3/30/2023 0 Comments Isoprene jmolHowever, leaching was found to be much less significant if the Pd( II) or Pd(0) precursor was bound to the resin via a bis(diphenylphosphino)propane or bis(diphenylphosphino)methane ligands. 35 Similar problem of leaching was also observed with Pd(acac) 2 catalyst heterogenised on Merrifield resin, where catalysis was due to the leached homogeneous Pd( II) species. A polystyrene-bound Pd(0) triphenylphosphene catalyst was shown to be active in butadiene telomerisation with a number of nucleophiles, but loss of activity was significant already on the second recycle. 34 Leaching of Pd was found to be severe, leading to rapid deactivation of the catalysts. Thus, silica bound Pd( II) (SiO 2-acac-Pd( II)) was found to be active in the telomerisation of butadiene with methanol, with a two-fold excess of free alkyl phosphine ligand necessary to ensure selectivity to telomerisation. There have been a number of attempts to develop solid heterogeneous telomerisation catalysts. However, the high boiling point of carbonates does not allow easy product separation by flash distillation, which is a general problem of high-boiling point solvents. ![]() In the latter case the use of a cyclic carbonate solvent was advantageous over acetonitrile in terms of catalyst activity and selectivity. Gas–liquid telomerisation represents another type of heterogeneity telomerisation of gaseous butadiene with liquid acetic acid 33 and of gaseous carbon dioxide with liquid butadiene 27 have been demonstrated. In addition to aqueous phase, catalysts may also be immobilised in ionic liquids (IL), leading to the organic-IL biphasic systems. 4,6,17 A number of transition metal catalysed bi-phasic reactions involving decantation as the main separation stage have been successfully commercialised on a large scale, most notably the SHOP process, see e.g., ref. Because of the significant difference in molecular weight of reactants and products, and the possibility of using aqueous solutions of catalysts, the telomerisation reaction can be readily implemented as a bi-phasic aqueous/organic reaction, with the catalysts remaining in the aqueous phase. The feasibility of developing heterogeneous telomerisation reactions, with a considerably simplified work-up compared to homogeneous single phase catalysis, increases its attractiveness from an industrial point of view. 29 Recent advances in catalyst development, especially the introduction of highly active carbene catalysts with the means of controlling the selectivity between dimerisation and telomerisation products, see e.g., 7,30 should promote a variety of novel applications. It was also shown that longer-chain conjugated dienes could be telomerised 28 and internal long-chain dienes could be converted in tandem isomerisation–telomerisation reactions. 5 The synthetic potential of telomerisation reaction is exemplified by the wide range of nucleophiles shown to be active in telomerisation, such as alcohols, 7–10 amines and ammonia, 11–19 sugars 20–23 and polyols, 24–26 carbon dioxide, 27 etc. 1–5 However, thus far there are few commercial realisations, notably the Kuraray process of telomerisation of butadiene with water with consecutive double bond reduction to yield a linear 1-octanol 6 and the Dow Chemical Company telomerisation of butadiene with methanol to produce methoxyocta-2,7-diene which is then reduced to 1-octene. Introduction Dimerisation of dienes with a nucleophile, known as telomerisation, is a well-studied reaction with a significant synthetic potential. ![]() The loss of activity upon recycling the catalyst is due to the loss of associated (dvds) ligand during reaction and during the recycle process. The high activity is a function of the presence of the (dvds) ligand while the regioselectivity is attributed to the combination of (dvds) and PPh 3 ligands associated with the palladium centre. High catalytic activity for the telomerisation of isoprene with methanol with an unusual regioselectivity towards tail-to-tail telomerisation products was achieved with a heterogeneous (DVB-resin-PPh 3-Pd-dvds) catalyst synthesised by supporting palladium-(dvds) on a commercial triphenylphosphine–divinylbenzene resin, with potential for continuous operation.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |