The carbonylation of allylic halides and prop-2-en-1-ol catalysed by triethylphosphine complexes of rhodium 

Abstract

In ethanol, [RhX(CO)(PEt₃)₂] added directly or formed in situ from [Rh₂(OAc)₄]·2MeOH (OAc = O₂CMe) and PEt₃ or [Rh(OAc)(CO)(PEt₃)₂] catalysed the carbonylation of CH₂=CHCH₂X (X = Cl, Br or I) to ethyl but-3-enoate with CH₂=CHCH₂OEt as a side product. Small amounts of the isomerisation product, ethyl but-2-enoate were produced but no base was required for the reaction. The selectivity of the reaction is in the order Cl > Br > I and prop-2-en-1-ol can be successfully carbonylated to prop-2-enyl but-3-enoate by the same system using 3-chloroprop-1-ene as a promoter. 3-Fluoropropene was not carbonylated, but in the presence of H₂ underwent hydroformylation to produce acetals. 3-Chlorobut-1-ene and 1-chlorobut-2-ene both produced ethyl pent-3-enoate and 3-ethoxybut-1-ene. In situ and ex situ NMR and IR spectroscopic studies have been used to show that the first step of the reaction is oxidative addition to give [Rh(CH₂CH=CH₂)Cl₂(CO)(PEt ₃)₂] for which thermodynamic parameters have been obtained. Both 3-chlorobut-1-ene and 1-chlorobut-2-ene give [Rh(CH₂CH=CHMe)Cl₂(CO)(PEt₃) ₂] but with different E∶Z ratios. The detailed mechanism of the oxidative addition is discussed. The CO inserts into the Rh–C bond to give [Rh(COCH₂CH=CH₂)Cl₂(CO)(PEt ₃)₂], from which but-3-enoyl chloride reductively eliminates to react with ethanol to give the observed products. High-pressure IR and high-pressure NMR studies reveal that [RhX(CO)(PEt₃)₂] (X = Cl or Br) reacts with CO to give [RhX(CO)₂(PEt₃)₂], which exists as two isomeric forms. The compound [Rh(OAc)(CO)(PEt₃)₂] catalyses the formation of prop-2-enyl ethanoate from 1-chloroprop-2-ene and sodium ethanoate. A mechanism is proposed.

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