1,5‐Sigmatropic shifts of bromine over a cyclopentadiene ring

Abstract

The intrinsic mechanism of circumambulatory rearrangements of 5‐bromo‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene, 5‐bromo‐1,2,3,4 5‐pentamethoxycarbonylcyclopentadien and 5‐bromo‐ 1,2,3,4,5‐pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic 13C and 1H NMR technique, to be governed by successive intramolecular 1,5‐sigmatropic shifts. Semi‐empirical AM1 and MINDO/3 calculations of reactions paths performed for fluoro‐, chloro‐ and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5‐ over a 1,3‐shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5‐sigmatropic shifts of chlorine in 5‐chloro‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG 298+ = 26.1 kcal mol−1 were studied using 1H NMR spectroscopy.

Quantum Chemistry of Organic Compounds

“…An analysis [98] of the results of a number of other nonempirical and semiempirical calculations on ion adducts in the reactions of electrophilic aromatic substitution (X = H, F, CH 3 ) has led to the conclusion that generally the symmetric type XL VIII structures of n-complexes are not stable. This conclusion has been supported by semiempirical and ab initio calculations of the mechanisms governing the gas-phase reactions of nitrosation [137,138] and nitration [139][140][141] of benzene.…”

“…The MINDO/3 calculations [137] are in accord with basic outline of the proposed scheme showing at the same time that the conventional views on the structure of the n-complexes as the high-symmetry systems should be modified. The structure XLIX corresponds, similarly to XLVIII, to a point at the top of a flat hill on the PES of C6H6NO +.…”

“…The conclusion as to the preferability of the type XLIXa n-complex structure and its fluxionality, at which the calculations [137] arrived, has recently found a strong experimental support. X-ray structural data [142] have been obtained on the durene and hexamethylbenzene n-complexes, with nitrosonium cation counter-ioned by the tetrachloroaluminate-anion, showing close similarity to the gas-phase structure XLIXa.…”

Isomerization study of C5H5NO molecules

“…Cyclopentadiene is a nonbenzenoid aromatic compound with a ring system that is smaller than the benzene ring. The reaction pathways for the 1,5-sigma tropic displacement of the nitroso group along the perimeter of 5-nitroso-1,3-cyclopentadiene and its valence isomerization were studied by Minkin et al [5], using a semiempirical MINDO/3 method. Since this molecule has more than one conformer, the quantum chemical calculations and the conformational analysis of the nitroso compounds are more interesting and a useful exercise for theoretical chemists.…”

“…Since this molecule has more than one conformer, the quantum chemical calculations and the conformational analysis of the nitroso compounds are more interesting and a useful exercise for theoretical chemists. Calculations have been carried out by Minkin et al [5] using the MINDO/3 [6] and UMINDO/3 [7] schemes, respectively, revealed three stable valence isomers corresponding to the PES minimum for 5-nitroso-1,3-cyclopentadiene, which includes pyridine-N-oxide, bicyclo-laza-7-oxa-2,4 heptadiene, and 1,2-oxazepin [5]. Oxazepin has been identified as the most stable valence isomer of 5-nitroso-1,3-cyclopentadiene by MINDO/3 calculations.…”