Metal insertion bonding interactions

Polymerization occurs at active sites formed by interaction of the metal alkyl with metal chloride on the surface of the metal chloride crystals. Monomer is chemisorbed at the site, thus accounting for its orientation when added to the chain, and propagation occurs by insertion of the chemisorbed monomer into the metal—chain bond at the active site. The chain thus grows out from the surface (31). Hydrogen is used as a chain-transfer agent. Chain transfer with the metal alkyl also occurs. 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

In contrast, methyl cyclopropenone is reported283) to react with the Pt-olefin complex 455 at low temperature with replacement of the olefin ligand. In the resulting complex 456 the cyclopropenone interacts with the central atom via the C /C2 double bond according to spectroscopic evidence284). At elevated temperatures a metal insertion to the C1<2)/C3 bond occurs giving rise to 457. Pt complexes of a similiar type were obtained from dimethyl and diphenyl cyclopropenone on reaction with 455 and their structures were established by X-ray analysis285). 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

The electronic influence of the other ligands in the metal s coordination sphere is a major consideration when exploring the coordination chemistry of carbon dioxide. Since the metal-C02 bond is stabilized mainly by back-donative interactions, it would be expected that good donor ligands (e.g., the ubiquitous phosphine ligands) would enhance the binding ability of C02 to the metal center. This has been verified both experimentally (10) and theoretically (20), and has as well been demonstrated to be of importance in C02 insertion processes (see below). 

Because of the possibility that they might exist as intermediates in catalytic reactions, compounds with M-H-C interactions constitute perhaps the most important class of molecules of the M—H—X type. However, it was not until recently that molecules having such interactions were isolated and structurally characterized. As an example of a process in which M-H-C interactions are involved, consider metal insertion into a C—H bond. For this process one could envisage the following sequence of events ... 

The y-agostic interaction is such that the electron pair in the C, -He bond of the alkyl substituent at the metal is partly donated to the electron-poor metal this may play an important role in making more simple the insertion of a coordinated olefin into the metal-alkyl bond. This hypothesis has been tested in some instances [346-350], showing metal-hydrogen interaction via an oc-agostic bond. 

Generally, insertion of the alkyne into a metal-P bond is observed (Scheme 10).188,190 When aminoalkynes are used, the formation of a C=N double bond inhibits the interaction of that carbon with the metal centers of the cluster.186 187 When two PR groups are present, the alkyne has been observed to bridge between them as seen in Scheme 10.195,285 A second equivalent of diphenylacetylene can substitute for two carbonyl groups on the iron triangle.195 The hetero-main group element species Fe3(CO)9(NPh) (P Bu) and Fe3(CO)9(NPh)2 have been reacted with diphenylacetylene.273 Some of the products involved in the acetylene addition reaction are shown here (241-243). 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

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