- Palladium-Catalyzed Oxidative Allylation of Sulfoxonium Ylides: Regioselective Synthesis of Conjugated Dienones
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The first examples of palladium-catalyzed allylic C-H oxidative allylation of sulfoxonium ylides to afford the corresponding conjugated dienones with moderate to good yields have been established. The features of this novel conversion include mild reaction conditions, wide substrate scope, and excellent regioselectivity.
- Li, Chunsheng,Li, Meng,Zhong, Wentao,Jin, Yangbin,Li, Jianxiao,Wu, Wanqing,Jiang, Huanfeng
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supporting information
p. 872 - 875
(2019/05/16)
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- Nickel-Catalyzed Cross-Coupling of Allyl Alcohols with Aryl- or Alkenylzinc Reagents
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Nickel-catalyzed cross-coupling of allyl alcohols with aryl- and alkenylzinc chlorides through C-O bond cleavage was performed. Reaction of (E)-3-phenylprop-2-en-1-ol and 1-aryl-prop-2-en-1-ols with aryl- or alkenylzinc chlorides gave linear cross-coupling products. Reaction of 1-phenyl- or 1-methyl-substituted (E)-3-phenylprop-2-en-1-ol with aryl- or alkenylzinc chlorides resulted in 3-aryl/alkenyl-substituted (E)-(prop-1-ene-1,3-diyl)dibenzenes or 3-aryl/alkenyl-substituted (E)-(but-1-enyl)benzene. Reaction of allyl alcohol with p-Me2NC6H4ZnCl resulted in a mixture of normal coupling product 4-allyl-N,N-dimethylaniline and its isomerized product N,N-dimethyl-4-(prop-1-en-1-yl)aniline.
- Yang, Bo,Wang, Zhong-Xia
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p. 4542 - 4549
(2017/05/12)
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- Flow Metal-Free Ar-C Bond Formation via Photogenerated Phenyl Cations
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A convenient photochemical flow protocol for the formation of aryl-carbon bonds via photogenerated phenyl cations has been developed. A wide range of phenylated products, including biaryls, allylarenes, 2-arylacetals and benzyl γ-lactones, was smoothly synthesized in satisfactory yields under metal-free conditions. The adoption of a flow reactor often allowed us to adopt higher concentrations of substrates and shorter irradiation times compared to those usually employed in batch systems.
- Bergami, Matteo,Protti, Stefano,Ravelli, Davide,Fagnoni, Maurizio
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supporting information
p. 1164 - 1172
(2016/04/19)
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- Trifluoromethylchlorosulfonylation of alkenes: Evidence for an inner-sphere mechanism by a copper phenanthroline photoredox catalyst
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Abstract A visible-light-mediated procedure for the unprecedented trifluoromethylchlorosulfonylation of unactivated alkenes is presented. It uses [Cu(dap)2]Cl as catalyst, and contrasts with [Ru(bpy)3]Cl2, [Ir(ppy)2(dtbbpy)]PF6, or eosin Y that exclusively give rise to trifluoromethylchlorination of the same alkenes. It is assumed that [Cu(dap)2]Cl plays a dual role, that is, acting both as an electron transfer reagent as well as coordinating the reactants in the bond forming processes. Double role: The trifluoromethylchlorosulfonylation of unactivated alkenes was developed using [Cu(dap)2]Cl as catalyst (dap=2,9-bis(para-anisyl)-1,10-phenanthroline). [Cu(dap)2]Cl plays a dual role; acting as an electron transfer reagent as well as coordinating the reactants in the bond forming processes.
- Bagal, Dattatraya B.,Kachkovskyi, Georgiy,Knorn, Matthias,Rawner, Thomas,Bhanage, Bhalchandra M.,Reiser, Oliver
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supporting information
p. 6999 - 7002
(2015/06/08)
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- Aryl imidazylates and aryl sulfates as electrophiles in metal-free ArSN1 reactions
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Some oxygen-bonded substituents were investigated as leaving groups in photoinduced ArSN1 reactions. Irradiation of aryl imidazylates and of the corresponding imidazolium salts mainly caused homolysis of the ArO-S bond. However, previously unexplored trifluoroethoxy aryl sulfates were found to undergo efficient metal-free arylation. The sulfates were conveniently generated in situ by dissolving the corresponding imidazolium salts in basic 2,2,2-trifluoroethanol.
