Four-atom concerted

Four-atom concerted transition state (results in syn addition)... 

Four-atom concerted Syn addition of H and B transition state... 

The 4ne component in a (4tc + 2n) cycloaddition need be neither a four-atom system (as in 1,3-dienes), nor involve carbon atoms only, so long as the HOMO/LUMO symmetry requirements for a concerted pathway can be fulfilled. The most common of these non-dienic 4ne systems involve three atoms, and have one or more dipolar canonical structures, e.g. (34a), hence the term—1,3-dipolar addition. They need not, however, possess a large permanent, i.e. residual, dipole, cf. diazomethane (34a 34f>) ... 

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. 

The C H activation processes were proposed to proceed via a trimolecular pathway, involving a linear four-centered concerted breaking of the C—H bond between two Rh(II) metalloradicals [see Fig. 59 (a)]. In principle, this reaction could also proceed via two subsequent separate steps hydrogen-atom abstraction from the hydrocarbon R3C H bond, thus generating a Rh H species and a carbon-centered radical wCRs, which is rapidly followed by capture of the thus... 

In the process, a ring is also formed, and the process is therefore a cycloaddition. Specifically, the Diels-Alder reaction is called a [4-p2] cycloaddition because the reaction takes place between two different it systems, one of which is associated with four atoms, while the other is associated with two atoms. The product of a Diels-Alder reaction is always a substituted cyclohexene. As is the case for all pericyclic reactions, the Diels-Alder reaction is a concerted process. 

Generally speaking, branching in these systems has been discussed in a largely intuitive way in terms of the relative importance of sequential vs. concerted motions. TTie controlling factor is the development of the system with time. This concept could be followed up quantitatively by trajectory calculations, but none appear to have been done on these four-atomic systems. 

The PES found by Smedarchina et al. [1989] has two cis-form local minima, separated by four saddle-points from the main trans-form minima. The step-wise transfer (trans-cis, cis-trans) - because of endoergicity of the first stage - displays Arrhenius behavior even at T < T. . The concerted transfer of two hydrogen atoms was supposed to become prevalent at sufficiently low temperatures. However, because of too high a barrier for the concerted trans-trans transition, this... 

When the Marcus analysis is corrected to use rate constants and driving forces characteristic of the CuL species the derived self-exchange rate constants are much more self consistent. We caution, however, that the CuL + complex likely has all six thiaether atoms coordinated to the Cu11 center, while the CuLj complex is probably four coordinate. Since it is rather unlikely that electron transfer occurs in concert with this change in coordination number, a further correction will probably be required in order to obtain physically meaningful self-exchange rate constants. 

The cycloaddition of an atom or group X to an olefine to form a three-membered ring and the reverse process constitutes an example of four electron cycloaddition or elimination and if the reaction is concerted it becomes an example of cheletropic reaction. 

Abstract 1,3-Dipolar cycloaddition reactions (DCR) are atom-economic processes that permit the construction of heterocycles. Their enantioselective versions allow for the creation of up to four adjacent chiral centers in a concerted fashion. In particular, well-defined half-sandwich iridium (111) catalysts have been applied to the DCR between enals or methacrylonitrile with nitrones. Excellent yield and stereoselectivities have been achieved. Support for mechanistic proposals stems from the isolation and characterization of the tme catalysts. 

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