Insertion and Deinsertion

Equations 8.1 and 8.2 describe the general process of insertion of a ligand into an M-Y bond. The former shows a process known as 1,1-insertion and the latter its 1,2 counterpart. The reverse reactions are known interchangeably as deinsertion, extrusion, or elimination. 

The prototypical example of carbonyl insertion, and the one most thoroughly investigated, involves reaction of CO with (CH3)Mn(CO)5 to yield the acetyl manganese complex, which is shown in equation 8.4. 

From the net equation we might expect that the CO inserts directly into the Mn-CH3 bond if such were the case, the label CO insertion would be entirely appropriate for this reaction. Other mechanisms, however, are possible that would give the overall reaction stoichiometry while involving steps other than insertion of an incoming CO. The following three mechanisms have been suggested as plausible for this reaction. 

Mechanism 1 CO insertion Direct insertion of CO into metal-carbon bond. 

Migration of CO to give intramolecular CO insertion. This would give rise to a 5-coordinate intermediate, with a vacant site available for attachment of an incoming CO. 

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Evidence was shown for migration of an alkyl group in carbonyl insertion, and deinsertion steps between the methyl carbonyl rhodium complex [ r/Vi/ -Indenyl-l -(CH2)3PPh2 Rh(CO)-Me](BF4) and the acetyl rhodium complex [ r/5 r/l-(Indenyl-l -(Cl I2)3PPh2) RhI(COMe)] by crystallography as well as by 1H NMR spectroscopy.28... 

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

Charge-discharge studies of the product obtained by a 1 h pyrolysis at 600°C at a heating rate of 20°Cmin-1 exhibited first-cycle lithium insertion and deinsertion capacities of 825 and 564 mAh g-1, respectively. Subsequent cycles showed a remarkable improvement in cycling efficiency with a 10th cycle efficiency of 100% (105). 

There is a pronounced hysteresis between the voltage profiles of the insertion and deinsertion processes. This hysteresis is unique to these hard-disordered carbons and is not seen with graphitic materials and soft disordered carbons. 

Hydrozirconation occurs with yyn-addition of the Zr-H bond across a C=C or C=C bond (equation 8.16). Due to lower steric hindrance, the addition also tends to be regiospecific, with the zirconium attached to the less substituted position (just as in hydroboration). Internal alkenes and alkynes isomerize to 1-alkyl and 1-alkenyl complexes, respectively—presumably by alternating reactions of insertion and deinsertion—until the complex with the least steric hindrance is formed. 

Insertion and deinsertion (extrusion) of unsaturated compounds such as olefin, CO and isocyanide. 

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

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