Olefin-forming fragmentations

Similar fragmentations occur when y-aminoalcohols are treated with nitrous acid . If the nucleofuge is a neutral group, then fragmentation of hydroxyl compounds requires basic catalysis, e.g.  

In a number of fragmentations, the electrofugal group is generated only after an initial addition of a nucleophilic species. Typical of this class of fragmentation are the reactions of /3-halogenocarbonyl compounds, e.g.  

In weakly alkaline solution, dehalogenative decarboxylation of /3-halo-carboxylic acids occurs readily . In the case of //-arts-cinnamic acid dibromides, the stereochemical fate of the /3-bromostyrene is determined by the solvent and substituents in the benzene ring (274) .  

The c -alkene is formed alone in a stereospecific concerted reaction, analogous to an ctnti-E2 elimination, which is encouraged by electron-withdrawing substituents (Y) and a poorer ionising medium. However, electron-releasing substituents and a more ionising solvent promote benzylic carbonium ion character and a mixture of alkenes, in which the thermodynamically more stable trans product predominantes, is formed, e.g. 

As might be predicted for a reaction in which an increase in charge occurs in attaining the transition state, a positive salt effect on the rate coefficient is observed. In the zwitterion, rotation about the C -C bond is presumably more rapid than recombination of ions, as added bromide ion exerts only the same influence on the rate coefficient as perchlorate and acetate ions. 

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In a number of olefin-forming fragmentations, the electrofuge is the lone pair of an oxygen or nitrogen atom. One of the most well-known examples is the solvolysis of 4-bromoquinuclidine, wz. ... 

The mechanism of the polymerization contains ionic intermediate steps. The free H+ goes to a carbenium ion and, as shown in route B, rearranges to form tetrapropylene. It is highly likely that this actual tetrapropylene exists only in very small concentrations. The product variety is explained by the rearrangement of the carbenium ion to dodecene isomers according to route C. In addition, short-chain olefins formed by fragmentation (route D). Polymerization proceeds at almost 100% to mono olefins. Aromatics, paraffins, and diolefins exist only in trace amounts. The propylene tetramer is best characterized by its distillation range. 

If the side chain is in an iso form, a more complex aromatic olefin forms. Isopropyl benzene leads to a methyl styrene and styrene [70], The long-chain alkylate aromatics decay to styrene, phenyl, benzyl, benzene, and alkyl fragments. The oxidation processes of the xylenes follow somewhat similar mechanisms [71, 72],... 

In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review to the olefin-forming eliminations of the type... 

Elimination reactions which give rise to multiple bonds between carbon and a heteroatom occur with particular facility and show many of the characteristics of olefin-forming processes. Most often one of the eliminating fragments is a hydrogen atom, which is easily removed from a heteroatom and the choice of mechanism, between an anion or a concerted process, will depend on the lability of the beta carbon-X bond, e.g. 

In one case, a Wacker oxidation of an olefin to form a ketone was used to prepare fragments for the synthesis of Calyculins. The ketone was then converted via the vinyl tri-flate to the vinyl bromide. In a second example, Wacker oxidation was used in the synthesis of a taxol derivative. In this case, the Wacker oxidation of an olefin was conducted at the later stage of a synthesis. Oxidation of a pendant olefin formed a ketone, which cyclized by condensation with a second keto fimctionality. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Similar fragmentations to produce S-cyclodecen-l-ones and 1,6-cyclodecadienes have employed l-tosyloxy-4a-decalols and 5-mesyloxy-l-decalyl boranes as educts. The ringfusing carbon-carbon bond was smoothly cleaved and new n-bonds were thereby formed in the macrocycle (P.S. Wharton, 1965 J.A. Marshall, 1966). The mechanism of these reactions is probably E2, and the positions of the leaving groups determine the stereochemistry of the olefinic product. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Low molecular weight olefins ranging from to Q, are produced by the polymerization of propylene or butylenes over a supported phosphorie acid catalyst. The product of this polymerization is a series of highly branched olefins ranging from dimers to pentamers. Some fragmentation of the polymers formed takes place in the reactor, so appreciable quantities of olefins are obtained which are not integral multiples of the monomer units. 

A possible alternative mechanism for the formation of fragment alkyl ions comes immediately to mind—namely, beta fission of the Ci8 ion formed in Reactions 6 and 7 to form a smaller alkyl ion and an olefin. Thus we write as a typical example ... 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

The mechanism of this reaction has not been studied in detail. However, it can be represented as a sequence of reactions. The first reaction is, in fact, [3+ 2]-cycloaddition of olefin to furoxan (161). Under severe conditions, the resulting intermediate A undergoes fragmentation to give five-membered cyclic nitronate B. The latter is involved in the usual addition reaction with an excess of olefin to form isolable bicyclic product (162) (301, 378, 379). 

C NMR of the carbonyl groups also is consistent with this latter step, but may equally be interpreted as involving a rotation of the olefin about the metal triangle, a process not envisaged to occur with the 1 1 adducts (96). A recent study of the H-NMR spectra, particularly of the methylene protons of the 1 2 adduct formed by indene and Os3(CO)12, also indicates a rotation of the olefin fragment about the metal triangle (97). 

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