Base-induced rearrangement mechanism

Harger has studied the rearrangement of A-substituted N-phosphinoylhydroxylamines in the presence of base . He proposed a concerted mechanism based on the observed retention of the configuration at the phosphorous center during the transposition , and on studies with 0-labelled compounds . Similar cyclic transition states 572 were proposed in the base-induced rearrangement of A,0-bis(diphenylphosphinoyl)hydroxylamines (571) (equation 254). However, in the rearrangement of O-benzoyl-A-(diphenylphosphino-thiol)hydroxylamine where a transposition of O and S atoms occurs, the proposed cyclic transition state has sulfur participation . 

The base-induced rearrangement of aziridines has been examined using a combination of calculations and experiments. The calculations show that the substituent on nitrogen is a critical feature that greatly affects the favourability of both a-deprotonation and /3-elimination to form an allylic amine. Af-tosyl aziridines were found to deprotonate on the tosyl group, preventing further reaction. A variety of A-benzenesulfonyl aziridines having both a- and /3-protons decomposed when treated with lithium diisopropylamide (LDA) in various solvents have been reported. However, when a-protons were not present, allylic amine was formed, presumably via an 5 2-like mechanism. 

In the 1,2,4-triazole series, base-induced degenerate rearrangements have been reported with the aroylarylazo-l,2,4-triazolium salts 185 (87MI3) (Scheme IV.72). The reaction product is 3-aroyl-l-aryl-1,2,4-triazole 187, and its formation can be described as involving an ANRORC mechanism, initiated by an addition of the nucleophile at C-5, ring opening into 186, and subsequent heterocyclization with the former CNN side chain of the rearranging 1,2,4-triazole. 

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

Some of the recent work in contact mechanics is focused on understanding the adhesion of viscoelastic polymers and dynamic contributions to the adhesion energy this work is summarized in Section 5. Sections 6.1 and 6.2 include some of the current applications of contact mechanics in the field of adhesion science. These include possible studies on contact induced interfacial rearrangements and acid-base type of interactions. 

As reviewed so far, the contact-mechanics-based techniques (JKR and SFA methods) have been effective in the understanding molecular level mechanisms related to the adhesion of elastomers and in measuring the surface and interfacial energies of polymers and self-assembled monolayers. The current work in this area is aimed at understanding contact induced interfacial rearrangements and the role of specific interactions. The recent progress of these studies is discussed in this section. 

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