Elucidation of Nonstatistical Dynamic Effects in the Cyclization of Enyne Allenes by Means of Kinetic Isotope Effects

The thermal C2–C6 (Schmittel)/ene cyclization of enyne–allenes – crossing the boundary between classical and nonstatistical kinetics

“…However, a classical behavior is not warranted in this case, as "on the fly" trajectory calculations on 90 supported the conclusions drawn by Singleton, Lipton et al, [87] The smooth energy profile behind TS1 sustains strongly oscillating trajectories between diradical and product-like structures, thus pointing to nonstatistical dynamic behavior. [89] Aryl group at the allene terminus For enyne-allenes 89c and 89 d, each equipped with an aryl group at the allene terminus, the observed intermolecular KIEs are close to unity (1.17, 1.08, Table 2), but not within the range expected for a stepwise process, that is, 1.00-1.05, according to a classical interpretation. The observed intermolecular KIEs suggest that the transition states are composed of major contributions of stepwise nature and minor contributions of concerted reaction character.…”

“…On the basis of these observations, they suggested that the stepwise and concerted ene reaction of enyne–allenes proceeded via a single transition state and that dynamic effects at a post‐transition‐state valley–ridge inflection point would decide on concerted versus stepwise pathways. Although at that time the Singleton‐Lipton hypothesis was based on computational results exclusively, it was experimentally verified by a recent work investigating intramolecular and intermolecular KIEs in a series of enyne–allenes . According to the substitution pattern, four categories were identified: (i) enyne–allenes without radical stabilizing substituents; (ii) enyne–allenes with aryl groups at the allene unit; (iii) enyne–allenes with aryl groups at the alkyne terminus; and (iv) enyne–allenes with aryl groups at both the allene and alkyne terminus .…”

“…5) at the (U)B3LYP/6-31 G(d) level disclosed a very broad transition state zone, TS1, which comprises both the concerted (TS conc ≈ 32 kcal) and stepwise process (TS step ≈ 31 kcal). [89] Assuming a classical behavior for 90, the calculated intermolecular KIE (k CH3 /k CD3 ) would be 1.61 and the intramolecular KIE (k CH3 /k CH2D ) = 1.57. However, a classical behavior is not warranted in this case, as "on the fly" trajectory calculations on 90 supported the conclusions drawn by Singleton, Lipton et al, [87] The smooth energy profile behind TS1 sustains strongly oscillating trajectories between diradical and product-like structures, thus pointing to nonstatistical dynamic behavior.…”

“…Although at that time the Singleton-Lipton hypothesis was based on computational results exclusively, it was experimentally verified by a recent work investigating intramolecular and intermolecular KIEs in a series of enyne-allenes. [89] According to the substitution pattern, four categories were identified: (i) enyne-allenes without radical stabilizing substituents; (ii) enyne-allenes with aryl groups at the allene unit; (iii) enyne-allenes with aryl groups at the alkyne terminus; and (iv) enyne-allenes with aryl groups at both the allene and alkyne terminus. [90] The KIE results are summarized in Table 2.…”

Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene

“…2 Thus far, the intermediates known to exhibit such behavior are singlet biradicals, 3 radical cations, 4 radicals, 5 and carbocations. 6 In the present study, we sought to investigate whether carbene chemistry might also be subject to these dynamical effects.…”