Alkenes by P-elimination

Except for the biochemical example just cited the stractures of all of the alcohols m Section 5 9 (including those m Problem 5 13) were such that each one could give only a single alkene by p elimination What about ehmmahon m alcohols such as 2 methyl 2 butanol m which dehydration can occur in two different directions to give alkenes that are conshtutional iso mers Here a double bond can be generated between C 1 and C 2 or between C 2 and C 3 Both processes occur but not nearly to the same extent Under the usual reachon con dihons 2 methyl 2 butene is the major product and 2 methyl 1 butene the minor one... 

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

A detailed recent study has been made to try and elucidate whether metallacyclobutanes rearrange to alkenes by p elimination or by a elimination to give an intermediate ylide complex, which can rearrange to an alkene complex. Using deuterium-labeled platinum(IV) platinacyclobutanes it is concluded that the pathway involves a [1,3] H shift (a elimination) rather than a [1,2] H shift ()3 elimination). Platinacyclopentanes have also been formed by an alkene coupling between Pt(cod)2 and butadiene. Addition of PMcs gives complex (66 equation 2(K)). ... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

SAMPLE SOLUTION (a) Two alkenes are capable of being formed by p elimination methylenecyclopentane and 1-methylcydopentene. 

Now, the Wacker oxidation (p. 1337), by whatever detailed mechanism you prefer, must invoh the addition of water to a Pd(II) tt-complex of the alkene and p-elimination of palladium to gi, Pd(0), which is recycled by oxidation with oxygen mediated by copper. 

The full paper on the conversion of 1,2-diols into alkenes by reduction-elimination of appropriate 1,3,2-dioxaphospholans has appeared (see Organophosphorus Chemistry , Vol. 8, p. 123). 2 method was examined with particular regard to the conversion of cyclodecane- and cyclododecane-1,2-diols into the cycloalkenes, as mixtures of cis- and tmns-isomers, and consists of the treatment of the cyclic ethyl esters or AW-dimethylamides with lithium in liquid ammonia or with Ti-THF. The kinetics of the methanolysis of thiophosphoryl chlorides have been studied. The rather unusual isomerization of 2-hydroxyethyl phosphorothioates (70) yields the 2-mercaptoethyl isomers after several days at room temperature instead of the expected thiono-esters. This observation should be compared with those reported in ref. 83. 

This well known reaction can deceive us into thinking that o-complexes of Pd are stable. They are not. All the steps between 86 and 89 are reversible and in the absence of hydrogen, the reaction runs backwards. Palladium readily does the oxidative insertion into an alkyl halide to give the o-complex 92, a carbanion complex of Pd(II), but this immediately loses hydrogen by p-elimination and the alkene 86 is formed with the loss of a PdHBr complex. Notice that this Pd(II) complex immediately reverts 93 to Pd(0). 

The second ring is closed by an amido-palladation reaction. The nitrile in 151 is reduced to a primary amine and protected with a Boc group. Reaction with Pd(II) allows nucleophilic attack by the amide on the nearer end of the alkene and P-elimination again pushes the alkene round the ring 153, this time because there is no syn H atom in the intermediate 154 at the site of addition. Palladium is released as Pd(0) but this reaction needs Pd(II) so the quinone and MnOz are there to carry out the required oxidation. 

Synthesis of Alkenes by Reductive Elimination. The treatment of 2-halo-3-hydroxy esters and amides with samarium iodide leads to the corresponding di- or trisubstituted E)-a,p-unsaturated derivatives in high yields and diastereoselectivities (eqs 39 and 40). The precursors are readily accessible by condensation of the lithium enolate of a-haloesters or amides. If the substrate contains y,i5-unsaturation, the /3,y-unsaturated ester is generated in the process (eq 41). 

A photochemical version of the system has also been reported. In this case, irradiation of PtClg " and hexane in AcOH gives a Pt(II) complex of hexene. The alkene is almost certainly formed by P elimination of the intermediate alkyl, a reaction to be discussed in more detail in the next section. With Mc2CO as substrate a stable alkyl, [(H3N)Cl4Pt(CH2COMe)] was isolated by addition of NH3. 

