Metal atom reactions with pyridines

Many pyridine-indole compounds are biologically active. A growing number of methods for the preparation of indolylstannanes have been developed. 2-Trialkylstannylindoles, for example, have been synthesized via directed metalation followed by reaction with tin chloride [91-93]. The latest indolylstannane syntheses include Fukuyama s free radical approach to 2-trialkylstannylindoles from novel isonitrile-alkenes [94], and its extension to an isonitrile-alkyne cascade [95]. Assisted by the chelating effect of the SEM group oxygen atom, direct metalation of 1-SEM-indole and transmetalation with BujSnCl afforded 2-(tributylstannyl)-l//-indole 108, which was then coupled with 2,6-dibromopyridine to give adduct 109. 

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 reaction of [W2(OBut)6] with NO in pyridine leads to cleavage of the W—W tripk bond,303 and the formation of [W(OBu )3(NO)py]. In this structure, the geometry around the metal atom is trigonal bipyramidal, with the py ligand trans to NO. 

It should be mentioned that in many cases it turns out to be impossible to isolate the bimetallic mixed-ligand complex. The isolable reaction products appear to be homometallic heteroleptic species. The composition of the reaction products is often dependent on the polarity and donor properties of the solvent as the molecules of the latter compete with the bridging alkoxo-, car-boxylato-, or p-diketonatoligands for the place in the coordination sphere of metal atoms. Thus if the reaction of Pb(OAc)2 with Sn(OBu )4 mentioned above is carried out in pyridine, it gives Sn(OBu )3(OAc)Py and Pb(OAc)(OBu ) as the major products and not the bimetallic complex [298]. Sn(OBu )3(OAc)Py turns also to be the major product of the reaction of Sn(OBu )4with Me3SiOAc... 

Kinetic studies of the substitution reaction of 2-chloro-l-methylpyridinium iodide with phenoxides are consistent with the SnAt mechanism, with rate-determining nucleophilic attack.38 The effects of a variety of ring substituents on the reactivities of 2-fluoro- and 2-chloro-pyridines in reactions with sodium ethoxide in ethanol have been examined. The results were discussed in terms of the combination of steric, inductive, and repulsive interactions.39 Substitution in 2,4,6-trihalopyridines normally occurs preferentially at the 4-position. However, the presence of a trialkylsilyl group at the 3-position has been shown to suppress reaction at adjacent positions, allowing substitution at the 6-position.40 Methods have been reported for the introduction and removal of fluorine atoms for polyfluoropyridines. Additional fluorine atoms were introduced by metallation, chlorination, and then fluorodechlorination, while selective removal of fluorine was achieved by reduction with either metals or complex hydrides or alternatively by substitution by hydrazine followed by dehydrogena-tion-dediazotization.41... 

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