- Qrareya, Hisham,Protti, Stefano,Fagnoni, Maurizio
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p. 11527 - 11533
(2015/01/16)
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- Practical iron-catalyzed allylations of aryl grignard reagents
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An operationally simple iron-catalyzed reductive cross-coupling reaction between aryl halides and allyl electrophiles has been developed. The underlying domino process exhibits high versatility with respect to the allylic leaving group (acetate, tosylate, diethyl phosphate, methyl carbonate, trimethylsilanolate, methanethiolate, chloride, bromide) and high economic and environmental sustainability with respect to the catalyst system (0.2-5 mol% tris(acetylacetonato)iron(III), ligand-free) and reaction conditions (tetrahydrofuran, 0°C, 45 min).
- Mayer, Matthias,Czaplik, Waldemar M.,Von Wangelin, Axel Jacobi
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supporting information; experimental part
p. 2147 - 2152
(2010/12/18)
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- Solarylations via 4-aminophenyl cations
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(Chemical Equation Presented) The application of the photo-SN1 reaction on some 4-chloroanilines was explored under solar irradiation in view of obtaining a convenient metal-free arylation method. Several reactions previously carried out by UV irradiation, as well as some new ones, where either a new trap (α-methylstyrene) or a new halide (N,N-dimethyl-4- fluoroaniline) were adopted, were studied under these conditions and found to occur conveniently. Furthermore, at least in some cases the halide starting concentration could be raised up to 0.2 M, the excess trapping agent reduced from 20:1 to 2.5:1, and the solvent replaced by more environmentally friendly (co)solvents including water. Under these improved conditions, the photoarylation was carried out in a gram scale by merely exposing the solution to solar irradiation. This process has a low impact on the environment and can be considered a serious competitor of metal-catalyzed arylations. 2010 American Chemical Society.
- Dichiarante, Valentina,Fagnoni, Maurizio,Albini, Angelo
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experimental part
p. 1271 - 1276
(2010/04/26)
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- Development of highly chemoselective bulky zincate complex, tBu 4ZnLi2: Design, structure, and practical applications in small-/macromolecular synthesis
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We present full details of the unique reactivities of the newly developed dianion-type bulky zincate, dilithium tetra-ferf-butylzincate (tBu 4ZnLi2). With this reagent, halogen-zinc exchange reaction of variously functionalized haloaromatics and anionic polymerization of N-isopropylacrylamide (NIPAm)/styrene with excellent chemoselectivity were realized. Halogen-zinc exchange reaction followed by electrophilic trapping with propargyl bromide provided a convenient route to functionalized phenylallenes, particularly those with electrophilic functional groups (such as cyano, amide and halogens). Spectral and computational studies of the structure in the gas and liquid phases indicated extraordinary stabilization of this dianion-type zincate by its bulky ligands.
- Furayama, Taniyuki,Yonehara, Mitsuhiro,Arimoto, Sho,Kobayashi, Minoru,Matsumoto, Yotaro,Uchiyama, Masanobu
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supporting information; experimental part
p. 10348 - 10356
(2009/10/10)
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- Cross-coupling of aromatic bromides with allylic silanolate salts
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The sodium salts of allyldimethylsilanol and 2-butenyldimethylsilanol undergo palladium-catalyzed cross-coupling with a wide variety of aryl bromides to afford allylated and crotylated arenes. The coupling of both silanolates required extensive optimization to deliver the expected products in high yields. The reaction of the allyldimethylsilanolate takes place at 85°C in 1,2-dimethoxyethane with allylpalladium chloride dimer (2.5 mol %) to afford 73-95% yields of the allylation products. Both electron-rich and sterically hindered bromides reacted smoothly, whereas electron-poor bromides cross-coupled in poor yield because of a secondary isomerization to the 1-propenyl isomer (and subsequent polymerization). The 2-butenyldimethylsilanolate (E/Z, 80:20) required additional optimization to maximize the formation of the branched (γ-substitution) product. A remarkable influence of added alkenes (dibenzylideneacetone and norbornadiene) led to good selectivities for electron-rich and electron-poor bromides in 40-83% yields. However, bromides containing coordinating groups (particularly in the ortho position) gave lower, and in one case even reversed, selectivity. Configurationally homogeneous (E)-silanolates gave slightly higher γ-selectivity than the pure (Z)-silanolates. A unified mechanistic picture involving initial γ-transmetalation followed by direct reductive elimination or σ-π isomerization can rationalize all of the observed trends.
