Alkene oxides, elimination

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. 

Romakhin et al. [49] showed that anodically generated phosphoniumyl radicals can add onto alkenes to yield phosphonylated alkenes through an anodic oxi-dation/addition/anodic oxidation/elimination/nucleophilic attack sequence (Scheme 17). 

The unique feature of the Horner-Wittig reaction is that the addition intermediate can be isolated and purified, which provides a means for control of the reaction s stereochemistry. It is possible to separate the two diastereomeric adducts in order to prepare the pure alkenes. The elimination process is syn, so the stereochemistry of the alkene that is formed depends on the stereochemistry of the adduct. Usually the anti adduct is the major product, so it is the Z-alkene that is favored. The syn adduct is most easily obtained by reduction of (3-ketophosphine oxides.269... 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

This reaction depends upon the facile solvolysis of (J-haloselenides and the facile oxidative elimination of a selenoxide, which was discussed in Section 6.6.3. An alternative method, which is experimentally simpler, involves reaction of alkenes with a mixture of diphenyl diselenide and phenylseleninic acid.189 The two selenium reagents generate an electrophilic selenium species, phenylselenenic acid, PhSeOH. 

Solutions of nitric acid in chlorinated solvents can add to some alkenes to give nitrate esters. Some tertiary nitrate esters can be prepared in this way isobutylene (49) reacts with fuming nitric acid of 98.6 % concentration in methylene chloride to give ferf-butyl nitrate (50). However, the products obtained depend on both the substrate and the reaction conditions /3-nitro-nitrate esters, vic-dinitrate esters, /3-nitroalcohols and nitroalkenes have been reported as products with other alkenes. Oxidation products like carboxylic acids are also common, especially at elevated temperatures and in the presence of oxygen. The reaction of alkenes with fuming nitric acid is an important route to unsaturated nitrosteroids, which assumedly arise from the dehydration of /3-nitroalcohols or the elimination of nitric acid from /3-nitro-nitrate... 

Another example of oligomer preparation by C-C bond formation is outlined in Figure 16.30. In this synthesis, nitroalkyl phenyl selenides are converted into nitrile oxides in the presence of support-bound terminal alkenes, forming isoxazolines. Oxidative elimination of the selenide yields a new alkene, which can then be subjected to further 1,3-dipolar cycloaddition with a new nitrile oxide. Although this synthesis is short and easy to perform, the cycloadditions proceed with low diastereoselectivity... 

Tellur oxide elimination. sec-Alkyl phenyl tellurides on oxidation with CIQH4CO3H in ether afford alkenes as the major product. In some cases a stable Te(IV) compound is formed, which undergoes elimination on pyrolysis.5... 

Alkenes from organomercurials.1 Photoinitiated reaction of diphenyl diselenide with the organomercurial 1 provides a mixture of the corresponding a- and P-phenyl selenides, which undergo oxidative elimination to 2. The reaction provides a key step in a total synthesis of K-76 (3), which counteracts the inflammatory... 

The oxidative elimination of SePh gave the alkene 126 in moderate yield which was found to undergo gradual decomposition to undesired products (Scheme 13) (see Section 2.13.7.1). 

Some a-eliminations have already been discussed, like the formation of dichlorocarbene from chloroform and base. Others will be presented in certain contexts later. 1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as diazoalkanes or a-diazoketones and nitrile oxides (Chapter 15). Chapter 4 is limited to a discussion of the most important eliminations, which are the alkene-forming, /3-eliminations. Note that /3-Eliminations in which at least one of the leaving groups is removed from a heteroatom are considered to he oxidations. Eliminations of this type are therefore not treated here hut in the redox chapter (mainly in Section 17.3.1). 

A variation of the Hofmann elimination, where a tertiary amine oxide eliminates to an alkene with a hydroxylamine serving as the leaving group, (p. 907)... 

Nitrophenylselenation of the primary alcohol in 291, followed by in situ oxidative elimination, furnished alkene 292 in 95% yield. Stereoselective osmylation of the exocyclic alkene in 292, using 0s04 -NM0 gave diol 293, which was additionally characterized as its pentaacetate 294, formed in 65% yield. Acid hydrolysis of 294 furnished the target aminocyclopentitol (+)-5 in 87% yield. 

This was not observed in the diolate cycloreversion chemistry (Fig. 4).82 The measured (for moderately strained alkenes) and calculated (for more highly strained alkenes) enthalpy of activation for extrusion was approximately constant for a half-dozen examples at 30 1 kcal/mol. (All such examples were disubstituted alkenes to eliminate electronic differences due to substitution.) Oxidations showed the expected decrease in AH with increasing strain the near-unity slope indicates that the reacting carbon is fully sp3 hybridized in the transition state. 

Selenosulfonates (see also Section 3.6.2.3) add to alkenes when catalyzed by Lewis acids. These adducts are transformed via oxidative elimination to vinyl sulfones in good overall yields. Similar to other methods only trans addition is observed. Mixtures of regioisomers are often produced with unsyminetri-cal unactivated alkenes. All other alkenes (i.e. styrene and acrylonitrile) provide Maikovnikov additions. Radical processes yield anti-Markovnikov adducts. Thus, either regioisomeric vinyl sulfone may be prepared from almost any activated alkene by simply varying the mode of addition. ... 

The one-electron oxidation of cyclohexenes by S04 in aqueous solution has been studied from kinetic and stereochemical standpoints [49]. It was found that the alkene oxidation proceeds by an addition-elimination mechanism with 804" adding to the C=C double bond in the first step followed by C -OSOs" heterolysis to give a solvent-separated alkene radical cation-sulfate ion pair (SnI mechanism) (Scheme 9). 

The intermediate p-hydroxyphosphine oxide is isolated only if lithium is used to deprotonate the phosphine oxide. Sodium or potassium anions eliminate in situ to form the alkene direedy. Eliminations of... 

In a more direct comparison of the phosphine oxide elimination with the sulfone, Schreiber employed a identical system to Danishefsky, but used the phosphine oxide (422). Reaction with isobutyralde-hyde and subsequent elimination resulted in a 1 1 mixture of the ( )- and (Z)-alkenes (421 equation 98). It appears from the more complex example of the Merck synthesis and from this example, that the Julia coupling proceeds with higher ( )-selectivity, in similar yield. 

More recently, these concepts have been extended to include a range of elegant amidoselenation processes. Alkenes react with benzeneselenenyl chloride and nitriles to yield 3-amido phenyl selenides in excellent yield. - Oxidative elimination of the latter provides the corresponding allylic amides (Scheme 60). Since the reaction proceeds via an episelenonium intermediate, Ae amido selenides are formed with tram stereospecificity. However, when applied to nonsymmetrical alkenes (equation 50) it only proceeds with regioselectivity. 

As with amine oxides and sulfoxides, acyclic 1,2-disubstituted alkenes are usually obtained with the ( )-stereochemistry, although the formation of a,(3-unsaturated nitriles is reported to give a mixture of ( )- and (Z)-isomers. For cyclic alkenes, the stereochemistry of double bond formation depends upon ring size. However, it can be affected by conformational factors, e.g. cyclododecyl phenyl selenide gives a mixture of cis- and fra/is-cyclododecenes on oxidative elimination (equation 38) but only the (El-isomer (101) was obtained from the acetoxycyclododecyl selenide (100 equation 39). ... 

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