Replacing Chloride by Alkoxide: Cp2Zr(H)OR, Searching for Alternatives to Schwartz's Reagent

Methoxide-Enabled Zirconium-Catalyzed Migratory Alkene Hydrosilylation

A zirconocene dichloride-catalyzed alkene hydrosilylation is reported that can be applied to non-activated and conjugated terminal and internal alkenes. It involves a catalytic Zr-walk process and leads to a selective conversion to the linear product. Lithium methoxide serves as mild catalyst activating agent, which significantly increases the applicability and operational simplicity in comparison to earlier zirconium(II)-based protocols. Supported by additional experiments and calculations, a mechanism via zirconium(IV) intermediates is proposed. Due to the benign nature and ready-availability of the zirconium catalyst, the reaction is an attractive alternative to established alkene hydrosilylation methods.

Direct conversion of esters to imines/enamines and applications to polyester waste upcycling

Semi-reductive transformations of esters remain an underdeveloped but valuable class of functional group interconversions. Here, we describe the development of a highly selective method for the interconversion of esters to imines, enamines, aldehydes or amines through an amine-intercepted zirconocene hydride (ZrH)-catalyzed reduction. This protocol employs an inexpensive zirconium catalyst in combination with hydrosilanes and simple unprotected amines. A variety of aryl, benzylic, and aliphatic esters are directly transformed to imines and enamines in up to 99% yield or aldehydes in up to 84% yield, with little-to-no reduction to the corresponding alcohols. The utility of this method for the direct catalytic chemical upcycling of polyester plastic waste is demonstrated through multiple unprecedented depolymerization transformations. Further, the efficient preparation of nitrogen-containing products is also presented, including single-flask multicomponent reactions and the reductive amination of esters.

Hydrogenation of titanocene and zirconocene bis(trimethylsilyl)acetylene complexes

Reactions following the addition of dihydrogen under maximum atmospheric pressure to bis(trimethylsilyl)acetylene (BTMSA) complexes of titanocenes, [(η5-C5H5-nMen)2Ti(η2-BTMSA)] (n = 0, 1, 3, and 4) (1A-1D), and zirconocenes, [(η5-C5H5-nMen)2Zr(η2-BTMSA)] (n = 2-5) (4A-4D), proceeded in diverse ways and, depending on the metal, afforded different products. The former complexes lost, in all cases, their BTMSA ligand via its hydrogenation to bis-1,2-(trimethylsilyl)ethane when reacted at 80 °C for a prolonged reaction time. For n = 0, 1, and 3, the titanocene species formed in situ dimerised via the formation of fulvalene ligands and two bridging hydride ligands, giving known green dimeric titanocenes (2A-2C). For n = 4, a titanocene hydride [(η5-C5HMe4)2TiH] (2D) was formed, similarly to the known [(η5-C5Me5)2TiH] (2E) for n = 5; however, in contrast to this example, 2D in the absence of dihydrogen spontaneously dehydrogenated to the known Ti(iii)-Ti(iii) dehydro-dimer [{Ti(η5-C5HMe4)(μ-η1:η5-C5Me4)}2] (3B). This complex has now been fully characterised via spectroscopic methods, and was shown through EPR spectroscopy to attain an intramolecular electronic triplet state. The zirconocene-BTMSA complexes 4A-4D reacted uniformly with one hydrogen molecule to give Zr(iv) zirconocene hydride alkenyls, [(η5-C5H5-nMen)2ZrH{C(SiMe3)[double bond, length as m-dash]CH(SiMe3)}] (n = 2-5) (5A-5D). These were identified through their 1H and 13C NMR spectra, which show features typical of an agostically bonded proton, [double bond, length as m-dash]CH(SiMe3). Compounds 5A-5D formed equilibria with the BTMSA complexes 4A-4D depending on hydrogen pressure and temperature.

Reaction environment and ligand lability in group 4 Cp2MXY (X, Y = Cl, OtBu) complexes

Ligand transfer during halide metathesis with group 4 metallocenes depends on whether a polar solvent is used or not.

Iminoacyl Alkyl Complexes of Zirconium Supported by a Ferrocene-Linked Diphosphinoamide Ligand Scaffold

Zirconium dialkyl complexes of the general formula fc(NPiPr2)2ZrR2 (where fc = 1,1′-ferrocenyl, R = CH3, CH2Ph, CH2tBu, tBu) have been synthesized and characterized via the addition of alkyl lithium or potassium benzyl derivatives to the dichloride complex fc(NPiPr2)2ZrCl2(THF). Addition of 2,6-dimethylphenylisocyanide to these alkyl derivatives generates the corresponding mono iminoacyl alkyl zirconium complexes. On thermolysis, the iminoacyl moiety containing a benzyl substituent undergoes rearrangement to yield a new complex that contains an alkene-amido fragment. Mechanistic studies point to a 1,2 hydrogen shift as the rate-determining step.