Homogeneous Catalysis

We are studying catalysts which will give very high selectivity to desirable products often via cascade reactions. In Scheme 1, α,ω-diesters are formed in a single cascade reaction by the methoxycarbonylation of alkynes. The same catalyst catalyses the methoxy-carbonylation to the α, Β-unsaturated ester (the reaction can be stopped here), the double bond isomerisation and the second methoxycarbonyation, which only occurs when the double bond is in the least thermodynamically favoured terminal position.1

Squeme 1
          Scheme 1. Formation of dimethyl adipate from 1- butyne by a methoxycarbonylation - isomerisation - isomerisation Cascade sequence; also shown is the reductive amination of dimethyl adipate to N-phenylazacycloheptane

In a separate reaction, also shown in Scheme 1, α,ω-diesters can be converted by reductive amination to heterocycles. Related reactions are the formation of long chain α,ω-diesters from natural feedstocks (Scheme 2)2 and the catalytic hydrogenation of amides to amines (Scheme 3).3

Squeme 2
Scheme 2. Upgrading of methyl oleate to a polymer precursor
Squeme 3
Scheme 3. Hydrogenation of amides to amines

In addition, we have a major programme on new approaches to separating products from the catalyst in homogeneous reactions using biphasic systems involving aqueous, ionic liquid and supercritical fluid solvents. Recent studies have included additives to aqueous biphasic systems which give rate enhancements of 100 times without catalyst leaching or emulsion formation, catalysts which can be switched between water and organic phases by bubbling or removing CO2 (Figure 1) and supported ionic liquid phase catalysts with supercritical flow.4



Figure 1. Catalyst separation by transfer into and from water using CO2
Figure 1
Supported ionic liquid phase catalysts are composed of a thin layer of ionic liquid adsorbed onto the support pores, being the catalyst dissolved in the ionic liquid. This type of catalyst has been used to perform different reactions in a continuous flow mode with supercritical CO2 as carrier gas. The substrates, fed continuously, are dissolved into the supercritical CO2 and diffused into the ionic liquid layer to react with the catalyst and the products being also removed by the stream of supercritical CO2. The ionic liquid is insoluble in CO2 avoiding leaching of it and the catalyst (Figure 2). 
Figure 2. Squeme of  continuous flow SILP catalysis
This system has been succesfully applied to different reactions as metathesis, hydrogenations and carbonylations.5


    References:
  1. A. A. N. Magro, L. Robb, P. J. Pogorzelec, A. M. Z. Slawin, G. R. Eastman and D. J. Cole-Hamilton, 2010, 1(6), 723-730.
  2. C. Jimenez-Rodriguez, Graham R. Eastham and D. J. Cole-HamiltonInorg. Chem. Commun., 2005, 8, 878-881.
  3. A. A. Nez Magro, G. R. Eastham and D. J. Cole-Hamilton, Chem. Commun, 2007, 3154-6.
  4. S. L. Desset and D. J. Cole-Hamilton, Angew. Chem. Int. Ed., 2009, 48, 1472-1474.
  5. R. Duque, E. Ochsner, H. Clavier, F. Caijo, S. P. Nolan, M. Mauduit and D. J. Cole-Hamilton, Green Chem., 2011, 13, 1187-1195.
 
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