Te(II)/Te(IV) Mediated C–N Bond Formation on 2,5-Diphenyltellurophene and a Reassignment of the Product from the Reaction of PhI(OAc)2 with 2 TMS-OTf

A decade of lessons in the activation of ArIL2 species

Hypervalent iodine(iii) compounds of the general structure ArIL2 are widely used as oxidizing agents for a variety of applications across both organic and inorganic chemistry. Considerable work has been done on the activation of these compounds by tuning the ligands at the iodine centre. This perspective summarises the work of our and other groups on rectification of historically misidentified iodine(iii) reagents of this class, and the syntheses of activated species. Recent advances focusing on increasing the oxidative capacity of I(iii) moieties using Lewis and Brønsted acids and Lewis bases as well as the activation of halogens with I(iii) are discussed.

C-H, Si-H and C-F abstraction with an extremely electron poor I(III) reagent

The recently discovered I(III) reagent NO2-C6H4-I(OTf)2 is found to rapidly react with hydride sources, including HSiEt3 and relatively hydridic C-H precursors. These represent the first distinct reactions involving direct hydride abstraction by I(III) under ambient conditions. Direct C-F abstraction is also demonstrated, as well as oxidation of cyclic alkenes to aromatic rings, all representing new reactions for I(III) demonstrating the very high reactivity of NO2-C6H4-I(OTf)2.

Structural verification and new reactivity for Stang's reagent, [PhI(CN)][OTf]

The structure of Stang's reagent [PhI(CN)][OTf] is confirmed by X-ray crystallography and is determined to be best described as an ion-pair in organic solution. It is found to be a strong Lewis acid, but reaction with pyridine ligands gives [Pyr-CN][OTf] salts via oxidation of pyridine giving a new derivative of the CDAP reagent widely used as an activation agent for polysaccharides.

Synthesis and Structural Verification of an ArI(OTf)2 , NO2 -Ph-I(OTf)2

PhI(OTf)₂ and related ArI(OTf)₂ species have been incorrectly invoked as intermediates in oxidation reactions for many years. We recently established that such compounds did not yet exist but remain an attractive target. Here we describe the synthesis, isolation, and structural characterization of NO₂ -PhI(OTf)_2, which is resistant to decomposition and more reactive than PhI(OTf)(OAc), the species previously misidentified as PhI(OTf)₂ .

Lewis acid activation of Weiss' reagents ([PhI(Pyr)2]2+) with boranes and isolation of [PhI(4-DMAP)]2

Abstraction of a pyridine ligand from Weiss' reagent ([PhI(Pyr)₂]²⁺) using BF₃-Et₂O was found to activate Weiss' reagent towards electrophilic aromatic substitution reactions. The activated species can be isolated when 4-DMAP is used as the pyridine ligand and was determined to be [PhI(4-DMAP)]²⁺ in solution. The isolated cation was reactive in electrophilic aromatic substitution reactions towards mesitylene, xylene and toluene that Weiss' reagent itself does not react with.

Morita-Baylis-Hillman-Type [3,3]-Rearrangement: Switching from Z- to E-Selective α-Arylation by New Rearrangement Partners

α-aryl α,β-unsaturated carbonyls represent an important class of derivatizable synthetic intermediates, however, the synthesis of such compounds still remains a challenge. Recently, we showcased a novel Z-selective α-arylation of α,β-unsaturated nitriles with aryl sulfoxides via [3,3]-rearrangement involving an Morita-Baylis-Hillman (MBH) process. Herein, we demonstrate the feasibility of reversing the stereoselectivity of such MBH-type [3,3]-rearrangement by switching to a new pair of rearrangement partners consisting of aryl iodanes and α,β-unsaturated oxazolines. As a result, the two protocols complement each other in approaching E- or Z-α-aryl α,β-unsaturated carbonyl derivatives. Mechanistic studies reveal a possible reaction pathway and provide an explanation for the opposite stereoselectivities.

Bidentate Nitrogen-Ligated I(V) Reagents, Bi(N)-HVIs: Preparation, Stability, Structure, and Reactivity

Hypervalent iodine(V) reagents are a powerful class of organic oxidants. While the use of I(V) compounds Dess-Martin periodinane and IBX is widespread, this reagent class has long been plagued by issues of solubility and stability. Extensive effort has been made for derivatizing these scaffolds to modulate reactivity and physical properties but considerable room for innovation still exists. Herein, we describe the preparation, thermal stability, optimized geometries, and synthetic utility of an emerging class of I(V) reagents, Bi(N)-HVIs, possessing datively bound bidentate nitrogen ligands on the iodine center. Bi(N)-HVIs display favorable safety profiles, improved solubility, and comparable to superior oxidative reactivity relative to common I(V) reagents. The highly modular synthesis and in situ generation of Bi(N)-HVIs provides a novel and convenient screening platform for I(V) reagent and reaction development.

