Synthesis of Superbulky Amide Ligands by Addition of Polar Reagents to Sila-Imine

The chemistry of extremely bulky amide ligands is troubled by difficulties in deprotonation of the parent amine. As an alternative route to superbulky amide reagents, the addition of polar reagents to a sila-imine has been investigated. Attempts to synthesize the superbulky amide anion (tBu3Si)2N- by addition of tBuLi to tBu2Si=N(SitBu3) failed and gave tBu3Si(tBu2HSi)NLi and isobutene. Reaction of the sila-imine with KOtBu successfully led to tBu3Si[tBu2(tBuO)Si]NK which crystallized as a separated ion-pair. Reaction with the slightly bulkier KOAd (Ad=1-adamantyl) led in presence of THF to ether ring-opening. Reaction with tBuOH gave tBu3Si[tBu2(tBuO)Si]NH but this amine cannot be easily deprotonated. Reaction with (BDI*)MgnBu in presence of THF gave (BDI*)Mg+ ⋅ (THF)2 and the non-coordinating anion tBu3Si[tBu2(nBu)Si]N-; BDI*=ß-diketiminate ligand HC[C(tBu)N-DIPP]2, DIPP=2,6-diisopropylphenyl. Reaction of Mg(nBu)2 with tBu2Si=N(SitBu3) led to a Mg complex with one amide ligand: tBu3Si[tBu2(nBu)Si]N-. The other superbulky amide anion isomerized by internal deprotonation of a tBu-substituent to give a primary carbanion that is also coordinated to Mg. Although the amide-to-carbanion isomerization is highly contrathermodynamic, it allows for coordination of both anions to a single Mg center. The new bulky amides are rare cases of halogen-free weakly coordinating anions.

Planar Hexacoordinate Beryllium: Covalent Bonding Between s-block Metals

Achieving a planar hypercoordinate arrangement of s-block metals through covalent bonding with ligands is challenging due to the strong ionicity involved. Herein, we report the first case of a neutral binary global minimum containing a planar hexacoordinate beryllium atom. The central Be atom is coordinated by six active Be atoms, the latter in turn are enclosed by an equal number of more electronegative chlorine atoms in the periphery, forming a star-like phBe cluster (Be©Be6 Cl6 ). Importantly, the cluster exhibits dynamically stabilized stemming geometrically from the appropriate matching of metal-ligand size and electronically from adherence to the octet rule as well as possessing a 6σ/2π double aromaticity. Remarkably, energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) analysis reveals a significant covalent interaction between the ligand and the central metal beryllium atoms, a fact further supported by a large Wiberg bond index. This cluster is a promising synthetic as its excellent electronic, dynamic and thermodynamic stability.

Isolation and Reactivity of an Antiaromatic s-Block Metal Compound

The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation-state flexibility of the s-block metals has long stood in the way of their participation in sophisticated π-bonding arrangements, and truly antiaromatic systems containing s-block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis-base-coordination chemistry.

Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar₂P(O)H (Pudovik reaction) yields E/Z‐isomer mixtures of phenylethenyl‐di(aryl)phosphane oxides (1). Alkali and alkaline‐earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s‐block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P‐bound alkyl and alcoholate substituents impede the P−H addition across alkynes. The demanding mesityl group favors the single‐hydrophosphorylated products 1‐Ar whereas smaller aryl substituents lead to the double‐hydrophosphorylated products 2‐Ar. Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes₂P(O)H does not form the bis‐phosphorylated product 2‐Mes, activation of an ortho‐methyl group and cyclization occurs yielding 2‐benzyl‐1‐mesityl‐5,7‐dimethyl‐2,3‐dihydrophosphindole 1‐oxide (3).

Structural and Mechanistic Insights into s-Block Bimetallic Catalysis: Sodium Magnesiate-Catalyzed Guanylation of Amines

To advance the catalytic applications of s-block mixed-metal complexes, sodium magnesiate [NaMg(CH₂ SiMe₃ )₃ ] (1) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed-metal system is much greater than that of its homometallic components [NaCH₂ SiMe₃ ] and [Mg(CH₂ SiMe₃ )₂ ]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N'-diisopropylcarbodiimide with 4-tert-butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine-assisted rate-determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.