Fredric Davidson

Carbene adducts of magnesium and zinc

Abstract The syntheses and characterizations of adducts of dialkylmagnesium and dialkylzinc with nucleophilic carbenes are described. These carbene adducts are available directly from the reaction of the stable nucleophilic carbenes 1,3-dimesitylimidazol-2-ylidene or 1,3-di(1-adamantyl)imidazol-2-ylidene and the dialkylmetal compounds. The X-ray crystal structures of the 1,3-di(1-adamantyl)imidazol-2-ylidene · diethylzinc adduct and 1,3-dimesitylimidazol-2-ylidene · diethylmagnesium are reported. The adamantyl substituted carbene gave a monomeric metal adduct whereas the mesityl substituted carbene shows a structure with bridging alkyl groups.

C−H Insertion Reactions of Nucleophilic Carbenes

Syntheses and characterizations are described for C−H insertion products derived from 1,3-dimesityldihydroimidazol-2-ylidene (1) with acetylene, acetonitrile, methyl phenyl sulfone, and chloroform. In the reaction with acetylene, both acetylenic H-atoms are reactive so that 1 : 1 and 2 : 1 adducts can be obtained. The acetylene and methyl-phenyl-sulfone adducts are structurally characterized by means of single-crystal X-ray structure determinations. The reactions of 1,3,4,5-tetramethylimidazolidin-2-ylidene (8) with chloroform or chlorodifluoromethane are shown to yield 2-(dihaloalkyl)imidazolium salts that arise from a failure of the intermediate 2-protioimidazolium salt to capture the initially formed halocarbanion.

Reactions of imidazol(in)-2-ylidenes with electron deficient fluoroolefins

GRAPHICAL ABSTRACT ABSTRACT 1,3-Bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene forms stable 1:1 adducts with tetrafluoroethylene (2), hexafluorocyclobutene (3), and octafluorocyclopentene (4). Adduct 2 shows properties typical for nonpolarized olefins, as indicated by NMR spectroscopy and X-ray crystallography. By contrast, adducts 3 and 4 are best described as ylides with a significant charge separation between the imidazoline ring and the perfluorocycloalkyl unit. Similarly, 1,3-di-1-adamantylimidazol-2-ylidene reacts with tetrakis(trifluoromethyl)allene to form a polarized trimethylenemethane derivative, and bis(trifluoromethyl)ketene to form an imidazolium enolate zwitterion. The synthesis and characterization of a number of fluorinated methyleneimidazolines are described herein.

Carbene Adducts of Dimethylcadmium

The first examples of carbene-cadmium complexes are reported from the reactions of a variety of imidazol-2-ylidenes or imidazolin-2-ylidenes with dimethylcadmium. Four new carbene complexes are characterized by NMR spectroscopy (1H , 13C and 113Cd). The cadmium centers are strongly shifted downfield (100 -150 ppm) by interaction with the carbenes. X-ray structures are reported for three carbene-cadmium 1:1 adducts. The cadmium centers exhibit distorted trigonal-planar geometries in which the carbene ligands have an average 18.2 pm longer bond distance to cadmium compared to the methyl groups. The planes of cadmium coordination are twisted with respect to the plane of the imidazole ring. The more basic imidazolin-2-ylidene is shown to displace imidazol-2-ylidenes from the cadmium center.

Preparation of a cross-linked polycarbosilane and its conversion to silicon carbide ceramics

SummaryCross-linked polycarbosilanes are obtained from the reaction of Cl2MeSiCHCl2 and Mg in tetrahydrofuran, followed by reduction with LiAlH4. Analysis by NMR spectroscopy shows that most polycarbosilane is of the formula [MeSiCH]n.ZusammenfassungDurch Reaktion von Cl2MeSiCHCl2 mit Mg in Tetrahydrofuran und anschließende Reduktion mit LiAlH4 erhält man vernetzte Polycarbosilane. NMR-spektroskopische Untersuchungen zeigen, daß das Polycarbosilan überwiegend als [MeSiCH]n vorliegt.