Hydride transfer competition with alkylation

A case of the addition of an allylstannane to aldehydes has been reported by Tagliavini to proceed with appreciable enantioselectivity (Scheme 6.15) [40]. A notable feature of the Zr-catalyzed transformations is that they proceed more rapidly than the corresponding Ti-catalyzed processes reported by the same research team (see Scheme 6.16). Furthermore, C—C bond formation is significantly more efficient when the reactions are carried out in the presence of 4 A molecular sieves the mechanistic rationale for this effect is not known. It should be noted that alkylations involving aliphatic aldehydes are relatively low-yielding, presumably as the result of competitive hydride transfer and formation of the reduced primary alcohol. 

The strong competition between alkylation and hydride transfer appears in the alkylation reaction of propane by butyl cations, or butanes by the propyl cation. The amount of C7 alkylation products is rather low. This point is particularly emphasized in the reaction of propane by the terf-butyl cation, which yields only 10% of heptanes. In the interaction of isopropyl cation 31 with isobutane 2 the main reaction is hydride transfer from the isobutane to the isopropyl ion followed by alkylation of propane by the isopropyl ions (Scheme 5.20). 

As expected, initial studies on alkane hydrogenolysis found that these tantalum hydride species display a different alkane product distribution than the Zr-H species. Moreover, this catalyst was able to hydrogenolyse ethane into methane, suggesting a novel elementary step for this group-V transition metal an a-alkyl transfer occurs in competition with the observed fi-alkyl transfer for ZrH /SiOj [31]. These observations led to the discovery of silica-supported tantalum hydride [TaH /Si02] as an efficient catalyst for alkane metathesis [32]. To conduct this... 

Spontaneous proton elimination is unlikely, but /8-protons can be abstracted by basic compounds (solvent, counteranion, additives) in the system, including the monomer in competition with propagation (94). Mayr and co-workers (91) found that in the case of 1,2-dialkyl-substituted ethylenes, electrophilic attack on the double bond and hydride abstraction have comparable activation energies, so these systems are riddled with chain transfer, while in the case of 1,1-dialkyl and higher alkylated ethylenes the attack of carbocations at the C=C double bond is preferred. 

Some reactions can enter in competition with Reactions (3.1) to (3.3) described previously. These reactions are charge transfer (Reaction (3.4)) and the abstraction of a hydride or alkyl carbanion (Reactions (3.5) and (3.6)), the latter mainly observed with methane as the reagent. 

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