The reactions of bare Fe" " ions and related species in the gas phase continue to attract much interest. The remote functionalisation of 1,6-hexanediol by Fe occurs by C-H activation at C(3) and C(4).26 Functionalisation of 3-methyl-2-pentanone at C(4) is diastereoselective, probably because of the conformation of a chair-like intermediate. Reactions of Fe with anisoles and phenols have also been studied.28 Interaction of Fe with silanes gives both silene and silylene species, though the two are not interconvertible. The reactions of Fe(alkene)+ complexes with pentane were found to differ dramatically from those of bare Fe" , and C-H and C-C activation were also observed in reactions of Fe(C2H4) with oxygen. 0,31 interaction of Fe(benzyne)+ with alkyl halides led to C-X or C-C addition followed by p-elimination and loss of HX.32 The gas phase reaction of Fe(NH2)Me" with C2H4 is best explained by insertion into the Fe-C bond followed by P-elimination and loss of propene. The reaction of FeMe with 1-octyne also leads to C-C bond forming processes. 

Insertion of the alkene into the M—H bond to give M—CHMe(nPr), followed by p elimination to give MeCH=CHMe insertion requires prior binding of the alkene and so does not happen in the 18e case. 

In this section we shall closely inspect the retrosynthetic approach to simple alkenes, which includes disconnection of the C=C bond in two synthons, which have useful building blocks for synthetic equivalents. We shall not consider the formation of a double bond by p-elimination of two substituents on the vicinal carbon atoms, as for instance dehydration. More about these approaches is presented in the Sect. 4.3.1. 

Palladium acyl species can also undergo intramolecular acyl-palladation with alkenes to form five- and six-membered ring y-keto esters through exocyclic alkene insertion (eq 12). The carbonylative coupling of >-iodoaryl alkenyl ketones is also promoted by Pd(dba)2 to give bicyclic and polycyclic quinones through endocyclization followed by p- elimination. Sequential carbonylation and intramolecular insertion of propargylic and allylic alcohols provides a route to y-butyrolactones (eq 13). ... 

Pd -catalyzed aerobic oxidations of alkenes with nucleophiles other than water have met with limited success. Most successful reactions utilize nucleophiles that also serve as solvent such as alcohols and acetic acid. In contrast, the use of a heteroatom or carbon nucleophile generally requires stoichiometric quantities of palladium or the secondary oxidant, often Cu salts. Oxidation of alkenes in alcohols with Pd salts in the presence of a base can afford an acetal or a vinyl ether. Alkoxypalladation, which is the first step in both cases, can be followed by 1,2-hydride shift and attack of alkoxide anion on the resulting oxonium cation, affording the corresponding acetal (eq 5, path a). Formation of the vinyl ether can be understood by p elimination of the palladation intermediate (path b). The acetal of acetaldehyde is the main product in the oxidation of ethylene (R = H) while j8-H ehmination is the main path with higher alkenes. 

In 1875 Alexander M Zaitsev of the University of Kazan (Russia) set forth a gen erahzation describing the regioselectivity of p eliminations Zaitsev s rule summarizes the results of numerous experiments m which alkene mixtures were produced by p elim matron In its original form Zaitsev s rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the f3 carbon hav mg the fewest hydrogens... 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes. The principal methods by which alkenes are prepared in the laboratory are two other p eliminations the dehydration of alcohols and the dehydrohalo-genation of alkyl halides. A discussion of these two methods makes up the remainder of this chapter. 

Epoxides can be converted to alkenes by treatment with triphenylphosphine or triethyl phosphite P(OEt)3. The first step of the mechanism is nucleophilic substitution (10-50), followed by a four-center elimination. Since inversion accompanies the substitution, the overall elimination is anti, that is, if two groups A and C are cis in the epoxide, they will be trans in the alkene ... 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

Nitroisoxazolines were prepared from ALalkoxy-3,3-dinitroisoxazolidines by thermally induced P-elimination. For example, isoxazolidines 42 synthesized by a three-component reaction of tetranitromethane with two equivalents of alkenes 41, were converted into isoxazolines 43 by heating in boiling chlorobenzene <06S706>. 

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