An unprecedented rearrangement of salicylanilide derivatives: imidazolinone intermediate formation

The preparation of new prodrug forms of anti-tuberculosis active salicylanilide esters with amino acids led to an unexpected rearrangement. The isolation and the structure determination of 2-(5-chloro-2-hydroxyphenyl)-3-(3-chlorophenyl)-5-substituted-3,5-dihydro-4H-imidazol-4-ones unambiguously confirm one of the two proposed reaction mechanisms.     

The formation of a stable o-semidine rearrangement intermediate

Treatment of 5,6,11,12-tetrahydrodibenzo-[b, f][1,2] diazocine with dilute acids affords a spiro-1,2,3,4-tetrahydroquinoline derivative which represents the first isolated stable o-semidine rearrangement intermediate.     

An efficient modification of the Hofmann rearrangement: synthesis of methyl carbamates

A series of methyl carbamates was synthesized using NaOCl as an oxidant in the presence of KF/Al₂O₃/MeOH at reflux in excellent yields.     

A microfluidic flow chemistry platform for organic synthesis: the Hofmann rearrangement

We report on the use of commercially available chemical microreactors to effect the Hofmann rearrangement of aromatic amides to the corresponding carbamates.     

浅谈Hofmann重排反应在有机合成中的应用

讨论了Hofmann重排反应的机理以及烃基的结构、介质中的亲核试剂对反应的影响,并介绍了此重排在有机合成中的应用。     

Efficient synthesis of methyl carbamate via Hofmann rearrangement in the presence of TsNBr2

An efficient method has been developed for the synthesis of carbamates from the corresponding amides via the Hofmann rearrangement using N,N-dibromo-p-toluenesulfonamide (TsNBr₂) in the presence of DBU in methanol. The reaction goes into completion in 10–20 min at 65 °C to produce the corresponding carbamate in excellent yield.     

Reaction of iminopropadienones with amines: mechanistic explanations of zwitterionic intermediate, ketene and ketenimine formation

The complex reaction of thermally generated iminopropadienones with amines in the gas phase and upon matrix deposition and its varying product composition is investigated using density functional theory. In the high energy gas phase addition a single amine molecule reacts readily with iminopropadienone with the decisive step being a 1,3-hydrogen shift and activation barriers of at least 100 kJ/mol. In accordance with the experiment, the formation of ketenes is favored. In the condensed phase of an amine matrix, the utilization of amine dimers both as reagents and as explicit solvents lowers the activation energy required to a feasible 20-30 kJ/mol and predicts ketenimines as the main products, as observed experimentally.     

The mechanism of selective purine C-nitration revealed: NMR studies demonstrate formation and radical rearrangement of an N7-nitramine intermediate

Modified purine derivatives are of great importance in biomedical sciences, and substitution reactions on the purine skeleton are intensively studied. In our laboratory, an efficient and selective purine C2-nitration reaction was developed using a mixture of tetrabutylammonium nitrate and trifluoroacetic anhydride. The resulting 2-nitro moiety appeared to be a versatile handle to introduce a variety of pharmacophores onto the purine skeleton. Since the mechanism of this selective purine C2-nitration reaction has remained unclear, we now present an extensive NMR study leading to its elucidation, using N9-Boc-protected 6-chloropurine as a model compound. Direct electrophilic aromatic nitration of the highly electron-deficient C2 position was excluded, and we demonstrate that this reaction occurs in a three-step process. Electrophilic attack by trifluoroacetyl nitrate on the purine N7 position results in a nitrammonium species that is trapped by a trifluoroacetate anion furnishing N7-nitramine intermediate 11. This intermediate was characterized at -50 degrees C by (1)H, (13)C, (15)N, and (19)F NMR. At T > -40 degrees C, the N7-nitramine intermediate undergoes a nitramine rearrangement, which generates a C2-nitro species that immediately eliminates TFA to give 2-nitro-6-chloro-9-Boc purine 10. The involvement of radicals during the nitramine rearrangement was unequivocally established by (15)N-CIDNP. Moreover, the emission signal observed for the rearranged product, 2-nitropurine 10, showed that it is primarily formed in an intermolecular process. A quantitative radical trapping experiment finally disclosed that 65-70% of the nitramine rearrangement takes place intermolecularly.     

