Enantioselective Catalysis, XV Preparative and Structural Chemistry of Diastereomeric Derivatives of 3‐Phosphanylpyrrolidine and Their Palladium(II) Complexes ‐ Asymmetric Grignard Cross‐Coupling Reaction

Abstract

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...

Chiral P,N-bidentate ligands in coordination chemistry and organic catalysis involving rhodium and palladium

“…The synthesis of P,N-bidentate ligands 67a ± k, containing a pyrrolidine ring, has been described. 102,103 Compound 67…”

“…67b, the chemical yield is 98% and the optical yield is 75%. 103 P,N-Bidentate ligands 68 and 69, containing a phosphinite group, have been described. 104 Palladium complexes formed by the ligands 68 and 69 proved to be inefficient in the cross-coupling of 1-phenylethylmagnesium bromide with vinyl bromide; the optical yield was only 6% ± 9% for a degree of conversion of 60% ± 74%.…”

New phosphine ligands with the OPN donor configuration

“…[7][8][9][10][11][12] Problems of functional group interference may be overcome in some cases by choosing an appropriate order for the introduction of the functional substituents; protection of the functional group before following substitutions; or by varying the type of reaction used for subsequent sequential substitutions at phosphorus. 11, [13][14][15][16][17][18] In the preparation of phosphines bearing two or three different substituents the selectivity of sequential substitutions is clearly important, 19-22 especially since mixtures of such phosphines having similar types of substituents also have similar physical and chemical properties often resulting in difficulties in isolation of the desired product. 23 Purification and synthetic techniques have notably improved, 2,9 and preparations involving protection of the phosphine as the borane adduct are becoming more common, especially for the synthesis of functionalised or asymmetric phosphines (e.g.…”

“…Phosphines which may potentially co-ordinate in a linear, tridentate manner with P, N and O atoms take one of three forms depending upon the order of the donors: PNO, 13,15,16,28-37 PON 15,16,38-40 or OPN. 16,26,[41][42][43][44][45][46][47] The syntheses of the PNO and PON type phosphines are relatively straightforward in most cases as they require only single phosphorus substitution to yield the desired product and reported examples are typically diphenylphosphine based. Notably, only four different types of OPN phosphines have been reported.…”

“…16,47 In most cases the ligands have been designed for homogeneous catalysis and therefore often have weaker donor arms which act as hemilabile chelates during catalysis, or to provide a chiral centre at phosphorus for purposes of asymmetric catalysis. 16,26,42,44,45 The syntheses of oxygen and nitrogen functionalised phosphines bearing one pyridine substituent and one cyclic ether or ester substituent are described herein. Asymmetric syntheses of these phosphorus chiral ligands were not attempted.…”