Isomerization thermal

Fatemi F K, Fatemi D J and Bloomfield L A 1996 Thermal isomerization in isolated oesium-halide olusters Phys. Rev. Lett. 77 4895... 

As shown in Figure 1, the equiHbrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equiHbrium can be achieved when zeoHtes are used, owing to shape selectivity (see Molecularsieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

Uses ndReactions. a-Pinene (8) is useful for synthesizing a wide variety of terpenoids. Hydration to pine oil, acid-catalyzed isomerization to camphene, thermal isomerization to ocimene and aHoocimene, and polymerization to terpene resins are some of its direct uses. Manufacture of linalool, nerol, and geraniol has become an economically important use of a-pinene. 

Fig. 2. Acid treatment and thermal isomerization of a-pinene where R = CH. The dipentene (15) is 4 -lhnonene (18) is a-pyronene (19) is P-pyronene.

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

Uses ndReactions. Some of the principal uses for P-pinene are for manufacturing terpene resins and for thermal isomerization (pyrolysis) to myrcene. The resins are made by Lewis acid (usuaUy AlCl ) polymerization of P-pinene, either as a homopolymer or as a copolymer with other terpenes such as limonene. P-Pinene polymerizes much easier than a-pinene and the resins are usehil in pressure-sensitive adhesives, hot-melt adhesives and coatings, and elastomeric sealants. One of the first syntheses of a new fragrance chemical from turpentine sources used formaldehyde with P-pinene in a Prins reaction to produce the alcohol, Nopol (26) (59). 

Myrcene Manufacture. An important commercial source for mycene is its manufacture by pyrolysis of p-piaene at 550—600°C (87). The thermal isomerization produces a mixture of about 75—77 wt % myrcene, 9% limonene, a small amount of T -limonene [499-97-8] and some decomposition products and dimers. The cmde mixture is usually used without purification for the production of the important alcohols nerol and geraniol. Myrcene may be purified by distillation but every precaution must be taken to prevent polymerization. The use of inhibitors and distillation at reduced pressures and moderate temperatures is recommended. Storage or shipment of myrcene in any purity should also include the addition of a polymerization inhibitor. 

Oxidation of aHoocimene in the presence of a catalyst produces a polymeric peroxide, which can be thermally isomerized to produce aHoocimene diepoxide [3765-28 ] (56) in 70—75% yield (99). The diepoxide has been used in the manufacture of resins and as an acid scavenger for halogenated solvents (100). 

Another important process for linalool manufacture is the pyrolysis of i j -pinanol, which is produced from a-pinene. The a-pinene is hydrogenated to (73 -pinane, which is then oxidized to cis- and /n j -pinane hydroperoxide. Catalytic reduction of the hydroperoxides gives cis- and /n j -pinanol, which are then fractionally distilled subsequendy the i j -pinanol is thermally isomerized to linalool. Overall, the yield of linalool from a-pinene is estimated to be about 30%. 

Hoffmaim-La Roche has produced -carotene since the 1950s and has rehed on core knowledge of vitamin A chemistry for the synthesis of this target. In this approach, a five-carbon homologation of vitamin A aldehyde (19) is accompHshed by successive acetalizations and enol ether condensations to prepare the aldehyde (46). Metal acetyUde coupling with two molecules of aldehyde (46) completes constmction of the C q carbon framework. Selective reduction of the internal triple bond of (47) is followed by dehydration and thermal isomerization to yield -carotene (21) (Fig. 10). 

In the BASF synthesis, a Wittig reaction between two moles of phosphonium salt (vitamin A intermediate (24)) and C q dialdehyde (48) is the important synthetic step (9,28,29). Thermal isomerization affords all /ra/ j -P-carotene (Fig. 11). In an alternative preparation by Roche, vitamin A process streams can be used and in this scheme, retinol is carefully oxidized to retinal, and a second portion is converted to the C2Q phosphonium salt (49). These two halves are united using standard Wittig chemistry (8) (Fig. 12). 

Fig. 3. Photochemical and thermal isomerization products of vitamin D manufacture (49). The quantum yields of the reactions ate hsted beside the arrows...

