Catalysis by Lithium Salts

Anhydrous lithium salts are soluble in organic solutions which can be considered to be air-stable catalysts. The most common is lithium perchlorate which is generally used in solution in diethyl ether (LPDE) [9], nitromethane [15], and dichloro-methane [16]. Its catalytic role comes from the properties of the lithium cation [17] which gives rise to specific solute-Liinteractions modulated by complexation to appropriate solvents and counterions [18]. A recent proposal denies lithium catalysis and emphasizes electrostatic stabilization of the transition state by LPDE 

Catalysis by LPDE was applied to some high-pressure reactions including Diels-Alder cycloadditions (entries 1-4), homo-Diels-Alder reactions (entry 5) and ene reactions (entry 6) (Table 10.4) [22]. 

In a general way, combination of pressure and LPDE catalysis promotes the reactions. The most sensitive are the [4 + 2] cycloadditions involving carbonyl bonds (Table 10.4 entries 1, 2 and 3). The Diels-Alder adduct (entry 1) is obtained in quantitative yield when the substrates are reacted in 1 M LPDE under 300 MPa. The cycloaddition of the hindered 2,6-dimethylbenzoquinone to isoprene leads to 68 % yield under LPDE conditions whereas the system is unreactive at ambient pressure in acetone solution (entry 2). An excellent yield of the 7-oxabicyclo(2.2.1] adduct is obtained in 3 M LPDE solution by reacting furan and methyl vinyl ketone at 300 MPa (entry 3). It should be emphasized that such yields are usually obtained in organic solvents only under pressures in excess of 1000-1500 MPa. A more re- 

Lithium perchlorate displays explosive properties. The hazards of LPDE accelerated the search for alternative lithium compounds. Lithium trifluoromethane sulfonamide was used in some Diels-Alder reactions [23]. The limited access to this compound prompted the consideration of other lithium salts such as lithium tetrafluoroborate (LB) and lithium triflate (LS) [24]. They have seemingly similar catalytic activity to LPDE. Table 10.5 compares the catalytic merits of these lithium 

In conclusion of this section, organic solutions of lithium salts are a peculiar medium due to the properties of Li+ acting as a Lewis acid. The catalytic activity depends on the presence and the nature of the functional groups, on the counterion, and, as shown in Table 10.5, the type of reaction. 

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