The Newman-Kwart rearrangement of O-aryl thiocarbamates: substantial reduction in reaction temperatures through palladium catalysis

Photocatalytic O- to S-Rearrangement of Tertiary Cyclopropanols

Despite the importance of heteroatom-substituted cyclopropane derivatives in drug design and organic synthesis, cyclopropanethiols remain critically underexplored. Inspired by the wide use of the Newman-Kwart rearrangement to access valuable thiophenols from phenol feedstocks, we report the development of a photocatalytic approach for efficient ambient temperature aliphatic O- to S-rearrangement on tertiary cyclopropanol derivatives. After demonstrating that a range of cyclopropanethiols-that are difficult to access by other methods-can be obtained with this strategy, we show that these rearranged products can be easily hydrolyzed and further derivatized. We conclude this study with mechanistic findings that enabled an initial extension of this approach toward other classes of aliphatic alcohols.

Annulative Nonaromatic Newman-Kwart-Type Rearrangement for the Synthesis of Sulfur Heteroaryls

By employing enaminones and thiuram disulfides as starting materials, the frontiers of Newman-Kwart rearrangement have been expanded to the alkenyl system for the first time. In addition, instead of leading to the formation of simple carbamothioates, the rearrangement has led to the unprecedented construction of S-heteroaryls. Depending on the differences in the enaminone structure, the efficient synthesis of functionalized isothiazoles and thiophenes has been achieved.

Metal-Free Thiocarbamation of Quinolinones: Direct Access to 3,4-Difunctionalized Quinolines and Quinolinonyl Thiocarbamates at Room Temperature

A novel and practical method for the preparation of difunctionalized quinolines, bearing a thiocarbamate group at the C3-position and an acyloxyl group at the C4-position, and quinolinonyl thiocarbamates from quinolinones, tetraalkylthiuram disulfides, and hypervalent iodine(III) reagents has been developed via thiocarbamation of quinolinones at room temperature. The present method features mild reaction conditions, good tolerance with diverse functional groups, and a wide substrate scope, providing the desired products in good yields. Furthermore, this transformation is easy to scale up, and the desired products can be readily converted to heterocyclic thiols. Most importantly, this protocol allows for the late-stage thiocarbamation of bioactive compounds. Mechanistic studies show that radicals may be involved in this transformation, water is probably the oxygen source of thiocarbamates, and difunctionalized quinolines are possibly formed via nucleophilic attack of carboxylic anions, which derive from hypervalent iodine(III) reagents.

Photothermal Conversion by Carbon Black Facilitates Aryl Migration by Photon-Promoted Temperature Gradients

The Newman Kwart Rearrangement (NKR) offers an efficient and high-yielding method for producing substituted thiophenols from phenols. While an industrially important protocol, it suffers from high activation energy barriers (35-43 kcal/mol), requiring the use of extreme temperatures (>200 °C) and specialty equipment. This report details a highly efficient and straightforward method for facilitating the NKR using photothermal conversion. This underused, unique reactivity pathway arises from the irradiation of nanomaterials that relax via a non-radiative decay pathway to generate intense thermal gradients. We show carbon black (CB) can be an inexpensive and abundant photothermal agent under visible light irradiation to achieve a facile NKR under mild conditions. The scope includes a wide array of stereo- and electronically diverse substrates with increasing difficulty of rearrangement, including BHT and BINOL as effective substrates. Furthermore, we demonstrate the unique application for temporal control in a thermal reaction and tunability of thermal gradients by modulating light intensity. Ultimately, photothermal conversion enables high-temperature reactions with simple, visible light irradiation.

Synthesis of Novel Pyrazole Derivatives Containing Phenylpyridine Moieties with Herbicidal Activity

To discover new compounds with favorable herbicidal activity, a range of phenylpyridine moiety-containing pyrazole derivatives were designed, synthesized, and identified via NMR and HRMS. Their herbicidal activities against six species of weeds were evaluated in a greenhouse via both pre- and post-emergence treatments at 150 g a.i./hm2. The bioassay revealed that a few compounds exhibited moderate herbicidal activities against Digitaria sanguinalis, Abutilon theophrasti, and Setaria viridis in post-emergence treatment. For instance, compounds 6a and 6c demonstrated 50% inhibition activity against Setaria viridis, which was slightly superior to pyroxasulfone. Thus, compounds 6a and 6c may serve as the new possible leading compounds for the discovery of post-emergence herbicides.

