Dihydroxylations mechanism

Bassan A, MRA Blomberg, PEM Siegbahn (2004) A theoretical study of the cis-dihydroxylation mechanism in naphthalene 1,2-dioxygenase. J. Biol. Inorg. Chem. 9 (4) 439452... 

Key Words Nonheme, Iron, Biomimetic, Bio-inspired, Cytochrome P450, Rieske dioxygenases. Hydrogen peroxide. Peroxide activation. Homogeneous catalysis, c/s-Dihydroxylation, Mechanism, Catalytic additives. Asymmetric... 

Fig. 31. Proposed cis-dihydroxylation mechanism for the oxidation of olefins catalyzed by [Mn203(Me3tacn)2](PF6)2/gmha 155).

Furthermore, the competing dihydroxylation mechanisms were examined chemoselectivity). Figure 9 shows the most favorable calculated dihydroxylation pathways (left side) for R = Me, S02 Tol ( 8 L,. l) jjj comparison to... 

Scheme 3. The Criegee mechanism for the osmium tetroxide mediated dihydroxylation reaction.

Lazrak, T., A. Milon, G. Wolff, A.M. Albrecht, M. Miehe, G. Ourisson, and Y. Nakatani. 1987. Comparison of the effects of inserted C40- and C50-terminally dihydroxylated carotenoids on the mechanical properties of various phospholipid vesicles. Biochim. Biophys. Acta 903 132-141. 

DHQD-CL or DHQ-CL) was used as the chiral auxiliary.175,176 However, the enantioselectivity observed under catalytic conditions was inferior to that observed under stoichiometric conditions. The addition of triethylammonium acetate, which increases the rate of hydrolysis of the Osvm-glycolate intermediate, improved enantioselectivity. A further improvement in enantioselectivity was brought about by the slow addition of substrates (Scheme 44).177 These results indicated that the hydrolysis of the Osvm-glycolate intermediate (57) was slow under those conditions and (57) underwent low enantioselective dihydroxylation (second cycle). Thus, Sharpless et al. proposed a mechanism of the dihydroxylation including a second cycle (Scheme 45).177 Slow addition reduces the amount of unreacted olefin in the reaction medium and suppresses the... 

Concerted [3 + 2] cycloaddition was proposed as the mechanism for dihydroxylation originally by Boesken195 and Criegee166 and reiterated in a refined manner by Corey.172 196 197 Another mechanism, stepwise [2 + 2] cycloaddition, has been proposed by Sharpless (Scheme 50).198-200... 

Extensive studies have been made of the mechanism of asymmetric dihydroxylation, but it is still difficult to conclusively determine the mechanism. Thus, the experimental data supporting each mechanism are highlighted below. 

Abstract The reaction mechanism of the olefin dihydroxylation by transition metal... 

It can be concluded that the [3+2] pathway seems to be the only feasible reaction pathway for the dihydroxylation by permanganate. The study on the free activation energies for the oxidation of a. P unsaturated carboxylic acids by permanganate shows that the [3+2] mechanism is in better agreement with experimental data than the [2+2] pathway. Experimentally determined kinetic isotope effects for cinnamic acid are in good agreement with calculated isotope effects for the [3+2] pathway, therefore it can be concluded that a pathway via an oxetane intermediate is not feasible. 

Theoretical calculations have been fundamental in solving the controversy on the mechanism for the dihydroxylation of double bonds by transition metal oxo complexes. Nowadays, this topic which was the subject of a controversy just a few years ago seems to be solved in favor of the [3+2] pathway, at least in a vast majority of the cases. Despite this spectacular success there are still a number of open issues for this particular reaction which have not been solved, and which continue to be a challenge for computational chemists. Among this, one can mention the correlation between the nature of the substrate and its reactivity with permanganate, and the mechanisms leading to the proportion of products experimentally observed when CrC Cb is applied. Hopefully, these issues will be solved in the future with the help of theoretical calculations. 

A good fit between previously observed linear free Hammett plots and those based on a simple 3 + 2 FMO-based calculation fit well and therefore support a 3 + 2 mechanism for the addition of permanganate to a C=C double bond. A scaling factor allowed the lateness or earliness of the transition state to be adjusted, and thus allowed adjustments that are consistent with the ability of Mn04 to dihydroxylate both electron-rich and electron-poor alkenes. 

The muconate of arylglycerol 12-Et retains all six carbon atoms of the B-ring of the substrate 1-Et. Hence, this product seemed to be appropriate to examine mechanisms for the ring cleavage. Based on the results of tracer experiments, we proposed a ring cleavage mechanism for the enzyme (7,9), as now described. Degradation of l,3-dihydroxyl-l-(4-ethoxy-... 

In Fig. 1.6 a simplified mechanism for as -dihyroxylation of alkenes and ketohydroxylation of R CH=CHR by RuCl3/Oxone /aq. Na(HC03)/Et0Ac-CH3CN is shown. The cA-dihydroxylation route involves (3 + 2) cycloaddition of RuO to the alkene giving a Ru(VI) ester (1) which is oxidised by (HSOj) to the Ru(VIII) ester (2). Reversible nucleophilic addition of water to (2) gives the diol R CH(OH) CH(OH)R (3). Ketohydroxylation ensues when the activated Ru(VIII) ester... 

Recent developments in the understanding of the mechanisms of catalytic and asymmetric dihydroxylation reactions are discussed in Section V,E,l,b. An important aspect of this work is the kinetics and thermodynamics of the formation of adducts with N heterocycles, which have an important role in promoting many reactions. The crystal structure of the [0s04] adduct with the cinchona alkaloid ligand (dimethyl-... 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

In September 1997, Chemical and Engineering News summarized the ongoing discussion about the precise mechanism of the initial steps of the osmium-catalyzed olefin dihydroxylation in an... 

Corey also pointed out that 16 reflects the transition-state of an enzyme-substrate complex. Its formation was later supported by the observation of Michaelis-Menten-type kinetics in dihydroxylation reactions and in competitive inhibition studies [37], This kinetic behavior was held responsible for the non-linearity in the Eyring diagrams, which would otherwise be inconsistent with a concerted mechanism. Contrary, Sharpless stated that the observed Michaelis-Menten behavior in the catalytic AD would result from a step other than osmylation. Kinetic studies on the stoichiometric AD of styrene under conditions that replicate the organic phase of the catalytic AD had revealed that the rate expression was clearly first-order in substrate over a wide range of concentrations [38],... 

Corey collected further evidence against a [2+2] mechanism by investigating the rate acceleration in stoichiometric dihydroxylations with chiral 1,2-diamines [36]. The X-ray structure of a highly reactive heptacoordinate—formally 20-electron bisamine-0s04 com-... 

Dihydroxylation (especially asymmetric reactions) of alkenes is a very important synthetic tool for the introduction of a new functionality into organic molecules. The reader is referred to recent reviews on the synthetic utility and mechanisms of asymmetric dihydroxylation for useful background material regarding synthetic outcomes84- 88. 

In a seminal contribution [42a], Que and coworkers revealed insight into the reaction mechanisms of epoxidation and dihydroxylation using the pentadentate TPA and bpmen ligands. Some of those compounds had earlier shown remarkable... 

A solvent-dependent product distribution for iron-catalyzed dihydroxylation with N3Py-derived ligands was reported by Feringa and coworkers [72]. In acetonitrile, stereoselective ds-dihydroxylation was observed. On the other hand, acetone gave rise to trans-diols, thereby indicating that the choice of solvent determines the mechanism and hence the outcome of this reaction [72]. 

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