[54761-04-5] · C3F9O9S3Yb · Ytterbium(III) Trifluoromethanesulfonate · (MW 620.28)
(organometallics derived from addition of organolithiums or organomagnesiums to Yb(OTf)3 display enhanced diastereoselectivities in the carbonyl addition to chiral ketones and aldehydes;2 functions as a Lewis acid in promoting aldol reactions and hydroxymethylation reactions4)
Alternate Name: ytterbium(III) triflate.
Preparative Methods: prepared by addition of an excess of ytterbium(III) oxide (commercially available, 99.9%) to an aqueous solution of Trifluoromethanesulfonic Acid (50% v/v) followed by heating at boiling for 0.5-1 h. The mixture is then purified by filtration to remove unreacted ytterbium oxide; residual water is removed under reduced pressure. The resultant hydrate is dried by heating under vacuum (1 mmHg) at 180 to 200 °C for 48 h. The reagent is then used without further purification.1 The organoytterbium reagent is prepared at -78 °C from Yb(OTf)3 and organolithiums or organomagnesiums.2
Organoytterbium triflates display enhanced diastereoselectivity in carbonyl addition to chiral aldehydes and ketones. Because of the imposing steric bulk, organolanthanide reagents are suitable for achieving high diastereofacial selectivity in the addition to chiral aldehydes and ketones. Treatment of 2-methylcyclohexanone with the methylytterbium reagent produces a mixture of diastereomers in a 98:2 ratio, resulting from preferential equatorial attack of the bulky organolanthanide reagent (eq 1).2
Another attractive feature of the ytterbium triflate derived organometallic reagent is the diverse organoytterbiums which may be utilized in the process. Unlike other organometallic reagents possessing similar reactivity, the organoytterbium reagent is not limited to a narrow range of organic ligands which may be incorporated onto the metal center (eq 2). Specifically, high yields and diastereoselectivities may be achieved with both alkyl- and alkenylytterbium reagents. This is in contrast to organotitanium reagents, which may provide incomplete conversion and/or low diastereofacial selectivities in additions to carbonyl substrates.3 Additionally, tertiary organometallics can be generated and utilized without interference from b-hydride elimination processes, although enolization of the substrates does lower the yield significantly.
Organoytterbium triflates also show enhanced diastereofacial selectivity upon addition to acyclic chiral aldehydes (eq 3).
Yb(OTf)3 serves as catalyst in promoting the aldol-type reaction between silyl enol ethers and acetals or aldehydes (eqs 4 and 5).4
Unlike other Lewis acid catalysts capable of performing similar transformations, water does not interfere with the aldol reaction when ytterbium triflate is employed; hence, troublesome purification and dehydration steps are unnecessary when performing aldol reactions on water-soluble aldehydes. Furthermore, molecules with free hydroxyl and nitrogen functionalities are compatible with the ytterbium triflate promoted aldol reaction because these functionalities do not coordinate to ytterbium triflate. The Lewis acid thus remains active in the presence of such functional groups in contrast to other Lewis acid catalysts. Another important feature which makes ytterbium triflate attractive in these transformations is that the catalyst may be recovered from the aqueous layer after workup because Yb(OTf)3 is not hydrolyzed in the presence of water.
Gary A. Molander & Christina R. Harris
University of Colorado, Boulder, CO, USA