Isomerisation Intramolecular Processes

It was found that both the isomerisation and the racemisation are intramolecular processes, which occur at approximately the same rate and with activation energies that are identical within experimental error. It thus appears likely that the two processes have the same transition state. This exclude a twist mechanism as the principal pathway for racemisation. Moreover, it was found that isomerisation occurs mainly with... 

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

An example of identical functional groups with a different steric environment is found in the related dione 9, from which would not be possible to prepare the monoacetal 6 since the less hindered carbonyl group at C(6) is more reactive and leads to monoacetal 10. In fact, monoacetal 6 -prepared from 5- undergoes a smooth and clean acid-catalysed isomerisation to 10 through an intramolecular transacetalisation process (A) [7]. 

Rates for intramolecular A/A isomerisations have been reported for Cu(L)2+, where L = atropoisomer of di-imine benzimidazole-pyridine ligands. H, C and 31P NMR spectra suggest that MX[P(C7H7)3], where M = Cu or Ag, X = Cl, Br, are non-rigid in solution at room temperature.980 Dynamic 31P NMR spectroscopy was used to follow fluxional processes in [(3,5-di-/er/-butyl-l, 2,4-triphospholyl)Cu(PPh3)]. The preferred coordination mode at room temperature is r 5-7i coordination of copper. At low temperatures, two cr-isomers are seen.981... 

Inversion in the cyclic sulphite (65) and the cyclic sulphate (66) has been studied by ultrasonic techniques. Isomerisation of the cyclic iron-sulphur derivative (67) appears, from kinetic evidence, to take place by an intramolecular inversion process. ... 

The process from the FMC company involves as the pivotal step an intramolecular stereoselective [2 + 1 [-cycloaddition. In a Prins reaction [94] of chloral and isobutene, followed by an isomerisation, a racemic, trichloromethyl-substituted aUyl alcohol is obtained. Reaction with the isocyanate from (R)-naphthylethyl-amine enables separation ofthe diastereomers by crystallisation. The carbamate is cleaved by trichlorosilane/triethylamine, thus permitting the recycling of the chiral auxiliary. The optically pure (R)-aUyl alcohol is reacted with diketene, to produce the / -keto-ester. After diazo transfer and basic cleavage, the diazoacetate is obtained catalysed by a copper salt, this is converted in a [2 + 1 ]-cyclo-addition into a bicyclic lactone. The Boord reaction (discovered by Cecil E. Boord in 1930) [95] finally gives (IR)-cis-permethric acid. [96]... 

Proton transfer processes in the excited state are one of the most important photochemical deactivation routes in biological systems. It has been shown that an intramolecular proton transfer can suppress the double-bond isomerisation that would normally be expected in the excited salicylideneaniline (Fig. lOa). A similar effect was found in an analogue for a recently synthesised green fluorescence protein (GFP) chromophore, 4-(2-hydroxybenzylidene)-lFf-imidazol-5(4Ff)-one (OHBI,... 

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