Rotation about the CSe Bond as the Rate‐Determining Step in the Racemization of α‐Selenium‐Substituted Alkyllithium Compounds

Ruhland, Thomas; Dress, Ruprecht; Hoffmann, Reinhard W.

doi:10.1002/anie.199314671

Cited by 49 publications

(6 citation statements)

References 29 publications

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“…The groups of Reich and Hoffmann have also investigated the dynamics of carbanionic inversion in a variety of α-hetero and α,α 1 -dihetero organolithium compounds, where hetero = Te, Si, S, Se, see for example, 18 to 22a−c , both internally and externally solvated. In addition, by use of HMPT as external ligand(s), organolithium species RLi(HMPT) n ( n = 0−4) with different degrees of C−Li bond ionic character were generated and studied.…”

Section: Resultsmentioning

confidence: 99%

Partially Delocalized Allylic Lithium Compounds: Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Fraenkel

Qiu

1997

J. Am. Chem. Soc.

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While exo-exo-[1,3-bis(trimethylsilyl)allyl]lithium (15) and [1-(trimethylsilyl)allyl]lithium (16) were previously shown to be contact ion pairs containing delocalized carbanions, the corresponding species with a pendant ligand at the 2-position, [2-[[bis(2-methoxyethyl)amino]methyl]-1,3-bis(trimethylsilyl)allyl]lithium (14) and [2-[[bis(2-methoxyethyl)amino]methyl]-1-(trimethylsilyl)allyl]lithium (12), respectively, appear from their 13C NMR shifts and the first observation of 13C lithium spin coupling in an allylic lithium to be partially delocalized with detectable C−Li covalence. In proposed structures 12 and 14, lithium is tridentately complexed. N and Li lie within the allyl carbon plane with the two oxygens normal to it on opposite sides. NMR line shape analysis and 13C1 of signal averaging of the 13C−6Li coupling of 12 provides dynamics of intermolecular C−Li bond exchange with ΔH e ⧧ and ΔS ⧧ of 11.6 kcal/mol and −11.5 eu, respectively. Inversion at the lithium-bound carbon of 12 averages nonequivalent ligand shifts. Line shape analysis gives ΔH i ⧧ and ΔS i ⧧ of 8 kcal/mol and −10 eu, respectively. Line shape changes observed for the methylsilyl (13C and 1H) resonances as well as of the terminal 13C's of 14 due to a 1,3 Li sigmatropic shift yield activation parameters ΔH s ⧧ and ΔS s ⧧ of 18 kcal/mol and +15 eu. These results show that electronic structure of nominally conjugated organolithium compounds can be significantly altered by changing the stereochemistry of solvation, by use of pendant ligands, producing structures previously described in other systems as transition states for allylic rotation.

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Section: Resultsmentioning

confidence: 99%

Partially Delocalized Allylic Lithium Compounds: Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Fraenkel

Qiu

1997

J. Am. Chem. Soc.

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“…A very different mechanism for the inversion of α ‐heteroatom‐substituted organolithium reagents was reported independently by Hoffmann and co‐workers and by Reich et al in 1993 15, 16. The addition of hexamethyl phosphoramide (HMPA) to organolithium compounds 10 and 11 generates spectroscopically characterizable contact ion pairs (CIPs).…”

Section: Spectroscopic and Chemical Methods For Determining Configmentioning

confidence: 98%

Configurational Stability and Transfer of Stereochemical Information in the Reactions of Enantioenriched Organolithium Reagents

Basu

Thayumanavan

2002

Angew. Chem. Int. Ed.

233

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The conversion of a prochiral methylene group into a stereogenic center by means of a lithiation/substitution sequence has emerged as a powerful synthetic transformation over the past 15 years. This reaction proceeds via a chiral organolithium intermediate, and the stereochemical fidelity of the overall reaction sequence is intimately dependent on the stereochemical behavior of the chiral organolithium as well as on the rate and stereochemical sense of the electrophilic substitution step. Chiral organolithium reagents were first reported by Letsinger, Curtin, and Applequist half a century ago. The lithiated intermediates in these early studies were not highly configurationally stable, and applications in stereoselective synthesis were not immediately forthcoming. The two decades that followed the 1980 report by Still and Sreekumar of a configurationally stable α‐oxyorganolithium were marked by an increased interest in these reagents. As the synthetic applications of chiral organolithium reagents have grown, so have accompanying mechanistic studies of these intermediates which serve as the basis for this review.

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“…Studies by both Hoffmann and co-workers [59–60] and by Reich and co-workers [61] showed that the rate determining step for racemisation of α-thio, α-seleno and α-telluroorganolithiums is rotation about the C–S, Se or Te bond [62]. Simple inversion in α-thio-substituted carbanions has a barrier as low as 0.5 kcal·mol −1 [63] making inversion itself unlikely to comprise the rate determining step of the racemisation.…”

Section: Reviewmentioning

confidence: 99%

Asymmetric synthesis of tertiary thiols and thioethers

Clayden¹,

MacLellan²

2011

Beilstein J. Org. Chem.

186

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Rotation about the CSe Bond as the Rate‐Determining Step in the Racemization of α‐Selenium‐Substituted Alkyllithium Compounds

Cited by 49 publications

References 29 publications

Partially Delocalized Allylic Lithium Compounds: Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Partially Delocalized Allylic Lithium Compounds: Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Configurational Stability and Transfer of Stereochemical Information in the Reactions of Enantioenriched Organolithium Reagents

Asymmetric synthesis of tertiary thiols and thioethers

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