Michael K. Denk

Synthesis and reactivity of subvalent compounds: Part 11. Oxidation, hydrogenation and hydrolysis of stable diamino carbenes

Abstract The reactivities of the two stable diamino carbenes 1,3-di-tert-butylimidazol-2-ylidene (1) and 1,3-di-tert-butylimidazolin-2-ylidene (2) toward hydrogen, oxygen, water and carbon monoxide were investigated. The carbenes do not react with O2 or CO but are attacked by water to give the respective hydrolysis products tBu–NCHCH2–N(CHO)tBu (7) and tBu–NH–CH2CH2–N(CHO)tBu (8). While 2 is hydrolyzed instantaneously, the aromatically stabilized 1 reacts only very slowly and can be handled in air for brief periods of time. The carbenes 1 and 2 are unreactive towards H2 alone but can be catalytically hydrogenated to the respective aminals 2,3-dihydro-1,3-di-tert-butylimidazole (5) and 1,3-di-tert-butylimidazolidine (6). The reaction products were characterized by single crystal X-ray crystallography, multinuclear NMR spectroscopy (1H, 13C, 17O), IR spectroscopy and DFT methods. The extent of aromatic delocalization in 1 was investigated through isodesmic hydrogenation reactions at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. At this level of theory, the ‘carbene oxide’ (2,3-dihydro-imidazol-2-one) retains ca. 50% of the aromatic delocalization energy of the carbene. The oxidation of the diamino carbenes to the ‘carbene oxides’ (ureas) is calculated to be exothermic by −79.1 kcal mol−1 (1) and −86.4 kcal mol−1 (2). The oxygen affinity of the carbenes resembles that of trimethylphosphine (−79.7 kcal mol−1) and triethyl phosphite (−88.6 kcal mol−1) and is significantly higher than that of CO (−67.6 kcal mol−1). The aminals 5 and 6 are structurally related to Thauers hydrogenase (TH2/TH+) but do not show hydrogenase activity.

Aromatic phosphenium cations

Abstract The ionic chlorophosphane [L′P] + Cl − ( 1 ) was obtained for the ligand L′ = N,N′-bis (tert- butyl)-1,4-diaza-2- butene . By contrast, the CC-saturated analog LP-Cl ( 2 , L = N,N′-bis (tert- butyl)-1,4-diaza-2- butane ) is covalent. This remarkable difference can be attributed to the fact that the phosphenium cation derived from 1 possesses aromatic 6π-stabilization. As expected, the two chlorophosphanes differ sharply in their volatility and solubility in organic solvents.

Nucleophilic carbenes and the wanzlick equilibrium: A reinvestigation

Contrary to earlier studies, mixtures of enetetramines A=A and B=B readily exchange their carbene units upon heating (100 – 175 °C) to give the cross-over olefins A=B. The absence of such exchange reactions has formerly been used as evidence against the dissociation of enetetramines into free carbenes by Lemal et al. and Winberg et al.

Synthesis and reactivity of subvalent compounds: Part 10. Fast deuterium labeling and the basicity of stable diamino carbenes (imidazole-2-ylidenes)☆

Abstract The aromatic ring protons in imidazol-2-ylidenes (Arduengo carbenes) undergo rapid deuterium–hydrogen exchange in DMSO-d6, CD3OD and D2O.

C−H Activation with Elemental Sulfur: Synthesis of Cyclic Thioureas from Formaldehyde Aminals and S8

The C-H activation of cyclic formaldehyde aminals LCH2 (L = RN-CH2CH2CH2-NR and RNCH2CH2-NR, R = Me, Et, iPr, tBu, or Ph) with S8 proceeds at unusually low temperatures (T< 160 degrees C) and results in the formation of the respective thioureas LC=S and H2S. The reaction constitutes a new, solvent-free method for the synthesis of thioureas that eliminates the toxic and highly flammable CS2. For R = tBu, the ionic carbenium thiocyanates [LCH]+ SCN- dominate the product spectrum and the respective thioureas are obtained in low yield. The reactivity of the analogous sulfur and oxygen ring systems towards S8 was investigated. 1,3-Dithiolane is cleanly converted into 1,3-dithiolane-2-thione (S8, 14 d, 190 degrees C) and resembles the cyclic formaldehyde aminals in this respect. 1,3-Dioxolane (L = OCH2CH2O) is completely inert towards sulfur even under forceful reaction conditions (190 degrees C, 14 d). The formation of thioureas from aminals was investigated at the CBS-4 and B3LYP/6-31G(d) levels of theory.

Reaction of a Stable Silylene with Divalent Group 14 Compounds

The reaction of the stable silylene 1 (L′Si:) with CO, SnCl2, PbCl2, the stable carbene L′C: (2) and the stable germylene L′Ge: (3) was investigated as a possible approach for the synthesis of hetero-polysilanes (poly-carbosilanes, poly-germasilanes, poly-stannasilanes). The silylene 1 was found to be inert towards all substrates except SnCl2 and PbCl2. Reaction with SnCl2 led to the formation of the yellow, tris(silyl)stannane [(L′SiCl)3SnCl] (5) and elemental tin. The photolabile (5) was characterized by multinuclear NMR (1H, 13C, 29Si, 119Sn) and single crystal X-ray diffraction: C30H60Cl4N6Si3Sn, M = 849.60, orthorhombic, space group Pca21. The bond angles of 115.89 ° (ave. Si–Sn–Si) and 101.87 ° (ave. Cl–Sn–Si) show a slightly flattened tetrahedral geometry around the tin atom. – Thermolysis of 5 at 100 °C led to the deposition of thin films of tin; photolysis resulted in the formation of tin powder. All thermolysis products could be isolated and were characterized as SnCl2, elemental tin, L′Si: (1), L′SiCl2 (6) and the new disilane L′Si(Cl)-(Cl)SiL′ (7). Photolysis led to a reduced product spectrum (formation of Sn + SnCl2 + 7). Reaction of 1 with PbCl2 gave elemental lead and L′SiCl2 as the only products.

Transient ISEELS: a new probe of chemical reactions

Abstract An inner shell electron energy loss spectrometer (ISEELS) has been adapted as a probe of chemical reactions by coupling its gas phase interaction region to the output of a variety of chemical reactors including thermolysis, gas phase reaction, and heterogeneous reaction systems. Three examples of transient ISEELS studies are presented: in situ generation of a reactive carbene by thermolysis of its stable dimer, reaction of a silylene with N 2 O in the gas phase, and the heterogeneous reactions of H 2 S and H 2 O with crystalline boron in a heated quartz tube. In the latter example, temperature dependent inner shell excitation spectra, mass spectrometry and ab initio calculations were used to show that in the high temperature limit the reaction of H 2 S (g) and B (s) over SiO 2 (s) gives rise to borine (HBO), a species previously un-detected from this reaction chemistry.