Development of alkali-metal-mediated metallation, advances with potassium and aluminium

To date alkali-metal-mediated metallation (AMMM) chemistry has focused mainly on lithium and sodium methodologies. This project sought to develop the meagrely studied concepts of potassium-mediated zincation, magnesiation, and alumination. Five potassium dialkyl TMP-zincate bases [PMDETA·K(μ-TMP)(μ-A)Zn(A)] {A = Me (3a), Et (3b), iPr, nBu, or tBu (3e)} have been synthesised by a cocomplexation procedure. Reactions of 3b with a series of substituted 4-R-pyridines (R = Me₂N, H, Et, iPr, tBu, Ph) gave 2-zincated products of formulation [{PMDETA·K[2-Zn(Et₂)-4-R-C₅H₃N]}₂] (3j-3o, respectively). X-ray crystallographic and NMR spectroscopic characterisation of 3j-3o revealed a novel structural motif comprising a dianionic dihydroanthracene-like tricyclic ring with a central diazadicarbadizinca (NCZn)₂ ring, face-capped on either side by PMDETA-wrapped K+ cations. All the new zincated pyridine complexes share this dimeric arrangement. 3b selectively deprotonates 4-methoxypyridine at the 3-position affording a complex with a remarkably different, step-ladder structure catena-{PMDETA·K[3-Zn(Et)₂-4-MeO-C₅H₃N]}∞]. These studies established the importance of Zn-N bonds for efficient zincation. Direct zincation of ferrocene (FcH) and ruthenocene (RcH) by the synergic base 3a produced the monozincated complexes [PMDETA·K(μ-Me)₂Zn(Fc)]∞ and [{PMDETA·K(μ-Me)₂Zn(Rc)}₂] respectively. Dizincation of ferrocene can be achieved with base 3e to yield the complex [THF·{(PMDETA)K}₂·(C₅{1-Zn(tBu)₂}H₄)Fe(C₅{1'-Zn(tBu)₂}H₄)]. Potassium-mediated magnesiation of anisole via [PMDETA·K(μ-TMP)(μ-CH₂SiMe₃)Mg(TMP)] was followed by a combination of X-ray crystallographic and NMR spectroscopic studies. Departing from the behaviour of other heteroleptic alkyl-amido bases, the reaction stops first at an ortho-magnesiated anisole intermediate with elimination of amine, but finally ends at an ortho-magnesiated anisole complex with reincorporatation of amine ligand and elimination of alkane. Amide enriched iBu₂Al(TMP)₂Li acts as a single-fold amido base towards TMEDA or PMDETA to afford [Li{Me₂NCH₂CH₂N(Me)CH₂}(TMP)Al(iBu)₂] and [Li{Me₂NCH₂CH₂N(Me)CH₂CH₂N(Me)CH₂}(TMP)Al(iBu)₂] respectively. Long utilised as a strong Brønsted base, the TMP anion converts to a Brønsted acid under the persuasion of a potassium aluminate KTMP, TMEDA, iBu₂Al(TMP) mixture, in the most remarkable reaction of the project.To date alkali-metal-mediated metallation (AMMM) chemistry has focused mainly on lithium and sodium methodologies. This project sought to develop the meagrely studied concepts of potassium-mediated zincation, magnesiation, and alumination. Five potassium dialkyl TMP-zincate bases [PMDETA·K(μ-TMP)(μ-A)Zn(A)] {A = Me (3a), Et (3b), iPr, nBu, or tBu (3e)} have been synthesised by a cocomplexation procedure. Reactions of 3b with a series of substituted 4-R-pyridines (R = Me₂N, H, Et, iPr, tBu, Ph) gave 2-zincated products of formulation [{PMDETA·K[2-Zn(Et₂)-4-R-C₅H₃N]}₂] (3j-3o, respectively). X-ray crystallographic and NMR spectroscopic characterisation of 3j-3o revealed a novel structural motif comprising a dianionic dihydroanthracene-like tricyclic ring with a central diazadicarbadizinca (NCZn)₂ ring, face-capped on either side by PMDETA-wrapped K+ cations. All the new zincated pyridine complexes share this dimeric arrangement. 3b selectively deprotonates 4-methoxypyridine at the 3-position affording a complex with a remarkably different, step-ladder structure catena-{PMDETA·K[3-Zn(Et)₂-4-MeO-C₅H₃N]}∞]. These studies established the importance of Zn-N bonds for efficient zincation. Direct zincation of ferrocene (FcH) and ruthenocene (RcH) by the synergic base 3a produced the monozincated complexes [PMDETA·K(μ-Me)₂Zn(Fc)]∞ and [{PMDETA·K(μ-Me)₂Zn(Rc)}₂] respectively. Dizincation of ferrocene can be achieved with base 3e to yield the complex [THF·{(PMDETA)K}₂·(C₅{1-Zn(tBu)₂}H₄)Fe(C₅{1'-Zn(tBu)₂}H₄)]. Potassium-mediated magnesiation of anisole via [PMDETA·K(μ-TMP)(μ-CH₂SiMe₃)Mg(TMP)] was followed by a combination of X-ray crystallographic and NMR spectroscopic studies. Departing from the behaviour of other heteroleptic alkyl-amido bases, the reaction stops first at an ortho-magnesiated anisole intermediate with elimination of amine, but finally ends at an ortho-magnesiated anisole complex with reincorporatation of amine ligand and elimination of alkane. Amide enriched iBu₂Al(TMP)₂Li acts as a single-fold amido base towards TMEDA or PMDETA to afford [Li{Me₂NCH₂CH₂N(Me)CH₂}(TMP)Al(iBu)₂] and [Li{Me₂NCH₂CH₂N(Me)CH₂CH₂N(Me)CH₂}(TMP)Al(iBu)₂] respectively. Long utilised as a strong Brønsted base, the TMP anion converts to a Brønsted acid under the persuasion of a potassium aluminate KTMP, TMEDA, iBu₂Al(TMP) mixture, in the most remarkable reaction of the project