Methylideneoxiranes (allene oxides) (80T2269), e.g. (9) and (23), are also highly reactive, and undergo facile thermal isomerization to cyclopropanones, possibly via an oxyallyl intermediate (24 Scheme 20). 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

Azabicyclo[2.2.0]hexa-2,5-diene, pentakis-(pentafluoroethyl)-synthesis, 2, 304 2-Azabicyclop.2.0]hexadiene reactivity, 7, 360 thermal isomerization, 7, 360 2-Azabicyclo[2.2.0]hexa-2,5-diene synthesis, 2, 304 1 -Azabicyclo[3.2.0]hexadiene synthesis, 7, 361 1 - Azabicyclo[2.2.0]hexane reactions, 7, 344 ring strain... 

Diazabieyelo[2.2.0]hexa-2,5-diene, perfluoro-IR speetra, 7, 360 Diazabieyelo[2.2.0]hexadienes thermal isomerization, 7, 360 Diazabieyelo[3.1.0]hexane, 7, 56... 

Isopterin — see Pteridin-2-one, 4-atnino-Isopyrazole, 4-hydroxy-rearrangement, 5, 250 Isopyrazole, tetramethyl-thermal isomerization, 5, 249 Isopyrazole, 3,4,4,5-tetramethyl-mass spectrometry, 5, 204 Isopyr azoles H NMR, 5, 185, 188 Af-oxide... 

Isoxazole, 5-(p-anisyl)-3-phenyI-synthesis, 6, 63 Isoxazole, 4-aroyl-3,5-diaryI-synthesis, 6, 71 Isoxazole, aryl-UV spectra, 6, 4 Isoxazole, 3-aryl-5-unsubstituted ring cleavage, 6, 30 Isoxazole, 5-aryl-3-chloro-reactions, 5, 104 Isoxazole, 3-aryI-4-formyl-synthesis, 6, 84 Isoxazole, 3-aryl-5-methoxy-thermal isomerization, 6, 15 Isoxazole, 3-aryl-5-methyl-synthesis, 6, 63, 84 Isoxazole, 4-azorearrangement, 5, 719... 

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

An interesting preparation of substituted o-aminophenols has been developed by Birkofer and Daum (30). 2-Acylfurans (8) plus an aliphatic secondary amine presumably condense to give the eorresponding enamine (9) (not isolated), which undergoes thermal isomerization to the o-amino-phenol (10). 

Dewar and Turchi described the Comforth rearrangement of 5-alkoxyoxazole-4-thiocarboxylates as a potentially general method for the synthesis of 5-thiooxazole-4-carboxylic esters. Specifically, they found that thiol ester 13 underwent thermal isomerization to the corresponding 5-thiooxazole 14 in 94% yield. 

Thermal isomerization and decomposition reaction of indole behind reflected shoek waves at 1050-1650 K have been explained by a preequilibrium between 17/-indole and 3//-indole (97JPC(A)7787). 

Anastassiou has summarized in two reviews the knowledge about IH-azonine (41a) [72ACR281 78AHC(23)55]. Compound 41a as well as its salts (N M" ) are aromatic compounds which exist as such and not as imine polyenic forms. Tliis compound demonstrates a valence isomerism 41a/41b similar to that of l//-azepine (14a/14c see Section II,A,1) the transformation 41a 41b occurs upon irradiation. 9-Azabicyclo[6.1.0]nona-2,4,6-triene 41b displays no tendency to thermal isomerization to 41a at ambient temperature (72ACR281). 

Arylfuroxans 23 (R = Ar) underwent isomerization to 4-arylfuroxans 21 during electrolysis via formation of a cation radical (86BAU1543, 86IZV1691). The thermal isomerization of 4-arylfuroxans 21 afforded 3-arylfuroxans 23 (79BAU2118, 79IZV2295, 83G811). 

In agreement with the previously reported theoretical study, the results of semi-empirical calculations showed that the formation of the Dewar isomer is favored [99H(50)1115]. Probably, the observed formation of the azirine derives from a thermal isomerization of the first photoproduct, in line with that described in the case of furan and thiophene derivatives (Fig. 11). 

Further work by Adembri et al. describes the irradiation of 5-hydrazino-isoxazoles 291a-b under nitrogen, which gives a mixture of products analogous to that obtained by thermal isomerization 5-methylaminopyrazol-3-ones 292a-b, 4-aminopyrazol-3-ones 293a-b, and l,2,4-triazin-6-ones 294 -b (78TL4439). 

The following thermal isomerization occurs under relatively mild conditions. Identify the pericyclic reactions involved, and show how the rearrangement occurs. 

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