- Denmark, Scott E.,Werner, Nathan S.
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supporting information; experimental part
p. 16382 - 16393
(2009/05/08)
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- The β effect of silicon in phenyl cations
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Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The β effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N- dimethylaniline, the triplet cation remained in the ground state and trapping by π nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.
- Dichiarante, Valentina,Salvaneschi, Andrea,Protti, Stefano,Dondi, Daniele,Fagnoni, Maurizio,Albini, Angelo
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p. 15919 - 15926
(2008/09/20)
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- Characterizing ionic liquids as reaction media through a chemical probe
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The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the W-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by it nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C-H bond a to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.
- Dichiarante, Valentina,Betti, Cecilia,Fagnoni, Maurizio,Maia, Angelamaria,Landini, Dario,Albini, Angelo
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p. 1834 - 1841
(2008/02/04)
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- Metal-free cross-coupling reactions of aryl sulfonates and phosphates through photoheterolysis of aryl-oxygen bonds
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Photochemical cleavage of Ar-O bonds in phenyl esters substituted with electron-donating groups offers a convenient method for the arylation of alkenes and arenes. The reaction proceeds by an SN1 mechanism via a triplet phenyl cation to form allylbenzene or biphenyl derivatives under mild conditions (see scheme).
- De Carolis, Marco,Protti, Stefano,Fagnoni, Maurizio,Albini, Angelo
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p. 1232 - 1236
(2007/10/03)
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- (Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations. Chemoselectivity of the singlet and triplet 4-substituted phenyl cations
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(Chemical Equation Presented) The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X = H, t-Bu, and NMe2 and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X = Br, CN) or overcomes (X = COMe, NO2) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of π nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.
- Milanesi, Silvia,Fagnoni, Maurizio,Albini, Angeld
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p. 603 - 610
(2007/10/03)
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- Homolytic vs Heterolytic Paths in the Photochemistry of Haloanilines
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The photochemistry of 4-haloanilines and 4-halo-N,N-dimethylanilines has been studied in apolar, polar aprotic, and protic solvents. Photophysical and flash photolysis experiments show that the reaction proceeds in any case from the triplet state. It is rather unreactive in apolar media, the highest value being Π = 0.05 for the iodoanilines in cyclohexane. Changing the solvent has little effect for iodoanilines and for the poorly reacting bromo analogue, while it leads to a variation of over 2 orders of magnitude in the quantum yield for the chloro and fluoro derivatives. The triplets have been characterized at the UB3LYP/6-31G(d) level of theory, evidencing a deformation and an elongation (except for C-F) of the C-X bond. Homolytic fragmentation is in every case endothermic, but calculations in acetonitrile solution show that heterolytic cleavage of C-Cl and C-Br is exothermic. Experimentally, the occurrence of heterolytic fragmentation has been monitored through selective trapping of the resulting phenyl cation by allyltrimethylsilane. Heterolytic dechlorination occurs efficiently in polar media (e.g., Π = 0.77 in MeCN), while debromination remains ineffective due to the short lifetime of the triplet. Heterolytic defluorination is efficient only in protic solvents (Π = 0.48 in MeOH), in accord with calculations showing that in the presence of an ancillary molecule of water fragmentation is exothermic due to the formation of the strong H-F bond. The energy profile for both homo- and heterolytic dissociation paths has been mapped along the reaction coordinates in the gas phase and in acetonitrile. The conditions determining the efficiency and mode of dehalogenation have been defined. This is significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.
