Metallacycles isomerization

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

The distances and angles (70—74° at C-a and 147—150° at C-(3) in the metallacycle correspond closely to those calculated for organic cyclocumulenes such as cyclohexa- and cy-cloheptatrienes [23]. According to other theoretical calculations, titana- and zirconacyclocumulenes are thermodynamically more stable than the isomeric bis(o-acetylide) complexes [24], The calculated data are in good agreement with the obtained experimental values. All four carbon atoms of the former diyne are viewed as having p orbitals perpendicular to the plane of the cyclocumulene. The sp-hybridized internal C atoms possess additional p orbitals in that plane, which are used to establish a coordination of the relevant bond to the metal center. 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

This catalyst system was the first to utilize both terminal alkynes and olefins in the intramolecular reaction. Although a mechanistic rationale for the observed stereoselectivity was not offered, the formation of the single stereoisomer 26 may be rationalized through the diastereotopic binding of the rhodium complex to the diene moiety (Scheme 12.3). This facial selective binding of the initial ene-diene would then lead to the formation the metallacycle III, which ultimately isomerizes and reductively eliminates to afford the product [14]. 

A plausible mechanism that diverges at a common intermediate and may account for the product distributions is shown in Scheme 1. Reaction between the Ni catalyst and cyclopropylen-yne 1 would ultimately afford eight-membered metallacycle 6, which could result from either initial oxidative coupling between an alkene and alkyne (5a) [8-10] or initial isomerization of the VCP (5b). Ultimately, (i-hydride elimination from 6 and reductive elimi-... 

Vinylidene osmium(O) (187) (Section II,B,3,g) reacts with sulfur, selenium, or copper chloride to give complexes 233 via electrophilic addition to the osmium-carbon bond (116) (Scheme 18). Complex 187 also reacts with benzoylazide to form the five-membered metallacycle 234 (Z) which isomerizes 234 (E) on heating in benzene (141). 

Consistent with this picture, reaction of pure metallacycles with the 2,4-disubstitution pattern with nitriles gives isomerically pure 2,4,6-trisubstituted pyridines Y. Wakatsuki and H. Yamazaki, 7. Chem. Soc., Dalton Trans., 1978, 1278. 

The heterometallacyclic alkyne-containing complex 85 (M = Mo or W, R = Pr or /7-MeQH4, R = Me or Ph) isomerizes into 86 above 20°C (90). At 55°C the tungsten compound with R being Pr and R Ph rearranges, with incorporation of the alkyne ligand into a metallacycle and extrusion of the thiolate from the chelate ring, to form 87. 

However, we have demonstrated the formation of a metallacycle [(dipy)(Cl)Rh-(0-CH2-CHPh) or (dipy)(Cl)Rh-(0-CHPh-CH2)] from styrene and dioxygen. These intermediates could give rise to both styrene oxide and the carbonate. The higher reaction rate when starting from styrene and dioxygen with respect to the epoxide can be, thus, justified. High temperature (> 353 K) often cause decomposition of the catalyst. Two mutually free cis positions are necessary for the formation of the metallacycle, that interacts with carbon dioxide and yields the carbonate so, in the presence of Rh(diphos)2Cl and Rh(dipy)2Cl, no conversion at all into the carbonate has been observed, either starting from styrene or from styrene oxide. In the latter case, only a minor isomerization into acetophenone and phenylacetaldehyde has been observed. 

A few molecular orbital calculations have been reported for these metallacycles. An ab initio study of a postulated azoniumrhodacyclopropane produced a structure in good agreement with known azoniummetallacyclopropanes. This as yet unknown heterocycle was studied because it has been proposed as an intermediate in the stereoselective isomerization of allyl amines to enamines... 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

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