Density Functional Study of the [2+2]- and [2+3]-Cycloaddition Mechanisms for the Osmium-Catalyzed Dihydroxylation of Olefins

Abstract

The postulated intermediates in the base-free and base-assisted addition of OsO4 to olefins have been optimized using density functional theory (DFT). Ammonia was chosen as the base and ethylene as the olefin. The corresponding transition states have been characterized fully. Further, the activation barriers have been computed at the nonlocal level, and special attention has been given to the two different mechanistic hypothesis proposed for this reaction. In particular, the hypothesis by Sharpless of a [2+2]-cycloaddition pathway involving the formation of a four-member ring as an intermediate has been ruled out since the corresponding activation barrier was calculated to be as high as 39 kcal mol-1. The addition of a NH3 ligand to the osmium catalyst does not reduce significantly the [2+2] energy barrier. By contrast, it seems perfectly feasible that the dihydroxylation reaction proceeds through a [2+3] mechanism leading to the formation of a five-member ring intermediate as claimed by Corey. Such a process is found to be clearly exothermic and to involve a very small activation barrier of less than 2 kcal mol-1. A detailed analysis of the sequence describing exactly how the cycloaddition proceeds along the reaction path has also been performed by means of intrinsic reaction coordinate (IRC) calculations for the two studied mechanisms.

Facile Oxy‐Functionalization of a Nucleophilic Metal Alkyl with a cis‐Dioxo Metal Species via a (2+3) Transition State

“…One intriguing possibility is the use of cis-metal dioxo compounds that could react with MÀR s bonds in a potentially facile addition reaction via a (3+2)-type transition state (TS, see Figure 2). Related mechanisms are well known for the cis-dihydroxylation of alkenes by OsO 4 [5] and the oxidation of the s bonds of hydrogen, [6] silanes, [7] and alkanes. [8] Herein we report the quantitative and instantaneous oxyfunctionalization of MTO to methanol by reaction with OsO 4 in aqueous basic media at room temperature [Eq.…”

Cinchona Alkaloids as Chiral Ligands in Asymmetric Oxidations

“…The initial model (Figure 3.2a) for the transition state was based on a [2 þ 2] cycloaddition reaction model and, although NMR studies support this, there is now evidence and consensus that a [3 þ 2] model is followed [123][124][125][126][127][128][129][130]. Computational studies suggest that solvent can play a key role in account for the apparent reversal of reaction stereochemical outcome compared to that predicted by Sharplesss original mnemonic (Figure 3.2a).…”

Quantum chemical studies of asymmetric reactions: Historical aspects and recent examples

“…The calculations indicated that formation of a metallaoxetane is possible and that its geometry fit with empirical (mnemonic) models used to explain the stereoselectivity in these reactions . However, DFT calculations performed a few years later by different groups on osmium models concluded that the [3+2] pathway is strongly favored …”