Anup Rana

Quantification of Nonstatistical Dynamics in an Intramolecular Diels–Alder Cyclization without Trajectory Computation

Experimental and computational (DFT) investigations reveal that enyne-allenes with an aryl group as probe at the allene terminus follow a dynamic non-IRC Diels-Alder cyclization pathway. Starting from two separate C(2)-C(6) (Schmittel) transition states (TS), two distinct reaction paths originate that share a common diradical intermediate, however, without mixing! Because the momentum of the initial TS is transmitted into product formation, we suggest a simple protocol without trajectory computations to estimate the fraction of molecules that follow nonstatistical dynamics: It was calculated from the partitioning at the TSs, as derived from DFT computations, and the experimental ratio. The thus-determined percentage of dynamically reacting molecules only slightly depends on the depth of the intermediate well but rather on ΔΔG(‡) of the initial and the follow-up transition states.

Nonstatistical Dynamics in the Thermal Garratt–Braverman/[1,5]-H Shift of One Ene–diallene: An Experimental and Computational Study

A mechanistic study of the thermal Garratt-Braverman/[1,5]-H shift of ene-diallene 6 leading to alkenes (E)-8 and (Z)-8 is reported. The product ratio was found to be temperature-independent and does not agree with the computed energy barriers (i.e., TS2E(⧧) and TS2Z(⧧)). On the basis of experimental and DFT computational results, we propose a mechanism that is strongly controlled by nonstatistical dynamic effects.

Influence of Rotator Design on the Speed of Self-Assembled Four-Component Nanorotors: Coordinative Versus Dispersive Interactions

Four-component nanorotors are prepared by the self-assembly of stator [Cu4(4)]4+ with its four copper(I)-loaded phenanthroline stations and various rotators carrying one, two, or three pyridine terminals. The fourth component, 1,4-diazabicyclo[2.2.2]octane, serves as a connecting axle between rotator and stator. Capitalizing on the heteroleptic pyridyl and phenanthroline metal complexes concept, the rotators pyridine terminals are connected to the copper(I)-loaded phenanthroline stations (Npy → [Cu(phen)]+) in the STOP state and disconnected in the transition state of rotation. As the barrier of the thermally activated rotation, measured by variable-temperature 1H NMR, is mainly governed by attractive forces between stator stations and rotator terminals, it increases along the series Ea (monopyridine rotator) < Ea (dipyridine rotator) < Ea (tripyridine rotator). However, there are even distinct differences in rate between rotors with equal number of rotator terminals. The change from the 5,10-dipyridyl (cis) to 5,15-dipyridyl (trans) zinc porphyrin rotator enhances the rotational frequency by almost 1000-fold. Density functional theory computational results suggest that not only coordinative Npy → [Cu(phen)]+ interactions but also dispersive attraction influence the barrier of rotation.

Nonstatistical dynamics in the thermal C2-C6/diels-alder cyclization of enyne-allenes: effect of topology.

The thermal C(2)-C(6) (Schmittel) cyclization of an enyne-allene with two aryl rings at the allene terminus experimentally leads to three formal Diels-Alder (DA) cycloaddition products, two of which (involving the dimethylamino phenyl unit) are shown to form in a nonstatistical process. DFT computations on the reaction paths reveal that the two aryl rings (Ph vs PhNMe2) do not interact in a dynamic manner as their minimum energy pathways (MEPs) are separated by a large barrier. The preferential formation of the more-hindered DA product 8 (ortho to the dimethylamino group) over the less-hindered product 9 (para to the dimethylamino group), despite the higher energy TS for 8, suggests the occurrence of nonstatistical dynamics in the cyclization onto the dimethylamino phenyl unit, though. Potential energy surface (PES) computations indicate that the large amount of nonstatistical dynamics (97%) arises from facile IRC dynamics (left picture) that is compared with the non-IRC dynamics of a related system (∼76%, right picture, J. Org. Chem. 2014, 79, 2368).

Mechanistic Information from Nonstationary Points

The thermal cyclization of enyne-carbodiimides substituted at both the alkyne and carbodiimide terminus showed two curved Hammett correlations (log k/k0 against σp) that were fully reproduced by DFT (density functional theory) computational results. The latter suggest a concerted mechanism, but the transition state (TS) analysis failed to reveal any mechanistic insight about the reason for a curved Hammett correlation. Instead a preTS inspection, i.e., examination of the electronic and steric details on route between reactant and TS, furnished a detailed picture of the mechanism.

Solving the puzzling competition of the thermal C2–C6 vs Myers–Saito cyclization of enyne-carbodiimides

Summary The mechanism of the thermal cyclization of enyne-carbodiimides 7a–c has been studied computationally by applying the DFT method. The results indicate that enyne-carbodiimides preferentially follow the C2–C6 (Schmittel) cyclization pathway in a concerted fashion although the Myers–Saito diradical formation is kinetically preferred. The experimentally verified preference of the C2–C6 over the Myers–Saito pathway is guided by the inability of the Myers–Saito diradical to kinetically compete in the rate-determining trapping reactions, either inter- or intramolecular, with the concerted C2–C6 cyclization. As demonstrated with enyne-carbodiimide 11, the Myers–Saito channel can be made the preferred pathway if the trapping reaction by hydrogen transfer is no more rate determining.

A one-pot multistep cyclization yielding thiadiazoloimidazole derivatives.

Summary A versatile synthetic procedure is described to prepare the benzimidazole-fused 1,2,4-thiadiazoles 2a–c via a methanesulfonyl chloride initiated multistep cyclization involving the intramolecular reaction of an in-situ generated carbodiimide with a thiourea unit. The structure of the intricate heterocycle 2a was confirmed by single-crystal X-ray analysis and its mechanism of formation supported by DFT computations.