Alkyne derivatives metallated systems

The involvement of metalocyclopentadiene intermediates in the cyclotrimerization of alkynes (Scheme 6) has been established for some metal systems 162, 198, 201-204). However, there is no evidence to indicate participation of cobaltocyclopentadiene species in cyclotrimerization reactions involving carbonylcobalt complexes. It is worth noting, however, that 1,4-addition of an alkyne to such an intermediate would lead to the formation of 1,2,4- and 1,3,5-substituted benzenes. Moreover, the 1,2,4-derivative would be favored statistically. This line of reasoning has led to the incorporation of a cobaltocyclopentadiene intermediate in the mechanism proposed 120) (Scheme 7) for the Co4(CO)ia-catalyzed cyclotrimerization of PhC=CH. 

A number of other mechanistic sequences are thought to occur in alkyne trimerizations mediated by other metal systems. Catalysts derived from Co2(CO)s follow a characteristic sequence of steps that involves exclusively dinuclear complexes (Scheme 26). The nature of the final (alkyne)3C02(CO)4 intermediate, a so-called flyover complex, is supported by both X-ray crystal structure data as well as its involvement in forming other products besides benzenes. These dinuclear cobalt systems are note-... 

Cyclotrimerization and cyclotetramerization of alkynes to form benzene and cyclooctatetraene derivatives was first achieved with various nickel catalysts26. Several other metal systems are also active and numerous applications have been reported3 5. Various concerted or stepwise reaction paths have been suggested3,5,2 -29. 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

The formation of benzene (or substituted benzene derivatives) is a common transformation catalyzed by numerous homogeneous and heterogeneous metal catalysts, mainly Co, Rh, Pd, and Ni.63-69 Even highly crowded molecules, such as hexaisopropylbenzene, could be synthesized in the presence of metal carbonyls such as [Co(CO)4]2.70 A very simple catalyst system, Me3SiCl and Pd on carbon in refluxing tetrahydrofuran, has been shown to transform symmetrical alkynes into hexaalkylbenzenes in excellent yield.71... 

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

Introduction of two or more terminal diyne units, either at a metal center or, for example, on an q-ring-metal complex, gives the possibility of generating metal-based polymers. In the case of alkynes, many examples of polymers have been described, but studies of similar systems derived from di- or poly-ynes are in their infancy.90 Two- and three-dimensional polymers can be constructed in similar fashion, extension leading (conceptually, at least) to the formation of novel carbon allotropes. 

Nina A. Nedolya was born in Irkutsk (Russia) and educated in organic chemistry at the Irkutsk State University (Diploma 1972, PhD 1982, DSc 1998). From 1995 to 1999 she was associated with Prof. L. Brandsma at the Utrecht University (The Netherlands). In 1999 she obtained her second PhD from the Utrecht University. She is presently Head of the Research Group of Chemistry of Heterocyclic Compounds at A. E. Favorsky Irkutsk Institute of Chemistry. She is the author of over 210 review articles and research papers. She is also one of the inventors for 112 patents. She is interested in the chemistry of polyfunctional unsaturated heteroatomic systems (vinyl, allenyl, and alkynyl ethers and their derivatives, linear and cyclic heteropolyenes, hetero-cumulenes), including synthesis of important heterocycles, particularly pyrroles, thiophenes, thiazoles, imidazoles, dihydrofurans, dihydropyridines, pyridines, quinolines, dihydroazepines, and azepines, based on metallated allenes or alkynes and/or heterocumulenes. 

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