Anion-radical mechanism of bromine exchange by iodine in furan derivatives

The kinetics of bromine exchange by iodine in furan derivatives were studied; it is shown that the reaction is reversible and inhibited by oxidizing agents. Chloro derivatives of furan do not undergo this reaction. A cryptoreductive mechanism is proposed; the first stepof this mechanism consists of electron transfer to the halofuran molecule with subsequent reversible dissociation of the anion radical — inclusion of the iodide anion in the dissociation equilibrium leads to the substitution product.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1601–1606, December, 1976.     

Kinetics of deuterium exchange of furan and methylfurans in dimethylsulfoxide

Rates of exchange between deuterium in 2D-, 3D-, 2D-3CH₃-, and 2D-5CH₃ -furan and a solution of potassium tert-butoxide in dimethylsulfoxide are measured. Protophilic deuterium exchange at position 2 in the furan molecule is 600 times faster than that at position 3, while a methyl group at position 3 or 5 retards it 10-fold. Catalytic activities of potassium and lithium butoxides are compared.We thank A. E. Berzin for supplying 3-chloromercurifuran, and P. P. Alikhanov and L. Ya. Mashkova for checking the purities of the compounds by GLC.     

Kinetics of nucleophilic halogen exchange in 1-alkyl-2-(5′ -halo-2′- furyl) benzimidazoles

The kinetics of the reaction of 1-alkyl-2-(5-halo-2-furyl)benzimidazoles and their corresponding quaternary salts with secondary aliphatic amines were studied. The effect on the activation parameters of this reaction of the nature of the halogen being replaced by the secondary amine residue, the presence of a positive charge on the imidazole ring, the character of the anion in the quaternary salts, and the remoteness of the reaction center from the activating grouping on the activation parameters of this reaction is discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 252–257, February, 1971.     

Labelling of long chain fatty acids by non isotopic nucleophilic halogen exchange

The parameters of two exchange methods of preparing pure 97% labelled 17-Br-HDA in acetone and in benzene with dry NaI^* in a closed system are described. In aprotic solvents the need for a phase transfer catalyst up to 50g is necessary to dissolve the dry NaI^*. The use of aqueous medium in the exchange is totally prohibited. Energies of activation are calculated for both reactions.     

Kinetics and mechanism of the reaction of 5-bromo-2- carbonyl derivatives of furan with amines

The kinetics of exchange of bromine in 5-bromo-2-carbonyl derivatives of furan by dimethylamino and morpholino groups was studied. The reaction is kinetically complicated for 5-bromofurfural (I) — aldehyde I initially reacts simultaneously at both the carbonyl group to give the aminocarbinol and at the bromine atom with exchange by an amino group. The accumulation of the ammonium salt in the latter reaction leads to the development of an autocatalytic reaction, the product of which is 5-N,N-dialkylaminofurfurylidene-N,N-dialkylammonium bromide. Amines react with the ketones without complications; the second-order rate constants are presented. The rate of exchange decreases in the order CHO > COC₆H₅ > COCH=CHC₆H₅ > COCH₃ > CH=CHCOC₆H₅.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1465–1470, November, 1976.     

Synthesis and properties of furan derivatives 2. Role of the halogen in RMgX in reactions with alkoxytetrahydrofurans

Heating 2-methoxytetrahydrofuran with ethylmagnesium bromide in benzene at reflux leads to the formation of 2-ethoxytetrahydrofuran and slight amounts of 4-methoxy-1-hexanol. Under analogous conditions, 2-ethoxytetrahydrofuran is quantitatively cleaved by methylmagnesium iodide to give 4-ethoxy-1-pentanol.For Communication 1, see [1]Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 592–594, May, 1992.     

