Regioselectivity of fluorine substitution by alkoxides on unsymmetrical difluoroarenes

An efficient approach to unsymmetrical halogenated resorcinol diethers has been developed. This synthesis consists of two subsequent nucleophilic aromatic substitutions (S_NAr) of unsymmetrical difluoroarenes by alkoxides. The novelty of this approach is its control of regioselectivity during the first S_NAr, which occurs at room temperature. Interestingly, the reactivity of competing fluorines was correlated to their chemical shift in ¹⁹F NMR.     

Intramolecular aromatic nucleophilic substitution of the benzimidazole-activated nitro group

A wide range of 2-(2-nitrophenyl)-1H-benzimidazoles undergo high-yielding intramolecular S(N)Ar of nitrite with N-pendant alkoxides under mild conditions (DMF, rt). When this operationally simple process is carried out at elevated temperatures in the presence of excess NaH, the initially formed S(N)Ar products are converted to the corresponding N-vinyl-substituted 2-(2-hydroxyphenyl)-1H-benzimidazoles via base-catalyzed isomerization. [reaction: see text]     

Imidazole as leaving group in aromatic nucleophilic substitution reactions

The reaction of N-(2,4,6-trinitrophenyl) imidazole with n-butylamine is pH dependent. A mechanism involving acid catalyzed leaving group departure is suggested.     

Activation of carbon dioxide by divalent tin alkoxides complexes

A series of terminal tin(II) alkoxides have been synthesized utilizing the bulky β-diketiminate ligand [{N(2,6-(i)Pr(2)C(6)H(3))-C(Me)}(2)CH] (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)SnOR only reacts with highly activated aliphatic electrophiles such as methyl triflate, but reacts reversibly with carbon dioxide. Both the rate of reaction and the degree of reversibility is dependent upon minor changes in the alkoxide ligand, with the bulkier tert-butoxide ligand displaying slower reactivity than the corresponding isopropyl ligand, although the latter system is a more exergonic reaction. Density Function Theory (DFT) calculations show that the differences in the reversibility of carbon dioxide insertion can be attributed to the ground-state energy differences of tin alkoxides while the rate of reaction is attributed to relative bond strengths of the Sn-O bonds. The mechanism of carbon dioxide insertion is discussed.     

P4 activation by group 3 metal arene complexes

The direct P(4) activation using group 3 metal complexes was achieved for the first time under mild conditions. Two P(n)-containing products, P(8)(4-) and P(7)(3-), were isolated and characterized for scandium; however, P(7)(3-) was the sole product for yttrium.     

Ultrasound-promoted aromatic nucleophilic substitution of dichlorobenzene iron(II) complexes

The nucleophilic aromatic substitution under ultrasound irradiation of a dichlorobenzene iron η⁶-complex with various secondary amines is reported. The reaction time at moderate temperatures is considerably shortened (15 min) compared to non sonicated reaction conditions at room temperature (several days) or at solvent refluxing temperature (12-48 h). Controlled mono- or di-substitution was achieved by the tuning of the amine nucleophilicity and the solvent polarity. The method was successfully applied to the synthesis of differently substituted phenylenediamines.     

Enantioselective borane reduction of aromatic ketones catalyzed by chiral aluminum alkoxides

The asymmetric borane reduction of prochiral ketones with an alkoxide catalyst prepared in situ from aluminum tri-iso-propoxide and (R)-binaphthol was examined. Using these conditions, alcohols were obtained in high yield and e.e.'s of up to 83%.     

Investigation of the reaction order for nucleophilic substitution of dialkyl methylphosphonates by alkoxides

The rate of nucleophilic substitution at the phosphorus centre of dialkyl methylphosphonates by methoxide and ethoxide has been studied to investigate the possible involvement of hexacoordinated phosphorus species in this reaction. For alkoxide concentrations less than ca. 1.5 M the rate increases with the square of alkoxide concentration. However, consideration of the activity of the alkoxides, represented by an appropriate acidity function, reveals that only one equivalent of alkoxide is involved in the rate-determining step. Thus, there is no requirement to invoke the intermediacy of a hexacoordinated species in the reaction pathway. © John Wiley & Sons, Inc.     

Electrochemical substitution of a carbonyl group by a phosphorus ligand in arenechromium tricarbonyl complexes

Electrochemical oxidation of (η-1,3,5-Me₃C₆H₃)Cr(CO)₃ in the presence of P(OEt)₃ with subsequent electrochemical reduction results in the formation of a (η-1,3,5-Me₃C₆H₃)Cr(CO)₂[P(OEt)₃] and (η-1,3,5-Me₃C₆H₃)Cr(CO)[P(OEt)₃]₂ mixture. Under similar conditions (η⁶-arene)Cr(CO)₃, where arene = 3,5-Me₂C₆H₃(CH₂)₂OPR₂ (R = OEt,OPh,F), yields the corresponding arenephosphite chelate complexes.     

