Ben Conway

Closer Insight into the Reactivity of TMP−Dialkyl Zincates in Directed ortho-Zincation of Anisole: Experimental Evidence of Amido Basicity and Structural Elucidation of Key Reaction Intermediates

The new dialkyl(aryl) lithium zincates [(THF)2Li(C6H4−OMe)MeZnMe] (4), [(TMEDA)Li(C6H4−OMe)MeZnMe] (6), [(THF)3Li(C6H4−OMe)tBuZntBu] (7), and [(PMDETA)Li(C6H4−OMe)tBuZntBu] (8) have been prepared by co-complexation reactions of lithiated anisole with the relevant dialkylzinc compound and the relevant Lewis base. These new heterobimetallic compounds have been characterized in solution using 1H, 13C{H}, and 7Li NMR spectroscopy, and the molecular structures of 6 and 8 have been elucidated by X-ray crystallography. In 6 the distinct metals are connected through the anisole ligand which binds in an ambidentate fashion (through carbon−zinc and oxygen−lithium contacts) and also through one of the methyl groups, to close a [LiOCCZnC] six-membered ring; whereas 8 displays an open structure where anisole connects the two metals (in the same mode as in 6) but with the tert-butyl groups exclusively bonded terminally to zinc. Reactivity studies of zincates 4 and 7 with the amine TMP(H) supply experimental evidence that these heterobimetallic compounds are intermediates in the two-step deprotonation reaction of anisole by TMP−dialkyl zincates and show the relevance of the alkyl groups in the efficiency of TMP−dialkyl zincate bases. In addition, important solvent effects have also been evaluated. When hexane is added to THF solutions of compounds 4 or 7, the homoleptic tetraorganozincate [(THF)2Li2Zn(C6H4−OMe)4] (5) is obtained as the result of a disproportionation process. This lithium-rich zincate has also been spectroscopically and crystallographically characterized.

Alkali‐Metal‐Mediated Manganation(II) of Functionalized Arenes and Applications of ortho‐Manganated Products in Pd‐Catalyzed Cross‐Coupling Reactions with Iodobenzene

Extending the recently introduced concept of “alkali-metal-mediated manganation” to functionalised arenes, the heteroleptic sodium manganate reagent [(tmeda)Na(tmp)(R)Mn(tmp)] (1; TMEDA = N,N,N′,N′-tetra-methylethylenediamine, TMP = 2,2,6,6-tetramethylpiperidide, R = CH2 SiMe3) has been treated with anisole or N,N-diisopropylbenzamide in a 1:1 stoichiometry in hexane. These reactions afforded the crystalline products [(tmeda)Na(tmp)(o-C6H4OMe)Mn(tmp)] (2) and [(tmeda)Na(tmp){o-{C(O)N(iPr)2C6H4}Mn(CH2SiMe3)] (3), respectively, as determined from X-ray crystallographic studies. On the basis of these products, it can be surmised that reagent 1 has acted, at least partially and ultimately, as an alkyl base in the first reaction liberating the silane Me4Si, but as an amido base in the second reaction liberating the amine TMPH. Both of these paramagnetic products 2 and 3 have contacted ion-pair structures, the key features of which are six-atom, five-element (NaNMnCCO) and seven-atom, five-element (NaNMnCCCO) rings, respectively. Manganates 2 and 3 were successfully cross-coupled with iodobenzene under [PdCl2(dppf)] (dppf=1,1′-bis(diphenylphosphino)ferrocene) catalysis to generate unsymmetrical biaryl compounds in yields of 98.0 and 66.2%, respectively. Emphasizing the importance of alkali-metal mediation in these manganation reactions, the bisalkyl Mn reagent on its own fails to metalate the said benzamide, but instead produces the monomeric, donor–acceptor complex [Mn(R)2{(iPr)2NC(Ph)(=O)}2] (5), which has also been crystallographically characterised. During one attempt to repeat the synthesis of 2, the butoxide-contaminated complex [{(tmeda)Na(R)(OBu)(o-C6H4OMe)Mn}2] (6) was obtained. In contrast to 2 and 3, due to reduced steric constraints, this complex adopts a dimeric arrangement in the crystal, the centrepiece of which is a twelve atom (NaOCCMnC)2 ring.

Tuning the Basicity of Synergic Bimetallic Reagents: Switching the Regioselectivity of the Direct Dimetalation of Toluene from 2,5‐ to 3,5‐Positions

Meta-meta metalation: Remarkably, toluene can be directly dimanganated or dimagnesiated at the 3,5-positions using bimetallic bases with active Me3SiCH2 ligands (see scheme, blue). In contrast, n-butyl ligands lead to 2,5-metalation (red). tmp=2,2,6,6-tetramethylpiperidide.

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

Abstract 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.

Structural tracking of the potassium-mediated magnesiation of anisole.

The potassium-mediated magnesiation of anisole has been monitored by a combination of X-ray crystallographic and NMR spectroscopic studies. Departing from a heteroleptic alkyl-amido base and anisole, the reaction first stops at an ortho-magnesiated anisole intermediate, but the final destination is an ortho-magnesiated anisole complex with reincorporation of the amine ligand and elimination of alkane (see picture).

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.

Structurally Stimulated Deprotonation/Alumination of the TMP Anion†

Long utilized as a strong Bronsted base, the tetramethylpiperidine (TMP) anion is forced to convert to a Bronsted acid in the presence of a potassium–aluminate species. The new compound [(TMEDA)K(μ-TMP*)(μ-iBu)Al(iBu)] was obtained where TMP* represents a CH3- and NH-deprotonated dianionic variant of TMP.

Structural insights into mono-amido tris-alkyl potassium aluminates

Co-complexation of a potassium amide with iBu3Al in the presence of the tridentate chelating ligand PMDETA results in a series of potassium aluminates of general formula [PMDETA·K(μ-amide)(μ-iBu)Al(iBu)2] [where amide = TMP (6); DMP (7); HMDS (8)] which have been characterized by solution NMR spectroscopy and X-ray crystallography. Their solid-state structures contain a central KNAlC ring whose degree of puckering is dictated by the differing interactions between the potassium centre and the aluminium bound anionic ligands.