[13874-77-6] · Cl2Yb · Ytterbium(II) Chloride · (MW 243.94)
Physical Data: mp 720 °C; bp 1930 °C; d 5.08 g cm-3; green rhombohedral crystals.
Solubility: sol THF; readily oxidized in H2O.
Preparative Methods: can be obtained by reduction of the trichloride with various reagents (e.g. Na). In recent papers the trichloride is reduced in situ by Zinc.
Handling, Storage, and Precautions: unstable in water and humid air; oral toxicity of ytterbium trichloride: LD50 6.7 g kg-1 in rats.
Ytterbium dichloride is probably the reactive species formed by reduction of the trichloride with zinc, although it was not characterized as such. This reagent was shown to be more efficient than Ytterbium(0) metal in promoting the reductive cyclodimerization of a,b-alkenic ketones to cyclopentanols.2 Thus far, this reaction has only been observed with chalcone (eq 1).
The mechanism postulated involves a single-electron transfer to the conjugated system, followed by addition of the resulting enolate radical to the starting material. The reaction exhibits significant diastereoselectivity. The radical (or the enolate derived from its further reduction) can be trapped by an aldehyde to produce the acyl or hydroxyalkyl compound (eq 2).4 Starting from 2-propyn-1-ones, a similar process takes place, but the results are more complex. Dimerization occurs following a slightly different mechanism, with a third molecule being trapped to give an hemiacetal (eq 3). In other instances, dimerization predominates to afford either a linear (eq 4) or a cyclobutene product (eq 5).5
Addition of a perfluoroalkyl group to several alkenes is easily effected using in situ-generated ytterbium dichloride. Alkenes containing ester (eq 6), ether, alcohol, and phosphonate (eq 7) groups participate in excellent yield via a rapid (5-10 min) and smooth (50-75 °C) reaction.3
Université Paul Sabatier, Toulouse, France