[7791-03-9] · ClLiO4 · Lithium Perchlorate · (MW 106.39)
Physical Data: mp 236 °C; bp 430 °C (dec); d 2.428 g cm-3.
Solubility: 60 g/100 mL H2O (25 °C); 150 g/100 mL H2O (89 °C); 152 g/100 mL EtOH (25 °C); 137 g/100 mL acetone (25 °C); 182 g/100 mL MeOH (25 °C); 114 g/100 mL ether.
Form Supplied in: white solid; widely available in anhydrous form or as the trihydrate; it is usually used in solution in ether or MeOH.
Purification: anhydrous lithium perchlorate is prepared by heating the commercially available anhydrous material or its trihydrate at 160 °C for 48 h under high vacuum (P2O5 trap).
Handling, Storage, and Precautions: the anhydrous material should be used as prepared for best results. The decomposition of lithium perchlorate starts at about 400 °C and becomes rapid at 430 °C, yielding lithium chloride and oxygen. Perchlorates are potentially explosive and should be handled with caution.
This reagent, usually prepared as a 5.0 M solution in diethyl ether, produces a dramatic rate acceleration of Diels-Alder reactions (eq 1). Evidence shows that this rate acceleration, which was initially thought to be a result of high internal solvent pressure, is due to the Lewis acid character of the lithium ion.6
Under these conditions, reasonable levels of diastereoselectivity have been observed in the reaction between a chiral diene and N-Phenylmaleimide (eq 2).7 An interesting protecting group dependence of diastereoselectivities has also been observed in the hetero-Diels-Alder reaction of N-protected a-amino aldehydes with 1-methoxy-3-t-butyldimethylsilyloxybutadiene to produce dihydropyrones (eqs 3 and 4).8
O-Silylated ketene acetals undergo 1,4-conjugate addition to hindered a,b-unsaturated carbonyl systems3 and quinones9 in the presence of LiClO4.
In contrast to the [3,3]-sigmatropic rearrangement observed under thermal conditions, allyl vinyl ethers undergo [1,3]-sigmatropic rearrangements at rt when submitted to 1.5-3.0 M LiClO4 in Et2O (eq 5).10
LiClO4 is an efficient promotor for the regioselective nucleophilic opening of oxiranes with amines,11 cyanide,12 azide,13 thiols,14 halides,15 and lithium acetylides.16 The regioselective opening of oxiranes with lithium enolates derived from ketones has also been observed in the presence of LiClO4 (eq 6).17
The condensation of aldehydes and ketones with diphenylsulfonium cyclopropylide produces oxaspiropentanes which undergo ring expansion to produce cyclobutanones upon treatment with lithium perchlorate.18
André B. Charette
Université de Montréal, Québec, Canada