Sustainable Heck–Matsuda Reaction with Catalytic Amounts of Diazonium Salts: An Experimental and Theoretical Study

The palladium‐catalyzed Heck–Matsuda reaction with a catalytic amount of an in‐situ‐generated diazonium salt proceeded under mild and sustainable conditions. The reaction proceeded at room temperature, under base‐free conditions, and only generated tBuOH, H₂O, and N₂ as by‐products. Ortho‐substituted diazonium salts were more‐efficiently coupled to methyl acrylate than their corresponding para isomers, which required the addition of anisole as an additive. In support of these experimental data, we carried out theoretical studies to gain a deeper understanding of these reaction outcomes.     

Unprecedented substoichiometric use of hazardous aryl diazonium salts in the Heck-Matsuda reaction via a double catalytic cycle

The first Heck-Matsuda reaction using a catalytic amount of diazonium salts has been discovered. The reaction proceeds through an unprecedented double catalytic cycle in which the electrophile is in situ generated through the reaction advancement. This concept features mild and safe conditions as hazardous aryl diazonium salts are not isolated anymore. Importantly, this sustainable procedure generates only environmentally friendly byproduct such as t-BuOH, H(2)O, and N(2).     

Light‐Induced Mechanistic Divergence in Gold(I) Catalysis: Revisiting the Reactivity of Diazonium Salts

In a systematic study of the Au‐catalyzed reaction of o‐alkynylphenols with aryldiazonium salts, we find that essentially the same reaction conditions lead to a change in mechanism when a light source is applied. If the reaction is carried out at room temperature using a Au^I catalyst, the diazonium salt undergoes electrophilic deauration of a vinyl Au^I intermediate and provides access to substituted azobenzofurans. If the reaction mixture is irradiated with blue LED light, C?C bond formation due to N₂‐extrusion from the diazonium salt is realized selectively, using the same starting materials without the need for an additional photo(redox) catalyst under aerobic conditions. We report a series of experiments demonstrating that the same vinyl Au^I intermediate is capable of producing the observed products under photolytic and thermal conditions. The finding that a vinyl Au^I complex can directly, without the need for an additional photo(redox) catalyst, result in C?C bond formation under photolytic conditions is contrary to the proposed mechanistic pathways suggested in the literature till date and highlights that the role of oxidation state changes in photoredox catalysis involving Au is thus far only poorly understood and may hold surprises for the future. Computational results indicate that photochemical activation can occur directly from a donor–acceptor complex formed between the vinyl Au^I intermediate and the diazonium salt.     

Copper‐Catalyzed CH Bond Direct Chalcogenation of Aromatic Compounds Leading to Diaryl Sulfides,Selenides, and Diselenides by Using Elemental Sulfur and Selenium as Chalcogen Sources Under Oxidative Conditions

The reactions of aromatic compounds and elemental chalcogens catalyzed by a copper salt with molecular oxygen as an oxidant were carried out. The reaction of 3‐substituted imidazo[1,5‐a]pyridines and elemental sulfur in the presence of CuTC (copper(I) thiophenecarboxylate) gave the corresponding bisimidazopyridyl sulfides in good to quantitative yields. The reaction proceeded even under aerobic oxidation conditions. The use of a polar solvent was crucial for the reaction, and DMSO (dimethyl sulfoxide) in particular stimulated the reaction. The reaction could be applied to common aromatic compounds, such as N‐methyl indole and dialkyl anilines. The reaction of indole proceeded at the nucleophilic C3 position rather than at the acidic C2 position. In addition, the reaction of dialkyl anilines proceeded with an ortho, para orientation. The reactions of imidazopyridines and elemental selenium under similar conditions gave the corresponding bisimidazopyridyl diselenides along with bisimidazopyridyl monoselenides. The resulting diselenides were readily converted to the corresponding monoselenides with unreacted imidazopyridines under the same conditions. The reaction could be applied to the copolymerization of bifunctional bisimidazopyridines and elemental sulfur to give oligomeric copolymers in quantitative yield.     

