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

Abstract

In ethanol, [RhX(CO)(PEt 3 ) 2 ] added directly or formed in situ from [Rh 2 (OAc) 4 ]ؒ2MeOH (OAc = O 2 CMe) and PEt 3 or [Rh(OAc)(CO)(PEt 3 ) 2 ] catalysed the carbonylation of CH 2 ᎐ ᎐ CHCH 2 X (X = Cl, Br or I) to ethyl but-3enoate with CH 2 ᎐ ᎐ CHCH 2 OEt 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 H 2 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(CH 2 CH᎐ ᎐ CH 2 )Cl 2 (CO)(PEt 3 ) 2 ] for which thermodynamic parameters have been obtained. Both 3-chlorobut-1-ene and 1-chlorobut-2-ene give [Rh(CH 2 CH᎐ ᎐ CHMe)Cl 2 -(CO)(PEt 3 ) 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(COCH 2 CH᎐ ᎐ CH 2 )Cl 2 (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)(PEt 3 ) 2 ] (X = Cl or Br) reacts with CO to give [RhX(CO) 2 (PEt 3 ) 2 ], which exists as two isomeric forms. The compound [Rh(OAc)(CO)(PEt 3 ) 2 ] catalyses the formation of prop-2-enyl ethanoate from 1-chloroprop-2-ene and sodium ethanoate. A mechanism is proposed. ExperimentalThe NMR spectra were recorded on a Brüker AM300 spectrometer operating in the Fourier-transform mode with, † In honour of Sir Geoffrey Wilkinson, FRS, a great friend and mentor as well as a superb chemist.

A Novel Five‐Coordinate Rhodium(I) Complex

“…The NMR spectroscopic data for 1 are in a good agreement with the data obtained for the above compound [3] [ 31 P{ 1 H} 34 ppm, 13 C{ 1 H} 191.1 ppm, 1 J(Rh-P) = 91 ppm, 1 J(Rh-C) = 70 Hz, 2 J(C-P) = 16 Hz], and it seems very probable that in the early study [3] the high-frequency band has been missed in the spectrum either because of low sensitivity of the instrument used or due to low concentration of the sample. It has to be also noted that structurally analogous complexes of cobalt [15] and iridium, [16] as well as the chloride and bromide rhodium derivatives [5] {[Rh(CO) 2 -(PPh 3 ) 2 X]} also display two absorption bands in the carbonyl region with essentially similar location and relative intensities. A very similar trigonal-bipyramidal environment of the Rh I center has been recently found [17] in the dinuclear rhodium complex [Rh(CO) 2 I(PP)] 2 where two rhodium atoms are bridged by a long-chain diphosphane (PP) ligand with the phosphorus functionalities in axial positions.…”

“…Two other possibilities are represented by tetragonal pyramids with cis and trans arrangement of the ligands in the basal plane. [5] The disagreements in the experimental NMR spectroscopic data can be partially explained by fast exchange between the isomeric species [5] or by dissociative equilibria with the parent square planar [Rh(CO)(PPh 3 ) 2 (Hal)] complexes. [4] The importance of a clear assignment of the structures of this sort lies in the general significance of five-coordinate Rh I complexes in terms of the boundary between four-coordinate planar and five-coordinate (usually trigonal bipyramidal, sometimes square pyramidal) d 8 complexes in ge-neral, and their occurrence as intermediates in associative substitution reactions of planar Rh I complexes and in similar reactions proposed for catalytic systems.…”

“…Five-coordinate [Rh(CO) 2 (PR 3 ) 2 (Hal)] rhodium complexes had previously been reported [2][3][4][5] but were characterized only by NMR and IR spectroscopy. The spectroscopic data obtained for these complexes were not in good agreement but they are all compatible with four symmetrical structures described below.…”

“…[4] The importance of a clear assignment of the structures of this sort lies in the general significance of five-coordinate Rh I complexes in terms of the boundary between four-coordinate planar and five-coordinate (usually trigonal bipyramidal, sometimes square pyramidal) d 8 complexes in ge-neral, and their occurrence as intermediates in associative substitution reactions of planar Rh I complexes and in similar reactions proposed for catalytic systems. [6] In particular, it has been clearly shown [5] + cation [7] (trigonal-bipyramid with trans P donors), and the other is a closely analogous dinuclear [Rh(CO) 2 X(PP)] 2 neutral complex, where two Rh I centers are bridged by two long-chain bidentate diphosphane (PP) ligands. This has a trigonal-bipyramidal structure, with trans P donors.…”

“…The spectroscopic data obtained for these complexes were not in good agreement but they are all compatible with four symmetrical structures described below. Two structures are based on a trigonal-bipyramidal motif [2][3][4][5] with either two COs or two phosphane ligands in the axial positions. Two other possibilities are represented by tetragonal pyramids with cis and trans arrangement of the ligands in the basal plane.…”