Metal-alkyl complexes Metallacycles

Metallacyclic complexes such as the reactant in Equation 10.7b are much more stable thermally than their acylic analogues. This metallacycle decomposes 10 times more slowly than the dibutyl compotmd, and it decomposes by pathways that do not include p-hydrogen elimination. Additional metal-alkyl complexes that are stable to p-hydrogen elimination because of their metallacyclic structure have been isolated as part of studies on complex molecule s mthesis. One example of such a complex is shown in Equation 10.9. ... 

The polymerization and oligomerization of alkenes has been one of the most successful applications of organometallic chemistry to the synthesis of organic products on a large scale. As noted in the introduction to this chapter, organometallic complexes are involved in the synthesis of close to, or in excess of, fifty to one hundred million metric tons of polyolefins and a-olefins per year. In most cases, these products are formed by a series of alkene insertions into metal alkyl complexes in competition with p-hydrogen elimination processes. In other cases, selective dimerization or trimerization of alkenes occurs by the intermediacy of metallacyclic intermediates. 

C(O)CHCH) and 27.5 Hz (C(O)CHCH) shifts in the resonances for the matallacyclic ring H resonances can be observed upon the addition of 0.1 M Li[CF3S03] to a THF-d8 solution of 2c. Similarly, alkyl cations and H+ are known to attack the O atom of the metallacyclic ring carbonyl of 2b and 2c, and the product formed from the methylation of 2b has been crystallographically characterized (73). Furthermore, alkali cations, through an interaction similar to that proposed in equation (2), are known to inhibit the decarbonylation of transition metal acyl complexes (26). 

Other reversible y6-alkyl eliminations cause the transformation of ruthenacy-clobutanes to methyl allyl ruthenium derivatives (Eq. 6.26) [152], or alkyl exchange by a rare formal -alkyl elimination in a metal alkenyl complex (Scheme 6.52) [153]. Reversible propene extrusion by /1-alkyl elimination has also been described for some zirconium metallacycles [154]. 

A simple way to lower the reactivity of metal-alkyl bonds is to incorporate them in a metallacycle. Along these lines, complexes incorporating the 0-CH2-C6H4-P (o-tolyl)2-KC,P ligand on platinum have been used as robust building blocks for constructing dative Pt°—>Ag [31, 33] and Pt Hg bonds [34-37]. 

A common theme in many reactions is the generation of an equivalent of CP2M through the reduction of a tetravalent precursor metallocene with a metal alkyl such as n-butyllithium. The chemistry of olefin complexes of CP2M is characterized by a facile interconversion between formally M(II) and M(IV) manifolds, as shown in equation 34. Another central motif of metallocene chemistry, especially that of titanocenes, is the accessibility of pathways connecting metallacycles and metal alkylidene complexes, eg in alkene metathesis (eq. 35). 

As stated earlier, (11.3.1), the multiple insertion of carbon monoxide into the same metal-hydrocarbyl bond is a rather elusive reaction. On the other hand, multiple insertion of isocyanide has been reported for nickel(II). For example, when the nickelfO) derivative Ni(t-BuNC)4 was treated with Mel in hexane at RT, consecutive insertion of three RNC groups was observed to give the product of reaction (e), as a consequence of a primary oxidative addition of the alkyl iodide to the nickel(O) complex. It is interesting that one of the two terminal fragments of the five-membered metallacycle is reminiscent of an arrangement of the first insertion product. 

Osmacycle (48) arises from an alkyl osmium compound by a complex thermal rearrangement which has been studied in detail (Equation (19)) <86JAI346, 90JA704). A phosphonium-platinacyclopropane arises by intramolecular addition to an alkene (Equation (20)) <90OMl2ll>. Metallacycles are also available by addition of a sulfoxonium ylide to a metal-phosphorus double bond (Equation (21)) <90CB739>. 

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

Neither the Pd(II) nor the Pt(II) complexes of allylphosphines react with nucleophiles tp form -alkyl-metal complexes. Instead, longer aliphatic chains are required to form chelate-stabilized ring systems. Thus, both 1-butenyl- and 1-pentenylphosphines react with PtClj to give stable 6- and 7-metallacycles ... 

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