Theoretical Investigations on Mechanisms of Pd(OAc)2-Catalyzed Intramolecular Diaminations in the Presence of Bases and Oxidants

Regio- and diastereoselective Pd-catalyzed aminochlorocyclization of allylic carbamates: scope, derivatization, and mechanism

The regio- and diastereoselective synthesis of oxazolidinones via a Pd-catalyzed vicinal C-N/C-Cl bond-forming reaction from internal alkenes of allylic carbamates is reported. The oxazolidinones are obtained in yields of 44 to 95% with high to excellent diastereoselectivities (from 6 : 1 to >20 : 1 dr) from readily available precursors. This process is scalable, and the products are suitable for the synthesis of useful amino alcohols. A detailed theoretical and experimental mechanistic study was carried out to describe that the reaction proceeds through an anti-aminopalladation of the alkene followed by an oxidative C-Pd(ii) cleavage with retention of the carbon stereochemistry to yield the major diastereomer. The role of Cu(ii) in a C-Cl bond-forming mechanism step has also been proposed.

Pd(II)-Catalyzed Intramolecular Acetoxylative (3 + 2) Annulation of Propargylic Alcohol and Alkene: Polycyclic Oxa-heterocycle Synthesis and Mechanistic Insight

A Pd(II)-catalyzed intramolecular and highly selective acetxoylative (3 + 2) annulation of propargylic alcohol and alkene is reported. Mechanistically, a hydroxy-guided regioselective alkyne acetoxypalladation is followed by 6-exo-trig alkene insertion to form an alkyl-Pd(II) intermediate. After oxidation, the resulting cyclometalated alkoxy-Pd(IV)-alkyl undergoes direct reductive elimination to afford a polycyclic oxa-heterocycle. When an additional coordinating site or ligand accessible by palladium is present, an S_N2-type C-C reductive elimination of alkyl-Pd(IV) instead occurs along with hydroxy acetylation, affording 3-bicyclo[4.1.0]heptan-5-one products.

Pd(II)-Catalyzed Cyclization-Oxidation of Urea-Tethered Alkylidenecyclopropanes

A Pd(OAc)₂-catalyzed intramolecular oxidative cyclization of urea-tethered alkylidenecyclopropanes with urea as a nitrogen source through a Pd(II)/Pd(IV) catalytic cycle has been presented, giving the corresponding cyclobuta[ b]indoline derivatives in moderate to good yields with a broad substrate scope. The reaction proceeds through a ring expansion of alkylidenecyclopropane along with the nucleophilic attack of nitrogen atom onto the in situ generated palladium carbenoid species as well as an oxidation process.

Mechanistic Unveiling of C═C Double-Bond Rotation and Origins of Regioselectivity and Product E/Z Selectivity of Pd-Catalyzed Olefinic C-H Functionalization of (E)-N-Methoxy Cinnamamide

Density functional theory (DFT) calculations have been performed to study the Pd-catalyzed C-H functionalization of (E)-N-methoxy cinnamamide (^E1), which selectively provides the α-C-H activation products (^EP as minor product and its C═C rotation isomer ^ZP' as major product). Three crucial issues are solved: (i) The detailed mechanism leading to ^ZP' is one issue. The computational analyses of the mechanisms proposed in previously experimental and theoretical literature do not seem to be consistent with the experimental findings due to the high barriers involved. Alternatively, we present a novel oxidation/reduction-promoted mechanism featuring the Pd(0) → Pd(II) → Pd(0) transformation. The newly proposed mechanism involves the initial coordination of the active catalyst PdL₂ (L = t-BuCN) with the C═C bond in ^EP, followed by the oxidative cyclization/reductive decyclization-assisted C═C double-bond rotation processes resulting in ^ZP' and regeneration of PdL₂. (ii) The origin of the product E/Z selectivity is the second issue. On the basis of the calculated results, it is found that, at the initial stage of the reaction, ^EP is certainly completely generated, while no ^ZP' formation occurred. Once ^E1 is used up, ^EP immediately acts as the partner of the new catalytic cycle and sluggishly evolves into ^ZP'. A small amount of generated ^ZP' would reversibly transform to ^EP due to the higher barrier involved. (iii) The intrinsic reasons for the regioselectivity are the third issue. The calculated results indicate that the regioselectivity for α-C-H activation is mainly attributed to the stronger electrostatic attraction between the α-C and the metal center.

