The α-Effect in Benzyl Transfers from Benzylphenylmethyl Sulfonium Salts to <i>N</i>-Methylbenzohydroxamate Anions

Fountain, K. R.; Tad-Y, Darlene B.; Paul, Timothy W.; Golynskiy, Mikhail V.

doi:10.1021/jo981902+

Cited by 46 publications

(14 citation statements)

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“…of 4NP-O − were obtained in all buffers studied. Plots of the corresponding first-order-rate constants k obs versus the concentration of the reactive oximate species were linear with negligible intercepts, indicating no appreciable contribution of hydroxide ion and/or water to the rates and suggesting that k obs is simply given by eqn (3). Determination of the second-orderrate constants k Ox from the slopes of the various buffer plots was straightforward (Fig.…”

Section: Kinetic Studiesmentioning

confidence: 86%

“…The origin of this behaviour, the so-called a-effect, has received considerable attention in the last two decades but it is as yet incompletely understood. [1][2][3][4][5][6][7] Recent work suggests, however, that groundstate and transition-state effects, together with solvation effects, play a major role in determining the exalted reactivity of anucleophiles. [8][9][10][11][12][13] Among different a-nucleophiles, oximates have featured widely because they represent a class of nucleophilic catalysts which has proved to be very efficient in promoting such important processes as acyl, phosphyl and sulfyl transfers, as well as proton transfers.…”

Section: Introductionmentioning

confidence: 99%

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Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

et al. 2006

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Following a potentiometric determination of the relevant pKa values of the (R1R2)C=NOH functionality, the second order rate constants (k(Ox)) for reaction of a large set of oximate bases with two model organophosphorus esters, i.e. bis-(4-nitrophenyl)phenylphosphonate (BNPPP) and bis-(4-nitrophenyl)methylphosphonate (BNPMP), and three toxic compounds, i.e., sarin (GB), soman (GD) and diisopropylphosphorofluoridate (DFP), in aqueous as well as a 30 : 70 (v/v) H2O-Me2SO mixture have been measured. The corresponding Brønsted-type nucleophilicity plots of log k(Ox)vs. pKa(Ox) reveal a clear tendency of the reactivity of the oximates to suffer a saturation effect with increasing basicity in aqueous solution. In the case of BNPMP and the three toxic esters, this behaviour is reflected in a levelling off at pKa approximately 9 but a more dramatic situation prevails in the BNPPP system where the attainment of maximum reactivity at pKa approximately 9 is followed by a clear decrease in rate at higher pKa's. Interestingly, a number of data reported previously by different authors for the sarin, soman and DFP systems are found to conform rather well to the curvilinear Brønsted correlations built with our data. Based on this and previous results obtained for reactions at carbon centers, it can be concluded that the observed saturation effect is the reflection of an intrinsic property of the oximate functionality. An explanation of this behavior in terms of an especially strong requirement for desolvation of the oximates prior to nucleophilic attack which becomes more and more difficult with increasing basicity is suggested. This proposal is supported by the observed changes in pKa(Ox) and k(Ox) brought about by a transfer from H2O to a 30 : 70 H2O-Me2SO mixture. The implications of the saturation effect on the efficiency of oximates as nucleophilic catalysts for smooth decontamination are emphasized. Also discussed is the effect of basicity on the exalted (alpha-effect) reactivity of these bases.

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Section: Kinetic Studiesmentioning

confidence: 86%

Section: Introductionmentioning

confidence: 99%

Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

et al. 2006

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“…Compounds 6a, 6b, 6d, 6e, 6f, 6i, 6j and 6k are commercially available and used as purchased without purification. 6c 26 , 6h 27 , 6r 28 and 6v 29 are known compounds from literature.…”

Section: Methodsmentioning

confidence: 99%

Concise synthesis of functionalized benzocyclobutenones

2014

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A concise approach to access functionalized benzocyclobutenones from 3-halophenol derivatives is described. This modified synthesis employs a [2+2] cycloaddition between benzynes generated from dehydrohalogenation of aryl halides using LiTMP and acetaldehyde enolate generated from n-BuLi and THF, followed by oxidation of the benzocyclobutenol intermediates to provide benzocyclobutenones. The [2+2] reaction can be run on a 10-gram scale with an increased yield. A number of functional groups including alkenes and alkynes are tolerated. Coupling of benzynes with ketene silyl acetals to give 8-substituted benzocyclobutenones is also demonstrated.

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“…The 2 fluoroborate salt dissolved immediately in the solution. After 3 h a gcms sample was introduced into the inlet of the GC equipped with an MSD that we have previously used for our work. ,− The column was a methylsilicon gum cross-linked capillary (12 ft × 0.33 μm). Analysis of the MS and results was obtained on software supplied with the GCMS.…”

Section: Methodsmentioning

confidence: 99%

The α-Effect in Methyl Transfers from S-Methyldibenzothiophenium Fluoroborate to Substituted N-Methylbenzohydroxamates

et al. 2003

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Studies of the R-effect show increased reactivity of nucleophiles having lone pairs of electrons on atoms neighboring the lone pair involved in reactivity when compared to the basicity of the nucleophiles. Hammett-type plots and Brönsted-type plots of substituted methylphenyl sulfates vs hydrogen peroxide anions and substituted N-methylbenzohydroxanates (NMBH) with substituted methylarenesulfonates or substituted arenedimethylsulfonium ions have large F or nuc values, indicating a putative tightening of the usual S N 2 transition states (anti-Hammond effect). Electrochemical studies of S N 2-SET or reactivity indicate that SET character occurs in looser transition states, whereas S N 2 transition states are associated with greater tightness. The R-effects for the series of sulfonium salts in completion reactions for 3-ClNMBH anions and 3-nitrophenolate anions are (log k R /k normal ) 1.124 for dimethylphenyl sulfonium, 1.512 for dimethyl-1-naphthyl sulfonium, 1.835 for dimethyl-9-anthracenyl sulfonium, and 1.137 for S-methyldibenzylthiophenium. Correlations of the sizes of R-effects with typical SET (or ET) experimental parameters and the inverse dependence of the size of the R-effect on electron demand indicate inclusion of SET character in these S N 2 transition states, vs no (or at least diminished) SET character in normal transition states. This dichotomy of tighter S N 2 transition states, but looser SET transition states indicated in the R-effect, is examined in the present work.

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The α-Effect in Benzyl Transfers from Benzylphenylmethyl Sulfonium Salts to N-Methylbenzohydroxamate Anions

Cited by 46 publications

References 14 publications

Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

Concise synthesis of functionalized benzocyclobutenones

The α-Effect in Methyl Transfers from S-Methyldibenzothiophenium Fluoroborate to Substituted N-Methylbenzohydroxamates

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