By Boschke F.L.

Content material: Brown, H. C. Meerwein and equilibrating carbocations. -- Olah, G. A. From boron trifluoride to antimony pentafluoride looking for good carbocations. -- Hogeveen, H. and van Kruchten, E. M. G. A. Wagner-Meerwein rearrangements in long-lived polymethyl substituted bicyclo(3.2.0)heptadienyl cations. -- Kirmse, W. Rearrangements of carbocations

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In Memory of H.L. Meerwein

Content material: Brown, H. C. Meerwein and equilibrating carbocations. -- Olah, G. A. From boron trifluoride to antimony pentafluoride looking for reliable carbocations. -- Hogeveen, H. and van Kruchten, E. M. G. A. Wagner-Meerwein rearrangements in long-lived polymethyl substituted bicyclo(3. 2. 0)heptadienyl cations.

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Because of the relatively large fluorine chemical shifts, anisotropy and ring current effect play a relatively much smaller role than they do in the case of proton shifts. Therefore, a better correlation of charge distribution with chemical shifts can be obtained. The trifluorocyclopropenium ion 81 was also reported 142). F 81 A series of chloromethyl cations were observed, including phenyldichloromethyl cations120-122,143), and perchlorotriphenylmethyl ion 82144). Cts 82 West has characterized the perchloroallyl cation 8 3 .

7 3 * 9 45 lo The bridgehead congressane (diamantane) cation 46 has also been observed 1o3) Although the bridgehead l-norbornyl cation was not directly observed, 1-chloronorbornane yields the stable 2-norbomyl cation in SbFs-SO2 solution 1~ Thus, ionization to the bridgehead carbonium ion must be followed by a fast shift of hydrogen from C-1 to C-2, the driving force for which is obviously the tendency to relieve strain in the carbenium ion. 12. Aryl- and Alkylarylmethyl Cations The first stable long-lived carbocation observed was the triphenylmethyl cation 1o8-1 ~o) 47.

The 2-fluoro-2-propyl and 1-phenylfluoroethyl cations 79 and 80 are representative examples of the many reported similar ions 138). 91 Jnv=25-4 Hz Fig. 10B. 4 Hz The Search of Stable Carbocations Trifluormethy1139) and perfluorophenyl substituted carbocations have also been prepared and studied 14~ 140. Because of the relatively large fluorine chemical shifts, anisotropy and ring current effect play a relatively much smaller role than they do in the case of proton shifts. Therefore, a better correlation of charge distribution with chemical shifts can be obtained.

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