Palladium (II/IV) catalyzed cyclopropanation reactions: scope and mechanism

Deracemization of Binaphthyl by Suzuki Diarylation: The Role of Electronic and Steric Effects

We report a Suzuki 2,2'-diarylation of the racemic 2,2'-diiodo-1,1'-binaphthyl which proceeds with deracemization via a pallada(IV)cyclic intermediate, induced by a simple chiral ligand─BINAP [2,2'-bis(diphenylphosphino)-1,1',-binaphthyl]. A systematic study of the reaction scope, using 45 arylboronic acids, reveals that the diarylated product is formed when meta- and/or para-substituted phenylboronic acids are functionalized with a substituent with the Hammett constant from -0.5 to +0.4. Multiparametric analysis accounting for the effect of geometry on the reactivity using Boltzmann-weighted Sterimol parameters and electronic effects described by Hammett descriptors shows that the enantioselectivity depends on steric effects only, with enhanced enantioselectivity observed for substituents with a larger length, wL, and reduced for substituents with a larger maximum width, wB₅. We show that careful tuning of these parameters, with the aid of the presented mathematical model, can lead to excellent enantioselectivity. Additional factors that are investigated and found to affect the stereoselective course of the reaction include the reaction temperature, palladium source, palladium to ligand ratio, and the type of boronic acid derivative. During the chromatographic separation of diarylated products on an achiral silica gel, we observed a rare phenomenon: the diarylated products undergo self-disproportionation of enantiomers, with the major enantiomer being eluted first.

Palladium-Catalyzed Organic Reactions Involving Hypervalent Iodine Reagents

The chemistry of polyvalent iodine compounds has piqued the interest of researchers due to their role as important and flexible reagents in synthetic organic chemistry, resulting in a broad variety of useful organic molecules. These chemicals have potential uses in various functionalization procedures due to their non-toxic and environmentally friendly properties. As they are also strong electrophiles and potent oxidizing agents, the use of hypervalent iodine reagents in palladium-catalyzed transformations has received a lot of attention in recent years. Extensive research has been conducted on the subject of C—H bond functionalization by Pd catalysis with hypervalent iodine reagents as oxidants. Furthermore, the iodine(III) reagent is now often used as an arylating agent in Pd-catalyzed C—H arylation or Heck-type cross-coupling processes. In this article, the recent advances in palladium-catalyzed oxidative cross-coupling reactions employing hypervalent iodine reagents are reviewed in detail.

Hypervalent Iodine Reagents in Palladium-Catalyzed Oxidative Cross-Coupling Reactions

Hypervalent iodine compounds are valuable and versatile reagents in synthetic organic chemistry, generating a diverse array of useful organic molecules. Owing to their non-toxic and environmentally friendly features, these reagents find potential applications in various oxidative functionalization reactions. In recent years, the use of hypervalent iodine reagents in palladium-catalyzed transformations has been widely studied as they are strong electrophiles and powerful oxidizing agents. For instance, extensive work has been carried out in the field of C–H bond functionalization via Pd-catalysis using hypervalent iodine reagents as oxidants. In addition, nowadays, iodine(III) reagents have been frequently employed as arylating agents in Pd-catalyzed C–H arylation or Heck-type cross-coupling reactions. In this review, recent advancements in the area of palladium-catalyzed oxidative cross-coupling reactions using hypervalent iodine reagents are summarized in detail.

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.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Palladacycle catalysis: an innovation to the Suzuki–Miyaura cross-coupling reaction

A new approach to the Suzuki cross-coupling reaction based on the catalysis of a thiosemicarbazone palladacycle is presented.

Palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes: facile synthesis of bicyclo[4.1.0]heptan-5-ones

We here describe a new palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes at room temperature using tBuONO as an oxidant for the synthesis of 3-bicyclo[4.1.0]heptan-5-ones.

Asymmetric chiral ligand-directed alkene dioxygenation

A Pd-catalyzed asymmetric alkene 1,2-dioxygenation reaction is described. The diastereoselectivity of the reaction is controlled by tethering a chiral oxime ether directing group to the alkene substrate. The best selectivities are obtained with 8-substituted menthone-derived oxime ether auxiliaries.

