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Low-coordinate Organosilicon Chemistry: Fundamentals, Excursions Outside the Field, and Potential Applications
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Physical Organic Chemistry. (Henrik Ottosson)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis reports on unsaturated silicon compounds, as well as excursions from these into germanium chemistry, single molecule electronics, and silyl protective group chemistry. Both experimental and computational investigations were performed.

Potassium germenolates were synthesized through reactions of tris(timethylsilyl) substituted acyl- and carbamylgermanes with potassium tert-butoxide. The potassium germenolates calculated by density functional theory have pyramidal structures at the Ge atoms, similar to the Si in the corresponding potassium silenolates, indicating negative charge on germanium rather than on oxygen. Germenolates also display germyl anion-like reactivity instead of germene-like reactivity as they are alkylated at Ge and initiate anionic polymerization of dienes rather than form [4+2] cycloadducts. The NMR chemical shifts reveal more negative charge at Ge in germenolates than at Si in analogous silenolates.

Computations indicate that silabenzenes and silapyridines are reachable via [1,3]-silyl shifts from cyclic conjugated acylsilanes. Differently sized substituents were considered to prevent dimerizations, and 1-triisopropylsilyl-2-triisopropylsiloxy-6-tert-butylsilabenzene is a good synthetic target. Computations also show that silaphenolates are species with negative charge primarily localized at oxygen atom. Their planar structures, bond lengths, and NICS values reveal significant influence of aromaticity. Electrostatic repulsion should increase their stability, however, steric bulk is also important.

Furthermore, it was found computationally that [1,3]-silyl shift from an acylsilane to a silene can function as a molecular switch reaction. Conductance calculations support this proposition.  

Finally, tris(trimethylsilyl)silylmethaneamide (hypersilylamide) together with catalytic amounts of triflic acid were found to be efficient for protection of a range of alkyl and aryl alcohols and thiols in good to excellent yields. The protocol can be used to protect the less hindered OH group of a diol and has a broad functional group tolerance. A catalytic cycle is proposed. Hypersilyl protected alcohols and thiols are deprotected efficiently under photolytic conditions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 907
Keyword [en]
organosilicon, silene, silaaromatics, silenolate, hypersilyl group, alcohol protection, molecular switch
National Category
Organic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-169796ISBN: 978-91-554-8296-1 (print)OAI: oai:DiVA.org:uu-169796DiVA: diva2:507721
Public defence
2012-04-21, B7:101a, BMC, Husargatan 3, Uppsala, 13:30 (English)
Opponent
Supervisors
Available from: 2012-03-30 Created: 2012-03-06 Last updated: 2012-04-19Bibliographically approved
List of papers
1. Remarkably Stable Silicon Analogues of Amide Enolates: Synthesis, Structural Characterization, and Reactivity Studies
Open this publication in new window or tab >>Remarkably Stable Silicon Analogues of Amide Enolates: Synthesis, Structural Characterization, and Reactivity Studies
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Potassium 2-N,N-dialkylamino-1,1-bis(trimethylsilyl)silen-2-olates (or amide silenolates, silicon analogues of amide enolates) were synthesized through reaction of N,N-dialkyl-tris(trimethylsilyl)silylmethaneamides with potassium tert-butoxide, and these 2-N,N-dialkylaminosilen-2-olates display remarkable thermal stabilities (e.g., merely 37% decomposition after 8 h at 90 ºC).  The crystal structure of one of the potassium 2-N,N-dialkylaminosilen-2-olates, without potassium ion chelating agent, reveals a more pyramidal configuration around the Si atom than found in previously reported silenolates, indicating a strong localization of the negative charge to this atom. The reactivities of the potassium 2-N,N-dialkylaminosilen-2-olates are in part similar to those of previous lithium and potassium silenolates as they are alkylated with MeI at Si. However, they do not react with dienes to yield [4+2] cycloadducts, the customary adducts of silenolates and reverse polarized silenes, but instead initiate anionic diene polymerization.  Consequently, they display silyl anion-like rather than silene-like reactivities. Finally, we find that potassium 2-aminosilen-2-olates with N,N-diphenylamino instead of N,N-dialkylamino substitution decompose rapidly to potassium diphenylamide, carbon monoxide, and silylenes. Clearly, if the substituent at the 2-position of a silenolate is able to accept and stabilize negative charge, such as NPh2, then this silenolate will be prone to decompose.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-169783 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2012-04-19
2. Formation and Fundamental Properties of Potassium Germen-2-olates
Open this publication in new window or tab >>Formation and Fundamental Properties of Potassium Germen-2-olates
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Potassium 1,1-bis(trimethylsilyl)germen-2-olates (2a - 2d) with different substituents at the carbon atom were generated in good yields through the treatment of the correspondingly substituted tris(trimethylsilyl)acyl- and tris(trimethylsilyl)carbamyl-germanes (1a - 1d) with potassium tert-butoxide at room temperature in dry THF. Comparisons between the 29Si and 13C NMR chemical shifts of the germenolates and the analogous silenolates (4a4d) were performed. The recorded 13C and 29Si NMR chemical shifts of the potassium germenolates were also compared to those obtained from GIAO-B3LYP/6-31+G(d)//B3LYP/LANL2DZp calculations. The chemical reactivities of potassium germenolates were compared with silenolates. In this regard, the reactions of 2a - 2d were performed with methyliodide at -40 oC and the germanium methylated products (5a - 5c) were obtained in yields of 54 - 77 %. The reactions of these germenolates with 1,3-butadiene at low temperatures, however, lead to polymerization of dienes (2,3-dimethyl-1,3-butadiene, isoprene, and 1,3-pentadiene) revealing a reactivity resemblance to aminosilenolates, species which in return are comparable to silyl anions in reactivity.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-169795 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2012-04-19
3. Computational Investigation of Brook-Type Silabenzenes and Their Possible Formation through [1,3]-Si -> O Silyl Shifts
Open this publication in new window or tab >>Computational Investigation of Brook-Type Silabenzenes and Their Possible Formation through [1,3]-Si -> O Silyl Shifts
2013 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 32, no 1, 16-28 p.Article in journal (Refereed) Published
Abstract [en]

