Neues aus der Chemie superbasischer Protonenschwämme auf Basis von Guanidin-, Iminophosphoran- und Sulfoximin-Pinzettenliganden für Protonen

Proton sponges are neutral organic bases showing a chelating proton-binding site. The classical example of a proton sponge is 1,8-bis-(dimethylamino)naphthalene (DMAN) first introduced by Alder et al. The pKBH+ value of DMAN in acetonitrile is 18.2. Over the years new and innovative proton sponges on the basis of Alders proton sponge concept were synthesized and characterized. These new proton sponges show a higher basicity as well as a higher kinetic activity than DMAN. In our work group the new-generation proton sponges 1,8-bis-(tetramethylguanidino)naphthalene (TMGN), 1,8-bis-(dimethylethylenguanidino)-naphthalene (DMEGN) and 1,8-Bis-(hexamethyliminophosphoranyl)naphthalene (HMPN) were synthesized and fully characterized. The intention of this work was to increase the thermodynamic basicity of proton sponges with the naphthalene skeleton and to evaluate new potential applications in Chemistry. This was achieved by the successful synthesis of a new proton sponge from the group of 1,8-bis-(guanidino)naphthalene. This third guanidine-based proton sponge is called 1,8-bis(1,3-dimethyl-1,3-imidazol-2-ylidenamino)naphthalene (DIAN). Its structural and spectroscopic data were discussed, showing the properties of a perfect pincer ligand for a proton. The thermodynamic basicity was determined using NMR titration. In addition, the hydrolysis and the nucleophilic properties of DIAN towards ethyl iodide have been studied. The kinetic activation was investigated using the proton self exchange reaction of DIAN with [DIAN]PF6 via NMR line shape analysis. The resultant energy barrier is 49.6 kJ/mol. The pKBH+ value of DIAN was determined using the 1H-NMR titration with a reference base like TMGN. Compared to TMGN (pKBH+(MeCN) = 25.1) and DMEGN (pKBH+(MeCN) = 23.00 (calculated)) DIAN had a higher pKBH+ value of 26.4 in acetonitrile. The stronger basicity of DIAN compared to TMGN or DMEGN is based on the higher polarity of the exocyclic C-N bond, which corresponds to the stabilization of the carbenium centre in the heteroaromatic imidazolium moiety. Another goal was to enhance of the thermodynamic basicity of bisphosphazen-based proton sponges. Three synthetic strategies for building 1,8-disubstituted naphthalenes and sterically demanding phosphorous compounds of the types PR3, R3P=NH and X-+PR3 (R = NMe2, NMeR, N=C(NMe2)2 and soon) were tested. This work also engages in studies on the synthesis of P-electrophiles, P-nucleophiles and N-nucleophiles. The Staudinger reaction proved to be the most promising route. Although to date the superbasie phosphorus(III)-guanidine could not be isolated, the reaction of 1,8-diazidonaphthalene with the phosphorus(III)-amide provided a new path to HMPN via the bisphosphazide. In this Work could be among other things bis-(triazenido)- phosphoranylnaphthaline isolated and characterized. The Staudinger reaction with the Verkade azaphosphatrane derivate produces a new proton sponge called 1,8-bis-(azaphosphatranyl)naphthalene (APAN). According of DFT calculations of Maksić et al. 1,8-bis-(azaphosphatranyl) naphthalene has a proton affinity in the gas phase of 278.0 kcal/mol. This value is higher than that of HMPN (274.0 kcal/mol), so that APAN should have a higher pKBH+-value than HMPN (pKBH+(MeCN) = 29.9). The free base (APAN) was protonated with the acid (CF3SO2)2NH and thus formed into its conjugate acid. In addition to proton sponges based on iminophosphorane pincer ligands, sulfoximine ligands moved into the center of our attention. Bolm et al. synthesized 1,8-bis-((S)-S-methyl-S-phenylsulfoximino)naphthaline ((S,S)-MPSIN) using a double cross coupling reaction. In this work, the basicity, the hydrolysis and the nucleophilic behavior (S,S)-MPSIN towards ethyliodide are experimentally investigated and discussed. Single crystal structure of (S,S)-MPSIN and its corresponding acid [(S,S)-MPSIN-H]BF4 were obtained and discussed. It turned out that the thermodynamic basicity of (S,S)-MPSIN is