Hydride shift, isomerisation

Studies in deuterated water have shown that the hydroxyl proton does not end up in the ethanal formed. The decomposition of the 2-hydroxyethyl is not a simple P-elimination to palladium hydride and vinyl alcohol, which then isomerises to ethanal. Instead, the four protons stemming from ethene are all present in the initial ethanal product [6] (measured at 5 °C in order to suppress deuterium/hydrogen exchange in the product) and most authors have therefore accepted an intramolecular hydride shift as the key-step of the mechanism (see Figure 15.2). There remains some doubt as to how the hydride shift takes place. 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

As it had been found that the enzyme specifically reacts with the a-anomer, but that the substrate detected in crystals of enzyme substrate complex was the open-chain form, the authors suggested, similar to Collyer and Blow, that the mechanism involves three major steps in the catalytic isomerisation of D-glu-cose ring opening of the substrate a-o-xylopyranose aided by interaction of His-101 with 0-5 of the aldose, isomerisation by a metal ion catalysed hydride shift, and ring closure of the product. 

Detailed kinetic analysis of the catalytic mechanism of the Arthrobacter enzyme by Rangarajan and Hartley [58] also supported the hydride shift mechanism. However, these workers imply different reaction pathways for the Mg and the Co + enzymes. The rate-limiting step was inferred to be the isomerisation and not the ring opening, in contrast to the results of Zeikus and co-workers. Reports from Withlow and co-workers [59] as well as Collyer and his group [60] provided further support for the hydride shift mechanism. 

The above reaction scheme could not explain the marked effect of water on the reaction rate. Another drawback of the above reaction scheme is that if each monomer addition were followed by the shift of a hydride ion and isomerisation, the two charges should always remain in the vicinity and it should allow the elimination of HC1 or the addition of chloride ion to proceed easily otherwise the reaction scheme will not be feasible. 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

The first reaction involves interaction of a hydrocarbon with the catalyst surface. Hydride abstraction occurs to form a carbonium ion. Abstraction can be of any suitable hydrogen atom but if this results in a primary ion as shown, this will rapidly isomerise by hydrogen shift to the more thermodynamically stable secondary ion. This may be further isomerised by carbon shift to a tertiary ion. This contrasts with free radicals and although isomerisation occurs it is relatively slower. The carbonium ions can also undergo inter-molecular transfer (not shown) when a carbonium ion meets another hydrocarbon molecule. 

Cationic classical polyhydrides, [LmMHn], form as the result of proton transfer to the core metal of neutral LmMHn 1 hydrides or of cationic T -H2 complex isomerisation. Their IR spectra feature Vmh bands, which are shifted to higher frequencies by more than 60 cm relative to their neutral precursors. The intensity of these bands is usually lower than that of the neutral analogues (LmMHn) or neutral precursor (LmMHn 1) but they still can be observed in the IR spectra. This difference was first analysed for Cp2MH complexes and traced later for many other hydrides. For... 

---