Quantum chemical calculations with the M062X hybrid meta density functional theory method were performed in order to examine formation of Brook-type silabenzenes 4a 4l, silapyridines 6a 6d, and five-membered ring silaheteroaromatics 8a8d through [1,3]-trimethylsilyl (TMS) and [1,3]-tri(isopropyl)silyl (TIPS) shifts from a tetrahedral silicon atom to an adjacent carbonyl oxygen of cyclic conjugated acylsilane precursors. All Brook-type silabenzenes and silapyridines, having a 2-trialkylsiloxy substituent, are at lower relative energies than their precursors, whereas silaheteroaromatics 8a 8d are found at slightly higher energies. The free energies of activation for the thermal [1,3]-TMS shifts range from 29 to 44 kcal/mol, with the lowest for a Brook-type silapyridine and the highest for a silafuran. The geometries of the Brook-type silabenzenes, silapyridines, silafuran and silathiophene indicate aromatic character, but the silapyrroles are nonaromatic. At M062X/6-311+G(d)//M062X/6-31G(d) level all Brook-type silabenzene dimers studied herein are more stable than two silabenzenes, also for a silabenzene with bulky TIPS, OTIPS and tert-butyl substituents (4l). Yet, comparisons of the B3LYP/6-31G(d) dimerization energies of 4l with that of the isolable 1-Tbt-silabenzene (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) of Tokitoh [J. Chin. Chem. Soc. 2008, 55, 487] indicate that 4l will also be a monomeric silabenzene, and thus, a suitable synthetic target.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-169784 (URN)10.1021/om300023s (DOI)000313606200005 ()
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2017-12-07Bibliographically approved
4. Silaphenolates and Silaphenylthiolates: Two Unexplored Unsaturated Silicon Compound Classes Influenced by Aromaticity
Open this publication in new window or tab >>Silaphenolates and Silaphenylthiolates: Two Unexplored Unsaturated Silicon Compound Classes Influenced by Aromaticity
2012 (English)In: Molecules, ISSN 1420-3049, E-ISSN 1420-3049, Vol. 17, no 1, 369-389 p.Article in journal (Refereed) Published
Abstract [en]

Monosilicon analogs of phenolates and phenylthiolates are studied by quantum chemical calculations. Three different silaphenolates and three different silaphenylthiolates are possible; the ortho-, meta-, and para-isomers. For the silaphenolates, the meta- isomer is the thermodynamically most stable, regardless if the substituent R at Si is H, t-Bu or SiMe3. However, with R = H and SiMe3 the energy differences between the three isomers are small, whereas with R = t-Bu the meta- isomer is similar to 5 kcal/mol more stable than the ortho- isomer. For the silaphenylthiolates the ortho- isomer is of lowest energy, although with R = H the ortho- and meta- isomers are isoenergetic. The calculated nucleus independent chemical shifts (NICS) indicate that the silaphenolates and silaphenylthiolates are influenced by aromaticity, but they are less aromatic than the parent silabenzene. The geometries and charge distributions suggest that all silaphenolates and silaphenylthiolates to substantial degrees are described by resonance structures with an exocyclic C=O double bond and a silapentadienyl anionic segment. Indeed, they resemble the all-carbon phenolate and phenylthiolate. Silaphenylthiolates are less bond alternate and have slightly more negative NICS values than analogous silaphenolates, suggesting that this compound class is a bit more aromatic. Dimerization of the silaphenolates and silaphenylthiolates is hampered due to intramolecular Coulomb repulsion in the dimers, and silaphenolates with a moderately bulky SiMe3 group as substituent at Si should prefer the monomeric form.

