Perchloric acid protonation kinetics with

The detritiation of [3H]-2,4,6-trimethoxybenzene by aqueous perchloric acid was also studied, the second-order rate coefficients (107/c2) being determined as 5.44, 62.0, and 190 at 0, 24.6, and 36.8 °C, respectively, whilst with phosphate buffers, values were 3.75, 13.8, and 42.1 at 24.6, 39.9, and 55.4 °C, respectively. The summarised kinetic parameters for these studies are given in Table 134, and notable among the values are the more negative entropies of activation obtained in catalysis by the more negative acids. This has been rationalised in terms of proton transfer... 

Kresge et a/.498 have drawn attention to the fact that detritiation of [3H]-2,4,6-trihydroxy- and [3H]-2,4,6-trimethoxy-benzenes by concentrated aqueous perchloric acid gives correlations of log rate coefficient with — H0 with slopes of 0.80 and 1.14 respectively. Protonation to give the carbon conjugate acids is, however, governed by h0lA0 and h0l 9S, respectively, which suggests that the difference in kinetic acidity dependence is a property of the substrate and should not be interpreted as a major difference in mechanism. The kinetic difference can be eliminated by an appropriate comparison of kinetic and equilibrium acidity dependencies. In equation (230)... 

When sample components having ionizable groups are chromatographed the use of a background electrolyte and control of the eluent pH with an m>propridte buffer are mandatory. It is advisable to maintain a fairly high concentration of buffer in the medium in order to rapidly reestablish protonic equilibria and to thereby avoid peak sjditting or asymmetrical peaks due to slow kinetic processes. Acetic acid, phosphoric acid, and perchloric acid and their salts have been used for the control of pH. 

Eastham and Derwent474 have also studied the kinetics of the perchloric acid-catalyzed reaction of ethylene oxide with pyridine. In excess of pyridine the rate was found to be dependent on the Conor Titrations of ethylene oxide and perchloric add. Addition of stronger bases,. g. ammonia, triethylamine, or benzylamiae, depressed the vum of cleavage, presumably by competing with ethylene oxide for thr-available proton source, believed to be pyridinium perchlorate in this case. Other acids examined included nitric acid and hydroiodie irireaction rate depended to a certain extent... 

A formal total synthesis of ( )-morphine has been achieved by adopting the above synthetic route (Scheme 18). The tetrahydropyridine 91, prepared from the reaction of A/ -methyl-4-piperidone with 2,3-dimethoxy-phenyllithium, followed by dehydration, was converted to the bicyclic en-amine 92 by treatment with the ylic dibromide. Kinetic protonation of 92 with perchloric acid gave the trans-fused immonium salt, which upon dissolution in methanol equilibrated to the thermodynamically prefered cis isomer 93. Treatment of 93 with diazomethane brought about the formation of the aziridinium salt 94, which was readily transformed into the a-amino aldehyde 95 by its oxidation with dimethyl sulfoxide. It is also worth noting that the Komblum oxidation of aziridinium salts leads to the construction of a-amino aldehydes efficiently. Lewis-acid-catalyzed cyclization of 95 afforded the morphinan carbinol 96 in 80% yield. Successive mesylation and reduction of the mesylate derived from 96 with LiBEtjH afforded morphinan (97) in excellent yield. In this instance, direct conversion of 93 to 97 by treatment with diazomethane gave approximately 1 % of the desired product. Lemieux-Johnson oxidation of 97 under acidic conditions furnished the ketone 98, which was previously transformed into ( )-morphine by Gates. In order to confirm the structure of 98, its conversion to the known... 

Acid hydrolysis of oxaziridines was investigated kinetically and found to be of first order and Ho-dependent. 2-t-Butyl oxaziridines with an alkyl or aryl group in position 3 (16) were studied. The acidity-rate-profile approximated a constant rate given by complete protonation of 16. pK values are between -1-0.13 and —1.81, rate constants of hydrolysis in dilute perchloric acid at 25°C are 43.4 x 10 min (R = p-nitrophenyl) and 1.49 x 10 min (R = phenyl). O-Protonation is assumed to be followed by C—O bond cleavage (16a). In the case of 3-alkyloxaziridines there is considerable competition from O—N bond cleavage. 

Kinetic studies of oxidation of ce-amino acids (arginine, treonine, and glutamic acid) with CBT, leading to nitriles in 90% yields, were carried out in aqueous acetic acid in the presence and absence of perchloric acid and chloride ions [87JCS(P2)1569] (Scheme 97). Reaction rate decreases with the increase in the acidity of the medium and increases with the increase in the concentration of chloride ion. Under the studied conditions amino acids exist in a protonated form, and the complex formed by protonated CBT and the chloride ion acts as an oxidant. The mechanism of the reaction, which coincides with the kinetic data, includes initial slow chlorination of the amino group followed by subsequent formation of /V,A/-dichloramino derivatives and their transformations to the final reaction products (Scheme 98). 

Deprotonation to the enamine anion, selective coupling with the allylic terminus of dibromide 114, followed by an intramolecular enamine alkylation, afforded reduced isoquinoline 119. A rather elegant conversion to aminoaldehyde 122 ensued. Immonium ion formation in 119 via protonation with perchloric acid at first yielded the kinetic trans isomer, which underwent equilibration upon reflux in methanol to give the corresponding crystalline cis product 120. Diazomethane treatment led to aziridinium salt 121, which upon exposure to DMSO, ring opened with concomitant oxidation in a Komblum fashion to the aldehyde 122.63 Treatment with Lewis acid effected B-ring closure, thus... 

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

When Ni(LL)(Me4-l,4-benzoquinone), where LL = a bidentate cyclic alkene such as cot, cod, nor, or en /o-dicyclopentadiene, reacts with trimethyl phosphite, it is the alkene LL which is replaced first, en route to the product Ni(P OMe 3)4. This first step is bimolecular, producing an intermediate containing unidentate alkene. Some kinetic parameters are reported, particularly for LL = cyclo-octa-1,5-diene. The formation of the cation [NiH(P OEt 3)4]+ in perchloric acid-methanol solution appears to involve direct protonation of the nickel. Activation parameters for this are = 13 1 kcalmoL and AS = — 2 3 cal deg mol . 

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