Allylpalladium Complexes with P-Stereogenic Monodentate Phosphines. Application in the Asymmetric Hydrovinylation of Styrene

Abstract

A group of P-stereogenic monodentate phosphines S-PPhRR′ (R ) 1-naphthyl, 9-phenanthryl, or o-biphenylyl and R′ ) CH 3 -, i-C 3 H 8 -, and Ph 3 SiCH 2 -) have been prepared by succesive substitution reactions on the oxazaphospholidineborane obtained from (-)ephedrine and bis(N,N-diethylamino)phenylphosphine. The reaction with binuclear allyl compounds [Pd(µ-Cl)(allyl)] 2 gives neutral [PdCl(allyl)P*] complexes. When allyl ) 2-CH 3 -C 3 H 4 (5), two isomers appeared in solution due to the R-or S-geometry around the palladium atom. The discrimination effect of the phosphines is small and the maximum isomeric ratio is observed for PPh(o-Ph 2 )(CH 2 SiPh 3 ). The molecular structure determined by X-ray diffraction of two complexes with P* ) PPh(o-Ph 2 )(i-Pr) and PPh(o-Ph 2 )(OMe) showed a very similar nonsymmetric coordination of the allyl moiety according to the greater trans influence of the phosphorus atom. When allyl ) 1-C 6 H 5 -C 3 H 4 (6), the NMR spectroscopy showed up to four isomers due to the R-or S-geometry around palladium and the Z-or E-disposition of P* and the phenyl substituent of the allyl moiety. The E-isomers are the major species in solution, unique with PPh(o-Ph 2 )(CH 2 SiPh 3 ). The usual, well-defined dynamic exchanges by π-σ-π and pseudorotation of the allyl moiety have been observed. The codimerization reaction between styrene and ethylene has been tested using filtered CH 2 Cl 2 solutions of [PdCl(2-CH 3 -C 3 H 4 )P*] (5) complexes and AgBF 4 as catalytic precursors. Moderate activity (TOF < 225 h -1 at 25 °C) and good selectivities to 3-Ph-1-butene (∼90% at 80% conversion) are obtained. The ee is moderate (<40% ee) and different from the discrimination effects observed in the solutions of neutral complexes [PdCl(ally)P*]. The reaction carried out with deuterated styrene shows the clean C-H addition to the vinyl double bond of stryrene and confirms the irreversible nature of the insertion of styrene in the palladium hydride intermediate. The hydrovinylation reaction using substituted styrene with a potentially secondary coordination atom occurs only when the substitution is in the phenyl ring and without significant improvements of the ee.

Better Performance of Monodentate P‐Stereogenic Phosphanes Compared to Bidentate Analogues in Pd‐Catalyzed Asymmetric Allylic Alkylations

“…[14] (3) 13 C chemical shifts for the allylic carbon atoms show that the central atom resonates at a lower field than the terminal ones and that the carbon atom located at a position trans to phosphorus is less shielded than the cis atom, and is coupled to the phosphorus atom. Compared with the analogous [PdCl(η 3 -2-Me-C 3 H 4 )(P)] complexes, [26] the carbon atoms trans to phosphorus are about 10 ppm more deshielded in the diphenylallyl complexes. Complex 6d shows a large shift to lower fields in the allylic carbon atom cis to phosphorus, possibly as a result of the proximity of the ophenyl group of the phosphane, as reflected in the crystal structure (see below).…”

“…[26] From borane-protected methylphosphanes aЈ, cЈ and eЈ diphosphanes were prepared by the activation of a methyl proton by sec-BuLi, followed by a coupling reaction using CuCl 2 , or directly with SiCl 2 Me 2 . Both methods have been described elsewhere.…”

“…Complex [26] For this reason the neutral allylic complex [PdCl(η 3 -2-Me-C 3 H 4 )(f)] was tested as a precursor in the reaction (entries 7 and 8). It showed a lower but still remarkable ee and an initially lower rate of reaction, as expected for the less favoured initial nucleophilic attack on a neutral intermediate.…”

“…Discrimination between them increases with the size of the phosphane (6d Ͼ 6a), but it is similar to that observed in the analogous [PdCl(η 3 -2-Me-C 3 H 4 )(P)] complexes. [26] NOESY spectra showed dynamic exchange between the groups of hydrogen atoms. In particular, the exchange between anti-allylic protons of the two isomers indicates that the allyl pseudorotation is the main operating mechanism of exchange.…”

A Versatile Synthesis of β‐Lactam‐Fused Oxacycles through the Palladium‐Catalyzed Chemo‐, Regio‐, and Diastereoselective Cyclization of Allenic Diols

“…It has been reported that solutions of related complexes [29] contain only the isomer in which the PR 3 is trans to the more substituted allylic carbon. In other studies, both trans and cis isomers were observed, [30] such as for complexes PdI[CH 2 C(Ph)CHR](PPh 3 ).…”