Insertion, elimination and isomerisation of olefins at alkylaluminium hydride: an experimental and theoretical study

Abstract

The insertion, elimination and isomerisation of octenes with di-n-octylaluminium hydride [HAl(Oct)2], tri-n-octylaluminium [Al(Oct)3] and sec-octylaluminium species have been studied as individual steps in a putative aluminium based contrathermodynamic olefin isomerisation process. While elimination of 1-octene from [Al(Oct)3] is energetically unfavourable, the process is driven by high temperature vacuum distillation, leading to very high selectivity to 1-octene (>97%). At high conversions the [HAl(Oct)2] so obtained exists predominately as hydride-bridged cyclic oligomers, whereas at low conversion the mixed alkyl/hydride-bridged dimer [(Oct)2Al(μ-H)(μ-Oct)Al(Oct)2] is the major species. Di-n-octylaluminium hydride recovered after olefin elimination may be recycled and is active toward re-insertion of octenes. Internal octenes (cis- and trans-2-, 3- and 4-octene) only partially insert however, and even after prolonged heating there is no significant secondary to primary alkyl isomerisation evident.

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

“…14 In contrast, in the presence of catalytic 5 or 6 (5-10 mol%), insertion proceeds rapidly at room temperature and is essentially complete (>97%) within 15 minutes. We previously demonstrated that at 75°C 1-octene inserts with a half-life of around 30 minutes.…”

“…[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.…”

“…In the final step, β-hydrogen elimination together with rapid removal of the olefin furnish the α-olefin selectively. 14 As a consequence, the use of transition metal catalysts to promote insertion and isomerisation has been investigated. 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%).…”

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

“…12 The conversion of internal olefins into α-olefins, which is a contrathermodynamic process, is therefore required in order to develop an overall n-alkane to LAO conversion process. For this reason, we have recently 13,14 been investigating the individual steps of the cycle shown in Scheme 1 as a potential method for internal to α-olefin conversion, which involves three steps. The first is insertion of an internal olefin into a metal-hydride bond to yield a sec-alkylmetal compound.…”

“…17,18 On the other hand, we showed that α-olefins can be removed from tri-n-alkylaluminium with very high selectivity (>97%), but in this case insertion and isomerisation of internal olefins was unsuccessful. 14 The reaction of [Al(n-Alk) 2 H] with internal olefins reaches an equilibrium position (ca. 50% insertion), and isomerisation to the more stable primary alkyl is too slow to drive the reaction to completion.…”