Organolanthanide-induced carbon-carbon bond formation. Preparation and properties of monomeric lanthanide aldolates and enolates

Abstract

The reactivity of early-lanthanide carbyls Cp*2LnCH(SiMe3)2 (1, Ln = La; 2, Ln = Ce) toward ketones has been studied. No reaction was observed with the bulky di-tert-butyl ketone. The corresponding lanthanum hydride [Cp*2LaH]2 (3) is more reactive and afforded the addition product Cp*2LaOCH(t-Bu)2.O = C(t-Bu)2 (4). Hydrogen transfer and formation of lanthanide adolates Cp*2LnOCMe2CH2C(= O)Me (5, Ln = Ce; 6, Ln = La) together with the protonated carbyl CH2(SiMe3)2 were observed when 1 or 2 was treated with acetone. The molecular structure of the cerium analogue 5 was determined by X-ray diffraction and was found to be a monomeric bent metallocene compound with a eta(2)-coordinated aldol fragment. Cell data: monoclinic, P2(1)/n, a = 10.386 (5) angstrom, b = 17.32 (1) angstrom, c = 14.488 (3) angstrom, beta = 99.33 (3)-degrees, D(calcd) (Z = 4) = 1.36 g.cm-3. Least-squares refinement based on 3343 reflections converged to R = 0.035 and R(w) = 0.041. Addition of 3-pentanone to the carbyls 1 and 2 did not result in C-C coupling but provided enolate-ketone adducts Cp*2LnOC(Et) = C(H)Me.O = CEt2 (7, Ln = La; 8, Ln = Ce). Reactions of the carbyls 1 and 2 with 4-methyl-5-hydroxy-5-ethyl-3-hepatanone yielded the enolate-ketone adducts as well and suggest that the differences in reactivity between acetone and 3-pentanone are thermodynamic and not kinetic in origin.