The Tripropynylcarbonium Ion. Charge Delocalization in Ethynyl- and Propynylcarbonium Ions

Abstract

nated as a "protonated cyclopropane"). By way of 1 or a properly substituted derivative, an endo-hydrogen at C-6 will migrate to the two possible configurations at C-2 with exactly or approximately equal probability, whereas in 2a, 2b, or related species it will migrate exclusively to become bound endo to C-2. We now report experiments which show that, in the 2-carboxy-3methyl-5-norbornyl cation, intramolecular transannular hydride shift is exclusively endo + endo.trans-Crotonic acid-3-d, prepared6 from acetaldehyde-1 -8 and malonic acid, reacts with cyclopentadiene to give a mixture of 3-exo-methyl-5-norbornene-2endo-carboxylic acid-3-endo-d (4b) and 3-endo-methyl-5-norbornene-2-exo-carboxylic acid-3-exo-d (Sb), which can be separated by the iodolactone method, as in the undeuterated series 4a and Sass The position of the deuterium in 4b and Sb is evident from the method of synthesis, from the collapse of the n.m.r. methyl doublets of 4a and Sa to singlets, and from the disappearance of the one-proton multiplets of the C-3 hydrogen of 4a and Sa in the spectra of 4b and Sb.Under conditions (50% by volume sulfuric acid, 21 hr.) previously reported8 to give the lactones 6a and 7a from either 4a or Sa, either deuterated acid 4b or Sb gives the unrearranged lactone 6b (that from 4b being isolated in quantity sufficient to show that deuterium is exclusively at '2-3) and the rearranged lactone 7b, with deuterium exclusively at C-2.The total deuterium content of products 6b and 7b is the same as that of the starting materials, and, accordingly, the deuterium shift, which on purely structural grounds must have occurred, is entirely intramolecular. The complete disappearance in the n.m.r. spectrum of 7b of the one-proton signal at 4.18 p.p.m. (downfield from tetramethylsilane), which is associated with the C-2 hydrogen of 7a, indicates that less than about 3 % of C-2 protio species can be present. The results are in accord with the indicated mechanism (in which the question of nonclassical or classical nature of the cations is temporarily begged).OThe intermediates or transition states for the two hydride shifts required to transform 4b to rearranged lactone 7 by way of nortricyclonium species would be represented as 10 and 11. Formation of exclusively 2-deuterio lactone 7b then becomes inexplicable; this product would arise from 11, but 10, which is formed earlier in the reaction scheme and differs from 11 only in that the positions of deuterium and the C-2 hydrogen are reversed, would have been expected to form 2-protio rearranged lactone, in conflict with experiment. l o

Stable carbocations. 279. Carbon-13 NMR spectroscopic study of .gamma.-substituted tris(ethynyl)methyl cations

“…However, when one of the terminal acetylenic hydrogens in 2e is replaced by methyl, phenyl, or cyclopropyl groups, stable carbocations have been obtained using fluorosulfuric acid in sulfur dioxide. 8 In summary, a variety of stable 7-substituted tris(ethynyl)methyl cations were prepared under stable ion conditions in fluorosulfuric acid-sulfuryl chloride fluoride at low temperatures. The 13C NMR chemical shifts of the studied ions 1 appear to indicate that electronic effects due to different 7-substituents are only marginal.…”

Electronic spectrum and photodissociation chemistry of the linear methyl propargyl cation H2C4H3+

“…[1][2][3] These hydrocarbon cations are also relevant from an astrophysical standpoint having been identified in the atmosphere of Titan and possibly existing in interstellar clouds. 4 Alkynyl type carbocations, such as C 4 H 5 + , were originally observed in super-acidic media using NMR, [5][6][7][8] with subsequent computational studies suggesting that the methyl cyclopropenyl cation (Fig. 1a) is the lowest energy isomer of C 4 H 5 + , with the linear methyl propargyl cation, H 2 C 4 H 3 + (Fig.…”

Alkynylcarbenium ions and related unsaturated cations

“…Table 1 presents the 1 H NMR spectra of these mixtures recorded at 760 8C. 96 The chemical shifts of the methyl group protons in the compounds 9a ± d, 105 and 108a and the ethyne protons in 9e ± h, 108b as well as the downfield shifts of these signals (Dd) with respect to the signals of the corresponding protons in the initial alcohols point to substantial deficiency of the electron density on the carbon atoms of the acetylenic fragment. This deficiency becomes more pronounced as the carbenium ions become less stable, i.e.…”

“…Therefore, the researchers 98 used again the corresponding tertiary alcohols, which were treated as described in the study cited above. 96 As a result, they recorded the 1 H NMR spectra of the cations 9b and 9i ± n (Table 2).…”

“…Chemical shifts of the protons of substituent R 3 in the1 H NMR spectra of ethynyl-and propenylcarbenium ions 96. …”