Reaction of 1,4-phthalazinedione with furfural: formation of the [5,6]benza-3a,7a-diazaindane system via an unusual skeletal rearrangement

[reaction: see text] Oxidation of phthalahydrazide (1) with lead tetraacetate in the presence of furfural (3a) in methylene chloride gives [5,6]benza-3a,7a-diaza-3-carboxylindane-4,7-dione-1-ene (7a) in 64% yield. 5-Methylfurfural (3b) also reacted similarly to give the product 7b in 46% yield. Reaction of phathalazine-1,4-dione with thiophene-2-carboxaldehyde gives N-(2-formylthiophene)phthalahydrazide.     

Intramolecular azide trapping of the Nazarov intermediate: formation of peroxy-bridged indolizidinones via a deep-seated rearrangement and aerobic oxidation

Cross-conjugated dienones with pendent azide side chains undergo interrupted Nazarov trapping, leading to peroxy-bridged indolizidinones in good yields. This process is proposed to involve skeletal rearrangement of the initial trapping product, with loss of dinitrogen, to give an intermediate 1,4-betaine, which then undergoes reaction with atmospheric oxygen. The endoperoxide products can be reduced under catalytic hydrogenation conditions to furnish alpha-hydroxylactams. [reaction: see text].     

Iron complexes with monocarboxylate anions: Models of their formation

The composition, stability, existence ranges, and degrees of accumulation of iron complexes in solutions of formic, acetic, propionic, butyric, and valeric acids have been determined with the use of the oxidation function and suggested principle of equilibrium modeling from the experimental data obtained by the oxidation potential method. The calculated model parameters can serve as the reference material for targeted synthesis and different thermodynamic calculations.     

聚丙烯酰胺分子量对其Hofmann反应的影响

协同效应;高分子效应;聚丙烯酰胺分子量对其Hofmann反应的影响     

The stereochemistry of the methylenecyclopropane rearrangement: evidence against the orthogonal-allylic intermediate

The rearrangements of two optically pure derivatives of Feist's acid were studied: that of dimethyl trans-methylenecyclopropane-2,3-dicarboxylate (TRANS-diester) and trans-2,3-dicyano-methylenecyclopropane (TRANS-dinitrile). The optical purity and configuration of the products, methyl (Z)- and (E)2-carbomethoxyclopropylideneacetate (SYN- and ANTI-diesters) and (Z)- and (E)-2-cyanocyclopropylideneacetonitrile (SYN- and ANTI-dinitriles), establish that the rearrangements occur with predominant, but not exclusive, inversion of configuration at the migrating center. Investigation of the interconversions of (SYN)- and ANTI-diesters and dinitriles reveal that racemic product is not obtained, as would be expected from an orthogonal-allylic diradical intermediate, but that the enantiomer corresponding to antarafacial migration is slightly favored. All of the stereochemical results are explicable by application of the Doering-Sachdev diradical transition state hypothesis     

Stepwise cope rearrangement of cyclo-biphenalenyl via an unusual multicenter covalent pi-bonded intermediate

Multicenter covalent pi-bonding between pi-conjugated radicals has been recently recognized as a novel and important bonding interaction. The Cope rearrangement of cyclo-biphenalenyl 9 is studied by exploring its potential energy surface with density functional theory (DFT), and it is found that pi-bonding plays a critical role in the rearrangement process. Affected by this, the rearrangement of 9 takes place by a stepwise mechanism through an unusual pi-intermediate 10, of C2h symmetry, which can be characterized as a 2 x 13pi + 2 x 2pi system. The pi-intermediate has a long inter-phenalenyl distance of R approximately 2.8 angstroms, which is shorter than the sum of the van der Waals radii displaying multicenter covalent pi-bonding between the two phenalenyl units. The energy of the pi-intermediate 10 is higher than that of the sigma-bonded reactant 9 by approximately 2 kcal/mol according to the employed spin-restricted DFT. NMR chemical shift calculations support the sigma-bonded 9 as the global minimum. The calculated activation barrier of approximately 6 kcal/mol for the Cope rearrangement is consistent with the stepwise mechanism. A covalent pi-bonding effect in the pi-intermediate 10 is demonstrated indirectly by the shortening of inter-naphthalene distance of the dianion and dication of the cyclophane 14 compared to that of its neutral counterpart. The unusual pi-bonded structure with a long inter-phenalenyl distance becomes the most stable structure for the ethano-bridged derivative 13, which should have observable paramagnetism according to the calculated paramagnetic susceptibility.