Rearrangement circumambulatory

Note first that the shift of a substituent from atom j to atom 1 - or equivalently to atom j — 1), can be described as either a [1,2]- or a [l,j]-shift it is universally regarded to be the latter. Moreover, unless the ring is extremely large, only the suprafacial pathway need be considered. The same Rules are conventionally applied to it as to the linear [l,j]-rearrangement, so the thermal rearrangement of a single migratory atom is forbidden when j = 3,7. (4n — 1), 

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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." ... 

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... 

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. 

A more detailed description of such circumambulatory rearrangements was published recently . 

Other problems dealt with by the Hehre-Pople method are internal rotation in vinylcyclopropane and vinylcyclobutane 153>, the structure of homoallyl cation 154> and ethylenebenzenium cation 155>, torsional barriers in -substituted phenols 156), inversion barriers in -substituted anilines 157>, the effects of a-substitution in keto-enol tautomerism 158> and the circumambulatory rearrangement in bicyclo [3.1.0] hex-3-en-2-yl cation 159> ... 

Hehre predicted by quantum chemical calculations that in the 8,8-dimethyl-homotropylium ion, contrary to the unsubstituted ion 455, the circumambulatory rearrangement is feasible. In 1978 Childs succeeded in generating 8,8-dimethyl-substituted ions and made the bridged atom migrate in them > ... 

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