Why different ligands can control stereochemistry selectivity of Ni-catalyzed Suzuki–Miyaura cross-coupling of benzylic carbamates with arylboronic esters: a mechanistic study

Transmetallation is the rate-determining step for the whole catalytic cycle, and oxidative addition controls the stereoselectivity of products.

The Quest for Palladium-Catalysed Alkyl-Nitrogen Bond Formation

Our interest in the development of transition-metal catalysis for the realisation of vicinal diamination reactions of alkenes started about a decade ago. A number of successful transformations in this area have been developed using palladium catalysis. As a challenging aspect of major importance, the palladium-catalysed coupling of alkyl-nitrogen bonds constitutes the second step in diaminations of alkenes. We here discuss the details that led us to consider high-oxidation-state palladium catalysis as a key feature in such C-N bond-forming reactions. This work discusses both our own contributions and the ones from colleagues and combines the discussion of catalytic reactions and stoichiometric control experiments. It demonstrates that reductive alkyl-nitrogen bond formation from palladium(IV) proceeds with a low activation barrier and through a linear transition state of nucleophilic displacement.

Methods for direct alkene diamination, new & old

The 1,2-diamine moiety is a ubiquitous structural motif present in a wealth of natural products, including non-proteinogenic amino acids and numerous alkaloids, as well as in pharmaceutical agents, chiral ligands and organic reagents. The biological activity associated with many of these systems and their chemical utility in general has ensured that the development of methods for their preparation is of critical importance. While a wide range of strategies for the preparation of 1,2-diamines have been established, the diamination of alkenes offers a particularly direct and efficient means of accessing these systems. The purpose of this review is to provide an overview of all methods of direct alkene diamination, metal-mediated or otherwise.

Synthetic and mechanistic studies on Pd(0)-catalyzed diamination of conjugated dienes

Various dienes and a triene can be regioselectively diaminated at the internal double bond with good yields and high diastereoselectivity using di-tert-butyldiaziridinone (5) as the nitrogen source and Pd(PPh(3))(4) (1-10 mol %) as the catalyst. Kinetic studies with (1)H NMR spectroscopy show that the diamination is first-order in total Pd catalyst and inverse first-order in PPh(3). For reactive dienes, such as 1-methoxybutadiene (6g) and alkyl 1,3-butadienes (6a, 6j), the diamination is first-order in di-tert-butyldiaziridinone (5) and zero-order in the olefin. For olefins with relatively low reactivity, such as (E)-1-phenylbutadiene (6b) and (3E,5E)-1,3,5-decatriene (6i), similar diamination rates were observed when 3.5 equiv of olefins were used. Pd(PPh(3))(2) is likely to be the active species for the insertion of Pd(0) into the N-N bond of di-tert-butyldiaziridinone (5) to form a four-membered Pd(II) complex (A), which can be detected by NMR spectroscopy. The olefin complex (B), formed from intermediate A via ligand exchange between the olefin substrate and the PPh(3), undergoes migratory insertion and reductive elimination to give the diamination product and regenerate the Pd(0) catalyst.

Mechanism of N-fluorobenzenesulfonimide promoted diamination and carboamination reactions: divergent reactivity of a Pd(IV) species

The mechanism of the Pd-catalyzed diamination and carboamination of alkenes promoted by N-fluorobenzenesulfonimide (NFBS) was investigated. Stereochemical labeling experiments established that the diamination reaction proceeds via overall syn addition of the two nitrogen groups, whereas carboamination is the result of an anti addition of arene and nitrogen to the alkene. The intermediate Pd-alkyl complex arising from aminopalladation was observed, and an X-ray crystal structure of its 2,2'-bipyridine (bipy) complex was obtained, revealing strong chelation of the amide protecting group to palladium. Aminopalladation was shown to be an anti-selective process in both the presence and the absence of added ligands, proceeding via external attack of the nitrogen on a Pd-coordinated alkene. The intermediate Pd-alkyl complex was converted to diamination product upon exposure to NFBS with inversion of configuration via oxidative addition followed by dissociation of the benzenesulfonimide anion and S(N)2 displacement of the Pd-C bond. Conversely, arylation of the Pd-alkyl complex proceeds via retention of stereochemistry, consistent with C-H activation of the arene at the Pd(IV) center. A small intermolecular isotope effect (k(H)/k(D) = 1.1) and a large intramolecular isotope effect (k(H)/k(D) = 4) were measured for this process, indicating that C-H activation occurs via a poorly selective product-determining coordination of the arene followed by a highly selective C-H activation. Competition between arenes reveals an unusual reactivity order of toluene > benzene > bromobenzene > anisole.