A Rare-Earth-Metal Ensemble of the Tebbe Reagent: Scope of Coligands and Carbonyl Olefination

Abstract

Treatment of the half-sandwich complexes Cp R Ln-(AlMe 4 ) 2 (Cp R = C 5 Me 5 , C 5 Me 4 SiMe 3 ; Ln = Y, La, Lu) with the mild halogenido transfer reagents SiMe 3 X (X = Cl, Br, I) resulted in efficient and selective halogenido/tetramethylaluminato exchange. Depending on the size of the rare-earth metal, dimeric [Cp R Ln-(AlMe 4 )(μ-X)] 2 (Ln = Y, Lu) and decametallic [Cp R 3 La 3 (AlMe 4 ) 2 (μ-X) 4 ] 2 could be obtained. Donor (THF)-induced tetramethylaluminato cleavage gave access to "methyl-free" mixed methylidene/halogenido complexes Cp R 3 Ln 3 (μ-X) 3 (μ 3 -X)(μ 3 -CH 2 )(THF) 3 for yttrium (X = Cl, Br) and lanthanum (X = Cl, Br, I) in good yields. Additionally, mixed methylidene/halogenido Y(III) complexes could be obtained via methyl/halogenido exchange employing (C 5 Me 5 ) 3 Y 3 (μ-Me) 3 (μ 3 -Me)(μ 3 -CH 2 )(THF) 2 and SiMe 3 X via tetramethylsilane elimination. All methylidene complexes were probed in olefination reactions and found to act as efficient Schrock-type nucleophilic carbenes converting ketones and aldehydes into the respective terminal alkenes. Such reactivity is as high as that of the prominent Tebbe reagent but is less tolerant toward sterically demanding and functionalized substrates (such as esters). In contrast to the Tebbe reagent, complexes Cp R 3 Ln 3 (μ-X) 3 (μ 3 -X)(μ 3 -CH 2 )(THF) 3 polymerize δ-valerolactone in an efficient manner, generating polylactones with molecular weight distributions M w /M n as low as 1.13. Moreover, the bromido variant of the Tebbe reagent, Cp 2 Ti(μ-CH 2 )(μ-Br)AlMe 2 , is described, underlying similar synthesis limitations: that is, the coformation (and hence cocrystallization) of the trivalent species Cp 2 Ti(μ-Br) 2 AlMe 2 .

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(iii) methylene complex

“…2 , the solid-state structure of 3 displayed a C 2 symmetric dinuclear titanium structure bridged by a methylene ligand (Ti1–C1: 2.084(6) Å; Ti1–C1–Ti1′: 78.4(3)°) and chlorides (Ti1–Cl1: 2.409(2) Å; Ti1–Cl2: 2.405(2) Å, 2.472 (2) Å). The crystal structures of the Tebbe complex [Cp 2 Ti(μ-CH 2 )(μ-Cl)AlMe 2 ] have been reported by Mindiola 24 a and more recently by Anwander, 24 b and the Ti–CH 2 bond length in complex 3 (2.084(6) Å) is comparable to the reported Ti–CH 2 distances (2.095(5) Å, 24 a 2.058(3) Å) 24 b of the Tebbe complex. The bridging mode of the methylene ligand, where the hydrogen atoms were located from the difference map and refined isotropically, is oriented to avoid the steric repulsion with the methyl groups of TMEDA ( Fig.…”

[(CH3)Al(CH2)]12: Methylaluminomethylene (MAM‐12)

“…Compound 2 b promotes carbonyl methylenation as efficiently as the Tebbe reagent or related rare‐earth‐metal variants [11] . Treatment of compound 2 b with 12 equiv.…”

“…[5,2f] More recently, the solid-state structure of the Tebbe reagent could be elucidated by X-ray diffraction (XRD) analyses. [10,11] Interestingly, a more detailed mechanistic investigation of the system (C 5 H 5 ) 2 TiCl 2 /AlMe 3 by Grubbs and co-workers from 1984 also pointed to the formation of red toluene-insoluble species ("poly-TiCH 2 Al") when III was allowed to stand with excess AlMe 3 or in neat AlMe 3 for long reaction times. [12] Discrete [Cl 2 Al-CH 2 -AlCl 2 ] (IVa) and polymeric chloridoaluminomethylene species IVb (Figure 1), reminiscent of polymeric II were obtained in 1966 by Lehmkuhl and Schäfer from Al/CH 2 Cl 2 mixtures.…”

“…Tebbe could selectively synthesize his compound in 1974 using (C 5 H 5 ) 2 TiCl 2 /AlMe 3 in a 1 : 2 ratio (toluene, RT, 60 h) [5,2f] . More recently, the solid‐state structure of the Tebbe reagent could be elucidated by X‐ray diffraction (XRD) analyses [10,11] . Interestingly, a more detailed mechanistic investigation of the system (C 5 H 5 ) 2 TiCl 2 /AlMe 3 by Grubbs and co‐workers from 1984 also pointed to the formation of red toluene‐insoluble species (“poly‐TiCH 2 Al”) when III was allowed to stand with excess AlMe 3 or in neat AlMe 3 for long reaction times [12] .…”

Half‐Sandwich Complexes [Cp′4Ln4I8]4 (Ln=Ce, Pr):Emergence of Donor‐Free Organolanthanide Nanowheels

“…Alternatively, cluster 5 was obtained according to standard synthesis routes pursuing a stoichiometry-controlled reaction pathway. [35,58,59] Unsurprisingly, the tetramethylaluminatobridged compound 5 is isostructural to [Cp' 6 La 6 I 8 (AlMe 4 ) 4 ]. [35] Apparently, low temperature hampers a complete alkyl/halogenido exchange.…”