Isomerization Including Heteroatoms

Production of ally alcohol by the isomerization of propylene oxide is an industrialized process. Lithium phosphate is specifically selective for this reaction. It is believed that the appropriate bdance of the acidity and basicity of catalyst is essential for high selectivity. If the acidity is dominant, isomerization to aldehyde becomes the main reaction (eq. 9a), and a path to acetone is favored on basic catalysts (eq. 9c), as shown below. 

The above relationships have been confirmed between the acid-base properties of several metal phosphates and their catalytic activities and selectivities.  

According to the patent literature, the method of preparation of Li3P04 is crucial for obtaining selectivity higher than 90%. A correlation exists between the line broadening of XRD (002) line and catalytic performance of Li3P04 prepared by several methods. This indicates the importance of a crystal plane as well as high surface area for efficient isomerization. 

Isomerization of ethylene oxide to acetaldehyde is an undesirable reaction in the oxidation of ethylene over Ag catalyst. Addition of Cs has been reported to suppress the isomerization by weakening the Lewis acidity of Ag (electronic effect).  

Beckmann rearangement of cyclohexemone oxime to e-caprolactam is catalyzed by sulfuric acid in the industrial process. Several attempts have been reported to substitute sulfuric acid by suitable solid acids,but it is rather difficult to obtain high yields. Recendy, it was reported the silica-supported boria catalyst prepared by vapor phase decomposition method was very efficient (oxime conversion 98%, lactam selectivity 96 % at 523K), with slight deactivation with reaction time. (see Section 3.1.11)  

---

Isomerization is a reaction in which a molecule is transformed into a molecule having the same molecular formula but a different structure, i.e., isomers. Industrially important isomerization reactions are rearrangements of the carbon skeleton of C4-C8 hydrocarbons, and isomerization among alkyl benzene isomers such as xylenes and ethylbenzene.Isomerization including heteroatoms, such as propylene oxide to ally alcohol and Beckmann rearrangement of cyclohexanone oxime to e-caprolactam, are also significant industrial processes. 

The 1,2,3-thiadiazole 1 possesses three contiguous heteroatoms in a five-membered ring and exists as a remarkably stable neutral aromatic compound. It is isomeric with the ring-opened a-diazothioketone 2 (Equation 1) although there is evidence that it reacts through this intermediate, all structural methods, including X-ray diffraction, point to 1 as the structure for a 1,2,3-thiadiazole. 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

Because of their high reactivity, these complex salts react rapidly and regiospecifically, at low temperature, with a number of carbon and heteroatomic nucleophiles, including thiols, amines, and alcohols. Finally, exposure of the double bond takes place under particularly mild conditions so that isomerization of the (3,Y-unsaturated carbonyl system may be avoided. The present scope of reactions with these vinyl cation synthons is summarized in [able I. 

The same authors116 showed that 1,3-diketones 198 react easily with benzonitrile in the presence of a HC104-Ac20 mixture to form 3-azapyrylium perchlorates 199 (equation 63). This reaction did not occur without acetic anhydride, which is a catalyst since the acylium ions are not included in the end products. The application of the C-acylated ketones 198 leads to 3-azapyrylium salts 199 which are isomeric to those mentioned above (194), i.e. products 199 have an inverted orientation of heteroatoms comparatively to the three-carbon fragment of the cycle116. The azapyrylium salts 199 can be hydrolyzed to the enamides 200. 

Many photochemical isomerizations of five-membered heterocycles having two or more heteroatoms including ring transformations have been... 

Amino-iminophosphines have been known for more than a decade and have always been isolated as the trans isomers. The first example of a cis amino-iminophosphine, (144), has now been isolated (144) does not isomerize when heated or exposed to UV irradiation New, thermally stable heteroatom substituted iminophosphines include the aryloxy-iminophosphines e.g. (145), and... 

The Keggin-structure anion q -[XWi204o]", particularly the tungstophosphates and tungstosilicates, are the most frequently investigated because of the potential and real catalytic applications. The central heteroatoms, X, in the Keggin and isomeric anions [XWi204o]" include, in the a-form (H)2, Be, B, Al, Ga, Si, Ge,, ... 

Quite differently, a methoxyl-group participates in the solvolysis of endo isomer 119. This prevents the comparison of the behaviour of exo 118 and endo isomer 119. To avoid these complications with electronegative substituents, Gassman and Marshall included the heteroatoms of C in the dioxolane ring. The solvolysis of isomeric tosylates 122 and 123 ended in the case of the endo isomer 123 by simultaneously splitting the ring with the oxygen function involved. 

The Dimroth rearrangement is an isomerization process whereby atoms on a heterocyclic ring are translocated. The reaction can be catalyzed by a variety of acids and bases, with or without the presence of heat or light. Several factors may influence the rearrangement, including the number and types of heteroatoms in the ring, the number and types of substituents on the ring, and the kinetic and thermodynamic stabilities of the reactants and translocation products. 

Carbon-Heteroatom (N, S, O, Sn, Si, Se, P) Bond Formation. Primary and secondary amines (but not ammonia) undergo reaction with allylic acetates, haUdes, phosphates, and nitro compounds in the presence of Pd(PhsP)4 to provide the corresponding allylic amines (eq 26). A variety of ammonia equivalents have been demonstrated to be useful in this Pd(Ph3P)4-catalyzed alkylation, including 4,4 -dimethoxybenzhydrylamine, NaNHTs, and NaNs (eq 26). Both allylic phosphates and chlorides react faster than the corresponding acetates and (Z)-alkenes are isomerized to the ( )-isomers. The use of primary amines as nucleophiles in the synthesis of secondary allyl amines is sometimes problematic since the amine that is formed undergoes further alkylation to form the tertiary amine. Thus hydroxylamines have been shown... 

---