Structural Isomerism - Rearrangement

Trialkyl esters of phosphonic acid exist ia two structurally isomeric forms. The trialkylphosphites, P(OR)2, are isomers of the more stable phosphonates, 0=PR(0R)2, and the former may be rearranged to resemble the latter with catalytic quantities of alkylating agent. The dialkyl alkylphosphonates are used as flame retardants, plasticizers, and iatermediates. The MichaeUs-Arbusov reaction may be used for a variety of compound types, including mono- and diphosphites having aryl as weU as alkyl substituents (22). Triaryl phosphites do not readily undergo the MichaeUs-Arbusov reaction, although there are a few special cases. 

Cationic complexes are key intermediates in a great variety of organic transformations such as isomerizations, rearrangements, addition reactions, aromatic substitutions, polymerization and others. Long-lived cationic complexes are important structural models for these intermediates. Studies of such complexes by modem physical methods provide valuable insight regarding their structure and reactivity. 

Similar enamine cyclization processes occur in several other successful heterocycle syntheses, e.g. in the Fischer indole synthesis. In this case, however, a labile N—N bond of a l-aryl-2-vinylhydrazine is cleaved in a [3,3]-sigmatropic rearrangement, followed by cyclization and elimination of ammonia to yield the indole (B. Robinson, 1963, 1969 R. J. Sundberg, 1970). Regioselectivity is only observed if R2 contains no enolizable hydrogen, otherwise two structurally isomeric indoles are obtained. Other related cyclization reactions are found in the Pechmann synthesis of triazoles (T.L. Gilchrist, 1974) and in G. Bredereck s (1939) imidazole synthesis (M.R. Grimmett, 1970). 

Okada, Le Trong, et. al. have found that the combination of opsin and N-retylidene in crystalline form is unstable when exposed to visible light206. It appears the excitation of the retinoid, or the subsequent de-excitation through an isomeric rearrangement, distorts the individual molecule and degrades the overall crystalline structure of the... 

The way in which all 10 hydrogens (or carbons) in bullva-lene become equivalent involves a series of degenerate (i.e., leading to an equivalent structure) isomerizations known as Cope rearrangements (or [3.3] sigmatropic shifts). One such rearrangement is shown below (the dot is there to keep track of Cd) ... 

Tautomerism Special case of structural isomerism in which two isomers are directly interconvertible, the reversibility of the change is due to mobility of a group on atom, which can move from one position to another in the molecule, often with the rearrangement of a double bond. 

The competing side reaction is a cyclopropyl anionic ring opening (for a review see ref 13). In the case of isomeric l-benzoyl-2,3-diphenylcyclopropanes, no base-induced isomerization (MeOH/ NaOMe, 65 °C, 4 h) was detected. In many cases these rearrangements require rather drastic conditions but the interconversion can be facilitated by the participation of an adjacent substituent. For example, with an acyl-substituted cyclopropane, enolization permits cisitmns isomerization to proceed without ring opening and therefore without competing anionic structural isomerization. 

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