Synthesis of functionalized iodoalkenes using a multicomponent reaction triggered by electrophilic iodination of alkenyldiazoacetates

Diazo compounds are frequently used as precursors of metal carbenoids and act as soft nucleophiles even without the use of metal catalysts. The resulting diazonium species may also be trapped by various nucleophiles. The introduction of an iodine functionality applicable for the coupling reaction into an alkenyl diazo compound, however, has not been reported. We developed iodoalkoxylation reactions of alkenyl diazoacetates using an electrophilic iodinating reagent and oxygen nucleophile. This catalyst-free multicomponent reaction proceeded regioselectively, furnishing trisubstituted vinyl iodides in 31%–71% yield. The synthesized iodoalkenes were converted into the corresponding olefins with various functionalities in good yield via Suzuki-Miyaura coupling reactions with arylboronic acids under palladium catalysis to demonstrate the synthetic utility of the developed reaction sequence.     

Copper‐Promoted Sandmeyer Difluoromethylthiolation of Aryl and Heteroaryl Diazonium Salts

An efficient copper‐promoted difluoromethylthiolation of aryl and heteroaryl diazonium salts is described. The reaction is conducted under mild reaction conditions and various functional groups were compatible. In addition, reactions of heteroaryl diazonium salts such as pyridyl, quinolinyl, benzothiazolyl, thiophenyl, carbazolyl, and pyrazolyl diazonium salts occurred smoothly to afford the medicinally important difluoromethylthiolated heteroarenes. Furthermore, a more practical one‐pot direct diazotization and difluoromethylthiolation protocol was developed, and it converts the aniline derivatives into difluoromethylthiolated arenes. The utility of the method is demonstrated by difluoromethylthiolation of a number of natural products and drug molecules.     

Tandem and chemoselective synthesis of benzil derivatives from styrene and arene diazonium salts

A facile and practically applied protocol for synthesis of benzil derivatives using styrene and arene diazonium salts is reported. Pd(OAc)₂/SeO₂ catalytic system was found to be efficient for chemoselective synthesis of benzil. Selenium dioxide works well as an oxidant under milder reaction conditions. Moderate to very good yields of the desired products were obtained.     

Palladium-imidazolium carbene catalyzed Mizoroki-Heck coupling with aryl diazonium ions

[reaction: see text] Catalyst formed from N,N-bis(2,6-diisopropylphenyl)dihydroimidazolium chloride and palladium(II) acetate (2 mol %) was used, without added base, to efficiently produce Heck coupled products with olefins and aryl diazonium tetrafluoroborate substrates. The reactions were performed at room temperature, giving product in 2-4 h with 80-90% yields for isolated materials. Diazonium ions, formed in situ directly from anilines, also couple under these conditions.     

Metal-free regioselective construction of 2-aryl-2H-tetrazol-5-yl difluoromethylene phosphonates

Three bench-stable difluoromethylene phosphonate hydrazones were prepared from simple diethyl(difluoromethyl)phosphonate within two steps in good yields. The [3 + 2] cycloaddition reaction of these diazo precursors with aryl diazonium salts has been accomplished under metal-free conditions with exclusive regioselectivity. This transformation provides practical access to a broad panel of 2-aryl-2 H-tetrazol-5-yl difluoromethylene phosphonates, including the corresponding derivatives of amino acid...     

Addition of heteroaromatics to alkylidenecyclopropanes catalyzed by palladium

The reaction of heteroaromatics, such as furans, thiophenes, thiazoles, and pyrroles, with alkylidenecyclopropanes proceeded smoothly in the presence of palladium catalysts, producing the corresponding alpha-allylated products in good to high yields. For example, the reaction of 3-butylpentylidenecyclopropane (1a) with 2-methylfuran (2a), ethyl 2-thiophenecarboxylate (4a), 2-isobutylthiazole (6a), and 1-methylpyrrole (11b) gave the alpha-allylated products in 70%, 66%, 77%, and 30% yield, respectively. The reaction proceeded predominantly through distal bond cleavage. The order of reactivity of heteroaromatics toward 1a is as follows: furan > thiophene approximately thiazole (5-position) > thiazole (2-position) approximately pyrrole. This methodology provides a means for introducing an allylic group to various kinds of heteroaromatics under mild conditions.     