Modular synthesis of unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene molecular semiconductors for organic transistors

A modular approach to underexplored, unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene (BTBT) scaffolds delivers a library of BTBT materials from readily available coupling partners by combining a transition-metal free Pummerer CH-CH-type cross-coupling and a Newman-Kwart reaction. This effective approach to unsymmetrical BTBT materials has allowed their properties to be studied. In particular, tuning the functional groups on the BTBT scaffold allows the solid-state assembly and molecular orbital energy levels to be modulated. Investigation of the charge transport properties of BTBT-containing small-molecule:polymer blends revealed the importance of molecular ordering during phase segregation and matching the highest occupied molecular orbital energy level with that of the semiconducting polymer binder, polyindacenodithiophene-benzothiadiazole (PIDTBT). The hole mobilities extracted from transistors fabricated using blends of PIDTBT with phenyl or methoxy functionalized unsymmetrical BTBTs were double those measured for devices fabricated using pristine PIDTBT. This study underscores the value of the synthetic methodology in providing a platform from which to study structure-property relationships in an underrepresented family of unsymmetrical BTBT molecular semiconductors.

Unraveling the role of the electron-pair density symmetry in reaction mechanism patterns: the Newman–Kwart rearrangement

There is an underlying intimate relationship between Thom's catastrophe theory and the electron-pair density evidenced along a reaction pathway.

Insights into the kinetics and molecular mechanism of the Newman–Kwart rearrangement

Kinetics and molecular mechanism of the Newman–Kwart rearrangement (NKR) of N,N-dimethyl O-arylthiocarbamate into N,N-dimethyl S-arylcarbamate have been investigated theoretically.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Electrochemically Catalyzed Newman-Kwart Rearrangement: Mechanism, Structure-Reactivity Relationship, and Parallels to Photoredox Catalysis

The facilitation of redox-neutral reactions by electrochemical injection of holes and electrons, also known as "electrochemical catalysis", is a little explored approach that has the potential to expand the scope of electrosynthesis immensely. To systematically improve existing protocols and to pave the way toward new developments, a better understanding of the underlying principles is crucial. In this context, we have studied the Newman-Kwart rearrangement of O-arylthiocarbamates to the corresponding S-aryl derivatives, the key step in the synthesis of thiophenols from the corresponding phenols. This transformation is a particularly useful example because the conventional method requires temperatures up to 300 °C, whereas electrochemical catalysis facilitates the reaction at room temperature. A combined experimental-quantum chemical approach revealed several reaction channels and rendered an explanation for the relationship between the structure and reactivity. Furthermore, it is shown how rapid cyclic voltammetry measurements can serve as a tool to predict the feasibility for specific substrates. The study also revealed distinct parallels to photoredox-catalyzed reactions, in which back-electron transfer and chain propagation are competing pathways.

Iron(II)/Persulfate Mediated Newman-Kwart Rearrangement

Herein, we report that iron(II)/ammonium persulfate in aqueous acetonitrile mediates the Newman-Kwart rearrangement of O-aryl carbamothioates. Electron-rich substrates react rapidly under moderate heating to afford the rearranged products in excellent yields. The mild conditions, rapid reaction rates, and suitability for scale up offers immediate practical benefits to access functionalized thiophenols.

An Electrocatalytic Newman-Kwart-type Rearrangement

An electrochemical approach toward rearrangement of O-aryl thiocarbamates to the corresponding S-aryl thiocarbamates is presented. The protocol requires only catalytic amounts of electric charge and allows for operation at room temperature. The electrolysis can be carried out with the simplest equipment, i.e., under galvanostatic conditions in an undivided cell. Furthermore, it is demonstrated that when the electrolysis is performed in a microflow reactor, almost quantitative yields can be achieved without using supporting electrolyte.

