Scandium complexes alkyl compounds

Two notable points from the aforegoing discussion are as follows. (1) By far the majority of the known monohalogenoalkyl compounds are of the group VTII transition elements there are very few early transition metal halogenoalkyl compounds known. (2) Very few monofluoroalkyl metal complexes have been prepared. The lack of early transition metal halogenoalkyl compounds may be in part due to the high electropositivity of these metals, which facilitates a- and / -elimination reactions. Related hydride elimination reactions almost certainly occur more easily for early transition metal alkyl compounds than for later transition metal compounds. In this regard it is particularly noteworthy that one of the only early transition metal haloalkyl compounds mentioned is the fluoroethyl scandium com-... 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

Much work in the review period has concerned enantioselective substitution in five-membered heterocyclics. The enantioselective alkylation of some pyrroles by unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(in) triflate complex (31) has been reported.39 Compound (33) is formed in 98% yield and 94% ee from the 2-acylimidazole (32) and pyrrole at —40 °C. A series of enantiomer- ically pure aziridin-2-ylmethanols has been tested as catalysts in the alkylation of /V-mclhylpyrrolc and (V-methylindole by ,/l-unsalura(cd aldehydes.40 Enantiomeric excesses of up to 75% were observed for the alkylation of /V-mcthylpyrrole by ( >crotonaldehyde using (2.S ,3.S )-3-mclhylazirin-2-yl(diphenyl)methanol TFA salt as catalyst to form (34). 

In this article the term organometallic compound includes alkyl and aryl derivatives of the rare earths—the transition metals of group III, scandium, yttrium, lanthanum and the lanthanides cerium to liitetium with covalent metal-to-carbon a-bonds, as well as the so-called 77-complexes with more than monohapto metal-to-carbon bonds, for example cyclopentadienyl and olefin complexes, metal acetylides, but not carbonyls, cyanides and isocyanide complexes. Derivatives of scandium, yttrium and lanthanum are included and discussed together with the compounds of the lanthanides, because of many similarities in the synthesis and the chemistry of these organometallic derivatives of the rare earths. 

Some of the first reactions of soluble metal complexes with methane occurred by a-bond metathesis. Like the first examples of oxidative addition of alkyl C-H bonds, the first examples of a-bond metathesis with alkyl C-H bonds were intramolecular. Yet, the lute-tium- and yttrium-methyl complexes, Cp MMe (M = Lu and Y) were shown by Watson to react intermolecularly with C-labeled methane to form the labeled methyl complexes and unlabeled methane at 70 °C (Equation 6.51). Related scandium compounds have now been shown to undergo similar reactions with alkanes, and a thoracyclobutane... 

Metal vinyl and aryl complexes are two typical classes of products resulting from sp C—H bond activation from alkene and arene substrates, respectively. Bercaw and coworkers conducted a systematic study of the reactions between Cp 2ScR (R = alkyl or hydride) and various hydrocarbons and found that while ethylene underwent rapid polymerization with Cp 2ScMe, substituted alkenes, such as styrene and isobutene, were readily activated through cr-bond metathesis to result in scandium vinyl compounds (Scheme 8A). One... 

Further experimental proof for the cationic active species with an t -coordinated ring and an alkyl ligand was given by employing the isolated and structurally characterized cationic complex 50 (with the borate anion used in the activation process, Fig. 7.5). Virtually the same polymerization behavior is found when activation of the neutral complex is performed. The addition of 1 equiv of 1,2-dimethoxyethane (DME) to that complex results in a well-defined compound but renders the cation inactive as the DME coordinates to the scandium and efficiently blocks the active center. 

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