Pd/light-accelerated atom-transfer carbonylation of alkyl iodides: applications in multicomponent coupling processes leading to functionalized carboxylic acid derivatives

The atom-transfer carbonylation reaction of various alkyl iodides thereby leading to carboxylic acid esters was effectively accelerated by the addition of transition-metal catalysts under photoirradiation conditions. By using a combined Pd/hν reaction system, vicinal C-functionalization of alkenes was attained in which α-substituted iodoalkanes, alkenes, carbon monoxide, and alcohols were coupled to give functionalized esters. When alkenyl alcohols were used as acceptor alkenes, three-component coupling reactions, which were accompanied by intramolecular esterification, proceeded to give lactones. Pd-dimer complex [Pd(2)(CNMe)(6)][PF(6)](2), which is known to undergo homolysis under photoirradiation conditions, worked quite well as a catalyst in these three- or four-component coupling reactions. In this metal/radical hybrid system, both Pd radicals and acyl radicals are key players and a stereochemical study confirmed the carbonylation step proceeded through a radical carbonylation mechanism.     

Significant enhancement of monooxygenase activity of oxygen carrier protein hemocyanin by urea

Oxygenation of a series of p-substituted phenols to the corresponding catechols (phenolase activity) by the (mu-eta2:eta2-peroxo)dicopper(II) species of Octopus hemocyanin has been directly examined for the first time by using a UV-vis spectroscopic method in a 0.5 M borate buffer solution containing 8 M urea under anaerobic conditions. Preliminary kinetic studies have indicated that the reaction involves an electrophilic aromatic substitution mechanism as in the case of phenolase reaction of tyrosinase. The oxygenation of phenols by hemocyanin also proceeded catalytically when the reaction was carried out under aerobic conditions.     

Spontaneous functionalization of carbon black by reaction with 4-nitrophenyldiazonium cations

The mechanism of the spontaneous chemical functionalization of Vulcan carbon black by reaction with 4-nitrophenyl diazonium cations was investigated by varying the reaction conditions. First, the carbon black was oxidized by nitric acid reflux to introduce oxygenated functionalities onto the surface prior to the functionalization step. Second, a reducing agent (H3PO2) was added to a solution containing 4-nitrobenzene diazonium tetrafluoroborate to generate 4-nitrophenyl radicals homogeneously in the bulk solution. The functionalized carbons were characterized by elemental analysis, X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption isotherms using the BET isotherm and DFT Monte Carlo simulations. These characterization methods were employed to determine the grafting yield as a function of the reaction conditions. Interestingly, the grafting yield was not affected by a change in the reaction conditions. An average nitrogen content of 1.4 +/- 0.1 atom % was found by elemental analysis, and XPS showed a nitrogen surface concentration of about 3.5%. XPS also indicated an important decrease in the concentration of oxygenated functionalities upon grafting 4-nitrophenyl moieties onto the oxidized carbon black. Presumably, in this case the grafting involves either the coupling of carboxylate and 4-nitrophenyl radicals or, more likely, a concerted decarboxylation where the diazonium cation, acting as an electrophile, replaces the oxygenated groups and loss of CO2. The nitrogen adsorption isotherms of the functionalized carbon blacks suggested that the grafted groups were most probably localized at the entrance of the micropores.     

AgONO‐Assisted Direct CH Arylation of Heteroarenes with Anilines

A novel copper‐catalyzed C?H arylation of heteroarenes with anilines by an in situ diazonium reaction is established by using silver nitrite (AgONO) as an unconventional nitrosating reagent under acid‐free conditions. It provides a complementary approach for the C?H arylation of electron‐rich heteroarenes with aromatic amines affording a variety of heterobiaryls in moderate to good yields.     

Synthesis of functionalized diaryldimethylstannanes from the Me2Sn2- dianion by SRN1 reactions

[reaction: see text] The reaction of different ArCl with the Me(2)Sn(2)(-) dianion in liquid ammonia under irradiation afforded Ar(2)SnMe(2) in good yields (68-85%). The results obtained clearly indicate that the reaction proceeded through an S(RN)1 reaction. As a synthetic application of these Ar(2)SnMe(2), the homocoupling is described in the presence of Cu(NO(3))(2).2.5H(2)O to afford the biaryls. These reactions proceeded almost quantitatively.     

