Quantum Chemical Study of Degenerate Hydride Shifts in Acyclic Tertiary Carbocations

Vrček, Ivana Vinković; Vrček, Valerije; Siehl, Hans‐Ullrich

doi:10.1021/jp013158u

Cited by 45 publications

(31 citation statements)

References 40 publications

(29 reference statements)

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“…Figure 5 shows the most important bond lengths of the hyperconjugated carbocation molecule complexes with a carbenium ion center at C3. The C1-C2 bond is significantly elongated ( Figure 5) relative to a typical C-C bond of 1.53Å that was calculated at the same level of theory for ethane; such elongation of a β C-C (relative to the carbenium ion center) has been previously reported [36]. Also, the calculated C2-C3 bond lengths are significantly shorter than a normal C-C single bond length ( Figure 5).…”

Section: Modeling Hyperconjugated Cation Molecule Complex Esupporting

confidence: 70%

“…Therefore new complexes of E involving hyperconjugated carbocations were modeled that led to stable ion molecule structures. Hyperconjugation has been reported to contribute to the stability of carbenium ions (R 3 C + ) [36]. In our case, hyperconjugative stability is attributed to the overall stability of the gas-phase carbocation molecule complex.…”

Section: Modeling Hyperconjugated Cation Molecule Complex Esupporting

confidence: 47%

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Density Functional Theory and Mass Spectrometry of Phthalate Fragmentations Mechanisms: Modeling Hyperconjugated Carbocation and Radical Cation Complexes with Neutral Molecules

Jeilani

Cardelino

Ibeanusi

2011

J. Am. Soc. Mass Spectrom.

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This is the first ab initio study of the energetics of the fragmentation mechanisms of phthalate, by mass spectrometry, leading to protonated phthalic anhydride (m/z 149). Phthalates fragment by two major pathways; namely, the McLafferty+1 rearrangement and the loss of alkoxy. Both pathways involve a carbonyl oxygen attack to the ortho-carbonyl carbon leading to structures with tetrahedral carbon intermediates that eventually give m/z 149. These pathways were studied by collision induced dissociation (CID) using triple quadrupole mass spectrometry. The proposed McLafferty+1 pathway proceeds through a distonic M•+ , leading to the loss of an allylic-stabilized alkene radical. The McLafferty rearrangement step proceeds through a six-membered ring transition state with a small activation energy ranging 0.4-6.2 kcal/mol; the transfer of a second H from the distonic ion of the rearrangement step proceeds through a radical cation molecule complex. Based on quantum chemical modeling of the cation molecule complexes, two kinds of cation molecule complexes were identified as radical cation molecule complex and hyperconjugated cation molecule complex. This distinction is based on the cation and simplifies future modeling of similar complexes. Optimization of important fragments in these pathways showed cyclized and hydrogen-bonded structures to be favored. An exception was the optimized structure of the protonated phthalic anhydride (m/z 149) that showed a structure with an open anhydride ring.

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Section: Modeling Hyperconjugated Cation Molecule Complex Esupporting

confidence: 70%

Section: Modeling Hyperconjugated Cation Molecule Complex Esupporting

confidence: 47%

Density Functional Theory and Mass Spectrometry of Phthalate Fragmentations Mechanisms: Modeling Hyperconjugated Carbocation and Radical Cation Complexes with Neutral Molecules

Jeilani

Cardelino

Ibeanusi

2011

J. Am. Soc. Mass Spectrom.

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“…It is well known that the [1,2]-hydride shift for the carbocation is very facile with activation barriers of about 5 kcal mol À1 . [33] This process is a two-electron process and its transition state has a two-electron-two-orbital stabilizing interaction between the proton and the p orbital of the alkene. In contrast, the [1,2]-proton shift for the carbanion is a four-electron process and is difficult as the corresponding transition state involves a four-electron-two-orbital destabilizing interaction between the hydride and the p orbital of the alkene.…”

mentioning

confidence: 99%

Mechanism, Regioselectivity, and the Kinetics of Phosphine‐Catalyzed [3+2] Cycloaddition Reactions of Allenoates and Electron‐Deficient Alkenes

