Isomerization thermally-induced

Further expansion of 13-vertex species or thermal metal transfer reactions leads to the 14-vertex cluster [(T -C H )Co]2C2B2qH22 [52649-56-6] and [52649-57-7] (199). Similar 14-vertex species have been obtained from tetracarbaboranes (203) and show unusual stmctures. The isomeric bimetallic cobaltacarborane complexes /(9j (9-(Tj -CpCo)2C2BgH2Q (cp = C H ) can be formed by either polyhedral expansion or contraction reactions. Six isomers of this cluster are formed in the thermally-induced intermolecular metal transfer and polyhedral expansion of the 11-vertex f/oj o-(ri -C H )CoC2BgH Q. 

A variety of 1-azirines are available (40-90%) from the thermally induced extrusion (>100 °C) of triphenylphosphine oxide from oxazaphospholines (388) (or their acyclic betaine equivalents), which are accessible through 1,3-dipolar cycloaddition of nitrile oxides (389) to alkylidenephosphoranes (390) (66AG(E)1039). Frequently, the isomeric ketenimines (391) are isolated as by-products. The presence of electron withdrawing functionality in either or both of the addition components can influence the course of the reaction. For example, addition of benzonitrile oxide to the phosphorane ester (390 = C02Et) at... 

The facile, photoinduced valence isomerization of ethyl 1//-azepine-l-carboxylate to ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylatehas been studied as a potential solar energy storage system.101102 Unfortunately, the system proved to be inefficient due to build up of polymeric material during the thermally induced, exothermic retro-reaction. 

Finally, thermally induced isomerizations which generate carbon-centered biradical organic molecules have been shown to serve as alternative for conventional chemical and photochemical methods [71]. A straightforward procedure to accomplish such biradicals was described by Myers using a thermal conversion of yne-allenes [72]. According to this scheme, Wang and coworkers [73] heated 3-178 in 1,4-cyclohexadiene to 75 °C and obtained 3-181 in 22% yield via the biradicals 3-179 and 3-180 (Scheme 3.48). 

The bora-2,5-cyclohexadienes 25, 43, and 44 have been used in experimental studies. Photochemical reaction of 25 and 43 with Fe(CO)s produces the robust complexes 45 (29) and 46 (92) with 7)5-divinylborane structures (92), whereas thermally induced complex formation of 44 with Fe2(CO)9 is accompanied by ligand isomerization and affords complex 47 (67). We note in passing that S(UB) = 38.8 ppm for 47 is at rather low field. The only strictly comparable boraolefin known is l-methoxy-6-(trimethylsilyl)bora-2,4-cyclohexadiene [8(nB) = 47.1 ppm] (26). On this basis, the high field shift upon complexation (only 8.3 ppm) indicates weak Fe-B interaction. 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

The thermally induced Cornforth rearrangement can be rationalized by postulating the dicarbonylnitrile ylid 103 as intermediate. It is especially the natures of the substituents R and R" that determine the direction of the rearrangement. For example, heating of 5-alkoxy-4-(aminocarbonyl)oxazole (102, R = OAlkyl, R" = NR2) readily gives in a good yield rearrangement into the isomeric 5-(substituted amino)-4-(alkoxycarbonyl) oxazole (104, R = OAlkyl, R" = NR2) (Scheme IV.40). 

Some nitro derivatives of benzofurazan have been investigated for their explosive properties. 4-Amino-5,7-dinitrobenzofurazan (56) has been prepared by a number of routes including (1) the thermally induced cyclodehydration of l,3-diamino-2,4,6-trinitrobenzene (55), (2) the nitration of 4-amino-7-nitrobenzofurazan and (3) the reduction of 4-amino-5,7-dinitrobenzofuroxan with triphenylphoshine. The isomeric 5-amino-4,7-dinitrobenzofurazan (57) has been prepared along similar routes. ... 

Enaminone 128 (Scheme 33) is obtained, together with an isomeric indo-lizine derivative, by flash vacuum thermolysis of aminomethylene Meldrum s acid derivative through intermediate ketene and delocalized azomethine ylide (85TL833). The thermally induced cyclization of semi-cyclic dienamines to afford, for instance, tricyclic 129 is also believed to start with an azomethine ylide (97JOC7744) the p-chlorophenyl substituent is essential for the reaction. Unstabilized ylide 130, on the other hand, is generated from pipecolinic acid and /1-phenylcinnamaldehyde by the decarboxylation method target base 131 is formed by 1,7-electrocycliza-tion and [l,5]-hydrogen shift (99J(P1)2605). 

Both experiment and calculations have shown that the diazo compounds are the more thermodynamically stable isomers and in some cases, notably for A -silyl substituents, thermally induced isomerizations from 149 to give 148 can be an important factor. 

On the basis of a study of the thermally induced isomerizations of several 2H- and 3//-azepines a generalization concerning the stability of azepines has been proposed which states that sigmatropic 1,5-H shifts in the isomerizations of azepines always proceed in the direction of the C=N rather than in the direction of the N=C bond (74JOC3070). Occasionally mixtures of 3H-azepines are produced. 

The well known thermally induced isomerization of an isoimide to an imide was the chemistry selected to test the concept. A series of high molecular weight polyisoimides was prepared based on PMDA and pendent aromatic diamines that on thermal treatment would exhibit the required geometry for reinforcement. Polymerizations of the diamines with PMDA were carried out in DMAC (10% by weight) at room temperature in a dry nitrogen atmosphere. Subsequent cyclodehydration of the polyamic acid to the corresponding polyisoimide was... 

Like isoconjugate mesomeric betaines, which have been discussed in preceding sections, phthalazinium-l-olates (196) give 1,3-dipolar adducts with acetylenes. Thermally induced isomerization reactions of these adducts (255) make them of special interest. The A-methyl derivative and diphenyl-... 

A unique example is a thermally induced cyclotrimerization catalyzed by Si2Cl6.538 In this case, terminal alkynes are transformed to isomeric mixtures. 

Thermally induced isomerization of 33 was suggested to take place through the formation of the acyclic intermediate 128 [66JCS(C)2031]. Recyclization of the latter gives the isomeric l,2,4-triazolo[l,5-a]pyrimidines 121 (Scheme 53). 

Thermally induced isomerization of the benzocyclobutane (83) gives the pyrroloisoquino-line (84), the pyridine ring of which can be aromatized with iron(III) chloride (Scheme 29). Similarly, the aldehyde (85 X = 0) or its O-methyl oxime (85 X = NOMe) gives the cis-trans mixture (86). If p- toluenesulfonic acid is included in the reaction mixture the pure cis compounds are obtained (72AG(E)l03l). 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to /ra/M-(109)] and racemization [i.e. (/< )-( 109) to (S)-( 109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

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