Reaction pathways trajectory model

Substrate probes have aided mechanistic understanding of the key C— H activation step in the MMOH reaction cycle. Chiral alkanes and radical-clock substrate probes " " were used to discriminate between radical recoil/rebound and nonsynchronous concerted insertion pathways. A short lifetime (< 150 fs) estimated for the putative radical species derived from cyclopropane-based radical-clock substrates favors the latter process,whereas partial racemization of chiral ethane substrate is consistent with the former scenario. A unifying model was proposed, in which both recoil/rebound and concerted reaction channels are available for a bound radical intermediate and the partitioning between each trajectory is dependent on the substrate. Formation of carboca-tion-derived products from certain probes implicates yet another route involving a formal OH+ insertion.Participation of multiple species capable of oxygen transfer is an emerging mechanistic view in both heme and nonheme systems, as exemplified by the studies of cP450s and their synthetic models.Scheme 3 depicts various density functional theory (DFT) models of MMOHq and their computed reaction pathways, which are reviewed in detail elsewhere. 

The sense of asymmetric induction in the a-oxygenated aldehydes, which shows a strong kinetic preference for the formation of the syn diol, is consistent with the classic Gram model as in 27 (Scheme 32). Once the complexation occurs, the allyl group is transferred to the carbonyl carbon from the less hindered 7r-face opposite to that occupied by the R group. In 28, the chelation pathway is able to adopt a chair conformation which accommodates the favored formation of the syn-diol. For /3-chelate reactions, the factors which influence the product formation appear to be the same. When 29 forms, the intramolecular attack occurs syn to the hydroxyl group. This reaction trajectory leads preferentially to the anti-dio, provided that a chairlike transition states such as 30 is followed. 

A solution to the problem of constructing a strict theoretical model that could harmoniously accomodate the above-considered notion concerning the pathway of chemical reaction was suggested by Fukui [62]. He introduced the concept of the intrinsic reaction coordinate (IRC) defined as a classical trajectory of a system s motion over the PES crossing the saddle point of a transition state... 

Fig. 17. Solid-angle differential cross section for the reaction 0( D) + H OH + H proceeding via the insertion-decomposition pathway. The best fit from the osculating complex model (OCM) is the solid curve which is shown superimposed on the histogrammic representation of the trajectory data. The collision energy is 2 kcal/mol. Arrows indicate the range of scattering angles obtained in the trajectory calculations.

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