Alkali‐Metal‐Mediated Magnesiations of an N‐Heterocyclic Carbene: Normal,Abnormal, and “Paranormal” Reactivity in a Single Tritopic Molecule

Herein the sodium alkylmagnesium amide [Na₄Mg₂(TMP)₆(nBu)₂] (TMP=2,2,6,6‐tetramethylpiperidide), a template base as its deprotonating action is dictated primarily by its 12 atom ring structure, is studied with the common N‐heterocyclic carbene (NHC) IPr [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]. Remarkably, magnesiation of IPr occurs at the para‐position of an aryl substituent, sodiation occurs at the abnormal C4 position, and a dative bond occurs between normal C2 and sodium, all within a 20 atom ring structure accommodating two IPr^2?. Studies with different K/Mg and Na/Mg bimetallic bases led to two other magnesiated NHC structures containing two or three IPr^? monoanions bound to Mg through abnormal C4 sites. Synergistic in that magnesiation can only work through alkali‐metal mediation, these reactions add magnesium to the small cartel of metals capable of directly metalating a NHC.     

Direct C?H Metalation with Chromium(II) and Iron(II): Transition‐Metal Host / Benzenediide Guest Magnetic Inverse‐Crown Complexes

Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low‐polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.

     

Cover Picture: Direct C?H Metalation with Chromium(II) and Iron(II): Transition‐Metal Host / Benzenediide Guest Magnetic Inverse‐Crown Complexes (Angew. Chem. Int. Ed. 18/2009)

Chromation and ferration are the latest additions to the concept of alkali‐metal‐mediated metalation, as described by J. Klett, R. E. Mulvey, and co‐workers in their Communication on page 3317 ff. While the more electropositive sodium is essential for the reaction, it is the less electropositive chromium or iron that actually performs deprotonation of benzene. This novel reactivity can be likened to a game of chess in which the queen (Na) holds the king in check, while the knight (Cr, Fe) scores checkm(etal)ate.

     

Recent synthetic applications of manganese in organic synthesis

While the organometallic compounds derived from many metals have found a broad application in organic synthesis, the use of organomanganese compounds has only recently been developed due to the passivity exhibited by commercial Mn in the direct metalation of organic compounds. In this Concept article, we highlight the potential of manganese and its organometallic compounds in organic synthesis by illustration of the studies previously reported by others and our laboratory in this field. Based on the transformations reported herein, organomanganese compounds could become important tools in the future of the organic synthesis, due to their high selectivity.     

Structural and Magnetic Insights into the Trinuclear Ferrocenophane and Unexpected Hydrido Inverse Crown Products of Alkali‐Metal‐Mediated Manganation(II) of Ferrocene

With the aim of introducing the diisopropylamide [NiPr₂] ^? ligand to alkali‐metal‐mediated manganation (AMMMn) chemistry, the temperature‐dependent reactions of a 1:1:3 mixture of butylsodium, bis(trimethylsilylmethyl)manganese(II), and diisopropylamine with ferrocene in hexane/toluene have been investigated. Performed at reflux temperature, the reaction affords the surprising, ferrocene‐free, hydrido product [Na₂Mn₂ (μ‐H)₂{N(iPr)₂}₄]?2 toluene ( 1 ), the first Mn hydrido inverse crown complex. Repeating the reaction rationally, excluding ferrocene, produces 1 in an isolated crystalline yield of 62 %. At lower temperatures, the same bimetallic amide mixture leads to the manganation of ferrocene to generate the first trimanganese, trinuclear ferrocenophane, [{Fe(C₅H₄)₂}₃{Mn₃Na₂(NiPr₂)₂ (HNiPr₂)₂}] ( 2 ) in an isolated crystalline yield of 81 %. Both 1 and 2 have been characterised by X‐ray crystallographic studies. The magnetic properties of paramagnetic 1 and 2 have also been examined by variable‐temperature magnetisation measurements on powdered samples. For 1 , the room‐temperature value for χT is 3.45 cm³ K mol^?1, and on lowering the temperature a strong antiferromagnetic coupling between the two Mn ions is observed. For 2 , the room‐temperature value for χT is 4.06 cm³ K mol^?1, which is significantly lower than the expected value for three isolated paramagnetic Mn^II ions.     

Deprotonative metalation using ate compounds: synergy, synthesis, and structure building

Historically, single-metal organometallic species such as organolithium compounds have been the reagents of choice in synthetic organic chemistry for performing deprotonation reactions. Over the past few years, a complementary new class of metalating agents has started to emerge. Owing to a variable central metal (magnesium, zinc, or aluminum), variable ligands (both in their nature and number), and a variable second metallic center (an alkali metal such as lithium or sodium), "ate" complexes are highly versatile bases that exhibit a synergic chemistry which cannot be replicated by the homometallic magnesium, zinc, or aluminum compounds on their own. Deprotonation accomplished by using these organometallic ate complexes has opened up new perspectives in organic chemistry with unprecedented reactivities and sometimes unusual and unpredictable regioselectivities.     

Insights into the Metalation of Benzene and Toluene by Schlosser’s Base: A Superbasic Cluster Comprising PhK,PhLi, and tBuOLi

The metalation of benzene by Schlosser’s base (nBuLi/tBuOK) occurs smoothly in THF at low temperatures to afford a discrete mixed‐metal Li₂K₄ cluster that contains phenyl anions and tert‐butoxide. The aggregate itself exhibits superbasic behavior by metalating toluene. The delocalized benzyl anion obtained this way π bonds to potassium counterions, thereby creating a 2D coordination polymer.     

Synthesis and Ambiphilic Reactivity of Metalated Diorgano-Phosphonite Boranes

Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.     

Synergic Deprotonation Generates Alkali-Metal Salts of Tethered Fluorenide-NHC Ligands Co-Complexed to Alkali-Metal Amides

Synergic combinations of alkali-metal hydrocarbyl/amide reagents were used to synthesise saturated N-heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li₂ derivatives displayed a bridging amide between two Li atoms within the fluorenide-NHC pocket, whereas the Na₂ and K₂ analogues displayed extended solid-state structures with the fluorenide-NHC ligand chelating one alkali metal centre.