Gerhard Raabe

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Organization: RWTH Aachen University , Germany

Department: Institute of Organic Chemistry

Title: (PhD)

TOPICS

Organic Chemistry

Chemicals
References

Chiral Fluorinated -Sulfonyl Carbanions: Enantioselective Synthesis and Electrophilic Capture, Racemization Dynamics, and Structure

Co-reporter:Dr. Gunther Hellmann;Dr. Achim Hack;Dr. Eric Thiemermann;Dr. Olaf Luche;Dr. Gerhard Raabe;Dr. Hans-Joachim Gais

Chemistry - A European Journal 2013 Volume 19( Issue 12) pp:3869-3897

Publication Date(Web):

DOI:10.1002/chem.201204014

Abstract

Enantiomerically pure triflones R¹CH(R²)SO₂CF₃ have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes. Key steps of the syntheses of the sulfanes are the photochemical trifluoromethylation of the thiols with CF₃Hal (Hal=halide) or substitution of alkoxyphosphinediamines with CF₃SSCF₃. The deprotonation of RCH(Me)SO₂CF₃ (R=CH₂Ph, iHex) with nBuLi with the formation of salts [RC(Me)SO₂CF₃]Li and their electrophilic capture both occurred with high enantioselectivities. Displacement of the SO₂CF₃ group of (S)-MeOCH₂C(Me)(CH₂Ph)SO₂CF₃ (95 % ee) by an ethyl group through the reaction with AlEt₃ gave alkane MeOCH₂C(Me)(CH₂Ph)Et of 96 % ee. Racemization of salts [R¹C(R²)SO₂CF₃]Li follows first-order kinetics and is mainly an enthalpic process with small negative activation entropy as revealed by polarimetry and dynamic NMR (DNMR) spectroscopy. This is in accordance with a C_αS bond rotation as the rate-determining step. Lithium α-(S)-trifluoromethyl- and α-(S)-nonafluorobutylsulfonyl carbanion salts have a much higher racemization barrier than the corresponding α-(S)-tert-butylsulfonyl carbanion salts. Whereas [PhCH₂C(Me)SO₂tBu]Li/DMPU (DMPU = dimethylpropylurea) has a half-life of racemization at −105 °C of 2.4 h, that of [PhCH₂C(Me)SO₂CF₃]Li at −78 °C is 30 d. DNMR spectroscopy of amides (PhCH₂)₂NSO₂CF₃ and (PhCH₂)N(Ph)SO₂CF₃ gave NS rotational barriers that seem to be distinctly higher than those of nonfluorinated sulfonamides. NMR spectroscopy of [PhCH₂C(Ph)SO₂R]M (M=Li, K, NBu₄; R=CF₃, tBu) shows for both salts a confinement of the negative charge mainly to the C_α atom and a significant benzylic stabilization that is weaker in the trifluoromethylsulfonyl carbanion. According to crystal structure analyses, the carbanions of salts {[PhCH₂C(Ph)SO₂CF₃]Li⋅L}₂ (L=2 THF, tetramethylethylenediamine (TMEDA)) and [PhCH₂C(Ph)SO₂CF₃]NBu₄ have the typical chiral C_αS conformation of α-sulfonyl carbanions, planar C_α atoms, and short C_αS bonds. Ab initio calculations of [MeC(Ph)SO₂tBu]⁻ and [MeC(Ph)SO₂CF₃]⁻ showed for the fluorinated carbanion stronger n_Cσ* and n_Oσ* interactions and a weaker benzylic stabilization. According to natural bond orbital (NBO) calculations of [R¹C(R²)SO₂R]⁻ (R=tBu, CF₃) the n_Cσ*_SR interaction is much stronger for R=CF₃. Ab initio calculations gave for [MeC(Ph)SO₂tBu]Li⋅2 Me₂O an O,Li,C_α contact ion pair (CIP) and for [MeC(Ph)SO₂CF₃]Li⋅2 Me₂O an O,Li,O CIP. According to cryoscopy, [PhCH₂C(Ph)SO₂CF₃]Li, [iHexC(Me)SO₂CF₃]Li, and [PhCH₂C(Ph)SO₂CF₃]NBu₄ predominantly form monomers in tetrahydrofuran (THF) at −108 °C. The NMR spectroscopic data of salts [R¹(R²)SO₂R³]Li (R³=tBu, CF₃) indicate that the dominating monomeric CIPs are devoid of C_αLi bonds.

Theoretical investigations of β-trimethylsilyl-substituted iminium ions

Co-reporter:Stefanie Mersmann, Gerhard Raabe, Carsten Bolm

Tetrahedron Letters 2011 Volume 52(Issue 42) pp:5424-5426

Publication Date(Web):19 October 2011

DOI:10.1016/j.tetlet.2011.07.118

Calculated reaction energies, NBO analyses, and natural charge distributions show that a trimethylsilyl group at the β carbon atom stabilizes the positive charge of an iminium cation more effectively than a tert-butyl group in the same position. Different from the classical β-silicon effect, where theoretical studies suggest that the positive charge is stabilized hyperconjugatively by interaction of the Si–C σ-bond with a formally empty 2p-orbital of suitable symmetry at the carbonium carbon, the stabilization in the case of iminium ions occurs by interaction of the Si–C σ-bond with the π∗ orbital of the CN double bond. Stabilization of the silyl-substituted derivatives by hyperconjugation might be enhanced by interaction of the positive charge with a neighboring induced dipole.

Quantum-chemical calculations on the electronic circular dichroism of (−)-dibromophakellin and (−)-dibromophakell-statin

Co-reporter:Iuliana Atodiresei;Thomas Lindel;Jörg Fleischhauer;Michael Zöllinger

Chirality 2007 Volume 19(Issue 7) pp:542-549

Publication Date(Web):8 MAY 2007

DOI:10.1002/chir.20388

The CD spectra of the pyrrole-imidazole alkaloids, (−)-dibromophakellin and (−)-dibromophakellstatin, have been calculated employing the quantum-chemical time-dependent density functional theory. Comparison of calculated and measured spectra showed that this well-established method is also a useful tool to elucidate the absolute stereochemistry of this class of compounds. The computational results have further been used to analyze the spectra measured in methanol and to explain the remarkable red shift of one CD band when trifluoroethanol is used as a solvent instead of methanol. Chirality, 2007. © 2007 Wiley-Liss, Inc.