Deprotonated pyridines Deprotonative metalation/deprotonation

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

In the dihapto mode the pyridine ring can be protonated intermolecularly at nitrogen, or even intramolecularly deprotonated at carbon. The first evidence for metal C—N insertion is the reaction of the metallaaziridine complex (111) with homogeneity LiHBEt3 in THF at low temperature that yields (112) (Scheme 49).251-254 Experiments with carbon nucleophiles (RMgCl, MeLi) in place of LiHBEt3 have provided valuable information to allow discrimination between... 

Direct metalation of 2,6-disubstituted pyridines has also been reported to occur at the 4-position under certain conditions. Thus the 2,6-bistrimeth-ylsilyl chromium tricarbonyl compound 96 gave the 4-lithio derivative 102 [91JCS(P1)501], and 2,6-dichloropyridine gave mainly the 4-lithio derivative 103 under kinetic deprotonation conditions, in contrast to the thermodynamic situation where 3-lithiation was preferred (91JOC4793). 

In general, reactive carbon electrophiles have been shown to react preferentially at the nitrogen atoms of the purine bicycle (see Section 10.11.5.2.1). However, 9-(2,3,5-tris-0-/r /T-butyldimethylsilyl)-a-D-ribofuranosyl-6-chloro-2-(tri-butylstannyOpurine reacted with benzoyl chloride to substitute the 2-tributylstannyl group (PhCOCl, pyridine, toluene, 60% yield) <1997JOC6833>. Indirect C-alkylations have been achieved through deprotonation and alkylation see Section 10.11.5.3.4. The major routes to (7-alkyl and (7-aryl substitution are through nucleophilic displacement or transition metal-catalyzed reactions of halopurines see Sections 10.11.7.4.1 and 10.11.7.4.2. 

The formation of 295 prompts a mechanistic comparison with results observed in the metalation of 3-OMOM pyridine and N,(V-dialkyl 3-carbamoyloxypyridine, in which only 4-metalation results were observed (see later). Whereas 2-deprotonation of 3-ethoxypyridine may be driven by the greater coordination ability of RLi base to the ring nitrogen 299, such interaction may be less significant for the corresponding 3-OMOM and carbamoyloxy systems because of the greater coordination effects of two heteroatoms, 300 and 301, respectively (Scheme 90). 

Early work by Abramovitch showed that treatment of 3- N,N-diethylaminosulfonyl)pyridine with PhLi leads to C-2 addition (65CJC1752), a result which discouraged used of alkyllithiums for deprotonation of pyridyl sulfonamides. In a detailed investigation, Queguiner found that (a) the piperidine sulfonamide is the best DMG (b) LDA is a useful metalation base in 2 equiv. amount (80% yield) rather than 1 equiv. (27% yield) on the basis of PhCHO quench (83S822). 

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Complexes of pyridine-2-carboxylic acid (picolinic acid, picH) and its substituted derivatives commonly exhibit the N—O bidentate nature of this aromatic amino acid. From aqueous solutions chelates are obtained with the coordinated carboxylic group deprotonated, or neutral ligand forms may be isolated from non-aqueous media.26 Bis chelates are common in either case with bivalent metal ions. The tris chelates of trivalent cobalt27 and manganese28 have been structurally characterized recently. The latter is tetragonally distorted in a structure similar to Mnin(oxine)3. 

The 2-pyridylhydrazone of pyridine-2-carbaldehyde yields tridentate metal complexes which can be deprotonated to neutral complexes (81).175-177 These in turn can be reprotonated, alkylated or acylated on nitrogen and reacted with benzenediazonium fluoroborate on carbon (Scheme 33).178... 

The iodination of pyridine, quinoline, and isoquinoline via a-metalation using lithium di-fert-butyltetramethylpi-peridinozincate (TMP-zincate) proceeds smoothly at room temperature using iodine as the electrophile. The chemoselective deprotonative zincation generated 2-iodopyridine 70 and 1-iodoisoquinoline 71 in 76% and 93% yield, respectively. Quinoline metalated preferentially at the 8-position to give 61% yield of the 8-iodo derivative 72 and 26% yield of 2-iodoisoquinoline 73 (Equations 25-27) <1999JA3539>. 

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