Lithium salts catalysts

Although it is known that in some cases the lithium salts of chiral amino alcohols are even better catalysts than the chiral ligands themselves, the use of metals other than lithium has rarely been investigated. The oxazaborolidines A and B and the aluminum analog C have been used as catalysts for the enantioselective addition of diethylzinc to benzaldehyde35 (Table 32). 

Synthesis of comb (regular graft) copolymers having a PDMS backbone and polyethylene oxide) teeth was reported 344). These copolymers were obtained by the reaction of poly(hydrogen,methyl)siloxane and monohydroxy-terminated polyethylene oxide) in benzene or toluene solution using triethylamine as catalyst. All the polymers obtained were reported to be liquids at room temperature. The copolymers were then thermally crosslinked at 150 °C. Conductivities of the lithium salts of the copolymers and the networks were determined. 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

Enantioselective addition of R2Zn to aldehydes. Corey and Hannon2 have prepared the diamino benzylic alcohol 1 from (S)-proline and (lS,2R)-( + )-ephed-rine and report that the chelated lithium salt of 1 is an effective catalyst for enantioselective addition of diethylzinc to aromatic aldehydes. Thus benzaldehyde can be converted into (S)-( - )-3 with 95% ee, via an intermediate tridentate lithium complex such as 2 formed from 1. Similar reactions, but catalyzed by diastereomers of 1, show that the chirality of addition of dialkylzincs to aldehydes is controlled by the chirality of the benzylic alcohol center of 1. 

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-... 

Both form nickel carbonyl complexes (36). The lithium salt of triphenylstannide, which can readily be formed from tetraphenyltin and lithium salts, reacts violently with nickel carbonyl to give the presumably efficient catalyst Li(Ni(C0)3Sn(Ph)3). This complex possibly catalyzes the carbonylation of methyl iodide in a manner similar to that of the phosphine complex. 

A more recent synthesis for (14-9) takes quite a different course. The first step comprises the displacement of one of the halogens in 1,4-dibromobenzene by the alkoxide from A-2-hydroxyethylpyrrolidine (15-2) in the presence of 18-crown ether to afford (15-3). Condensation of the lithium salt from (15-3) with 6-methoxy-tetralone (15-4) followed by dehydration of the initially formed carbinol give the intermediate (15-5), which incorporates the important basic ether. Reaction of that compound with pridinium bromide perbromide leads to the displacement of the vinylic proton by halogen and the formation of bromide (15-6). Condensation of that product with phenylboronic acid in the presence of a tetrakistriphenyl-phosphine palladium catalyst leads to the coupling of the phenyl group by the formal displacement of bromine. The product (14-9) is then taken on to lasoxifene (14-11) as above [16]. 

TABLE 2. Summary of Olefin Metathesis Catalysts Prepared by Reacting a Selected Schiff Bases with RuCI2( p-cymeneb and then Postreacting with Alkyl or Aromatic Lithium Salt"... 

Virtually quantitative conversions were observed in the hydroformylation of 1-tetradecene with rhodium complexes generated from the lithium salt of tppms or the lithium (sodium) salts of 21 (Table 2 R=Ph n=3,4) and 22 (Table 2) in methanol as solvent.127,334 Catalyst recycling involved evaporation of methanol and addition of water to form a two phase system, separation of the aqueous phase, evaporation to dryness and addition of MeOH. 

It uses a fixed bed of platinum-based catalyst promoted by a lithium salt which can tolerate sulfur contents up to 300 ppm in the benzene and whose LSHV in relation to liquid benzene is about 1.5 [63],... 

In 2000, Kagan and Holmes reported that the mono-lithium salt 10 of (R)- or (S)-BINOL catalyzes the addition of TMS-CN to aldehydes (Scheme 6.8) [52]. The mechanism of this reaction is believed to involve addition of the BI NO Late anion to TMS-CN to yield an activated hypervalent silicon intermediate. With aromatic aldehydes the corresponding cyanohydrin-TMS ethers were obtained with up to 59% ee at a loading of only 1 mol% of the remarkably simple and readily available catalyst. Among the aliphatic aldehydes tested cyclohexane carbaldehyde gave the best ee (30%). In a subsequent publication the same authors reported that the salen mono-lithium salt 11 catalyzes the same transformation with even higher enantioselectivity (up to 97% Scheme 6.8) [53], The only disadvantage of this remarkably simple and efficient system for asymmetric hydrocyanation of aromatic aldehydes seems to be the very pronounced (and hardly predictable) dependence of enantioselectivity on substitution pattern. Furthermore, aliphatic aldehydes seem not to be favorable substrates. 

In this system, the chiral phase transfer catalyst (PTC) is able to recognize one aldolate selectively. There is an equilibrium between syn- and anti-aldolates via retro-aldol addition, and the formation of a stable, chelated lithium salt blocks the non-catalyzed subsequent reaction from yielding the epoxide product ... 

Mono- or di-lithium salts of (7 )-BINOL give high yields and good ees in cyanations of aromatic aldehydes.262 Formation of an aqua (or alcohol) complex of the catalyst gives higher and reversed ee, and non-linear effects in some cases. 

The work with which we are chiefly concerned here is an extension of these investigations of the effects of water on the thermodynamic properties of electrolytes in DPA solvents. The electrolytes considered are acids (HA), whose importance as a class of electrolytes derives from their involvement in many chemical reactions, either as reactants or as catalysts. In conjunction with these investigations, a parallel study was carried out water was replaced by diethyl ether (Et20) to determine the extent to which the hydrogen bond donor properties of the water molecule affect the interactions between HA, H20, and the solvent. For comparison, some additional experiments were included that used as electrolytes a lithium salt and a chloride salt and H2S instead of H20. 

Cyanosilylation. The chiral titanium reagent, prepared from the lithium salt of BINOL with TiCL, has been used as a catalyst for the asymmetric addition of cyanotrimethylsilane to aldehydes. In the example shown, the cyanohydrin is obtained with <82% ee (eq 9). 

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