significantly lower than that of the guanidine proton sponges. The basicity of (S,S)-MPSIN ranges between triflate and water, which is able to deprotonate [(S,S)-MPSIN-H]+. Although (S,S)-MPSIN is structurally comparable with the guanidine proton sponges, it is not a good base. In fact the corresponding acid of (S,S)-MPSIN is a moderate acid, as it clearly is a stronger acid than the ammonium cation. The investigation of the kinetic activation was achieved using the proton self exchange reaction of (S,S)-MPSIN with [(S,S)-MPSIN-H]+ via NMR line shape analysis according to the Eyring equation. The resultant energy barrier is 39.5 kJ/mol. In the second part of this work efforts have been made to find new applications for proton sponges in organic and organometallic chemistry. The nucleophilic reactivity of proton sponges towards ethyl iodide has been determined experimentally. Despite the high basicity of the proton sponges they show a relatively low nucleophilicity. In contrast to ethyl iodide dihalomethanes CH2X2 (X = Cl, Br) are inert towards the proton sponges. No reaction occurs with dichloromethane at 25 °C. The first bisiminophosphorane proton sponge HMPN shows conversion with dibromomethane at 25 °C. In addition to an acid-base reaction between HMPN and dibromomethane was observed a nucleophilic reaction competitor. The activation product of nucleophilic reaction is formed in the ratio of two to one in comparison to the conjugate acid of HMPN. In contrast to HMPN both the guanidine proton sponges (DIAN, TMGN) are unreactive towards CH2Br2. The guanidine and iminophosphorane proton sponges are perfect proton acceptor ligands. The properties of these ligands towards Lewis acids were hardly known. For the investigation of the donor properties, TMGN has been used to represent the class of guanidine-proton sponges, as a Lewis acid BeCl2was selected. By coordinating BeCl2 to the chelating ligand TMGN a tetrahedral beryllium complex was formed. [(TMGN)BeCl2] is the first complex of beryllium with a guanidine ligand. The preparation of a stable, cationic, trigonal planar beryllium complex [(TMGN)BeCl]+ by chloride abstraction failed. Recent works by Himmel et al. show that TMGN is able to coordinate Pd(II) and Pt(II) cations. These catalysts were used in a Heck reaction. In this work the Buchwald-Hartwig reaction was tested as an example for a C-N cross coupling reaction. The amidation of iodobenzene was chosen, which is usually performed with a copper(I) salt and a bidentate diamine ligand. In this work the diamine ligand was replaced by a bisguanidine ligand. In this context the complexes of copper(I)-halogenides with TMGN were prepared. These and other complexes with the ligands DIAN, HMPN, 1,2-bis-(1,1,3,3-tetramethylguanidino)ethane (TMGE) and 1,2-bis-(1,3-diisopropylguanidino)ethane (IGE) were tested in a C-N cross coupling reaction. It was shown that guanidine-copper(I) complexes can catalyze such an amidation reaction of aryl halides. Guanidine ligands shows a lower activity compared to diamine ligands such as N, N-dimethylethylenediamine. The highest activity of the tested gunidine ligands shows IGE. Another potential application is the in this study for the first time observed protolysis of molecular hydrogen with the guanidine proton sponges 1,8-bis-(dimethylethylenguanidino)-naphthalenn (DMEGN) and TMGN in the presence of tris(pentafluorophenyl)borane BCF. The salts [TMGN-H][H-BCF] und [DMEGN-H][H-BCF] were isolated and completely characterized. It is interesting to note that the less basic “classical” proton sponge DMAN do not react under the same conditions with molecular hydrogen. Other proton sponges like HMPN and DIAN are stronger bases than TMGN and DMEGN. They react nonselectively with BCF even in the absence of molecular hydrogen. This work expanded the class of proton sponges by the two new representatives APAN and DIAN. The third “proton sponge” (S,S)-MPSIN based on a sulfoximine. It turned out to be slightly alkaline. The heterolytis splitting of molecular hydrogen by proton sponges opens interesting perspectives for future work.