Isomerization in solution

Velsko S P and Fleming G R 1982 Photochemical isomerization in solution. Photophysics of diphenylbutadiene J. Chem. Phys. 76 3553-62... 

Armand and coworkers have shown that, while 1,4-dihydropyrazines are the initial products of the electrochemical reduction of pyrazines, they could not be isolated and readily isomerize in solution into 1,2- or 1,6-dihydropyrazines depending on the substitution pattern in the heterocyclic ring (74CJC3971 84MI1). The rate of the isomerization depends on the type of pyrazine as well as the pH and the nature and amount of the cosolvent. 

Other thietane derivatives have been obtained by isomerization of nitrogen-containing heterocycles. The reaction of an acyl isothiocyanate (RCONCS) with diphenyldiazomethane gave 20 and this isomerized in solution to the tetraphenyl-3-thietanone 21 <96BSB253>. Additionally, the isoxazolidine 22 was converted into 23 by the action of trimethylsilyl iodide and zinc iodide <96H1211>. 

This equation has been used by Sundstrom and coworkers [151] and adapted to the analysis of femtosecond spectral evolution as monitored by the bond-twisting events in barrierless isomerization in solution. The theoretical derivation of Aberg et al. establishes a link between the Smoluchowski equation with a sink and the Schrodinger equation of a solute coupled to a thermal bath. The reader is referred to this important work for further theoretical details and a thorough description of the experimental set up. It is sufficient to say here that the classical link is established via the Hamilton-Jacobi equation formalism. By using the standard ansatz Xn(X,t)= A(X,i)cxp(S(X,t)/i1l), where S(X,t) is the action of the dynamical system, and neglecting terms in once this... 

Aberg, U., Akesson, E., Alvarez, J.-L., Fedchenia, I. and Sundstrom, V. Femtosecond spectral evolution monitoring the bond- twisting event in barrierless isomerization in solution, Chem.Phys., 183 (1994), 269-288... 

The 1,3,2-dioxaphosphorinans (101 R = Cl, NMe2, or OMe), derived from trans-2-hydroxymethylcyclopentanol, condensed with perfluorobiacetyl to give the expected phosphoranes.64 That from (101 R = OMe) probably had the relationship of methoxy to ring junction shown in (102) it slowly isomerized in solution at 40 °C to a 60 40 mixture with the isomer (103). The corresponding isomerizations... 

Insertion of 1,3-diynes into NiXMe(PMe3)2 (X = Cl, Br, I) gives the corresponding 1-alkynylalkenyl-nickel complexes 293, which undergo E/Z isomerization in solution, the former predominating. ... 

A -Oxatriazolines (66) are thermally unstable, undergoing rapid isomerization in solution with formation of (68) <64TL887,66TL405,72CB284l>. Betaines (67) are considered likely intermediates. The azo compound (68) may undergo a further reaction with the nitrile oxides used for oxatriazoline formation, most likely to form a new oxatriazoline (69) which isomerizes to (71) (Scheme 17)... 

The initial product 38, from aqueous basic potassium hexacyanofer-rate(III) oxidation, was first formulated as the 4,4 -adduct and later corrected to the 2,2 -isomer on the basis of IR spectra, although it is still apparently given the 4,4 -structure in later papers. It is piezochromic, and it also rapidly isomerizes in solution in the cold, via the triarylimidazolyl radical, to the l,2 -diraer 39, which is both photo- and thermochromic. The method of choice for preparing compounds 39, which are important materials in photo- and thermocopying processes (Section IV,A,2), is to carry out the oxidation by using basic hexacyanoferrate(III) at room temperature in the presence of benzene. ... 

A rate and equilibrium study of the cis/trans isomerization of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) in the vapour phase has been reported.761 The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 °C, the same value being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-1 the Cr—O bond energy is 210 kJ mol-1. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. 

The questions that have arisen in regard to isomerizations in solution also apply to isomerizations in clusters. One of these questions, which has now been addressed, is the threshold energy and its variation with size of the cluster. Data on the threshold energies were obtained in the microcanonical study by Heikal et al. [1], New questions, however, also arise for clusters How rapidly is the energy transferred between the solute and the solvent molecules in the cluster Outside the threshold region a reduction in krale(E) with... 

The activation energy of the thermal cis-trans isomerization in solution had been measured193 to be 37 kcal/mole and... 