A tellura-Baeyer-Villiger oxidation: one-step transformation of tellurophene into chiral tellurinate lactone

Baeyer-Villiger (BV) oxidation is a fundamental organic reaction, whereas the hetero-BV oxidation is uncharted. Herein, a tellura-BV oxidation is discovered. By oxidizing a tellurophene-embedded and electron-rich polycycle (1) with mCPBA or Oxone, an oxygen atom is inserted into the Te-C bond of the tellurophene to form tellurinate lactone mono-2. This reaction proceeds as follows: (i) 1 is oxidized to the tellurophene Te-oxide form (IM-1); (ii) IM-1 undergoes tellura-BV oxidation to give mono-2. Moreover, the hybrid trichalcogenasumanenes 7 and 8 are, respectively, converted to tellurinate lactones mono-9 and mono-10 under the same conditions, indicating that tellura-BV oxidation shows high chemoselectivity. Due to the strong secondary bonding interactions between the Te[double bond, length as m-dash]O groups on tellurinate lactones, mono-2, mono-9, and mono-10 are dimerized to form U-shaped polycycles 2, 9, and 10, respectively. Notably, mono-2, mono-9, mono-10, and their dimers show chirality. This work enables one-step transformation of tellurophene into tellurinate lactone and construction of intricate polycycles.

PhI(OTf)2 Does Not Exist (Yet)*

PhI(OTf)₂ has been used for the past 30 years as a strong I(III) oxidant for organic and inorganic transformations. It has been reported to be generated in situ from the reactions of either PhI(OAc)₂ or PhI=O with two equivalents of trimethylsilyl trifluoromethanesulfonate (TMS-OTf). In this report it is shown that neither of these reactions generate a solution with spectroscopic data consistent with PhI(OTf)₂ , with supporting theoretical calculations, and thus this compound should not be invoked as the species acting as the oxidant for transformations that have been associated with its use.

Reactions of PhIX2 I(iii) oxidants with heavy triphenyl pnictines

The reactions of [PhI(pyridine)2]2+, PhI(OAc)2 and PhI(OTf)(OAc) with Ph3As, Ph3Sb and Ph3Bi are described. The reactions of [PhI(pyridine)2]2+ with Ph3Sb and Ph3Bi afford dicationic Pn(v) complexes ligated by pyridine in one step. These were previously reported by Burford in multi-step syntheses. Reactions with PhI(OAc)2, which were already known for Sb and Bi giving Pn(v) diacetates, were confirmed to give the same type of compound for As. Reactions with PhI(OAc)(OTf) were less selective, resulting in the isolation of iodonium cations [Ph-I-Ph]+ for As and Bi, while Ph3Sb gave an oxobridged di-antimony species characteristic of the decomposition of a high valent triflate bound species.

Redox chemistry of π-extended tellurophenes

In the past decade, the incorporation of tellurophene motifs into organic devices has been a promising strategy for the design of advanced materials. However, fundamental redox behavior of tellurophene-containing materials have never been comprehensively explored. Here, we report unique redox behavior of π-extended tellurophenes. The facile coordination of solvent molecules and/or anions becomes evident, in addition to the attachment of nucleophilic halides. This indicates that the tellurium center in oxidized 2,5-diphenyltellurophene is highly electron-deficient and easily yields coordinated structures. This coordination appears to trap the positive charge on the tellurium center rather than delocalizing it over the π-system. When no coordinating counter ion is present, however, oxidation appears to be delocalized over the entire π-system. Additionally, by using more delocalized structures, we show that coordination and charge-delocalization can co-exist. These results provide important insights to understand the properties of tellurophene-containing molecules and materials with extended π-systems.

Photoactivity and optical applications of organic materials containing selenium and tellurium

Sulfur-containing compounds, particularly derivatives of thiophene, are well studied for organic optoelectronic applications. Incorporating selenium or tellurium in place of sulfur imparts different physical properties due to the fundamental differences of these atoms relative to their lighter analogues. This has a profound influence on the properties of molecules and materials that incorporate chalcogens that may ultimately lead to new opportunities and applications. This mini-review will focus on the quantitative and qualitative photophysical characteristics of organic materials containing selenium and tellurium as well as their emerging applications as molecular photoactive species, including light-emitting sensors, triplet sensitizers, and beyond.

Chemoselective Oxidation of Equatorial Alcohols with N-Ligated λ3-Iodanes

The site-selective and chemoselective functionalization of alcohols in complex polyols remains a formidable synthetic challenge. Whereas significant advancements have been made in selective derivatization at the oxygen center, chemoselective oxidation to the corresponding carbonyls is less developed. In cyclic systems, whereas the selective oxidation of axial alcohols is well known, a complementary equatorial selective process has not yet been reported. Herein we report the utility of nitrogen-ligated (bis)cationic λ3-iodanes (N-HVIs) for alcohol oxidation and their unprecedented levels of selectivity for the oxidation of equatorial over axial alcohols. The conditions are mild, and the simple pyridine-ligated reagent (Py-HVI) is readily synthesized from commercial PhI(OAc)2 and can be either isolated or generated in situ. Conformational selectivity is demonstrated in both flexible 1,2-substituted cyclohexanols and rigid polyol scaffolds, providing chemists with a novel tool for chemoselective oxidation.

