The preparation of both diastereomeric derivatives of 3-(diphenylphosphany1)pyrrolidine with chiral (tetrahydrofuran-2-y1)methyl and [ (N-neopentyl)~iyrrolidin-2-yl]methyl groups as substituents on the pyrrolidine nitrogen atom and of (2S,4S) -l-benzyl-4-(diphenylphosphanyl)-2-(methoxymethy1)pyrrolidine is reported. [3S,P(RS)]-3-(phenylphosphanyl)pyrrolidine, bearing an additional chiral center on phosphorus, is the starting material for the preparation of phosphancs, in which one phenyl group of the PPh2 moiety is substituted by an 2-methoxyphenyl (L An) or 2,4,6-trimethoxyphenyl (= TMP) group. PdIz complexes of these ligands were separated into diastereomers by chromatography on silica gel columns. The structural chemistry of these novel phosphane diastereomers and their PdIa complexes is investigated by X-ray crystallography and NMR. At the P,Ncoordinated palladium center displacement of an iodide anion is found for P,N,N' ligands only. In the nickel complex catalysed cross-coupling reaction, yielding 3-phenyl-l-butene, we obtain the highest enantioselectivities in the case of simple l-alkyl-3-(diphenylphosphanyl)pyrrolidine ligands. The enantioselectivity obtained with diastereomeric dcrivatives, bearing additional ether or amine ligating sites is mainly determined by the chiral center in 3-position of the 3-(phosphany1)pyrrolidine part of these ligands. Optimisation of enantioselcctivity with these iigands can be carried out by a variation of the ligand to nickel ratio and by the choice of the vinyl halide used as starting compound. The catalytic cycle must contain at least one catalytically active species, bearing more than one 1kiminoalkylphosphane ligand.Carbon -carbon bond formation by cross-coupling of main group organometallics with carbon electrophiles catalysed by transition metal complexes is a valuable tool in organic chemistry"]. The cross-coupling of vinyl halides l a , l b with Grignard compound 2a preparcd from raccmic 1-chloro-1-phenylethane yields 3-phenyl-1 -butene (3)C2s31 (cf. Scheme 1). The generally accepted catalytic cycle of Grignard cross-coupling catalysid2] involves oxidative addition of a vinyl halide to a nickel(0) complex, transmetallation from the Grignard compound and reductive elimination of 3-phcnyl-1 -butem (3) regenerating thc nickel(0) complcx. The cross-coupling reaction using achiral nickel monophosphane (NILL,) coinplexes141 as catalysts has been investigated by Yamamotolsl. Thermodynamically more stable NiL2 complexes with tmns-coordinated phosphane ligands have to be isoinerised by associative mechanisms to the cis complexes before reductive elimination can occur. This truns to cis isomerisation can be promoted by the coordination of l'urther phosphane ligands or by transmetallation steps. Reductive elimination from the resulting NIL-, complcx in which alkyl groups arc cis-oriented may also be induced by coordination of further phosphane ligandd51.In the asymmetric version of this cross-coupling reaction enantioselectioii can take place cither during reductive elimina...
An investigation of the asymmetric synthesis of 3-phenyl-lbutene by the Grignard cross-coupling reaction is presented. The reaction is catalysed by nickel complexes of bisphosphane and 3-diphenylphosphanylpyrrolidine-type ligands. A comparison of the results obtained with our P,N monophosphane ligands with those obtained with the most effective known amino acid derived P,N ligands shows a similar enantioselectivity but an inverse sense of optical induction. An Xray structural analysis of the 1 -phenylethyl Grignard compound is reported. Quantitative analysis and the detennination of enantiomeric composition of the catalytic samples is accomplished using a novel enantioselective GLC separation. Compared to the popular model reaction that uses a chloride-containing Grignard compound and vinyl bromide as starting compounds, we obtain improved enantioselectivities of P,N monophosphane catalysts by substituting vinyl bromide with vinyl chloride. With two of the new P,N ligands we find a nonlinear dependence of enantioselectivity on the enantiomeric purity of the ligands (asymmetric amplification). Catalytic results subject to such nonlinear effects show a dependence on the ligand to nickel ratio.
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The preparation of both enantiomers of 3-diphenylphosphanylpyrrolidine (2) and several N-substituted derivatives together with two Pdn complexes of this ligand is reported. From L-malic acid and L-hydroxyproline both enantiomers of 3-hydroxypyrrolidine are prepared without any problems due to epimerization. KPPh, in the presence of LiCl is shown to be the most effective reagent for the synthesis of 2. The reported X-ray stnicture determinations of Pd12 complexes show a rather rigid bicyclic hetero-norbornane skeleton. The flexibility of the other parts of the molecules is obvious in several polymorphs revealed by this method. This polymorphism is additionally investigated by a 31P-CP-MAS study. From solution IH-, 13C-and 31P-NMR studies it is concluded that the bicyclic hetero-norbornane skeleton is retained in solution.Over the past two decades we have used rhodium complexes with ligands of the (3 R,4R)-3,4-bis(diphenylphosphany1)pyrrolidine (1) family"] (shown in Scheme 1) as very efficient catalysts in the enantioselective hydrogenation of a-acetylaminocinnamic acid. The nitrogen atom in the backbone has been used for the synthesis of water-solubld21 or polymer-atta~hed [~] ligands. In all of these investigations we never found any strong influence of this nitrogen atom on the enantioselectivity achieved in hydrogenation. A second family of pyrrolidinebisphosphanes e.g. BPPM has been used by AchiwaL41. In this paper, we report on the synthesis of a third, structurally related family of chiral pyrrolidinephosphanes, which are derivatives of 3-diphenylphosphanylpyrrolidine (2). Our synthetic strategy is shown in Scheme 1. The synthesis of enantiomerically pure 2 should facilitate an easy access to many N-substituted derivatives (e. g. 2a-c). Furthermore, the synthesis of both enantiomers of 2 was required for further extension of this chemistry.The application of derivatives of 2 in enantioselective catalysis is of great interest because of the bridged p-aminoalkylphospane skeleton. Kumada and Hayashi showed that nickel complexes of monophosphanes with an amino group in the P-position are clearly superior to 1,2-bisphosphanes (e.g. 1) in the enantioselective cross-couplingWe were further interested in the structural chemistry of N-alkylated derivatives of 2 and their Pd12 complexes. A recent paper by Pregosin and co-workersI61 clearly proved a different solution structure of the PdCI2 complex of a "Hayashi-type'' P,N ligand compared to the structure obtained by X-ray crystallography. We report here our results. Io1Part XII: U. Nagel, J. Leipold, Chem. Bet 1996,129, 815-821. Synthesis of the Ligands and PdIz ComplexesThe synthesis of the (R)-3a enantiomer is known in the literature [7]. Therefore, it is only necessary to mention the modifications made to the well-known synthesis shown in Scheme 2. The crude (R)-3hydroxypyrrolidine, obtained by 2-cyclohexen-1 -one catalyzed decarboxylation of L-hydroxyproline was extracted with aqueous acetic acid. Block-
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