Room‐Temperature,Ligand‐ and Base‐Free Heck Reactions of Aryl Diazonium Salts at Low Palladium Loading: Sustainable Preparation of Substituted Stilbene Derivatives

The Pd(OAc)₂‐catalyzed Heck reaction of aryl diazonium salts with 2‐arylacrylates led to cis‐stilbenes with good to excellent stereoselectivity. The environmentally friendly protocol developed in this work features low palladium loading in technical‐grade methanol at room temperature under base‐, additive‐, and ligand‐free conditions. The same protocol applied to simple Heck coupling of aryl diazonium salts with methyl acrylate allows astonishingly low palladium loading, down to 0.005 mol %. The stereoselectivity experimentally observed for the synthesis of cis‐stilbenes has been rationalized by DFT calculations. Moreover, the role of methanol in promoting the reaction has been clarified by a computational study.     

Synthesis of organofunctionalized polysiloxane from polymeric tributylstannyl ester of silicic acid and chlorosilanes

Polymeric tributylstannyl ester of silicic acid (PTBS) was demonstrated to be an effective material for the preparation of functionalized polysiloxane derivatives (2). The reaction of PTBS and monochlorosilane (1), which possessed an organofunctional group such as the acetoxy or methylmethacrylate group, proceeded readily at room temperature under neutral conditions to give 2 with the elimination of the tributyltin group. The molecular weight of 2 estimated by GPC measurement was almost the same as that of starting PTBS. Such metal-exchange reaction was applicable for the incorporation of two kinds of silyl units into polysiloxane 2 by employing these monochlorosilanes 1 at the same time in the reaction. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2363–2369, 1999     

Dediazoniations of Arene Diazonium Ions in Homogenous Solution. Part IV: Change-over from a heterolytic to a homolytic mechanism in 2,2,2-trifluoroethanol pyridine mixtures

There are only two dediazoniation products of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE), namely phenyl 2,2,2-trifluoroethyl ether ( 1 ) and fluorobenzene ( 2 ). The reaction kinetics are strictly first-order with respect to the diazonium salt. The addition of increasing amounts of pyridine to the system results in a gradual decrease in the yields of 1 and 2 and an increase in the yields of the homolytically formed products, benzene ( 3 ), biphenyl ( 4 ), isomeric phenylpyridines ( 5 ) and diazo tar ( 6 ). The reaction kinetics show that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine. The reaction with added pyridine is no longer first-order with respect to the diazonium ion. The product analyses and the kinetic data are consistent with the view that in pure TFE this diazonium salt decomposes completely by a heterolytic mechanism. The addition of pyridine brings about a competitive homolytic mechanism which becomes increasingly dominant as the concentration of pyridine increases.     

An Easy Route to (Hetero)arylboronic Acids

An unprecedented spontaneous reactivity between diazonium salts and diboronic acid has been unveiled, leading to a versatile arylboronic acid synthesis directly from (hetero)arylamines. This fast reaction (35 min overall) tolerates a wide range of functional groups and is carried out under very mild conditions. The radical nature of the reaction mechanism has been investigated.     

几种芳香萘环重氮盐热解性质的研究

重氮盐是日益广范应用的光敏和热敏材料。国内外学者对重氮盐热解稳定性等问题的研究早有报道.前人研究认为:重氮盐热分解时发生C—N键的断裂,分为异裂和均裂两种.在酸性水溶液中为异裂反应,生成芳烃正离子;在碱性水溶液或醇溶液中,大多发生均裂反应,生成游离基.Brown及角田等研究发现:在苯环重氮基对位上引入强正极性基能增加重氮盐的热稳定性,但Crossley等人发现,在重氮基对位上的取代基,无论是吸电子的硝基(-NO_2)或是给电子的甲氧基(-OCH_3),都能增加重氮盐的热稳定性.然而人们还未能从理论上对于上述问题予以深入明确的探讨.取代基的不同、热解产物的不同以及