Inverting the Selectivity of the Newman-Kwart Rearrangement via One Electron Oxidation at Room Temperature

The discovery that the Newman-Kwart rearrangement can be performed at room temperature by action of a simple and readily available oxidant, cerium ammonium nitrate, is described. The conditions give clean conversion when using electron-rich aromatic substrates, and the reactions are often quantitative. Computational studies support a reaction mechanism where the O-thiocarbamate is first oxidized to the radical cation, followed by nucleophilic attack by the ipso carbon of the aromatic system.

Microwave-mediated Newman–Kwart rearrangement in water

For the first time the unimolecular Newman–Kwart rearrangement is performed in pure water.

Ambient-Temperature Newman-Kwart Rearrangement Mediated by Organic Photoredox Catalysis

The Newman-Kwart rearrangement is perhaps the quintessential method for the synthesis of thiophenols from the corresponding phenol. However, the high thermal conditions required for the rearrangement of the requisite O-aryl carbamothioates often leads to decomposition. Herein, we present a general strategy for catalysis of O-aryl carbamothioates to S-aryl carbamothioates using catalytic quantities of a commercially available organic single-electron photooxidant. Importantly, this reaction is facilitated at ambient temperatures.

Ruthenium-Catalyzed O- to S-Alkyl Migration: A Pseudoreversible Barton-McCombie Pathway

A practical ruthenium-catalyzed O- to S-alkyl migration affords structurally diverse thiooxazolidinones in excellent yields. Our studies suggest this catalytic transformation proceeds through a pseudoreversible radical pathway drawing mechanistic parallels to the classic Barton-McCombie reaction.

Tailored near-infrared contrast agents for image guided surgery

The success of near-infrared (NIR) fluorescence to be employed for intraoperative imaging relies on the ability to develop a highly stable, NIR fluorescent, nontoxic, biocompatible, and highly excreted compound that retains a reactive functionality for conjugation to a cancer-recognizing peptide. Herein, systematic modifications to previously detailed fluorophore ZW800-1 are explored. Specific modifications, including the isosteric replacement of the O atom of ZW800-1, include nucleophilic amine and sulfur species attached to the heptamethine core. These novel compounds have shown similar satisfactory results in biodistribution and clearance while also expressing increased stability in serum. Most importantly, all of the synthesized and evaluated compounds display a reactive functionality (either a free amino group or carboxylic acid moiety) for further bioconjugation. The results obtained from the newly prepared derivatives demonstrate that the central substitution with the studied linking agents retains the ultralow background in vivo performance of the fluorophores regardless of the total net charge.

4-Dodecylbenzenesulfonic acid (DBSA) promoted solvent-free diversity-oriented synthesis of primary carbamates, S-thiocarbamates and ureas

A simple and efficient solvent-free preparation of primary carbamates, S-thiocarbamates and ureas from alcohols, phenols, thiols and amines in the presence of 4-dodecylbenzenesulfonic acid, as a cheap and green Brønsted acid, has been described.

Analogues of the Lavallo-Grubbs compound Fe3(C8H8)3: equilateral, isosceles, and scalene metal triangles in trinuclear cyclooctatetraene complexes M3(C8H8)3 of the first row transition metals (M = Ti, V, Cr, Mn, Fe, Co, and Ni)

