Dienes, isomerisation

Stereochemical studies have established [206] that the thermal reaction of thiobenzophenones with (Z),( )-2,4-hexadiene leads to dihydro-(2H)-thiapy-rans with cis/tmns ratios usually superior to 98 2. On the other hand, achieving the addition at 12 kbar and ambient temperature (with a special technique to avoid diene isomerisation) provided mixtures in which the trans isomer was predominant. It is remarkable that, with pressure, conservation of the diene geometry with a concerted pathway is observed whereas, at normal pressure, a stepwise mechanism has to be assumed. This unprecedented dichotomy is probably linked to the low reactivity of the cis,trans diene isomer. 

The formation of the benzocyclo-octatetraene (26) undoubtedly involves a singlet species which undergoes benzo-vinyl bridging rather than the vinyl-vinyl bridging of the triplet which leads, eventually to compound (27) 51>52>. In accord with this the photo-isomerisation of the adduct (28) formed by the reaction of tetrafluorobenzyne with cyclohexa-1,3-diene, results in the formation of tetrafluorobenzo-dihydrosemibull-valene (29) 53>. 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

Another application of an isomerisation reaction can be found in the production of the third monomer that is used in the production of EPDM rubber, an elastomeric polymerisation product of Ethene, Propene and a Diene using vanadium chloride catalysts. The starting diene is made from vinylnorbomene via an isomerisation reaction using a titanium catalyst. The titanium catalyst is made from tetravalent salts and main group hydride reagents, according to patent literature. 

Industrially this diene is made the same way as ethylidenenorbomene from butadiene and ethene, but now isomerisation to 2,4-hexadiene should be prevented as the polymerisation should concern the terminal alkene only. In both systems nickel or titanium hydride species react with the more reactive diene first, then undergo ethene insertion followed by (3-hydride elimination. Both diene products are useful as the diene component in EPDM rubbers (ethene, propene, diene). The nickel hydride chemistry with butadiene represents one of the early examples of organometallic reactions studied in great detail [22] (Figure 9.14). 

However, too few examples are available to allow a general prediction of the position for hydrogen attachment. Electrochemical reduction of 1,4-diphenylbutadiene gives products from both 1,2- and 1,4-attachment of hydrogen [6], The 1,2-addition product predominates in absence of a good proton donor because electro-chemically generated base catalyses isomerisation of the 1,4-addition product. Reduction of a steroid diene 4 gives the 1,4-addition product [17],... 

Recently we discovered [11] that photolysis of the diphenylallyl carbanion with white light causes isomerisation of the trans,trans conformer to the cis,trans (scheme 1). When the source of the illumination is removed, conformational relaxation proceeds at a rate which is markedly sensitive to the nature of the ion pairing, being much slower for loose than for tight ion pairs. The results of an extension of this work will be presented in this paper together with an outline of the possible relevance of this photochemical phenomenon to the stereochemistry of the polymerisation of dienes. 

With palladium—alumina, the products of the reaction of but-l-yne with deuterium [189] were but-l-ene, 99.1% frans-but-2-ene, 0.2% cis-but-2-ene, 0.2% n-butane, 0.5%, until at least 75% of the but-l-yne had reacted. But-l-ene hydrogenation and hydroisomerisation were observed to occur when all the but-l-yne had reacted. The formation of but-2-ene as an initial product was postulated as being the result of a slow isomerisation of but-l-yne to absorbed buta-1 2-diene... 

The formation of but-2-ene and the more highly deuterated cis- and frans-but-2-enes (-d3 and above) were considered to arise from the hydrogenation of adsorbed buta-1 2-diene formed by isomerisation of but-... 

