Lithium catalysis addition

The role of HMPA as solvent in the addition of organolithium reagents to enones has been explored by applying a multinuclear NMR technique to quantify the amount of solvent-separated ion pairs (SIP) in solution and to corrolated this with changes in regioisomeric and diastereomeric product ratios.Contact ion pairs (CIP) have been found to react exclusively by 1,2-addition, presumably via a four-centre transition state as hypothesized. However, the situation for SIPs is more complicated and clean 1,4-addition occurs only in the absence of lithium catalysis. Well stabilized anions react by 1,2- and 1,4-addition in the absence of HMPA, when lithium catalysis is possible and SIPs are energetically accessible intermediates. 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

In addition to the formulation parameters mentioned above, selection of the base used for catalysis has strong implications. Bases commonly used are sodium hydroxide, potassium hydroxide, lithium oxide, calcium hydroxide, barium hy-... 

The Diels-Alder reaction of nonyl acrylate with cyclopentadiene was used to investigate the effect of homochiral surfactant 114 (Figure 4.5) on the enantioselectivity of the reaction [77]. Performing the reaction at room temperature in aqueous medium at pH 3 and in the presence of lithium chloride, a 2.2 1 mixture of endo/exo adducts was obtained with 75% yield. Only 15% of ee was observed, which compares well with the results quoted for Diels-Alder reactions in cyclodextrins [65d]. Only the endo addition was enantioselective and the R enantiomer was prevalent. This is the first reported aqueous chiral micellar catalysis of a Diels-Alder reaction. 

Systems based on complex lithium tetraborohydrate are of interest because of its large hydrogen capacity (ca. 18.3 wt%). However the material requires large energy input to release stored hydrogen (AH = 66.9 kJ mol"1 H2 7). Work into the catalysis of the system has focused on chloride and oxide additions however these reduce the overall system capacity 7. 

Table 2 shows the results of the addition of silicon and lithium enolates of methyl acetate to 2b and 2c (Eq. 3). Under the Fe-Mont catalysis, the t-butyldimethylsi 1yl ketene acetal of 6 is far less reactive than the trimethyl si 1 yl ketene acetal of 1, requiring higher reaction temperature moreover, it caused exclusive 1,2-addition to 2b in a good yield, but was inactive to 2c. Satisfactory yields of the expected products could not be... 

The hydrolysis of these model precursors was studied at 37 , with catalysis by hog liver esterase. The major product, isolated in 60-70% yield from the hydrolysis of a-acetoxyNPy, was 2-hyd oxy-tetrahydrofuran. This compound was identified by comparison to a reference sample,prepared by lead tetraacetate oxidation of 1,2,5-pentanetriol (53). Additional evidence was obtained by lithium aluminum hydride reduction of the product to 1,4-butanediol. Minor amounts of butenals were also identified as products of the hydrolysis of a-acetoxy IPy. 

With 5-33.3 vol.% water/acetone mixtures, it is found136 that common-ion salts have no effect on the rate of hydrolysis of benzoyl chloride whereas the rate in 15% (but not 33.3%) water is increased by the addition of neutral salts such as lithium bromide or potassium nitrate. The increase in ionic strength on the addition of neutral salts is not the major reason for the increase in rate and nucleophilic catalysis via the more easily hydrolysed benzoyl bromide was postulated. 

The advantage of the LiC104/ether system is its equal or greater polarity compared to that of water. Therefore it promotes solvolysis of 12. Furthermore the Lewis acidity of the lithium cation activates the ketone.19 As expected, the addition occurred from the sterically less hindered a-face of the molecule. Previously these transformations could be carried out only thermally or under high pressure with the aid of Lewis acid catalysis, e. g. TiCl4 or Ti(OiPr)4. Grieco also points out that it is very important to keep the exact concentrations of before diluted substrate and added LiC104 in ether. In disrespect the rate of the formation of the 1,2 addition product is enhanced over that of the 1,4 adduct. 

In addition to this, there is another lithium salt promoted pathway (Fig. 4.9) that contributes signihcantly to product formation. Here the product-forming reaction between lithium acetate and acetyl iodide is followed by the reaction between Lil and methyl acetate. These reactions are shown by the inner loop on the left-hand side. In fact, the inner loop is the dominant product-forming pathway, and lithium salts play a crucial role in the overall catalysis. Note that the right-hand-side loop of the catalytic cycle is exactly the same as in Fig. 4.1(a). 

The basic organometallic reaction cycle for the Rh/I catalyzed carbonylation of methyl acetate is the same as for methanol carbonylation. However some differences arise due to the absence of water in the anhydrous process. As described in Section 4.2.4, the Monsanto acetic acid process employs quite high water concentrations to maintain catalyst stability and activity, since at low water levels the catalyst tends to convert into an inactive Rh(III) form. An alternative strategy, employed in anhydrous methyl acetate carbonylation, is to use iodide salts as promoters/stabilizers. The Eastman process uses a substantial concentration of lithium iodide, whereas a quaternary ammonium iodide is used by BP in their combined acetic acid/anhydride process. The iodide salt is thought to aid catalysis by acting as an alternative source of iodide (in addition to HI) for activation of the methyl acetate substrate (Equation 17) ... 

Much more impressive rate accelerations for several Diels-Alder (and other) reactions have been observed by employing solutions of lithium perchlorate (up to 5 m) in diethyl ether (LPDE solutions) [802-806]. The dramatic rate accelerations found for Diels-Alder reactions in LPDE solutions appear to stem from Lewis acid catalysis by the coordinative unsaturated Li+ ion (see the end of Section 3.1). The Lewis acid catalysis by LPDE is applicable to those Diels-Alder reactions in which the lithium cation can coordinate with suitable functional groups in the reactants e.g. Li+---0=C). Addition of lithium-specific crown ethers e.g. [12]crown-4) leads to a loss of the catalytic activity of the Li+. For a recent extensive review of salt effects on Diels-Alder reactions, see reference [802]. 

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