Circumambulatory

Note that the first example bears out the stereochemical prediction made earlier. Only the two isomers shown were formed. In the second example, migration can > continue around the ring. Migrations of this kind are called circumambulatory rearrangements. Such migrations are known for cyclopentadiene, pyrrole, and phosphole derivatives.[1,5] Hydrogen shifts are also known with vinyl aziridines." ... 

Noteworthy NMR studies involving nuclei other than phosphorus have been carried out for some P-chloro-NHPs where the possible occurrence of spontaneous P-Cl bond dissociation was probed by II NMR titrations and 35C1 NMR [20], and for P-cyclopentadienyl derivatives where measurement of solid-state 13C CP-MAS NMR spectra allowed one to substantiate the preservation of the circumambulatory ring migration of cyclopentadienyl groups in the solid state [47], Several neutral and cationic derivatives have also been studied by 15N NMR [20, 53],... 

A quantum-chemical study has been undertaken on the isomerization of ctx-1-vinyl-, -1-formyl-, -1-thioformyl-, and -l-iminomethyl-2-vinylcyclopropane to cyclohepta-1,4-diene, 2,5-dihydrooxepine, 2,5-dihydrothiepine, and 2,5-dihydroaze-pine, respectively. Reaction pathways for circumambulatory rearrangements of main group migrants (NO, PO, NCS, SCN, NCO, OCN, SR, Cl, Br, and XX where X... 

All attempts of preparing the parent secondary dication (63), a t -tiicyclo(5.1.0.0 ) octa-2,6-diyl dication, were unsuccessful. The dication was originally anticipated to undergo the circumambulatory rearrangement, as in cyclopropylmethyl cation rearrangements However, it spontaneously rearranged into the thermodynamically more stable homotropylium cation (64). 

Typical examples of circumambulatory rearrangements of bicyclo[3.1. Ojhexenyl cations are shown in Schemes 19 and 20. Swatton and Hart reported the isomerization shown in Scheme 19 in 1967 and proposed that the observed deuterium scrambling could be accounted for on the basis of a cyclopropyl walk reaction153. This circumambulation is comparable to that proposed by Zimmerman and Schuster as part of the sequence of reactions involved in the type A photorearrangement of 2,5-cyclohexadienones. ... 

SCHEME 20. Degenerate circumambulatory rearrangement of a hexamethylbicyclo[3.1.0]hexenyl cation... 

Berson and coworkers95 observed that the parent bicyclo[3.1.0]hexenyl cation 86 undergoes circumambulatory migration with an activation barrier of 15.1 kcalmol" (equation 55). 

The tricyclo[5.3.1.0]undecatrienyl anion (97) undergoes circumambulatory rearrangement at —78°C to form anion (98) which is happed by D20 at the least hindered anti face to give (99 )-t/1.170 At higher temperature (98) is converted to (102), as indicated in Scheme 7. 

Attempts to observe circumambulatory rearrangement in the 2,6-unri-tricyclo [5.1.0.03,5]octane-2,6-diyl dication 178 have been unsuccessful.397 The dication 178 would appear to rearrange instantaneously to the homotropylium ion 179 by proton elimination. However, substituted dications of type 178 (e.g., 180) are quite stable they are static, and a substantial part of the charge is delocalized into the cyclopropane rings. 

Hart and Kusuya have documented the remarkable properties of the nona-methylbicyclo[3.2.1]octa-3,6-dien-2-yl cation (75) as generated by ionization of the alcohol in magic acid.127 Below —60 °C in this solvent system, 75 undergoes both a rapid circumambulatory process (NMR observable) and a slower bridge shift (deuterium labelling necessary for observation). At or above -60 °C, 75 rearranges irrever-... 

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