Evidence for a concerted SN2? mechanism in the gas-phase acid-induced nucleophilic substitutions on allylic substrates

Dezi, E.; Lombardozzi, Antonietta; Pizzabiocca, Adriano; Renzi, Gabriele; Speranza, Maurizio

doi:10.1039/c39950000547

Cited by 7 publications

(4 citation statements)

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“…(A) The present results confirm previous experimental [27][28][29] and theoretical [23][24][25] indications that concerted acid-induced S N 2′ reactions are feasible in the gas phase and efficiently compete with the classical S N 2 processes. Competition between the S N 2 and S N 2′ is essentially determined by the orienting properties of the oxonium intermediate toward the incoming nucleophile.…”

Section: Discussionsupporting

confidence: 90%

“…Carrion and Dewar suggest that the predominance in solution of the S N 2 mechanism over the S N 2‘ one is primarily due to energy-demanding ion desolvation in the S N 2‘ transition structure, which mostly contributes to the building up of the relevant activation barrier, whereas the more favored S N 2 barrier is essentially determined by electronic factors. In light of these modern concepts, these and other authors conclude that there is no real reason concerted S N 2‘ reaction should not be feasible in the gas phase. − This indication has been followed in a recent gas-phase study of acid-induced nucleophilic substitution on some allylic alcohols that showed that the concerted S N 2‘ reaction actually competes with the classical S N 2 pathway in the absence of solvation and ion-pairing factors. − The same modern concepts consider restrictive any theoretical rationale of the S N 2‘ stereochemistry simply based on stereoelectronic factors, − since an important role may be played by Coulombic interactions between NuH and LG and among these and the reaction medium.…”

Section: Introductionmentioning

confidence: 99%

“…[23][24][25][26] This indication has been followed in a recent gas-phase study of acid-induced nucleophilic substitution on some allylic alcohols that showed that the concerted S N 2′ reaction actually competes with the classical S N 2 pathway in the absence of solvation and ion-pairing factors. [27][28][29] The same modern concepts consider restrictive any theoretical rationale of the S N 2′ stereochemistry simply based on stereoelectronic factors, [30][31][32][33][34][35][36][37][38][39][40] since an important role may be played by Coulombic interactions between NuH and LG and among these and the reaction medium.…”

Section: Introductionmentioning

confidence: 99%

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Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Speranza

Troiani

1998

J. Org. Chem.

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The regio- and stereochemistry of the nucleophilic attack of (S)-trans-3-hexen-2-ol (MS ) and (S)-trans-4-hexen-3-ol (ES ) on the corresponding O-protonated (L = H) and -methylated (L = CH3) derivatives (MSL+ and ESL+ ) are investigated in the gas phase at 40 °C (720 Torr). The MSL+ and ESL+ intermediates are produced in the gas phase by the attack of the ionic Brønsted and Lewis acids, formed by stationary γ-radiolysis of bulk CH3Cl, on the corresponding chiral alcohols, i.e., MS and ES . In these systems, firm evidence in favor of the concerted SN2‘ pathway, accompanying the classical SN2 one, is obtained by excluding the following: (i) the isomerization of MSL+ (or ESL+ ) before the attack by the nucleophile NuH = MS (or ES ); (ii) the isomerization of the (C6H11)2OH+ substitution intermediates before neutralization; (iii) the intermediacy of allylic cations. The regioselectivity factors (SN2‘/SN2 = 1.4 (MS ), 1.1 (ES )) confirm previous experimental and theoretical evidence about the prevalence in the gas phase of the SN2‘ pathway, over the competing SN2 one. Orientation of NuH by MSL+ (or ESL+ ) determines the regiochemistry of the allylic substitution. When NuH approaches the oxonium intermediate from the direction syn to the leaving moiety LOH, a frontside SN2 displacement takes places favored by preliminary proton bonding between LOH and NuH. The SN2‘ reaction instead follows attack on the π-LUMO of the oxonium ion by the NuH juxtaposed anti to the leaving LOH group. Observation of a predominant anti-SN2‘ orientation provides the first experimental basis of modern concepts pointing to Coulombic interactions as the main intrinsic factors governing the SN2‘ stereochemistry and to solvation and ion pairing as the factors determining the low efficiency of SN2‘ reactions and their preferred syn stereochemistry in solution.

