Reversible CO2 Addition to a Si=O Bond and Synthesis of a Persistent SiO2 -CO2 Cycloadduct Stabilized by a Lewis Donor-Acceptor Ligand.

The donor-stabilized sila-ß-lactone 1 reacts with CO2 via a remarkable reversible [2+2]-cycloaddition reaction to form the spiro-cyclic silicon carbonate derivative 2. Furthermore, photolysis of 2 under pressure of CO2 affords the first persistent SiO2 -CO2 cycloadduct 3, presenting a Si2 O4 -like structure, which is stabilized by a Lewis donor-acceptor type ligand. As predicted by theoretical calculations, in marked contrast to the thermodynamically stable SiO2 dimer, the SiO2 -CO2 mixed cycloadduct 3 is labile and readily releases CO2 .

Cyclic (Amino)(Phosphonium Bora-Ylide)Silanone: A Remarkable Room-Temperature-Persistent Silanone.

A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium bora-ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t1/2 =4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t1/2 =5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr2 increases further its stability (no degradation after 3 weeks at room temperature).

Stereodivergent Mannich reaction of bis(trimethylsilyl)ketene acetals with N-tert-butanesulfinyl imines by Lewis acid or Lewis base activation, a one-pot protocol to obtain chiral ß-amino acids.

We report a one-pot synthesis of chiral ß2,2,3-amino acids by the Mannich addition of bistrimethylsilyl ketene acetals to N-tert-butanesulfinyl imines followed by the removal of the chiral auxiliary. The synthesis and isolation of pure ß-amino acid hydrochlorides were conducted under mild conditions, without strong bases and this method is operationally simple. The stereoselective reaction was promoted by two different activation methods that lead to different stereoisomers: (1) Lewis Acid (LA) catalysis with boron trifluoride diethyl etherate and (2) Lewis Base (LB) catalysis with tetrabutylammonium difluorotriphenylsilicate. The reaction presented good diastereoselectivity with LB activation and moderate to good dr with LA catalysis. The exceptions in both protocols were imines with electron donating groups in the aromatic ring.

Exceptionally Strong Electron-Donating Ability of Bora-Ylide Substituent vis-à-vis Silylene and Silylium Ion.

Electropositive boron-based substituent (phosphonium bora-ylide) with an exceptionally strong π- and σ-electron donating character dramatically increases the stability of a new type of N-heterocyclic silylene 2 featuring amino- and bora-ylide-substituents. Moreover, the related silylium ion 4 and transition-metal-silylene complexes, with trigonal-planar geometries around the silicon center, are also well stabilized. Therefore, the N,B-heterocyclic silylene 2 can be used as a strongly electron-donating innocent ligand in coordination chemistry similarly to N-heterocyclic carbenes.

A Fairly Stable Crystalline Silanone.

Silanones 2 substituted by bulky amino- and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron-donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2 =0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2 Si=O owing to the strong π-electron donation from the phosphonium ylide substituent.

The Lightest Element Phosphoranylidene: NHC-Supported Cyclic Borylidene-Phosphorane with Significant B=P Character.

A borylidene-phosphorane, the lightest phosphoranylidene, which is stabilized by an N-heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π-back bonding related to the less electronegative boron compared to phosphorus.

Cyclic Amino(Ylide) Silylene: A Stable Heterocyclic Silylene with Strongly Electron-Donating Character.

An isolable heterocyclic silylene (4) with two different π-donating substituents, namely a classical amino group and a more electropositive and stronger carbon-based π-donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π-donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4. Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N-heterocyclic carbenes, in contrast to classical N-heterocyclic silylenes, which generally present a weak donating character.