Keyword
silicon, aromaticity, quantum chemical calculations
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-166896 (URN)10.3390/molecules17010369 (DOI)000299535700025 ()
Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2017-12-08Bibliographically approved
5. The [1,3]-Si→O Silyl Shift from a Nonconducting Acylsilane to a Conducting Brook-Silene as Basis for a Molecular Switch
Open this publication in new window or tab >>The [1,3]-Si→O Silyl Shift from a Nonconducting Acylsilane to a Conducting Brook-Silene as Basis for a Molecular Switch
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

By usage of density functional theory (DFT) calculations we explored if the [1,3]-silyl shift leading from an acylsilane with two p-conjugated substituents to a silene (a Si=C double bonded compound) can be used as a basis for a molecular conductance switch. In such a switch, the acylsilane, with a tetrahedral saturated silicon atom disrupting the conjugation through the molecule, acts as the OFF state, whereas the silene with a conjugated path running through the complete molecule represents the ON state. Our requirements are (i) the silenes should be slightly higher in relative energy than the acylsilane so as to promote a thermal backrearragment, (ii) the barrier for the backtransfer of the silyl group should be 25-30 kcal/mol, (iii) the ON/OFF conductance ratio should be high, and (iv) the switch should be realistic. According to our calculations using non-equilibrium Green’s function theory, a 1,2-bis(4-thiophenylethynyl)silene has a conductance which is 270 times higher than that of the corresponding acylsilane at zero bias voltage. However, at a voltage of +1 V the ON/OFF ratio decreases to ~40.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-169782 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2012-04-19
6. Highly Efficient and Convenient Acid Catalyzed Hypersilyl Protection of Alcohols and Thiols by Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide
Open this publication in new window or tab >>Highly Efficient and Convenient Acid Catalyzed Hypersilyl Protection of Alcohols and Thiols by Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide
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2012 (English)Article in journal (Other academic) Submitted
Abstract [en]

Tris(trimethylsilyl)silyl-N,N-dimethylmethaneamide, herein named hypersilylamide, is a convenient and efficient source of the hypersilyl group in the first widely applicable acid catalyzed protocol for silyl group protection of primary, secondary, tertiary alkyl as well as aryl alcohols and thiols in high yields. The sole by-product is N,N-dimethylformamide (DMF) and a range of solvents can be used, including DMF. A high selectivity in the protection of diols can be achieved, also for diols with very small differences in the steric demands at the two hydroxyl groups. Moreover, in the protection of equivalent alcohol and thiol sites the protection of the alcohol is faster, allowing for selective protection in high yields. Quantum chemical calculations at the M062X hybrid meta density functional theory level give insights on the mechanism for the catalytic process. Finally, the hypersilyl group is easily removed from all protected alcohols and thiols examined herein by irradiation at 254 nm.

National Category
Organic Chemistry Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-169781 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2013-03-07Bibliographically approved
7. Scope and Limitations of an Acid Catalyzed Protocol for Hypersilyl Protection of Alcohols
Open this publication in new window or tab >>Scope and Limitations of an Acid Catalyzed Protocol for Hypersilyl Protection of Alcohols
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

A highly efficient and convenient triflic acid (TfOH) catalyzed protocol for the protection of various functionalized alcohols in CH2Cl2 at ambient temperature using tris(trimethylsilyl)silyl-N,N-dimethyl-methaneamide (hypersilylamide) 1 as the protecting reagent is developed. Herein, results on the scope and limitations of this protocol for a number of functionalized alcohols are presented. This method was found to be effective for the selective protection of less hindered OH groups in different classes of diols containing both pri/tert, sec/tert, or aromatic/aliphatic hydroxyl groups. In general, our protocol exhibited excellent functional group tolerance in the protection of alcohols containing alkoxy, keto, amino, as well as halo substituents in good to excellent yields.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-169780 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2012-04-19

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