Experimental and Theoretical Kinetic Isotope Effects for Asymmetric Dihydroxylation. Evidence Supporting a Rate-Limiting “(3 + 2)” Cycloaddition

DelMonte, Albert J.; Haller, Jan; Houk, K. N.; Sharpless, K. Barry; Singleton, Daniel A.; Straßner, Thomas; Thomas, Allen A.

doi:10.1021/ja971650e

Cited by 241 publications

(164 citation statements)

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“…Electrophilic multiply-bonded ligands are less common but such ligands are important in various processes, including atom and group transfers, and dihydroxylation and aminohydroxylation of olefins. 4 Complex 1 is unreactive with protic acids, methyl triflate (MeOTf), BF 3 ·Et 2 O, and [Ph 3 C][BF 4 ], but it reacts with PPh 3 and carbanions at nitrogen. 3 Despite its lack of reaction with simple electrophiles, we report here that 1 is a good ligand for Co(I) and Pt(II) centers.…”

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confidence: 99%

“…Indeed, addition of 1 to a benzene solution of CpCo(h 4 (Fig. 1) shows a two-legged piano stool geometry about the cobalt atom with the two legs being octahedral osmium centers, connected to the cobalt by m-nitrido ligands.…”

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confidence: 99%

“…The bonding in 2 and 3 could alternatively be described as a resonance hybrid of A or B with a multiple bond form OsNN=MA or OsNNNMA, the latter well known for homonuclear m-nitrido complexes. 1a The ability of 1 to act as a strong ligand to late transition metals is perhaps surprising in light of its lack of reaction with main group Lewis acids such as BF 3 ·Et 2 O and [Ph 3 C][BF 4 ]. Complex 1 should be contrasted with the more nucleophilic hydrocarbyl derivatives TpOs(N)Ph 2 3a,12 and CpOs(N)(CH 2 SiMe 3 ) 2 .…”

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An osmium nitrido complex as a π-acid ligand for late transition metals

Crevier¹,

Lovell²,

Mayer³

1998

Chem. Commun.

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An osmium nitrido complex as a π-acid ligand for late transition metals

Crevier¹,

Lovell²,

Mayer³

1998

Chem. Commun.

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“…Moreover, NMR experiments in [D 8 ]toluene confirmed that the isolated products did not epimerize between À80 to 90 8C. These results prove the configurational stability of the chiral-at-metal osmaimidazolidines reported.There has been a pronounced interest in the mechanism of the Sharpless AD reaction over the past years [8,16,17] which was Table 1: Diastereomeric ratios for syntheses of chiral-at-metal-osmaimidazolidines. …”

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Stereochemically Defined Osmium Centers from Asymmetric Diamination of Olefins: Mechanistic Implications for Osmium‐Mediated Acrylate Functionalization

2003

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Investigations on transition-metal complexes with stereogenic metal centers were pioneered by Brunner in 1969, who achieved the resolution of a complex with a pseudo-tetrahedrally coordinated chiral manganese atom.[1] Such complexes, which are also referred to as "chiral-at-metal complexes", [2] have gained important application in organic synthesis, [3] chiral recognition, [4] material sciences, [5] and fundamental basic research.[6] For example, in the area of enantioselective catalysis seminal work by Noyori on transfer hydrogenation of ketones and imines relies on a catalyst with a continuously regenerated stereogenic ruthenium center.[7] All these compounds usually originate either from stereoselective modification of racemic chiral-at-metal complexes with enantiomerically pure auxiliaries or from complexation of prochiral metal centers to chiral, non-racemic ligands.Herein we report on a conceptually novel approach for the synthesis of stereogenic, tetrahedrally coordinated osmium atoms in an asymmetric diamination. Osmium(viii) complexes have been most successful in oxidative C-C double-bond transformations. Among these reactions, Sharpless dihydroxylation and aminohydroxylation have been highly acclaimed, [8] whereas related diamination reactions have received only scant attention. [9][10][11] We have recently reported on the stereoselective diamination of olefins by reaction of bis(N-tert-butylimido)dioxoosmium(viii) with fumaric and acrylic ester derivatives [10] to give stable monomeric osmaimidazolidines.[11] When the corresponding trisimidoosmium complex 1 is allowed to react with unsymmetrical esters such as methyl cinnamate 2 a, clean formation of two new compounds is observed (Scheme 1). These were identified as a pair of diastereomeric chiral-at-metal osmaimidazolidines.[12] The two diastereomers, which, according to NMR spectroscopy, were formed in the ratio of 62:38, could be separated by conventional column chromatography. The (2R,4R,5S)/(2S,4S,5R) configuration for the major diastereomer 3 a was determined by X-ray structure analysis. [13] Other cinnamates and crotonates gave comparable diastereomeric ratios. In contrast, diastereomeric ratios for osmaimidazolidines obtained from the methyl and tert-butyl esters of acrylic and methacrylic acid were significantly higher (Table 1). Apparently, steric bulk in the 3-position has a deteriorating effect on the diastereomeric ratio, whereas it is advantageous when placed in the ester group. A Hammett correlation revealed a 1 value of + 0.28 for diamination of various 4-aryl substituted ethyl cinnamates. [14] This points to an increase in the rate of the reaction with increasing electrophilicity of the olefin in the presence of the imidoosmium reagent 1 with strongly nucleophilic character.[15] For all reactions of cinnamic esters, diastereomeric ratios were unchanged between 10 and 95 % conversion within NMR accuracy and did not show temperature dependence between À5 and 30 8C. Moreover, NMR experiments in [D 8 ]toluene confirmed that the isol...

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“…[3] The oxidation of alkenes by osmium tetroxide has been well known for a long time and osmium is one of the most efficient transition metals for catalytic dihydroxylations. It is not just the possibility of an asymmetric version of this reaction that has attracted synthetic [4] and theoretical [5] chemists. Several cooxidants have been investigated in osmium-catalyzed dihydroxylations to replace the ferricyanide in the reoxidation of Os VI to Os VIII .…”

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confidence: 99%