Complex reactions transition state switching

Higher temperatures and polar solvents are considered to switch the reaction mechanism of transmetalation from a four-centered transition state (Se2 (cyclic)) to a back-side attack of the palladium(II) complex (Se2 (open)) (Fig.1). 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

Scheme I further indicates the tendency of the Ln(III) cations to form the mofe unusual oxidation states in solution [73]. Hitherto, organometallic compounds of Ce(IV), Eu(II), Yb(II) and Sm(II) have been isolated. Charge-dependent properties, such as cation radii and Lewis acidity, significantly differ from those of the trivalent species (Table 4). Ln(II) and Ce(IV) ions show very intense and ligand-dependent colors which is attributed to Laporte-allowed 4/-+ 5d transitions [65b]. Complexes of Ce(IV) and Sm(II) have acquired considerable importance in organic synthesis due to their strong oxidizing and reducing behavior, respectively their reaction patterns have been reviewed in detail [40, 44-47, 74], Catalytic amounts of compounds containing the hot oxidation states also initiate substrate transformations as a rule this implies switch to the more stable, catalytically-acting Ln(III) species [75],...

In this chapter, crystalline-state photochromic dynamics of rhodium dithionite complexes are reviewed. The chemistries described here have been achieved not only by recent developments of the analytical technique but also by discovery of a new class of transition-metal based photochromic compounds. One of the advantages of transition-metal complexes is structural diversity. In order to find the rule of an exquisite combination of metal ions and ligands, we are currently synthesizing various dithionite derivatives with other metal ions and/or modified Cp ligands. As shown in this chapter, dithionite complexes are a very useful photochromic system to investigate crystalline-state reaction dynamics. We believe that dynamics studies of newly synthesized dithionite derivatives provide useful insight into the construction of sophisticated molecular switches. A dithionite complex may appear in a practical application field in the near future. 

The luminescence and excited state electron transfer reactions of (dppe)Pt S2C2(2-pyridine(ium))(H) and (dppe)Pt S2C2(4-pyridine(ium))(H) are dependent on the protonation state of the pyridine [30-35]. The switching on of the luminescence in these compounds results from a change in the ordering of the electronic transitions in the pyridine and pyridinium substituted complexes. Unlike the quinoxaline-substituted complexes, the neutral pyridine complexes have a lowest lying d-to-d transition, which leads to rapid nonradiative decay of the ILCT excited states. However, upon protonation the ILCT becomes the low-lying transition. The pyridinium complexes are room temperature lumiphores with emission from ILCT and ILCT excited states (see Table Ic). 

Depending on the wave-length of irradiation diverse electronically excited states of both transition metal complexes and organometallic compounds are accessible. Under certain circumstances one may switch between homolytic and heterolytic reaction pathways. 

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