Organometallic heterocycles aromaticity

Heterocyclic aromatic compounds such as pyrrole are readily metallated with Grignard reagents. The resulting compounds have N"Mg bonds and are, therefore, not organometallic compounds, but on reaction with electrophiles give 2-substituted pyrroles [14] (eq (4)). The reaction of chloroform or bromoform with PrMgCl at -78 °C in THF-HMPA (4 1) is mild and convenient method for the generation of an unstable carbenoid in the solution [15] (eq (5)). 

The reaction of A-acyliminium ions with nucleophilic carbon atoms (also called cationic x-amidoalkylation) is a highly useful method for the synthesis of both nitrogen heterocycles and open-chain nitrogen compounds. A variety of carbon nucleophiles can be used, such as aromatic compounds, alkcncs, alkyncs, carbcnoids, and carbanions derived from active methylene compounds and organometallics. 

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8,167 Fluorocarbon Derivatives of Metals, 1, 143 Heterocyclic Organoboranes, 2, 257... 

Most reactions discussed in this chapter rely on the formal attack of an electrophilic organometallic species on the electron-rich aromatic core of a five membered heterocycle. Depending on the way the transition metal... 

Some Reactions of Aromatic Heterocycles Involving the Catalytic Action of the Cyanide Ion E. Hayashi, Shizuoka Yakka Daigaku Kaigaku, 25-Shunen Kinen, 1978, 1-12. Reactions of Organometallic Compounds in the Heterocyclic Field A. Marxer, Led. Heterocycl. Chem., 1976, 3, 131-139. 

The inclusion of the nitrogen donor atom in an aromatic heterocycle allows the possibility of -bonding with the metal centre, giving these ligands some similarities to tertiary phosphines. Thus pyridyl and bipyridyl ligands stabilize organometallic complexes of palladium (see companion volume).2 ... 

For example, unbranched aliphatic acids with an even number of carbon atoms will be generously represented, while one finds a nearly complete absence of odd-numbered and branched acids. Nature produces an incredible diversity of the most ingeniously constructed cyclic products containing cycloaliphatic, aromatic, or heterocyclic moieties, but such derivatives like aniline or thiophe-nol, as well as plethora of other simple representatives of these classes, are not in the list of naturally occurring substances. Such important types as alkyl halides, nitro compounds, and diazo compounds would be sparsely represented by very rare (if any) examples. Even the simplest compounds like formaldehyde, chloroform, diethyl ether, dioxane, etc., which are trivial to organic chemists, turn to be rather exotic for Nature. In the list of items provided by Nature one will notice the almost complete absence of various organometallic compounds, as well as many other classes of structures of immense scientific and practical significance. 

The majority of the known heterocyclic dienes are pentadienes, largely as a result of the discovery that alkynes react with alkali metals and form dianions via dimer addition of the acetylene anion radical. A method that has become useful for the generation of larger cyclic dienes is the addition of an organometallic dihydride to a suitable dialkyne. Synthetic approaches to six-membered dienes tend to be unique for each system, and no general method has been developed. Some of the six-membered heterocyclic dienes have been converted to trienes for the study of systems that may be aromatic. 

The reaction of LVII" and some related heterocycles with a large number of nucleophiles has been investigated thoroughly. Among the nucleophiles, which with few exceptions form sulfonium salts by their reaction with LVII", are water [194, 195], carboxylate ions [196, 197], pyridine [198], organometallic compounds like dialkylmercury and dialkylzinc [199], and Grignard reagents [200], alkenes and alkynes [201], and various aromatic compounds [202-204]. 

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