The methoxycarbonylation of alkenes by palladium(II) complexes with P,O‐donor ligands [(2‐methoxyphenyl)diphenylphosphine (L‐2), bis(2‐methoxyphenyl) phenyl phosphine (L‐3) and tris(2‐methoxyphenyl) phosphine (L‐4)] has been investigated. The results show that the Pd complexes derived from these ligands provide high regioselectivity for the branched esters from 1‐pentene and 1‐hexene (>80 %). Various parameters (including temperature, pressure, acid concentration) were optimized to improve the performance of the catalyst system. Higher temperatures afforded higher regioselectivity; but effected rapid catalyst decomposition. Acceptable turnover frequencies, conversions as well as catalyst stability could be obtained at higher L/Pd ratios. The dramatic change in regioselectivity is rationalised on the basis of the hemi‐lability of the o‐methoxy moiety, which may lead to ligand dissociation from L2PdX2 (L=ligand, X=Cl) rather than the expected dissociation of X. In support of our hypothesis, direct evidence for the coordination of the o‐methoxy to the Pd centre was demonstrated by the crystal structure. To the best of our knowledge, this work provides the first reported route to valuable branched esters through the methoxycarbonylation of alkenes at suitable rates.

The methoxycarbonylation reaction provides ar oute to the synthesis of esters from medium-chain alkenes that may be used as fuel supplements. However,t he known productivec atalytic systems are expensive and/or unstablea te levated temperatures. Most of the data availableo nt he methoxycarbonylation of alkenes is derivedf rom ethylene and styrene as substrates. To broaden the scope, we conducted ac omparative study of ar ange of phosphine ligandsu nder comparable conditions fort he methoxycarbonylation of 1-octene. The results demonstrate that anumber of ligand structural motifs facilitate the process effectually.F urthermore, the critical importance of alkene isomerization and the acid/liganda nd Pd/ligand ratios are presented.Carbonylation reactions provide an efficient route to generate av ariety of functionalg roups from CÀCu nsaturation by using readily available carbon monoxidea nd other ancillary reagents.[1] Amongst others, these metal (Pd, Co, Ni, Rh, etc.) [2] catalyzed carbonylation reactions include the syntheses of aldehydes (hydroformylation), [3] carboxylic acids (hydrocarboxylation), [4] and esters (hydromethoxycarbonylation or methoxycarbonylation for short, Scheme 1). [5] Twom echanisms,n amely,P d ÀHa nd PdÀOMe, have been proposed for the methoxycarbonylationr eaction, each with much study and evidencei nits favor.[5b] However,m ost evidence collected thus far indicatesapreference for the PdÀH mechanism, as shown in Scheme 2. [5c,d] Despite the fact that long-chain esters play ac rucial role in the manufacture of lubricants and surfactants [6] and are eligible candidates for petroleum derivatives, these higher-carbonl inear aliphatic esters are typicallyp roduced by furtherm anipulations of the products of the hydroformylation reaction.[7] However,m uch effort is being devotedt oc atalytic methoxycarbonylation with respect to the utilization of renewable resources. [8] As South Africah as no naturals ource of liquid fuel, there is an opportunity and an eed to convert its (Sasol) FischerTropsch medium-chaino lefins into their correspondingm ethyl esters as substitutes for diesel fuels. Althoughanumber of catalytic systemsh ave been developed for the required methoxycarbonylation, industrial implementation with regard to medium-chain alkenes has not yet been realized. In addition to patent protection,t he known catalysts suffer from an umber of disadvantages, includingh igh cost. Most often, these systems comprise Pd 2 (dba) 3 (dba = dibenzylideneacetone)a nd the ligands( andt heir analogues) shown in Figure 1. [8, 9] The use of ligand A in the methoxycarbonylation of ethylene is aw ellknown industrial process (Lucite process).[10i]The requirements for ap otential industrial catalystw ould include low cost, efficiency in terms of reactionr ates and selectivity,h ight emperature stability, and the capability of recycling. The development of ac atalysts ystem of this kind can be expedited by ab etter understanding of the critical factors, in particular, ligand structure and reaction con...

The unnatural selection: The use of aluminum triflate as a promoter for the Pd catalyzed methoxycarbonylation of 1‐octene allowed an investigation into the role of the acid and counterion on the activity and selectivity of the catalyst. With this promoter there is no counterion effect at all for AcO−, p‐TsO−, MsO−, or TfO− counterions. The results imply that the nature of the acid determines catalyst activity, assuming the presence of a weakly coordinating counterion.

The palladium diacetate–aluminium triflate combination is an effective catalyst for the hydromethoxycarbonylation reaction of isolated alkenes and alkynes. However, its application in the hydromethoxycarbonylation of oxygen atom‐bearing alkynes is non‐trivial: the position relative to the alkyne (e.g., allylic, homoallylic) and nature of the oxygen atom determine the success of the transformation. We detail the minimum structural requirements for a successful outcome with ether‐containing alkyne substrates and hydroxyl‐containing analogues. In general, the oxygen atom has a detrimental effect on the outcome of the hydromethoxycarbonylation reaction, which can be overcome in many instances. The presence of the oxygen atom(s) causes decreases in the rate of the reaction compared to aliphatic alkynes and the closer the oxygen atom is to the triple bond, the more pronounced the effect. The chemistry is demonstrated on complex substrates derived from 5 and 6‐carbon sugars in optimised reactions.

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