Investigation of the mechanism of 1,2-migrations in methoxysiloxycarbene with the electron localization function

Loncke, Paul G.; Gadosy, Timothy A.; Peslherbe, Gilles H.

doi:10.3998/ark.5550190.0002.c17

Cited by 3 publications

(7 citation statements)

References 22 publications

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“…Scheme ). As mentioned earlier, we have investigated the mechanism for 1,2-silyl migration in methoxy(siloxy)carbene. , However, we will briefly review some of the findings for completeness and then focus on decarbonylation via the Brook rearrangement.…”

Section: Resultsmentioning

confidence: 99%

“…In terms of frontier molecular orbital (FMO) theory, 1,2-silyl migration is expected to occur via front-side nucleophilic attack by the carbene lone pair at silicon rather than an anion-like shift to the “vacant” carbene p orbital (cf. Scheme ). , This is because the HOMO has carbene lone-pair n C6 orbital character while the LUMO is essentially the σ* O7 - Si orbital (cf. Figure ), and the geometries of conformers 1 and 3 are ideal for HOMO/LUMO interaction.…”

Section: Resultsmentioning

confidence: 99%

“…Figure 1), the ∠C6O7Si angles shrink considerably, while there is modest O7-Si bond lengthening and C6-O7 bond shortening in TS 14 and TS 35 . These geometric changes are accompanied by the appearance of a new C6-Si bond with low F b (r) and positive ∇ 2 F b (r) in TS 14 and TS 35 (cf. Table 1), in concert with modest decreases and increases in F b (r) for the O7-Si and C6-O7 bonds, respectively.…”

Section: Ground-state Methoxy(siloxy)carbene Conformersmentioning

confidence: 99%

“…However, there are questions regarding the mechanisms of these intramolecular carbene rearrangements. We recently carried out thorough theoretical investigations of the mechanisms for 1,2-silyl migration and 1,2-methyl migration in methoxy(siloxy)carbene (CH 3 OC̈OSiH 3 ) as a model system for carbene II . , Our results suggest that 1,2-silyl migration occurs via nucleophilic attack by the carbene lone pair at silicon, whereas 1,2-methyl migration appears to involve an anion-like shift of the methyl group from oxygen to the “vacant” carbene p orbital. We also found that 1,2-silyl migration is significantly more favorable than 1,2-methyl migration, in accord with experimental findings.…”

Section: Introductionmentioning

confidence: 96%

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A Theoretical Investigation of the Decarbonylation of Methoxy(siloxy)carbene

Loncke¹,

Peslherbe²

2004

J. Phys. Chem. A

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The decarbonylation of methoxy(siloxy)carbene (CH3OC̈OSiH3) has been investigated by means of ab initio molecular orbital theory and hybrid density functional theory calculations, in conjunction with the quantum theory of atoms in molecules. Pathways involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon and by the siloxy oxygen at the methyl carbon have been examined. The pathway involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon has been found to be concerted (one step, no intermediates) and synchronous (bond formation and dissociation occur at the same rate). In contrast, although the pathway involving intramolecular front-side nucleophilic attack by the siloxy oxygen at the methyl carbon appears to be concerted, it is asynchronous, with transition-state features that resemble a tight ion-pair intermediate comprising a methyl cation and a siloxycarbonyl anion. Pathways for decarbonylation involving the intermediacy of methyl silyl formate, as well as radicals, have also been investigated. The most energetically viable pathway for decarbonylation appears to be that involving intramolecular front-side nucleophilic attack by the methoxy oxygen at silicon.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Ground-state Methoxy(siloxy)carbene Conformersmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

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A Theoretical Investigation of the Decarbonylation of Methoxy(siloxy)carbene

Loncke¹,

Peslherbe²

2004

J. Phys. Chem. A

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“…This expression can be re-written in terms of Gibbs free energies as eqn. (2), where DG TS and DG H TS are the Gibbs free-energy barriers for the substituted and parent compounds.…”

Section: Hammett Free-energy Relationshipsmentioning

confidence: 99%

Substituent effects and the role of negative hyperconjugation in siloxycarbene rearrangements

Loncke

Peslherbe

2005

Org. Biomol. Chem.

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Substituent effects and the role of negative hyperconjugation in 1,2-silyl migration and decarbonylation of methoxy(substituted-siloxy)carbenes have been investigated using quantum chemical calculations and natural bond orbital analysis. It has been found that sigma-electron-withdrawing substituents generally lower the barriers for 1,2-silyl migration and decarbonylation, consistent with symmetry-forbidden concerted rearrangements involving intramolecular front-side nucleophilic attack by the carbene lone pair at silicon and by the methoxy oxygen at silicon, respectively. However, while good linear Hammett correlations are obtained for 1,2-silyl migration, those obtained for decarbonylation are poor. In addition, there appears to be a relationship between the extent of pertinent hyperconjugative interactions in the siloxycarbene conformers and the ease of intramolecular reactivity. As a matter of fact, the finding that 1,2-silyl migration is more favorable than decarbonylation seems to be primarily related to stronger negative hyperconjugation between the carbene lone pair and the O-Si antibonding orbital, compared to that between the methoxy oxygen n(sigma) lone pair and the O-Si antibonding orbital. Moreover, the activation enthalpies for 1,2-silyl migration decrease linearly with stronger negative hyperconjugation, although no such correlation could be established for decarbonylation.

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Mechanisms of Intramolecular Rearrangements of Cyclic Siloxycarbenes

Loncke

Peslherbe

2004

J. Phys. Chem. A

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Intramolecular rearrangements of 3-sila-2-oxacyclohexylidene have been investigated using hybrid density functional theory calculations, the quantum theory of atoms in molecules, and the electron localization function. Mechanisms for 1,2-H migration, ring contraction, and decarbonylation have been examined. The mechanism for 1,2-H migration was shown to involve a typical hydride-like shift from the migration origin to the “vacant” carbene p orbital, while ring contraction was found to occur via a concerted pathway involving a front-side nucleophilic attack by the carbene lone pair at silicon as opposed to a stepwise pathway involving an acyl−silyl biradical intermediate. Decarbonylation, on the other hand, was shown to be a stepwise reaction that preferentially occurs via the intermediacy of silacyclopentanone rather than acyl−silyl and alkyl−silyl biradicals. Computed energetics and thermodynamics indicate that 1,2-H migration and ring contraction are considerably more favorable than decarbonylation. Finally, AIM analysis reveals that the changes in molecular structure associated with 1,2-H migration involving a hydride-like shift to the “vacant” carbene p orbital occur via a conflict mechanism, whereas those associated with ring contraction (or 1,2-silyl migration) involving front-side nucleophilic attack by the carbene lone pair at silicon occur via a bifurcation mechanism. The latter findings further suggest that AIM analysis may be a viable approach to unambiguously distinguish between reaction mechanisms.

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Investigation of the mechanism of 1,2-migrations in methoxysiloxycarbene with the electron localization function

Cited by 3 publications

References 22 publications

A Theoretical Investigation of the Decarbonylation of Methoxy(siloxy)carbene

A Theoretical Investigation of the Decarbonylation of Methoxy(siloxy)carbene

Substituent effects and the role of negative hyperconjugation in siloxycarbene rearrangements

Mechanisms of Intramolecular Rearrangements of Cyclic Siloxycarbenes

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