Synthesis of 6-[18F]fluoroveratraldehyde by nucleophilic halogen exchange at electron-rich precursors

Summary Fluorodehalogenation reactions were used to prepare 6-[¹⁸F]fluoroveratraldehyde. The synthesis of 6-[¹⁸F]fluoroveratraldehyde is the first step in the multi-step synthesis of the clinically important tracer 6-[¹⁸F]fluoro-L-dopa. In the literature yields ranging from 20-50% are reported when using nitro and trimethylammoniumtriflate precursors. However, no data exist concerning the use of different leaving groups such as halogens. Therefore, 6-bromo, 6-chloro and 6-fluoroveratraldehyde were tested in the nucleophilic aromatic substitution by [¹⁸F]fluoride. In DMF, 6-[¹⁸F]fluoroveratraldehyde was obtained with radiochemical yields of (57±1.0)% and (66±3.6)% in 20 minutes at 160 °C using 50 mg/ml bromo and chloro precursor, respectively. The fluoro precursor gave a radiochemical yield of (87±0.8)% at 140 °C. Temperature, solvent and concentration strongly affected the ¹⁸F-labeling. Among the halogens the ability as a leaving group was F>>Cl>Br. The halogenated veratraldehydes provide a good alternative for the synthesis of ca and nca 6-[¹⁸F]fluoroveratraldehyde, as the first step of the synthesis for [¹⁸F]FDOPA since they are inexpensive, commercially available, stable, sustain hard conditions in the labeling step, and give yields better or equal to other precursors previously reported.     

Theoretical study of the mechanism of nucleophilic substitution in cyclobutane derivatives

The mechanism of nucleophilic substitution in cyclobutane derivatives has been studied by means of ab initio molecular orbital computations on a model system. The results are in favour of a reaction which proceeds with inversion of configuration, in agreement with experimental data.     

Investigation of the lability of furan and its derivatives in nucleophilic substitution reactions by the mindo/3 method

The a complexes of furan, thiophene, pyrrole, pyridine, and benzene with the hydride ion were calculated by the MINDO/3 method. The affinities of the indicated aromatic rings and a number of substituted furans for the H^– ion were estimated. It is shown that the relative magnitude of the affinity correlates qualitatively with the experimental data on the relative rates of the reaction of the examined compounds with nucleophilic reagents. The following order of increasing reactivities of unsubstituted aromatic rings was obtained: pyrrole相似文献   

Electron paramagnetic resonance studies on oxidative polymerization mechanism of furan derivatives

The oxidative polymerization of furan, 3-methylfuran and 2,2′,5′,2″-terfuran, induced by iodine in organic solvents having different donicity, was investigated by time-resolved electron paramagnetic resonance (EPR) spectroscopy. The reaction was monitored starting from the onset of the process and radical species arising from the monomers in early stages of the polymerization reaction were detected. The comparison between experimental and theoretical hyperfine coupling constants, obtained by simulated EPR spectra and theoretical ab initio calculations, respectively, allowed one to structurally characterize the above radical species, thus enabling mechanistic conclusions about oxidative coupling. Moreover, the analysis of the time evolution of EPR spectrum during the oxidative polymerization process was used for evaluating the relative amount of localized and mobile radical species along the polymer backbone, thus getting preliminary information about conjugation extent and electroconducting properties of the final material. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1901–1910, 1998     

Kinetics and mechanism of nucleophilic addition to nitric oxide: secondary amine diazeniumdiolation

The kinetics of nitric oxide (NO) addition to dipropylamine and pyrrolidine are determined for a range of solvents and NO pressures. A new isobaric technique is utilized to determine the order of NO, and the kinetics are shown to conform to reactions which are either mono- or bimolecular in NO depending upon the conditions. The mechanism is interpreted in terms of competitive addition of amine to either NO or its dimer.     

Kinetics and mechanism of nucleophilic substitution of dichloroarylazopyridinepalladium(II) by pyridine bases

The reaction of dichloroarylazopyridinepalladium(II) [Pd(aap)Cl2, aap=4-RC6H4N-N-2-C5H4N; R= H (1), Me (2), Cl (3)] with pyridine bases [RPy: R-H (a), 2-Me(b), 4-Me(c), 4-Cl(d), 2-Ph(e), 2-PhCH2(f)] has been studied spectrophotometrically in MeCN at 400nm. The products (4) have been isolated and characterized as trans-Pd(RPy)2Cl2. The kinetics of the nucleophilic substitution have been examined under pseudo-first-order conditions with respect to base at 298K and follow the rate law, Rate=k[RPy]2 [Pd(aap)Cl2]. The rate data supports a nucleophilic association path. External addition of Cl– (LiCl) suppresses the rate, which follows the order: k(3)> k(1)>k(2), where k values are linearly related to Hammet constants. 2-Substitution in the pyridine base remarkably reduces the rate compared with 4-substitution and is attributed to a steric effect that destabilizes the transition state. The rate decreases with increasing steric crowding at the ortho-position and follows the order: (e)>(f)>(b). The 4-substituted pyridines control the rate via the inductive effect and follow the order: (d)>(a)>(c).     

