Olefin isomerisation

AS As are prepared from alk-l-enes (cr-olefins) by catalytic isomerisation, followed by an addition reaction with maleic anhydride (Figure 7.20). The location of the double bond in the alkene is important and it has been shown that the internal alkenes are much more effective than or-olefins. The probable explanation for this is that the AS As derived from cr-olefins, being solids at room temperature, require higher temperatures for emulsification than the ASAs derived from isomerised olefins. The main advantage of ASA is that it is very reactive and, unlike AKD, no heat treatment is necessary and full sizing develops immediately off the paper machine. 

Two novel routes to triethylborane have been reported. Irradiation of bromoethane and aluminium powder with ultrasound gives ethyl aluminium sesquibromide which on treatment with triethoxyborane gives triethylborane in good yields and a laser initiated gas phase reaction between diborane and ethene gives yields of upto 91%. Allylic boranes have been prepared from allylpotassiiim derivatives and chloroboranes. Hydrolysis leads to the Isomerised olefin and the technique has been used to transform (+)-a-pinene into (+)-3-pinene. Condensation reactions between allylboranes and acetylenes have been developed into a convenient method for the synthesis of bicyclo[3.3.l]nonane derivatives. Mainly linear alkyl derivatives of 9-BBN have been synthesised from the, reaction of iron carbonyls and the organoborane in a Fischer-Tropsch type reaction. ... 

The addition of HCo(CO)4 [or HCo(CO)3] to unsynunetrical olefins can occur in two directions, and both possible isomeric aUcylcobalt tetra-carbonyls are often obtained. Moreover HCo(CO)4 can isomerise olefins, and its subsequent addition to these isomerized olefins would give further isomeric alkyl cobalt tetracarbonyls. Therefore mixtures of aldehydes and ketones are often obtained under the conditions of the 0X0 reaction. The products are subsequently reduced to alcohols, the present production capacity for these being about 200 000 tons per year in Britain. 

Terminal to interior double olefins catalyst cyclopentadienyl-tantalum. Isomerisation catalyst based on Al-silicate 55... 

The add-catalyzed cyclodehydration of (Z)- and ( )-6-hydroxy-a,p-unsaturared ketones offers a mild synthesis of substituted furans. In the case of ( )-olefins, photochemical isomerisation was found to accelerate the reaction <96TL6065>. Reaction of alkynyl(phenyl)iodonium tetrafluoroborates with tropolone in the presence of a base yields 2-substituted furotropones (Scheme 16, <96TL5539>). 

The Pd(II)-catalysed asymmetric carbonylation of olefins with a chiral thiourea as the hgand has been reported recently. Since these ligands are stable in the presence of oxidising agents, they prevent Pd precipitation and double-bond isomerisation (Scheme 35) [141]. 

Am (PIB=). Of the four isomeric olefinic groups which can be formed by removal of CH3 from the polyisobutyl ion and subsequent isomerisation, the structure -CMe2 CMe = CH2 will have the largest AMe+ which is taken as the same as that of isobutene this can be estimated as -70 kcal less than the proton affinity, i.e., -130 kcal. 

In spite of this, the hydride catalyses the isomerisation of higher a-olefins to an equilibrium mixture of all the possible internal isomers and catalyses also the methoxycarbonylation of a-olefins and of internal olefins to the linear esters with 99% regioselectivity. 

Since the addition of methylene to an olefin should be exothermic, with the evolution of about 90 kcal/mole, isomerisations of the initially formed cyclo-propanes are very likely, since they only need about 64 kcal/mole. RRKM-studies demonstrate that this isomerisation should be faster than the rearrangement of cyclopropanes 32, 33 to the pentenes 34, 35 Numerous studies of the photochemical generation in the gas phase provided conclusive evidence in favour of these findings uo.iii.iis). 

When heated under reflux in benzene or methanol, in the absence of hydrogen, isomerisation of the terminal olefin of the pent-4-enyl phosphine n=3) is completed in less than 5 hours to yield the cis-pent-3-enyl phosphine complex. For the but-3-enyl complex ( =2) the isomerisation to the but-2-enyl phosphine complex is incomplete even after 88 hours. The mechanism involves a i-allyl hydride intermediate, whose stability, as in the case of the hydrogenation studies, is controlled by the... 

The isomerisation of terminal olefins is catalysed by many transition metals (56). Two mechanisms have been proposed to account for these... 

Several cases of isomerisation have been noted on reaction of olefinic phosphines with transition metals. The ligand ap reacts with (norbom-adiene)M(CO)4 (M = Cr, Mo, W) to give initially (cts-pp)M(CO)4. This may be followed by a slow equilibration to give a mixture of (cis-pp)M-(CO)4 and (fraMS-pp)M(C0)4. This ligand has not yet been found to isomerise with other transition metals. 

Not only acetylene derivatives do undergo palladium catalysed intarmolecular carbon-nitrogen bond formation with amines. The similar reaction of olefins in a Wacker-type process also leads to ring closure. (0-Aminopentenes bearing a suitable leaving group in the 4-position were converted to pyrroles in a cyclization-isomerisation-elimination sequence (3.65.),82... 

The processes of olefin exchange, double bond migration and cis—trans isomerisation, observed to occur concomitant with hydrogenation, may be accounted for by considering that the formation of the half-hydrogenated state is reversible. For olefin exchange we can write... 

Abilities of various metals to catalyse olefin isomerisation and exchange... 

For olefins possessing one or more a-CH2 groups, it is possible that olefin exchange and isomerisation could occur through the formation of an adsorbed 7r-allylie intermediate [87]. The appropriate mechanism for olefin exchange and double bond migration is... 

Fig. 18. The effect of mercury coverage ( Hg) upon the rates of hydrogenation (O), olefin exchange ( ) and isomerisation ( ) of but-l-ene over rhodium—silica at 48°C [129].

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation... 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

The hydrogenations of dialkylacetylenes are of particular interest from a stereochemical viewpoint since, as noted in Sect. 4.3, the adsorbed state of the acetylene is expected to adopt a c/s-configuration and, consequently, upon hydrogenation to yield the cis-olefin. Wide use of this fact has been made in preparative organic chemistry as noted by Burwell [192] and by Campbell and Campbell [193]. Although early studies of the catalytic hydrogenation of disubstituted acetylenes [194—196] revealed the formation of trans- as well as c/s-olefins, it was generally assumed that the trans-isomer was formed by isomerisation of the cis-ole-fin. However, more recent studies have shown that this view may have... 

On solid acid—base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 

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