Facile introduction of SH group on aromatic substrates via electrophilic substitution reactions

Herein, we describe a mild and efficient two-step procedure to introduce a thiol group on aromatic substrates. First, reaction with an activated sulfoxide leads to an arylsulfonium salt intermediate. Then, two successive beta-elimination-based dealkylation reactions afford the desired arylthiols in good to excellent yields.     

Room temperature metathesis of aryl isocyanates and aromatic aldehydes catalyzed by group(IV) metal alkoxides: An experimental and computational study

Aromatic aldehydes and aryl isocyanates do not react at room temperature. However, we have shown for the first time that in the presence of catalytic amounts of group(IV) n-butoxide, they undergo metathesis at room temperature to produce imines with the extrusion of carbon dioxide. The mechanism of action has been investigated by a study of stoichiometric reactions. The insertion of aryl isocyanates into the metal n-butoxide occurs very rapidly. Reaction of the insertion product with the aldehyde is responsible for the metathesis. Among the n-butoxides of group(IV) metals, Ti(OⁿBu)₄ (8aTi) was found to be more efficient than Zr(OⁿBu)₄ (8aZr) and Hf(OⁿBu)₄ (8aHf) in carrying out metathesis. The surprisingly large difference in the metathetic activity of these alkoxides has been probed computationally using model complexes Ti(OMe)₄ (8bTi), Zr(OMe)₄ (8bZr) and Hf(OMe)₄ (8bHf) at the B3LYP/LANL2DZ level of theory. These studies indicate that the insertion product formed by Zr and Hf are extremely stable compared to that formed by Ti. This makes subsequent reaction of Zr and Hf complexes unfavorable.     

Inter- and intramolecular activation of aromatic C-H bonds by diphosphine and hydrido-bridged dinuclear iridium complexes

Reactions of [(Cp*Ir)2(mu-dmpm)(mu-H)2]2+ (1) with NaOtBu in aromatic solvent at room temperature give [(Cp*Ir)(H)(mu-dmpm)(mu-H)(Cp*Ir)(Ar)]+ [Ar = Ph (3), p-Tol (4a), m-Tol (4b), 2-furanyl (5a), 3-furanyl (5b)] via intermolecular aromatic C-H activation. Treatment of [(Cp*Ir)2(mu-dppm)(mu-H)2]2+ (2) with base (Et2NH) results in intramolecular C-H activation of the phenyl group in the dppm ligand to give [(Cp*Ir)(H){mu-PPh(C6H4)CH2PPh2}(mu-H)(Cp*Ir)]+ (6). The structures of 3, 5a, and 6 have been determined by X-ray diffraction methods.     

Nucleophilic aromatic substitution of hydrogen through lithiated phosphine borane complexes and N-phosphorylphosphazenes

Organophosphorus compounds containing nitroaryl and cyanoaryl groups have been prepared in good yield through nucleophilic aromatic substitution of hydrogen using α-lithiated phosphazenes and phosphine borane complexes as nucleophiles. In all cases, nearly exclusive replacement of the hydrogen in the para position with respect to the activating group has been observed.     

Practical synthesis of aromatic ethers by SNAr of fluorobenzenes with alkoxides

Aromatic fluorines have been substituted by alkoxides in a variety of activated and unactivated aromatic systems.     

Novel catalyst compositions for the syndiospecific polymerization of styrene prepared by the combination of cyclopentadenyl complexes of group IIA or group IIIA elements with titanium alkoxides

A highly reactive catalyst system, which induces the syndiospecific polymerization of styrene with high activity, has been found by the combination of cyclopentadienyl (Cp) complexes of group IIA or group IIIA elements with titanium alkoxides. The ¹H NMR monitoring of these reactions reveals the occurrence of a novel Cp‐transfer reaction that leads to the generation of Cp‐containing titanium complexes as catalysts for promoting the syndiospecific polymerization of styrene. Detailed in situ ¹H NMR studies reveal that the rate of the Cp‐transfer reaction is highly dependent on the steric bulkiness of the titanium alkoxide complexes, the structures of the Cp complexes of group IIA or group IIIA elements, and the polymerization temperature. Styrene polymerization studies also reveal that a more effective Cp‐transfer reaction can typically lead to the generation of a more highly reactive catalyst for sPS polymerization. This study provides a convenient method for the in situ generation of variable structures of Cp/titanium alkoxide complexes, which are difficult to synthesize by other methods. Most importantly, the mixture of a catalyst precursor can be directly used as an sPS polymerization catalyst without isolation and purification of Cp/titanium complexes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2304–2315, 2005