Solvolysis rates and .beta.-deuterium secondary kinetic isotope effects of some tertiary and secondary alk-5-enyl derivatives. Evidence for .pi.-participation

Abstract

0.5 mL of S02 was slowly added a clear, homogeneous solution of magic acid (1 mL) in S02 (ca. 1 mL) at dry ice-acetone temperature with efficient vortex mixing. A cold aliquot was withdrawn with a precooled pipet (S02) and transferred into a 5-mm NMR tube under nitrogen, to which ca. 5 drops of cold CD2C12 had been added (vortex). NMR spectra were recorded at -65 °C.Molecular Modeling. MMX force field energy calculations on the sulfoxide substrates were carried out by using the pcmodel program (Serene Software). Good convergence was achieved after 100 iterations. The CSC angles were obtained directly from the minimized structures.search support and the Ohio Academic Challenge Program for the funds for a high-field NMR spectrometer.Solvolysis Rates and /9-Deuterium Secondary Kinetic Isotope Effects of Some Tertiary and Secondary Alk-5-enyl Derivatives. Evidence for ir-Participation

Solvolysis of 1,1-Dimethyl-4-Alkenyl Chlorides: Evidence for π-Participation

“…10 The lower values for 2 compared with 4 (∆∆H q ) -17 kJ mol -1 ; ∆∆S q ) -33 J mol -1 K -1 ; 80E) 19 are consistent with the mechanism involving a two π-electron assistance. Also, chloride 2 solvolyzes with a lower secondary -deuterium KIE than the referent chloride 4 (k H /k D ) 1.37 ( 0.03 vs k H /k D ) 1.79 ( 0.01; 80E, Table 2) demonstrating that the partial positive charge located on the reaction center in the transition state is smaller.…”

“…1) can be taken as a valid proof of neighboring group participation. 10 Modest or even inverse rate effects can conceal some considerable assistance, and if this is a case, the participation or its lack must be proven by other methods. Solvolytic behavior of the tertiary chlorides 1 and 3 is such an example where the relative rates obtained are not decisive parameters (Table 1).…”

“…These values can be used not only to compare the results obtained for chloride 1 but also for chloride 3, since, regardless of the chain length, essentially the same values of activation parameters (∆H q ≈ 90 kJ mol -1 , ∆S q ≈ -30 J K -1 mol -1 ; 80E) and exactly the same values of secondary -deuterium KIEs (k H /k D ≈ 1.8; hexadeuterated at methyl groups adjacent to the reaction center) were obtained in solvolysis of numerous dimethylalkyl chlorides in which the alkyl group varied form ethyl to squalanyl. 10,18 This typical value of the -deuterium KIE for the k S process is due to the rate depression for the deuterated isotopomer caused by hyperconjugative electron release from the neighboring C-H(D) bond to the incipient empty p orbital in the transition state.…”

“…9 Even more surprising was that the KIE observed for 1 was smaller than the secondary -deuterium KIE obtained for chloride 2 (k H /k D ) 1.37 ( 0.03; 80E), in which the assistance of the double bond has been proven without ambiguity. 10 We proposed the result to be rationalized in terms of charge distribution in the reaction transition state. Thus, in the case of 1, the transition state may be more akin to a tertiary cyclopentyl cation with relatively less charge at the C2, i.e., a "late" transition state.…”

Biomimetic rearrangement of the 2-cyclopentyl-2-propyl cation

“…Unlike the model cation 6, the secondary cations 2 and 4 enjoy additional stabilization of the positive charge, either by internal interaction with the p-electrons of the remoted double bond 11,12 or by external interactions with p-electrons of aromatic amino acid residues of the enzyme. The structure of the model cation 6 is closely related to the secondary cation 2 and the baccharenyl cation 4, which arise under enzymatic conditions (Scheme 1).…”