Liang

Liu

Xia

et al. 2008

Chemistry A European J

352

151

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With the aid of computations and experiments, the detailed mechanism of the phosphine-catalyzed [3+2] cycloaddition reactions of allenoates and electron-deficient alkenes has been investigated. It was found that this reaction includes four consecutive processes: 1) In situ generation of a 1,3-dipole from allenoate and phosphine, 2) stepwise [3+2] cycloaddition, 3) a water-catalyzed [1,2]-hydrogen shift, and 4) elimination of the phosphine catalyst. In situ generation of the 1,3-dipole is key to all nucleophilic phosphine-catalyzed reactions. Through a kinetic study we have shown that the generation of the 1,3-dipole is the rate-determining step of the phosphine-catalyzed [3+2] cycloaddition reaction of allenoates and electron-deficient alkenes. DFT calculations and FMO analysis revealed that an electron-withdrawing group is required in the allene to ensure the generation of the 1,3-dipole kinetically and thermodynamically. Atoms-in-molecules (AIM) theory was used to analyze the stability of the 1,3-dipole. The regioselectivity of the [3+2] cycloaddition can be rationalized very well by FMO and AIM theories. Isotopic labeling experiments combined with DFT calculations showed that the commonly accepted intramolecular [1,2]-proton shift should be corrected to a water-catalyzed [1,2]-proton shift. Additional isotopic labeling experiments of the hetero-[3+2] cycloaddition of allenoates and electron-deficient imines further support this finding. This investigation has also been extended to the study of the phosphine-catalyzed [3+2] cycloaddition reaction of alkynoates as the three-carbon synthon, which showed that the generation of the 1,3-dipole in this reaction also occurs by a water-catalyzed process.

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“…However, this observation may also result from ammonium adduct formation (or proton adduct after loss of ammonia) with the double bond via cation-π interaction [44, 45] and charge-remote dissociation involving a six-membered ring transition state [46, 47] followed by hydride ion transfer (or a series of sequential transfers) [34, 49] (Supplementary Scheme S1a). …”

Section: Resultsmentioning

confidence: 99%

Characterization of Wax Esters by Electrospray Ionization Tandem Mass Spectrometry: Double Bond Effect and Unusual Product Ions

Chen

Green

Nichols

2015

Lipids

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A series of different types of wax esters (represented by RCOOR′) were systematically studied by using electrospray ionization (ESI) collision-induced dissociation tandem mass spectrometry (MS/MS) along with pseudo MS3 (in-source dissociation combined with MS/MS) on a quadrupole time-of-flight (Q-TOF) mass spectrometer. The tandem mass spectra patterns resulting from dissociation of ammonium/proton adducts of these wax esters were influenced by the wax ester type and the collision energy applied. The product ions [RCOOH2]+, [RCO]+ and [RCO – H2O]+ that have been reported previously were detected; however, different primary product ions were demonstrated for the three wax ester types including: 1) [RCOOH2]+ for saturated wax esters, 2) [RCOOH2]+, [RCO]+ and [RCO – H2O]+ for unsaturated wax esters containing only one double bond in the fatty acid moiety or with one additional double bond in the fatty alcohol moiety, and 3) [RCOOH2]+ and [RCO]+ for unsaturated wax esters containing a double bond in the fatty alcohol moiety alone. Other fragments included [R′]+ and several series of product ions for all types of wax esters. Interestingly, unusual product ions were detected, such as neutral molecule (including water, methanol and ammonia) adducts of [RCOOH2]+ ions for all types of wax esters and [R′ – 2H]+ ions for unsaturated fatty acyl-containing wax esters. The patterns of tandem mass spectra for different types of wax esters will inform future identification and quantification approaches of wax esters in biological samples as supported by a preliminary study of quantification of isomeric wax esters in human meibomian gland secretions.

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Quantum Chemical Study of Degenerate Hydride Shifts in Acyclic Tertiary Carbocations

Cited by 45 publications

References 40 publications

Density Functional Theory and Mass Spectrometry of Phthalate Fragmentations Mechanisms: Modeling Hyperconjugated Carbocation and Radical Cation Complexes with Neutral Molecules

Density Functional Theory and Mass Spectrometry of Phthalate Fragmentations Mechanisms: Modeling Hyperconjugated Carbocation and Radical Cation Complexes with Neutral Molecules

Mechanism, Regioselectivity, and the Kinetics of Phosphine‐Catalyzed [3+2] Cycloaddition Reactions of Allenoates and Electron‐Deficient Alkenes

Characterization of Wax Esters by Electrospray Ionization Tandem Mass Spectrometry: Double Bond Effect and Unusual Product Ions

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