The preparation and racemization of an optically active sulfonium ylide. (-)-Ethylmethylsulfonium phenacylide

Darwish, David; Tomilson, Ralph L.

doi:10.1021/ja01023a072

Cited by 49 publications

(13 citation statements)

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“…Prior studies have established a pyramidal structure for the sulfur center in acyclic sulfonium ylides, 19,20 and the pyramidal inversion barrier for ethylmethylsulfonium phenacylide(5)has beendetermined to be23.3 kcal/ mo1.20 Our estimate of a lower limit for pyramidal inversion in 2 of 16.8 kcal/mol and in 3 of 22.3 kcal/mol is therefore consistent with the view that thiabenzenes are ylide-like in character. 9, lo…”

Section: Department Of Chemistry Princeton Uizioersity Princeton Nesupporting

confidence: 83%

Thiabenzenes. III. Pyramidal stability at sulfur

Maryanoff¹,

Senkler²,

Stackhouse³

et al. 1974

J. Am. Chem. Soc.

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Section: Department Of Chemistry Princeton Uizioersity Princeton Nesupporting

confidence: 83%

Thiabenzenes. III. Pyramidal stability at sulfur

Maryanoff¹,

Senkler²,

Stackhouse³

et al. 1974

J. Am. Chem. Soc.

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“…Calcd for C I~H I~S (mol wt 274): C, 83.17; H , 5.14; S, 11.69. Found [mol wt 831 (osmometry in b e n~e n e ) ] :~'~ C, 83.07; H, 5.08; S, 11.33.…”

Section: 6-diphenyl-4-o-tolylthiopyryliummentioning

confidence: 99%

Thiabenzenes. Reassessment of their chemical and physical properties

Maryanoff¹,

Stackhouse²,

Senkler³

et al. 1975

J. Am. Chem. Soc.

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The syntheses of divers thiabenzene systems are described, and several chemical and physical properties of this class of compounds are evaluated. Observations are recorded which point to the similarity between thiabenzenes and acyclic sulfonium ylides: both classes of compounds are stabilized toward thermal decomposition by substituents which delocalize or stabilize electronic charge, both classes undergo thermal Stevens rearrangements, and both classes are stably pyramidal at sulfur with a barrier to pyramidal inversion >20 kcal/mol. These observations, as well as the marked upfield chemical shifts noted for protons on carbons CY to sulfur in thiabenzene systems, rule out an aromatic bonding description for thiabenzenes and can be reconciled only with the view of thiabenzenes as cyclic sulfonium ylides.In the course of our studies on pyramidal i n~e r s i o n ,~ we had occasion to develop a specially parametrized C N D 0 / 2 scheme4 which proved useful in providing reliable quantitative estimates for barriers to pyramidal inversion in systems containing elements from the first and second row of the periodic table. These calculations also offered many predictions which, it was hoped, would stimulate and direct further experimentation. c)One such prediction was that 1 -methylthiabenzene (1) should have a relatively high barrier to pyramidal inversion at sulfur (ca. 43 kcal/m01).~ As was subsequently pointed this high barrier contrasts sharply with a previously expressed opinion that 1-phenylthiabenzene (2), and other similar unhindered thiabenzenes, should be planar or have a very low bending barrier for the sulfur-phenyl b~n d .~-~ To resolve these conflicting views, we decided to resort to an experimental test. Nuclear magnetic resonance spectroscopy is a convenient technique for gaining insight into the question of pyramidality. By incorporation of groups containing enantiotopic nuclei, which become diastereotopic in the chiral molecular environment that would be associated with appropriately substituted pyramidal sulfur, one could probe the conformation of thiabenzenesg If thiabenzenes proved to be nonplanar, on the time scale of the N M R observations, then dynamic nuclear magnetic resonance (DNMR) would provide a means of determining the inversion barrier so long as the activation energies were in the range of ca. 5-25 kcal/mol. Hence, the synthesis of an appropriately substituted thiabenzene was undertaken.Since 2 was reported5 to be quite stable, and since Sarylthiabenzenes might be expected'O to be more stable than S-alkylthiabenzenes (and thus 2 more stable than l), we felt that a derivative of 2 would be ideally suited for a preliminary study along the lines described above. Accordingly, we attempted to synthesize 2-isopropyl-1 -phenylthiabenzene (3b), in which the sulfur atom serves as a potential center of chirality, by the route that was employed for the preparation of 2 (eq The reaction of 3a" with phenyllithium was examined by NMR spectroscopy. In an N M R tube under a dry nitrogen atmosphere, 3a (...

