Oxazoline-Mediated Interannular Cyclopalladation of Ferrocene: Chiral Palladium(II) Catalysts for the Enantioselective Aza-Claisen Rearrangement

Selective Manipulation of Well-Defined Trinuclear Pd(II)-Complexes

Several strategies are available to design well-defined multimetallic molecular entities bearing functional ligands. Substoichiometric exchange reactions in the coordination sphere of pre-existing multinuclear precursors are relatively underexploited in this context. Palladium(II) acetate is not a mononuclear compound in the solid state but rather exists as a trimer, i. e. [Pd3(OAc)6]. Although this material is ubiquitously used to synthesize mononuclear Pd species, it may principally also lend itself to selective exchange of some of the edge-sharing acetate units in its triangular motif, whilst keeping the overall multinuclear architecture intact. Strikingly, little is known about the controlled manipulation and substoichiometric substitution chemistry of this well-defined conglomerate. We herein conclusively demonstrate that, for the first time, the targeted exchange of two or four acetate units from the Pd3(acetate)6 platform is possible, thereby installing either one or two new tridentate ligands onto this trinuclear architecture. Follow-up exchange and substitution chemistry is available without disrupting the multimetallic nature of the core structure. New complexes 2-7 are all conclusively characterized using multinuclear NMR spectroscopy, UV-vis and IR spectroscopy as well as X-ray diffraction analysis.

Advanced Application of Planar Chiral Heterocyclic Ferrocenes

This manuscript is reviewing the superior catalytic activity and selectivity of ferrocene ligands in a wide range of reactions: reduction of ketones, hydrogenation of olefins, hydroboration, cycloaddition, enantioselective synthesis of biaryls, Tsuji–Trost allylation. Moreover, the correlation between a ligand structure and its catalytic activity is discussed in this review.

Recent Advances in Transition-Metal-Catalyzed C-H Functionalization of Ferrocene Amides

During the past decades, in synthetic organic chemistry, directing-group-assisted C-H functionalization is found to be a key tool for the expedient and site-selective construction of C-C and hybrid bonds. Among C-H functionalization of ferrocene derivatives, the directed group strategy is undoubtedly the most commonly used method. Compared to the other directing groups, ferrocene amides can be synthesized easily and are now recognized as one of the most efficient devices for the selective functionalization of certain positions because its metal centre permits fine, tuneable and reversible coordination. The family of amide directing groups mainly comprises monodentate and bidentate directing groups, which are categorized on the basis of coordination sites. In this review, various C-H bond functionalization reactions of ferrocene using amide directing groups are broadly discussed.

Stereospecific Asymmetric Synthesis of Tertiary Allylic Alcohol Derivatives by Catalytic [2,3]-Meisenheimer Rearrangements

Chiral acyclic tertiary allylic alcohols are very important synthetic building blocks, but their enantioselective synthesis is often challenging. A major limitation in catalytic asymmetric 1,2-addition approaches to ketones is the enantioface differentiation by steric distinction of both ketone residues. Herein we report the development of a catalytic asymmetric Meisenheimer rearrangement to overcome this problem, as it proceeds in a stereospecific manner. This allows for high enantioselectivity also for the formation of products in which the residues at the generated tetrasubstituted stereocenter display a similar steric demand. Low catalyst loadings were found to be sufficient and the reaction conditions were mild enough to tolerate even highly reactive functional groups, such as an enolizable aldehyde, a primary tosylate, or an epoxide. Our investigations suggest an intramolecular rearrangement pathway.

Bi and trinuclear complexes in palladium carboxylate-assisted C-H activation reactions

The role of polynuclear species in C-H activations assisted by palladium carboxylates has not been clear so far. The summary of the key findings covering this issue shows its important role under certain conditions. However, much more effort is necessary for a deeper understanding of the whole issue.

Palladium(II)-Catalyzed Enantioselective Reactions Using COP Catalysts

Allylic amides, amines, and esters are key synthetic building blocks. Their enantioselective syntheses under mild conditions is a continuing pursuit of organic synthesis methods development. One opportunity for the synthesis of these building blocks is by functionalization of prochiral double bonds using palladium(II) catalysis. In these reactions, nucleopalladation mediated by a chiral palladium(II) catalyst generates a new heteroatom-substituted chiral center. However, reactions where nucleopalladation occurs with antarafacial stereoselectivity are difficult to render enantioselective because of the challenge of transferring chiral ligand information across the square-planar palladium complex to the incoming nucleophile. In this Account, we describe the development and use of enantiopure palladium(II) catalysts of the COP (chiral cobalt oxazoline palladacyclic) family for the synthesis of enantioenriched products from starting materials derived from prochiral allylic alcohols. We begin with initial studies aimed at rendering catalyzed [3,3]-sigmatropic rearrangements of allylic imidates enantioselective, which ultimately led to the identification of the significant utility of the COP family of Pd(II) catalysts. The first use of an enantioselective COP catalyst was reported by Richards' and our laboratories in 2003 for the enantioselective rearrangement of allylic N-arylimidates. Shortly thereafter, we discovered that the chloride-bridged COP dimer, [COP-Cl]₂, was an excellent enantioselective catalyst for the rearrangement of (E)-allylic trichloroacetimidates to enantioenriched allylic trichloroacetamides, this conversion being the most widely used of the allylic imidate rearrangements. We then turn to discuss S_N2' reactions catalyzed by the acetate-bridged COP dimer, [COP-OAc]₂, which proceed by a unique mechanism to provide branched allylic esters and allylic phenyl ethers in high enantioselectivity. Furthermore, because of the unique nucleopalladation/deoxypalladation mechanism of these S_N2' reactions, they provide exclusively the branched allylic product. Importantly, both enantiomers of the [COP-Cl]₂ and [COP-OAc]₂ catalysts are commercially available. We also briefly consider several other enantioselective reactions catalyzed by COP complexes. The mechanism of enantioselective COP-catalyzed allylic rearrangements and allylic substitutions is discussed in some detail. In both reactions, nucleopalladation is found to be the enantiodetermining step. The cyclobutadienyl "floor" of the COP catalyst is critical for transmitting chiral information across the palladium square plane in these reactions. This structural feature enables high enantioselection to be realized in spite of the nearly 180° angle between the catalyst, electrophile and nucleophile in the enantiodetermining step. Our discussion concludes by considering several uses of the COP family of catalysts by other researchers for the enantioselective synthesis of biologically active chiral molecules. We anticipate that additional uses for COP catalysts will emerge in the future. In addition, the structural features of these catalysts that we have identified as important for achieving high enantioselection should be useful in the future development of improved enantioselective Pd(II) catalysts.

