Trapping, stabilization, and characterization of an enolate anion of a 1,6-adduct of benzophenone chelated by a sodium alkylamidozincate cation

There has been a recent upsurge of activity in the study of alkali metal zincate reagents due to their often special reactivity/selectivity in, for example, deprotonative metalation and nucleophilic addition reactions. Heteroleptic dialkylamidozincates, [M+Zn(R)2(NR'2)-], usually transfer selectivity of the amide ligand to electrophiles. Here, in contrast, it is reported that the sodium zincate [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)] reacts as an alkylating agent toward the diaryl ketone benzophenone (Ph2C=O), selectively adding one of its tBu ligands to the para-C atom of one of the Ph rings. The reaction can be carried out at room temperature, which is a decided advantage over lithium reagents as these are generally utilized at subambient temperatures. The stabilizing effect of the bimetallic (Na, Zn) cationic residue of the starting zincate reagent in coordinating to the dearomatized enolate anion of the 1,6-addition adduct allows the adduct to be isolated in a pure crystalline form. An X-ray crystallographic study of the adduct reveals a molecular structure based on a near-planar, four-element (NaOZnN) ring with a TMP-N and an enolato-O bridge. The Na and Zn atoms also carry terminal TMEDA (N,N'-attached) and tBu (C-attached) ligands, respectively. Also included are 1H/13C NMR spectroscopic data for the adduct when dissolved in cyclohexane-d12 solution.     

Structurally-defined potassium-mediated regioselective zincation of amino- and alkoxy-substituted pyridines

The new synergic base [PMDETA.K(TMP)(Et)Zn(Et)] selectively zincates 4-(dimethylamino)pyridine at the 2-position and 4-methoxypyridine at the 3-position, to afford bimetallic potassium pyridylzinc complexes each displaying a novel, but remarkably different, structure.     

Synthesis and characterisation of new bimetallic alkali metal-magnesium mixed diisopropylamide-acetylides: structural variations in bimetallic lithium- and sodium-heteroleptic magnesiates

The reactivity of the Br?nsted basic mixed-metal tris-amide compounds of empirical formula [MMg(N(i)Pr2)3] [where M = Li (1), Na (2)] towards phenylacetylene (HC[triple bond, length as m-dash]CPh) has been investigated and has led to the synthesis of a series of mixed-metal acetylido-amido-magnesiates. Thus, 1 and 2 molar equivalents of the alkyne with [MMg(N(i)Pr2)3] produce heteroanionic bis(amido)-mono(acetylido) [LiMg(N(i)Pr2)2(C[triple bond, length as m-dash]CPh)]2 (3) and mono(amido)-bis(acetylido) [(TMEDA) x Na(C[triple bond, length as m-dash]CPh)2Mg(N(i)Pr2)](2) (4) (TMEDA = N,N,N',N'-tetramethylethylenediamine) respectively. X-Ray crystallographic studies reveal that the new compounds adopt a different structural motif. Complex can be defined as an inverse crown structure, having a cationic eight-atom [(NaNMgN)2]2+ ring which hosts in its core two acetylido ligands. On the other hand, adopts a tetranuclear NaMgMgNa near-linear chain arrangement, held together by acetylido and amido bridges. The metal coordination geometries in both structures are distorted tetrahedral, and the sodium cations at the end of the mixed-metal chain carry terminal chelating TMEDA ligands. 1H and 13C NMR spectral data recorded in C6D6 solutions are also reported for and , and are consistent with the solid-state structures being retained in solution.     

Highly Reactive Hydrocarbon Soluble Alkylsodium Reagents for Benzylic Aroylation of Toluenes using Weinreb Amides

Deaggregating the alkyl sodium NaCH₂SiMe₃ with polydentate nitrogen ligands enables the preparation and characterisation of new, hydrocarbon soluble chelated alkylsodium reagents. Equipped with significantly enhanced metalating power over their organolithium counterparts, these systems can promote controlled sodiation of weakly acidic benzylic C−H bonds from a series of toluene derivatives under mild stoichiometric conditions. This has been demonstrated through the benzylic aroylation of toluenes with Weinreb amides, that delivers a wide range of 2-arylacetophenones in good to excellent yields. Success in isolating and determining the structures of key organometallic intermediates has provided useful mechanistic insight into these new sodium-mediated transformations.     

