Electron-deficient carbocations. Direct observation of .alpha.-carbonylmethyl cations by laser flash photolysis

Johnston, Linda J.; Kwong, P. C.; Shelemay, Avshalom; Lee‐Ruff, Edward

doi:10.1021/ja00058a009

Cited by 37 publications

(34 citation statements)

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“…Carbocations are key intermediates in many organic reactions. [1][2][3][4][5] They can be generated by photoinduced heterolysis of organic halides such as RÀCl [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] or other precursors. [6-8, 21, 23-26] The photolysis of these precursors can also lead to the generation of carbon-centered radicals.…”

Section: Introductionmentioning

confidence: 99%

“…0, see Table 1) can be obtained by photolysis of the corresponding benzhydryl chlorides in CH 3 CN but not in CH 2 Cl 2 [9] and the photogeneration of these benzhydryl cations in CH 2 Cl 2 requires other photoleaving groups, such as PPh 3 . [25,26] In view of the important role of photolytic techniques in the characterization of benzhydryl ions [19,20,25,26,[35][36][37] and other reactive carbocations, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] it is desirable to find out whether an inhibition of the electron transfer or an efficient depletion of the ion pairs by geminate recombination is responsible for the negligible quantum yields in solvents of low polarity. Herein, we present ultrafast broadband pump-probe experiments on the photolysis of a set of benzhydryl chlorides (shown in Table 1).…”

Section: Introductionmentioning

confidence: 99%

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A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

et al. 2013

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Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

et al. 2013

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“…[6,[15][16][17] This method has widely been employed to determine the rates of fast reactions of carbocations with nucleophiles. Common precursors include alkyl halides R-Hal, [18][19][20][21][22][23][24] acetates R-OAc, [24][25][26][27][28][29][30][31][32][33] aryl ethers R-OAr, [24][25][26][27][28][29][30][31][32][33] ammonium salts R-NR 0 3 + , [18,34,37] and phosphonium salts R-PR 0 3 + . [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]…”

Section: Introductionmentioning

confidence: 99%

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

Ammer

Mayr

2013

J of Physical Organic Chem

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The generation of carbocations by laser flash photolysis of suitable precursors provides information about their reactivities toward nucleophiles on the nanosecond and microsecond time scale that cannot be obtained by conventional methods. We discuss the requirements that must be met by the precursors to achieve sufficient yields of the carbocations under the conditions of kinetic experiments. These include (i) efficient photogeneration of the carbocations; (ii) the stability of the precursor in the sample solution; (iii) the absorption of the precursor at the excitation wavelength; and (iv) sufficient lifetimes of the photogenerated carbocations for the observation of their reactivities toward the nucleophiles of interest. We provide an overview of the advantages and disadvantages of some precursors that are commonly employed for the generation of carbocations R + . This includes alkyl halides R-Cl, acetates R-OAc, aryl ethers R-OAr, ammonium salts R-NR 0 3 + , and phosphonium salts R-PR 0 3 + .

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“…7 In a related studies, the carbonyl substituted cations 3 have been studied by Lee-Ruff and Johnston under laser flash photolytic conditions. 8,9 Recent computational studies by Schleyer and Tidwell suggest that the parent 9-fluorenyl cation is non-aromatic (as opposed to antiaromatic). 10 Our recent experimental and computational studies on fluorene system 4 suggests that the carbonyl group in this cation is rotated 90°out of conjugation with the cationic center.…”

Section: Introductionmentioning

confidence: 99%

Effect of oxime substituents on 9-fluorenyl carbocations

Creary¹,

Wolf²

2000

J. Phys. Org. Chem.

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Electron-deficient carbocations. Direct observation of .alpha.-carbonylmethyl cations by laser flash photolysis

Cited by 37 publications

References 0 publications

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

Effect of oxime substituents on 9-fluorenyl carbocations

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