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.

Steric Effects on the Structures, Reactivity, and Coordination Chemistry of Tris(2‐pyridyl)aluminates

“…Am ixture of compounds is observed by in situ 1 H and 13 (7), N2-Li1 2.046 (7), O1-Li1A1.964 (7), O1-Li1 1.980 (7);C py -Al1-C py 104.28 (15),C py -Al1-O1101.41 (14) in the chemical shifts of C2 of the 2-py group in the 13 CNMR spectrumc ompared to 5 suggested that at least some transfer of the 2-py group onto Zn 2 + had occurred. [19] This was confirmed by the isolation of af ew crystals of dimeric [LiZn(6-Brpy) 3 ] 2 (6)f rom the NMR-scale reaction and their structural characterization (see Figure 7). Compound 6 consists of ad imeric zincate [Zn(6-Br-2-py) 3 ] 2 2À dianion ion-paired with two Li + cations, in which full transfer of the 6-Br-2-py groups has occurred from Al to Zn.…”

“…However, the ca. 20 ppm increase in the chemical shifts of C2 of the 2‐py group in the 13 C NMR spectrum compared to 5 suggested that at least some transfer of the 2‐py group onto Zn 2+ had occurred 19. This was confirmed by the isolation of a few crystals of dimeric [LiZn(6‐Br‐py) 3 ] 2 ( 6 ) from the NMR‐scale reaction and their structural characterization (see Figure 7).…”

“…[12] The extent to which theses econdary interactions contribute to the coordina- tion of Li + is unclear at this stage. In the case of 3,f or example, variable-temperature 19 Fa nd 7 Li NMR spectroscopy (298-213 K) indicated no Li···F coupling and the presence of asinglet in the 19 FNMR spectrum at all temperatures shows that the CF 3 groups rotate freely. We next moved on to exploret he coordination chemistry of the new aluminate ligands of 2 and 3.I nc ontrast to the reaction of 1 with FeCl 2 ,whichproduces agreen solution presumably of the half-sandwichc ompound [{EtAl(2-py) 3 }FeCl], [8] neither 2 nor 3 coordinate FeCl 2 .F urther attempts to coordinate other metal ionsw ith 2 and 3 also failed.…”

Catalytic Direct‐Type Addition Reactions of Alkylarenes with Imines and Alkenes

“…By using ac hiral potassium complex prepared from trimethylsilylmethyl potassium (KCH 2 SiMe 3 ) [22] and chiral diamine L, [23] the addition reaction of toluene with 1b proceeded in high yield with significant enantioselectivity (Scheme 2). By using ac hiral potassium complex prepared from trimethylsilylmethyl potassium (KCH 2 SiMe 3 ) [22] and chiral diamine L, [23] the addition reaction of toluene with 1b proceeded in high yield with significant enantioselectivity (Scheme 2).…”

Exploiting σ/π Coordination Isomerism to Prepare Homologous Organoalkali Metal (Li, Na, K) Monomers with Identical Ligand Sets

“…While lithium complexes are often smaller (and consequently more soluble) oligomers, heavier sodium and potassium derivatives have a tendency to give larger insoluble polymers; for example, hexameric LiCH 2 SiMe 3 [10] is commercially available as a hexane solution yet its K congener is insoluble in the same medium. [11] Whereas judicious choice of anion and Lewis donor can disrupt the polymerization of lithium species, it is much more arduous to do so with Na or K complexes of simple organic anions. [12] As a consequence of this there is a paucity of successful structural studies pertaining to simple organopotassium complexes in the literature.…”