Isomerisation of double bonds

BINAP has been extensively used for the asymmetric hydrogenation, transfer hydrogenation and isomerisation of double bonds using both ruthenium and rhodium complexes. 

The occurrence of this chain transfer reaction results in a cis-trans isomerisation of double bonds in the polymer chains however the cis or trans structure of these double bonds has no essential influence on their susceptibility to a backbiting reaction. An important implication of the intermolecular secondary metathesis reaction is, instead, the tendency of the molecular weight distribution in the resulting polymer to attain the equilibrium condition Mw/Mn = 2 [122]. 

Bases under PTC conditions promote both the conversion of acetylenes into allenes and the isomerisation of double bonds. H/D exchange can be easily obtained on every substrate capable of generating an intermediate carbanion. Compounds with... 

Similarly, the introduction of double bonds, isomerisation or hydrolysis are also frequently encountered reactions in central metabolism. Many of these reactions have their analogues in sterol/steroid interconversions. Below we will confine ourselves to a limited number of examples. 

Although many isomerisations are possible, relatively few are of commercial value. Most involve the movement of double bonds, typically illustrated by the conversion of... 

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... 

A process with potential practical applicability is the hydrogenation of edible oils. Reduction of multiply unsaturated triglycerides with hydrogen over Ni-based catalysts is frequently used to gain autoxidative stability of edible oils. According to the Polanyi-Horiuti mechanism, multiple 1,2 or 1,4 diadsorption of the fatty acid tail with exclusively c/s-configuration around the double bonds causes cis-trans isomerisation, whilst the number of double bonds is being reduced. The trans-fatty acid chains have adverse effects on the human metabolism and must be minimized. 

Another potential area of application of FTIR spectroscopy is in the determination of the oxidative status or stability of an oil. Autoxidation is a major deteriorative reaction affecting edible fats and oils, and it is of major concern to processors and consumers from the standpoint of oil quality, as the oxidative breakdown products cause marked off flavours in an oil. A wide range of end products are associated with the autoxidative deterioration of fats and oils, the most important being hydroperoxides, alcohols, and aldehydes. Moisture, hydrocarbons, free fatty acids and esters, ketones, lactones, furans, and other minor products may also be produced, with the free fatty acids becoming more important in thermally stressed oils. In addition, there is significant cis to trans isomerisation and conjugation of double bonds in the hydroperoxides formed as an oil oxidises. 

A very interesting group of random copolyethers is obtained by anionic copolymerisation of EO (a highly hydrophilic monomer) with BO (a highly hydrophobic monomer). Because EO does not isomerise to double bond structures and BO has a much lower tendency to isomerise to allyl structures than PO (see Chapter 12.2), the BO-EO copolyethers have a very low unsaturation level compared to PO homopolymers or even PO-EO copolymers [82]. This variant of polyether polyols synthesis in the form of BO-EO copolymers is a very interesting way to obtain low unsaturation polyether polyols directly from synthesis. Another group of low unsaturation polyether polyols, obtained directly from synthesis, are the tetrahydrofuran (THF)-EO and THF-PO copolymers synthesised with cationic catalysts (see Chapter 7.3). 

A study comparing, under a range of conditions, diene synthesis by the two alternative Wittig routes, allylic ylide-saturated aldehyde (route 1) and reactive ylide-a,P-unsaturated aldehyde (route 2), has been reported. 9 por the system chosen (Scheme 2) the reactive ylide-a,P-unsaturated aldehyde route is clearly preferred in that the stereochemistry of the new double bond can be controlled more easily and there is little or no isomerisation of the double bond already present in the aldehyde. A route to symmetrically substituted polyenes containing an odd or even number of double bonds has been reported (Scheme 3).20 The Wittig reactions of ylides derived from... 

The impetus for research in this filed stems from the industrial importance of metal-olefin complexes as intermediates and catalysts in a wide range of reactions, especially in the petrochemical industry. Major uses include the Wacker Process (oxidation of ethylene to acetaldehyde in the presence of PdCl2), the OXO process (hydroformalation of olefins), the specific hydrogenation of double bonds and the isomerisation of olefins (e.g. but-l-ene to but-2-ene in the presence of [ (C2H4 )2 RhCl ]2 ). 

Sonesson, C., Larhed, M., Nyquist, C. and Hallberg, A. (1996) Regiochemical control and suppression of double bond isomerisation in the Heck arylation of l-(methoxycarbonyl)-2,5-dihydropyrrole. J. Org. Chem., 61, 4756-63. 

Shezad, N., Clifford, A.A. and Ra5mer, C.M. (2001) Suppression of double bond isomerisation in intramolecular Heck reactions using supercritical carbon dioxide. Tetrahedron Lett., 42, 323-5. 

Not surprisingly, decarboxylation is a common reaction of / -lactones and can be initiated either photochemically or thermally. The low-temperature photolysis of 2-oxabicyclo[2,2,0]hex-5-en-3-one (159), prepared photochemically from a-pyrone, has been used as a solid matrix preparation of cyclobutadiene. An alternative to the Wittig reaction for the stereospecific introduction of double bonds has been reported by Adam and co-workers. Thermal decarboxylation of the /S-lactone (160), prepared by cyclization of the corresponding /3-hydroxy-acid with benzenesulphonyl chloride, takes place with retention of initial geometry and without isomerisation of the... 

In 2013, Terada and Toda reported a relay catalysis for a ternary reaction sequence composed of double bond isomerisation, protonation of the double bond, and enantioselective Pictet-Spengler-type cyclisation, which was accomplished using a binary catalytic system consisting of a ruthenium hydride complex and a chiral phosphoric acid. As shown in Scheme 7.47, the intramolecular reaction of allylamides led to the corresponding chiral tetrahydroisoquinoline derivatives in moderate to good yields and insufficient enantioselectivities of 18 to 53% ee. 

In some early work, conjugated trienes were reported to undergo cis-trans isomerisation and double bond migration on packed columns containing polyester stationary phases [609,619]. On the other hand, when an "all-glass" WCOT system was used with the non-polar stationary phase, OV-1, it proved possible to separate a number of geometrical isomers, eluting in the order cfc-9,11,13- (ECL = 18.95), cff-9,11,13- (18.99), c-8,10,12- (19.09), ttc-9,11,13- (19.10), fff-9,11,13- (19.36) and fff-8,10,12-18 3 (19.39), without any losses [900]. Comparable results have been obtained by others [226,282,394]. Indeed, somewhat better resolution was obtained with a WCOT column coated with Carbowax 20M (ECLs = 21.50 to 22.19), from which components eluted without loss in the same order as on an OV-1 column [282]. 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

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]. 

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