Rearrangements in Allylpalladium Complexes with Hemilabile Chelating Ligands

Abstract

In memoriam Professor Luigi M. VenanziIn addition to h 3 -to h 1 -allyl rearrangement, h 2 -to h 1 -chelating-ligand rearrangements can affect the dynamic properties in allylpalladium complexes containing hemilabile ligands. These rearrangements have the potential of altering stereochemistry, which can ultimately affect the stereochemistry and regiochemistry of reactions upon the allyl group, e.g., allylic alkylation. Apparent rotation of an h 3 -allyl in four-coordinate Pd IIcomplexes can be the result of ligand exchange or intramolecular processes. NMR Studies indicate that the effects of h 2 -to h 1 -chelating-ligand stereochemistry and rearrangements may be hidden or subtle, and both intraand intermolecular processes may be important.Introduction. ± The synthetic utility of transition-metal-catalyzed alkylations involving allyl intermediates has been shown to be effective for the regio-and stereocontrolled formation of CÀC bonds over the past thirty years [1] [2]. Asymmetric variants have been developed more recently [3]. Early work focused on the use of bisphosphane ligands, such as chiraphos [3], where C 2 -symmetric ligands were considered to have special advantages [4]. More recently, however, some of the most impressive enantioselectivities have been obtained with asymmetric P,N ligands, e.g., phosphinoaryloxazolines [5 ± 13]. In general, the complexes show dynamic NMR spectra, which indicate that the complexes are rearranging rapidly. The stereodynamics in these systems can be important in enantioselective allylic alkylations and has been reviewed recently [14], but many of the principles were established previously [3] [15 ± 19]. A number of rearrangements are possible that can affect the geometry. In earlier work, there was a tendency to overemphasize the importance of a C 2 -symmetric ligand for obtaining high levels of asymmetric induction. However, the excellent levels of enantioselectivity obtained with P,N ligands has demonstrated that asymmetric bidentate ligands can be as effective as, if not superior to, C 2 -symmetric ligands [5 ± 13].With a C 2 -symmetric ligand, such as chiraphos [3], a rotation of the allyl group or the ligand will yield an equivalent structure; hence, the presence or absence of a rotation mechanism may be irrelevant. With a non-C 2 -symmetric ligand, however, a different isomer will be produced and this has the potential to have significant consequences with regard to the resulting stereochemistry of the allyl product (see Fig. 1). Although, regiochemistry has often been determined on the basis of substitution patterns of the termini of the allyl, Hayashi et al. have recently shown that the regiochemistry of nucleophilic attack in some cases can be determined by differences in trans influence of the phosphine and the other ligand [20].

Reactivity of Rhodium(I) Iminophosphine Carbonyl Complexes with Methyl Iodide

“…If the 14-electron Rh species is highly reactive toward methyl iodide, the observed trend could result. Although we found no direct evidence for a chelate ring opening mechanism, hemilability of an iminophosphine ligand has been suggested to occur in Pd(II) allyl complexes . Another possibility, suggested by a reviewer, is that an agostic interaction between the Rh center and an ortho alkyl C−H bond can stabilize the S N 2 transition state for oxidative addition and hence accelerate the reaction.…”

Modular Approach to New Chiral Monodentate Diamidophosphite Ligands. Application in Palladium-Catalyzed Asymmetric Hydrovinylation of Styrene

“…Some relevant NMR spectral data are summarized in Tables S1 and S2. 31 P NMR spectra showed two sharp signals, with approximately the same ratio, arising from the two well-known isomers formed due to the different disposition of the chiral ligand and the allylic group around the metal center. For complexes 11f , 13b -( R,R al ) and 13b -( R,S al ), the ratio of isomers obtained from the 31 P NMR spectra was 1:1.5, 1:1.4, and 1:1.7, respectively.…”

Nickel(II) and Palladium(II) Complexes of the Small-Bite-Angle P-Stereogenic Diphosphine Ligand MaxPHOS and Its Monosulfide

“…The presence of isomers is due to the unsymmetrical nature of the MaxPHOS ligand and of the allyl group in the case of Pd5 . That means that two diastereomeric species can exist for Pd4 and Pd6 , depending on the orientation of the allyl ligand (which can be designated as R Pd and S Pd according to Faller and co-workers), , and in the case of Pd5 two more are possible, taking into account the two possible dispositions of the Ph group of the allyl ligand (cis/trans with respect to the P( t -Bu) 2 moiety). Each isomer features a pair of doublets in the 31 P NMR spectrum, and duplication of signals can also be observed in 1 H and 13 C NMR spectra, although these spectra are more complicated to analyze due to severe overlapping.…”