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“…Such a supposition, if it were true, would seem contradictory to the expectation that chiral sulfur ylides are conformationally stable. An early measurement of the rate constant for stereoinversion of stabilized sulfure ylides found that the rate constant for the stereoinversion of 6 is 3.52 × 10 -5 s -1 at 25 °C in dichloromethane 31. Temperature dependence of the rate constant for inversion yielded an enthalpy of activation of 23.3 kcal/mol.…”

Section: Resultsmentioning

confidence: 99%

Mechanism of the Swern Oxidation: Significant Deviations from Transition State Theory

Giagou

Meyer

2010

J. Org. Chem.

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Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-β-elimination of the ylide. In the present study, intramolecular 2 H kinetic isotope effects (KIEs) are used to gain insight into this syn-β-elimination step. As a result of the stereogenic sulfur center in the ylide intermediate, two diastereomeric transition states (endo-TS1 and exo-TS1) must be assumed to contribute to the intramolecular KIE. The intramolecular 2 H KIE determined at -78 °C is 2.82 ± 0.06. Attempts to reproduce this measurement computationally using transition state theory and a Bell tunneling correction yielded a value (1.58) far below that determined experimentally. Computational analysis is complicated by the existence of two distinct transition structures owing to the stereogenic center. Two extremes of Curtin-Hammett kinetics are explored using energies, vibrational frequencies, and moments of inertia from computed transition structures. Neither Curtin-Hammett scenario can reproduce the observed KIE to any acceptable degree of fidelity. Evidence based upon previous kinetics measurements and calculations upon a model system suggest that the stereogenic sulfur center is not likely to undergo inversion to a significant degree at the temperatures at which the Swern oxidation is performed here. Proceeding under the assumption of no stereoinversion at the sulfur center, calculations predict a nearly linear Arrhenius plot for the KIE -even with the inclusion of a 1-dimensional tunneling correction. By contrast, the experimentally-determined temperature dependence shows a significant concave upward curvature indicative of the influence of tunneling. Notably, KIEs measured in CCl 4 , CHCl 3 , CH 2 Cl 2 , dichloroethane, and chlorobenzene at -23 °C showed little Correspondence to: Matthew P. Meyer, mmeyer@ucmerced.edu. Supporting Information Available: Details of the experimental and computational methods, derivations of eqs. 1-6, and Cartesian coordinates for all computed structures. This material is available free of charge via the Internet at http://pubs.acs.org. variance. This finding discounted the possible influence from dynamical effects due to incomplete vibrational relaxation. An ad hoc amplification of the imaginary frequencies corresponding to the first order saddle points corresponding to endo-TS1 and exo-TS1 allowed us to reproduce the experimental temperature dependence of the KIE using only two adjustable parameters applied to a kinetic scenario that involves four isotopomeric transition states. The cumulative data and computational modeling strongly suggest that, even though the intramolecular 2 H KIE observed in these experiments is small, this reaction requires a multi-dimensional description of the tunneling phenomenon to accurately reproduce experimental trends. NIH Public AccessAuthor Manuscript J Org Chem. Author manuscript; available in PMC 2011 December 3.

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The preparation and racemization of an optically active sulfonium ylide. (-)-Ethylmethylsulfonium phenacylide

Cited by 49 publications

References 1 publication

Thiabenzenes. III. Pyramidal stability at sulfur

Thiabenzenes. III. Pyramidal stability at sulfur

Thiabenzenes. Reassessment of their chemical and physical properties

Mechanism of the Swern Oxidation: Significant Deviations from Transition State Theory

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