Using boryl-substitution and improved Suzuki-Miyaura cross-coupling to access new phosphorescent tellurophenes

A new di(isopropoxy)boryl -B(OⁱPr)₂ tellurophene precursor is described, from which several previously inaccessible phosphorescent borylated tellurophenes are formed via exchange of the -OⁱPr groups. One such tellurophene Mes(ⁱPrO)B-Te-6-B(OⁱPr)Mes, bearing a sterically encumbered mesityl (Mes) substituent at each boron center, exhibits bright yellow-orange phosphorescence in the solid state at room temperature and in the presence of the known quencher O₂. Furthermore, Suzuki-Miyaura cross-coupling between the newly prepared borylated tellurophenes and the test substrate 2-bromothiophene was examined with the pre-catalyst Cl(XPhos)Pd(aminobiphenyl). While more electron deficient boryl groups such as catecholatoboryl (-Bcat) yield significant protodeboronation in place of productive C-C bond formation, efficient formation of the desired thiophene-capped tellurophene thienyl-Te-6-thienyl was noted from tellurophenes bearing the readily accessible pinacolatoboryl (-Bpin) and 1,8-naphthalenediaminatoboryl (-Bdan) functional groups. These findings open the door for the efficient synthesis of aryl tellurophenes and polytellurophenes via the ubiquitous Suzuki-Miyaura coupling of borylated tellurophenes, which was previously hampered by protodeboronation.

Diverse Reactions of Thiophenes, Selenophenes, and Tellurophenes with Strongly Oxidizing I(III) PhI(L)2 Reagents

We report the outcomes of the reactions of aromatic group 16 thiophene, selenophene, and tellurophene rings with the I(III) oxidants PhI(OAc)(OTf) and [PhI(Pyr)₂][OTf]₂ (Pyr = pyridine). In all reactions, oxidative processes take place, with generation of PhI as the reduction product. However, with the exception of tellurophene with PhI(OAc)(OTf), +4 oxidation state complexes are not observed, but rather a variety of other processes occur. In general, where a C-H unit is available on the 5-membered ring, an electrophilic aromatic substitution reaction of either -IPh or pyridine onto the ring occurs. When all positions are blocked, reactions with PhI(OAc)(OTf) give acetic and triflic anhydride as the identifiable oxidative byproducts, while [PhI(Pyr)₂][OTf]₂ gives pyridine electrophilic aromatic substitution onto the peripheral rings. Qualitative mechanistic studies indicate that the presence of the oxidizable heteroatom is required for pyridine to act as an electrophile in a substantial manner.

Reactions of Trivalent Iodine Reagents with Classic Iridium and Rhodium Complexes

In this work, the reactions of iodine(iii) reagents (PhI(L)2: L = pyridine, acetate (OAc−), triflate (OTf−)) with iridium(i) and rhodium(i) complexes (Vaskas’s compound, Wilkinson’s catalyst, and bis[bis(diphenylphosphino)ethane]rhodium(i) triflate) are reported. In all cases, the reactions resulted in two-electron oxidation of the metal complexes. Mixtures of products were observed in the reactions of Iiii reagents with Vaska’s compound and Wilkinson’s catalyst via ligand exchange and anion scrambling. In the case of reacting Iiii reagents with chelating ligand-containing bis[bis(diphenylphosphino)ethane]rhodium(i) triflate, no scrambling was observed.

Acid Activation in Phenyliodine Dicarboxylates: Direct Observation, Structures, and Implications

The use of the hypervalent iodine reagents in oxidative processes has become a staple in modern organic synthesis. Frequently, the reactivity of λ³ iodanes is further enhanced by acids (Lewis or Brønsted). The origin of such activation, however, has remained elusive. Here, we use the common combination of PhI(OAc)₂ with BF₃·Et₂O as a model to fully explore this activation phenomenon. In addition to the spectroscopic assessment of the dynamic acid-base interaction, for the first time the putative PIDA·BF₃ complex has been isolated and its structure determined by X-ray diffraction. Consequences of such activation are discussed from a structural and electronic (DFT) points of views, including the origins of the enhanced reactivity.

Access to the Parent Tetrakis(pyridine)gold(III) Trication, Facile Formation of Rare Au(III) Terminal Hydroxides, and Preliminary Studies of Biological Properties

In this paper we report on the use of [NO][BF4] to access tricationic tetrakis(pyridine)gold(III) from Au powder, a species inaccessible using the more traditional (tetrahydrothiophene)AuCl route. It is then demonstrated that this family of compounds can be used to access new terminal Au(III) hydroxides, a challenging class of compounds, and the first crystallographically characterized examples employing bidentate ligands. Finally, preliminary biological studies indicate good activity for derivatives featuring polydentate ligands against the HeLa and PC3 cell lines but also strong inhibition of primary HUVEC cells.

A Mechanistic Study of Halogen Addition and Photoelimination from π-Conjugated Tellurophenes

The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η(1) association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.