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Section: Discussionsupporting

confidence: 90%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Speranza

Troiani

1998

J. Org. Chem.

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“…Amines were used as the nucleophiles, and not surprisingly, the amount of elimination increases as more basic amines are used. Finally, Speranza and co-workers − have recently used their radiolytic method to study the competition between S N 2 and S N 2‘ reactions in the acid-induced substitutions of allylic alcohols. The results show that S N 2‘ reactions can compete successfully with S N 2 reactions in these systems.…”

Section: Sn2 and Related Reactionsmentioning

confidence: 99%

Mass Spectrometric Studies of Organic Ion/Molecule Reactions

2001

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In the past 25 years, a tremendous amount of work has been published on the ion/molecule reactions of organic species. This review provides an overview of the areas where gas phase ion chemistry has made a contribution to our understanding of fundamental organic reaction processes. It is clear that the gas phase work can provide insights into subtle features of reaction mechanisms that could not be addressed by conventional condensed phase methods. The study of ion/molecule reactions has already had a major impact on the way that organic chemists think about reaction mechanisms and interpret substituent effects. Moreover, it has heightened our awareness of the importance of solvation effects and how they can alter not only absolute rates but also relative rates, leading in some cases to complete reversals in reactivity patterns. A large body of work could not be included in this review due to space limitations. For example, the study of thermochemistry in the gas phase (i.e., acidities, basicities, bond strengths, binding energies, etc.) has provided a wealth of data that has been exceptionally useful in interpreting organic reaction mechanisms. This has spilled over into the study of organometallic systems, and several groups are making major headway in using mass spectrometry to probe the stability and reactivity of transition metal species. Finally, work involving chemical ionization has provided abundant information on gas phase reaction mechanisms. The future appears to be very promising for the study of gas phase organic reaction mechanisms. In particular, the emergence of new ionization techniques and more powerful mass analyzers will allow chemists to explore a wider range of species. Although still at an early stage, the gas phase study of biochemical transformations offers great promise and has been facilitated by electrospray and matrix assisted laser desorption ionization methods. In addition, these techniques provide a means for introducing important, metal-centered catalytic species into the gas phase and exploring the details of their reactivity. Finally, mass spectrometry continues to play a major role in the study of atmospheric ion chemistry and is providing important kinetic as well as mechanistic data.

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Gas‐Phase Acid‐Induced S_N2′ versus S_N2 Mechanism in Allylic Alcohols

Renzi

Lombardozzi

Dezi

et al. 1996

Chemistry A European J

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A first demonstration of the existence of the concerted SN2' mechanism in the gas phase was obtained by establishing the regioselectivity of the attack of a neutral nucleophile, such as MeOH, on several allylic oxonium ions. These were generated in the gas phase by the reaction of radiolytically formed GA' acids (GA+ = C,H: (n = I , 2), iC,H:, and (CH,),F+) with trans-(1) and cis-2-buten-1-01 (2) as well as with I-buten-3-01 (3). Firm evidence in favor of the concerted SN2' pathway accompanying the classical SN2 one in these systems was obtained after careful evaluation of the extent of conceivable intramolecular isomerization both of the primary oxonium ions from GA' attack on 1-3 before nucleophilic displacement by MeOH and of their substituted intermediates before neutralization. The intermediacy of free allylic ions in the nucleophilic substitution was ruled out by generating the ions by protonation of 1,3-butadiene and by investigating their behavior in exactly the same media employed in the substitution reactions. The regioselectivity of MeOH with the ionic substrates investigated showed the occurrence of nearly equally extensive SN2' and SN2 pathways in the oxonium ions from 1 (SN2' (57f2%) and SN2 (43 2 YO)) and 3 (SN2' (54 + 2 YO) and SN2 (46+2%)), whereas, with 2, the SN2 (66 2 YO) reaction prevailed over the SN2' one (34 2 %). The role of intrinsic structural factors in determining the SN2'/SN2 branching in the selected oxonium ions is discussed.

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Evidence for a concerted SN2? mechanism in the gas-phase acid-induced nucleophilic substitutions on allylic substrates

Cited by 7 publications

References 7 publications

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Mass Spectrometric Studies of Organic Ion/Molecule Reactions

Gas‐Phase Acid‐Induced S_N2′ versus S_N2 Mechanism in Allylic Alcohols

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Evidence for a concerted SN2? mechanism in the gas-phase acid-induced nucleophilic substitutions on allylic substrates

Cited by 7 publications

References 7 publications

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols1

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols1

Mass Spectrometric Studies of Organic Ion/Molecule Reactions

Gas‐Phase Acid‐Induced SN2′ versus SN2 Mechanism in Allylic Alcohols

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Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols¹

Gas‐Phase Acid‐Induced S_N2′ versus S_N2 Mechanism in Allylic Alcohols