PMR detection of carbocation intermediates in the pinacol rearrangement in a superacid

Chikinev, A. V.; Bushmelev, V. A.; Шакиров, М. М.; Shubin, V. G.

doi:10.1007/bf00960288

Russ Chem Bull

1984

DOI: 10.1007/bf00960288

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PMR detection of carbocation intermediates in the pinacol rearrangement in a superacid

A. V. Chikinev

V. A. Bushmelev

М. М. Шакиров

et al.

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Cited by 4 publications

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“…The 1 H NMR spectrum of a solution of nitrile I in HSO 3 F-SO 2 ClF-CD 2 Cl 2 at -105°C displayed signals obviously belonging to two conformers of 10-hydroxy-9,9-dimethylphenanthrenyl cation, IIIa and IIIb at a ratio of 5 : 1 (Scheme 2), δ, ppm: IIIa: 1.90 s (9-CH 3 ), 12.11 s (OH); IIIb: 1.93 s (9-CH 3 ), 12.62 s (OH) (cf. [2]). Signals from aromatic protons were…”

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Reaction of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile with acids

et al. 2011

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The behavior of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile in trifluoromethanesulfonic acid and acid systems CF 3 SO 3 H-CD 2 Cl 2 , HSO 3 F-SO 2 ClF-CD 2 Cl 2 , and CF 3 COOH-CD 2 Cl 2 were studied by NMR spectroscopy. Principal reaction schemes were determined; the first step in these schemes is protonation of the initial compound at the oxygen or nitrogen atom.We found [1] that 10,10-dimethyl-9-trimethylsilyloxy-9,10-dihydrophenanthrene-9-carbonitrile (I) in superacid media undergoes ionization with formation of long-lived 9-cyano-9,10-dimethylphenanthrenyl cation (II) (Scheme 1). Apart from signals belonging to cation II, the NMR spectra contained a number of additional signals indicating other schemes of reactions of nitrile I with superacids. The goal of the present work was to elucidate these schemes. located in the region δ 7.6-8.9 ppm. In the 13 C NMR spectrum we observed only signals from conformer IIIa, δ C , ppm: 27.1 (9-CH 3 ), 46.1 (C 9 ), 215.7 (C 10 ); aromatic carbon atoms resonated in the range δ C 120-160 ppm. According to the NMR data, the ratio of cations II and III was about 1 : 1.

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confidence: 93%

Reaction of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile with acids

et al. 2011

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“…However, the assumption that the rearrangement is hindered as a result of protonation of cyano group (cf. [19,20]) is untenable. This is supported by the following.…”

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“…This is supported by the following. As shown in [20], the degree of protonation of the methoxy group in structurally related 9-methoxy-9,10-dimethylphenanthrenylium in HSO 3 F-SO 2 ClF is relatively small (20-30%). Acetonitrile is a weak base; it undergoes protonation by half in 99.6% H 2 SO 4 , i.e., the H 0 value for the conjugate acid is -10.12 [21].…”

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Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation. Migration of cyano group in carbocations

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“…* The experimental α/β ratio for R = p-MeOC 6 H 4 at -40°C (~1 : 4) was not taken into account, for it should be obviously distorted as a result of protonation of the R group in strongly acidic medium [8]. a The α/β ratios for the other groups R (Table 1) were not determined experimentally, but the equilibrium was found to be displaced toward β-isomer.…”

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confidence: 97%

“…Long-lived 9-R-9,10-dimethylphenanthrenyl cations are capable of undergoing degenerate rearrangement via 1,2-shift of the R substituent (Scheme 1) and are therefore convenient models for studying relative migration abilities of various atoms and groups [2][3][4][5][6][7][8][9][10][11][12].…”

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Relative stability of isomeric carbocations with migrating groups in the neighboring positions

Genaev

Shubin

2011

Russ J Org Chem

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The relative stabilities of isomeric 9,9-dimethyl-10-R-and 9-R-9,10-dimethylphenanthrenyl, as well as of 3-R-2,3-diphenylbutan-2-yl and 1-R-2-methyl-1,2-diphenylpropan-1-yl cations, were determined in terms of the density functional theory. The equilibrium isomer ratio was found to depend to an appreciable extent on steric requirements of the substituents.Data on the energy barriers to rearrangements of carbocations and relative stabilities of the initial and rearranged ions are necessary for the determination of structural and kinetic relations holding in cationoid rearrangements, in particular for the prediction of the direction of multistep isomerizations involving carbocationic intermediates [1]. Long-lived 9-R-9,10-dimethylphenanthrenyl cations are capable of undergoing degenerate rearrangement via 1,2-shift of the R substituent (Scheme 1) and are therefore convenient models for studying relative migration abilities of various atoms and groups [2][3][4][5][6][7][8][9][10][11][12].(α-I) is formed under the conditions ensuring "long life" of such cationic species. For the sake of simplicity, cations with the group R located in the β-and α-positions relative to the cationic center (Scheme 2), are referred to as β-and α-isomers, respectively. When R = H [6], Et [2], ClCH 2 [7], Cl 2 CH [5], CH 2 =C(Me) [11], Ph, p-MeC 6 H 4 , p-FC 6 H 4 , m-FC 6 H 4 , p-F 3 CC 6 H 4 , C 6 F 5 [3], O 2 N [7], the equilibrium is displaced toward β-isomer, whereas α-and β-isomeric carbocations with R = p-MeOC 6 H 4 [3], CH 2 =CH [10], trans-MeCH=C(Me) [11], cyclopropyl [12], and Cl [4] are characterized by comparable stabilities. Therefore, for most of the above listed migrating groups we succeeded in determining by dynamic NMR the rate of 1,2-shift, not being concerned about almost complete transformation of β-isomer I into α-isomer as a result of 1,2-shift of methyl group. In addition, in many cases we were able to synthesize precursors of carbocations in one step according to Scheme 3.

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PMR detection of carbocation intermediates in the pinacol rearrangement in a superacid

Cited by 4 publications

References 3 publications

Reaction of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile with acids

Reaction of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile with acids

Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation. Migration of cyano group in carbocations

Relative stability of isomeric carbocations with migrating groups in the neighboring positions

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