The synthesis and spectroscopic characterization of previously unprecedented sulfur-stabilized silicon cations are reported. Several 1,3-dithiolan-2-yl- and 1,3-dithian-2-yl-substituted silanes were prepared and successfully transformed into the corresponding silicon cations by hydride abstraction. The silicon–sulfur interaction creates three consecutive stereocenters at three different elements. It is remarkable that the present stereocenter at the silicon atom determines the stereochemical outcome at the formerly prochiral sulfur and carbon atoms with excellent diastereoselectivity. All sulfur-stabilized silicon cations are shown to be potent catalysts in a challenging Diels–Alder reaction. Moreover, structurally related oxazoline-stabilized silicon cations were generated and characterized but found to be unreactive.

[NiFe] hydrogenases catalyze the reversible oxidation of molecular hydrogen into two protons and two electrons. A key organometallic chemistry feature of the NiFe active site is that the iron atom is co-coordinated by two cyanides (CN–) and one carbon monoxide (CO) ligand. Biosynthesis of the NiFe­(CN)2(CO) cofactor requires the activity of at least six maturation proteins, designated HypA–F. An additional maturase, HypX, is required for CO ligand synthesis under aerobic conditions, and preliminary in vivo data indicated that HypX releases CO using N 10-formyltetrahydrofolate (N 10-formyl-THF) as the substrate. HypX has a bipartite structure composed of an N-terminal module similar to N 10-formyl-THF transferases and a C-terminal module homologous to enoyl-CoA hydratases/isomerases. This composition suggested that CO production takes place in two consecutive reactions. Here, we present in vitro evidence that purified HypX first transfers the formyl group of N 10-formyl-THF to produce formyl-coenzyme A (formyl-CoA) as a central reaction intermediate. In a second step, formyl-CoA is decarbonylated, resulting in free CoA and carbon monoxide. Purified HypX proved to be metal-free, which makes it a unique catalyst among the group of CO-releasing enzymes.

The preparation of a chiral derivative of [B(C6F5)4]– in which the fluorine atom in the para position of each of the C6F5 groups is replaced by a 1,1′‐binaphthalen‐2‐yl group is described. The new counteranion was isolated as its lithium, sodium, and trityl salts. The chiral trityl salt was then used as a catalyst in selected counteranion‐directed Diels–Alder reactions and a Mukaiyama aldol addition, but no asymmetric induction was achieved. Application of the chiral trityl salt to the generation of silicon cations by silicon‐to‐carbon hydride transfer from hydrosilanes failed, presumably as a result of the incompatibility of the relatively electron‐rich naphthyl groups in the borate and the cationic silicon electrophiles.

The synthesis and characterization of chiral [B(C 6 F 5 ) 4 ]derivatives bearing a myrtanyl group instead of a fluoro substituent in the para position are described. These new chiral borates were isolated as their bench-stable lithium, sodium, and cesium salts. The corresponding trityl salts were prepared and tested as catalysts in representative counter- [a] 7240 Scheme 3. Representative trityl-cation-catalyzed Diels-Alder reactions of cyclohexa-1,3-diene (14) with different enophiles.

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