Sandra Künzler

Inorganic versus organic: the molecular structures and Lewis basicities of triphenylphosphine and tris(2‐borazinyl)phosphine appear similar but their hydrolysis and oxygenation chemistry could hardly be more contrasting.

Abstract

A completely inorganic version of one of the most famous organophosphorus compounds, triphenylphosphine, has been prepared. A comparison of the crystal structures of inorganic triphenylphosphine, PBaz₃ (where Baz=B₃H₂N₃H₃) and PPh₃ shows that they have superficial similarities and furthermore, the Lewis basicities of the two compounds are remarkably similar. However, their oxygenation and hydrolysis reactions are starkly different. PBaz₃ reacts quantitatively with water to give PH₃ and with the oxidizing agent ONMe₃ to give the triply‐O‐inserted product P(OBaz)₃, an inorganic version of triphenyl phosphite; a corresponding transformation with PPh₃ is inconceivable. Thermodynamically, what drives these striking differences in the chemistry of PBaz₃ and PPh₃ is the great strength of the B−O bond.

Various unprotected carboxylic acids undergo enantioselective aldol reactions in the presence of a chiral phosphine oxide as a Lewis base catalyst. The carboxylic acids were activated with silicon tetrachloride to form the bis(trichlorosilyl)enediolates in situ, which subsequently underwent an aldol reaction with an aldehyde or a ketone to produce β‐hydroxycarboxylic acids in high enantioselectivities of up to 92 % ee.

Abstract

The first catalytic enantioselective aldol reaction of various unprotected carboxylic acids is described. In the presence of a chiral bis(phosphine oxide) as a Lewis base catalyst, carboxylic acids were activated with silicon tetrachloride to form the corresponding bis(trichlorosilyl)enediolates in situ, which subsequently underwent an aldol reaction with an aldehyde or a ketone to produce β‐hydroxycarboxylic acids in high enantioselectivities of up to 92 % ee.

Ruthenium complexes are reactive to NCp7 and exhibit significantly different reactivity. Ruthenium binding results in zinc‐ejection from NCp7 and disrupts recognition of the protein to nucleic acids. In addition, the C‐terminal domain of NCp7 is much more reactive than the N‐terminal domain to ruthenium complexes. This work found a new type of inhibitors of NCp7. The different reactivity of the ruthenium complex suggests that targeting NCp7 could be modulated by ligand modification.

Abstract

Nucleocapsid protein 7 (NCp7) is an attractive target for anti‐HIV drug development. Here we found that ruthenium complexes are reactive to NCp7 and various Ru‐agents exhibit significantly different reactivity. Interestingly, the zinc‐finger domains of NCp7 also demonstrate different affinity to Ru‐complexes; the C‐terminal domain is much more reactive than the N‐terminal domain. Each zinc‐finger domain of NCp7 binds up to three Ru‐motifs, and the ruthenium binding causes zinc‐ejection from NCp7 and disrupts the protein folding. Therefore, ruthenium complexes interfere with the DNA binding of NCp7 and interrupt the protein function. The different reactivity of Ru‐agents suggests a feasible strategy for improving the targeting of NCp7 by ligand design. This work provides an insight into the mechanism of ruthenium complex with NCp7, and suggests more potential application of ruthenium drugs.

Building polystannanes: A simple light‐ and moisture‐stable semi‐crystalline hypercoordinated polystannane is revealed: A key design feature for asymmetrical polystannanes is the use of strongly dative‐bonded ligands that render the majority of tin(IV) centers of this main‐group inorganic polymer pentacoordinate, electron rich and protected from nucleophilic attack.

Abstract

The synthesis and solid‐state molecular structures of two dichlorido(aryl)(alkyl) tin compounds, 5 and 8, both key intermediates to tunable polystannane architectures, are reported. The materials were further investigated by single‐crystal XRD and a DFT analysis of their preferential “open and closed” geometries. Conversion of said compounds to their dihydride analogues was undertaken, followed by their application as monomers for polystannane polymer synthesis. The properties of two asymmetrical polystannanes prepared by transition‐metal‐catalyzed dehydropolymerization of dihydrido(aryl)alkylstannanes (6 and 9) were investigated. The first product was the structurally simple, modest molecular weight, semi‐crystalline light‐ and moisture‐stable polystannane 10 with NMR (¹¹⁹Sn) evidence of prominent Sn←O hypercoordination along the polymer backbone. The second was the lower molecular weight, tosylated four‐coordinate polystannane 11 with no evidence of hypercoordination. Differential scanning calorimetry (DSC) of polymer 10 revealed a reversible semi‐crystalline nature, whereas an amorphous character was detected for polymer 11. Polystannane 10 was also found to be exceptionally stable to both moisture and light (>6 months) and a promising candidate for the design of readily modified (i.e., tunable) polystannane materials.

Double duty: The combination of B(C₆F₅)₃ and an organic base, such as triethylenediamine (DABCO), serve as an excellent catalytic system for the C(sp)−H silylation of terminal alkynes with hydrosilanes. DABCO plays two crucial roles (Lewis base and Brønsted base) as revealed by experimental and DFT studies.

Abstract

Transition metal catalyzed C−H functionalization of organic compounds has proved to be a useful atom‐efficient strategy in organic synthesis. In contrast, main‐group‐element‐based catalytic processes for C−H functionalization have remained underexplored to date. Reported herein is the catalytic C(sp)−H silylation of a wide range of terminal alkynes with hydrosilanes by using a combination of B(C₆F₅)₃ and an organic base such as triethylenediamine (DABCO). This protocol constitutes the first example of boron‐catalyzed C(sp)−H functionalization, offering a convenient route for the synthesis of a variety of alkynylsilanes. Experimental and computational studies have revealed that DABCO plays two crucial roles (Lewis base and Brønsted base) in this catalytic transformation.

Activating H₂ : The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me₂E‐CH₂‐E′Ph₂; see scheme) is analyzed in detail by means of computational methods. The geminal N/Al FLP is identified as the most active system for the dihydrogen activation reaction.

Abstract

The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me₂E‐CH₂‐E′Ph₂) has been computationally explored within the density functional theory framework. To this end, the dihydrogen‐activation reaction, one of the most representative processes in the chemistry of FLPs, has been selected. It is found that the activation barrier of this transformation as well as the geometry of the corresponding transition states strongly depend on the nature of the E/E′ atoms (E=Group 15 element, E′=Group 13 element) in the sense that lower barriers are associated with earlier transition states. Our calculations identify the geminal N/Al FLP as the most active system for the activation of dihydrogen. Moreover, the barrier height can be further reduced by replacing the phenyl group attached to the acidic atom by C₆F₅ or 3,5‐(CF₃)₂C₆H₃ (Fxyl) groups. The physical factors controlling the computed reactivity trends are quantitatively described in detail by means of the activation strain model of reactivity combined with the energy decomposition analysis method.

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