Alkene isomerization catalysis

With the renaissance in alkene chemistry engendered by the rising versatility of olefin metathesis in both fine chemical and commodity production, new methods for alkene isomerization are of increasing interest and importance. Alkene isomerization can be performed using Bronsted-Lowry acid or base catalysis (1). However, these reactions are limited to substrates which tolerate carbanionic or carbocation intermediates, and are susceptible to undesired side reactions. 

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

Osmium carbonyls on MgO and on y-ALO, among other oxides, are catalysts (or catalyst precursors) for alkene isomerization and hydrogenation (Li et al., 1984). The activity depends on the metal oxide used as a support. The ligands present on the metal during catalysis have not yet been elucidated. 

Another approach to alkene isomerization would be to use a catalyst. Base catalysis is of no use as there are no acidic protons in the alkene. Acid catalysis can work (Chapter 19) if a carbocation is formed by protonation of the alkene. 

Ammonia N-donor Ligands Asymmetric Synthesis by Homogeneous Catalysis Decarbonylation Catalysis Hydride Complexes of the Transition Metals Hydroboration Catalysis Hydrogenation Isomerization of Alkenes Hydrosilation Catalysis P-donor Ligands Rhodium Organometallic Chemistry. 

Zr and Hf hydrides have been proposed as intermediates or by-products in heterogeneous and homogeneous catalysis of alkene isomerization, polymerization, and trimerization reactions.388 657-659 For example, heating the well-characterized silica-bound Zr(neopentyl)ra (n 1, 2) with dihydrogen afforded silica supported Zr-hydrides as indicated by IR spectroscopy. 58 Interestingly, this silica-supported Zr hydride exchanged via cr-bond metathesis with alkanes to produce the corresponding Zr alkyl derivatives.388,660,661... 

Supported molecular species. Although there are several examples of intact oxide-supported metal clusters acting as catalysts at relatively low temperatures, e.g., alkene isomerization at < 100 C, there is at least one example whereby, for specific combinations of metal, support, pre-treatment, reaction conditions, etc., catalysis can be carried out at up to 275°C. At this temperature, on the basis of the structural characterization at present available, the structure of the cluster is maintained. The catalytic properties of such stable support-... 

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