Distinction of Push,pull effect and steric hindrance in disubstituted alkynes

A new functionalised alkynylsilane, Cl-Si(CC-CMe3 )3 (3), was obtained by a facile multistep synthesis. Treatment of 3 with equimolar quantities of the hydrides H-M(CMe3 )2 (M=Al, Ga) gave the mixed alkenyl-di(alkynyl)silanes, in which the chlorine atom adopts a bridging position between the aluminium and silicon atoms. Dual hydrogallation of 3 resulted in the formation of a di(alkenyl)-alkynylsilane containing two gallium atoms, one of which is coordinated to the chlorine atom, and the second is bonded to the α-carbon atom of the remaining alkynyl group. A tert-butylsilane was unexpectedly formed by a unique 1,3-dyotropic chlorine-tert-butyl exchange for the corresponding dialuminium compound. One aluminium atom is bonded to a tert-butyl group, a terminal chlorine atom and the α-carbon atom of the ethynyl moiety; the second is coordinatively unsaturated, with two terminal tert-butyl substituents. High-level quantum-chemical calculations favour a stepwise dyotropic rearrangement with an intermediate cationic silicon species over a simultaneous tert-butyl-chlorine migration via a five-coordinate silicon atom in the transition state.

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“…and 3. [17] Absorptions at about 2200 and 2150 cm À1 in the IR spectra were characteristic of the CC triple bonds.…”

Section: Resultsmentioning

confidence: 98%

Hydroalumination of a Chlorotrialkynylsilane: Spontaneous Stepwise 1,3‐Dyotropic Rearrangement via an Intermediate Silyl Cation

Uhl

Bohnemann

Layh

et al. 2014

Chemistry A European J

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“…In terminal fragments, monomeric compounds (c. f. compound 4 and ref. [17 -19]) or terminal alkynes [24,25] the difference is usually much smaller. The characteristic shifts δ ( 1 H) of the bridging hydride anions in compounds 2 and 3 are found between δ = 3 and 4.…”

Section: Resultsmentioning

confidence: 99%

Hydroalumination versus Deprotonation of Alkynes with Sterically Demanding Substituents

Uhl

Layh

Rhotert

et al. 2013

Zeitschrift Für Naturforschung B

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Treatment of sterically highly shielded terminal alkynes, H-C≡C-aryl, with dialkylaluminium and dialkylgallium hydrides, R 2 E-H, afforded by hydrogen release dimeric dialkylelement alkynides with a four-membered E 2 C 2 heterocycle independent of the bulk of the aryl groups. A rare example of a monomeric alkynylaluminium compound was only obtained with very bulky CH(SiMe 3 ) 2 groups attached to the metal atoms and by salt elimination reaction. The steric shielding by the bulky aryl groups did not prevent condensation reactions. Hydroalumination of 1-(trimethylsilyl)-2-(2,6-dimethylphenyl)ethyne using Me 2 Al-H resulted in a divinyl compound by elimination of trimethylaluminium.

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“…The type of reaction pathways is determined by several factors that include, the electrophile–nucleophile pair, the nature of the nucleophile, the leaving group ability of the nucleofuge, the permanent group electrofugality, solvent effects and the electrophilicity of the C = X group . The latter is not yet completely understood and its effect on the reaction mechanism has been discussed on the basis of experimental evidence using a qualitative push–pull effect at the C = X group (see Scheme ) …”

Section: Introductionmentioning

confidence: 99%

Predicting the reaction mechanism of nucleophilic substitutions at carbonyl and thiocarbonyl centres of esters and thioesters

Ormazábal‐Toledo

Castro

Santos

et al. 2012

J of Physical Organic Chem

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In nucleophilic substitution reactions at carbonyl centres, there are two possible channels. The first one occurs when the attack of nucleophilic agents takes place simultaneously with the departure of the nucleofuge. This process is named as concerted. The second possibility is the formation of a reaction intermediate, typically a tetrahedral intermediate from which the nucleofuge departs after passing through a second transition state. This second mechanism is defined as stepwise. Whether a concerted or stepwise mechanism is to be expected for a given reaction depends on several factors. Among these determinants are the nucleophilicity of the attacking group, the leaving group ability of the nucleofuge, and the solvent, which affects both the stability of the intermediate or the transition states involved. The role of the electrophilic centre can however become an important factor that can determine the reaction mechanism. In this work we show that the group nucleophilic Fukui function model may be used to rationalize and to predict the reaction mechanism of the title compounds towards alicyclic amines. In general, when the electrophilic carbon centre is attached to the soft sulfur atom, the reaction mechanism is predicted to follow a stepwise route. When the electrophilic carbon atom is attached to a harder oxygen centre, the reaction mechanism is determined by chemical substitution at the nucleofuge moiety. Experimental verification for a set of four substrates is presented.

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Distinction of Push,pull effect and steric hindrance in disubstituted alkynes

Cited by 24 publications

References 12 publications

Hydroalumination of a Chlorotrialkynylsilane: Spontaneous Stepwise 1,3‐Dyotropic Rearrangement via an Intermediate Silyl Cation

Hydroalumination of a Chlorotrialkynylsilane: Spontaneous Stepwise 1,3‐Dyotropic Rearrangement via an Intermediate Silyl Cation

Hydroalumination versus Deprotonation of Alkynes with Sterically Demanding Substituents

Predicting the reaction mechanism of nucleophilic substitutions at carbonyl and thiocarbonyl centres of esters and thioesters

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