Synthesis and transformations of furan derivatives

Condensation of thiosemicarbazones of furfural, 3-(2-furyl)acrolein, as well as their 5-nitro derivatives with chloroacetone by boiling in alcohol or acetic acid gives the corresponding 4-methylthiazolyl-(2)hydrazones. 4-Methylthiazolyl-(2)-hydrazones of 5-nitro-2-acetylfuran and 5-nitro-2-furfurilydeneacetone can be prepared by condensing the corresponding thiosemicarbazones with chloroacetone by heating with glacial acetic acid containing fused sodium acetate.For Part IV see [1].     

Kinetics and mechanism of mercuridestannylation of phenylstannatranes and related derivatives

The kinetics and mechanism of mercuridestannylation of phenylstannatrane derivatives, PhSn(OCH₂CH₂)₃N and PhSn(OC₆H₄)₃N have been investigated relative to model systems PhSnBu₃, PhSn(OMe)₃ in solvent MeOH. For the latter two, the reaction proceeded via the following patternn with k₂ > 10 k′₂

For the stannatranes, step (iii) was not observed. The order of reactivity was shown to be PhSnBu₃ > PhSn(OCH₂CH₂)₃N = PhSn(OC₆H₄)₃ N ⪢ PhSn(OMe)₃.Arguments are made for rate-determining CSn bond fission in reaction (i). Stabilisation of the developing the charge on Sn is important and the enhanced reactivity of the stannatranes is explained by SnN transannular interactions.Values of k₂ and k′₂ were obtained for PhSnBu₃ and PhSn(OMe)₃ which enabled K to be evaluated in each case.     

Synthesis of carbaalane halogen derivatives

The carbaalane halogen derivatives [(AlX)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (X = F (9), Cl (7), Br (10), I (11)) were prepared in toluene from [(AlH)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (6) and BF(3).OEt(2), BX(3) (X = Br, I), Me(3)SnF, and Me(3)SiX (X = Cl, Br, I), respectively. A partially halogenated product [(AlH)(2)(AlX)(4)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (12) (X = Cl (approximately 40%), Br (approximately 60%)) was obtained from 5 and impure BBr(3). [(AlH)(6)(AlNMe(3))(2)(CCH(2)Ph)(6)] (5) was converted to [(AlX)(6)(AlNMe(3))(2)(CCH(2)Ph)(6)] (X = F (13), Cl (14), Br (15), I (16)) using BF(3).OEt(2) and Me(3)SiX (X = Cl, Br, I), respectively. The X-ray single-crystal structures of 11.C(6)H(6), 12.3C(7)H(8), 13.6C(7)H(8), and 15.4C(7)H(8) were determined. Compounds 7 and 9-11 are soluble in benzene/toluene and could be well characterized by NMR spectroscopy and MS (EI) spectrometry. The results demonstrate the facile substitution of the hydridic hydrogen atoms in 5 and 6 by the halides with different reagents.     

Reactivity of nucleophilic uracil derivatives

The reactivity of uracil derivatives bearing acidic methylene groups at N1 was investigated. On treatment with strong base, compounds with nitrile as the activating group underwent extensive dimerisation to give new tricyclic systems. Ester-activated compounds also produced dimers in the absence of an external electrophile, but their monomeric carbanions could be trapped by prompt addition of benzyl bromide to give normally alkylated products.     

Multistep nucleophilic substitution of the halogen atom in 2-halo-2-alkenals

pso-Substitution of the halogen atom in 2-halo-2-alkenals under the action of secondary amines proceedsvia three consecutive steps: addition, nucleophilic substitution, and -elimination (the Ad_N-S_N-E mechanism). The main intermediates arising in the reactions of 2-chloropropenal and 2-chloro-2-butenal with piperidine were detected by NMR spectroscopy. A general scheme of the reaction is proposed.Translated fromIzvestiya Akademii. Seriya Khimiicheskaya, No 1.pp. 139–142.January, 1996.