Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer.

The reactivity of the geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3 CC(O)CH=CH2 , Ph[C(O)]2 Ph, PhN=NPh and Me3 SiCHN2 ), featuring polar or non-polar multiple bonds and/or represent α,ß-unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV-induced photoisomerization, which results in the highly selective complexation of cis-azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2- or 1,4-addition products, but to the non-stereoselective (tBu)2 PCH2 -transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X-ray diffraction experiments.

A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination.

The reactivity of the frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F5 C2 )2 SnCH2 PH(tBu)2 (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X-ray diffraction. NMR experiments with the two isotopologues H2 and D2 showed it to be formed via an H2 adduct (F5 C2 )3 HSnCH2 PH(tBu)2 (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen-induced polarization experiments revealed the formation of a second product of the reaction of 1 with H2 , [HP(tBu)2 Me][Sn(C2 F5 )3 ] (4), in 1 H NMR spectra, whereas 2 was not detected due to its transient nature.

An Adduct of Sulfur Monoxide to a Frustrated Sn/P Lewis Pair.

The geminal frustrated Lewis pair (F5 C2 )3 SnCH2 P(tBu)2 (1) reacted with N-sulfinylaniline PhNSO to afford the first sulfur monoxide adduct of a main group metal, (F5 C2 )3 SnCH2 P(tBu)2 ⋅SO (2), which contains a SnCPSO ring. The second product is a phenylnitrene adduct of 1. The surprising stability of 2 was compared with the stabilities of the so far inaccessible O2 and S2 adducts of 1. Attempts to prepare these from 1 and the elemental chalcogens (O2 , S8 , Se∞ , Te∞ ) led to four-membered SnCPE ring systems. Quantum-chemical investigations of 2 demonstrate the bond polarity of the SO unit to stabilize 2.

A Neutral Geminal Tin/Phosphorus Frustrated Lewis Pair.

The geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (2) was prepared by reacting (F5 C2 )3 SnCl with LiCH2 P(tBu)2 . It is neutral and contains an extremely electronegatively substituted, but relatively soft (hard-soft acid-base, HSAB) acidic tin function. Its FLP-type reactivity was proven by reaction with a variety of small molecules (CO2 , SO2 , CS2 , PhNCO, HCl, (Ph3 P)AuCl). However, it shows no reaction in H/D scrambling experiments with H2 /D2 mixtures and binds CO2 reversibly, as was observed by VT-NMR spectroscopy. Compound 2 and all its adducts were completely characterized by means of multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction experiments.

Asymmetric synthesis of propargylamines as amino acid surrogates in peptidomimetics.

The amide moiety of peptides can be replaced for example by a triazole moiety, which is considered to be bioisosteric. Therefore, the carbonyl moiety of an amino acid has to be replaced by an alkyne in order to provide a precursor of such peptidomimetics. As most amino acids have a chiral center at Cα, such amide bond surrogates need a chiral moiety. Here the asymmetric synthesis of a set of 24 N-sulfinyl propargylamines is presented. The condensation of various aldehydes with Ellman's chiral sulfinamide provides chiral N-sulfinylimines, which were reacted with (trimethylsilyl)ethynyllithium to afford diastereomerically pure N-sulfinyl propargylamines. Diverse functional groups present in the propargylic position resemble the side chain present at the Cα of amino acids. Whereas propargylamines with (cyclo)alkyl substituents can be prepared in a direct manner, residues with polar functional groups require suitable protective groups. The presence of particular functional groups in the side chain in some cases leads to remarkable side reactions of the alkyne moiety. Thus, electron-withdrawing substituents in the Cα-position facilitate a base induced rearrangement to α,ß-unsaturated imines, while azide-substituted propargylamines form triazoles under surprisingly mild conditions. A panel of propargylamines bearing fluoro or chloro substituents, polar functional groups, or basic and acidic functional groups is accessible for the use as precursors of peptidomimetics.