Chiral Ions in the Gas Phase. 1. Intramolecular Racemization and Isomerization of O-Protonated (S)-trans-4-Hexen-3-ol

Abstract

The acid-catalyzed racemization and regioisomerization of (S)-trans-4-hexen-3-ol (1S) has been investigated in gaseous CH4 and C3H8 at 720 Torr and in the 40−120 °C temperature range. The contribution to the racemization and isomerization products by free 1-methyl-3-ethylallyl cations, arising from unimolecular fragmentation of excited O-protonated (S)-trans-4-hexen-3-ol (IS), was evaluated by generating them from protonation of isomeric 2,4-hexadienes and by investigating their behavior toward H2 18O under the same experimental conditions. The rate constant of the gas phase racemization of IS (1.4−21.3 × 106 s-1) was found to exceed that of its isomerization (1.0−9.9 × 106 s-1) over the entire temperature range. The experimental results, combined with ab initio theoretical calculations on the model [C3H5 +/H2O] system, are consistent with a gas phase intramolecular IS racemization and isomerization involving the intermediacy of structured ion−molecule complexes, wherein the H2O molecule is coplanarly coordinated to the hydrogen atoms of the 1-methyl-3-ethylallyl moiety. The rate of formation of these structured complexes, their relative stability, and the dynamics of their evolution to the racemized and isomerized products depend, in the gas phase, on the specific conformation of IS. The relevant activation parameters point to transition structures wherein a substantial fraction of the positive charge is located on the allyl moiety. The results obtained in the present gas phase investigation confirm previous indications about the occurrence and the role of intimate ion−molecule pairs in acid-catalyzed racemization and isomerization of optically active alcohols, including allylic alcohols, in solution.

Gaseous Ion Activation Dynamics: The Role of the Bulk Gas in the Racemization of Chiral Oxonium Ions

“…[17,18] Indeed, as already pointed out for the CH 3 Cl systems, [3,13] the n G = 1021 AE 22 cm À1 matches the n 6 (e) CH 3 rocking mode of unperturbed CH 3 Cl molecules (n 6 (e) = 1015 cm…”

Regioselective Synthesis of Acycliccis-Enediynes via an Acid-Catalyzed Rearrangement of 1,2-Dialkynylallyl Alcohols. Syntheses, Computational Calculations, and Mechanism

“…90:10 for 29 ). We consider that the two reactive intermediates 43 and 44 might be involved in the product-forming step (Scheme ) . A nucleophile attacks at the protonated allyl alcohol in either an S N 2 or an S N 2‘ fashion to form two regioisomers.…”

Intracomplex CH3OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

“…In the preceding paper of this series, the kinetics and the dynamics of the unimolecular racemization and regioisomerization of O-protonated ( S )- trans -4-hexen-3-ol (the IS anti / IS syn mixture in Scheme ) have been investigated in the dilute gas state, i.e., in a reaction environment entirely free from the interference of solvation, ion-pairing, etc., that normally complicates analogous studies in solution. − The results are consistent with a gas-phase intramolecular racemization and regioisomerization involving the intermediacy of the hydrogen-bonded complexes B and C in Scheme , In the particular case of B and C , the moving H 2 O molecule is coplanarly coordinated to the in-plane hydrogens of the allyl cation moiety. The formation of these hydrogen-bonded complexes is determined by kinetic factors as well as by the specific conformation of their oxonium ion precursors.…”

“…To enlarge our knowledge of the factors governing acid-promoted unimolecular racemization and regioisomerization of chiral allyl alcohols in the gaseous state, the kinetic investigation has been now extended to the rearrangement of ( S )- trans -4-hexen-3-ol (the 1S anti / 1S syn mixture, henceforth denoted as 1S ) and ( R )- trans -3-hexen-2-ol (the 2R anti / 2R syn mixture, henceforth denoted as 2R ) promoted by a pure Lewis acid, such as the dimethylchloronium ion (CH 3 ) 2 Cl + (Scheme ). By this approach, the methylated oxonium ions IS‘ anti / IS‘ syn (henceforth denoted as IS‘ ) and IIR‘ anti / IIR‘ syn (henceforth denoted as IIR‘ ) can be readily prepared in the dilute gas phase and their rearrangement kinetics evaluated with the same procedures employed in previous studies . Thus, stationary concentrations of the (CH 3 ) 2 Cl + Lewis acid have been generated from γ-radiolysis of gaseous mixtures, containing CH 3 Cl, as the bulk component (720 Torr), together with traces of the chiral alcohol (either 1S or 2R ; 0.5−0.7 Torr), of O 2 , as a radical scavenger (10 Torr), and of (CH 3 O) 3 PO (0.8−0.9 Torr), as a powerful base (proton affinity (PA)=212.0 kcal mol -1 ) .…”