X-ray crystal structure of complex (205) (R=Bu R = Ph) revealed a distorted octahedral environment, with a monodendate OOBuc group and a bidendate carboxylate.635... 

A density functional study of the transition structures of Ti-catalyzed epoxidation of allylic alcohol was performed, which mimicked the dimeric mechanism proposed by Sharpless et al.5 Importance of the bulkiness of alkyl hydroperoxide to the stereoselectivity, the conformational features of tartrate esters in the epoxidation transition structure, and the loading of allylic alcohol in the dimeric transition structure model were pointed out. 

Chemical structure of hydroperoxide forming the initial complex. This alters the structure and spin state of the Fe" + complex and, consequently, affects dominant product pathways (73). H2O2 forms low spin complexes that undergo heterolytic scission, whereas alkyl hydroperoxides form high spin complexes that release alkoxyl radicals in homolytic scissions (81). 

---