The trinuclear derivative Fe(3)(C(8)H(8))(3) was synthesized in 2009 by Lavallo and Grubbs via the reaction of Fe(C(8)H(8))(2) with a bulky heterocyclic carbene. This fascinating structure is the first example of a derivative of the well-known Fe(3)(CO)(12) in which all 12 carbonyl groups have been replaced by hydrocarbon ligands. The density functional theory predicts a structure having a central Fe(3) equilateral triangle with ∼2.9 Å Fe-Fe single bonded edges bridged by η(5),η(3)-C(8)H(8) ligands. This structure is close to the experimental structure, determined by X-ray crystallography. The related hypoelectronic M(3)(C(8)H(8))(3) derivatives (M = Cr, V, Ti) are predicted to have central scalene M(3) triangles with edge lengths and Wiberg bond indices (WBIs) corresponding to one formal single M-M bond, one formal double M═M bond, and one formal triple M≡M bond. For Mn(3)(C(8)H(8))(3), both a doublet structure with one Mn═Mn double bond and two Mn-Mn single bonds in the Mn(3) triangle, and a quartet structure with two Mn═Mn double bonds and one Mn-Mn single bond are predicted. The hyperelectronic derivatives M(3)(C(8)H(8))(3) have weaker direct M-M interactions in their M(3) triangles, as indicated by both the M-M distances and the WBIs. Thus, Ni(3)(C(8)H(8))(3) has bis(trihapto) η(3),η(3)-C(8)H(8) ligands bridging the edges of a central approximately equilateral Ni(3) triangle with long Ni···Ni distances of ∼3.7 Å. The WBIs indicate very little direct Ni-Ni bonding in this Ni(3) triangle and thus a local nickel environment in the singlet Ni(3)(C(8)H(8))(3) similar to that observed for diallylnickel (η(3)-C(3)H(5))(2)Ni.

SAR studies of acidic dual γ-secretase/PPARγ modulators

A novel set of dual γ-secretase/PPARγ modulators characterized by a 2-benzyl hexanoic acid scaffold is presented. Synthetic efforts were focused on the variation of the substitution pattern of the central benzene. Finally, we obtained a new class of 2,5-disubstituted 2-benzylidene hexanoic acid derivatives, which act as dual γ-secretase/PPARγ modulators in the low micromolar range. We have explored broad SAR and successfully improved the dual pharmacological activity and the selectivity profile against potential off-targets such as NOTCH and COX. Compound 17 showed an IC(50) Aβ42=2.4 μM and an EC(50) PPARγ=7.2 μM and could be a valuable tool to further evaluate the concept of dual γ-secretase/PPARγ modulators in animal models of Alzheimer's disease.

Suzuki-Miyaura cross-coupling of aryl carbamates and sulfamates: experimental and computational studies

The first Suzuki-Miyaura cross-coupling reactions of the synthetically versatile aryl O-carbamate and O-sulfamate groups are described. The transformations utilize the inexpensive, bench-stable catalyst NiCl(2)(PCy(3))(2) to furnish biaryls in good to excellent yields. A broad scope for this methodology has been demonstrated. Substrates with electron-donating and electron-withdrawing groups are tolerated, in addition to those that possess ortho substituents. Furthermore, heteroaryl substrates may be employed as coupling partners. A computational study providing the full catalytic cycles for these cross-coupling reactions is described. The oxidative addition with carbamates or sulfamates occurs via a five-centered transition state, resulting in the exclusive cleavage of the aryl C-O bond. Water is found to stabilize the Ni-carbamate catalyst resting state, which thus provides rationalization of the relative decreased rate of coupling of carbamates. Several synthetic applications are presented to showcase the utility of the methodology in the synthesis of polysubstituted aromatic compounds of natural product and bioactive molecule interest.

Highly unsaturated binuclear butadiene iron carbonyls: quintet spin states, perpendicular structures, agostic hydrogen atoms, and iron-iron multiple bonds

The highly unsaturated binuclear butadiene iron carbonyls (C₄H₆)₂Fe₂(CO)_n (n = 2, 1) have been examined using density functional theory. For (C₄H₆)₂Fe₂(CO)_n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C₄H₆)₂Fe₂(CO)_n (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C₄H₆)₂Fe₂(CO)₂ structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C₄H₆)₂Fe₂(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C₄H₆)₂Fe₂(CO)_n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH₂ groups. Singlet (C₄H₆)₂Fe₂(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.

Silver-mediated allylic disulfide rearrangement for conjugation of thiols in protic media

Alkyl and aryl allyl disulfides are induced to undergo the desulfurative allylic rearrangement by silver nitrate in protic solvents at room temperature, thereby removing the necessity for the use of phosphines as thiophilic reagents. The silver-mediated reaction functions at ambient temperature in protic solvents and in the absence of protecting groups.