On the Relative Acidities of Organic Compounds:  Electronic and Geometric Relaxation Energies

Bökman, Fredrik

doi:10.1021/ja992759k

“…More insight into the influence of strain on acidity of silanes A – C (Figure 4 A) and 2 can be obtained by decomposing the deprotonation reaction into two formal stages (Figure 4 B). [45] In the first stage, a proton is abstracted from the silane with all other atomic coordinates frozen. The energy of this process (Δ E 1 ) gauges the penalty of charge separation in the course of heterolytic Si−H bond dissociation followed by electronic relaxation.…”

Section: Resultsmentioning

confidence: 99%

Strain‐Modulated Reactivity: An Acidic Silane

Tretiakov

¹

,

Witteman

²

,

Lutz

³

et al. 2021

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Compounds of main‐group elements such as silicon are attractive candidates for green and inexpensive catalysts. For them to compete with state‐of‐the‐art transition‐metal complexes, new reactivity modes must be unlocked and controlled, which can be achieved through strain. Using a tris(2‐skatyl)methylphosphonium ([TSMPH3]+) scaffold, we prepared the strained cationic silane [TSMPSiH]+. In stark contrast with the generally hydridic Si−H bond character, it is acidic with an experimental pKaDMSO within 4.7–8.1, lower than in phenol, benzoic acid, and the few hydrosilanes with reported pKa values. We show that ring strain significantly contributes to this unusual acidity along with inductive and electrostatic effects. The conjugate base, TSMPSi, activates a THF molecule in the presence of CH‐acids to generate a highly fluxional alkoxysilane via trace amounts of [TSMPSiH]+ functioning as a strain‐release Lewis acid. This reaction involves a formal oxidation‐state change from SiII to SiIV, presenting intriguing similarities with transition‐metal‐mediated processes.

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“…More insight into the influence of strain on acidity of silanes A-C (Figure 4A)a nd 2 can be obtained by decomposing the deprotonation reaction into two formal stages (Figure 4B). [45] In the first stage,aproton is abstracted from the silane with all other atomic coordinates frozen. Theenergy of this process (DE 1 )gauges the penalty of Figure 4.…”

Section: Computational Analysis Of Acidity Of [Tsmpsih] + (2)mentioning

confidence: 99%

Strain‐Modulated Reactivity: An Acidic Silane

Tretiakov

¹

,

Witteman

²

,

Lutz

³

et al. 2021

View full text Add to dashboard Cite

Compounds of main‐group elements such as silicon are attractive candidates for green and inexpensive catalysts. For them to compete with state‐of‐the‐art transition‐metal complexes, new reactivity modes must be unlocked and controlled, which can be achieved through strain. Using a tris(2‐skatyl)methylphosphonium ([TSMPH3]+) scaffold, we prepared the strained cationic silane [TSMPSiH]+. In stark contrast with the generally hydridic Si−H bond character, it is acidic with an experimental pKaDMSO within 4.7–8.1, lower than in phenol, benzoic acid, and the few hydrosilanes with reported pKa values. We show that ring strain significantly contributes to this unusual acidity along with inductive and electrostatic effects. The conjugate base, TSMPSi, activates a THF molecule in the presence of CH‐acids to generate a highly fluxional alkoxysilane via trace amounts of [TSMPSiH]+ functioning as a strain‐release Lewis acid. This reaction involves a formal oxidation‐state change from SiII to SiIV, presenting intriguing similarities with transition‐metal‐mediated processes.

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“…Die Ursachen der Acidität eines Teilchens HA wurden schon vielfach analysiert und in der Literatur beschrieben. Häufig wurden isolierte Moleküle durch quantenchemische ab initio Rechnungen untersucht [7]. Veränderungen in der Molekülgeometrie bzw.…”

Section: Thermodynamische Betrachtungen Zur Aciditätunclassified

Is a polar bond a prerequisite for an acid? Analysis of a widely accepted explanation

Fleischer

¹

2020

Chemkon

1

0

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Die Erklärung der Acidität eines Moleküls HA durch die Polarität einer Element‐Wasserstoff‐Bindung („Polaritätskonzept“) ist nicht tragfähig. Die Analyse der Protolyse verschiedener Säuren HA mit der Base H2O in einem thermodynamischen Kreisprozess zeigt drei wesentliche Beiträge, die für Aciditätsunterschiede verantwortlich sind: Die freie Standardbindungsenthalpie, ΔBG0(H−A), die freie Standardhydratationsenthalpie der korrespondierenden Base A−, ΔhydG(A−), und die Ionisierungsenergie von A−, IE(A−). In vielen Fällen steigt der Betrag von ΔBG0(H−A), d.h. die Bindungsstärke der Element‐Wasserstoff‐Bindung, mit deren Polarität. Das steht dem Polaritätskonzept entgegen. Je größer hingegen IE(A−), desto stabiler ist A− und umso acider ist HA. Es wird die Hypothese aufgestellt, dass IE(A−) mit der Möglichkeit zur Delokalisierung der negativen Ladung zunimmt. Eine Analyse der Orbitale von CH3−, NH2−, OH− und F− zeigt, dass die Delokalisierung der Valenzelektronen und damit auch der negativen Ladung in dieser Reihe zunimmt. Dieser Effekt, und nicht die Polarität der H−A‐Bindung, bedingt den Anstieg der Acidität in der Reihe CH4−NH3−H2O−HF. Die Delokalisierung der negativen Ladung lässt sich mit dem Kugelwolkenmodell im Chemieunterricht der Sekundarstufe 1 anschaulich darstellen. Daher wird ein „Delokalisierungskonzept“ statt des „Polaritätskonzepts“ zur Vorhersage und zur Erklärung unterschiedlicher Aciditäten vorgeschlagen.

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On the Relative Acidities of Organic Compounds: Electronic and Geometric Relaxation Energies

Cited by 17 publications

References 22 publications

Strain‐Modulated Reactivity: An Acidic Silane

Strain‐Modulated Reactivity: An Acidic Silane

Strain‐Modulated Reactivity: An Acidic Silane

Is a polar bond a prerequisite for an acid? Analysis of a widely accepted explanation

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