Negative hyperconjugation in silicon

<h2 id="experimental-evidence">Experimental evidence</h2>
In 1946, Leo Sommer and <a href="/facts/Frank_C._Whitmore/kc79RQGY">Frank C. Whitmore</a> reported that <a href="/facts/Free-radical_halogenation/cC3L0mdK">radically chlorinating</a> liquid ethyltrichlorosilane gave an <a href="/facts/Positional_isomer/vEQEBbWQ">isomeric mixture</a> with exhibited unexpected reactivity in <a href="/facts/Aqueous_solution/eRNr3MH2">aqueous</a> base. All chlorides pendant to silicon hydrolyze, but the geminal chlorine on carbon failed to hydrolyze, and the vicinal chlorine eliminated to <a href="/facts/Ethylene/QMNBYLYn">ethene</a>: The same behavior appeared with n-propyltrichlorosilane. The α and γ isomers resisted hydrolysis, but a <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> group replaced the β chlorine: They concluded that silicon inhibits <a href="/facts/Electrofuge/BakzY7hA">electrofugal</a> activity at the α carbon.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>
The silicon effect also manifests in certain compound properties. Trimethylsilylmethylamine (Me3SiCH2NH2) is a stronger <a href="/facts/Base_(chemistry)/M9wA7qeM">base</a> (<a href="/facts/Conjugate_acid/hohgQ07M">conjugate pKa</a> 10.96) than <a href="/facts/Neopentylamine/2khz6VBB">neopentylamine</a> (conjugate <a href="/facts/PKa/t4YyRtKC">pKa</a> 10.21); trimethylsilylacetic acid (pKa 5.22) is a poorer acid than <a href="/facts/Trimethyl_acetic_acid/dDnDBbo0">trimethylacetic acid</a> (pKa 5.00).<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
In 1994, Yong and coworkers compared the <a href="/facts/Free-energy_relationship/oa87Np7N">free-energy</a> effects of α- and β-<a href="/facts/Trimethylsilyl_group/RJuCYXfk">Si(CH3)3 moieties</a> on C–H homo- and heterolysis. They, too, concluded that the β silicon atom could stabilize carbocations and the α silicon destabilize carbocations.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>

<h2 id="orbital-structure">Orbital structure</h2>
The silicon α and β effects arise because 3rd period heteroatoms can stabilize adjacent carbanions charges via (<a href="/facts/Negative_hyperconjugation/WWxyp82p">negative</a>) <a href="/facts/Hyperconjugation/9aMXnxT2">hyperconjugation</a>.
In the α effect, reactions that develop negative charge adjacent to the silicon, such as <a href="/facts/Metalation/oqsSXEUI">metalations</a>, exhibit accelerated rates. The C–M σ orbital partially overlaps the C–Si σ* <a href="/facts/Anti-bonding_orbital/9lhLNyt3">anti-bonding orbital</a>, which stabilizes the C–M bond. More generally, (i.e. even for "naked" carbanions) the Si σ* orbitals help stabilize the electrons on the α carbon.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>[unreliable source?]

In the β effect, reactions that develop positive charge on carbon atoms β to the silicon accelerate. The C–Si σ orbital partially overlaps the with the C–X (leaving group) σ* orbital (2b): This electron-density donation into the anti-bonding orbital weakens the C–X bond, decreasing the <a href="/facts/Activation_energy/GqkgPvo0">barrier</a> to the cleavage indicated 3, and favoring formation of the <a href="/facts/Carbenium_ion/vwRVNmxp">carbenium</a> 4.
<h2 id="in-silyl-ethers">In silyl ethers</h2>

The silicon α‑effect described above is mainly focused on carbon. In fact, the most <a href="/facts/Chemical_industry/3M6gfHer">industrially</a>-important silicon α‑effect instead occurs with <a href="/facts/Silyl_ether/k6y8Th9E">silyl ethers</a>. Under hydrolysis condition, certain α-silane-terminated prepolymers <a href="/facts/Crosslink/zeQ9hEAA">crosslink</a> 10-1000 times faster than the corresponding <a href="/facts/Prepolymer/40he8EfT">prepolymers</a> produced from conventional Cγ-functionalized trialkoxypropylsilanes and dialkoxymethylpropylsilanes.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>

<h3>History</h3>
This silicon α-effect was first observed in the late 1960s by researchers at <a href="/facts/Bayer/fnYepBcG">Bayer AG</a> as an increase in reactivity at the silicon atom for hydrolysis and was used for cross-linking of α-silane-terminated prepolymers. For a long time after that, people attributed this reactivity as silicon α-effect. However, the real mechanism beneath it had been debated for many years after this discovery.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> Generally, this effect has been rationalized as an intramolecular donor-acceptor interaction between the lone pair of the organofunctional group (such as NR2, OC(O)R, N(H)COOMe) and the silicon atom. However, this hypothesis has been proved incorrect by Mitzel and coworkers<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> and more experiments are needed to interpret this effect.

