Mechanism of the Anionic Cyclopolymerization of Bis(dimethylvinylsilyl)methane

Suga, Yasuhiro; Oku, Jun-ichi; Masuda, Hideki; Takaki, Mikio; Mukai, Masanori; Kitagawa, Teizo

doi:10.1021/ma9801016

Cited by 5 publications

(6 citation statements)

References 15 publications

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“…The IR spectrum of 2 features a CC stretch at 1628 cm –1 that is 13 cm –1 lower than the 1641 cm –1 frequency seen for its conjugate acid 4,4-dimethyl-1-pentene . This observation is consistent with previous infrared and resonance Raman studies of lithium–olefin complexes. ,, However, the lowered CC stretching frequency is not definitive evidence of the presence of a Li–olefin interaction because this mode can be affected by other factors such as coupling to other vibrations, strain, and inductive effects …”

Section: Resultssupporting

confidence: 91%

“…We briefly consider here the question of the rate-determining step in carbolithiation reactions. ,− Carbolithiation reactions are thought to proceed through a monomeric (i.e., Lewis-base-coordinated) lithium species in which the anionic carbon and the olefin are simultaneously bound to lithium. , Although ab initio studies of gas phase carbolithiation reactions have suggested that coordination of the olefin to lithium is downhill thermodynamically by 12 kcal/mol, , these calculations involved unsolvated organolithium monomers, which are quite different from the species actually present in solution. Our studies suggest that, in THF, which is a common solvent for carbolithiation reactions, the dominant species in solution before carbolithiation occurs are ones in which no Li–olefin interactions are present.…”

Section: Resultsmentioning

confidence: 99%

“…36 This observation is consistent with previous infrared and resonance Raman studies of lithium−olefin complexes. 14,25,26 However, the lowered C C stretching frequency is not definitive evidence of the presence of a Li−olefin interaction because this mode can be affected by other factors such as coupling to other vibrations, strain, and inductive effects. 37 The 7 Li NMR spectrum of 2 in cyclohexane-d 12 consists of a singlet at δ 2.01.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…1−5 In such carbolithiation reactions, the addition step is immediately preceded by formation of an intermediate in which there is a direct lithium−olefin interaction; 4,6−17 the structure of this intermediate is thought to be principally responsible for the chemo-, regio-, and stereoselectivity of the addition step. 8,10,[13][14][15]17,18 Computational studies support this view: the intermolecular reaction between methyllithium and ethylene occurs through initial formation of a Li−ethylene π-complex, followed by synaddition of methyllithium across the carbon−carbon double bond. 19 Theoretical studies have shown that intermediates with similar lithium−olefin interactions are also crucial for the steric control of intramolecular carbolithiation reactions.…”

Section: ■ Introductionmentioning

confidence: 94%

“…15,20 The experimental study of lithium−olefin interactions involving alkyllithium reagents is challenging: 1,13 in ether solvents, simple olefins such as ethylene insert rapidly into the Li−C bond of alkyllithium reagents even at −50 °C. 21 One approach is to study such interactions at low temperatures by techniques such as 1 H− 6 Li HOSEY 22 and resonance Raman 14 spectroscopy. Another approach is to disfavor insertion of the olefin into the Li−C bond, for example, by employing ωalkenyl lithium species with five or fewer carbon atoms in the chain (for which the open-ring form is thermodynamically more stable owing to the ring strain of the cyclized reaction product), 23 or by carrying out the studies in hydrocarbon rather than ether solvents.…”

Section: ■ Introductionmentioning

confidence: 99%

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Lithium–Olefin π-Complexes and the Mechanism of Carbolithiation: Synthesis, Solution Behavior, and Crystal Structure of (2,2-Dimethylpent-4-en-1-yl)lithium

et al. 2019

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We describe the first crystallographically characterized example of a nonconjugated olefin bound in a simple dihapto fashion to a lithium center, as part of a study of two alkyllithium compounds that contain CC double bonds at the alkyl chain terminus: (2,2-dimethylbut-3-en-1-yl)lithium (1) and the related pentenyl compound (2,2-dimethylpent-4-en-1-yl)lithium (2). The Li–olefin interactions in the crystal structure of 2 serve as a model for those proposed to be present in the [RLi···olefin] intermediate in olefin carbolithiation reactions. As seen in other systems, the Li–olefin interaction is correlated with deshielding of the 1H NMR resonances of the olefinic hydrogen atoms. DOSY and NOE measurements show that 1 and 2 remain tetrameric in cyclohexane and that the lithium–olefin interactions persist in solution. Addition of a Lewis base such as THF to these ω-alkenyllithium species has two effects: the THF displaces the lithium–olefin interactions while accelerating the rate of carbolithiation. A deuteration experiment shows that compound 2 undergoes reversible carbolithiation to the corresponding cyclobutylmethyllithium species in the presence of Lewis bases, but this transformation is thermodynamically uphill owing to ring strain. In comparison, the longer chain hexenyl species (2,2-dimethylhex-5-en-1-yl)lithium is thermodynamically unstable with respect to the intramolecular carbolithiation product [(3,3-dimethylcyclopentyl)methyl]lithium (3). We suggest that rate-determining step in carbolithiation reactions may not always be formation of the C–C bond, as is often assumed, but in some cases may be formation of the lithium–olefin complex; the coordination of the olefin to lithium may occur in a concerted fashion with disaggregation of lithium clusters. Finally, we point out that activation enthalpies can be obtained solely from NMR line shapes above the coalescence point.

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