The hydrogenation of buta-1 2-diene appears to have received relatively little attention. Over palladium—alumina at room temperature, the products of the gas phase hydrogenation were c/s-but-2-ene, 52% but-l-ene, 40% frans-but-2-ene, 7% and n-butane, 1% [189]. Some isomerisation of the buta-1 2-diene to but-2-yne (10%) together with traces of but-l-yne and buta-1 3-diene was also observed. A similar butene distribution (namely, cis-but-2-ene 52%, but-l-ene 45% and frans-but-2-ene 3%) was observed in the liquid phase hydrogenation over palladium [186]. 

The observed deuterobutene distributions together with the calculated AAprofiles, for those metals where unique A/-profiles could be obtained, the surface D/H ratio and the calculated deuterobutene distributions are shown in Table 29. One of the major features of these results is that, over all the metals studied, the trans- and cis-but-2-ene profiles show pronounced maxima at -N2 This clearly shows that the predominant route to the formation of but-2-ene was direct 1 4-addition of two hydrogen atoms to adsorbed buta-1 3-diene. 1 2-Addition of hydrogen to yield but-l-ene-A/j followed by isomerisation would have led to a zero value for but-2-ene-A/j and a maximum at but-2-ene-AA3 or higher depending upon the number of butene—butyl interconversions before desorption of the but-2-ene. The detailed interpretation of the A/-profiles has been discussed fully by Wells and co-workers [166,167] who have proposed the two mechanisms shown in Fig. 37. 

More recently, the degradation of a-pinene by Pseudomonas jluorescens NCIMB 11671 was described [97,98]. A novel pathway for the microbial breakdown of a-pinene (119) was proposed, Fig. (23). The attack is initiated by enzymatic oxygenation of the 1,2-double bond to form the epoxide (127). This epoxide then undergoes rapid rearrangement to produce a novel diunsaturated aldehyde, occurring as two isomeric forms. The primary product of the reaction (Z)-2-methyl-5-isopropylhexa-2,5-dien-l-al (trivial name isonovalal) (128) can undergo chemical isomerisation to the -form (novalal) (129). Isonovalal, the native form of... 

There are rather few reactions that can be described as fully atom economical , i.e. when there are no co-products and all the atoms in the starting material(s) appear in the product(s). However, all isomerisation reactions necessarily fall into this category. The use of a transition metal to catalyse such a process with an appropriate substrate brings the possibility of effecting asymmetric isomerisation, a very efficient method to generate enantiomerically enriched products. Indeed, the asymmetric Rh-catalysed isomerisation of an allylamine to an enamine, which proceeds in over 96% ee, was scaled up a number of years ago for industrial production. The enamine product forms a multi-tonne feedstock for menthol and perfumery synthesis. In contrast, the cyclo-isomerisation of dienes, an equally atom-economical process that generates synthetically useful cyclic products, has seen relatively little development despite the reaction having been known for some 30 years. 

Scheme 12.6 Pro-catalyst-dependent regioselectivity in the Pd-catalysed cyclo-isomerisation of 1,6-dienes DCE = 1,2-dichloroethane (solvent) see the text for catalysts.

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

Scheme 12.7 Conclusions from the studies of the kinetics of the Pd-catalysed cyclo-isomerisation of 1,6-diene 10 (E = CO2R) to give 11, 12 and 13.

The result of acid-catalyzed isomerisation of F-dienes depends on several factors structure of substrate, catalyst, and temperature. Action of SbF5 on terminal dienes under mild conditions causes a 1,3 fluorine shift occurring stereoselectively to give trans-, frans-, and cis-, trans- isomers of the corresponding internal dienes [160] ... 

In 1957, Golub [47] first reported the cis-trans isomerisation of an unsaturated macromolecule. He showed the successful transformation of the cis 1,4 BR into corresponding trans units by means of ultraviolet irradiation in the presence of a suitable sensitiser, which may be any one of a wide variety of organic bromine or sulfur compounds. In the photoisomerisation of BR or other diene elastomers, there is direct excitation of 7t-electrons of the double bonds to an antibonding state in which free rotation and hence geometrical interconversion can occur. 

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