Termination with rearrangement

Another innovative approach to controlling amine-initiated NCA polymerizations was reported in 2003 by Schlaad and coworkers [20]. Their strategy was to avoid formation of NCA anions, which cause significant chain termination after rearranging to isocyanocarboxylates [11, 12], through use of primary amine hydrochloride salts as initiators. The reactivity of amine hydrochlorides with NCAs was first explored by the group of Knobler, who found that they could react... 

Ozonolysis of a double bond leads first to a so-called primary ozonide 40 through 1,3-dipolar cycloaddition. Rearrangement of primary ozonide 40 with ring cleavage produces a carbonyl oxide 42 and a carbonyl compound 41, which then recyclize to secondary ozonide 43. The reaction terminates with a redox process involving... 

Acetylenes can react with metal complexes in a number of ways. Internal acetylenes usually coordinate to metal centers in a q -fashion, in whieh the C=C triple bond donates electrons to the metal center, and both carbon atoms form metal-carbon bonds. Terminal acetylenes can also coordinate to metal centers in this fashion, but ean react in other ways as well. In particular, because the acetylenic hydrogen atom is relatively acidic for a hydrocarbon, the C—H bond of terminal acetylenes ean also add oxidatively to metal centers to form metal acetyhde complexes. In some cases, however, a third kind of reaction is seen, in which the terminal aeetylene rearranges to a vinylidene complex, M=C=CHR. 

The mechanism of this reaction was discussed based on the preliminary results of control experiments and analysis of GC-MS data (Scheme 4.35). The reaction is initiated by the addition of azido free radical and terminated with molecular oxygen to generate the peroxide intermediate 65 [136]. Then this species would undergo continuous multi step rearrangement to give product 56. The details of this rearrangement were disclosed in their ensuing research in which DFT calculations were carried out (Scheme 4.36). This research provides concise and practical access to synthetic important molecules, which may find broad applications in synthesis. 

Fig. 14 A conceptual model of Nafion as a micellar structure. The PTFE groups segregate to the core with the perfluoroether groups forming a shell terminated with the sulfonic acid groups forming a hydrophilic channel for water and proton transport. The PTFE chains form cross-links between hydrophilic domains imparting mechanical strength. The surface of Nafion can rearrange when in contact with vapor or liquid water to be either hydrophilic or hydrophobic...

A recent reporP on the photochemistry of the 3-(l-naphthyl)propenyl acetate derivatives 12 (Scheme 3 yields normalized to 100%) is clearly related to the arylmethyl ester reactivity. Along with rearranged acetate and to 2 isomerization, photosolvolysis in methanol results in the ethers 13. Two observations are surprising First, no ethers resulting from trapping of the aUylic cation at the terminal end were reported, and, second, no products derived from allylic radical intermediates were observed. 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

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