Lewis base stabilized lithium TMP-aluminates: an unexpected fragmentation and capture reaction involving cyclic ether 1,4-dioxane

Three Lewis base variations of the synthetically useful aluminate [L x Li(TMP)((i)Bu)Al((i)Bu)2], where L is TMPH, Et3N or PhC(=O)N(i)Pr2, are reported, together with the reaction of the benzamide complex with 1,4-dioxane, which surprisingly leads to fragmentation of the cyclic ether and capture of its alkoxy vinyl ether residue within a novel dilithium dialuminium hexaalkyl aggregate.     

Isolation and structural elucidation of a key aluminoaromatic intermediate and evidence for dismutation phenomena in TMP-alumination chemistry

Lithium TMP-aluminate "(i)Bu(3)Al(TMP)Li" undergoes dismutation in THF solution to precipitate the tetraalkylaluminate [{Li.(THF)(4)}(+){Al((i)Bu)(4)}(-)], but reacts kinetically as a TMP base towards N,N-diisopropylbenzamide to afford the crystalline ortho-aluminated species [(THF)(3).Li{O([=C)N((i)Pr)(2)(C(6)H(4))}Al((i)Bu)(3)] and TMPH.     

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 IPr²⁻. 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.     

Expanding Mg-Zn hybrid chemistry: inorganic salt effects in addition reactions of organozinc reagents to trifluoroacetophenone and the implications for a synergistic lithium-magnesium-zinc activation

Numerous organic transformations rely on organozinc compounds made through salt-metathesis (exchange) reactions from organolithium or Grignard reagents with a suitable zinc precursor. By combining X-ray crystallography, NMR spectroscopy and DFT calculations, this study sheds new light on the constitution of the organometallic species involved in this important synthetic tool. Investigations into the metathesis reactions of equimolar amounts of Grignard reagents (RMgX) and ZnCl(2) in THF led to the isolation of novel magnesium-zinc hybrids, [{(thf)(2)Mg(μ-Cl)(3)ZnR}(2)] (R=Et, tBu, nBu or o-OMe-C(6)H(4)), which exhibit an unprecedented structural motif in mixed magnesium-zinc chemistry. Furthermore, theoretical modelling of the reaction of EtMgCl with ZnCl(2) reveals that formation of the mixed-metal compound is thermodynamically preferred to that of the expected homometallic products, RZnCl and MgCl(2). This study also assesses the alkylating ability of hybrid 3 towards the sensitive ketone trifluoroacetophenone, revealing a dramatic increase in the chemoselectivity of the reaction when LiCl is introduced as an additive. This observation, combined with recent related breakthroughs in synthesis, points towards the existence of a trilateral Li/Mg/Zn synergistic effect.     

Structurally Defined Potassium‐Mediated Zincation of Pyridine and 4‐R‐Substituted Pyridines (R=Et,iPr, tBu,Ph, and Me2N) by Using Dialkyl–TMP–Zincate Bases

Two potassium–dialkyl–TMP–zincate bases [(pmdeta)K(μ‐Et)(μ‐tmp)Zn(Et)] ( 1 ) (PMDETA=N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine, TMP=2,2,6,6‐tetramethylpiperidide), and [(pmdeta)K(μ‐nBu)(μ‐tmp)Zn(nBu)] ( 2 ), have been synthesized by a simple co‐complexation procedure. Treatment of 1 with a series of substituted 4‐R‐pyridines (R=Me₂N, H, Et, iPr, tBu, and Ph) gave 2‐zincated products of the general formula [{2‐Zn(Et)₂‐μ‐4‐R‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 3 – 8 , respectively) in isolated crystalline yields of 53, 16, 7, 23, 67, and 51 %, respectively; the treatment of 2 with 4‐tBu‐pyridine gave [{2‐Zn(nBu)₂‐μ‐4‐tBu‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 9 ) in an isolated crystalline yield of 58 %. Single‐crystal X‐ray crystallographic and NMR spectroscopic characterization of 3 – 9 revealed a novel structural motif consisting of a dianionic dihydroanthracene‐like tricyclic ring system with a central diazadicarbadizinca (ZnCN)₂ ring, face‐capped on either side by PMDETA‐wrapped K⁺ cations. All the new metalated pyridine complexes share this dimeric arrangement. As determined by NMR spectroscopic investigations of the reaction filtrates, those solutions producing 3 , 7 , 8 , and 9 appear to be essentially clean reactions, in contrast to those producing 4 , 5 , and 6 , which also contain laterally zincated coproducts. In all of these metalation reactions, the potassium–zincate base acts as an amido transfer agent with a subsequent ligand‐exchange mechanism (amido replacing alkyl) inhibited by the coordinative saturation, and thus, low Lewis acidity of the 4‐coordinate Zn centers in these dimeric molecules. Studies on analogous trialkyl–zincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn? N bonds for efficient zincation.