Metallation heterocyclic systems

Prefixes and Suffixes Used in Naming Metal Heterocyclic Systems... 

Compared to the previously described transition metal-catalyzed transformations in this chapter, microwave-assisted Stille reactions [74] involving organotin reagents as coupling partners are comparatively rare. A few examples describing both inter- and intramolecular Stille reactions in heterocyclic systems are summarized in Scheme 6.38 [47, 75-77]. Additional examples involving fluorous Stille reactions are described in Section 7.3. 

His present interests include the development of new synthetic methodologies in carbohydrates, free radical chemistry, organometallic chemistry (Pauson-Khand reaction, transition metal (PtCl2, AuCl)-mediated cycloisomerization of polyunsaturated precursors), and synthesis/biologi-cal evaluation of heterocyclic systems (CSIC reaction, tacrine analogs). 

In laboratory-scale homogeneous catalysis applications, in the last decade further investigations have been carried out in which a less soluble organo-metallic catalyst system was utilized for metathesis reactions [46]. Under RCM-conditions, it was possible to convert substrates with functional groups that were problematic due to their potential to inactivate the rutheniiun catalyst here, the conversion in supercritical carbon dioxide avoids the protection of critical amino groups as an additional synthetic step. Consequently, it was possible to synthesize a number of carbo- and heterocyclic products with varying ring size (C4 to Cie). 

Ru(CO)3(GeMe)3]3 0.072 Triangle of Ru atoms form part of first established six-membered heterocyclic metal ring system. i... 

Ring and substituent carbanions situated not only a-, but also /3- and y-to unsaturated (sp ) heterocyclic nitrogen are discussed. Some of the heterocyclic systems mentioned here have also been individually reviewed elsewhere, and in these cases the present work concentrates on more recent aspects. However, earlier work is still discussed where it is felt appropriate or necessary, in order to provide a unified coverage of the subject. The greatest emphasis is placed on methods for overcoming the reluctance often shown by nitrogen heterocycles toward carbanion formation, and the direction of metalation to specific sites where more than one is available. 

Finally, the primary site of reaction with some heterocyclic systems may not be one that is desired for final elaboration, and in these cases blocking of the initial reaction site is necessary. This can often be achieved by a consecutive metalation and alkylation sequence, but as with N—H protection the overall process, including final removal of the blocking group, must be achievable in reasonable yield. Trialkylsiyl groups have so far received the most attention in this area, because of their ready addition, relative stability under metalation conditions, and facile hydrolysis. 

The structural diversity associated with bicyclic 5-5 fused heterocyclic systems containing two heteroatoms in each ring has been noted previously in both CHEC(1984) <1984CHEC(6)1027> and CHEC-II(1996) <1996CHEC-11(7)115>. In recent years this diversity has also been reflected in the number of applications that have been found for these compounds which include organic metals, molecular clips and receptors and molecular magnets. These fields will be examined in due course through this chapter. 

It is not surprising, that the transition metal catalyzed carbon-heteroatom bond forming reactions constitute a major part of the transformations used for the preparation of heterocyclic systems. This chapter summarises all those catalytic transformations, where the six membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom. Annulation reactions, involving the formation of a carbon-heteroatom bond are discussed in Chapter 4.4. 

Transition metal catalyzed insertion reactions offer a variety of alternate approaches for the preparation of heterocyclic rings, of which Heck reactions were utilised extensively to prepare rings with more than 6 atoms. At the end of this Chapter some examples of the use of insertion reactions in the formation of the carbacyclic part of condensed heterocyclic systems will also be presented. 

When the donor atom is a part of the aromatic system, one would expect more obvious differences in reactivity. At present relatively little comparative information is available on such heterocyclic systems. Only on pyridine and its derivatives are there any reasonably extensive data. For pyridine a wide variety of coordination processes are available and pyridine-N-oxide as well as metallic complexes and complexes with nonmetallic Lewis acids must be considered. For comparative purposes the great reluctance with which pyridine undergoes electrophilic... 

Carbanionic Heterocyclic Systems R. R. Schmidt, Led. Heterocycl. Chem., 1978,4,97-114. Heterocyclic tt-Complexes of the Transition Metals K. H. Pannell, B. L. Kalsotra and C. 

Predictably, the five-membered heterocyclic rings are readily reduced by metal-acid systems via the heterocyclic cations (B-77MI30507, 72HC(25-l)i, 8lAHC(29)34l). Indoles are reduced, for example, by zinc dust and phosphoric acid or by tin and hydrochloric acid, to give indolines. Under these conditions 1,2,3,4-tetrahydrocarbazole is reduced to the c/s-fused l,2,3,4,4a,9a-hexahydrocarbazole whilst the corresponding reduction of 2,3-dimethylindole yields a mixture of the cis- and trans- 2,3-dimethylindolines (72HC(25-i)l). 

Heterocyclic systems can be named by various methods, depending on ring size, the presence or absence of metal atoms, the availability of trivial names, etc. For this index the naming of the heterocyclic parents has generally followed the practices of IUPAC and Chemical Abstracts. Thus Hantzsch-Widman names have been employed for all monocyclic systems and for the other systems when the smallest ring size is five atoms or more, but replacement nomenclature has usually been followed for bicyclic and higher systems when they contain three- and four-membered rings. 

Claisen rearrangements in heteroaromatic systems, 42, 203 in nitrogen heterocyclic systems, 8, 143 Complex metal hydrides, reduction of nitrogen heterocycles with, 6, 45 39, 1 Concept of aromaticity in heterocyclic chemistry, 56, 303 Condensed heterocyclo[n,m- , b, or cjquinazolines, 52, 1 Condensed 4-thiazolidinones, 49, 1... 

The 1,2,4,3-triazaphospholes are colorless or pale yellow distillable liquids or crystalline solids. They are not oxidized by air and are reluctant to react with sulfur. Three isomeric heterocyclic systems of 277-1,2,4,3-triazaphospholes 15, 177-1,2,4,3-triazaphospholes 16, and 477-1,2,4,3-triazaphospholes 17 are known and they differ considerably in their behavior <1996CHEC-II(4)771>. The synthesis of 1,2,4,3-triazaphospholes and reactivity of different isomers of 1,2,4,3-triazaphospholes in the reactions at a ring nitrogen, in the addition to the P=N bond, oxidative addition to the ring phosphorus, cycloaddition reactions, and the formation of transition metal complexes are systematically covered in CHEC-II(1996) <1996CHEC-II(4)771>. The 1,3,4,2-thiadiazaphospholium ions 18 are only briefly mentioned in CHEC-II(1996) and no new results on their chemistry have been published in the last decade. 

The heterocyclic system of 1,2,3-azadiphosphole 20 is relatively uncommon. The preparation and properties of the 1,3-diphospholyl-substituted 1,2,3-azadiphospholes and 1,2,3-benzazadiphospholes are briefly discussed in CHEC-11(1996) <1996CHEC-II(4)771>. In a more recent research, several representatives of this structural class (e.g., compounds 116 <2000CC1659> and 117 <2005EJI1955, 1999JOM(580)386, 1997JOM(529)243>) were prepared and used as ligands in the transition metal complexes. 

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