Olefin dihydroxylation mechanism

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

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... 

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

The author proposed that the reaction presumably proceeded via [3+2] mechanism, which is quite similar to [3+2] mechanisms for olefin dihydroxylation. Insertion of OH into OsOa at pH = 12.1 led to the formation of 0s04(0H) species. Addition of C-H bonds into two oxo-groups of 0s04(0H) provided an intermediate, which can be hydrolyzed by aqueous base, and liberated a corresponding alcohol and reduced osmate [0s02(0H)4 ] (eq 80). 

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... 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

The fundamental mechanism of the dihydroxylation of olefins remained unclear for a long time. Two different mechanisms, concerted [3+2] and stepwise [2+2] were popular, plausible, mechanisms for the reaction [84, 85]. The controversy about the reaction mechanism was finally resolved after the thorough work of several experimental [84, 85] and theoretical [86] research groups. A scientific consensus emerged considering the [3+2] mechanism as the operative one [87]. Nevertheless, in very special cases and using other metal oxides, the activation barrier for the stepwise mechanism is lower in energy than for the concerted one [88, 89]. Other mechanisms, such as a diradical mechanism, have been also discussed [90]. 

Mechanism 8-10 Acid-Catalyzed Opening of Epoxides 362 8-14 Syn Dihydroxylation of Alkenes 364 8-15 Oxidative Cleavage of Alkenes 366 8-16 Polymerization of Alkenes 369 8-17 Olefin Metathesis 373... 

The reaction mechanisms shown in Scheme 2 have been the subject of several computational studies. Of particular interest has been the dihydroxylation by osmium tetroxide,26-29 where the above mentioned controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally.10,12,13,16,19,22,24,25,35,36... 

Fig. 4.31 Mechanism of osmium-catalyzed vicinal dihydroxylation of olefins.

Of particular interest are asymmetric dihydroxylation (AD) reactions. Depending on the oxidant employed, which is commonly either NMO or K3[Fe(CN)6]/K2C03, two different mechanisms may apply (see Schemes 5.12 and 5.13).[701 One essential difference is that A-morpholinc-A -oxide is typically used in a homogeneous acetone-water solvent mixture while a biphasic reaction mixture is usually observed with aqueous K3[Fe(CN)6 /K.2C03 as oxidant. In the latter case the olefin is oxidised... 

Torrent, M., Deng, L., Duran, M., Sola, M., Ziegler, T. Density Functional Study of the [2+2]- and [2+3]-Cycloaddition Mechanisms forthe Osmium-Catalyzed Dihydroxylation of Olefins. Organometallics 1997,16,13-19. 

The use of resin-supported sulfonic acid, an easily recyclable catalyst, makes it possible to conduct the dihydroxylation of cyclohexene with 30% HP without any solvent, at 70 °C, with 98% yield to trows-l,2-cyclohexandiol [36uj. The mechanism indudes the in situ generation of the resin-supported peroxysulfonic acid, which oxidizes the olefin to cyclohexene epoxide the latter is then quickly hydrolyzed by water to yield the final product. 

Two reaction mechanisms have been proposed for these dihydroxylations (pathway a or b, Figure 7.23), either a concerted [3+2] cycloaddition of the olefins on osmium-diamine complex 7.33 or a stepwise reversible [2+2] cycloaddition followed by a rearrangement [559,1350], An X-ray crystal structure of the resulting osmic ester 2.89A shows its symmetrical structure. Houk s calculations [1351] are in favor of a concerted reaction, and his transition state model is reactant-like, with steric interactions dictating the face selectivity of osmylation. 

Scheme 8.14. Simplified mechanism for the dihydroxylation of olefins. A more complete description is available [63].

Abstract The asymmetric dihydroxylation of olefins by osmium tetroxide is one of the most useful reactions in organic synthesis. Apart from the enormous experimental work, an extensive theoretical effort has been applied to study this reaction. A vast number of computational methods like QM, MM, Q2MM, QM/MM, and those commonly applied to enzymatic studies like docking. Molecular Dynamics (MD) and Genetic Algorithms (GA) have been employed. The computational studies performed to date in order to understand the mechanism of this reaction are reviewed here, with special focus on those directed to study the origin of the high enantioselectivity. 

The asymmetric dihydroxylation of olefins by 0s04 NR3 catalysts is an extremely useful reaction in organic synthesis. It is able to introduce two vicinal functional groups simultaneously on olefins not functionahsed. The apphca-tion of theoretical methods to study this reaction has proven to be critical in order to determine the reaction mechanism, and to identify the origin of the enantioselectivity. 

The discussion about the possible formation of metalla-2-oxetanes in transition metal-mediated oxidation reactions began with the ground breaking work of Sharpless in the field of enantioselective dihydroxylation of olefins with osmium tetraoxide using cinchona alkaloids as ligands [6]. The transfer of the stereochemical information of the chiral ligand to the substrate was explained by Sharpless with a two-step mechanism for the addition reaction, which should occur rather than a concerted [3+2] addition as originally proposed [110] (Fig. 15). 

A similar scenario as for the dihydroxylation has recently been opened for the diamination Muniz et al. reported experimental studies on the asymmetric dia-mination of olefins with osmium(VIII) imido complexes using (-)-8-phenyl-menthyl esters as chiral auxiliaries [114-116]. The reaction of (Nt-BuljOsO with enantiopure acrylates and methacrylates yielded only two out of four possible stereoisomers, whose absolute configurations are inconsistent with a concerted [3-1-2] mechanism of olefin face differentiation. The authors considered a stepwise mechanism via an osma-2-azetidine intermediate as reasonable pathway (Fig. 16). 

The first quantum chemical calculations which provided quantitative data about the mechanism of the ds-dihydroxylation of olefins with OSO4 were reported in 1994 by Sharpless etal. [122] andbyVeldkampandFrenking [123]. Previous work by Jorgensen and Hoffmann [ 124] focused on a quahtative orbital analysis using EHT calculations. 

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