Influence of substituents on NMR and barriers to rotation in tert-benzyllithium compounds

Fraenkel, Gideon; Geckle, J. M.

doi:10.1021/ja00529a001

Cited by 35 publications

(12 citation statements)

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“…16 These results are also consistent with Boche's X-ray crystallographic structure. 3e The 13 C NMR shifts for the R-secondary benzylic lithium compounds reported here and previously 12 the well-known linearity of 13 C shift with π charge, 14 the model system which exhibits shifts to be expected for a delocalized benzylic anion is R-methyl-R-neopentylbenzyllithium‚TMEDA. 13 These 13 C shifts are, in δ units; R, 71; ipso, 131; o, 105; m, 128; and p, 87.…”

Section: Resultssupporting

confidence: 56%

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Stereochemistry of Solvation of Benzylic Lithium Compounds: Structure and Dynamic Behavior

et al. 1999

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Several sec-benzylic lithium compounds, both externally coordinated, [α-(trimethylsilyl)benzyl]lithium·PMDTA (12) and p-tert-butyl-α-(dimethylethylsilyl)benzyllithium·TMEDA (13), and internally coordinated, [α-[[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsilyl]-p-tert-butylbenzyl]lithium (14) and [α-[[[(S)-2-(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsilyl]benzyl]lithium (15), have been prepared. Ring 13C NMR shifts indicate that 12−15 have partially delocalized structures. Externally solvated allylic lithium compounds are found to be delocalized, and only some internally coordinated species are partially delocalized. Compound 15 exists as >95% of one stereoisomer of the two invertomers at Cα. This is in accord with a published ee of >98% in products of the reactions of 15 with aldehydes. All four compounds show evidence of one-bond 13C−6Li spin coupling, ca. 3 Hz, which indicates a small detectable C−Li covalence. Averaging of the 13C−6Li coupling of 12 with increasing temperature provides the dynamics of intermolecular C−Li bond exchange, with Δ = 9 ± 0.5 kcal mol-1. Carbon-13 NMR line shape changes due to geminal methyls, and ligand carbons gave similar rates of inversion at Cα in 13 (externally solvated) and 14 (internally solvated), Δ ≈ 4.9 ± 0.5 kcal mol-1. By contrast, barriers to rotation around the ring-Cα bonds vary widely, depending on the mode of lithium coordination, Δ ≈ 8 ± 0.5 to 19 ± 1.0 kcal mol-1. Some mechanisms for these processes are proposed.

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Section: Resultssupporting

confidence: 56%

“…9 Effects similar to those described above have been observed for some benzylic lithium compounds. 12 However, as will be shown below, only the R-disubstituted benzylic lithium compounds exhibit properties expected of delocalized carbanions; 13,14 only the R-monosubstituted species appear to be partially delocalized. Their behavior will be described below.…”

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confidence: 96%

Stereochemistry of Solvation of Benzylic Lithium Compounds: Structure and Dynamic Behavior

et al. 1999

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“…Over the years, the rotational barriers and delocalization energies of allylic and benzylic systems have been studied both experimentally and theoretically . However, we note at the outset that these rotational barriers are predicted by Hückel MO (HMO) theory to be independent of charge.…”

Section: Introductionmentioning

confidence: 89%

Variations in Rotational Barriers of Allyl and Benzyl Cations, Anions, and Radicals

Bally

Houk

et al. 2016

J. Org. Chem.

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High accuracy quantum chemical calculations show that the barriers to rotation of a CH 2 group in the allyl cation, radical, and anion are 33, 14, and 21 kcal/mol, respectively. The benzyl cation, radical, and anion have barriers of 45, 11, and 24 kcal/mol, respectively. These barrier heights are related to the magnitude of the delocalization stabilization of each fully conjugated system. This paper addresses the question of why these rotational barriers, which at the Huckel level of theory are independent of the number of nonbonding electrons in allyl and benzyl, are in fact calculated to be factors that are of 2.4 and 4.1 higher in the cations and 1.5 and 1.9 higher in the anions than in the radicals. We also investigate why the barrier to rotation is higher for benzyl than for allyl in the cations and in the anions. Only in the radicals is the barrier for benzyl lower than that for allyl, as Huckel theory predicts should be the case. These fundamental questions in electronic structure theory, which have not been addressed previously, are related to differences in electron−electron repulsions in the conjugated and nonconjugated systems, which depend on the number of nonbonding electrons.

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“…However, the same difficulties with over-hydrolysis were encountered here as well. A successful synthesis of 2 was accomplished by employing a more traditional approach using 2-(4-bromophenyl)propene ( 5 ) as the primary starting material [ 11 , 12 ]. The Grignard reagent of 5 was reacted with TEOS to give 2-(4-(triethoxysilyl)phenyl)propene ( 6 ) in 81% yield.…”

Section: Resultsmentioning

confidence: 99%

The Synthesis of 1-(4-Triethoxysilyl)phenyl)-4,4,4-trifluoro-1,3-butanedione, a Novel Trialkoxysilane Monomer for the Preparation of Functionalized Sol-gel Matrix Materials

2008

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Influence of substituents on NMR and barriers to rotation in tert-benzyllithium compounds

Cited by 35 publications

References 12 publications

Stereochemistry of Solvation of Benzylic Lithium Compounds: Structure and Dynamic Behavior

Stereochemistry of Solvation of Benzylic Lithium Compounds: Structure and Dynamic Behavior

Variations in Rotational Barriers of Allyl and Benzyl Cations, Anions, and Radicals

The Synthesis of 1-(4-Triethoxysilyl)phenyl)-4,4,4-trifluoro-1,3-butanedione, a Novel Trialkoxysilane Monomer for the Preparation of Functionalized Sol-gel Matrix Materials

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