Migration of methylethynyl group in a long-lived carbocation

Abstract

Degenerate 1,2-shift of methylethynyl group in long-lived 9,10-dimethyl-9-methylethynyl-phenanthrenium ion has been detected by NMR. This is the first example of ethynyl migration in a long-lived carbocation.

Brønsted Acid‐Catalyzed Straightforward Synthesis of Benzo[b]carbazoles from 2,3‐Unsubstituted Indoles

“…Interestingly, the observed [1,4]‐aryl or alkynyl shift has no precedent and opens the door to future developments in this field. Specially significant is the result of the migration of an alkynyl group in the formation of 14ac as the low migratory aptitude of these groups in carbocation rearrangements is known 25…”

“…Specially significant results the migration of an alkynyl group in the formation of 14ac as it is known the low migratory aptitude of these groups in carbocation rearrangements. [25] Scheme 4. Reactions of 1a with tertiary alcohols 13a-c.…”

Computational NMR of charged systems

“…Note: Compiled from Fadeev et al [71] S C H E M E 6 13 C and 19 F NMR chemical shifts of carbocations 84d and 85d (À50 C) in FSO 3 H-SO 2 ClF-CD 2 Cl 2 medium. Reproduced with minor editing privilege from Salnikov et al [75] with the permission of the Royal Society of Chemistry ; fast exchange between cations 84d and 85d (bottom). Reproduced from Salnikov et al [75] with the permission of the Royal Society of Chemistry experiment, as exemplified in Figure 27 for the heptafluorobenzyl cation (75).…”

“…Based on the performed calculations at the same level of theory, the authors of the former paper [73] found that kinetic stability of carbodication 82 was rather low: the energy barrier of its transformation into the isomeric fulvene dication 83 (shown in Figure 31) was about 10 kcal/mol, which corresponded to an estimated half-life time of about 10 min at À130 C. It followed that experimental detection of the viable carbodication 82 and its methylated derivative was hardly possible and might "remain one of the elusive goals in carbocation chemistry." [73] Salnikov and coauthors [75] studied migration of the methylethynyl group in the series of the long-lived 9,10-dimethyl-9-methylethynyl-phenanthrenium cations 84a-d. These carbocations were generated by dehydration of corresponding 10,10-dimethyl-9-(R-ethynyl)-9,10-dihydrophenanthren-9-ols (R = H, Me, Ph, CF 3 ) in FSO 3 H-SO 2 ClF at À95 C, as shown in Scheme 5.…”

“…Note:a Experimental values are given in parentheses.T A B L E 1 4 19 F-19 F spin-spin coupling constants (Hz) of heptafluorobenzyl cation(75), calculated at different levels of theory, as compared with experiment…”