<h3>Mechanism study</h3>

Reinhold and coworkers<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> performed a systematical experiment to study the kinetics and mechanisms of hydrolysis of such compounds. They prepared a series of α-silanes and γ-silanes and tested their reactivity in different pH (acidic and basic regime), functional group X and the spacer
between the silicon atom and the functional group X.
In general, they find that under basic conditions, the rate of hydrolysis is mainly controlled by the electrophilicity of the silicon center and the rate of the hydrolysis of the γ-silanes is less influenced by the generally electronegative functional groups than α-silanes. More electronegative the functional groups are, the higher the rate of hydrolysis. However, under acidic conditions, the rate of hydrolysis depends on both the electrophilicity of the silicon center (determining the molecular reactivity) and the concentration of the (protonated) reactive species. Under acidic conditions, the nucleophile changes from OH− to H2O, so it involves the process of protonation and the atoms are protonated could be either silicon or the functional group X. As a result, the general trend in acidic solution is more complicated.

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Colvin, E. (1981) Silicon in Organic Synthesis. Butterworth: London. <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Whitmore, Frank C.; Sommer, Leo H. (March 1946) 
"Organo-silicon compounds II: Silicon analogs of neopentyl chloride and neopentyl iodide: The alpha silicon effect." doi:10.1021/ja01207a036. ISSN 0002-7863. PMID 21015745.
"——— III: α- and β-chloroalkyl silanes and the unusual reactivity of the latter." doi:10.1021/ja01207a037.
Journal of the American Chemical Society, volume 68 issue 3. pp. 481–487. 
Sommer, Leo H.; Dorfman, Edwin; Goldberg, Gershon M.; and Whitmore, Frank C. "The reactivity with alkali of chlorine-carbon bonds alpha, beta and gamma to silicon." Ibid, pp. 488–489. doi:10.1021/ja01207a038. ISSN 0002-7863. PMID 21015747. <a href="/wiki/Frank_C._Whitmore" target="_blank">/wiki/Frank_C._Whitmore</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Colvin, E. (1981) Silicon in Organic Synthesis. Butterworth: London. <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Bausch, M. J.; Gong Yong (June 1994). "Effects of α- and β-silicon atoms on the free energies of C-H homolysis and heterolysis". Journal of the American Chemical Society. 116 (13): 5963–5964. doi:10.1021/ja00092a055. ISSN 0002-7863. <a href="https://pubs.acs.org/doi/abs/10.1021/ja00092a055" target="_blank">https://pubs.acs.org/doi/abs/10.1021/ja00092a055</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Zhe, orthocresol, and NotEvans (17 Jan 2022). Answers to "Stabilisation of anions by silicon". (Accessed 2022-11-29.) Chemistry Stack Exchange. Stack Exchange. Archived 2022-11-29 at the Wayback Machine <a href="https://chemistry.stackexchange.com/users/35555/" target="_blank">https://chemistry.stackexchange.com/users/35555/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">de Meijere, Armin; Diederich, François, eds. (2004-08-25). Metal-Catalyzed Cross-Coupling Reactions (1 ed.). Wiley. doi:10.1002/9783527619535. ISBN 978-3-527-30518-6. <a href="978-3-527-30518-6" target="_blank">978-3-527-30518-6</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Whitmore, Frank C.; Sommer, Leo H. (March 1946) 
"Organo-silicon compounds II: Silicon analogs of neopentyl chloride and neopentyl iodide: The alpha silicon effect." doi:10.1021/ja01207a036. ISSN 0002-7863. PMID 21015745.
"——— III: α- and β-chloroalkyl silanes and the unusual reactivity of the latter." doi:10.1021/ja01207a037.
Journal of the American Chemical Society, volume 68 issue 3. pp. 481–487. 
Sommer, Leo H.; Dorfman, Edwin; Goldberg, Gershon M.; and Whitmore, Frank C. "The reactivity with alkali of chlorine-carbon bonds alpha, beta and gamma to silicon." Ibid, pp. 488–489. doi:10.1021/ja01207a038. ISSN 0002-7863. PMID 21015747. <a href="/wiki/Frank_C._Whitmore" target="_blank">/wiki/Frank_C._Whitmore</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Mitzel, Norbert W.; Losehand, Udo; Richardson, Alan (1999-07-01). "Two Successive Steps of Hypercoordination at Tin: The Gas-Phase and Solid-State Structures of ( N,N- Dimethylaminoxy)trimethylstannane". Organometallics. 18 (14): 2610–2614. doi:10.1021/om990219q. ISSN 0276-7333. <a href="https://pubs.acs.org/doi/10.1021/om990219q" target="_blank">https://pubs.acs.org/doi/10.1021/om990219q</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Berkefeld, André; Guerra, Célia Fonseca; Bertermann, Rüdiger; Troegel, Dennis; Daiß, Jürgen O.; Stohrer, Jürgen; Bickelhaupt, F. Matthias; Tacke, Reinhold (2014-06-09). "Silicon α-Effect: A Systematic Experimental and Computational Study of the Hydrolysis of C α - and C γ -Functionalized Alkoxytriorganylsilanes of the Formula Type ROSiMe 2 (CH 2 ) n X (R = Me, Et; n = 1, 3; X = Functional Group)". Organometallics. 33 (11): 2721–2737. doi:10.1021/om500073m. ISSN 0276-7333. <a href="https://pubs.acs.org/doi/10.1021/om500073m" target="_blank">https://pubs.acs.org/doi/10.1021/om500073m</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
</ol>

Negative hyperconjugation in silicon open-in-new

Negative hyperconjugation in silicon