Lithium halides catalysts

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

Polymers containing 90-98% of a c 5-1,4-structure can be produced using Ziegler-Natta catalyst systems based on titanium, cobalt or nickel compounds in conjuction with reducing agents such as aluminium alkyls or alkyl halides. Useful rubbers may also be obtained by using lithium alkyl catalysts but in which the cis content is as low as 44%. 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Use of this technique results in an equivalent of lithium halide being present in the reaction mixture, unlike when the isolated copper arenethiolates are employed. Lithium salts can have very profound effects on copper-mediated reactions, but in this case a similar ee (40%) and complete y selectivity were still obtained for the reaction between 21 and n-BuMgl when the catalyst was prepared from Cut. Nei-... 

Vinyl isoxazolidine 84 was prepared by intramolecular 5-exo Pd-catalyzed allylic substitution of hydroxylamine 83. A comparative study on the Pd-based catalytic system proved that Pd(II) in the presence of lithium halides was the most selective catalyst giving the trans isoxazolidine 84 from syn-83 and the czs-84 from the isomeric anti-83 <07SL944>. 

Epoxytosylates are converted to the corresponding halohydrins with lithium halides (LiCl, LiBr, Lil) in the presence of Amberlyst 15 resin as catalyst <94TL797>. 

Enol germyl ethers are expected to be more stable than the corresponding enol stannyl ethers, but more reactive than the enol silyl ethers. A recent study shows that enol germyl ethers derived from a number of ketones e.g. 162) condense with benzaldehyde at -78 to -40 C without the need for a catalyst. However, the yield of the product (163) appears to be improved by the addition of BF.vOEt2. Interestingly, the presence of lithium halide also affects the reaction (Scheme 54). 

The 1,3-dioxolans (234 R = CH2CI, Me, or Et) have been prepared under neutral conditions by the reactions of the corresponding epoxides with benz-aldehyde. The reactions are catalysed by halide ion and provide a mixture of cis- and rra s-2,4-disubstituted compounds. Using Lewis acid catalysts, the ds-isomer was preferentially formed whereas catalysis by lithium halides favoured the rrans-isomer. 

It is interesting to develop a novel route to the CL precursor, PDHA, which was hitherto prepared by hydrogen peroxide oxidation of cyclohexanone (3) followed by treatment with ammonia [126,130]. Because ofthe ease of transformation of PDHA to a 1 1 mixture of CL and 3 under the influence of an appropriate catalyst such as lithium halides, the CL production via PDHA is considered to be a superior candidate for a next-generation waste-free process for CL. The NHPI-catalyzed aerobic oxidation of KA oil was applied to the synthesis of PDHA without formation of any ammonium sulfate waste. The strategy is outlined in Scheme 6.10. The NHPI-catalyzed oxidation of KA oil (a mixture of 3 and 2) with O2 produces 1,1 -dihydroxydicyclohexyl peroxide, which seems to exist in equilibrium with cyclohexanone and H2O2 (path 1). Subsequent treatment of the resulting reaction mixture... 

Shen et aL report the use of lithium halides under copper-catalyzed aerobic conditions to effect aromatic C—H bond activation resulting in bromo- and chloro-arenes. The catalyst employed Cu(N03>2 SHjO is cheap and readily available and, importantly, the reaction is compatible with both electron-donating and electron-withdrawing substituents on the aryl rings examples are shown with the optimized reaction conditions, using LiCl as the halogen source. LiBr and NaCl also resulted in halogenation albeit in modest yields (Scheme 7.31). 

There are three methods which are commonly used in the steroid field to replace a halogen atom by deuterium. These methods involve treatment of the halides— generally chloride, bromide or iodide—(a) with lithium aluminum deuteride, (b) with deuterium gas and a surface catalyst or (c) with zinc in O-deuterated acids or alcohols. 

Vinylic copper reagents react with CICN to give vinyl cyanides, though BrCN and ICN give the vinylic halide instead." Vinylic cyanides have also been prepared by the reaction between vinylic lithium compounds and phenyl cyanate PhOCN." Alkyl cyanides (RCN) have been prepared, in varying yields, by treatment of sodium trialkylcyanoborates with NaCN and lead tetraacetate." Vinyl bromides reacted with KCN, in the presence of a nickel complex and zinc metal to give the vinyl nitrile. Vinyl triflates react with LiCN, in the presence of a palladium catalyst, to give the vinyl nitrile." ... 

---