Photolysis (> 290 nm) induced quantitative conversion of (20c and d) to the crystalline 3-azaquadricyclanes (22c and d) that isomerize in solution at 20° to the azepines (23c and d) at measurable rates. The azepines rapidly dimerize. The thermally forbidden 47 reversal of 22 to 20 was not observed.80 Prinzbach and Eberbach81 have carried out numerous cycloadditions and acid-catalyzed rearrangements of 22c and d and of 23c and d. The 3-azaquadricyclanes 22 have diene properties but, as valence isomerization to azepines is so easy (above), cycloadditions were achieved only with the very reactive dienophiles DMAD and EP. Reactions with 22c and d at -15° to - 30° over 24 to 48 hours using a large excess of ester gave 24 (R = H or E). If equimolecular proportions of the quadricyclane and ester are used, the products (24) compete as dienophile for example, 24d (R = H) added to 20d to give 25.81... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

Obviously the investigator should also be aware of any ground state reactions peculiar to the system under study. For example 2-azidopyridines isomerize to tetrazoles which are usually less sensitive than the azide to irradiation (Fig. 3.1). The position of the equilibrium is both temperature, solvent and pH dependent. Even if the equilibrium constant for such an isomerization in solution is known the situation in a ligand binding site cannot be predicted. Azide-tetrazole isomerization was an important consideration in experiments using 2-azidoadenine derivatives which have recently been described by MacFarlane et al. (1982). 

A4-Oxatriazolines (122) are thermally unstable, undergoing rapid isomerization in solution with formation of (123) (72CB2841, 66TL405, 64TL887). Betaines are considered likely intermediates (equation 73). 

It is generally accepted that the oxidative power of charcoal is due to the chemisorbed layer of oxygen on its surface.1 This oxygen layer appears to be important also in other ways T. W. J. Taylor s studies on the interconversion of stereoisomeric oximes and sulphoxides show that oxygen is necessary for this reaction. The mechanism appears to be an exchange of the active, adsorbed oxygens on the surface of the carbon, with the molecules of the substances undergoing isomerization in solution. The nature of the layers adsorbed on charcoal influences, naturally, its behaviour as an electrode, and the kind of ions which it sends into solution ... 

Another way to obtain chiral clusters is to use mixed-metal systems (see Section II,A). Addition of acid to one of the two diastereomers of [WCo(/i-HC2CH(OH)Et)(CO)5Cp] leads to formation of two propargylium cations which do not isomerize in solution on the NMR time scale.75 Similarly, protonation of the separate diastereomers of [MoCo(/i-2-propynylborneol) (CO)5Cp] leads to formation of propargylium cations which are nonflux-ional. It has been shown that the diastereomer ratios for metal-stabilized cationic clusters can be directly correlated with the ionization process, and it is proposed that the elimination of water is anchimerically assisted by... 

The difference of total energies of the two conformations 7 and 8 with symmetry Z)2 and Cu> respectively, for the dibenzotetrathiocine 6 (X = F) was calculated by density functional theory (DFT) to be only 4.6 kj mol 1 in favor of the twist boat 7 <1998CJC1093>. Indeed, 6 (X = F) undergoes a slow conformational isomerization in solution (Section 14.09.3.1) but adopts the chair conformation 8 in crystalline form (cf. Section 14.09.4). 

X-Ray crystallography has confirmed the existence of 1,2,5,6-tetrathiocine 3 as the twisted conformer 4 (see Section 14.09.2) <1996AGE2357>. The fluoro-substituted dibenzo-l,2,5,6-tetrathiocine 6 (X = F) adopts the chair conformation 8 in the crystalline state, while the chloro compound 6 (X = Cl) was found in the twist boat conformation 7 in solid state (see Section 14.09.2) <1998CJC1093>. The calculated difference in the energies of the two conformers 7 and 8 for the fluoro-substituted derivative 6 is very small (<5 kj mol-1), and its slow conformational isomerization in solution was... 

The if geometry is compatible with the orbital template for a C2vML3( Cplanar sulfur bound coordination (dihedral angle = 60°) and spectroscopic studies indicate r Cpianar to -( r-N=S) linkage isomerization in solution ... 

In contrast, phenylimidoylcarbodiimides PhN=C(R)N=C=NR, isomerize in solution to fom aminoquinazolines. Other imidoylcarbodiimides also undergo intramolecular rearrangements. For example, RR CHN=C(CCl3)N=C=NAr 1 undergoes a rearrangement to form dihydro-1,3,5-triazines 2 ... 

The compound different methods (148) tlie compounds show no tendency to isomerize in solution (155). The compound Rh(C0)(PPhg)2NCSe... 

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