A Convenient Preparation of Organofluorosilanes. A Possible Involvement of Tetracoordinated Siliconium Ion Pairs

Organofluorosilanes and their complexes with weakly coordinating fluorine con- taining salts are frequently used in mechanistic studies of organosilicon compounds.1 Examples include studies of nucleophilic attack at silicon2 and attempts to generate silicenium ions of type R3Si+.3’4 Silyl fluorides are also used in key steps in the synthesis of sterically hindered tetraalkylsilanes5’6 or of highly congested trialkylsilyl halides (chlorides, bromides and iodides).6’7 In view of these important applications, it is surprising that until very recently the available synthetic methods for preparing silyl fluorides were limited and used quite inconvenient chemicals, procedures and reaction conditions. Thus, organofluorosilanes were prepared by the treatment of the corresponding silyl halides with anhydrous hydrofluoric acid,8 antimony trifluoride,9 or zinc fluoride.7,10 Only one report in the older literature indicated that triphenylfluorosilane can be prepared under much milder conditions, i.e., by the reaction of the corresponding chloride with sodium fluoroborate in acetone at room temperature.11 The lack of simple straightforward methods for preparing silyl fluorides is especially intriguing in view of the fact that the Si-F bond is the strongest known bond to silicon, and consequently fluorosilanes are expected to exhibit high thermodynamic stability.12 Progress towards a more simple synthesis of fluorosilanes was made in recent years. Bassindale and Stout noted that the reaction of Me3SiCl with silver tetrafluoroborate yields trimethylfluorosilane138 (not trimethylsilyl tetrafluoroborate as previously suggested).136 Della and Tsanaktidis reported in 1988 that silyl triflates react at 25-70 °C with potassium fluoride in DMF in the presence of 18-crown-6 to produce the corresponding silyl fluorides.1

Isolation of cationic and neutral (allenylidene)(carbene) and bis(allenylidene)gold complexes.

The one-electron reduction of a cationic (allenylidene)[cyclic(alkyl) (amino)carbene]gold(i) complex leads to the corresponding neutral, paramagnetic, formally gold(0) complex. DFT calculations reveal that the spin density of this highly robust coinage metal complex is mainly located on the allenylidene fragment, with only 1.8 and 3.1% on the gold center and the CAAC ligand, respectively. In addition, the first homoleptic bis(allenylidene)gold(i) complex has been prepared and fully characterized

Conjugation, Resonance, and Stability in N-Heterocyclic Silylenes and in Phosphorus Ylide Substituted Silylenes

The role of π conjugation in the thermodynamic stabilization of N-heterocyclic silylenes and phosphorus ylide substituted silylenes is analyzed using the block localized wave function (BLW) method. This method enables the <i>direct</i> calculation of the resonance stabilization energies caused by π-electron delocalization and/or by cyclic 6-π-electron delocalization (aromaticity). The major advantage of the BLW method is that there is no need for reference compounds, as the fully conjugated molecule itself serves as the reference compound. The observed high stability of the CC unsaturated N-heterocyclic silylenes <b>1</b> (E = Si; silaimidazol-2-ylidene) and the C–C saturated <b>2</b> (E = Si; silaimidazolin-2-ylidene) is rationalized by their <i>high resonance energies</i> (REs) of 79.4 and 53.4 kcal/mol, respectively. The nuclear independent chemical shift (NICS_<i>zz</i>(1.0)) value of −22.0 for <b>1</b> (E = Si) indicates significant diatropic ring current, implying the existence of aromaticity. The additional stabilization of <b>1</b> due to aromaticity (6-π-electron delocalization) is 14 kcal/mol, which is only 18% of the total RE of <b>1</b> but is 55% of the aromatic stabilization of benzene. Thus, aromaticity contributes to the stability of <b>1</b>, but it is not a prerequisite for the isolation of N-heterocyclic silylenes. Cyclic unsaturated silylenes substituted by exocyclic phosphorus ylide substituents, i.e., <b>12</b>, have calculated cyclic resonance energies and thermodynamic stabilization energies similar to those of <b>1</b> (E = Si), and they are significantly larger than those of saturated <b>2</b> (E = Si); i.e., the REs of <b>12</b> (R″ = CH₃, SiH₃) are 73.4 and 75.6 kcal/mol, respectively. These data suggest that <b>12</b> is sufficiently stable to be isolated, as indeed was recently reported. The energy of the lone pair orbital on Si in <b>12</b> (R″ = Ph) is higher by 1.9 eV than that of <b>1</b> (E = Si, R = <i>t-</i>Bu), suggesting that <b>12</b> are better σ donors and thus may exhibit higher activity in transition-metal catalysis