Palladium-Catalyzed Anti-Markovnikov Oxidation of Allylic Amides to Protected β-Amino Aldehydes

A general method for the preparation of N-protected β-amino aldehydes from allylic amines or linear allylic alcohols is described. Here the Pd(II)-catalyzed oxidation of N-protected allylic amines with benzoquinone is achieved in tBuOH under ambient conditions with excellent selectivity toward the anti-Markovnikov aldehyde products and full retention of configuration at the allylic carbon. The method shows a wide substrate scope and is tolerant of a range of protecting groups. Furthermore, β-amino aldehydes can be obtained directly from protected allylic alcohols via palladium-catalyzed autotandem reactions, and the application of this method to the synthesis of β-peptide aldehydes is described. From a mechanistic perspective, we demonstrate that tBuOH acts as a nucleophile in the reaction and that the initially formed tert-butyl ether undergoes spontaneous loss of isobutene to yield the aldehyde product. Furthermore, tBuOH can be used stoichiometrically, thereby broadening the solvent scope of the reaction. Primary and secondary alcohols do not undergo elimination, allowing the isolation of acetals, which subsequently can be hydrolyzed to their corresponding aldehyde products.

Chiral multidentate oxazoline ligands based on cyclophosphazene cores: synthesis, characterization and complexation studies

Chiral oxazoline derivatives of cyclophosphazenes were prepared and their complexation and catalytic studies for the asymmetric rearrangement of trichloroacetimidates to trichloroacetamides have been performed.

Electrochemical Properties and Regioselectivity of Cyclopalladation of Chiral Ferrocenylimines Deriving from (S)-Ferrocenylethylamine

Treatment of a cyclopalladated complex derived from chiral ferrocenylimine (η5-C5H5)Fe(η5-C5H4)–CH(CH3)–N=CH–2-C4H3S, (Sc)-1, with PPh3 produced the heteroannular palladacycle (Sc)-3 in which palladation occurred at the unsubstituted cyclopentadiene ring. While for (η5-C5H5)Fe(η5-C5H4)–CH(CH3)–N=CH–C6H5, (Sc)-4, cyclopalladation took place mainly at the phenyl ring; the heteroannular palladacycle (Sc)-6, as a minor product, was also obtained. Single crystal X-ray analysis, electrochemical, and computational studies have been performed, showing good agreement with experimental results.

Characterization of DMSO coordination to palladium(II) in solution and insights into the aerobic oxidation catalyst, Pd(DMSO)2(TFA)2

Recent studies have shown that Pd(DMSO)(2)(TFA)(2) (TFA = trifluoroacetate) is an effective catalyst for a number of different aerobic oxidation reactions. Here, we provide insights into the coordination of DMSO to palladium(II) in both the solid state and in solution. A crystal structure of Pd(DMSO)(2)(TFA)(2) confirms that the solid-state structure of this species has one O-bound and one S-bound DMSO ligand, and a crystallographically characterized mono-DMSO complex, trans-Pd(DMSO)(OH(2))(TFA)(2), exhibits an S-bound DMSO ligand. (1)H and (19)F NMR spectroscopic studies show that, in EtOAc and THF-d(8), Pd(DMSO)(2)(TFA)(2) consists of an equilibrium mixture of Pd(S-DMSO)(O-DMSO)(TFA)(2) and Pd(S-DMSO)(2)(TFA)(2). The O-bound DMSO is determined to be more labile than the S-bound DMSO ligand, and both DMSO ligands are more labile in THF relative to EtOAc as the solvent. DMSO coordination to Pd(II) is substantially less favorable when the TFA ligands are replaced with acetate. An analogous carboxylate ligand effect is observed in the coordination of the bidentate sulfoxide ligand, 1,2-bis(phenylsulfinyl)ethane to Pd(II). DMSO coordination to Pd(TFA)(2) is shown to be incomplete in AcOH-d(4) and toluene-d(8), resulting in Pd(II)/DMSO adducts with <2:1 DMSO/Pd(II) stoichiometry. Collectively, these results provide useful insights into the coordination properties of DMSO to Pd(II) under catalytically relevant conditions.

Rh(III)-catalyzed directed C-H bond amidation of ferrocenes with isocyanates

[RhCp*(OAc)₂(H₂O)] [Cp* = pentamethylcyclopentadienyl] catalyzed the C–H bond amidation of ferrocenes possessing directing groups with isocyanates in the presence of 2 equiv/Rh of HBF₄·OEt₂. A variety of disubstituted ferrocenes were prepared in high yields, or excellent diastereoselectivities.