Alkenes oxidative functionalization

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionaUties at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The abiUty to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubiUty of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

Although oxidation in general and of alkenes in particular is very well documented, it is rather surprising that there has been no systematic description of the oxidation of dienes or polyenes in the literature. In fact, more often than not an early literature example of the oxidation of a diene will be part of or an extension of a report dealing with the oxidation of monoalkenes. Only in recent years has oxidative functionalization of di- and polyenes become an important procedure in itself in fields such as natural product synthesis. 

In addition to the unfunctionalized alkene epoxides discussed in the previous subsection, various other types of epoxides exist that are also derived from unconjugated alkenes but that share two additional features, i. e., being characterized by the presence of one or more functional group(s) and having biological significance. Thus, the present subsection examines epoxy alcohols, epoxy fatty acids, allylbenzenes 2, 3 -oxides, as well as alkene oxide metabolites of a few selected drugs. 

The availability of 18 in one synthetic step via direct oxidative functionalization of HCTD [19-21] allows exploration of this valuable compound s chemistry, for the first time. Thus, the reaction sequence shown in Scheme 9 has been utilized to prepare a novel polycyclic alkene, 30, whose s unmetry properties require that its central, tetrasubstituted C=C double bond be completely planar (in the isolated molecule). The constraints imposed by the polycarbocyclic cage framework cause the CCC bond angles about the central C=C double bond in 30 to deviate significantly from the preferred value of 120°, thereby introducing additional steric strain in this molecule beyond that which is associated with its framework alone. 

Flo. 4. Activation enthalpies for alkene oxidation and extrusion as a function of strain in the double bond. Legend El, directly measured activation enthalpies , T calculated from the relationship AH (extrusion) — double bond strain — AH% (oxidation) = 5.2 kcal/mol. 

Ever since the initial discovery of the Wacker process [1], i.e. the Pd/Cu-catalyzed oxidation of ethylene to acetaldehyde (1) in water, methods for the palladium (II) - mediated oxidative functionalization of alkenes have found widespread application in the synthesis of complex molecules [2J. 

Various terminal alkenes with functional groups are oxidized to the corresponding methyl ketones. Since the oxidation proceeds under mild conditions, various functional groups, such as an aldehyde, carboxylic acid, ester, alcohol, " MOM ether, acetal, bromide, selenide, sulfonyl ester and amines, which are located at suitable positions, remain intact. Though it is known that alcohols are oxidized to aldehydes or ketones with PdCh, the oxidation of terminal alkenes is faster than that of alcohols under these conditions. 

An excess (5 1 mole ratio) of ethyl diazoacetate is used in these reactions to suppress cyclobutadimerization or Diels-Alder cyclodimerization. In difunctional molecules which have non-equivalent ionizable functionalities, cyclopropanation is highly selective for the more easily oxidized functionality. The latter selectivity is perhaps the most attractive aspect of the reaction. In contrast to transition metal (e.g. rhodium) catalyzed cyclopropanations, cation radical additions to electron deficient alkene moieties do not occur at all. The reaction is relatively sensitive to... 

Although it is difficult to predict which drugs are likely to be prone to photodegradation, there are certain chemical functions that are expected to introduce photoreactivity, including carbonyl, nitroaromatic and N-oxide functions, aryl halides, alkenes, polyenes and sulfides. The mechanisms of photodegradation are of such complexity as to have been fully elucidated in only a few cases. We will consider two examples - chlorpromazine and ketoprofen. 

In an attempt to determine the atmospheric oxidation processes that would result in an arene oxide functional group in PAHs, Murray and Kong (1994) studied the reaction of particle-bound PAHs with oxidants derived from the reactions of ozone with alkenes. Phenanthrene and pyrene were converted to arene oxides under these simulated atmospheric conditions. Control experiments indicated that the oxidant responsible for the transformation was not ozone, but a product of the reaction of ozone with tetramethylethylene (TME), probably the carbonyl oxide or the dioxirane derived from TME. 

Aryliodine(ni) derivatives have found a wide range of application in organic synthesis, particularly for oxidative functionalization of alkenes, amines, carbonyl derivatives, phenols. 2,i3,i5,25 Among the various mechanisms which have been invoked to explain these transformations, the ligand coupling mechanism has been implied in a number of these reactions carbon-carbon bond formation, carbonyl a-fimctionalisation, hydrazone a-functionalisation. 

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