Palladium-catalyzed 1,1-aryloxygenation of terminal olefins

This paper describes the 1,1-arylacetoxylation of diverse α-olefins using organostannanes and hypervalent iodine oxidants. The reaction provides a convergent approach for generating a C-C and a C-O bond as well as a new stereocenter in a single catalytic transformation.

Acetoxypalladation of unactivated alkynes and capture with alkenes to give 1-acetoxy-1,3-dienes taking dioxygen as terminal oxidant

A new and general protocol for the synthesis of 1-acetoxy-1,3-dienes by an acetoxypalladation/Heck cross-coupling/β-H elimination tandem process is described in which dioxygen is the terminal oxidant. Electron-rich and electron-deficient alkynes are both effective substrates in this system. It is the first example of acetoxypalladation of diarylalkynes.

Intramolecular scrambling of aryl groups in organopalladium complexes [ArPd(PPh3)2]+: from solution to the gas phase, back again, and in-between

Gas-phase experiments are used to probe the intramolecular scrambling of aryl groups between palladium and phosphorus in organopalladium complexes, [ArPd(PPh(3))(2)](+), generated by means of electrospray ionization (ESI). To this aim, ESI mass spectrometry, including tandem mass spectrometric experiments, were carried out on deuterated, non-deuterated, and substituted [ArPd(PPh(3))(2)](+) complexes. The fragment ions obtained from the deuterated parent ions clearly show the occurrence of intramolecular scrambling between the aryl group bound to palladium and the phenyl groups of the phosphine in the gas phase. Fragmentation pathways, supported by a statistical model, are proposed to explain these migrations and the implications for the condensed-phase chemistry are probed experimentally by using ESI mass spectrometry.

Palladium-catalyzed oxidative arylhalogenation of alkenes: synthetic scope and mechanistic insights

This article describes the development of a Pd-catalyzed reaction for the arylhalogenation (halogen = Cl or Br) of diverse alpha-olefins by oxidatively intercepting Mizoroki-Heck intermediates. These transformations afford synthetically useful 1,2- and 1,1-arylhalogenated products in good yields with good to excellent selectivities that can be modulated by changing the nature of the halogenating reagent and/or the reaction conditions. The selectivity of these reactions can be rationally tuned by (i) controlling the relative rates of oxidative functionalization versus beta-hydride elimination from equilibrating Pd(II)-alkyl species and (ii) stabilization of organometallic Pd(II) intermediates through the formation of pi-benzyl adducts. These arylhalogenations exhibit modest to excellent levels of stereoselectivity, and the key carbon-halogen bond-forming step proceeds with predominant retention of stereochemistry at carbon.

Synthetic and mechanistic studies of Pd-catalyzed C-H arylation with diaryliodonium salts: evidence for a bimetallic high oxidation state Pd intermediate

This contribution describes the substrate scope and mechanism of Pd-catalyzed ligand-directed C-H arylation with diaryliodonium salts. This transformation was applied to the synthesis of a variety of different biaryl products, using directing groups including pyridines, quinolines, pyrrolidinones, and oxazolidinones. Electronically and sterically diverse aryl groups (Ar) were transferred in high yield using iodine(III) reagents of general structure [Mes-I-Ar]BF(4). Mechanistic investigations have been conducted that establish the kinetic order of the catalytic reaction in each component, determine the resting state of the catalyst and the iodine(III) reagent, quantify the electronic influence of the arylating reagent on the reaction rate, and establish the intra- and intermolecular 1 degree H/D kinetic isotope effect. On the basis of these studies, this transformation is proposed to proceed via turnover-limiting oxidation of the Pd dimer [Pd(N~C)(OAc)](2) (N~C = 3-methyl-2-phenylpyridine) by [Mes-I-Ph]BF(4). This mechanism implicates a bimetallic high oxidation state Pd species as a key catalytic intermediate. The significance of this and other aspects of the proposed mechanism are discussed in detail.

Organopalladium and platinum chemistry in oxidising milieu as models for organic synthesis involving the higher oxidation states of palladium

This perspective focuses on the higher oxidation state (III, IV) organometallic chemistry of palladium involving a range of strong oxidants, with consideration of platinum chemistry where it is informative for the evaluation of structure and mechanism. Particular emphasis is placed on hypervalent iodine reagents, halogens and related oxidants of intense current interest in organic synthesis, together with linkages in concepts between this chemistry and recent advances in studies of diaryl disulfides, diaryl diselenides, and diaryl peroxides as oxidants.