- Freccero, Mauro,Fagnoni, Maurizio,Albini, Angelo
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p. 13182 - 13190
(2007/10/03)
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- Cationic arylation through photo(sensitised) decomposition of diazonium salts. Chemoselectivity of triplet phenyl cations
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The photodediazoniation of some 4-X-phenyldiazonium tetrafluoborates in MeCN leads to the singlet phenyl cations (X = H, tert-butyl, NMe2, CN), which add to the solvent yielding the corresponding acetanilides. Triplet sensitisation, however, leads to the triplet phenyl cation, which is reduced in neat solvent and is trapped by π nucleophiles (allyltrimethylsilane and benzene), resulting in an ionic analogue of the Meerwein or Gomberg arylations. With the 4-nitro derivative intersystem crossing prevails over dediazoniation from the singlet and with the 4-cyano competes with it, so that in those cases the triplet phenyl cation is formed also upon direct irradiation.
- Milanesi, Silvia,Fagnoni, Maurizio,Albini, Angelo
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p. 216 - 217
(2007/10/03)
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- Photoinduced, ionic Meerwein arylation of olefins
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Irradiation of 4-chloroaniline or of its N,N-dimethyl derivative in polar solvents generates the corresponding triplet phenyl cations. These are trapped by alkenes yielding arylated products in medium to good yields. B3LYP calculations show that the triplet cation slides with negligible activation energy to a bonded adduct with ethylene, whereas it forms only a marginally stabilized CT complex with water (chosen as a representative σ nucleophile). The structure of the final products depends on the preferred path from the adduct cation with the alkene. In the case of aryl olefins, this deprotonates to stilbene derivatives, while, from 2,3-dimethyl-2-butene and allytrimethylsilane, allylanilines are obtained by elimination of an electrofugal group in γ. In the case of mono- and disubstituted alkenes the cation adds chloride rather than eliminating and β-chloroalkylanilines are obtained. The regio- and sterochemistry of the addition across the alkene are best understood with a phenonium ion structure for the adduct. The nucleophile entering in fi can be varied under conditions in which the adduct cation is trapped more efficiently than the starting phenyl cation. Thus, β-methoxyalkylanilines are formed when the irradiation is carried out in methanol. β-Iodoalkylanilines are obtained in acetonitrile containing iodide and unsubstituted alkylanilines in the presence of sodium borohydride. A case of intramolecular nucleophilic trapping is found with 4-pentenoic acid. The reaction is a wide-scope ionic analogue of the radicalic Meerwin arylation of olefins.
- Mella,Coppo,Guizzardi,Fagnoni,Freccero,Albini
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p. 6344 - 6352
(2007/10/03)
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- Smooth synthesis of aryl- and alkylanilines by photoheterolysis of haloanilines in the presence of aromatics and alkenes
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Irradiation of 4-chloro-N,N-dimethylaniline in acetonitrile in the presence of benzene and of various alkenes leads to heterolytic dehalogenation and trapping of the cation. 4-(Dimethylamino)biphenyl is formed in the first case, while with alkenes β-chloroalkylanilines, stilbenes, or allylanilines are obtained depending on the alkene structure. 4-Fluoroaniline is similarly dehalogenated.
- Fagnoni, Maurizio,Mella, Mariella,Albini, Angelo
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p. 1299 - 1301
(2008/02/09)
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- Nickel catalyzed tellurium-zinc exchange reactions. A new preparation of arylzinc reagents
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Diaryltellurides 2 and diarylditellurides 3 undergo a smooth tellurium-zine exchange reaction in the presence of catalytic amounts of Ni(acac)2 (5-10 mmol %) leading to arylzinc derivatives 1. The reaction can be extended to the preparation of alkylzinc compounds and allows a stereoselective cyclization to a 2,4-disubstituted tetrahydrofuran by a radical ring closure of an unsaturated telluride.
- Stuedemann, Thomas,Gupta, Vijay,Engman, Lars,Knochel, Paul
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p. 1005 - 1008
(2007/10/03)
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