Insertion and isomerisation of internal olefins at alkylaluminium hydride: catalysis with zirconocene dichloride

Abstract

The insertion of internal olefins (hydroalumination) and chain walking isomerisation at di-n-octylaluminium hydride [Al(Oct)2H], promoted by zirconocene dichloride [Cp2ZrCl2] has been studied. The reaction between [Cp2ZrCl2] and [Al(Oct)2H] in non-polar solvents leads to clusters containing bridging hydride ligands between Zr and Al. This system promotes hydroalumination of 1-octene but is largely ineffective for internal octenes (2-, 3-, 4-octene). In tetrahydrofuran the Zr-Al hydride clusters formed are more reactive and catalyse insertion and isomerisation of internal olefins to primary metal-alkyls, although this is accompanied by catalyst deactivation. Elimination and removal of 1-octene from the system post insertion/isomerisation was attempted, but it was found that the presence of the Zr catalyst leads to back-isomerisation to internal octenes, along with further decomposition with n-octane formation. Some possible pathways of catalyst decomposition, involving reduction of Zr and alkane elimination, have been studied theoretically.

Zirconium-Catalyzed Hydroalumination of C═O Bonds: Site-Selective De-O-acetylation of Peracetylated Compounds and Mechanistic Insights

“…These preliminary mechanistic studies in THF also revealed that a mixed metal hydride should be involved. However, since Zr/Al clusters have not been identified in THF, , we wanted to transpose the de- O -acetylation reaction in toluene to identify the active species. Indeed, in this apolar solvent, extensive structural and reactivity studies have been performed to rationalize zirconium-catalyzed hydroalumination of alkenes by mixtures of Cp 2 ZrCl 2 and aluminum hydrides .…”

“…The formation of clusters from mixtures of Cp 2 ZrCl 2 and organoaluminum compounds in THF has previously been monitored by 1 H NMR spectroscopy, but the structure of these aggregates remains elusive in this polar solvent . On the other hand, several zirconium/aluminum mixed metal hydrides were fully characterized in apolar solvents, and extensive reactivity studies revealed their critical role in zirconium-catalyzed hydro- and carboaluminations of alkenes and alkynes .…”

Cobalt-bis(imino)pyridine complexes as catalysts for hydroalumination–isomerisation of internal olefins

“…15,[38][39][40][41][42] This may be one reason for the poor performance of this system in hydroalumination, relative to hydroboration. While the Co-hydride signal appears as a single peak, the ligand resonances show that two very similar species are present, a major and a minor component.…”

“…[13][14][15] The cycle relies on the tendency of some transition metal and group 13 metal secondary alkyls to rearrange to the thermodynamically preferred primary alkyl by way of chain walking. [13][14][15] The cycle relies on the tendency of some transition metal and group 13 metal secondary alkyls to rearrange to the thermodynamically preferred primary alkyl by way of chain walking.…”

“…13 For alkylaluminium the situation is reversed and it was demonstrated that α-olefins may be removed from tri-nalkylaluminium with very high selectivity (>97%). [22][23][24] Following on from an investigation into a zirconocene based catalyst, 15 we were drawn to the work of Chirik and others who have demonstrated that Fe and Co complexes of bis(imino)pyridine ligands are effective catalysts for hydroboration and hydrosilylation. 14 As a consequence, the use of transition metal catalysts to promote insertion and isomerisation has been investigated.…”